A2 Biology Notes for edexcel

1 of 1 Teacher/Lecturer

Practical 5.2 The effect of temperature on the hatching success of brine shrimps Purpose •

To investigate the effect of temperature on the hatching success of brine shrimp.

•

To develop certain experimental skills, namely considering the ethical issues arising from the use of living organisms, presenting results, producing reliable results, identifying trends in data and drawing valid conclusions.

Notes on the procedure If students are given g iven basic information on maintaining brine shrimps this activity could be planned before they are given the Student sheet. The need to keep conditions other than temperature constant should be appreciated. Brine shrimps hatch in salt water that is 2 to 5% salt (optimum 2.8%) and pH 8.5. Oxygen must be present. Light is an added (but not essential) factor for hatching. Datalogging Datalogging could be used to check that the conditions within each of the treatments are maintained at a constant level. This experiment could be completed by small groups of students. If each group completed the range of temperatures then the error between replicates could be investigated. Brine shrimps will hatch in 24 hours at temperatures between 25 and 30 C with an optimum of 28 C. °

°

This activity can be used to highlight experimental and investigative assessment objectives, in particular the ethical issues arising from the use of living organisms (including the disposal of the brine shrimps after the activity) and for the environment, and the need to identify both dependent and independent variables and, where possible, control or allow for them. The method suggested provides precise measurements. The need for valid, reliable results should be considered and the random nature of error could be investigated. The practical procedure and technical notes are based on an investigation that appears in the British Ecological Society publication Brine Shrimp Ecology by Michael Dockery and Stephen Tomkins. This book contains detailed information on the care and breeding of brine shrimps. It is available from the British Ecological Society or from: Homerton Brine Shrimp Project Dept of Biological Sciences Homerton College Cambridge CB2 2PH Tel: 01223 507175 Price from Blades Biological, £48.26 (2009) including post and packing, brine shrimp eggs and innoculum. For more details, including including the preparation of a salt water aquarium in which to keep the brine shrimps, see the teacher area of the British Ecological Society website.

Edexcel practical materials created by Salters-Nuffield Advanced Biology, ©University of York Science Education Group.

10

Technician 1 of 1

for patterns and and correlations correlations Practical 5.1 Looking for

Purpose •

Safety

To carry out a study on the ecology of

Teachers/lecturers Teac hers/lecturers must follow their LEA/school/colleg LEA/school/college e

a habitat.

policy and local rules for off-site visits, especially with regard to identification of hazards and risk assessments. Additional information is available in the DCSF guidance Health and Safety of Pupils on Educational Visits on Visits on the Teachernet Teac hernet website. The hazards will vary depending on the site chosen. Risk assessments will identify the risk.

The standard equipment required for carrying out an ecological study is detailed below. below. Additional items required to measure abiotic factors will depend on the site selected and type of study being undertaken. Requi Re quirem rement entss per stu studen dentt or grou group p of stud student entss

Notes Not es

A 50 m tape measure. If this is not available a 10–20 m tape measure can be used

A piece of rope with 0.5 m intervals marked on it will also work. Two will be needed if a grid layout is to be used.

A quadrat

Either square or point quadrats can be used. Square frame quadrats with subdivisions are useful. The ‘standard’ square frame quadrat is 50 cm 3 50 cm, but smaller and larger ones can be used.

Small, numbered pegs

At least 50.

A clipboard A clear plastic bag large enough to get clipboard and hand with pencil in

To protect students’ notes if it rains.

Ranging poles

The geography department may have these already.

A clinometer

The geography department may have one already.

Small plastic vials or dishes

For holding invertebrates while identifying them on site.

A key to to organis organisms ms likely likely to be found found in the the area area

The Field Field Studi Studies es Counc Councilil produce producess a range range of of excellen excellentt guides. guides.

Access to a compass

To describe the aspect.

Access to an OS map of the area

To accurately pinpoint the site and for background knowledge.

Jam jars

For pitfall pitfall traps. traps.

Pooter Sweep net Tullgreen and/or Baermann funnel Thermometer Whirling hygrometer Light meter


151 15 1

1 of 1 Technician

Practical 5.2 The effect effect of temperature temperature on the hatching success of brine shrimps Purpose •

To investigate the effect of temperature on the hatching success of brine shrimps.

•

To develop certain experimental skills, namely considering the ethical issues arising from the

use of living organisms, presenting results, producing reliable results, identifying trends in data and drawing valid conclusions.

For detailed guidance on the care and breeding of br ine shrimps see the publication Brine Shrimp Ecology by Michael Dockery and Stephen Tomkins, available from: Homerton Brine

Shrimp Project, Dept of Biological Sciences, Homerton College, Cambridge CB2 2PH Tel: 01223 507175. Price from Blades Biological, £48.26 (2009) including post and packing, brine shrimp eggs and innoculum. For more details see the teacher area of the British Ecological Society website.

General note This practical takes place over several days. The students set up the beakers with eggs to hatch on the first day. On subsequent days they count the number of eggs that have hatched. Requirements pe per st student or or gr group of of st students

Notes

Brine shrimp egg cysts

Available from pet shops. There are approximately 24 000 egg cysts per gram so only tiny quantities are required. Brine shrimps will breed and produce cysts that can be collected. They adhere to the sides of an aquarium tank.

100 cm3 beakers (one for each temperature to be tested) 100 cm3 de-chlorinated water for each treatment

Tap water can be de-chlorinated by leaving it to stand for 48 hours.

40 cm3 beaker of salt water 2 g sea salt for each treatment

Students could be supplied with the salt water already prepared.

Stirring rod Access to refrigerator Access to water baths or incubators (one for each temperature to be investigated)

A range of temperatures between 5 and 35 °C is recommended. The optimum for hatching is 28 °C.

White A4 sheet of paper Sheet of graph paper 3 cm × 4 cm Magnifying glass Pair of forceps Bright light

A lamp or light from one side. For counting larvae on the second day.

Fine glass pipette

For counting larvae on the second day.

Small beaker of salt water

For counting larvae on the second day.

Large beaker or tank of salt water 1 substrate and algae, set up well in advance. Details in the Dockery and Tomkins book

To hold the brine shrimps after hatching. This tank can be maintained for subsequent investigations of brine shrimps.


152 15 2

Student 1 of 8

Practical 5.1 Looking for patterns

Purpose •

To carry out a study on the ecology of a habitat.

Observing patterns Have you ever walked into a wood and noticed that the vegetation changes as you enter? Why do the bluebells only occur under the trees? Or have you been clambering over a rocky shore and spotted that the seaweeds grow in distinctive bands and that you only find mussels where the tide is far out? What causes these patterns in plant and animal distribution? When ecologists study habitats, they try to account for plant and animal distribution, correlating them to the abiotic and biotic factors that are affecting the habitat. Abiotic means ‘non-living’ and examples of abiotic factors include light intensity, slope, humidity, wind exposure, edaphic (soil) characteristics such as pH and soil moisture, and many more. Biotic means ‘living’ and examples of biotic factors include competition, grazing and predation. All species of plants and animals you encounter in the wild are well adapted to the set of conditions encountered in their usual habitat. If they weren’t they would either grow somewhere else or become extinct!

Studying patterns Look around your local habitats and spot any patterns in distr ibution and abundance of organisms. You don’t need to go far; you might notice something in your school grounds or the local park. You might have a look at the distribution of plants in trampled areas of the sports field or grass paths; are there any patterns? Once you have identified a pattern, think about why it might have come about. Describe the pattern and use appropr iate biological ideas to suggest an explanation. Now you need to plan a fieldwork investigation to test your idea. When planning any investigation you need to: •

decide what data you are going to collect

•

select suitable apparatus and methods

•

ensure you are going to collect valid and reliable data

•

decide how you will analyse it once collected

•

complete a risk assessment and decide on steps to avoid or minimise the harmful effects of any hazards

•

conduct a trial to inform your planning.

Read the following section, which briefly mentions some of the techniques that you could use. There is more detail in Student Practical suppor t sheet Ecological sampling (page 26). You can also look at the British Ecological Society (BES) website education pages – the students 161 section contains detailed information about sampling techniques.Your teacher may also give you details of websites that can be used to help you prepare your plan.


23

2 of 8 Student

Practical 5.1 (cont.)

Looking for patterns

Completing a transect study One of the easiest patterns to sp ot is zonation in the vegetation and animal distribution – as you go from one place to another the vegetation and animal distribution changes. A zonation can often be explained by a gradual change (a gradient) in one or more physical or abiotic factors. A transect is often used to study zonation in vegetation or non-mobile animal distribution. A transect is a line along which systematic records can be made.

Comparing two sites Frequently ecologists may notice a distinct pattern that does not show a gradual change and may be related to one or more factors at the two sites. For example, the vegetation in one area of a field may be very different from the rest of the field, or the species found upstream and downstream of an outflow pipe discharging into a river may seem to differ. A transect may not be the best method for this type of investigation; instead sampling of each area may be more appropriate.

Procedure 1

Plan how you are going to collect reliable and valid data that will test your hypothesis. You need to make the following decisions. •

The most appropriate sampling method to use (e.g. random or systematic sampling).

•

The position and length of any transect to use (Figure 1).You need to make sure your transect extends far enough to sample all the possible zones.

•

The size and number of quadrats to use, and their positioning.

•

The species of plants and animals you are to record – you should focus on those which will enable you to test the hypothesis under investigation. (You may need to find out more about the species concerned using secondar y sources).

•

The method to use for measuring abundance.

•

The abiotic factor(s) you are going to record. Although you may be investigating the correlation between, for example, soil moisture and the distr ibution of plant species, there may be other factors that could affect the distribution of organisms. It is not possible to control these variables but you can measure them and take them into account when analysing your results.

•

The appropriate method for measuring the abiotic factor(s).

•

How the data will be analysed.

•

How to avoid or minimise any risks when completing the eldwork.

A pilot study in advance of the main data collection will help you make these decisions. peg marked ‘20’ 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

origin peg marked ‘0’

0.5 m � 0.5 m quadrat no. 6

21 22 23 24 25 26 27 28 29 30

transect continues

tape case

Figure 1 One way of laying out a tape measure for a transect study. Quadrats are laid down at regular intervals along the tape and the abundance of species within each quadrat is recorded. Edexcel practical materials created by Salters-Nuffield Advanced Biology, ©University of York Science Education Group.

24

Student 3 of 8



2

Collect the data.

3

When you have collected your data, you must present it in an appropriate way to help you identify any patterns in the data. For transect data you can draw kite diagrams by hand or use a computer programme such as FieldWorks which may be available from your teacher.

4

Analyse your data to reveal any patterns or significant differences, and explain the main relationships between species and abiotic factors using scientific knowledge. Determine if your original hypothesis was corr ect. If you have suitable data, you can calculate correlation coefficients between your biotic and abiotic data. For example, you can see if there is a significant positive or negative correlation between the factor you think is responsible for the pattern and the distr ibution of the organisms you have recorded. Remember that correlations do not prove cause and effect.

5

In your write-up interpret your results using biological principles and concepts. Support any conclusions you make with results. Discuss the limitations of your results and conclusions based upon them, and suggest modifications that you could make to the procedure.


25

4 of 8 Student

Practical support sheet Ecological sampling

Why sample in ecology? In an ideal world when investigating, say, the number of dandelions in two meadows, you would count every single dandelion in each. The problem is that this might take forever and become very, very boring. So, instead, you need to take a sample. You might estimate the number of dandelions in each meadow by counting the number in several small areas and then multiplying up to calculate a value for each meadow. The idea is to maximise the usefulness of your data while minimising the effort required to collect them.

Random sampling Frequently, ecologists notice a distinct pattern that may be related to one or more factors at two sites. For example, the vegetation in one field may be very different to that in another field, or the species found under oak trees may be different to those under ash trees, or the species upstream and downstream of an outflow pipe discharging into a r iver may seem to differ. To make valid comparisons, samples need to be taken from both sites. If the investigator chooses where to sample, the sample will be subjective. Random sampling allows an unbiased sample to be taken.

Using a grid In a habitat, such as a meadow or heathland, tape measures put on the ground at right-angles to each other can be used to mark out a sampling area (Figure 1). Using a pair of random numbers you can locate a position within the sampling area to collect your data. The random numbers can be pulled from a set of numbers in a hat, come from random number tables, or be generated by a calculator or computer. The two numbers are used as coordinates to locate a sampling position within the area. The first random number gives the position on the first tape and the second random number gives the position on the second tape.

5

2 e r u s a e m e p a T

position of 0.25 m2 quadrat using random numbers 2, 4

4

3

2

1

1

2

3

4

5

Tape measure 1 Figure 1 Using measuring tapes to define a sample area.


26

Student 5 of 8

Practical support sheet (cont.)

Ecological sampling

If you are sampling fixed objects within an area, for example the area of

Pleurococcus (an

alga) on the shaded side of trees in a wood or the number of woodlice under rocks, you could number all the trees or rocks and then use random numbers to select which trees or rocks to sample. This sampling idea is also used when measuring the number o f cells in a culture. The culture is mixed to give a reasonably uniform distribution of cells and then a known volume is placed on a haemocytometer (a special cavity slide with a ruled grid in the centre). You then count the number of cells that occur in, say, 25 squares of the grid. Because you know the dimensions of the grid squares and the depth of the liquid above the square, you can work out the volume of culture in each square, and then calculate a mean number of cells per cm 3 of the culture.

Systematic sampling Random sampling may not always be appropriate. If conditions change across a habitat, for example across a rocky shore or in a sloping meadow that becomes more boggy towards one side, then systematic sampling along a transect allows the changes to be studied. A transect is effectively a line laid out across the habitat, usually using a tape measure, along which samples are taken. The sample points may be at regular intervals, say every 2 m across a field, or they may be positioned in relation to some morpholog ical feature, such as on the ridges and in the hollows in a sand dune system.

Sampling techniques Quadrats Quadrats are used for sampling plant communities and slow moving or stationary animals, for example many of those found on rocky shores. There are two types of quadrat: a frame quadrat and a point quadrat. A frame quadrat is usually square; the most commonly used is 50 cm by 50 cm (0.25 m 2) and may be subdivided into 25 smaller squares, each 10 cm by 10 cm. The abundance of organisms within the quadrat is estimated (see the section Methods of measuring abundance and Figure 3). Quadrats may be placed across the site to be sampled using random or systematic sampling methods. Throwing quadrats is not random and can be dangerous. It is important to sample enough quadrats to be representative of the site, but why do 1000 quadrats if 10 will give almost as accurate a result? To find out the optimum number of quadrats required, record the number of species in each quadrat and plot the cumulative results against number of quadrats until sampling additional quadrats does not substantially increase the number of species recorded. A point quadrat frame (Figure 2) enables pins to be lowered onto the vegetation below. Each species touched is recorded as a hit. The percentage cover for a particular species is calculated using the equation: % cover

5

hits ____________ 3 100 hits 1 misses


27

6 of 8 Student


Ecological sampling

knitting needle

holes metal spike (such as a tent peg) inserted in ground multiple hit

Figure 2 A point quadrat frame. Each plant species touched by the needle is recorded.

Methods of measuring abundance Density Count the number of individuals in several quadrats and take the mean to give number per unit area, for example per metre squared (m 22). In many plant species (e.g. grasses) it is ver y difficult to distinguish individual plants, so measuring density is not possible.

Frequency Frequency is the number or percentage of sampling units in which a particular species occurs. This avoids having to count the number of individuals. If clover was recorded in 10 of the 25 squares that make up a 0.25 m 2 quadrat frame, the percentage frequency would be 40%. You need to be consistent when determining presence or absence in a sampling unit. For example, you might decide that only plants rooted in the square are counted, or you might decide that any plant or animal in the quadrat is counted including any that touch or overhang the quadrat.

Percentage cover This is the percentage of the ground covered by a species within the sampling unit. Count the number of squares within the quadrat that the plant completely covers, then count those that are only partly covered and estimate the total number of full squares that would be completely covered by that species.

Estimating animal populations Quadrats cannot be used for mobile animals as these don’t stay in the quadrats. A variety of different nets and traps need to be used. Animals that occur on the soil surface may be sampled using a pitfall trap (Figure 3). Those in vegetation can be sampled using a pooter directly or indirectly (after being knocked from the vegetation onto a white sheet). Insects and other small invertebrates found in leaf litter can be collected using a Tullgren funnel. Mark–release methods can also be used.


28

Student 7 of 8


Ecological sampling

• Tullgren funnel for collecting organisms from the soil or leaf litter

• Pitfall trap for sampling arthropods flat stone prevents rainfall filling trap

25 W bulb

stick support soil sample

16 mesh flour sieve

jam jar sunk into soil

ground slopes away from trap for drainage

bait of meat or ripe fruit polythene funnel

• Pooter for collecting insects glass collecting tube

clear plastic tube

air and arthropods drawn into tube cork or rubber bung specimen tube where arthropods collect

Organisms move away from the heat and light, falling into the jar.

glass mouthpiece

80% alcohol

gauze covering tube opening air sucked through mouthpiece

• Alternatively a muslin bag of soil surrounded by water can be used to collect living organisms. This is a Baermann funnel.

60 W bulb • Sweep net This net is swept through low-growing vegetation, collecting any animals in the mesh net.

rod for supporting bag

soil sample in muslin bag water glass funnel

rubber tubing

clip

beaker

Figure 3 Net and traps for sampling animals.


29

8 of 8 Student


Ecological sampling

Measuring abiotic factors when sampling the environment Angle of slope Use a clinometer.

Aspect Use a compass.

Temperature Use a thermometer or temperature probe, but be aware that the time of day can influence the values obtained, as will cloud cover. The thermometer or probe should be placed in the same position each time a measurement is made to allow valid comparison of measurements.

Light Use a light meter. Light readings can vary widely with time of day and cloud cover. It is better to take all measurements over a short period or take regular readings over extended periods using a datalogger.

Oxygen concentration In aquatic systems, oxygen probes can be used to measure oxygen concentration.

Humidity Relative humidity can be measured using a whirling hygrometer. It needs to be spun for 60 seconds just above the vegetation before readings are taken from the wet and dry thermometer and used to determine the humidity from a calibration scale.

Conductivity The ability of a water sample to carry an electric current gives a measure of the dissolved mineral salts. The conductivity of pure water is zero; increasing ion concentration raises the conductivity.

Soil water A sample of soil is dried at 110 ºC until there is no fur ther loss in mass. The % soil moisture can be calculated using the equation: % soil moisture

5

mass of fresh soil 2 mass of dry soil ________________________________ 3 100 mass of fresh soil

Soil organic matter A dry soil sample of known mass is heated in a crucible for 15 minutes to burn off all the organic matter. The mass is re-measured after the soil sample has cooled. The % soil organic matter is calculated using the equation: % organic matter in soil

5

mass of dry soil 2 mass of burnt soil ________________________________ 3 100 mass of dry soil

pH Universal Indicator or a pH meter can be used to test pH after mixing a soil sample with water. If using Universal indicator in the field, it is best to use a proper soil testing kit that contains some long glass tubes, with lines engraved on the sides, to show levels for adding soil and chemicals. First, 1 cm3 of soil is shaken with distilled water before adding one spatula of barium sulphate (low hazard). This helps to flocculate (settle) the clay fraction, which is important as clay particles are very small and will otherwise cloud the water for days. Then 1 cm 3 of pH indicator solution is added and the pH recorded after the contents of the tubes have been allowed to settle. Edexcel practical materials created by Salters-Nuffield Advanced Biology, ©University of York Science Education Group.

30

Student 1 of 2

Practical 5.2 The effect of temperature on the hatching success of brine shrimps Purpose • •

To investigate the effect of temperature on the hatching success of brine shrimps. To develop certain experimental skills, namely considering the ethical issues arising from the use of living organisms, presenting results, producing reliable results, identifying trends in data and drawing valid conclusions.

Brine shrimps Brine shrimps are small, saltwater crustaceans; the adults are about 8 mm in length. They are relatively easy to keep in the laboratory and will produce dormant egg cysts that hatch to produce young shrimp larvae. first antenna male (blue/green)

female (brownish red) 1 mm eggs

second antenna Drawings to show features of brine shrimps

Procedure You will need: • Brine shrimp egg cysts • 2 g sea salt for each treatment • 100 cm3 de-chlorinated water for each treatment • 40 cm3 beaker of salt water • 100 cm3 beakers (one for each temperature to be tested) • Water baths or incubators (one for each temperature to be investigated)

• • • • • • • •

Stirring rod Magnifying glass Pair of forceps Fine glass pipette Bright light Access to refrigerator Sheet of A4 white paper Sheet of graph paper 3 cm 3 4 cm

1

Decide on a range of temperatures from 5 °C to 35 °C to be tested.

2

Place 2 g of sea salt into a 100 cm 3 beaker.

3

Add 100 cm3 of de-chlorinated water and stir until the salt completely dissolves.

4

Label the beaker with sea salt and the temperature at which it will be incubated.

5

Place a tiny pinch of egg cysts onto a large sheet of white paper. Edexcel practical materials created by Salters-Nuffield Advanced Biology, ©University of York Science Education Group.

31

2 of 2 Student


The effect of temperature on the hatching success

of brine shrimps 6

Wet the piece of graph paper using a few drops of salt water. Dab the paper onto the white sheet to pick up approximately 40 eggs. This will look like a tiny shake of pepper. Use a magnifying glass to count the eggs. Cut the graph paper so that there are exactly 40 eggs.

7

Put the paper with the 40 eggs into the beaker (eggs-side down). After 3 minutes, use a pair of forceps to gently remove the paper, making sure that all the egg cysts have washed off into the water.

8

Repeat steps 2 to 7 for all the temperatures that are to be investigated.

9

If possible replicate the treatments.

10

Incubate the beakers at the appropriate temperatures, controlling exposure to light as far as possible.

11

The next day count the number of hatched larvae in each of the beaker s. To do this, place a bright light next to the beaker. Any larvae will swim towards the light. Using a fine glass pipette catch the brine shrimps and place them in a small beaker of salt water. (It may be easier if the pipette is reversed with the tip inserted into the teat, providing a wider bore to take up the shrimp.) Repeat the counting daily for several days. Brine shrimps are very delicate and care must be taken when handling them. Finally, discuss with your teacher the best method for disposing of the brine shrimps.

12

Record the number of larvae that have successfully hatched at each temperature.

13

Write up your experiment making sure your report includes: •

a discussion of any health and safety precautions taken

•

comments on the ethical issues arising from the use of living organisms

•

results presented in the most appropriate way

•

an explanation of any patterns in the data using evidence from the data and your own biological knowledge

•

comments on how valid your conclusion is

•

comments on how you ensured that the results obtained in this experiment were valid and reliable

•

suggestions for how you could have made your results more reliable.

To find out more about brine shrimps, visit the British Ecological Society website.


32

Examzone: Topic tests 1 of 5

Unit 4 Topic 5

1 The diagram below shows an outline of the light-independent stage of photosynthesis,

together with some of the products. CO2

X

Y note

GALP is glyceraldehyde 3-phosphate. GALP

phosphorylated 6C sugar glucose

sucrose

a Using the information provided in the diagram, identify substances X and Y

(2)

and state the number of carbon atoms present in each. X Y

(2)

b Explain how substance Y is converted to GALP.

c GALP is converted to a phosphorylated 6-carbon sugar which in turn can be

converted to a number of products such as sucrose and glucose. Sucrose is translocated around the plant in phloem. Describe how phloem tissue is adapted for this function.

(6)

(Total 10 marks)

Edexcel A2 Biology Implementation and Assessment Guide for Teachers and Technicians ©Pearson Education Limited 2009

45

2 of 5 Examzone: Topic tests

Unit 4 Topic 5 (cont.)

2 A study of plant species near a sandy shore was carried out.

The table below shows some of the data collected. Sites were sampled at a range of distances inland from a reference point on the beach.

Site number

1

2

3

4

5

6

Distance from reference point/m

20

80

250

500

650

1800

Dead organic matter in soil (%)

0.4

0.5

0.9

2.8

6.4

23.4

Number of plant species found

1

1

8

16

7

2

Each site represents a stage in the succession from bare sand to a climax community. a (i)

Describe the changes in the percentage of dead organic matter and number of plant species as the distance from the reference point increases.

(2)

(ii) Suggest how changes in the percentage of dead organic matter in the soil

(4)

could account for the changes in the number of plant species.

(2)

b Explain what is meant by a climax community.

c Another way to study plant succession is to measure changes in the percentage

cover of plant species. Describe how you could measure percentage cover of a plant species in a site you have studied.

(2)

(Total 10 marks) Edexcel A2 Biology Implementation and Assessment Guide for Teachers and Technicians ©Pearson Education Limited 2009

46



3 Pollen grains buried in peat can be used to deduce what the climate was like in the past. a (i)

State how the age of a peat layer can be estimated.

(1)

(ii) The presence of a large amount of alder tree pollen in a layer of peat is taken to mean that there was high rainfall when the layer was formed. Suggest why it is thought that the presence of alder pollen in peat indicates high rainfall.

(1)

b The exoskeletons of many insect species are also found in peat. Exoskeletons are

resistant to decay. Suggest how insect exoskeletons from peat could be used to show that the climate became warmer over a period of years.

(2)

Arctic Ocean

Tundra

Russia

Finland

y a w r o N

Sweden

c Certain arctic plants are adapted to growing in very cold conditions in the treeless

tundra habitat of northern Norway, Sweden, Finland and Russia, near the coast of the Arctic Ocean. (i)

Suggest why the geographical location of arctic plants makes them especially vulnerable to extinction due to global warming.

(1)


47



(ii) Increase in temperature increases the rate of respiration more than it increases the rate of photosynthesis. Suggest why this might be a problem for arctic

(2)

plants affected by global warming.

(Total 7 marks) 4 An investigation was carried out into the mating preferences of cichlid fish from three

populations (A, B and C) taken from Lake Malawi. The fish were all the same species, but the males of each population showed distinct physical differences. Male fish were separated into different areas of a tank by transparent plastic sheets. The plastic sheets had holes which allowed any female to enter, but prevented the males from leaving. The diagram below shows the arrangement of the tank. male fish

female fish

A

B

C

transparent plastic sheets

hole big enough for female to pass through

glass tank

Females from each population were allowed to choose one mate, and their offspring were collected. The male parent of the offspring was determined using DNA analysis.


48



The table below shows the number of times mating occurred between individuals of the different populations in a range of trials.

Female from population Male from population

A

B

C

A

29

0

0

B

0

26

4

C

0

1

8

a Explain how the DNA analysis provides reliable evidence for the identity of

(3)

male parents.

b (i)

Calculate the percentage of the matings that were between individuals of the

(2)

same population. Show your working.

(ii) Describe the mating preferences shown by the female fish in this

(2)

investigation.

c Suggest how the data support the hypothesis that population A is the most likely

to become a separate species.

(4)

(Total 11 marks) [TOTAL for test 38 marks] Edexcel A2 Biology Implementation and Assessment Guide for Teachers and Technicians ©Pearson Education Limited 2009

49

Examzone: Topic test mark schemes 1 of 2

Unit 4 Topic 5

Note; the number of lines for students to write their answer is not always a true reflection of the space provided on the real examination papers or the maximum number of marks that may be awarded. You may wish to provide students with additional paper to complete their answers. 1

a

X Ribulose bisphosphate / RuBP 5 (carbon atoms) Y {PGA / GP / eq} 3 (carbon atoms) [1 mark for each two correct]

b

(2)

Y is reduced;

(2)

Using ATP and reduced NAD; c

1

Reference to sieve tubes (cells) and companion cells;

2

Sieve tube cell has {perforated end walls / sieve plates};

3

To allow passage of materials / eq;

4

Sieve tube cell has no nucleus / reference to nucleus present in companion cell;

5

No vacuole / few cell organelles / organelles pushed to the sides;

6

To provide more space for translocation;

7

Companion cell has {dense cytoplasm / many mitochondria / eq} for {metabolic activity / energy};

8

Plasmodesmata connecting sieve tube cell with companion cell;

9

For exchange of materials / eq;

10

Transfer cells;

11

To load the sieve tube cells;

(6) [Total 10 marks]

2

a

(i)

Organic matter increases with distance; Number of species rises; Up to {500 m / site 4} / falls {beyond 500 m / site 4 / at 650 m / site 5};

(2)

(ii) 1 Less organic matter, fewer nutrients; 2 Unstable substrate; 3 Few species can tolerate; 4 Reference to pioneer species; 5 Increased organic matter suits more species; 6 Reference to increased competition; 7 Fewer species where good soil due to

(4)

climax community established; b

(Plant community that) no longer changes (in species) composition / eq; (Due to) succession; Dominant species (for climate) most abundant;

c

(2)

Defined area / quadrat; Method for choosing position (e.g. random, transect); Method of estimating cover described;



69

2 of 2 Examzone: Topic test mark schemes


3

a

(i)

1 the deeper the layer the older (the layer) / eq;

(max 1)

2 use of (radio) carbon dating of the peat;

(1)

(ii) alder trees grow well in wet places / eq; b

1 identify the insects; 2 estimate insect numbers of each species in each layer; 3 find out which species of insect live in warm places (today); 4 reference to increased numbers of such insects in

(max 2)

upper layers of ( peat); c

(i)

1 (loss of habitat as) cannot migrate/disperse northwards;

(max 1)

2 competition from other plants advancing from the south; (ii) 1 will use up stored food / starch / sugar (in respiration) faster than it can be replaced (by photosynthesis); 2 therefore limiting growth/ increase in biomass / eq;

(max 2)

3 increased CO2 release further increases global warming;

[Total 7 marks] 4

a

1 male fish have unique DNA; 2 inherit DNA from male parent / eq; 3 share DNA fingerprint characteristics / eq; 4 doesn’t change during life / eq; 5 reference to microsatellites / restriction enzymes cut DNA predictably;

b

(i)

(max 3)

63 1 ___ 3 100; 68 or 42.6 1 38.2 1 11.8

(2)

2 92.6 / 92.64 / 92.65 / 93; (ii) 1 all females are more likely to mate with a male from same population / eq; 2 population A exclusively mates within the same population; 3 populations B and/or C will breed with each other; 4 reference to limitations in data (e.g. ref to small

(max 2)

sample size in population C); c

1 reference to definition of a species; 2 all mating within population / eq; 3 reference to reproductive isolation; 4 no gene flow; 5 reference to accumulation of differences / changes;

(max 4)

6 more likely to become genetically different;

[Total 11 marks] [TOTAL for test 38 marks] Edexcel A2 Biology Implementation and Assessment Guide for Teachers and Technicians ©Pearson Education Limited 2009

70

Teacher/Lecturer 1 of 3

Practical 5.1 Looking for patterns

Purpose •

Safety

To carry out a study on the ecology of

Teachers/lecturers must follow their LEA/school/college

a habitat.

policy and local rules for off-site visits, especially with regard to identification of hazards and risk assessments. Additional information is available in the DCSF guidance Health and Safety of Pupils on Educational Visits on the Teachernet website. The hazards will vary depending on the site chosen. Risk assessments will identify the risk.

Notes on the procedure Students need to carry out a study on the ecology of a habitat to produce valid and reliable data (including the use of quadrats and transects to assess the abundance and distribution of organisms and the measurement of abiotic factors). This activity outlines how to approach a fieldwork investigation. It briefly mentions some techniques that might be used. It does not attempt to provide detailed accounts of the techniques. Additional details are provided on the Student Practical support sheet Ecological sampling (page 26) and there is a wealth of information on the British Ecological Society student 16 1 education web pages about sampling techniques. There are also examples of projects with data and virtual tour s of the rocky shore and sand dune habitats. The Field Studies Council website also has online resources for students including information on different urban ecosystems. The methods used will depend on the habitat and factors under investigation. Any suitable methods could be used that give students the opportunity to have first-hand experience of field data collection. The school/college grounds can be a valuable resource. Other activities could be covered in a fieldwork context; for example, feeding relationships and the transfer of energy through ecosystems, or succession, could be investigated through a specific habitat. Fieldwork can be used as students’ investigation for A2 coursework. Teachers/ lecturers and students should ensure that the proposed investigation meets the assessment criteria. When planning coursework for Unit 6 there is a full description of the requirements illustrated with exemplars in the Tutor support material for Unit 6 on the Edexcel website.

Select a site carefully The site selected should show some clear relationships. Some examples where transects can be used: •

a playing field or grassy area from an area of high trampling to an area of low trampling.

The Field Studies Council online resources include a case study on the distribution of ribwort plantain and greater plantain in trampled and non-trampled areas.


7



•


a woodland margin – passing from a field or other example of grazed or mown grassland,

through brambles into a wood. The key gradient is likely to be light intensity – but it may not be the only one. •

a sand dune system from the shore across dunes into g rassland and scrub as you go further

inland. In this case the key f actors could be soil moisture, soil stability and organic matter, although succession is also involved here. •

a rocky shore from the low tide mark to the top of the beach. The key factor is the

proportion of time a part of the shore is left exposed to the air and to desiccation when the tide is out. •

a geological boundary between two types of rock, such as between limestone and millstone

grit. Some examples where two sites can be compared: •

grazed and ungrazed grassland

•

mowed and unmowed grassland

•

trampled and untrampled grassland

•

fertilised and unfertilised lawns

•

shaded and sunny sites

•

fast- and slow-owing streams

•

chalk and sandy soil sites

•

understories of beech and oak woodlands.

The two sites could be compared by random sampling.

Identification – don’t panic! Identification is not as big a problem as you may think. The only species students need focus on are those which would support the hypothesis under investigation. Trying to identify each species in a quadrat down to the tiniest piece of moss can be a poor use of time and can actually prevent students from ‘seeing the wood for the trees’. A transect at a woodland margin could be successfully done recording only tree cover, grass, brambles, nettles, ivy, bare ground, wild garlic (with give-away smell) and bluebells, together with good light meter readings and soil pH (often showing no pattern), especially if you start to consider something like leaf surface area of brambles at the same time. But the principles are: •

select a site where students can cope with the species identication

•

before teaching students to identify another species, ask yourself ‘What is the ecological point of teaching this?’

•

make use of expert help if available – this could include a Field Study Centre.

The Field Studies Council produces excellent laminated cards to aid with field identification in particular habitats. The students will be relying on you for identification so careful preparation is impor tant. You may like to produce a record sheet that includes the species you want to focus on. Edexcel practical materials created by Salters-Nuffield Advanced Biology, ©University of York Science Education Group.

8




Class organisation Students can work in groups if they are not using this investigation for their A2 coursework. The smaller the group the more ‘ownership’ by individuals, whilst the larger the group the more quadrats can be recorded and the bigger and statistically more meaningful the picture that can be produced. A good group size is six – working as three pairs. It helps if the whole group can have access to a computer as soon as possible after collecting the data and to a full set of equipment during data collection. Waiting to borrow equipment from other groups wastes a lot of time and loses momentum. Pace is also important – it is possible to be too slow and nit-picking about accuracy, in which case the students become bored and lose sight of the bigger picture. It is also possible for the students to feel it is somehow ‘efficient’ to get the job done as quickly as possible, subsequently data are produced that are so inaccurate and incomplete that they do not produce reliable results.

FieldWorks: an invaluable IT resource FieldWorks is available on CD and produced by Interpretive Solutions, Hallsannery Field Centre, Bideford, Devon EX39 5HE. It benefits from having grown out of the learning experiences of many students. Students can enter their own data, and present these using kite diagrams or other formats. It also contains information about species and habitats including rocky shores, sand dune, salt marsh, moorland, woodlands and fresh water. The data can be subjected to statistical analysis using correlation coefficients (Pearson’s parametric or Spearman rank non-parametric), particularly useful with transects. The programme can also calculate species diversity indices and carry out t-tests and z-tests. Other statistical packages are available for use in ecology from software companies and other suppliers.

Useful references Jones C. (1998) Fieldwork sampling – animals. Biological Sciences Review, 10(4), 23–25. Jones C. (1998) Fieldwork sampling – plants. Biological Sciences Review, 10(5), 6–8. Williams G. (1987) Techniques and fieldwork in ecology. London: HarperCollins. Field Studies Council identification sheets and booklets are very good. These can be obtained from: FSC Publications, Preston Montford, Montford Bridge, Shrewsbury SY4 1HW.


9


Unit 4 Topic 6


CCUU;

(1)

b

anticodon;

(1)

c

1 about 20 amino acids;

a

2 (triplet) gives 64 permutations / 43; 3 of 4 bases; 4 (lowest number of bases with) enough permutations / calculation shown / one or two bases not enough permutations / more codes than needed;

(3)

5 code is degenerate / some amino acids have more than one code; d

1 ensure that the correct amino acid (added to polypeptide) / converse; 2 for given {anticodon / codon (on mRNA)} / eq; 3 reference to correct sequence of amino acids / correct polypeptide structure;

(2)

4 reference to complementary base pairing; e

(i)

1 radioactivity in protein rises sharply / eq; 2 then falls (more) slowly; 3 protein peaks between 5 and 7 minutes; 4 radioactivity in tRNA falls throughout; 5 Compare rate before and after about 5 minutes;

(3)

6 reference to a relationship between protein and tRNA; (ii) 1 amino acids (had been) taken up (first) by tRNA;

2 tRNA {delivered / released} amino acids {at ribosome / passed to polypeptide};

(2)

3 became part of protein (during translation);

[Total 12 marks] 2

a

1 (HIV proteins act as) antigens; 2 stimulate immune response / antibody production by B cells; 3 clonal selection by the antigen / eq; 4 B cells {proliferate / clone / divide / clonally expand / undergo mitosis}; 5 to plasma cells; 6 (plasma cells) {produce / eq} (specific) antibodies;

(4)

7 reference to antigenic presentation; b

1 not much effect in first few years; 2 number of T helper cells falls (with time); 3 reference to role of cytokines; 4 less able to {respond to antigen / mount immune response / eq} / not enough cytokines produced; 5 more difficult to prevent infection; 6 health deteriorates / more infectious illness / opportunistic infection / example; 7 reference to T cells going below 200; 8 reference to AIDS; 9 death likely;

(4) [Total 8 marks]


71



3

a

An explanation to include six from:

1

increased use;

2

more bacteria exposed;

3

reference to mutation;

4

reference to plasmids;

5 6 7 8

b

reference to conjugation / sexual reproduction; reference to genetic variation / existence of different genes; reference to selection by the antibiotic; description of selection;

9

reference to resistance gene passed to offspring;

10

selected organisms become more common;

11/12 reference to graphs;

(6)

(i)

(1)

greater clear area;

(ii) An explanation to include two from: 1 larger molecules / antibiotic C {diffuse / eq} more slowly 2 through agar; 3 does not reach as many bacteria as the smaller antibiotic B; 4 clear area of C is reduced;

(2)

5 underestimates the effectiveness of C / converse; (iii) An explanation to include: 1 viruses no cell wall; 2 or ribosomes/ viruses use host cell for protein synthesis;


4

a

(i)

(1)

humidity / oxygen concentration / toxins /pH;

(ii) 1 time for fly to find body is due to chance / eq; 2 reference to succession; 3 earlier organisms change conditions; 4 (changed conditions) more suitable for later organisms / eq;

(max 4)

5 example of condition changes; b

(i)

1 (45), 9 / 9–10 days;

(2)

2 (1.5), 7.5–8.5; (ii) 1 (house fly 45 days at 22) 89 days at 12; 2 (flesh fly 4 days at 22) 8 days at 12; 3 (all figures suggest) within 89 days;

(max 3)

4 died within a day of each other;

[Total 10 marks] [TOTAL for test 41 marks]


72

Examzone: Answers to practice questions 1 of 3

Unit 4 Topic 5

1

a (i)

A stroma B starch (grain) / lipid droplet C thylakoid (membrane) / chlorophyll D granum (2) for all correct

(2)

(1) for 2 or 3 correct

(1)

(ii) C / thylakoid / granum / D; (iii) photolysis / splitting water using light energy; release of electrons to chlorophyll; releases hydrogen ions; oxygen released;

(3)

OH source of oxygen; b reduced production (of carbohydrate);

less carbon dioxide fixed; reference RuBP; reference Calvin cycle / eq; less carbon available for reduction to carbohydrate;

(3)

reference to enzyme / RuBPcarboxylase; c high light intensity;

reference to change in wavelength; high temperature; high humidity;

(2)

high carbon dioxide concentration;

[Total 11 marks] 2

a {chemical energy / carbohydrates stored} / {energy fixed / eq};

in {producers / green plants / by {photosynthesis / autotrophic nutrition}; b (i)

180 2 145 (5 / 35)

(2)

5 35;

3 100

or other intermediate stage;

5 14.3

(%);

(3)

(ii) 1 {decompose / breakdown / rot / eq} {dead bodies / remains / eq}; 2 using external digestion; 3 to release {nutrients / nitrates / eq} / recycling of nutrients;

(2)

(iii) 1 temperature lower; 2 lower enzyme activity; 3 shorter growing season / less sunlight / less suitable wavelength of light; 4 less photosynthesis; 5 less water;

(2)

c 1 replanting after harvesting trees;

2 selective felling of timber trees, leaving rest of forest intact; 3 {pollarding / coppicing} / harvesting / eq on rotation; 4 (coppicing) trees cut at ground level and allowed to regrow / (pollarding) cut leaving short trunk and regrow; Edexcel A2 Biology Implementation and Assessment Guide for Teachers and Technicians ©Pearson Education Limited 2009

83

2 of 3 Examzone: Answers to practice questions


5 for 4–25 years and then harvested;

(3)

6 plant fast-growing species of trees;

[Total 12 marks] 3

a (i)

(1)

C

(ii) Award one mark for each of the following points in context of whether or not statement was chosen to a maximum of three marks.

1 as Ascophyllum decreases, Chondrus increases / Chondrus has low percentage where Ascophyllum has high percentage / eq; 2 no data to say how Fucus grows when not covered by water;

(3)

3 Ascophyllum still present in low tidal regions; b Award one mark for each of the following points in context to

a maximum of two marks.

1 temperature; 2 light intensity; 3 nature of rock / substratum / eq; 4 slope / eq; 5 aspect / eq; 6 salinity / eq;

(2)

7 reference to pollution; c [S1C] Award one mark for each of the following points in context to a maximum

of four marks.

1 suitable named organism linked with habitat; 2 reference to suitable technique for the organism; 3 e.g. use of quadrat, transect; 4 reference to systematic sampling; 5 detail of method; 6 stated measurement; 7 reference to two abiotic measurements;

(4)

8 reference to a safety procedure;

[Total 10 marks] 4

a Remote sensing / satellite (images) / GIS / Global imaging

system / OTCS

(1)

b [S1C] (Algae supply polyps) with {sugar / carbohydrates / energy / food};

(Algae supply polyps) with oxygen; Any credible suggestion why colour is an advantage to the polyp;


84

(2)



c [S1C] Answers to include;

1 Some (species of) coral become less common / other types become more common (and take over); 2 Reference to competition; 3 Existing species {become adapted to change / evolve}; 4 Correct reference to mutation; 5 Improved conservation measures (to alleviate effects of human interference); 6 Increased sea temperature was not the real reason why the reef was in decline suggested credible alternative, e.g. {pollution / predation / disease} / whatever the original cause {has now declined / no longer the problem it was};

(2) [Total 5 marks]

5 a 1 63;

[max 2 from the following three marking points]

2 cannot form gametes / eq; 3 (because) chromosome pairing not possible / eq; 4 during {meiosis / eq};

(max 3)

b 1 (isolating mechanisms) prevent interbreeding (between

species / populations) / barrier to reproduction / eq; 2 (postzygotic mechanisms) allow {mating / fertilisation / eq} / eq; 3 but prevent production of {viable / fertile} offspring / prevent offspring developing / eq; c

(3)

1 idea that species is group of organisms that can interbreed to produce fertile offspring / eq; 2 but mule {is sterile / cannot produce offspring};

(2) [Total 8 marks]


85

Answers to student book text questions

Unit 4 Topic 5: Chapter 5.1

Stretch and challenge questions are indicated throughout the Student’s Book with the following icon

. These questions are aimed to stretch and challenge all students by using

different assessment strategies including: a variety of question stems, synopticity, extended extended writing opportunities oppor tunities and different question types to enable students to develop and use a variety of skills. All students should be able to give answers to these questions and a basic answer is outlined below for each of them. However it is expected that candidates working at higher grades will give a g reater depth of answer bringing in extended knowledge and understanding from a variety of sources.

pages 10–11 1

Marks awarded awarded for: Most important reaction because almost all life on earth depends on it, only large scale way in which new organic molecules are synthesised in living organisms and the only way in which a new supply of ATP can be built up. Any other valid point. Marks not awarded for: Without respiration, cells die so photosynthesis can’t take place. Without synthesis of proteins, there is no ATPase, ATPase, so no ATP cycle. Almost impossible to rank reactions in terms of impor tance. Any other valid point.

2

available to all cell enzymes. a Releases energy when ATP broken down to ADP in a form available ADP and inorganic phosphate can be reformed into ATP by ATPase using energy from universal reactions, such as redox reactions, within the cell. Any other valid points. include: presence in all cell types investigated, investigated, presence in every species b Evidence to include: investigated, investiga ted, impact on cells if ATP ATP synthesis or breakdown inhibited. Any other valid point.

pages 12–13 1

all the cells carry out photosynthesis – any any parts of the plant which are a Because not all not directly exposed to light will not contain chlorophyll. surface area. b Folded membranes give large surface Enzymes on membranes and in stroma to carr y out reactions. Presence of several photosynthetic pigments to absorb different wavelengt wavelengths hs of light. Any other valid point.

2

None of the pigments absorb well well in the green/yellow areas of the spectrum. As this light is not absorbed, it is reflected which is why plants appear g reen.

pages 14–17 1

Cyc ycli licc phot photop opho hossph phor ory yla lati tion on

Non on-c -cyc ycli licc phot photop opho hossph phor oryl ylat atio ion n

only involves PSI

involves PSI and PSII

electrons come from and return to chlorophyll in PSI

electrons lost on excitation from the chlorophyll molecules in PSII are replaced by electrons from the oxidation of water.

no reduced NAD produced as electrons return to chlorophyll

NADP reduced to reduced NAD during the process

ATP only formed

ATP, reduced NAD and oxygen formed

Edexcel A2 Biology Implementation and Assessment Guide for Teachers Teachers and Technicians Technicians ©Pearson Education Limited 2009

95


Chapter 5.1 (cont.)

2

The reactions take take place in the absence of light – they are not catalysed or triggered by light. As long as the enzymes of the cycle have a supply of reduced NAD and ATP ATP they will continue with or without light. The Calvin cycle continues in the absence of light but it is dependent on the products of the light reactions r eactions for its raw materials. Without light there is no long-term supply of reduced NAD or ATP and so the Calvin cycle cannot continue without the lightdependent reactions – it is dependent indirectly on light.

3

In a biochemical biochemical process that that depends on or is affected by a number of factors, the process will be limited by the factor that is nearest to its minimum value. The amount of light available affects the amount of chlorophyll that can be excited and therefore the amount of reduced NAD and ADP produced in the light-dependent stage. If there is a low level of light then insufficient reduced NAD and ATP ATP will be produced to allow the reactions of the light-independent stage to progress at their maximum rate, so light is the limiting factor. Low levels of carbon dioxide available for fixing in the Calvin cycle means that the reactions cannot proceed at the maximum rate. When this is the case, carbon dioxide is the limiting factor. In the natural situation situation of plants it is most often carbon dioxide that is the limiting factor. All of the Calvin cycle reactions and many of the light-dependent reactions of photosynthesis are controlled by enzymes and are therefore sensitive to temperature. This means that even when the light and carbon dioxide levels are suitable for a very high r ate of photosynthesis, if the temperature is low low,, the plant will be unable to take advantage of the conditions.


96


Chapter 5.2

pages 18–19 1

The habitat is is the ‘address’ of the organism – it describes where it lives. lives. Suitable examples of habitats should be included, such as tropical rainforest, under a log (many other possibilities). The niche describes the role of an animal within a habitat – so several organisms may inhabit the same habitat but occupy different niches within it. For example, food niches in woodland: top predator fox, primary consumer rabbit, large tree-dwelling herbivore squirrel, etc.

2

Full details should should be given of whichever biomes biomes chosen. A clear understanding of the link between adaptations and the conditions of the habitat should be shown.

pages 20–23 1

The process by by which communities of animals animals and plants colonise an an area and and then over time are replaced by other communities.The changes continue until a steady state is achieved, where the number and type of species remain roughly the same until conditions change again.

2

A community reached at the end of a series of stages of development that continues continues to consist of the same plant and animal species. Different habitats have different climax communities.

3

Primary succession from bare bare rock or sand dune, secondary succession on land that was was colonised by living things but was cleared.

4

[S1C] The answers should include some or all of the following. Other points may be valid.

changed and more a Initial steady growth in the number of species as conditions changed soil developed. Then a period per iod of stagnation from about 1974–90 as the island had developed as far as it could without new input of fertility. Once Once birds colonised islands they brought new species on their bodies and increased fer tility of the soil with their droppings and so the number of plant species increased again relatively rapidly. make it to the island and start to grow but cannot cannot survive to establish establish b Some species make a breeding population of plants. So more species of plants have been obser ved on the island at different times than are actually present at any one time – this is the cumulative species total. The cumulative total cannot fall, while the actual species total could drop considerably considerably.. predictions by scientists come true, the number of actual actual species could eventually c If the predictions drop as low as 10, but the cumulative total will remain at its present level or even go up if more new species become established but previously established ones die out.

pages 24–27 1

elements of the environment in which a population population of organisms is is a The non-living elements living. Examples Examples could include any of those listed or any other relevant points such as pH of the soil. availability b Abiotic factors determine the fertility of the soil, the temperature, water availability etc. This in turn decides which plants will be able to grow and thrive thr ive in an area, which in turn affects the other organisms which w ill be able to sur vive. In terms of the basic colonisers of an area, abiotic factors are cr ucial.


97


Chapter 5.2 (cont.)

2

Any suitable example, such as in a windy environment water will evaporate from the ground more rapidly so will be less available for living organisms, high light levels but low oxygen levels mean many organisms cannot grow well and reproduce etc.

3

Any suitable examples demonstrating an understanding of how the animal/plant is adapted to the particular abiotic factor selected.

pages 28–29 1

[S1C] The answers should include some or all of the following. Other points may be

valid. Any three suitable examples, including one bird and one mammal, with clear explanation of how terr itories are marked and defended, e.g. scent marking, physical display/dance, fighting. 2

With little biodiversity, if one organism is affected by disease it will have a major impact on the small number of other organisms in the ecosystem – they won’t get eaten or their prey numbers will fall substantially. With little biodiversity there will be few other available food resources. Also disease is more likely to spread between individuals where there are few buffer species. In a more diverse community, changes due to disease are likely to have less of an effect as there is much more variety of food choice and more buffering organisms to prevent spread of disease.

pages 30–33 1

Intraspecific – between members of the same species, e.g. for territor y, mates, food. Intraspecific competition tends to affect the abundance of a particular sp ecies of organism – low resources, much competition, less reproduction and more mortality – number decrease. Plenty of resources – little competition, lots of breeding, low mor tality, numbers increase. Interspecific – between members of different species, e.g. for space, food. Interspecific – this tends to affect the distribution of species in a habitat and also the abundance of species – the biodiversity. If one species competes very successfully against others, it will tend to drive them to extinction in the area.

2


valid. a Adding food – hare density more than tripled.

Excluding predators – hare density more than doubled. Fertilising the grass had no major effect. Adding food and removing predators – hare density went up by a factor of about 15. b If add food, hares breed more successfully, but more will also be taken by predators

which will also breed successfully. So the impact of the food on the hare population is not fully illustrated in the population numbers.


98


Chapter 5.2 (cont.)

c Where the hare population has increased as a result of the experimental manipulation,

other factors begin to limit, for example the natural food supply begins to be exhausted, disease/parasites begin to impact on the more crowded population, or any other sensible suggestion. This causes the hare numbers to fall. The fact that the population which showed the least g rowth, where predators were excluded, also shows only a very small dip, supports this idea, as in those enclosures the population did not reach levels which had a major impact on the natural resources and so population size could be maintained. The enclosures where fertiliser was applied to the grass did not see any sustained population growth and so did not experience a dip resulting from over-density. 3

[S1C] The answer should include some or all of the following. Other points may be

valid. Density-independent factors tend to limit the distribution of species, e.g. rainfall, temperature will affect the overall conditions and therefore which species can survive in a particular area. Can also affect abundance, e.g. if light levels, temperature and rainfall appropriate for a species, then numbers of species will affect how much light, water etc. is available to individuals, so becomes density-dependent also. Density-dependent factors are based on the numbers of organisms present so will tend to affect the abundance of an organism rather than whether it can sur vive in a particular habitat.

pages 34–37 1

a Algae and coral reefs 25 000 g m22 y21 production per 1% of Earth’s surface. b Open ocean 2.3 g m22 y21 production per 1% of Earth’s surface. c Although they have a low productivity there is an enormous amount of open ocean so

it has a big overall impact on total productivity. 2 a Producers to primary consumers: 16.7%

Primary to secondary consumers: 4.5% Secondary to tertiary consumers: 8.9% b (16.7

1 4.5 1 8.9)/3 5 30.1/3 5 10.0%

3 a Any suitable reason, such as would be too complicated to follow as a food web, can

ignore the minor species within a trophic level, much easier to compare with studies of other ecosystem studies. b Any reasonable assumptions, including using average body mass to calculate energy

within biomass of a particular species, estimates of population size, possibly ignoring species which are very small and/or few in number, making estimates of energy transfer into a species over a whole year.


99


Chapter 5.3

pages 38–39 1


valid. The link should be made between releasing carbon from sinks more rapidly than normal and more rapidly than photosynthesis etc. can remove it. 2

Atmosphere: increasing industrialisation, electricity generation, cars and other road vehicles, aeroplanes, all producing carbon dioxide emissions affect amount of carbon dioxide in the atmosphere. Soil organic matter: temperature (rate of activity of breakdown) death rate of organisms, any sensible alternative. Ocean: temperature, any sensible alternative. Marine sediments: erosion, acidity of water, acid rain etc., any sensible alternative. Terrestrial plants: season of the year in temperate areas has a big effect on global photosynthesis levels, deforestation, crop growth and harvesting.

pages 40–41 1

People could stop eating beef and using dairy products, because millions of people around the world rely on meat and milk from ruminants as a major part of their diet and big farming interests have a lot of economic clout and would object. Any other valid point.

2


valid. The greenhouse effect is vital to maintain the surface of the Earth at a temperature suitable for life. It is the enhanced greenhouse effect, due to increasing levels of greenhouse gases, that is potentially a problem.

pages 42–45 1

20.81%; allow 19–21%

2

Carried out over long period of time, very large number of readings, same or similar measuring equipment used throughout, area of low air pollution etc. Any other relevant points.

3

Gives a much longer time perspective (readings going back over a thousand years), shows natural falls and rises in carbon dioxide concentration and events of last couple of hundred years. Any other valid points. Reliability 0.2 ppm. Correlation with other evidence.

4

The data show that atmospheric carbon dioxide levels have increased. They do not show the source of the carbon dioxide.

5

a Student should question validity of data, statistical methods used, inclusion of tree-

ring data in original. b 2008 version contains a large amount of extra data from hundreds of studies, two

different statistical methods used, figures calculated both with and without tree ring data. Any other valid point. Edexcel A2 Biology Implementation and Assessment Guide for Teachers and Technicians ©Pearson Education Limited 2009

100


Chapter 5.3 (cont.)

pages 46–47 1

Graph A shows glaciation levels are inversely linked to carbon dioxide levels, i.e. lower carbon dioxide correlates with g reater glaciation, which suggests low temperatures and vice versa. Any sensible points. Graph B shows close cor relation in pattern between temperature and carbon dioxide levels. Which comes first – change in carbon dioxide or change in temperature?

2

86.7%, 70.7% – carbon dioxide from fossil fuels has increased more than other sources.

3

a Evidence of careful research and ability to weigh up the strengths and weaknesses in

terms of clarity of ideas, reliability of data etc. b Look for closeness of data, which comes first in rise on graph, temperature or possible

cause etc. Comments should be made on validity and reliability of data. Any other valid points.

pages 48–51 1

Any valid points such as: impossible to predict new technologies which may emerge, don’t know increase in production by developing economies, don’t know how effective drive to reduce carbon footprint will be.

2

a 4.3%, 2.7% b 150 mm c Increasing carbon dioxide levels thought to cause rise in global temperatures due to

enhanced greenhouse effect. Increasing temperatures causing melting of snow cover on mountains, less snow falling due to rising temperatures and melting of snow at poles. This causes r ise in average sea levels. 3

Particularly affects diseases with insect vectors. Linked to climate change – rising carbon dioxide and rising temperatures, means insect vectors can survive in areas where they couldn’t previously live and therefore carry diseases such as malaria and bluetongue to areas where they have previously not been an issue.

4

Look for clarity of thought and understanding of the possible sequences of events.

pages 52–53 1

Any valid points here. Important to show awareness of the fact that no one body can legislate for the whole world, difficult to say that developing countries cannot strive to have the same standard of living as that enjoyed by more developed countr ies even though that involves massive increase in carbon dioxide emissions, politicians don’t want to be unpopular at a national level and measures to reduce use of electricity/petrol etc are inevitably unpopular as they impact on individual choice or economics.

2


valid. Answer should cover the ways in which the evidence differs, that there is no complete answer, and that organisations may have vested interests.


101


Chapter 5.4

pages 54–55 1

Proteomics is the study of all the different proteins that can be made as a result of the genome throughout the life of an individual, at all the different stages of development as a result of differential gene expression. Should note that there are far more proteins than genes. Genomics is the study of the DNA , the sequence of the genes and the specific alleles in place in an individual.

2

Different alleles arise as a result of mutation – changes in the sequence of DNA bases. Changes in allele frequency in a population usually occur in response to environmental changes. If a particular allele or combination of alleles produces a sur vival or reproductive advantage for the organisms which have them so their offspring are more likely to survive and breed again, the frequency of the allele will increase (e.g. Malpeque oysters from AS level). Alternatively, if a particular allele becomes disadvantageous, the frequency will decrease (e.g.

Biston betularia from

AS level).

pages 56–57 1

In order to plant the apple orchards huge areas of natural bushes would have been ploughed up. This would have destroyed many hawthorns, the natural habitat of the flies, which in turn would have led to some females being left with no alternative but the apple trees on which to lay their eggs. Then some of these flies would have had alleles which helped them recognise the scent of the apples, or digest apples effectively, or lay eggs at times that fit the pattern of apple tree flowering and fr uiting rather than hawthorns. These flies would be most likely to sur vive and pass on their alleles, thus changing the allele frequency in the population and moving towards speciation.

2

Any suitable examples.

pages 58–59 1

In order to measure genetic diversity, to identify relationships between different species, to track the process of evolution, to identify fraud in foodstuffs such as caviar, to identify ways of controlling disease. Any other suitable examples.

2

Because they are present in a wide range of different organisms, and so they can be compared. The genes mutate at a measurable and suitable rate, which allows the time since even relatively closely related organisms shared the same genes to be calculated.

pages 60–61 1

a Fossil evidence is limited: only relatively small numbers of organisms are fossilised,

small numbers of fossils are found, fossils often not intact. There may also be similarities in anatomy due to convergent evolution. Any valid other point. b DNA starts to degrade immediately after death so limited application. Mutation rates

of different genes varies considerably so timing can be ver y different depending on which gene used. c Evidence from any one source has limitations and potential inaccuracies which reduce

its validity. If evidence from different sources gives similar results, the more valid are any conclusions drawn. Similarly, using different sources can show up discrepancies and prevent time and money being wasted on erroneous data. Any other valid point. Edexcel A2 Biology Implementation and Assessment Guide for Teachers and Technicians ©Pearson Education Limited 2009

102


Chapter 5.4 (cont.)

2


valid. Helps to validate new research by presenting it to peers for constructive criticism. When several teams produce similar results in relevant research, the validity of all their findings is confirmed. Allows new ideas to be aired and stimulates discussion and debate. Enables cooperative research ventures to be set up by colleagues in different countries, and exchange of different techniques. Disadvantages: pressure to publish and appear at conferences can skew research and drive people to publish before they have gathered all the data. Vested interests and large funding groups can squash unpopular papers. Timing of paper can minimise audience – early morning papers don’t always get good audiences! New ideas which go against the accepted view can be treated harshly, slowing down the development of radical new models. People need to be able to affo rd to go to conferences to present papers which mitigates against scientists in poorer countries.


103



pages 66–67 1

a The heat of the body is produced as a result of the metabolic reactions taking place,

particularly respiration in active tissues such as the muscles and the brain. After death these reactions slow down and stop, so no more heat energy is produced. At the same time heat energy is lost by radiation and conduction from the skin and latent heat of evaporation so the body cools down. b The metabolic reactions do not all stop immediately after death – many cells continue

to respire, contract, and so on, until all the available oxygen and ATP is used up. So heat continues to be generated in the first hours after death, although at a slower rate, so body temperature drops but relatively slowly. c The temperature gradient between the body and the environment will affect rate of

cooling, for example if someone dies inside a warm house the body will cool down more slowly than if it is on a cold windy hillside, so the external temperature and weather conditions have to be taken into consideration. A naked body will cool much faster by convection than a clothed body, and a body wrapped in blankets or a duvet will cool down even more slowly due to insulation. A wet body will cool faster than a dry body as a result of heat lost as the water evaporates, and death in a warm bath or beside some form of heating will also change the rate of cooling. Even the body position affects the rate of cooling – a stretched out body has a much bigger exposed surface area to volume ratio than a curled up one and so will cool down faster. All of these different factors have to be taken into consideration when using temperature as a guide to the time of death. 2 Very little difference as they are poikilothermic and so their body temperature is normally

very similar to the ambient temperature. 3 Limited value because, although there is a generalised pattern of rigor, there are large

variations from one individual to another depending on factors such as: •

amount of ATP stored in the muscles at the time of death which depends genetic tendency and levels of fitness

•

level of activity before death – how much ATP has been used

•

temperature of the individual at the point of death

•

temperature of the surroundings

•

speed at which rigor passes also depends on a variety of factors such as external temperature, activity of enzymes.

Other examples may be valid.

pages 68–71 1 The first stage is caused by the colonisers – anaerobic bacteria. Enzymes from lysosomes

break down cells releasing cell contents on which anaerobic bacteria grow. As more cells are broken down, the bacteria spread. •

The bacteria are followed by a number of species of ies, e.g. blowies, which lay eggs. The larvae (maggots) feed on the tissues, breaking them down further.

•

As the body liquees, adult ies can feed on it too.

•

Beetles arrive whose larvae feed on maggots. Parasitic wasps lay their eggs in the larvae.


104


Chapter 6.1 (cont.)

•

As the body is eaten away, it dries out and different species such as the cheese ies and coffin flies move in.

•

When the remains of the body are too dry to support y larvae any more, beetle species with strong chewing mouthparts move in, e.g. carcass beetles, ham beetles and hide beetles.

•

At the end, mites and moth larvae feed on the hair until only dry bones are left.

2 The obvious choice of factors are temperature and level of exposure of body (buried or

not, inside or outside), but any other valid factor could be chosen e.g. clothed/unclothed. Clear scientific explanations should be given of the effect on decomposition. For example, temperature affects the rate of chemical/enzymic reactions – higher temperature increases rate and lower temperature slows them down. Or, the level of exposure will affect the availability of the body to insect decomposers, etc. 3 Succession follows a regular pattern so, by examining the stage of succession in a body,

along with data on the conditions where the body has been found, forensic scientists can build up a fairly precise picture of how long the body has been dead. [S1C] 4 a Exposed body: peaks and troughs roughly follow ambient temperature but body

temperature gets higher than the air. This is because of the metabolic heat released by the bacteria and other organisms living on the body. When the temperature is higher, reaction rates increase and so more heat is released and the body temperature increases further. Underground: the temperature shows less variation as the body is insulated from changes in air temperature. Fluctuations in temperature are due to variations in the activity of the decomposing organisms. b Advantages: far fewer ethical issues, easy availability of pig carcasses, fewer problems

with obtaining burial sites, any other valid points. Disadvantages: pigs are not people, so there may be differences in colonising organisms, differences in skin thickness, fat layers etc. So data obtained will be only an approximation of human situation. Any other relevant points c Advantages: the results are directly relevant to forensic investigation of human bodies.

Disadvantages: ethical issues of using human bodies, impact on researchers of using human bodies, limits to where human remains can be buried/hidden for observation. Any other relevant points.

pages 72–77 1 It contains more information than it needs to. Often only the first two of the three

nucleotides seem to matter in determining which amino acid results. This is important because if each amino acid was produced by only one codon, then any error or mutation would cause havoc. With a degenerate code an error could still produce the same amino acid. Only methionine and tryptophan are represented by only one codon. A degenerate code partially protects living organisms from the effects of mutation and means there are several ways of instructing the cell to put a particular amino acid in a specific place in an amino acid chain. Edexcel A2 Biology Implementation and Assessment Guide for Teachers and Technicians ©Pearson Education Limited 2009

105


Chapter 6.1 (cont.)

2


Transcription: A region of DNA unravels. Only the 5’ prime DNA strand (the template or antisense strand) is transcribed to give a single strand of mRNA. The transcription is brought about by an enzyme called DNA-directed RNA polymerase (RNA polymerase). Every triplet code on the DNA gives rise to a complementary codon (triplet of bases) on the mRNA. Every thymine in the DNA template is replaced with uracil in RNA. Apart from this change, the newly synthesised RNA strand has the same sequence as the nontemplate, coding or sense strand of the DNA. Translation: mRNA molecules pass through the pores in the nuclear membrane and move to the surface of the ribosomes, transporting the instructions from the genes to the site of protein synthesis. Each tRNA molecule has a unit of three bases at one end of the molecule known as the anticodon. Each tRNA molecule also binds to a specific amino acid depending on the code of the anticodon. The ribosomes are made up of a large and a small subunit. They are 50% RNA. They surround those parts of the mRNA which is being actively translated, and then move along to the next bit. Their job is to hold together the mRNA, tRNA and enzymes controlling the process of protein synthesis in the correct positions. The ribosome starts reading the mRNA at a start codon (AUG). This codes for the amino acid methionine. Molecules of transfer RNA carr y individual amino acids to the surface of the ribosome. Each tRNA lines up its anticodon alongside a complementary codon in the mRNA and is bound in place to the ribosome while enzymes link the amino acid to the previous one in the amino acid chain by peptide bonds. Once its job is done, the tRNA returns to the cytoplasm to pick up another amino acid. The ribosome moves along the molecule of mRNA revealing one codon after another until the end is reached at a stop codon (UAA, UAC or UGA), producing a completed polypeptide chain. 3


They used the mould Neurospora crassa which has a short life cycle, is easily grown and mutates easily. The original strain of the mould could grow on minimal medium (with few nutrients) as it could synthesise all amino acids and sugars it needed. Moulds were exposed to X rays to cause mutation. Mutations in the chemical pathways that affected the ability to synthesise particular amino acids would show up easily because mutated strains needed one or more nutrients added to the growth medium otherwise they would die. Crosses of different strains were carried out, to show that the ability to synthesise all amino acids could return and be passed on. From the evidence of many experiments came the hypothesis that a single missing enzyme was the result of the mutation of one gene. 4 Scientists recognised that the end result of a gene was not always an enzyme, for example

that it could be a structural protein or some other type of protein and so the definition became broader. As the structure of complex proteins like haemoglobin were unravelled, scientists realised that some proteins were made up of several polypeptide strands, and that different mutations changed different polypeptide strands which showed the genes were coding for individual polypeptides, not always the same thing as the final protein. 5 We now know that different proteins can be manufactured from the same gene, from the

evidence that all those proteins are affected by mutation in that gene. We also understand that post-transcriptional changes may occur in mRNA, from obser vations of the action of spliceosomes. Other points may be valid. Edexcel A2 Biology Implementation and Assessment Guide for Teachers and Technicians ©Pearson Education Limited 2009

106


Chapter 6.1 (cont.)

pages 78–81 1 It is the production of an analysis of the DNA of an individual based on mini- or

microsatellite groupings from a limited number of introns which can be compared with similar profiles from other individuals. 2 Used to produce a DNA profile.

•

The strands of DNA from a sample are chopped into fragments using restriction endonucleases which cut the DNA at particular points in the intron sequences. Different restriction enzymes cut DNA molecules into fragments at specific base sequences known as recognition sites which are found at either side of mini and micro-satellite units, leaving repeated sequences intact.

•

The fragments of different length are separated by gel electrophoresis. The DNA fragments are placed in wells in an agarose gel medium in a buffering solution (to maintain a constant pH).

•

The gel contains a dye which binds to the DNA fragments. The dye will uoresce when placed under UV light, which makes the DNA bands visible when the electrophoresis is complete. The most commonly used dye is EtBr (ethidium bromide). A different dye is also added to the DNA samples to show the position of the samples as they move through the gel.

•

An electric current is passed through the apparatus and the DNA fragments move towards the positive anode, because of the negative charge on the phosphate groups. The fragments move at different rates depending on their mass and charge. Known DNA fragments are usually placed in a control well.

•

When electrophoresis is complete, the plate is placed under UV light. The DNA fragments fluoresce and show up clearly so they can be identified.

This is the original method of DNA fingerprinting, which needs a relatively large sample of DNA. It shows up large DNA fragments containing a minimum of 50 base pairs, i.e. minisatellites. However smaller regions of DNA (microsatellites) and specific genes can now be identified using extensions of this technique. 3

a The polymerase chain reaction repeats the normal replication of DNA using the

enzyme DNA polymerase. The reactants – the DNA sample to be amplified, DNA polymerase, primers (small sequences of DNA which must join to the beginning of the separated DNA strands before copying can begin) and many copies of the four nucleotide bases – are mixed together in a PCR vial and placed in a PCR machine. The reaction mixture is first heated to 90 –95 °C for about 30 seconds which causes the DNA strands to separate as the hydrogen bonds holding them together break down. The mixture is then cooled down to 55–60 °C when the primers bind (or anneal) to the single DNA strands. Finally the mixture is heated up again to 75 °C for at least a minute. This is the optimum temperature for the DNA polymerase enzyme which builds up complimentary strands of DNA identical to the original molecule. These three basic steps are repeated around 30 times to give around 1 billion copies of the original DNA.


107


Chapter 6.1 (cont.)

b It allows scientists to amplify tiny specimens of DNA to produce enough to carry out

DNA profiling. This is very important in forensic investigations, also in identifying different species of organisms, identifying tiny DNA fragments from fossils etc. Other points may be valid. 4

a Traces of biological material are used as a source of DNA which is amplified using

PCR and then used to produce a profile. Because the chances of two individuals having the same DNA profile are extremely low, when the DNA profile of a suspect matches DNA from the crime scene this is taken as strong evidence of their involvement in some way. b Only identical twins have identical DNA profiles, however family members show

many more similarities than non-related people. So, if the DNA of a suspect of one crime is checked, if DNA from another family member is on the database, it will come up as a close match. This can lead police to the right suspect, even if their DNA is not on the database.


108


Chapter 6.2

pages 82–83 1 •

•

Extremely small so can easily get into the body. Simple structure of protein coat and genetic material means the virus can withstand harsh conditions and retain infectivity.

•

Has virus attachment points (VAPs) to enable the virus to attach to specic types of cells.

•

Some have specic mechanisms for injecting their DNA into cells.

•

Viral genetic material is adapted to take over host cell mechanisms. For example, viral DNA acts directly as a template for both new viral DNA and for the mRNAs needed to induce synthesis of viral proteins. Viral RNA directs the synthesis of reverse transcriptase which proceeds to make DNA corresponding to the viral genome which is then used as a template for new viral proteins and ultimately a new viral RNA genome.

Other points may be valid. 2 [S1C] The answers should include some or all of the following. Other points may be

valid. a Like living organisms, they reproduce, contain genetic material, undergo evolution, are

obligate intracellular parasites (can only exist and reproduce as parasites in the cells of other living organisms). b Unlike living organisms, they don’t feed, respire, excrete, move, or show any

sensitivity. The only characteristic of living things shown is reproduction.

pages 84–85 1


valid. Lytic: Viral genetic material is replicated independently of the host DNA straight after entering the host cell. Mature viruses are made by host cell, which eventually bursts and releases large numbers of new virus particles. These go on to invade other host cells. The virus is said to be virulent (disease-causing). Lysogenic: Viruses are non-virulent when they first get into the host cell. They insert their DNA into the host DNA so it is replicated every time the host cell divides. No mRNA is produced from the viral DNA because one of the viral genes causes the production of a repressor protein which makes it impossible to translate the rest of the viral genetic material. The virus remains dormant. Viruses in the lysogenic state can become lytic under the right conditions. 2

a Retroviruses have RNA as genetic material instead of DNA. b They have more complex lifecycles as the RNA first has to be translated into DNA

by reverse transcriptase in the cytoplasm of the host cell. This DNA passes into the nucleus to be inserted into the host DNA. Viral DNA is then transcribed to make viral mRNA and viral genome RNA. The viral mRNA acts as a template for new viral proteins and other chemicals to make new viruses.


109


Chapter 6.2 (cont.)

pages 86–87 1

[S1C] The answers should include some or all of the following. Other comparative

points may be valid.

Structural features

Animal cells

Bacteria

Viruses

Average size (mm)

10–100

0.5–10

0.02–0.3

Genetic material

DNA

DNA

DNA or RNA

Outer layers

lipoprotein cell surface membrane

• cell surface membrane

• may have an outer

and bacterial cell wall made of peptidoglycan with other compounds such as teichoic acid, lipopolysaccharides

lipid envelope • protein coat or capsid

made up of repeating capsomeres

• may have slime

capsule or layer Main inclusions

• nucleus containing

genetic material (DNA) • • • • • • • • • •

ribosomes mitochondria Golgi body lysosomes vesicles cytoplasm centrioles cytoskeleton vacuoles endoplasmic

• cytoplasm • genetic material

• genetic material

(DNA) • plasmids • mesosomes

reticulum (RER and SER) Movement

• varies – may not

• may have agellae

move, may be amoeboid, may use cilia, flagella, etc.

• passive movement

only

pages 88–89 1

a Rapid, reliable – can occur every 20 minutes in ideal conditions. b Brings about genetic variation which can enable bacteria to survive unfavourable

conditions. 2

a It shows that genetic material can be taken up by bacteria in sufficient quantity to

bring about a change in character of the organism, and in this case make it infective. b When the dead bacteria were attacked by enzymes that destroyed carbohydrates

and proteins it had no effect on the ability of the transforming factor to change the R bacteria into S bacteria. This shows that the transforming factor is neither carbohydrate nor protein. However, when the DNA is destroyed, the transformation does not take place, providing strong evidence for DNA as the transforming factor. Edexcel A2 Biology Implementation and Assessment Guide for Teachers and Technicians ©Pearson Education Limited 2009

110


Chapter 6.2 (cont.)

pages 90–91 1


Viruses: Cause disease by taking over the metabolism of the cell and causing it to make new viruses. The host cells are usually destroyed by lysis as the new viruses break out of the cell, by the viruses triggering the release of the cell’s own lysozymes which digest the cell, or (relatively rarely) by the viruses triggering the production of toxins in the cell which inhibit cell metabolism. Bacteria: Cause disease through the toxins they make as they grow and reproduce. Endotoxins are lipopolysaccharides, part of the outer layer of Gram-negative bacteria. They often cause fever, diarrhoea and sickness. Exotoxins are usually soluble proteins produced by bacteria as they metabolise – they have specific effects on the host, causing disease. They are less likely to cause fevers but some of the diseases they cause can be fatal. 2

Diagram showing microorganisms involved at the following stages: dead organic matter carbon compounds in decomposers, carbon compounds in decomposers

→

CO2 in air

→

or water.

pages 92–95 1


valid. Suitable table along with comments such as: •

vectors: tough skin, blood clotting

•

fomites: natural skin ora and sebum

•

direct contact: skin, natural ora, sebum, saliva

•

inhalation: mucus, lysozymes, phagocytes

•

ingestion: saliva, mucus, stomach acid

•

inoculation: clotting

2 Lysozymes are enzymes that can destroy bacterial cell walls. Lysozymes are present in the

mucus that lines the respiratory system, the gut, the urinary and reproductive tracts. They act to destroy bacteria, and are particularly effective against Gram-positive bacteria. They are also present in tears and destroy bacteria that enter and might infect the eyes. 3 Physical barriers:

•

Skin is a tough waterproof outer layer impregnated with keratin that prevents pathogens from gaining entry to the moist, blood-rich tissues where they could invade cells and easily grow to cause disease.

•

Mucus forms a sticky layer which acts as a physical barrier to the entry of pathogens. It traps pathogens and prevents them reaching the moist tissues below in the respiratory tract and gut, for example.

Chemical barriers: •

Sebum is a layer of oil on top of the skin that contains chemicals which inhibit the growth of pathogenic bacteria but which support the growth of the natural healthy skin bacteria.

•

Acid in the stomach has a low pH and acts as a barrier to most pathogens which are destroyed if they are ingested. Edexcel A2 Biology Implementation and Assessment Guide for Teachers and Technicians ©Pearson Education Limited 2009

111


Chapter 6.2 (cont.)

Biological barriers: •

Skin and gut ora – these are the healthy bacteria growing on the surface and lining the tubes of the body. They outcompete pathogens and prevent them gaining entr y to the body.

•

Lysozymes are enzymes found in mucus and tears that break down bacterial cell walls and kill them before they can infect the tissues.

•

The vomiting reex ejects bacteria and viruses from the body before an infection can spread far.

Any other valid points.

pages 96–99 1 •

Mast cells and damaged white blood cells release chemicals known as histamines which cause the blood vessels in the area, particularly the arterioles, to dilate causing local heat and redness. Heat reduces the efficiency of reproduction of pathogens.

•

Histamines also make the walls of the capillaries leaky so uid, including plasma, white blood cells and antibodies are forced out of the capillaries causing swelling (oedema) and often pain.

•

White blood cells engulf pathogens by phagocytosis, antibodies inactivate pathogens, pain makes you take care of injured site.

2

Because they simply react to non-self – the response is not specific to a particular pathogen.

3

a A raised temperature can help the body combat infection by lowering the

reproduction rate of the pathogens. Also the immune system works better at higher temperatures and so will be more successful at combating the infection. b If body temperature rises above 40 °C, the denaturation of some enzymes may occur

causing permanent tissue damage. If the temperature is not lowered fairly quickly death may result.


112


Chapter 6.2 (cont.)

pages 100–103 1

Humoral response

pathogen with antigens

Cell mediated response


macrophage


B cell

APC

T helper cell

cytokines APC

B effector cells

plasma cells

binds and cytokine

B memory cells

T helper clones

infected body cell

cytokines

APC

T killer clones

T killer cell

T killer memory cells

bind to and destroy infected cells

antibodies

2 The immune system responds to foreign antigens on the surface of pathogens.The cells

of the body have antigens that the immune system recognises as self and so does not attack them. 3 B cells: These have receptor proteins which recognise the antigens on the surface of

invading pathogens. They give rise to clones of cells that produce antibodies to a specific pathogen. T cells: These come in two types. T helper cells produce chemicals that stimulate the production of antibodies. T killer cells produce chemicals that destroy pathogens. 4 Non-specific responses recognise the difference between self and non-self and initiate

general reactions against anything that is non-self. Examples are the inflammatory response involving the mast cells and the histamine reaction, and fevers. The specific responses of the immune system recognise not just non-self, but also very specific antigens on a pathogen. So the specific immune system targets particular pathogens, and the response is unique to that organism and will not affect others.


113


Chapter 6.3

pages 104–105 1 •

Disinfectants kill bacteria from the area, from instruments etc. This reduces the likelihood of bacteria being transferred from environment to the patient/wound.

•

Antiseptics are applied to skin and to wounds. They kill bacteria and so reduce the likelihood of wounds becoming infected, bacteria getting into blood etc.

•

Antibiotics destroy bacteria within a patient and so reduce the likelihood of infection passing from one person to another through either direct or indirect contact.

2

a In the 1800s, 300 women died per 100 000 babies born (600–900 women infected per

100 000 babies born). In the early 2000s deaths were 0.85 women per 100 000 babies born. b Their work was important because it showed clear evidence for the effectiveness of

antibiotic drugs in treating a bacterial infection which killed many young women each year. It fell short of modern ethics and standards because there were no lab trials on tissues and cells, no control patients who did not receive the drug or who received a placebo, no double-blind trial, probably no patient permission for the treatment sought, and no ethics committee decision on whether the trial should go ahead. Other points may be valid.

pages 106–107 1 The following describes the basic information. Other details may be found from research.

•

Ronald Hare, one of Fleming’s young assistants, found that penicillin mould grows best at quite low temperatures.

•

Dr Cecil Paine, another one of Alexander Fleming’s pupils, was the rst person to try penicillin out on people. A local miner got a stone in his eye and had a massive infection which was making him blind. Paine washed the eye with his penicillin extract, and the eye recovered. Paine also used his penicillin wash to treat a tiny baby which had picked up an eye infection as it was born. The baby was cured and its sight was saved. Paine never published what he had done, but he talked to Howard Florey, a new professor at the university, who went on to follow up the work.

•

Norman Heatley was a key man in the Oxford team. He worked out a way of making enough penicillin to text it and carr ied out the following experiment. Eight mice were infected with bacteria which would kill them in 24 hours. Four were given penicillin. The four treated mice stayed healthy – but the other four died. In 1940–41 Heatley developed special pot vessels so he could grow more mould and so collect more penicillin. In just a few weeks he had made enough penicillin for Florey to try it out on a fully grown man.

•

Mary Hunt brought a mouldy melon into the lab that she had found on a market stall. The mould was a new sort of Penicillium which produced much more penicillin than the original strain. The new mould would also grow in big fermentation tanks which was the breakthrough which enabled industrial production to go ahead. Ms Hunt got the nickname ‘Mouldy Mary’.


114


Chapter 6.3 (cont.)

2

a Any two examples of the action of bacteria clearly explained. b Bacteriocidal means it kills bacteria. Bacteriostatic means it stops bacteria growing/

reproducing. c Concentration – some antibiotics are bacteriostatic at relatively low concentrations,

sufficient to allow immune system to deal with the infection. However, with severe infections, a higher dose can be bacteriocidal.

pages 108–111 1 Mutation causes a change that affects the way the antibiotic gets into or affects the bacteria.

The antibiotic is a selection factor by killing all bacteria that are sensitive to it. Therefore there is rapid evolution so that the resistant type becomes the most common form. 2

a [S1C] The answer should include some or all of the following. Other points may be

valid. Keep antibiotic prescription to a minimum; ensure that patients complete the course of treatment with antibiotics; be especially careful with the use of broad-spectrum antibiotics in the case of C . difficile. b Minimise exposure to reduce the likelihood of resistant mutation being selected for.

Make sure that the whole course of antibiotics is used so that bacteria with only a slightly increased resistance are definitely killed or inactivated. 3

a Broad-spectrum antibiotics wipe out large numbers of the normal gut flora so C.

difficile can get established, produce toxins and cause disease symptoms. b Narrow-spectrum antibiotics are targeted at the organisms which are causing a

specific infection. So they are less likely to have an impact on gut flora and so minimise opportunities for C . difficile to become established. 4 Examples of good practice include:

•

Doctors, nurses and other health care professionals washing their hands or using alcohol based gels between seeing patients removes bacteria on the skin and so reduces chances of passing them on. Spores of C.difficile are not destroyed by the alcohol gels so these are of limited use in outbreaks of C.difficile infection.

•

Avoid clothing which might carry bacteria from patient to patient, e.g. long ties, wrist watches and long-sleeved shirts (the cuffs can carr y bacteria).

•

Health professionals wear sterilised scrubs all the time to avoid bringing in pathogens from the outside.

•

Monitoring patients for infection when admitting and treating and nursing in isolation avoids spread of pathogens.

•

Encourage visitors to wash hands and use alcohol gels to minimise risk of bringing infection into hospital or taking it out.

•

Thorough cleaning of hospital wards, toilets, bed pans etc, prevents and controls the spread of disease by removing bacteria, faecal traces etc.

•

Using chlorine-based disinfectant to be sure C. difficile is destroyed.

Any other valid points.


115


Chapter 6.3 (cont.)

pages 112–115 1 Natural active immunity: Infected by pathogen which may reproduce and cause

symptoms of disease. The immune system is activated and the immune cascade initiated: B cells, antibodies and T cells produced which destroy pathogen. Immunological memory ensures that if the pathogen is met again, the immune system is activated before disease results. Natural passive immunity: Antibodies against various diseases are passed from mother to baby through placenta or in breast milk. Short-term protection against these diseases as immunity is lost with time. Artificial passive immunity: Antibodies produced in one individual or animal are given to another individual if they have come into contact with a par ticularly dangerous or rapidly acting pathogen, e.g. tetanus. Short-term protection as no immunological memor y involved. Artificial active immunity: The immune system is exposed to the attenuated pathogen (e.g. dead, inactivated toxin, attenuated live organism, protein coat fragment or DNA fragment). This stimulates the immune response. The immunological memory ensures that if the live pathogen is encountered again, B cells, T cells and antibodies will deal with it before it can cause symptoms of disease. For example, vaccination against polio, whooping cough etc. 2 As the antigen portion of vaccine becomes increasingly pure (bits of protein coat, DNA

fragments etc.) something else is needed to stimulate inflammation and the full-blown immune response. This is the role of adjuvants – research into best material to use. Any other valid points. 3

a They are not immune to that disease, therefore if they meet the pathogen in this

country or abroad they are likely to become ill and may be permanently damaged or die. b If a child is not vaccinated, it may carry infection or become ill and put other

unvaccinated children (e.g. those allergic to eggs or with compromised immune systems) at risk of the disease. c •

From an individual point of view, completely ethical. In fact it could be regarded as unethical if parents did not have a choice over their children’s treatment.

•

From a societal point of view, it could be argued that it is unethical that individuals through ignorance, misconceptions, laziness or indifference could compromise not only the health of their own children but also the health of society as a whole and the weakest members of it by threatening the concept of herd immunity. So from this perspective it is unethical that parents can deny their child immunisation. Any other valid points.


116


Chapter 6.3 (cont.)

4 A link between the whooping cough vaccine and brain damage was suggested. The

media took hold of the story and published it, but ignored the f act that whooping cough is known to cause brain damage in some children with a quantified risk. Parents panicked when they heard the story and didn’t take their children for vaccination with the whooping cough vaccine. More children started suffering from whooping cough and cases of damage caused by the disease began to be seen again. The scare was shown to be no more than a correlation which had caught the eye of a doctor who became convinced of the problem, in spite of all evidence to the contrary. It took years for public confidence in the vaccine to be restored. Material on MMR as produced by students. This should cover how concerns over the MMR vaccine came about; the effect of the media and how the story was reported, including the lack of scientific analysis in the reporting; the response of parents; and the impact of the reduction in take-up of the vaccine on the number of cases of measles, mumps and rubella, including their worst side-effects.

pages 116–119 1 •

•

It is transmitted from one person to another by droplet infection. It can affect a wide variety of tissues, but particularly infects the lungs which means it is easily coughed out and spread.

•

It has evolved a protective wax outer covering. This allows bacteria with the random mutation which provides the thickest outer coat to remain dormant or grow very slowly for years until the host is in a low physical condition resulting in reduced immune response, at which point the bacteria can take hold and become active. Those bacteria best fitted for survival in these conditions are the ones which will be passed on.

•

Attacks the immune system of the host and so reduces the defences against it.

•

It has evolved resistance to many of the most commonly used antibiotics, which means it is more successful at surviving and spreading easily.

2 Primary infection: often symptomless but causes an inflammatory response from the

immune system. The immune response forms a mass of tissue called a tubercule, with dead bacteria and macrophages trapped in the centre where conditions are completely anaerobic. After about eight weeks the immune system controls the mycobacteria, the inflammation dies down and the lung tissue heals. Active tuberculosis: the bacteria multiply rapidly and destroy the lung tissue. Symptoms include fever, night sweats, loss of appetite, loss of weight, feeling tired and listless. As the infection invades the lungs, it causes a cough. As the cough get worse, the tissue of the lungs becomes damaged and blood may be coughed up in the sputum. The structure of the lungs is steadily destroyed, with the alveoli breaking down to produce large, inefficient air spaces. The T cells of the immune system are affected so antibody production is reduced. Eventually TB causes death, either because the individual cannot get enough oxygen from the air through their damaged lungs, their organs fail through lack of nutrition, or through opportunistic infections such as pneumonia.


117


Chapter 6.3 (cont.)

3


valid. •

Poverty: malnutrition therefore suppressed immune systems making infection easier.

•

Crowded living and working conditions: ease of infection.

•

High rates of illness from malaria, water-borne diseases etc, so immune system under pressure.

•

High rates of HIV/AIDS so the immune system is inactivated, making opportunistic infection easier.

•

Poor infrastructure and limited medical resources, so lack of vaccination programmes, lack of accurate diagnosis and lack of drugs to treat symptoms effectively for the time required.

• 4

Cattle infected with bovine TB and milk not pasteurised.

[S1C] The answers should include some or all of the following.

Any valid points, well-argued and well-researched material, showing awareness of the conflicting perspectives of scientists, farmers, politicians, the general public etc.

pages 120–123 1 Initially after infection people may have fevers, headaches, tiredness, and swollen glands

but may have no symptoms. About 3–12 weeks after infection, HIV antibodies appear in the blood so person is HIV positive. Once the infection is established, all symptoms disappear and this stage may last many years (in fit young people with access to effective anti-AIDS drugs), or months to years (people with little food or medicine). During this stage the virus replicates, infecting the CD4 T-helper cells but is kept in check by the T killer cells. This is the stage when other people may become infected through high-risk behaviour. Eventually secondary infections develop as the immune system begins to be overwhelmed and symptomatic disease returns. The viral load becomes so large that the immune system can no longer cope. The normal T-helper cell count falls from 500 per mm 3 of blood to about 200 per mm 3, and symptoms develop including weight loss, fatigue, diarrhoea, night sweats and low-grade infections such as thrush. This rapidly progresses to the final stage. As the T-helper cell numbers fall, severe symptoms begin to appear such as major weight loss, dementia as brain cells become infected, cancers (e.g. Kaposi’s sarcoma) and serious infections such as TB and cryptococcal meningitis. These serious diseases, along with opportunistic infection, such as pneumonia lead to death.


118


Chapter 6.3 (cont.)

2


valid. TB: Bacterial; infects a wide range of cells, including lungs, lymph and bone, as well as macrophages of the immune system; often remains dormant for months to years within the lungs but can cause immediate infection; causes a particular set of symptoms and can kill because of damage to lungs or malnutrition, but also leaves body open to opportunistic infections as damages the immune system; can be treated effectively and cured by antibiotics; can be prevented by vaccination. HIV/AIDS: Viral; infects the helper T cells of the immune system; after initial infection period remains dormant within the immune system for months to many years; causes a particular set of symptoms but the main impact is on the immune system; leaves host vulnerable to a wide range of opportunistic infections which usually result in death; the course of disease can be slowed but not prevented by medication; no cure; no effective vaccine. 3 Main mechanisms: the thick coat of M. tuberculosis enables them to survive inside

macrophages for many years in a dormant or slow-growing state. This is the result of evolution, as those with the thickest coats survive and will be able to infect other hosts, and so are the most successful pathogens. Multidrug-resistant strains are also the result of evolution. HIV: rapid mutations (and therefore evolution) mean changes in antigens which makes it very difficult for the immune system to attack the virus. The targetting cells of the immune system reduce the effectiveness of the host in fighting the virus. Rapidly evolving antigens makes vaccine production very difficult. Some people have genes which make them resistant to HIV. In countries where the disease is rife those genes will become dominant in a population as individuals with them survive to reproduce.


119


Practical 6.1 DNA gel electrophoresis

Purpose •

Safety

To use gel electrophores electrophoresis is to separate DNA

If gel electrophoresis electrophoresis is to be used a full risk

fragments of different sizes.

assessment should be obtained for the procedure and carefully followed by both staff and students.

Restriction Restr iction enzymes The use of restriction enzymes to cut DNA and electrophore electrophoresis sis to separate the resulting fragments is possible using equipment available from NCBE (National Centre for Biotechnology Education) Education) and Bio-Rad. Protocols Protocols for these practicals as pdf files can be downloaded from their websites. NCBE produce an electrophoresis base unit and a lambda DNA kit. Together these contain all necessary gel electrophoresis apparatus, dried lambda DNA, three dried restriction enzymes and both student and technical guides. The kit includes microsyringes and gel tanks but not batteries needed as a power supply. The kit for f or eight electrophoresis stations (16 runs) cost £52 in 2009. The The lambda protocol module, for use with eight students, cost £90 in 2009. Have a look at the technical guides by downloading the pdf files from the NCBE website. The NCBE publication Illuminating DNA also contains protocols for experiments using restriction enzymes and electrophoresis. This publication can also be viewed on the NCBE website. NCBE has developed a genetic screening simulation, Nature’s Nature’s dice. In this practical simulation, the inheritance of a single gene that is involv involved ed in a particular par ticular genetic trait is investigated. investiga ted. The kit contains 24 DNA samples donated by a large family who are affected by the genetic condition. Each student is given one or more of these DNA samples and they have to detect which allele is present. They cut the DNA with a restriction enzyme and examine the resulting DNA fragments by electrophoresis. The genetic data obtained is then combined with the family tree to look at how the genetic trait is inherited. This activity would provide an excellent alternative to the standard gel electrophoresis practical suggested above. Further information about the kit and cost of materials is on the website listed in the weblinks section for this activity activity.. Bio-Rad produces a Restriction Digestion and analysis of lambda DNA kit. This contains dried lambda DNA, three restriction enzymes, buffers, microtubes and both Student and Teacher’s guides. It does not include the micropipette, electrophoresis cell or power supply. The kit for eight stations (eight runs) cost £86 in 2009; the cell and power supply needed to run the gels cost an additional £400 1 but can be borrowed from some ITT centres (see below). You can have a look at the protocol by downloading the pdf file from the Bio-Rad website. Their website contains more detail and alternative kits. Go to the site and then click the Life Science education icon. Edexcel practical materials created by Salters-Nuffield Advanced Biology, ©University of York Science Education Group.

11



DNA gel electrophoresis

Pfizer and Bio-Rad have donated biotechnology equipment to 31 ITT centres around the country as part of a National Year of Science project. Each centre holds equipment that can be used to conduct biotechnology practicals for 25 students at a time. They have all the equipment necessary for completing DNA fingerprinting, bacterial transformation (genetically altering bacteria with a bioluminescent jellyfish gene), purification of a useful protein, and the polymerase chain reaction (PCR). It is envisaged that this equipment will be used for hands-on practical training of student science teachers. There are also plans for loan schemes to be established to enable local schools to borrow the equipment. To find out more about this project contact the BioEducation Project Manager at Bio-Rad Laboratories (details can be f ound on the BioRad website). Both organisations produce replacement replacement equipment for their kits and also supply the components separately. Note that some of the Bio-Rad manuals accompanying accompanying their kits (unless recently revised) may not carry full and appropriate health and safety guidance applicable in the UK (they were written for a US audience). Where the instructions conflict with good practice that is standard in the UK, additional precautions should be taken. It is most likely that gel electrophoresis equipment using batteries or a low-voltage supply is used. This is safe if the voltage is less than 40 V DC. If equipment is used that is operated by a power supply at a greater voltage, it is essential that the equipment is designed so that it is impossible to make skin contact with the electrodes or electrolyte when an electric current is flowing. This is usually achieved by a suitable design of lid for the gel tank which only permits a current to flow when the lid is in place. Also, although although it is very unlikely to be needed, ethidium bromide, which is very toxic, is not normally recommended for use in schools. For general CLEAPSS guidance on electrophoresis, schools should consult section 11.1.7 of the Laboratory Handbook.


12


Practical 6.2 DNA amplification using PCR

Purpose •

Safety

To use PCR to amplify DNA.

If PCR is to be used a full risk assessment should be obtained for the procedure and carefully followed by both staff and students.

PCR It is possible to carry out practical PCR using equipment available from a number of suppliers including NCBE (National Centre for Biotechnology Education), Bio-Rad and Edvotek Europe. Although all organisations supply automated PCR thermal cyclers, they are not absolutely necessary. Instead, Instead, PCR can be under taken using three separate thermostatically controlled water baths, although although care must be taken with the hottest as a risk of scalding exists. Note that during Science Year (September 2001–July 2002) many schools, colleges and initial teacher training institutions received free PCR equipment. Student and teacher/technician protocols for the PCR practicals can be downloaded as pdf files from the suppliers’ supplier s’ websites. websites. Equipment can be purchased either in class sets or individually individually,, with consumables also available in class-sized batches. All three organisations listed offer training courses, frequently in association with other institutions like botanic gardens or science centres, giving teachers and technicians the opportunity to car ry out the practicals themselves. There are social and ethical considerations to take into account when using DNA derived from students for PCR. Although the suppliers should have chosen STR sequences that have no biological significance, it may be socially and ethically less challenging to use plant DNA as the sample. But working w ith one’s own DNA can be uniquely motivating!

Edexcel practical materials created by Salters-Nuffield Advanced Advanced Biology, ©University of York Science Education Group.

13



DNA amplification using PCR

Contact details and examples of the protocols offered by NCBE, Bio-Rad and Edvotek Europe: Organisation

Example of suitable protocols

NCBE National Centre for Biotechnology Education Science and Technology Centre The University of Reading Whiteknights Reading, RG6 6BZ www.ncbe.reading.ac.uk 0118 987 3743 [email protected]

Amplifying lambda DNA (from the Illuminating DNA booklet) Investigating plant evolution Analysing mitochondrial DNA. This practical requires a microcentrifuge and the protocol is not downloadable from the NCBE website. It is based on a protocol developed for a DNA workshop at the Cold Spring Harbor Laboratory. It can be obtained direct from NCBE.

Bio-Rad Bio-Rad Laboratories Ltd. Bio-Rad House Maxted Road Hemel Hempstead Hertfordshire HP2 7DX www.biorad.com/ Follow the links to Life Science education, then About Biotechnology Explorer Freephone: 0800 181134

Crime scene investigator – PCR basics PV92 PCR infomatics kit GMO investigator kit (only useful if GM foodstuffs are easily available)

Edvotek Europe The Biotechnology Education Company PO Box 280 Hertford SG13 9DG www.edvotek.co.uk Follow links to Experiments and then Polymerase chain reaction [email protected]

332 Mitochondrial DNA analysis using PCR


14

Teacher/Lecturer

Practical 6.3

Which antibiotic is most effective?

Purpose •

1 of 1

Safety

To investigate the effect of

Eye protection is not necessarily required during microbial

different antibiotics on bacteria.

work, though it is good standard practice. The microorganisms are a potential biological hazard. Use aseptic techniques when transferring the bacteria to the Petri dishes. Clean the bench with antibacterial disinfectant. Do NOT open the Petri dishes once they have been incubated.

Notes on the procedure The aim of the practical is to show students a method of determining bacterial sensitivity to different antibiotics and to give them practice at using aseptic techniques. This could be done with antibiotic discs or the same technique could be used with antiseptics. The Student sheet suggests using different bacterial species but the activity could be done using just one species. However, using different species should illustrate that they are not all equally susceptible. Although allergic response by patients is considered in the questions, where antibiotics are already impregnated on to discs, the risk for students with allergic responses is not g reat. Discs are not handled directly and no airborne dust is created by the discs. The behaviour of students must be considered. If they might attempt to lift a piece of tape from an incubated dish, the agar plates should be sealed around the circumference after incubation but before being returned for observation. Also, to prevent agar plates being removed from the lab, count all plates back in again before students are allowed to leave.

Answers 1

Choice of strategy may vary, but it must be clear and easy to carry out and produce reliable results.

2

The rate of diffusion of the antibiotic will be influenced by the size of molecule, its concentration and the potency of the antibiotic. If the antibiotic is effective at lower concentrations the circle will be larger (all other things being equal). Gram-positive and Gram-negative bacteria respond differently to antibiotics.

3

Responses will depend on the data, but should show a clear understanding of the concepts of accuracy, validity and/or reliability when explaining variation. Suggestions for the presentation of variation in graphs may also var y but need to identify anomalous results clearly so that conclusion are obviously based on reliable results.

4

You may have to consider whether the patient is allergic to any of the antibiotics. For example, allergy to penicillin is not uncommon. You may consider the state of the patient’s immune system. In patients with a weakened immune system you would not want to use a bacteriostatic antibiotic, that is, one that stops bacterial reproduction but does not kill the bacteria. Some antibiotics can be used together and produce a larger effect when combined than if administered separately. This is known as synergism. Edexcel practical materials created by Salters-Nuffield Advanced Biology, ©University of York Science Education Group.

15

Technician

Practical 6.3

1 of 1


Purpose •

To investigate the effect of different antibiotics on bacteria.

This experiment could be done with antiseptics rather than antibiotics. Paper discs produced with a hole punch are autoclaved, then dipped in a range of antiseptics. CLEAPSS guidance on microbiology is in section 15.2 of the Laboratory Handbook. Sterilisation guidance is in section 15.12. Requirements per student or group of students

Notes

Agar plates seeded with bacteria or equipment for their preparation (i.e. access to one of two broth cultures of known bacteria; Universal McCartney bottle of nutrient agar; sterile Petri dish; sterile pipette)

Suitable bacteria are listed in various catalogues. Phillip Harris use E. coli (K12 strain) and Staphylococcus albus in their kit. The Student sheet with instructions for preparation of seeded agar plates, ‘Pouring agar plates’, can be found in Edexcel AS Biology Practical 4.3. Safety

The bacterial culture could present a biohazard and should be handled and disposed of in accordance with current guidelines on microbiology in schools. Disposal of agar plates and other used equipment, including forceps, must be in accordance with current best practice. Autoclaving is the preferred method of disposal over any chemical disinfection method.

Bunsen burner Bench spray of disinfectant

1% Virkon (preferred) or equivalent.

Soap or handwash

This does not need to be bactericidal.

Paper towels Marker pen for marking Petri dishes Autoclaved after placing in cotton wool stoppered boiling tube.

Forceps (metal) A Mast ring or separate antibiotic discs

Separate antibiotic discs are usually cheaper, but it is impossible to obtain more than two types of antibiotics using these.

Adhesive tape Space to keep dishes at room temperature

This will be needed for at least 48 hours.

Eye protection

Not essential.


153

Student 1 of 1

Practical 6.1 DNA gel electrophoresis

Purpose •

Safety

To use gel electrophoresis to separate DNA

If you undertake gel electrophoresis make sure

fragments of different sizes.

you are aware of the hazards and follow the instructions of your teacher very carefully.

Procedure You may have the opportunity to complete experimental work using restriction enzymes and gel electrophoresis or you may use the simulation of this.


33

1 of 1 Student

Practical 6.2 DNA amplification using PCR

Purpose •

Safety

To use PCR to amplify DNA.

If you undertake PCR make sure you are aware of the hazards and follow the instructions of your teacher very carefully.

Practical PCR Scientists in forensics laboratories carry out the polymerase chain reaction (PCR) using a machine called an automated thermal cycler. This is a programmable heating unit in which the DNA to be amplified is incubated in a buffer solution with thermo-stable DNA polymerase, primers and deoxyribonucleotides. The unit maintains the cyclical sequence of temperatures for the PCR process. Your school or college may be lucky enough to possess a thermal cycler but it is possible to carry out PCR without them, using three separate thermostatically controlled water baths. You simply have to move the DNA sample from bath to bath – and complete 30 cycles! You need a stopwatch, good teamwork and some sort of protection from the steam coming off the hottest bath. Having amplified the short tandem repeat sequences within your DNA sample, you will then separate out the fragments using gel electrophoresis (see Pr actical 6.1). Comparing the position of the bands on the gels to a standard or reference you will be able to draw conclusions about the DNA sample you star ted with. You will follow a practical protocol supplied by the company that produces the equipment and reagents your school or college has p urchased.


34

Student

Practical 6.3


Purpose •

1 of 2

Safety

To investigate the effect of different

Wear eye protection.

antibiotics on bacteria.

The microorganisms are a potential biological hazard. Use aseptic techniques when transferring the bacteria to the Petri dishes. Clean the bench with antibacterial disinfectant. Do NOT open the Petri dishes once they have been incubated.

Introduction When a bacterial infection is diagnosed it is useful to be able to tell to which antibiotics it is most susceptible. In some cases this information is known, but in other cases tests need to be carried out to find out which antibiotic will be most effective. In this activity you will be testing the effectiveness of several types of antibiotics on bacteria. The standard method of doing this is to put discs of chromatography blotting paper soaked in the various antibiotics onto an agar plate that has been inoculated with the bacteria. Alternatively a Mast ring (a r ing of paper with several ‘arms’, each treated with a different antibiotic) can be used.

Procedure You will need: •

Agar plate seeded with a known bacterium

•

Marker pen

•

Bunsen burner

•

Autoclaved forceps

•

Bench spray of disinfectant, 1% Virkon or

•

Mast ring or antibiotic-impregnated paper

equivalent

discs

•

Soap or handwash

•

Adhesive tape

•

Paper towels

•

Eye protection

1 Wash your hands with the soap or handwash. Spray the working area thoroughly with the

disinfectant spray. Leave for at least 10 minutes, then wipe with a paper towel. 2 Work very close to a lit Bunsen burner. Prepare an agar plate seeded with bacteria. This

may have already been done for you. If not, follow the instructions in the section ‘Pouring agar plates’ in Practical 4.3 Edexcel AS Biology. Label the Petri dish on the base at the edge with your name, the date and the type of bacterium it is inoculated with. 3 Flame the forceps and then use them to pick up an antibiotic disc or Mast ring. Raise the

lid of the Petri dish and place the Mast ring firmly in the centre of the agar; if individual discs are used they will need to be spaced evenly around the dish. 4 Tape the dish securely with two pieces of adhesive tape (but do not seal it completely),

then keep it upside down at room temperature for 48 hours. Edexcel practical materials created by Salters-Nuffield Advanced Biology, ©University of York Science Education Group.

35

2 of 2

Student

Practical 6.3 (cont.) Which antibiotic is most effective?

5 Wash your hands with soap or handwash and clean the bench again using the Virkon

spray. 6

After incubation, look carefully at the plate but do not open it . Where bacteria have grown the plate will look opaque, but where the antibiotics have inhibited growth, clear zones called inhibition zones will be seen. Measure the diameter of the inhibition zones in millimetres and use this information to decide which antibiotic is most effective at inhibiting the growth of the bacterium.

7 Collect data from other members of the class who used the other bacterial cultures. 8 Write a brief report of the results, comparing the different antibiotics and the effects on

the different bacterial cultures.

Questions 1 Are the inhibition zones circular? If not, what is a sensible measuring strategy? 2 What factors determine the diameter of the inhibition zones? 3 If class data are shared: a what is the overall spread of the data b do all individual results show the same trends – if not, why not, and how could this

variability be represented on your graphs? 4 If you were working in a hospital laboratory, and you had just carried out this test on

bacteria isolated from sick patients, would you always choose the antibiotic that gave the biggest inhibition zone? Are there any other factors you would need to consider?


36


Unit 4 Topic 6

1

The diagram opposite represents the

B

structure of a transfer RNA (tRNA) molecule. Transfer RNA has a similar structure to messenger RNA (mRNA), but the single strand of nucleotides is folded back on itself. The folds are held in place by base pairing. G G A A

A

amino acid

a

b

List the four bases in the region labelled A which pair with those already shown.

(1)

The three shaded bases in region B are responsible for binding to an mRNA molecule during translation. Give the name used to describe this group of three bases on the tRNA.

c

(1)

Each amino acid is coded for by three bases on the mRNA molecule. Explain why a group of three bases is the smallest number that can be used to code for all of the amino acids found in proteins.

d

(3)

The particular amino acid bound to a tRNA molecule depends on the three bases at position B. Explain why this is important during the translation of mRNA to form a polypeptide.


50

(2)



e

In an experiment to investigate the role of tRNA in the synthesis of proteins, cells were exposed for a brief time to radioactively labelled amino acids. Samples of the cells were then removed at intervals over a period of 20 minutes, and the level of radioactivity associated with tRNA and protein was measured as counts per minute (cpm). The graph below shows the results of the experiment. 350 protein 300 ) m p c ( y t i v i t c a o i d a r f o l e v e L

250 200 150 100 50

tRNA

0 0

5

10

15

20

Time/minutes

(i)

Using the information in the graph, describe the changes in the level of

(3)

radioactivity in protein and tRNA.

(ii) Suggest an explanation for the changes in radioactivity found in protein over the first 5 minutes.

(2)

(Total 12 marks)


51



2

A person suspected of being exposed to the Human Immunodeficiency Virus (HIV) may have their blood tested to check for infection. Some tests determine whether antibodies that act against the virus proteins are present in the blood. a

Explain how the presence of HIV proteins in the body results in the production

(4)

of specific antibodies.

b

The graph below shows the change in the numbers of T helper cells in a person infected with HIV. 900 800

d o o l b

3

m m r e p r e b m u n / s l l e c r e p l e h T

700 600 500 400 300 200 100 0 1

2

3

4

5

6 7 8 9 Time since infection/years

10

11

12

13

T helper cells are needed for all immune responses. Fewer than 200 cells per mm 3 prevents an effective immune response. Use the information in the graph to describe how HIV infection is likely to affect this individual’s health.

(4)

(Total 8 marks) Edexcel A2 Biology Implementation and Assessment Guide for Teachers and Technicians ©Pearson Education Limited 2009

52



3

The graphs below show changes in the quantities of an antibiotic used in a hospital and the percentage of infections caused by bacteria resistant to the antibiotic over the same time period. 14 12

g k / 10 d e s u 8 c i t o 6 i b i t n 4 A 2 0 87

88

89

90 91

92

93

94

) % ( 14 t n a t s 12 i s e r 10 s n o 8 i t c e 6 f n i f o 4 e g a 2 t n e c r 0 e P

Year

a

87

88

89

90 91

92

93

94

Year

Using the information in the graph, explain how bacteria become resistant to antibiotics. Use the information in the graphs to support your answer.

b

(6)

Bacteria were grown on an agar plate and incubated with four different antibiotics. The antibiotics were placed on paper discs. The resulting plate is shown in the diagram below.

A

B

C

D

area with no bacteria

control paper without antibiotic


53



(i)

Give

one

piece of evidence that suggests that antibiotic B is more effective at

(1)

killing these bacteria than antibiotic C.

(ii) Antibiotic C is a much larger molecule than antibiotic B. Explain why this method might not be appropriate when comparing the effectiveness of antibiotics

(2)

B and C.

(iii) Antibiotic C acts by inhibiting the growth of bacterial cell walls. Antibiotic D acts by interfering with the activity of bacterial ribosomes. Use this information to

(2)

explain why neither antibiotic would be effective against viruses.

(Total 11 marks) 4

The graph below shows the growth of two species of fly larvae on a dead body. The temperature was kept at 22 °C. 50 Flesh fly

45

House fly 40 35 m m 30 / h t g n e l y 25 d o b n a 20 e M

15 10 5 0 0

1

2

3

4

5

6

7

8

9

Time after death/days Edexcel A2 Biology Implementation and Assessment Guide for Teachers and Technicians ©Pearson Education Limited 2009

54

10

11

12



a

(i)

Give one factor, other than temperature, that could affect the growth of insect

(1)

larvae on a dead body.

(ii) Suggest and explain reasons why the time since death that larvae first appear on a

(4)

body is different for each species.

b

The growth of the larvae is affected by temperature as shown in Table 1. The effects of temperature are given as the number of days ahead ( 1) or behind (–) their development at 22 °C. Table 1

Effect on development/days Temperature/°C

House fly

Flesh fly

12

24

24

27

11

11.5

Two dead bodies were found at the same address and evidence was needed to decide whether they died at the same time. One was found in a boiler room with a temperature of 27 °C and the other was found in an outside shed where the temperature was 12 °C. Insect larvae from both bodies were collected, identified and measured. The results are shown in Table 2. Table 2

Mean length of larvae/mm Site of body

House fly

Flesh fly

Boiler room

23

45

Shed

6

23

(i)

Use the information in the graph, Table 1 and Table 2 for flesh fly larvae to estimate the time of death for the body in the boiler room. The estimate using house fly larvae has been done for you.

House fly Flesh fly

Length/mm

Suggested time since death at 22 °C/days

Adjustment for 27 °C/days

Estimated actual time since death /days

23

9

1

8

(2)


55



(ii) Use the measurements of larvae from the body in the shed to provide evidence that the two deaths occurred at approximately the same time.

(3)

(Total 10 marks) [TOTAL for test 41 marks]


56


Unit 4 Topic 6

1

a arginine alanine threonine glutamine glycine

accept arg ala thr glu gly [All correct

5

2 marks, one mistake

1 mark, more mistakes

5

(2)

no marks]

5

b ACT

(1)

accept adenine cytosine thymine c AGA GCC ACC CAG GGU [All correct

5

2 marks, one mistake

1 mark, more than one mistake

5

(2)

no marks]

5

d (i) Award one mark for each of the following points in context to a maximum of

two

marks.

1 alanine would replace threonine; 2 {primary / secondary} structure would be altered;

(max 2)

3 3D shape would not be correct / eq; (ii) Award one mark for each of the following points in context to a maximum of

two

marks.

1 a stop signal would be inserted; 2 the protein would be shorter / eq; 3 protein would be 46 amino acids long / eq;

(max 2) [Total 9 marks]

2 a Award 1 mark for each correct row in the following table.

Structural feature Mesosomes

Bacteria

Viruses

3

Capsid

3

Nucleic acid

3

Cytoplasm

3

Ribosomes

3

3

(5)

b (i) Award one mark for each of the following points in context to a maximum of two marks.

1 increase in number of new cases in Africa and Europe 2 decrease in number of new cases in Asia and South America 3 any relevant manipulation of data

(2)

(ii) Explanation: •

greater incidence of TB in the population / eq;

Award one mark for each of the following points in context to a maximum of two marks.

1 reference to opportunistic infection 2 HIV positive people have weakened immune system 3 a higher proportion of HIV positive people are infected by TB


86

(3)



c [S1C] Award one mark for each of the following points in context to a maximum of two

marks.

1 TB bacteria {mutate / become resistant to antibiotics} 2 immigration from countries with high incidence of TB 3 increased travel 4 increase in HIV infection 5 lower rates of immunisation against TB


3

a 1 C is bacteriocidal;

2 bacteriocidal kills bacteria; 3 B is bacteriostatic; 4 bacteriostatic prevents reproduction / growth;

(max 3)

b [S1C] Answers include;

1 bacterium is no longer affected by antibiotic A; 2 reference to mutation / changed {gene / DNA}; 3 reference to resistance; 4 reference to selection / eq; 5 reference to plasmid transmission / horizontal inheritance;

(max 4)

c [S1C] Answers include;

1 lawn bacteria / eq; 2 reference to agar plate / eq; 3 antibiotic in well / multidisc / eq; 4 incubation qualified; 5 measurement of clear area / eq; 6 bigger area implies more effective; 7 reference to safety / aseptic technique / eq;

(max 4) [Total 11 marks]

4 a organism that causes disease;

e.g. {virus / bacterium / fungus}; b (i)

(2)

(release) histamine; dilation of arterioles / increased blood flow / vasodilation; oedema / swelling / leakage of plasma; more white blood cells / eq (attack pathogens); mast cells;

(3)

(ii) enzyme; tears / saliva / nasal secretions; breaks down (cell walls of) bacteria / kills bacteria / lysis;

(2)

(iii) {destroys / prevents replication of} viruses or

secreted by infected cells;

(1) [Total 8 marks]


87



5

a Any two from:

1 DNA does not change; 2 DNA unique to individual; 3 test reliable / reference to probability;

(2)

b tissue / eq;

intact DNA / contain DNA / nuclei;

(2)

c An explanation to include:

1 sample from corpse; 2 match with family; 3 match with sample / previous test; 4 example of source of comparison (e.g. hairbrush, toothbrush);

(3) [Total 7 marks]

6 a [S1C]

1 insect development rate / life cycle / aspect of development / rate of growth / eq; 2 temperature dependent; 3 rate fixed at constant temperature; 4 reference to weather conditions affecting temperature of the room; b several values for development rate / eq; c

(2) (1)

1 do not know temperature throughout / temperature not constant; 2 so rate might vary; 3 might have died at night; 4 so infestation next morning / do not know time of infestation / entry to room difficult; 5 too late for other evidence; 6 other factor involved;

(2) [Total 5 marks]


88


Unit 5 Topic 7


a A myosin;

(2)

B Z line / disc; b 1 actin filaments {almost touching / overlapping};

2 Z lines closer together; 3 myosin and actin filaments approximately unchanged in length;

(3)

[accurate representation of printed diagram] c 1 binds to troponin;

2 cause tropomyosin to {move / change shape / be displaced}; 3 binding site (on actin) exposed;

(2)

4 allows {myosin to bind to actin / actomyosin bridges to form};

[Total 7 marks] 2 a (0.60

1 0.65 1 0.65 1 0.65 1 0.75)

5 0.66

(2)

dm ;

b breaths per minute 5 4 or 5

ventilation rate 5 24 / 30 3

÷5;

3

3 6 5 24

3 1.25

/ 30, volume of each breath

to 1.5

5 30.0

5 1.25

to 1.5;

to 45.0 (depends on values used)

21

(2)

dm min ; c tidal volume increases;

rate of breathing increases; greater variation in volume;

(3)

credit comparative manipulation use of figures; d (i)

increase in tidal volume after exercise not as great; increase in ventilation rate after exercise not as great; faster return to normal breathing pattern; respiratory muscles stronger; alveolar capillary network increased;

(4)

gaseous exchange more efficient; (ii) (because) volume pumped per beat / stroke volume, increased;

(2)

maximum / potential, cardiac output increased;

[Total 13 marks] 3

a hypothalamus;

(1)

b reference to water temperature lower than body temperature / converse;

heat lost by conduction (therefore body temperature decreased); c (i)

(2)

[S1C] decrease in temperature when swimming was faster than when lying still;

decrease in temperature when swimming was greater than when lying still; credit quantitative comparative difference;

(3)


73



(ii) 1 reference to muscles used (when swimming); 2 increased {blood flow / vasodilation} to {muscles / limbs}; 3 reference to increased ventilation; 4 (therefore) increases heat loss from body; 5 credit reference to the fact that the volunteer was thin therefore less insulating effect of fat; 6 idea that warm(ed) water is moved away from body’s surface and

(2)

temperature gradient maintained;

[Total 8 marks] 4 a (i)

1 reference to oxygen (concentration) decreasing / eq; 2 greater (decrease) when ADP is added; 3 (oxygen used to) convert ADP to ATP (in respiration); 4 oxygen is needed for respiration / eq; 5 correct reference to oxidative phosphorylation;

(max 3)

6 reference to {ADP concentration / eq} is limiting; (ii) reduced NAD / NADH / NADH2;

(1)

(iii) cristae / inner membrane / stalked particle;

(1)

(iv) 1 hydrogen atoms split into protons and electrons / eq; 2 electrons transferred along electron carriers / a series of redox reactions / eq; 3 oxygen is the terminal electron acceptor / water is formed; 4 {protons / eq} moved into intermembrane space / eq; 5 {protons / eq} move (into matrix) down a {concentration / electrochemical} gradient; 6 through stalked particles / ATP synthetase / eq; 7 correct reference to chemiosmotic theory; 8 (ATP)formed by {phosphor ylation of ADP / oxidative

(max 3)

phosphorylation } / eq; b (i)

1 correct reference to ATP (supplies energy) for active transport / reference to sodium-potassium pump / eq; 2 sodium ions pumped out (of the axon) / restores

(2)

(membrane to) resting potential; (ii) 1 correct reference to ATP (supplies energy) for active transport / reference to sodium/potassium pump / eq; 2 (pumps sodium ions out) of inner segment / maintains

(2)

(more) negative charge inside the membrane / eq;

[Total 12 marks] [TOTAL for test 40 marks]


74


Unit 5 Topic 8


One main function

Letter

Name of region

Initiating and controlling voluntary muscle movement

A

Cerebral hemispheres / cerebrum / cortex;

Coordination of skeletal muscle movement, balance and posture

C

Cerebellum;

Contains centres controlling heart and breathing rate

B

Medulla (oblongata) / brain stem;

[Total 4 marks]

1 mark only for correct letters; 2 a B depolarising;

C

repolarising;

[two correct 5 1 mark, all correct b (i)

5 2

D

resting;

(2)

marks]

1 reference to distribution of Na1 ions (at 0.5 msecs); 2 membrane stimulated; 3 voltage-gated channels open; 4 Na1 gates open; 5 Na1 floods in; 6 idea that more and more (Na1) gates open; 7 K1 gates open / K1 leaves / Na1 gates close (near 2 msecs); (4)

(ii) duration of action potential 5 {4.7 / 4.8 / 4.9} (msecs); [CE] 1000 ÷ {4.7 / 4.8 / 4.9}

5 {213

/ 208 / 204} (per second);

(2)

c 1 reference to hyperpolarisation / reference to K1 overshoot;

2 K1 gates close; 3 reference to Na1 / K1 pump (re)starting; 4 resting potential restored;

(3)

5 reference to refractory period;

[Total 11 marks] 3

a 1 reference to columns being {smaller / narrower / eq}

(in visual cortex for left eye) / converse; 2 (sensory) {neurone / axon} {shorter neurone / reduced growth} (for left / deprived eye) converse; 3 {fewer / shorter dendrites} / fewer {synapses / branches} (in left / deprived eye)

(max 2)

/ eq / converse;

b 1 reference to {critical / sensitive} period / critical window (in visual development);

2 idea that if one eye is deprived of {stimulation / light}, {neurones / dendrites/

(2)

synapses / columns} do not develop / eq;

c reference to visual deprivation studies, e.g. cataract removal from children, bandaging

of eyes / reference to development of distance perception e.g. Muller-Lyer

(1)

[Total 5 marks] Edexcel A2 Biology Implementation and Assessment Guide for Teachers and Technicians ©Pearson Education Limited 2009

75



4

a Award one mark for each of the following points in context to a maximum of three marks.

1 recovery time falls steeply initially

3 fluctuations

2 stays low

4 reference to reinforcement

5 credit appropriate manipulation of data reference to anomalous point 3

(3) (1)

b habituation c [S1C] Award one mark for each of the following points in context to a maximum of

four marks.

1 ignore unimportant stimuli

4 more time for oxygen uptake

2 more receptive to important stimuli

5 can remain active when being touched

3 less time wasted with gill covered

6 such as by wave action

(4) [Total 8 marks]

5 a Impaired synthesis / serotonin reabsorbed before it reaches post-synaptic membrane /

(1)

reference to vesicles; b [S1C] An explanation to include six fr om:

1

{nerve impulses / action potentials} arrive at {synapse / presynaptic membrane};

2

calcium ion channels open / calcium ions enter;

3

vesicles fuse with presynaptic membrane;

4

correct reference to exocytosis;

5

neurotransmitter diffuses across (synaptic cleft);

6

binds to receptors on post-synaptic membrane;

7

leads to opening of sodium ion channels / sodium ions enter;

8

depolarisation of post-synaptic membrane;

9

{threshold / action potentials / impulses} in post-synaptic neurone;

10 c

(6)

neurotransmitter broken down by enzymes / reabsorbed;

[S1C] A suggestion and an explanation to include two from:

1 increases serotonin synthesis; 2 blocks serotonin re-uptake into (presynaptic) neurones / correct ref. to SSRI; 3 therefore high levels of serotonin remain in cleft / eq; 4 binds with post-synaptic receptor; 5 {threshold / more impulses / more action potentials) in post-synaptic neurone; (2) d [S1C] A suggestion to include three from:

1 similar shape to serotonin / eq; 2 binds to molecules on {presynaptic membrane / re-uptake mechanism}; 3 therefore high levels of serotonin remain in cleft / eq; 4 impulses / action potentials continually generated;

(3)

5 correct reference to post-synaptic neurone;

[Total 12 marks] [TOTAL for test 40 marks] Edexcel A2 Biology Implementation and Assessment Guide for Teachers and Technicians ©Pearson Education Limited 2009

76



pages 128–129 1 It supplies energy in the form of ATP for all cellular reactions. It also supplies substrates

for other metabolic pathways, including amino acids and fatty acids. 2

Strengths: succinct summary; sums up the necessary reactants, the desired product and the waste products; gives a balance chemical equation to show the proportions of the reaction chemicals. Limitations: shows it as a single reaction, when it is a complex series of reactions; doesn’t show where in the process ATP is made; gives no indication of the enzymes involved; there is no indication that alternative substrates can be used; there is no mention of hydrogen acceptors, coenzymes etc. Any other valid points.

pages 130–131 1

a Respirometers measure carbon dioxide output and use that to calculate oxygen uptake

during respiration. Looking at the whole organism, it gives an overall picture but no detail of what is happening in individual cells, e.g. whether the carbon dioxide comes from aerobic or anaerobic respiration. Any other valid point. b The apparatus with the two tubes will give more reliable evidence because there is:

a clear scale to measure changes; syringe for recalibration; KOH on both sides to eliminate the amount of carbon dioxide in normal air from the results. Any other valid points. 2 Evidence is needed to associate the substrate molecules or enzymes involved in a

particular stage of respiration with the membranes or the contents of a particular area of a mitochondrion, e.g. blocking or poisoning of one mechanism and observing the buildup of product in a particular area of the mitochondrion.

pages 132–135 1 During vigorous exercise, muscles respire anaerobically, producing lactate which builds

up in muscles and is carried away in the blood. When exercise stops, as well as the normal oxygen demands of the body, lactate must also be oxidised to pyruvate and glucose in the liver, ATP and phosphocreatine levels need to be restored, myoglobin needs reoxygenating, and metabolic reaction rates are f aster due to raised temperature using more ATP. So the oxygen demands of the body remain high for some time after the completion of the exercise. The rate of breathing continues to be raised to supply the needs of the body and the hear t rate remains high to pump extra oxygenated blood around the body and remove the excess carbon dioxide. 2

Diagram similar to fig.7.1.6 with annotations such as the following: •

Glucose, 6C sugar, is starting point.

•

ATP used to phosphorylate glucose – 2 phosphate groups added to give phosphorylated 6C sugar, one reaction controlled by phosphofructokinase, ratecontrolling reaction for the whole process of cellular respiration.

•

Phosphorylated 6C sugar split to form 2 molecules of 3C glyceraldehyde-3-phosphate (GALP).


120


Chapter 7.1 (cont.)

•

Each molecule of GALP converted to pyruvic acid in a series of steps – for each molecule of GALP 2 hydrogen atoms removed to reduce NAD which is passed along electron transfer/transport system to produce 3 molecules of ATP, so 6 ATPs formed in total per glucose molecule.

•

The initial phosphorylation reactions are reversed before the nal intermediate is converted to pyruvate and the phosphate group released is used to produce ATP from ADP.

•

2 molecules of pyruvate enter mitochondrion and go into Krebs cycle for every glucose molecule which enters glycolysis.

•

If insufcient oxygen, pyruvate converted to lactate or ethanol.

3 During the process two hydrogen atoms are removed from the 3C sugars and taken up

by NAD to form reduced NAD which then enters the electron carrier system producing energy which is used to phosphorylate 3 molecules of ADP. ATP is also made directly when the 3C sugar is converted to pyruvate. This replaces the ATP used up to prime the 6C sugar, and the remainder is available as useful energy for cell metabolism, assuming that the substrate moves on into aerobic respiration.

pages 136–137 1 Glycolysis: doesn’t need oxygen to produce ATP; linear; relatively simple; takes place in

cytoplasm. Any other valid point. Krebs cycle: needs oxygen to produce ATP; cyclical; complex; takes place in mitochondria. Any other valid point. 2 Krebs cycle alone does not release energy, glycolysis also release energy for the cell. Krebs

cycle directly produces very little ATP for the cell – it produces reduced carrier molecules that enter the electron transport chain, which in turn releases energy that is used to drive the production of ATP. 3


valid. Krebs used enzyme inhibitors to block particular enzymes or stages in the pathway, resulting in a build-up of the reactants of that reaction and a lack of the products compared to an analysis of the normal process. This enabled Krebs and his team of scientists to work out exactly which chemicals are involved in a particular step of the process. Answer could include diagram of apparatus used.

pages 138–141 1


valid. Diagram should show all stages of aerobic respiration: glycolysis, link reaction, Krebs cycle and electron transport chain, making clear where and how many ATP molecules are used and formed, where reduced NAD and reduced FAD are formed and oxidised, and where oxygen is used and carbon dioxide formed. An indication of how many ATP may be formed for each molecule of glucose respired could be included. Clarity of layout of diagram is important. Edexcel A2 Biology Implementation and Assessment Guide for Teachers and Technicians ©Pearson Education Limited 2009

121


Chapter 7.1 (cont.)

2 The oxidation of glucose is a multi-step process during which the glucose molecule is

split into 3C units, built back into 6C molecules and then dismantled again. As hydrogen is removed, it is used to reduce carrier molecules that pass to the electron transport chain. As the components of the chain are reduced and then oxidised again sufficient energy is released to drive the production of ATP. By the end of the process glucose has been completely oxidised to carbon dioxide and water. 3 Glycolysis (at the start of aerobic and anaerobic respiration) has to expend ATP to move

reduced NAD into the mitochondria to reach the electron transport chain; one hydrogen is removed from each 3C sugar in glycolysis, while 5 hydrogen atoms are passed into the electron transport chain from each 3C pyruvate molecule that enters the Krebs cycle (aerobic respiration only); aerobic respiration involves complete oxidation of glucose while anaerobic results only in its partial breakdown. Any other valid points. 4 Energy from electrons as they pass along the electron transport chain is used to

actively transport hydrogen ions into the space between the inner and outer mitochondrial membranes. The hydrogen ions cannot pass through the inner mitochondrial membranes. So a concentration gradient for hydrogen ions is built up across the inner membrane. This also creates a pH gradient and an electrochemical gradient. Although there is a tendency for the hydrogen ions to move back into the matrix, the only way they can move back is through special pores. These pores are found on the stalked particles, and the movement of the hydrogen ions through the pores is linked to an ATPase enzyme. The energy from the gradients is used to drive the synthesis of ATP. This model is important because it provides a mechanism for the observed events in a wide variety of cells, and it shows how the removal of hydrogen atoms from glucose molecules can result in the production of ATP.


122


Chapter 7.2

pages 142–143 1 Whole muscle is made up of many muscle fibres. Different fibres might have different

thresholds for response, or different levels of response, so when the whole muscle is stimulated the contraction might be more or less than expected. 2

Striated muscle

Smooth muscle

Cardiac muscle

attached to the skeleton

not attached to skeleton

not attached to skeleton

involved in locomotion

involved in moving food through gut, controlling diameter of blood vessels etc.

pumps blood out of heart around lungs and body

controlled by voluntary nervous system

controlled by involuntary nervous system

contracts spontaneously, some voluntary and involuntary control

striated appearance under microscope

no striations seen under microscope

striations with cross connections seen under microscope

contracts rapidly

contracts slowly

average around 70 contractions a minute

fatigues rapidly

fatigues slowly

does not fatigue

pages 144–147 1 Aerobic respiration takes place in the mitochondria supplying the active muscle cells with

ATP as an energy supply. Myoglobin is a protein similar to haemoglobin, with one chain rather than four, which binds oxygen and has a much higher affinity for oxygen than haemoglobin. Myoglobin readily accepts oxygen from the blood and acts as an oxygen store in the muscles. 2 Fast twitch fibres have few mitochondria and little myoglobin: they produce a quick burst

of aerobic energy then anaerobic respiration continues: they fatigue quickly. Slow twitch fibres have many mitochondria and plenty of myoglobin; tend to have a rich blood supply bringing oxygen which is taken up and stored by the myoglobin; allow plenty of aerobic respiration over time. 3 Leg muscles contain lots of slow twitch fibres, with a good blood supply and lots of

myoglobin. Therefore these muscles are a dark colour, contain lots of mitochondria and so provide sustained activity for walking around but less speed and power in initial contraction. Breast meat contains more fast twitch fibres, with relatively little myoglobin and few blood vessels. Therefore they are a pale colour, good for short explosive bursts of activity such as a short flight.


123


Chapter 7.2 (cont.)

4

[S1C] The answer should include some or all of the following. Other points may be valid. Slow twitch muscles

Fast twitch muscles

deep red (lots of myoglobin)

pale pink/white (little myoglobin)

lots of capillaries

few capillaries

lots of myoglobin to store oxygen

little myoglobin

not much stored glycogen

large glycogen stores

little sarcoplasmic reticulum

lots of sarcoplasmic reticulum

doesn’t fatigue easily

fatigue easily

many mitochondria

relatively few mitochondria

pages 148–149 1 Diagrams as in fig. 7.2.7, fully labelled to show the H zone, I bands, A band, Z lines, actin

and myosin filaments. 2 Calcium ions released in response to nervous stimulation of the muscle set up contraction

of the sarcomeres. Calcium ions bind to troponin changing the shape of the molecule. This changes the shape of the troponin molecules, so they pull on the tropomyosin molecules to which they are attached. This moves the tropomyosin away from the myosin binding sites on the actin molecules, exposing them, so that they can bind with the myosin heads which sets up the contraction. Calcium ions also needed for the action of the ATPase enzyme in the myosin heads, which enables the heads to return to their resting position. 3

a ATP binds to the myoglobin head, and the release of energy when it is hydrolysed

allows the head to return to the resting position. The bonding of the ADP and inorganic phosphate results in changes in the shape of the myosin head so it can bind to the actin binding site. The release of the ADP and inorganic phosphate results in another shape change which results in the release of the myosin head from the actin binding site. ATP is also needed as the energy supply for the calcium pump which returns calcium ions to the sarcoplasmic reticulum, ending the contraction. b [S1C] The answer should include some or all of the following. Other points may be

valid. After death, once there is no more ATP, the myosin heads cannot return to the resting position but remain locked upright, so the muscle becomes stiff and rigid.

pages 150–151 1 Synovial fluid is produced by synovial membranes in joints with biggest range of

movement. It acts as a lubricant between the cartilage covering the ends o f the bones. Prevents wear and tear of the cartilage.


124


Chapter 7.2 (cont.)

2 •

Bones: form the framework and support; hard, strong tissue; act as levers.

•

Joints: where two bones meet; ; allow movement.

•

Ligaments: form capsules around the joints; hold bones in place so they can do useful work without becoming separated; also need to be elastic to allow the bones of the joint to move when necessary; yellow elastic tissue gives strength with elasticity; tightness of capsules varies with movement needed in jo int.

•

Muscles: provide the power to move the joints; tissue can contract and relax; are attached to bones by tendons which don’t stretch, so the contraction of the muscle is converted into movement; occur in antagonistic pairs as they can only pull, so one muscle moves a bone in one direction and when it relaxes, another muscle pulls the bone back to its original place.

Any other valid descriptions, diagrams etc.


125


Chapter 7.3

pages 152–153 1 The natural pacemaker should send impulses regularly through the conductive tissue of

the heart to the ventricles to trigger them to contract and pump blood out of the heart to the lungs and around the body. By delivering electric shock to the ventricles when the SA node fails, the artificial pacemaker triggers contraction in the ventricles and replicates the intrinsic rhythmicity of the heart beat. 2 P wave: electrical impulses travelling from SA node.

QRS complex: wave of excitation spreads from the AV node through the bundle of His and through the ventricles. T wave: recovery of contractile tissue and the ventricles.

pages 154–157 1 normal blood pressure

exercise raises blood pressure

→

detect rise in blood pressure in brain

baroreceptors send nerve impulses to cardiovascular centre

→

cardiovascular centre sends nerve impulses to heart and blood vessels

→

rate slows and blood vessels dilate 2

baroreceptors in carotid arteries

→

rest

exercise

heart

→

blood pressure falls back to normal.

→

recovery

20

/ e t t u u p n t i u m 15 o r c e a p i 10 d r 3 a m C d

5

0

3

a

10 Time/minutes

20

At rest

Vigorous exercise

16000

14000

14000

12000

12000

10000

10000

800

800

600

600 400

400

200

200

0

0

t r s i n u e y s e s i n t s l k r a r i v e a n d r a i s s i d n e u s c s r p a l b t h e l g k g l n h e l m n a a l u o t r e e t l d e a k s Blood flow at rest

t r s i n u e y s e s i n t s l k r a r i v e a n d r a i s s i d n e u s c s r p a l b t h e l g k g l n h e l m n a a l u o t r e e t l d e a k s

Blood flow during vigorous exercise heart liver adrenal glands brain lung tissue kidneys skeletal muscles skin other parts


126

heart liver adrenal glands brain lung tissue kidneys skeletal muscles skin other parts


Chapter 7.3 (cont.)

b When the body is at rest or carrying out normal activities, the skeletal muscles need

some blood to supply the oxygen needed to work against gravity and move around. However far more of the blood needs to go to the liver, which deals with the products of digestion, detoxifying urea and other processes, and the kidneys which are filtering the blood and removing excess water, salt, all urea etc. Only a relatively small amount of blood flows to the skin to maintain core body temperature. The brain has large blood flow for its size as it is responsible for the coordination of the body and thinking at all times. During exercise the needs of the body change and the blood supply is shifted. The flow to the gut, liver, kidneys is much reduced as these activities are less immediately important. Blood supply to the muscles is almost doubled to provide the food and oxygen for vigorous contraction. Blood flow to the hear t increases enormously to pump the blood to the muscles faster to supply the fo od and oxygen needed, and blood flow through lungs doubles to get more oxygen into the body. Blood flow to the skin also increases greatly to keep the body cool, while the brain gets a little more blood to deal with potential dangers, increased alertness etc. 4 Nervous control: involuntary; sympathetic nerves speed up heart rate, parasympathetic

nerves slow it down; nerves come from cardiovascular centre; impulses from these nerves affect the SAN and so change the rhythm of the heart to match the demands of the body. Cardiovascular centre in brain is affected by input from stretch receptors in the heart which are affected by volume of blood returning to the heart as exercise starts or stops; baroreceptors measure blood pressure in the arteries in the neck, which is particularly important as exercise ends; sensors that measure carbon dioxide in the blood also affect heart rate through the cardiovascular centre. Voluntary control: nerves from conscious parts of the brain can also stimulate or inhibit the SAN. Hormonal control : adrenaline stimulates the SAN, speeding up the excitation rate of the heart so it beats faster and more strongly to prepare for flight or fright. Any other valid points.

pages 158–161 1 Spirometers can be used to measure the breathing rate at rest and whilst exercising etc.

Also, the simple process of counting breaths with a stop watch can give a rough idea. 2

[S1C] The answer should include some or all of the following. Other points may be valid.

Diagram should include chemorecepors in the brain, chemoreceptors in the carotid bodies, chemoreceptors in the aortic bodies, stretch receptors in the intercostal muscles and diaphragm, stretch receptors in the bronchi lungs. all mentioned directly or by association.


127


Chapter 7.3 (cont.)

3

Carbon dioxide levels in the blood increase with exercise as more cellular respiration takes place (and pH levels which fall as carbon dioxide concentration goes up) and fall as hard breathing continues after exercise. Levels are detected by chemoreceptors in the hindbrain, the carotid bodies in the carotid arteries of the neck and the aortic bodies in the aorta. They respond to changes in levels by changing the rate of impulses sent to the respiratory centre in the brain, which in turn send impulses to the intercostals muscles and the diaphragm to change the rate and depth of breathing. Brain cortex: recognises movement has begun and stimulates the respiratory centre to cause an increase in breathing rate. Stretch receptors in muscles register movement, and in the lungs measure the degree of stretching, and respond with impulses to the respiratory centre to affect breathing rate. Voluntary nerves can be used to increase or decrease rate or depth of breathing within boundaries, beyond which the normal breathing reflex kicks in. Any other valid suggestions.

4


For: best way to collect data to help with treating breathing problems in other children; response in children is different to the response in adults, so can’t extrapolate from adults to children; children involved enjoyed the experience (and probably got a lot from it). Against: could have been very risky – not right to put children at risk unnecessarily; they weren’t benefitting directly from the research so not a good reason to do it; conditions were extreme and one child struggled with them.

pages 162–163 1 Metabolic reactions produce heat. During exercise the level of aerobic and anaerobic

respiration in the muscles increases so the amount of heat produced increases considerably. High temperatures can denature enzymes and damage the metabolic processes they control. Maintenance of relatively constant internal temperature by sweating, skin flushing and so on is vital to prevent the core body temperature from rising to high critical levels when positive feedback mechanisms respond by increasing the metabolic rate so producing even more heat and making temperature rise until it may reach lethal levels. 2


Strengths: gives direct evidence of individual human response to changing temperature; can control conditions quite closely so that results clearly indicate effect of changing conditions. Weaknesses: not particularly comfortable for perso n being studied; takes a long time to collect data on a large number of individuals in many different conditions in order to produce reliable conclusions.


128


Chapter 7.3 (cont.)

pages 164–167 1 Training programme: internal temperature will increase due to muscle activity and

cellular respiration. Negative feedback mechanisms trigger an increase in sweating so heat lost through evaporation, and increased blood flow to surface of skin means heat lost through conduction and radiation. Drinking ice cold water can cause problems because this tends to drop the blood temperature, but excess heat is still being produced by muscles and heart after exercise. Negative feedback means sweating rate lowered, vasoconstriction of blood vessels in skin to reduce heat loss by radiation and conduction. However temperature of blood would soon rise again as a result of the metabolic heat so the systems would revert to heat loss mode, but it would take longer to restore normal core body temperature. Any other valid points. 2 Thermoregulation is not as effective in elderly people as blood circulation may be poor.

34 °C is a high temperature and so the man will soon get hot both from the external temperature and the exercise. A raised external temperature makes it harder to lose body heat through conduction. He could have problems with the negative feedback system as prolonged walking means prolonged excess heat produced by muscles. Also, for the dog – exercising in heat with thick fur coat can lead to overheating as limited area for heat loss through sweat. Any other valid points. 3 Negative feedback: core temperature increases, sweating rates increase so heat lost by

evaporation and arteriole shunt opens up to allow blood to flow to the surface of the skin; heat is lost by radiation and conduction and core temperature falls back towards normal. Core temperature down, sweating reduces so less heat loss by evaporation, arteriole shunt closes so less blood flows near the surface of the skin reducing heat loss by radiation and conduction, core temperature rises towards normal again. Positive feedback: core temperature above high critical level, metabolic reactions speed up with increase in temperature so the temperature increases even more, which increases the rate of metabolic reactions further, including the release of even more heat energy.


129


Chapter 7.4

pages 168–169 1


valid. Fig. 7.4.1 shows data from the National Statistics site collected as part of a Health Survey for England. The sample size is very large (over 14 000 adults in 2003) so the evidence is reliable. The obesity data for fig. 7.4.2 came from a study by the International Obesity TaskForce in 2002, and the cycling data from the Department for Transport in 1996. These graphs summarise large-scale international and national studies, and are considered convincing evidence. The pie chart in fig.7.4.3 is based on data of all newly diagnosed cases of diabetes in the US, and again is a large sample size. The graph in fig 7.4.3 comes from National Institute of Aging in the US, with a sample size of about 1000 people, all of whom were overweight and had slightly raised bloo d glucose levels, both of which are good predictors for the development of Type 2 diabetes. 45% of the sample came from particular minority g roups known to have a higher risk of developing diabetes. The survey compared this group who increased their level of activity with others who either received drug treatments or placebos. Sample size is relatively small here, because of the breakdown by age, and the effect of ethnic origin might mean that these results aren’t as valid for other ethnic groups. 2


Correlation is when factors var y in a similar way, either directly (going up and down at the same time) or indirectly (one going up as the other goes down and vice versa). This suggests a link between the factors, e.g. something else might be affecting the factors in the same way. Causation is when a change in one factor leads to a change in another factor. A valid mechanism is needed to explain the causal link between them. Much of the evidence for a relationship between level of exercise, obesity, diabetes and heart disease is in the form of correlation, as shown by the figures on these pages. Increasingly though, scientific research is producing evidence of causal links, e.g. that moderate exercise strengthens the heart and circulatory system, so reducing the risk of heart disease. A causal link with obesity is more difficult to show because many studies ignore the impact of fitness which may be greater than simple BMI.

pages 170–171 1

a 8000 moderate swimmers: large sample; in spite of open age range, size of sample

increases validity and reliability. 2311, 35–37 year old marathon runners: big sample and clearly defined age range increases validity and reliability. 750, 40-81 runners: relatively large sample with defined, if large, age range. b The answers should include some or all of the following. Other points may be valid.

10 men running for three weeks: tiny sample, all one gender, very short time scale. 16, 60-72 year olds moderate exercise: small sample, gender not defined.


130


Chapter 7.4 (cont.)

2

a Four studies seem to confirm the hypothesis that moderate exercise benefits the

immune system shown by reducing the incidence of URTIs. Eight studies seem to suggest strongly, and one less strongly, that extreme exercise is damaging to the immune system as measured by incidence of URTIs. b [S1C] The answers should include some or all of the following. Other points may be

valid. In order to show causation, there needs to be either overwhelming evidence from many large-scale and reliable studies or there needs to be a clear causal link established from scientific research. Currently neither of these is available, although the data suggest that one day a causal link is worth looking for.

pages 172–173 1 P wave: electrical impulses travelling from SA node, contraction of the atria.

QRS complex: wave of excitation spreads from AV node through the bundle of His and through the ventricles, atrioventricular valve closes, ventricles start contracting as aortic valve opens and the ventricles contract fully and empty. T wave: repolarisation of the Purkyne tissue and recovery of the ventricles. Changes in the electrical activity of the various areas of the heart will change the way the heart muscle contracts and therefore interfere with the normal rhythm of the heart. 2 Atrial fibrillation: atria beat too fast and ineffectively; don’t fill and empty properly.

Increases the risk of clot formation and therefore a stroke. Heart doesn’t pump very effectively but people can live relatively normal lives with this condition. Ventricular fibrillation: ventricles of the heart lose their rhythm, no longer contract together, contract erratically and weakly. Little blood is pumped into the arteries which causes a rapid fall in blood pressure, starving the brain, body and heart itself of blood. Often leads to heart attack or death. Sometimes normal heart rhythm can be restored by an electric shock from a defibrillator.

pages 174–177 1

a In osteoarthritis the cartilage which normally reduces friction and cushions the

joints becomes roughened and erodes away. The bone ends thicken and the synovial membrane makes more fluid so the joint swells, and the joint capsule thickens too. The whole joint can become swollen and painful. Eventually the cartilage wears away completely and the bone ends touch. The bone then starts to wear away as well, changing the shape of the joint and causing severe pain. b [S1C] The answer should include some or all of the following. Other points may be

valid. In people over 40, osteoarthritis is usually the result of years of wear and tear on the joints and possibly the effect of earlier injuries. Being overweight increases the chance of osteoarthritis developing as it increases the strain on the joints. People under 40 occasionally suffer from osteoarthritis, particularly of the knee. This is almost always either the result of a genetic condition that affects the connective tissue, or as a delayed consequence of a tear injury in the knee. When cartilage has to be removed or repaired surgically, it is then a less effective cushion and the damage of osteoarthritis sets in. Edexcel A2 Biology Implementation and Assessment Guide for Teachers and Technicians ©Pearson Education Limited 2009

131


Chapter 7.4 (cont.)

2

a The answer should include some or all of the following. Other points may be valid. Case history 2

Case history 3

knee twisted following tackle

knee collapsed, no contact

much swelling around knee joint

knee swollen

joint tender

joint painful

sensation of something catching when knee straightened

movement very painful

medial meniscus tear

complete rupture of anterior cruciate ligament (ACL)

b Keyhole surgery: through a tiny incision using fibre-optic tube with camera and

miniaturised instruments; the damaged area is cut away so healing can occur; player should be playing again within days; no need for radical surgery. ACL: also uses keyhole surgery; uses graft of ligaments from patient or donor; needs two weeks rest to allow swelling to reduce enough to do surgery and then 9 months recuperation and physiotherapy. In either case, an ar tificial knee would not provide the same strength and flexibility. This treatment is excellent for patients with pain from osteoarthritis because they can withstand some strain, e.g. that of moderate exercise. Top class athletes can’t use them. They also have a finite useful time, whereas a well-repaired knee will last a lifetime. Any other valid points. 3


valid. The IAAF’s ruling in 2007 banned the use of any device incorporating springs, wheels or any other element that provides the user with an advantage over another athlete not using such a device. The ruling against Pistorius taking part in the 2008 Olympics was because tests showed athletes using the blades used less energy than athletes with legs to achieve the same speeds. Some issues: Should prostheses be considered just f rom what they can do biomechanically? Just because the prosthetic blades save energy, doesn’t necessarily mean that the rest of the body is saving energy, because it isn’t working in exactly the same way as someone with complete legs, for example other areas of the body may have to work harder to balance. If people using prosthetic limbs could all run faster than able-bodied athletes, where does that leave olympic competition? If someone using prosthetic limbs is able to compete on an equal basis with able-bodied runners, would that relegate the paralympics to a second-rate level of spor t, particularly for those who couldn’t afford the prostheses?


132


Chapter 7.4 (cont.)

pages 178–181 1

Some drugs can create a long-term change in the body, either increasing physical performance or by damaging the body. These are the ones that are banned completely. Other drugs have no permanent impact and are eventually excreted from the body. These are the ones that are banned only during competition.

2

a [S1C] The answer should include some or all of the following. Other points may be

valid. Genes that code for natural transcription factors, such as erythropoietin, can give an advantage in sport, and at competition level could make the difference between winning and not coming first. The pressure to win is so great that using ‘natural’ methods like this to improve performance may be very tempting. b Steroid hormones: pass through the cell membrane; once in the cell they bind with

receptor molecules and are carried into the nucleus of the cell where they modify gene expression; the hormone/receptor complex acts as a transcription factor, binding to the DNA and switching on particular genes linked to protein synthesis; this changes the RNA which is produced, which in turn affects the type and numbers of enzymes produced. (Diagram like fig. 7.4.18 in Student Book also useful.) Peptide hormones: do not enter the cell; bind to a receptor in the cell membrane; the membrane-bound complex activates a second messenger in the cell cytoplasm and triggers a protein kinase cascade which involves the activation of several different proteins until the final product enters the nucleus and acts as a transcription factor, switching on the genes linked to the synthesis of a particular protein (enzyme). (Diagram like fig. 7.4.20 in Student Book also useful.) 3

a Possible answers include: the number of students surveyed; the areas where the survey

was taken from (e.g. all cities, rural areas, different states?); who carried out the survey; who funded the survey; did the sur vey reflect the regular use of steroids. b It is inaccurate because the graph doesn’t identify if it reflects regular use, once-only

use, how drug use is prevalent in f emale students, as well as it doesn’t indicate where the samples were taken etc (as g iven in answer to part a).

pages 182–183 1 The error bar on the left of the graph indicates a range of between a 0.5% increase in

time (getting slower) and a decrease of about 2.5% in time (indicating improvement). The measured improvement in performance is 2–3%, but that is similar to the error range. So living high and training low may actually make little difference to performance. 2


valid. Absolutist: no drugs for proving performance should be allowed in sport and so maximum red cell count is not adequate as it allows for use of dr ugs. Relativist: setting a maximum level for total red blood cell count is the only fair way to make sure nobody is tempted to add to their natural count to a point where they might damage their bodies using either high-altitude training or er ythropoietin.


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1 of 1 Technician

Practical 7.1 Measuring the rate of oxygen uptake

Purpose •

To demonstrate the uptake of oxygen in respiration.

•

To measure the rate at which an organism respires.

Requirements per student or group of students

Notes

Respirometer

See the diagram on the Student sheet. If a pipette is used, the scale shown will not be needed. Ideally the syringes are attached with a three-way tap. If these are not available a rubber tube and clip can be used. U-tube respirometers would be even better, if there is a class set available. See also section 15.10 on respirometers in the CLEAPS Laboratory Handbook for details of other (bulk) suppliers.

5 g of an actively respiring organism

Use actively germinating peas, beans or other seeds, or maggots, or woodlice.

Roughly a tablespoon of soda lime

To absorb the carbon dioxide. Safety

Soda lime is corrosive, but much less of a hazard than solutions of potassium or sodium hydroxide. Even so, eye protection is needed when handling the soda lime. Do not handle directly: use a spatula. Soda lime can sometimes be ’dusty’. To avoid exposing invertebrates to corrosive dust, take the soda lime outside and pour from beaker to beaker to blow the dust away. Position yourself so that you cannot inhale the dust during this activity.

About 2 cm3 of coloured liquid

e.g. water and food colour or equivalent.

Dropping pipette

Or 1 cm3 syringe plus hypodermic needle (in cover until in use) if appropriate to students.

Permanent OHT marker, or chinagraph pencils

For marking the position of the coloured liquid.

Solvent to remove the marker Small amount of cotton wool to wipe pipette Stopclock Eye protection

Respirometers A respirometer is shown on the Student sheet. Many schools and colleges have at least one of the U-tube respirometers (Figure 7.1.4 on page 131 of the A2 textbook). These can be used but can be a lot more fiddly, and the connections often leak. If the apparatus works, the respiring organisms use up the oxygen and give off CO2. The CO2 is absorbed by the soda lime. This means there is less air in the test tube so the liquid gets sucked towards the tube with the organisms in it. If it does not work there is usually an air leak somewhere, or possibly the organisms are too cold, or dead!


154

Technician 1 of 2

Practical 7.2 Investigating breathing

Purpose •

To investigate lung volumes and rate of breathing.

Safety The safety guidelines in Sections 14.5 and 14.5 .1 in the CLEAPSS Laboratory Handbook should be followed when using a spirometer. This investigation is potentially hazardous. Students should be closely supervised at all times when using a spirometer. If the students are allowed to breathe through the spirometer for too long they can lose consciousness from lack of oxygen. Limiting the time spent breathing through the spirometer and carefully observing each student should prevent problems. When using oxygen and absorbing CO 2: maximum time 5 minutes; in any other situation: maximum time 1 minute. If a student becomes less alert or has any feeling of suffocation they should stop immediately. Since the levels of CO 2 are kept low by the soda lime, the students won’t be aware that they are running out of oxygen until it is potentially too late. A trained member of staff should use an oxygen cylinder to fill the spirometer. Use eye protection when handling soda lime. Soda lime is corrosive. Do not handle directly: use a spatula. Use a type of soda lime that changes colour when it is saturated, and replace it when it changes colour. Use 5–10 mesh particle size. Follow CLEAPSS guidelines on removing dust for spirometer use. A layer of polymer wool can be put at the inflow and outflow of the soda lime canister chamber to prevent dust getting into the chamber. Ensure that the soda lime canister is fitted so that air is breathed out through the canister. The subject will feel some resistance to breathing when using a spirometer. Because of this they should not use the spirometer while exercising. To investigate the effect of exercise, readings should be taken immediately after exercise. If resistance to breathing suddenly increases it may be due to valves in the spirometer sticking . If this occurs the valves may need to be replaced. Do not use oil, grease or glycerine on any part of the spirometer tubing as these may make explosive compounds with oxygen. Water with a little liquid detergent can be used to aid connection of tubes, etc. Keep all flames away from the spirometer. It is essential to follow good hygiene practice with regard to cleaning and disinfecting the mouthpiece and removing condensation and saliva from the tubes. Care should also be taken to choose the subj ect, avoiding any students with asthma or other breathing or circulatory problems. (Asthmatics may use a spirometer if they are otherwise in good health.) Read the manufacturer’s instructions carefully before u sing a spirometer.

In this activity students learn how a spirometer works and how to interpret the spirometer trace that is produced. Alternatively a datalogger can be used with a traditional water-filled spirometer or with an airflow spirometer. A range of different types of sensors can be used and some alternatives are described on the Teacher sheet. Edexcel practical materials created by Salters-Nuffield Advanced Biology, ©University of York Science Education Group.

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2 of 2 Technician


Investigating breathing

A spirometer can be used to investigate the effect of exercise on breathing rate and lung volumes. If you have not got access to a spirometer, measurement of vital capacity and tidal volume can be carried out using breath volume bags. These are a fraction of the cost of a spirometer. However, they cannot be used for measuring oxygen consumption.


Notes

Spirometer

If the spirometer has a pen it is worth checking that it is still working – they fail quite quickly. Always keep the ink reservoir with its piece of wire in place. Some new spirometers can be set up to feed the data into a computer. See notes on Teacher/ Lecturer sheet about using spirometers with dataloggers. Safety See notes on page 1.

Kymograph

See CLEAPSS Laboratory Handbook Section 15.1 for advice on speeds, etc. A chart recorder, computer or datalogger can be used in place of a kymograph.

Kymograph paper

Be aware that it matters which way up you attach the paper. If it is the right way up the pen moves smoothly over the join in the paper. If it is the wrong way up it catches on the join.

Soda lime for the spirometer canister

Disinfectant solution, ethanol or fresh 0.1% hypochlorite followed by rinsing with water

Safety See notes on page 1.

For rinsing mouthpiece and breathing tubes after use. To minimise an unpleasant taste on the mouthpiece, use Milton made up at the manufacturer’s dose. However, this takes 30 minutes to achieve disinfection.

Eye protection


156



Purpose • •

Safety

To demonstrate the uptake of oxygen in

Wear eye protection when handling soda lime.

respiration.

Soda lime is corrosive. Do not handle directly:

To measure the rate at which an organism

use a spatula.

respires.

Notes on the procedure Rate of respiration is not a learning outcome in the specification. The respirometer shown in the diagram is a very simple one; more complex ones (e.g. U-tubes) can be used if available. Question 1 can be used as a summary activity for student understanding of respiration if students are asked to complete it in as much detail as possible and to state the source of carbon dioxide as well as its fate. The choice of what respiring organisms to put in the tubes is left to you. Germinating peas, maggots or woodlice are commonly used. If you are using pipettes there is no need to do any volume calculations; students just read the change in volume off the scale on the pipettes. If you are using thick-walled capillary tubing it is worth reminding students of the formula for working out the volume of oxygen used, and explaining it again. volume of oxygen used

5

2

p r

3 distance

moved

where r 5 the radius of the capillary tubing or pipette. Three-way taps can cause confusion with some students. A diagram of how the three-way tap works is shown in the diagram below. Putting up an OHT of this diagram during the practical can help. 1

2

respirometer open to syringe

3

respirometer open to atmosphere

Tap positions for a three-way tap (viewed from the side)


16

respirometer isolated



Measuring the rate of oxygen uptake

Answers 1

Simple answer: Oxygen molecules are absorbed by the organism and used in respiration. The same number of carbon dioxide molecules are released but these are absorbed by the soda lime. This reduces the pressure inside the test tube (fewer molecules

5 lower

pressure). Atmospheric

pressure pushes the liquid along the tube, until the pressure in and outside the tube is equal. Detailed respiration review answer: As above but should include reference to the role of oxygen as the final electron acceptor, and the fact that it eventually combines with hydrogen to make water. The carbon dioxide comes from the carbon dioxide released in the link reaction and the Krebs cycle as the carbohydrate is broken down. 2

a b c

A drop in temperature inside the tube, or an increase in atmospheric pressure. An increase in temperature inside the tube, or a decrease in atmospheric pressure. The distance moved by the liquid in the control in each minute towards the organisms should be subtracted from the distance that the liquid in the experimental respirometer moved. Movement of the liquid in the control away from the organisms should be added to the distance in the experimental tube.

3

Diagram showing a very simple respirometer: Disadvantages – does not allow you to reset; it needs a control tube used alongside it; no scale so measurements likely to be less accurate. Advantages – very simple to set up; minimal number of connections makes a good seal easier to obtain. 1 cm3 syringe experimental tube screw clip three-way tap

small organisms

gauze KOH solution absorbs carbon dioxide

KOH solution manometer tube containing fluid

U-tube respirometer

Advantages – does not need to have an additional control as the second tube balances out the effects of changes in temperature or atmospheric pressure; the syringe allows you to move the liquid in the U to reset the apparatus. Disadvantages – tendency for the connections to leak in elderly school/college models (making the equipment useless); expense. 4

The experimental design should include either a U-tube respirometer or controls; a known mass of maggots; a range of temperatures (between 5 °C and 40 °C); a suitable increment between temperatures (say, 5 °C); repeated measurements (say, at least three at each temperature). Edexcel practical materials created by Salters-Nuffield Advanced Biology, ©University of York Science Education Group.

17



Purpose •


Spirometers A copy of a spirometer trace is provided at the end of these notes, for use when there is no access to a spirometer or as extra data to interpret. The practical describes the use of a spirometer with a kymograph. Alternatively a datalogger can be used with a traditional water-filled spirometer or with an airflow spirometer. A range of different types of sensors can be used and some alternatives are described below using a spirometer with a datalogger. If you have not got access to a spirometer, measurement of vital capacity and tidal volume can be carried out using breath volume bags. These are a fraction of the cost of a spirometer. However, they cannot be used for measuring oxygen consumption.

Safety Use eye protection when handling soda lime. Soda l ime is corrosive. Do not handle directly: use a spatula. A spirometer should only be used with supervision. Students with breathing or circulation (heart) problems or suffer from epilepsy should not use the spirometer. Read the manufacturer’s instructions and safety notes before using the equipment. CLEAPSS guidance is in section 14.5.1 of the Laboratory handbook. If the students are allowed to breathe through the spirometer for too long they can lose consciousness from lack of oxygen. Limiting the time spent breathing through the spirometer and carefully observing each student should prevent problems. When using oxygen and absorbing CO2: maximum time 5 minutes; in any other situation: maximum time 1 minute. If a student becomes less alert or has any feeling of suffocation they should stop immediately. Since the levels of CO 2 are kept low by the soda lime, the students won’t be aware that they are running out of oxygen until it is potentially too late. Stop using the spirometer at once if the student experiences any unusual breathing problems or feels dizzy or uncomfortable. (Asthmatics may use a spirometer if they are otherwise in good health.) A trained member of staff should use an oxygen cylinder to fill the spirometer. If only a few breaths are to be measured, then atmospheric air is acceptable instead. The subject will feel some resistance to breathing when using a spirometer. Because of this they should not use the spirometer while exercising. To investigate the effect of exercise, readings should be taken immediately after exercise. If resistance to breathing suddenly increases it may be due to valves in the spirometer sticking. If this occurs the valves may need to be replaced. After use, the tubing from the spirometer should be removed and placed in 70% ethanol, to disinfect the internal ribbed surfaces where microorganisms might remain.

Notes on the procedure Effects of exercise on breathing rate and volumes can be investigated using the spirometer. Although this is not detailed on the activity sheet, students do need to be able to describe how to investigate the effects of exercise on tidal volume and breathing rate. Edexcel practical materials created by Salters-Nuffield Advanced Biology, ©University of York Science Education Group.

18




Using a spirometer with a datalogger One advantage of collecting the data electronically is that the data can be moved from the datalogging software to a spreadsheet or a word processing program. The data collected can then be made available to students involved in the investigation. Sensors such as motion sensors can be used to record the up and down motion of the airfilled spirometer lid. A motion sensor can be mounted above the large top surface of the moving lid (see below). As the lid moves up and down the change in distance can be recorded. Make sure the limits of the motion sensor are known and the minimum recordable distance is available when the lid is at its closest to the sensor. If known series of gas volumes is introduced into the spirometer, and the distance between the motion sensor and the surface of the lid is recorded, it is possible to produce a calibration curve of distance versus volume. Most datalogging software can convert distance measurements recorded by the sensor to volume data. In a similar way, a linear motion sensor (e.g. rotary motion or pendulum sensor) can be attached to the spirometer as shown below. With a pendulum sensor, a string weighted with a small counterweight can be r un from the end of the lid over the sensor’s pulley wheel. When the lid moves up and down, the string moves over the pulley. The sensor records a changing distance or angle. Calibration relating the volume of gas in the spirometer tank to the sensor’s reading will be required. rotary motion/position sensor A

B

motion sensor

pully counterweight to tension cord

cord distance varies moving lid

moving lid

water

Carbon dioxide and oxygen levels in the spirometer lid can be datalogged. Monitoring these levels gives another safety check that the CO 2 absorber is working and that O 2 levels are not falling dangerously low. Alternatively, many of the datalogging companies now produce an airflow spirometer which measures volume by integrating the flow data over time. These devices are very lightweight, do not need carbon dioxide absorbers and do not need cylinders of medical oxygen (they use the oxygen from the atmosphere). Edexcel practical materials created by Salters-Nuffield Advanced Biology, ©University of York Science Education Group.

19




Answers 1

The values here are for the trace provided at the end of these notes. Obviously if a student’s own trace is used the answers may be different. a

Tidal volume – between 0.5 and 0.8 dm 3. Encourage students to take an average over several breaths.

b

Vital capacity

c

Breathing rate

5

2.55 dm3 averaged over the two breaths. between 18 and 20 breaths per minute (depending on the section of

5

the graph used). d 2

Minute ventilation

19 breaths per minute × 0.65 dm3

5

12 dm3 min –1.

5

The rate of oxygen consumption determined from the trace at the end of these notes is approximately 1.0 dm 3 min –1 for the first 30 seconds.

3

If the subject had been exercising, the rate of oxygen consumption would be higher, so the slope would be steeper. The trace would also show that the subject was breathing faster and more deeply.

4

This question allows students to think through the practicalities of using a spirometer in an investigation. They should think about the type of exercise: it is easier to use a spirometer immediately after running up and down stairs than after swimming or paragliding. They may also consider individual variation and sample sizes. They should also think about the limits imposed by the amount of oxygen available in the tank. Comments on the effect of the spirometer on the subject are also worth encouraging – a spirometer often causes breathing to become slightly more laboured particularly with older models that have smaller diameter tubing. The graph should show that breaths become deeper and more frequent after exercise, with a greater rate of oxygen consumption. Before exercise

After exercise

3

m d / 2 0 e m u l o V

Time/minutes


20




1 cm

Spirometer trace for spirometer used with soda lime and filled to 9 dm 3 with oxygen

9

8 ) 3

m d ( 2 0 e m u l o V

7

6

5 Kymograph speed: 64 mm min�1

Sample reference spirometer trace


21

Student 1 of 2


Purpose • •

Safety

To demonstrate the uptake of oxygen in respiration. To measure the rate at which an organism respires.

Wear eye protection when handling soda lime. Soda lime is corrosive. Do not handle directly: use a spatula.

Respirometers Respirometers range from relatively simple pieces of equipment used in school science labs with seeds or invertebrates, to elaborate devices the size of a room used to measure respiration rates in humans living near-normal lives over a period of several days. In this practical you will be using a very simple respirometer, while considering the advantages of some of the slightly more complex ones.

Procedure You will need: • Respirometer (see diagram below) • 5 g of an actively respiring organism

1

• • • •

Soda lime Coloured liquid Dropping pipette Permanent OHT marker pen

• • • •

Solvent (to remove the marker) Cotton wool Stopclock Eye protection

Assemble the apparatus as shown in the diagram below.

syringe

scale

three-way tap glass tubing

1 cm3 pipette or glass tube

coloured liquid

small organisms gauze soda lime A simple respirometer 2

Place 5 g of maggots or peas into the test tube and replace the bung.


37

2 of 2 Student


3

Measuring the rate of oxygen uptake

Introduce a drop of marker fluid into the pipette or glass tube using a dropping pipette. Open the connection (three-way tap) to the syringe and move the fluid to a convenient place on the pipette (i.e. towards the end of the scale that is fur thest from the test tube).

4

Mark the starting position of the fluid on the pipette tube with a permanent OHT pen.

5

Isolate the respirometer by closing the connection to the syringe and the atmosphere and immediately start the stopclock. Mark the position of the fluid on the pipette at 1 minute intervals for 5 minutes.

6

At the end of 5 minutes open the connection to the outside air.

7

Measure the distance travelled by the liquid during each minute (the distance from one mark to the next on your pipette). If your tube does not have volumes marked onto it you will need to convert the distance moved into volume of oxygen used. (Remember the volume used

5

r 2

p

3 distance

moved, where r 5 the radius of the hole in the pipette.) 9 10

Record your results in a suitable table. Calculate the mean rate of oxygen uptake during the 5 minutes.

Questions 1

Why did the liquid move? Explain in detail what happens to the oxygen molecules, the carbon dioxide molecules and the pressure in the tube.

2

It would have been better to set up a second, control tube that did not contain living organisms but had everything else the same. a b

What could cause a movement of the liquid in the control tube towards the respirometer? What could cause a movement of the liquid in the control tube away from the respirometer?

c

What could you do to correct your estimate of oxygen uptake if the liquid in the control had moved too?

Extension 3

The diagram below and Figure 7.24 on page 148 of A2 Biology show two other types of respirometer. What advantages and disadvantages do these have compared to the one you are using? soda lime

drop of liquid

wire mesh

organism to be studied

capillary tube

A very simple respirometer 4

Design an experiment to investigate the effect of different temperatures on the rate of oxygen uptake in maggots. Remember that the maggots will need time to acclimatise to each new temperature.


38

Student 1 of 4


Purpose •

Safety


Use eye protection when handling soda l ime. Soda lime is corrosive. Do not handle directly: use a spatula. A spirometer should only be used with su pervision. If you have breathing or circulation (heart) problems or suffer from epilepsy you should not use the spirometer. Read the manufacturer’s instructions and safety notes before using the equipment. Stop using the spirometer at once if you experience any unusual breathing problems or feel dizzy or uncomfortable. (Asthmatics may use a spirometer if they are otherwise in good health.) A trained member of staff should use an oxygen cylinder to fill the spirometer.

Using a spirometer The apparatus shown below is a spirometer. Spirometers allow us to study both breathing and respiration. In this activity you will learn how a spirometer works and how to interpret the spirometer trace that is produced.

A spirometer

The general principle behind a spirometer is simple. It is effectively a tank of water with an air-filled chamber suspended in the water. It is set up so that adding air to the chamber makes the lid of the chamber rise in the water, and removing air makes it fall. Movements of the chamber are recorded using either a kymograph (pen writing on a rotating drum), a chart recorder, computer or datalogger.


39

2 of 4 Student



Tubes run from the chamber to a mouthpiece and back again. Breathing in and out through the tubes makes the lid of the chamber fall and rise. The volume of air the person inhales and exhales can be calculated from the distance the lid moves. The apparatus can be calibrated so that the movement of the lid corresponds to a given volume. A canister containing soda lime is inserted between the mouthpiece and the floating chamber. This absorbs the CO 2 that the subject exhales. In which direction will the pen move when the subject inhales?

Procedure You will need: • Spirometer • Kymogr Kymograph, aph, chart recorder or computer • Soda lime (for the spirometer canister)

• •

Disinfectant solution Eye protection

Calibration In order to interpret the spirometer trace you need to know what both the vertical and the horizontal scales represent.

Finding the vertical scale The vertical scale measures the volume of air in the chamber. The spirometer’s floatingchamber lid has markings on it showing how much gas it contains. 1

First, empty empty the chamber completely completely and, if using a kymograph, make a mark on the paper while it is stationary, to show where the pen lies when there is no gas in the tank. Then force a known volume of air into the tank (e.g. 500 cm 3) and make a second mark on the kymograph trace.

2

Repeat this procedure until the the chamber has been been completely filled with with air. If using a kymograph, if the trace is too large or too small, the length of the arm supporting the pen can be adjusted so that the trace fits onto the paper.

3

Write the values next to your calibrating calibrating marks – they will help with with interpretation of the trace later. Once Once the marks have been made on the paper it should be possible to count how many squares on the trace represent 1 dm 3.

Finding the horizontal scale 4

On most kymographs there there is a switch allowing allowing you to set set the speed at which which the drum turns. Choose a speed close to 1 mm per second. This is your horizontal hor izontal scale. Make a note of the speed on your trace, so that the trace can be interpreted once the experiment is complete.

Collecting data on breathing 5

After calibration, calibration, the spirometer is filled with oxygen. A disinfected mouthpiece mouthpiece is attached to the tube, with the tap positioned so that the mouthpiece is connected to the outside air. The subject to be tested puts a nose clip on, places the mouthpiece in their mouth and breathes the outside air until they are comfortable with breathing through the tube.


40

Student 3 of 4


6

Investigating Investiga ting breathing

Switch on the recording recording apparatus apparatus and at the the end of an exhaled exhaled breath turn the the tap so that the mouthpiece is connected to the spirometer chamber. The trace will move down as the person breathes in. After breathing normally the subject should take as deep a breath as possible and then exhale as much air as possible before returning to normal breathing.

3

m d / 2 0 e m u l o V

tidal volume

vital capacity

Time/minutes A sketch of a trace trace showing normal breathing breathing and one forced breath breath in and out

A diagram of a spirometer trace is shown above. In this example the subject has breathed in and out normally three times, then taken as deep a breath in as possible, then forced the air from their lungs. Several pieces of information about the subject’s breathing can be read off this kind of trace, or worked out from it. •

The tidal volume is the volume volume of air breathed in and out in one breath at rest. The tidal volume for most adults is only about 0.5 dm3.

•

Vital capacity capacity is the maximum volume volume of air air that can can be breathed breathed in or or out of of the lungs in one forced breath.

•

Breathing rate is the the number number of of breaths breaths taken taken per per minute. minute.

•

Minute ventilation ventilation is is the volume volume of air breathed into into (and out of) the lungs in one one minute. Minute ventilation

5 tidal

volume

3 rate

of breathing (measured in number o f breaths

per minute). Some air (about 1 dm3) always remains in the lungs as residual air and cannot be breathed out. Residual air prevents the walls of the bronchioles and alveoli from sticking together. Any Any air breathed in mixes with this residual air.

Collecting data on rate of respiration Each time we take a breath, some oxygen is absorbed from the air in the lungs into our blood. An equal volume of carbon dioxide is released back into the lungs from the blood. When we use the spirometer, each returning breath passes through soda lime, which absorbs the carbon dioxide so, with the canister in place, less gas is breathed back into the spirometer Edexcel practical materials created by Salters-Nuffield Advanced Biology, ©University of York Science Education Group.

41

4 of 4 Student



chamber than was breathed in. If we breathe into and out of the spirometer for (say) 1 minute, a steady fall in the spirometer trace can be seen. The gradient of the fall is a measure of the rate of oxygen absorption by the blood, and so is a measure of the rate of respiration by the body. 1

Using the trace produced in class, or one provided by your teacher/lecturer, find the following values:

2

a

tidal volume

b

vital capacity

c

breathing rate

d

minute ventilation.

Use the trace produced in class, or one provided by your teacher/lecturer, to work out the rate of oxygen consumption in someone at rest.

3

What differences would you expect if the subject had been exercising before a trace was taken?

4

Describe how you could use the apparatus apparatus to measure changes changes in breathing breathing and respiration rates due to exercise. (Note that the apparatus you have used may not be suitable for use during exercise, and and that measurements need to be taken immediately after exercise has stopped. Discuss with your teacher which is the best method for use with your apparatus apparatus.) .) State what exercise would be appropriate, and any hazards involved. involv ed. Sketch the shape of the trace you would expect before and after exercise.


42


Unit 5 Topic 7

1

The diagram below shows part of a myofibril in a relaxed muscle. A

a

Name the parts labelled

B

(2)

A and B.

A B b

c

In the space below, make a drawing to show this part of the myofibril when it is fully contracted.

(3)

Describe the role of calcium ions in muscle contraction.

(2)

[Total 7 marks] 2

Diagram A and diagram B below show recordings of the breathing patterns of a person. In diagram A the person is at rest. In diagram B the person has just finished a period of strenuous exercise. Diagram A: at rest

3

m d / r i a f o e m u l o V

0.5 dm3

0

10

20

30

40

Time/s


57



Diagram B: after exercise

3

m d / r i a f o e m u l o V

0.5 dm3

0

10

20

30

40

Time/s

a

Calculate the mean tidal volume between 10 and 30 seconds when the person is at rest. Show your working.

Answer _________________ b

(2)

The ventilation rate of a person can be calculated by multiplying the rate of breathing by the depth of breathing. Use diagram B to calculate the ventilation rate of the person during the first 10 seconds after exercise. Show your working.

Answer _________________ c

Using the data in the recordings, compare the breathing patterns of this person before and after exercise.


58

(2) (3)



d

This person then undertook a physical training programme. (i)

Describe and explain

two differences

that you would expect to see in the

breathing patterns of this person following exercise as a result of the training

(4)

programme.

(ii) Suggest how the training programme might affect the cardiac output of the

(2)

person.

[Total 13 marks] 3

During investigations into the control of body temperature, the internal (core) body temperature of a thin human volunteer was recorded at intervals. In the first investigation, the volunteer sat still for 30 minutes. He then got into water at 16 °C, where he lay still for a further 30 minutes. In the second investigation, the body temperature of the same volunteer was recorded at intervals for 30 minutes while he was sitting still. He then went swimming in water at 16 °C for 30 minutes and his body temperature was again recorded while he was swimming, and immediately after he stopped. The results are shown in the table below.

Time/minutes

First investigation Body temperature/°C

Second investigation Body temperature/°C

0

37.5

37.4

15

37.6

37.5

30

37.5 [got into water]

36.9 [started swimming]

40

37.3 [lying in water]

36.0 [swimming]

50


35.2 [swimming]

60


34.2 [stopped swimming]


59



a

b

(1)

State where in the body the temperature-regulating centre is situated.

Explain why the volunteer’s body temperature decreased when he was lying in

(2)

water at 16 °C.

c

(i)

Compare the changes in body temperature when the volunteer was lying in

(3)

water with the changes when he was swimming.

(ii) Suggest an explanation for the difference in the change in body temperature

(2)

when the person was swimming, compared to lying still in water.

[Total 8 marks] 4

Isolated mitochondria in a solution containing inorganic phosphate and an electron donor can be used to study respiration. An electrode is used to record changes in oxygen concentration while mitochondria respire. The graph below shows changes in oxygen concentration for some isolated mitochondria. ADP added

f ) s o t n i o n i t u a r y r t a r n t e i c b n r a o c ( n n o e i g t u y l x o O s

Time/milliseconds


60



a

(i)

(3)

Describe and explain the trends shown on the graph above.

(3) (ii) Name an electron donor used in the electron transport chain in

(1)

mitochondria.

b

(iii) State the location of the electron transport chain in mitochondria.

(1)

(iv) Describe how ATP is synthesised in the electron transport chain.

(3)

ATP is used to provide an immediate supply of energy for biological processes. Describe the role of ATP in the following processes. (i)

Nerve impulse transmission

(ii) Hyperpolarisation of rod cells in the retina

(2)

(2)

[Total 12 marks] [Total for test 40 marks] Edexcel A2 Biology Implementation and Assessment Guide for Teachers and Technicians ©Pearson Education Limited 2009

61

Examzone: Reading assessment 1 of 3

Unit 5 Pre-released reading assessment questions

Unit 5 Exercise and Coordination includes question 8 which is based around a pre-released scientific article that you will have studied during the A2 course. As part of this question you may be asked to explain or comment upon the biology and other issues within the context of the article which may draw upon knowledge and understanding from Units 1, 2, 4 or 5 respectively that you have studied during the AS and A2 course. This part of the assessment accounts for 30 marks which is a third of the marks for this unit. The article provided is different each year. Use the following link to the Edexcel website http://www.edexcel.com/quals/gce/gce08/biology which will take you to the page where you can download the sample pre-released scientific article for Unit 5 from the Sample Assessment Materials section. Sample questions to go with this article are provided below. Note; the number of lines for students to write their answer is not always a true reflection of the space provided on the real examination papers or the maximum number of marks that may be awarded. You may wish to provide students with additional paper to complete their answer. 8

The scientific document you have studied is adapted from articles on disease and epidemics in New Scientist , Biological Sciences Review and the website of AVERT, an international HIV and AIDS charity. Use the information from the document and your own knowledge to answer the following questions. a Describe, using specific examples, evidence that the Black Death was caused

by a virus.

(3)

b Suggest reasons why it is likely that a vaccine for bird flu can be produced fairly easily,

whereas no effective vaccine for malaria has yet been produced.

(2)

c Explain how small samples of DNA from a burial site can be amplified and how such

samples might be used to find the identity of an unknown virus.

(4)


77

2 of 3 Examzone: Reading assessment

Unit 5 Pre-released reading assessment questions (cont.)

d Describe the risks of using genetically modified organisms.

(2)

e A hybrid virus with a mixture of genes from the H5N1 flu virus and the human flu virus

could be produced in cells infected with both. Explain how a hybrid virus could be (i)

(4)

particularly dangerous to humans

(ii) useful in producing a vaccine.

f Explain what is meant by a ‘breathtaking selection pressure’, and how this might have

led to very high frequency of the mutant form of CCR5.

(3)

g The South African government decided not to allow the use of ARV drugs for the

treatment of HIV infected people. Suggest possible reasons for their decision.


78

(5)


Unit 5 Pre-released reading assessment questions (cont.)

h Use information from the two studies of HIV infection in South Africa to describe the

current pattern of infection.You should include reference to changes in infection rates between 2000 and 2005 and the effect of gender. Suggest reasons for the trends you observe, including reasons for the different findings of the two studies for infection of women with HIV in 2005.

(7)

[Total 30 marks]


79


Unit 5 Topic 7

1

a sprinting requires rapid {release of energy / production of ATP} / eq;

slow fibres {generate ATP / respire} aerobically or fast twitch fibres can (produce ATP) {respire anaerobically / by glycolysis};

(2)

slow twitch fibres have more {mitochondria / myoglobin} / converse; b (i)

as distance of event increases, SDH (activity) increases / eq / converse; the longer the distance the more {energy / ATP} produced aerobically / eq / converse;

(2)

reference to SDH being involved in aerobic respiration; (ii) LDH (activity) is greater over shorter distances / eq; the shorter the distance the more {energy / ATP} produced anaerobically;

(2)

reference to LDH being involved in anaerobic respiration; c 1 correct reference to reduced {coenzymes / NADH2 / FADH2};

2 hydrogen splits into electrons and {protons / H1}; 3 electrons passed from one carrier to next; 4 from higher energy levels to lower energy levels; 5 {redox / oxidation} reactions release energy (for synthesis of ATP); 6 reference to more ATP being produced / eq;

(4)

7 oxygen as terminal electron acceptor / water is formed; d most of the lactate is converted back into pyruvate;

and is (oxidised) to carbon dioxide and water (via the Krebs cycle); releasing energy to synthesise ATP; some lactate may also be converted into glycogen (and stored in the muscle or liver);

(2)

reference to oxygen debt;

[Total 12 marks] [S1C] 2 a Award one mark for each of the following points in context to a maximum of four marks.

1 wave of excitation sent from SAN; 2 spreads over atria walls; 3 delayed at atrioventricular septum; 4 wave of excitation sent from AVN; 5 passes along bundles of His / Purkyne fibres; 6 spreading over walls of ventricles;

(4)

b duration of 1 beat or 60 / duration;

correct answer, 50 beats min 21 / bpm;

(2)

c Award one mark for each of the following points in context to a maximum of two marks.

1 interval between QRS phases longer (in normal rhythm); 2 credit correctly manipulated figures; 3 lower voltage during QRS phase in normal rhythm;

(2)


89



d Award one mark for each of the following points in context to a maximum of three marks.

1 cardiac output could decrease if there was insufficient time to fill the ventricles (between contractions); 2 cardiac output could increase if ventricles fill sufficiently; 3 the change in cardiac output will depend on whether the decrease in stroke volume

(3)

is compensated by the increase in heart rate / eq;

[Total 11 marks] [S1C] 3

a (i)

(1)

0.96%;

(ii) Any two from: 1 time of administering drug prior to exercise; 2 dose of drug; 3 body mass / age / sex; 4 {food / fluid} intake / smoking / eq; 5 rest period between trials; 6 correct reference to swimming conditions; 7 same three swimmers;

(2)

8 ability / experience (of swimming); (iii) reference to control / allows the effect of the drug alone to be measured /

b (i)

reference to psychological effect of placebo / eq;

(1)

{vasoconstriction / constriction} of arterioles;

(1)

(ii) less heat lost by convection / radiation / conduction;

(1)

(iii) An explanation to include three from: 1 hypothalamus detects increase in (core) temperature; 2 sweating cools body by evaporation; 3 hairs lie flat, therefore less insulation / more heat lost / eq; 4 correct reference to negative feedback; 5 correct explanation of behavioural mechanism;

(3)

c Against: reference to damaging effect on synapses / leads to abnormal behaviour or

eq / unfair advantage / correct reference to long-term effects on body; or For: individual choice / responsibility;


4

a (i)

X – CO2;

(1)

(ii) A – glycolysis B – Krebs cycle / TCA cycle


90

(1)



b An explanation to include four from:

1 reference to cristae / inner membrane of mitochondria / intermembrane space; 2 correct reference to {coenzyrnes / NADH2 / FADH2} / eq; 3 hydrogen splits into electrons and {protons / H1} 4

electrons / H (atoms) pass along series of carriers / down ETC;

5

from higher energy levels to lower energy levels / redox / oxidation;

6

(reactions) release energy (for synthesis of ATP) / correct reference to chemiosmotic theory;

7

(4)

oxygen acts as terminal electron acceptor / water is formed;

c {broken down / hydrolysed} into ADP

1 Pi

/ ATP → ADP 1 Pi (1 energy)

(1)


a (only a small cut) because damage is less / less bleeding / less pain;

recovery is rapid / shorter stay in hospital / eq;

(2)

less risk of infection / inflammation; b reference to pathogens / disease-causing organisms;

through {travel / team sports / idea of runners meeting from other areas} / eq; weakened / suppressed immunity (with hard exercise); through fall in natural killer cells / phagocytes / lymphocytes / T helper cells / B and T cells; reference to airborne / droplet infection;

(3) [Total 5 marks]

6 a spontaneous / automatic (contraction) / self-stimulating;

no {nerve impulse / eq} required / no ner ve stimulation; b (i)

(2)

A SAN / sinoatrial node / pacemaker; B AVN / atrioventricular node; C bundle of His / Purkyne fibres / Purkinje tissue;

(3)

(ii) 1 {SAN / A / pacemaker} initiates impulse / eq; 2 {impulse / eq} to {AVN / B} then delay; 3 so atrial {systole / contraction} before ventricular systole / ventricles fill with blood before systole; 4 {bundle of His / C} conducts impulse to (base of) ventricles; 5 which contract from {apex / eq} upwards;

(3) [Total 8 marks]


91

1 of 2 Examzone: Reading assessment

Unit 5 Pre-released reading assessment answers

8

a Award one mark for each of the following points in context to a maximum of three

marks. 1. Increased frequency of mutant CCR5 allele; 2. Pattern of disease similar to flu; 3. Symptoms eg bleeding consistent with known effect of virus; 4. Incubation period matches known viruses; 5. Contagion matches known viruses;

(3)

6. Other valid point. b Award one mark for each of the following points in context to a maximum of two

marks. 1. Methods for flu vaccine production already exist; 2. Harmless version of virus can be produced; 3. Can’t use whole

Plasmodium since

too complex;

4. Antigens continually changing / mutations; 5. Several stages to life cycle.

(2)

c Award one mark for each of the following points in context to a maximum of four

marks. 1. Polymerase chain reaction / PCR; 2. Replication of DNA; 3. Many copies of same sequence of bases / nucleotides; 4. DNA / base sequence of unknown matched to sequence of known virus; 5. Ref. to gene probe.

(4)

d Award one mark for each of the following points in context to a maximum of two

marks. 1. unforeseen consequences; 2. danger of transmission of genetic material; 3. release of resistance markers/eq. e (i)

(2)

Award one mark for each of the following points in context to a maximum of two marks 1. have genes for replicating in human cells; 2. can infect human cells; 3. and genes for damaging humans / haemagglutinin; 4. passed directly from human to human / more easily passed

(2)

(ii) Award one mark for each of the following points in context to a maximum of two marks. 1. have H5N1 surface genes (producing antigens for immune system to recognise); 2. antigens stimulate immune system/eq; 3. but less dangerous human flu genes. Edexcel A2 Biology Implementation and Assessment Guide for Teachers and Technicians ©Pearson Education Limited 2009

80

(2)


Unit 5 Pre-released reading assessment answers (cont.)

f Award one mark for each of the following points in context to a maximum of three

marks. 1. Likelihood of surviving much greater with mutation / converse/eq; 2. To reach reproductive age; 3. Mutant allele much more likely to be passed to offspring; 4. Higher proportion of offspring have mutated allele; 5. In future generations; the mutant form of CCR5.

(3)

g Award one mark for each of the following points in context to a maximum of five

marks. 1. High cost; 2. Drug companies profiteering; 3. Campaign for generic, cheap drugs; 4. Not necessarily best treatment; 5. Other methods such as diet control may be effective; 6. Traditional methods may be effective; 7. Drugs may be unsafe; 8. Patients receiving drugs fall ill; 9. AIDS not caused by HIV alone; 10. Other factors affecting immune system important; 11. AVRs may cause AIDS; 12. People taking drugs get AIDS.

(5)

h Award one mark for each valid points in context to a maximum of seven marks.

Possible points include: 1. Infection rate stable under 24; 2. Fewer infected from birth due to drugs; 3. Better education; 4. Still rising over 24; 5. Sexually active; 6. Must be unprotected sex if pregnant; 7. Lower over 35; 8. Higher death rate of infected individuals; 9. Fewer males infected under 35; 10. Young males less likely to take the test; 11. DOH study shows higher infection rate; 12. Not all of National study having unprotected sex; 13. Those at risk might be more likely to refuse test; 14. Under 20 group includes children in National study.

(7)


81



pages 188–189 1 Light: needed for photosynthesis; important that shoots and leaves move towards it; roots

need to be in the soil so response away from light helps make sure they grow in the right direction; respond to direction, intensity and length of exposure. Gravity: growth movements of plant parts towards or away from the pull of gravity; roots grow towards gravity, shoots away; this helps to orientate the young plant as the seed germinates below ground – shoots grow up, roots down, whichever way up the seed is planted. Chemicals: plant roots will grow towards some chemicals and away from others. Water: roots grow towards water in the soil; water needed for growth. Temperature: some plants or parts of plants respond to changes in temperature; there may be a positive movement towards lower temperature, e.g. many roots, or parts of the plant may respond to protect the cells and tissues, e.g. rhododendron leaves curl in the cold; important to help roots grow in the right direction, and also to protect plant tissues from damage in extreme cold. Touch, thigmotropism: plants (particularly climbing plants) grow in response to touch, e.g. runner beans curving up canes. 2

Plants respond to stimuli by growth. They need to respond to stimuli, such as direction and intensity of light, throughout life so that they can continue to grow. Animals respond to stimuli often by movement, largely through muscle contractions, in response to nerves so they can continue to respond when growth has stopped.

pages 190–193 1


valid.

Part of plant affected

Effect of red light (light, build-up of P FR)

Effect of far red light (dark, build-up of P R)

seed

stimulates germination

inhibits germination

stem

elongation inhibited by red light

elongation is stimulated by far red light exposure to far red light gives the same effect as etiolation

leaf

expansion is stimulated by red light chlorophyll formation stimulated

expansion is inhibited by far red light chlorophyll formation inhibited

lateral roots

growth of lateral roots is inhibited

growth of lateral roots is stimulated

flowering

in SDPs red light inhibits flowering in LDPs red light stimulates flowering

in SDPs far red light promotes flowering in LDPs far red light inhibits flowering


134


Chapter 8.1 (cont.)

2


valid. •

Suggests that a chemical message is produced in leaf exposed to periods of light and dark which travels to the flowering apical meristem.

•

If it is a chemical message, takes time to be synthesised and to travel through plant from leaf to buds. So if the leaf is removed immediately, there is no time for the chemical to be made and moved.

•

Suggests chemical message moves from one plant to another to stimulate owering in both.

•

A chemical from a leaf exposed to light and dark moves out into tissues of host plant – again suggests chemical message.

3


valid. Light detected by phytochromes in leaves

then split the flow chart for long-day and

→

short-day plants. Short day: dark period of appropriate length or far red light → conversion of P FR to PR → balance of phytochromes moves to more P R than P FR → triggers production of florigen in leaf cells

florigen travels to cells in shoot apex

→

stimulates flowering.

→

Long day: dark period of appropriate length or red light → conversion of P R to PFR → balance of phytochromes moves to more P FR than PR leaf cells

florigen travels to cells in shoot apex

→

triggers production of florigen in

→

stimulates flowering.

→

pages 194–195 1 Photoperiodism: involves a major physiological change, such as flowering, which involves

the transfer of a message from the leaves to the flowering regions of the plant; once it has been set up, it continues; depends on phytochromes and florigen. Phototropisms: much more localised; rely on auxins; can easily be reversed when light source moves. 2 Because cocoa butter is a fat, only fat-soluble substances are able to pass through it. The

message passes through agar (which is water-based) but cannot penetrate the fat, so the message must be water-soluble. 3


valid. In all the experiments shown, the dependent variable is growth. The independent variables were: A presence/absence of shoot tip; B block to transmission of chemicals on one side of shoot; C substance diffused from shoot tip into agar; D: substance diffused from shoot tip into agar, placed asymmetrically. Delicate shoots so can easily be damaged; simple design makes investigations very clear; capable of being repeated many times very cheaply and easily.


135


Chapter 8.1 (cont.)

pages 196–197 1

[S1C] Unilateral light seems to cause the IAA molecules to move laterally across the

shoot, so that the side of a shoot exposed to light contains less IAA than the side which is not illuminated. This means that the shoot tip acts as a photoreceptor. More hormone diffuses down to the region of cell elongation on the dark side. The IAA molecules bind to specific receptor sites on the cell surface membranes, activating the active pumping of hydrogen ions into the cytoplasm. This changes the hydrogen ion concentration, providing the optimum pH for the enzymes that break bonds between adjacent cellulose microfibrils and keep the wall flexible. The cells absorb water by osmosis and the very flexible cell walls stretch and allow the cells to expand. Eventually, as the cells mature, the IAA is destroyed by enzymes, the pH of the cell walls rises and bonds form between the cellulose microfibrils. So the cell wall becomes more r igid and the cell cannot expand any more. Because there is more IAA on the dark side, there is more cell elongation and so growth on the dark side, resulting in the shoot bending towards the light. Once the shoot is growing directly towards the light, the unilateral stimulus is removed. The asymmetric transport of IAA ends and the shoot g rows straight towards the light. 2

The answers should include some or all of the following. Other points may be valid. a Went’s bioassay showed that the amount of growth (bending when the block was

placed asymmetrically) was directly related to the amount of chemical in the block. This makes it possible to say that growth towards unilateral light must be due to higher levels of the chemical on the shaded side than on the light side of the shoot. b The current model of phototropisms explains the difference in elongation of the cells

on the shady and light sides in terms of the movement of auxin from the light to the shady side, where auxin allows the cell walls to remain flexible for longer.


136


Chapter 8.2

pages 198–201 1 Nerve fibre: axon of an individual nerve cell; may carry impulses to (sensory) or from

(motor) the brain but not both. Nerve: bundle of nerve fibres, may be all sensory, all motor or a combination of both. 2


valid. Nerve cell in CNS with dendrites to synapse with and receive input from other nerve cells, long axon/fibre to reach tissues, myelinated to give rapid speed of impulse transmission, synapses on effector to pass on impulse. 3 In invertebrates, the speed of transmission of a nerve impulse is directly related to

diameter of nerve fibre, and there is a limit to how big a nerve fibre can grow. Most vertebrate neurones are associated with Schwann cells and therefore have a myelin sheath. As a result of the nodes of Ranvier, the transmission of a nerve impulse is much faster. So vertebrate nerves that need to carry impulses fast are myelinated, with relatively small diameters. Those that are not myelinated don’t need to carr y impulses very fast so they can still have small diameters. 4

Squid giant axons are large as they carry impulses relatively quickly during an escape response. This means that they are easy to find and access and easy to insert micropipettes into. Squids are invertebrates, so there are also fewer ethical issues with using them in experiments of this type.

pages 202–205 1

The resting potential of a neurone is produced by the differential exchange of ions across the cell membrane. The membrane is relatively impermeable to sodium ions, but quite freely permeable to potassium ions. There are also very active sodium/potassium pumps which use ATP to move sodium ions out of the axon and potassium ions in. This lowers the concentration of sodium ions inside the axon, because they are pumped out and cannot diffuse back in again. Potassium ions are moved in to the cell by the pumps but they then diffuse out again along a concentration gradient through open potassium ion channels. Eventually the movement of positively charged potassium ions out of the cell along the concentration gradient is opposed by the electrochemical gradient. As a result, the inside of the cell is slightly negative relative to the outside. This gives a potential difference across the membrane of around –70 mV.


137


Chapter 8.2 (cont.)

2

a An action potential is the result of a sudden and dramatic increase in axon membrane

permeability to sodium ions. Specific sodium channels or sodium gates open up, allowing sodium ions to diffuse rapidly into the cell. The potential difference across the membrane is briefly reversed, with the cell becoming positive on the inside with respect to the outside, at about

140

mV. This depolarisation lasts about 1 millisecond.

The sodium channels then close again and excess sodium ions are rapidly pumped out by the sodium pump, using ATP to supply energy. The permeability of the membrane to potassium ions is temporarily increased as voltage-dependent potassium ion channels open. Potassium ions diffuse out of the axon down their concentration gradient and an electrochemical gradient, attracted by the negative charge on the outside of the membrane. The inside of the cell becomes negative relative to the outside once again. It takes a few milliseconds before the resting potential is restored and the axon is ready to carr y another impulse. The refractory period is the recovery time after an action potential during which an axon membrane cannot be restimulated. It is the time taken for the sodium pump and other ionic movements to restore the balance of positive and negative ions that produce the resting potential. Absolute refractory period is when the sodium channels are completely blocked and no resting potential. As recovery progresses there is the relative refractory period. Its importance is that it limits the rate at which impulses travel along a nerve fibre, preventing constant stream of impulses, and preventing the action potential from travelling backwards along the fibre. b An action potential can be recorded accurately by inserting a very fine glass

microelectrode into an axon. Another electrode records the electrical potential from the outside. The results are shown on an oscilloscope. 3 The graph shows that DNP prevents the pumping of sodium ions out of the axon. This

suggests that the ATP is being used to power the sodium pump – when it runs out, the pump no longer works. As the DNP is washed away the pumping of sodium ions out begins again, suggesting that ATP production has started again. 4


valid. The recording from the whole nerve shows an increasing level of response to an increasing stimulus. This is because more and more nerve cells within the nerve are responding to the stimulus. The response from the single axon shows the typical all-ornothing response – where the response is the same to a stimulus beyond a threshold level.

pages 206–209 1 The movement of a nerve impulse along the fibre is the result of local ion movements

both in front of and behind the action potential. In an unmyelinated nerve fibre these events occur all along the membrane. Although each event takes only milliseconds, the time adds up as the action potential travels along the fibre. In myelinated neurones, ions can only pass in and out of the axon at the nodes of Ranvier,. So an action potential will jump from one node to the next. This speeds up transmission as the ionic movements occur much less frequently, taking less time.


138


Chapter 8.2 (cont.)

2 They are involved in making the neurotransmitter substances in the presynaptic knobs;

involved in the production of vesicles; involved in the breakdown of neurotransmitters in the synaptic cleft; involved in the production of ATP to power the various ion pumps and synthesis and breakdown of neurotransmitters. Any other valid points. 3


valid. Electron micrographs: show the presence of vesicles in the synaptic knob of the presynaptic neurone before an action potential; after repeated action potentials these vesicles are no longer visible, implying that they have released their contents as a result of stimulation. Also shows large numbers of mitochondria that supply the energy for the synthesis of the neurotransmitters etc. Botulinus toxin: blocks the release of acetylcholine and so shows that acetylcholine from the presynaptic membrane is needed for the transmission of an action potential across a synapse. Nicotine: stimulates the nervous system by binding to the post-synaptic membrane, mimicking the effect of acetylcholine and so suggesting that acetylcholine does the same. Strychnine etc.: show that acetylcholine causes the setting up of action potentials in postsynaptic neurones, because preventing the breakdown of acetylcholine causes the neurones to respond continuously. Curare: shows that blocking of acetylcholine stops transmission of action potentials from nerve cells to muscle cells confirming that acetylcholine needs to bind to acetylcholine receptors to initiate post-synaptic potential.

pages 210–211 1

Accommodation occurs when all of the synaptic vesicles in a synaptic knob are discharged as a result of too many action potentials in rapid succession. The rate of synthesis of new vesicles cannot keep up and the neurone can no longer respond to the stimulus. A short rest restores the response as new vesicles are made. Accommodation allows organisms to ignore repeated harmless stimuli so that the CNS does not become overwhelmed with input.

2

Spatial and temporal summation make an organism more responsive to small stimuli which might not on their own trigger a response. A response coming into several sensory receptors at once, for example, can be added together to give awareness, e.g. the rods of the eye. Similarly, if a small stimulus which would not trigger a post-synaptic action potential on its own is repeated several times in quick succession, an organism becomes aware of it, thus increasing sensitivity and responsiveness.

pages 212–213 1

a When a receptor cell receives a stimulus, sodium ions move rapidly across the

cell membrane setting up a generator potential. A small stimulus results in a small generator potential and a large stimulus results in a large generator potential. If the generator potential is large enough to reach the threshold of the receptor neurone, an action potential will result in that neurone. If it is not, there will be no action potential.


139


Chapter 8.2 (cont.)

b In convergence, even if the generator potential from an individual receptor cell is

too small to set up an action potential, the generator potentials from several may add together or summate and trigger an action potential. This makes it possible for the sensory system to respond to low-level stimuli. 2


valid. Accommodation: overstimulation of any presynaptic neurone releases so many synaptic vesicles that further action potentials in the neurone can’t release neurotransmitter molecules into the synaptic cleft and no action potential is generated in the postsynaptic neurone. This makes it possible for the animal to concentrate on new, and potentially more important, stimuli. Response returns as new neurotransmitter is synthesised. Adaptation: constant stimulation of receptor cells results in a gradual decline in response. Again this makes it possible for the animal to concentrate on new, and potentially more important, stimuli. Animal will not respond again regardless of amounts of neurotransmitter.

pages 214–217 1

a Rhodopsin is formed from opsin and retinal. In the dark, retinal is all in the cis form.

Light converts this to trans-retinal. This changes the shape of the retinal, and the rhodopsin breaks up into opsin and retinal. b The break-up of rhodopsin triggers a cascade reaction that results in the closing of

the sodium channels in the rod membrane. As the sodium pump continues to pump Na1 out of the cell, the interior becomes more negative than usual. This produces a generator potential in the rod. If the generator potential is large enough to reach the threshold, or if several rods are stimulated at once, neurotransmitter substances are released into the synapse with the bipolar cell. This sets up an action potential in the bipolar cell which passes across the synapse to cause an action potential in the sensory neurone leading to the brain. In the visual areas of the brain this visual information is converted into an awareness of the image. 2

a Several rods synapse with a single bipolar cell, so summation of generator potentials is

possible. In low light levels which would not result in an action potential in the bipolar cell from a single rod, summation of generator potentials from several rods can result in an action potential. Each cone synapses with a single bipolar cell so it is less likely to trigger an action potential in dim light. In addition, iodopsin in cones needs to be hit with more light energy than rhodopsin in the rods before it will break down. So, again, the rods will respond to dimmer light. b Several rods synapse (converge) on a single bipolar cell. This means that subtle

changes in light level as something moves are detected by rods although not necessarily clearly. Cones need much bigger light differences, and there is no convergence, so they are less likely to respond to movement. c Cones respond only to bright light, and each individual cone synapses onto a single

bipolar cell. As a result any image is usually clear and crisp.


140


Chapter 8.2 (cont.)

pages 218–219 1 The need to move the whole body around in animals means faster responses are required

– electrical coordination makes this more possible. Chemical coordination depends on diffusion and movement in mass flow systems such as blood and phloem and so speed is limited. It usually involves growth as the mechanism of response. It is sufficient for plant responses. 2


valid.

Plants

Animals

chemical coordination

plant hormones (growth substances) control growth, movement, flowering examples: auxins (IAA) gibberellins, florigen relatively slow changes

animal hormones control growth, development, sexual maturity, response to stress, blood sugar levels etc. examples: adrenaline, testosterone, oestrogen, human growth hormone, insulin relatively slow changes

electrical coordination

none

nervous system composed of conducting nerve cells controls rapid response to stimuli, e.g. reflexes, and brain allows coordination of many nervous inputs allowing learning, habituation etc. includes chemical transmission at synapses can be very rapid, long-term changes in synapses of brain related to memory

3 Light falling on the sensory cells of the retina causes impulses to travel along neurones

in the optic nerve to the brain. The impulses are detected in a control centre in the midbrain. The impulses travel along neurones to further control centres. These synapse with branches of the parasympathetic cranial nerve (the oculomotor) which transmits impulses to the iris. The impulses in the oculomotor nerve fibres stimulate the effectors (the muscles of the iris) causing the circular muscles to contract and the radial muscles to relax so the pupil constricts. If light levels drop, frequency of action potentials from the retina falls, impulses travel from the control centres along sympathetic nerves to the iris, the circular muscles relax and the radial muscles contract and widen the pupil. By reducing the amount of light entering the eye in bright conditions this reflex avoids damage to the delicate rods and cones by overstimulating them. In dim light, the reflex causes the pupil to open wide so as much light as possible falls on the rods to maximise what you can see.


141


Chapter 8.3

pages 220–221 1

The cerebral hemispheres are associated with higher brain functions – seeing, thinking, learning and emotions for example. Folding of the cerebral hemispheres increases the surface area of these regions of the brain. It is thought that greater folding and thus greater surface area is associated with greater intelligence, more complex emotions and the ability to learn more complex skills. Humans have greater abilities than other primates, and more folding. The volume of tissue is important too which is a function not just of folding but of thickness. So dolphins have big surface area but less volume because the tissue layer is thinner. They are very intelligent but it is thought that their development does not rival humans.

2


Brain area

Function

cerebral hemispheres

vision, sight, thinking, learning, emotions

frontal lobe

emotional responses, planning, reasoning, decision-making

primary motor cortex controls many movements temporal lobe

sound recognition, hearing, speech, many memory functions

occipital lobe

vision, shape recognition, colour vision, sense of perspective

cerebellum

coordinates smooth movements, maintains posture and balance

hypothalamus

coordinates autonomic nervous system, including thermoregulation, and controls many basic drives, e.g. thirst, hunger, aggression, reproductive behaviour

medulla oblongata

controls reflex centres for heart rate, breathing rate, blood pressure, coughing, sneezing, swallowing, saliva production, peristalsis

pages 222–227 1 X-rays: pass through body, absorbed differentially by different tissues, make an image on

photographic film. Good for taking images of hard tissue, e.g. bones, but much less useful for producing images of soft tissues such as the brain. Computerised tomography (CT scan): thousands of tiny beams of X-rays passed through an area of the body, e.g. head. Beams attenuated by the density of the tissue. The X-rays which make it through are detected and measured. A computer collates the data to produce a cross-sectional image of a thin slice through the body. Special dyes can make areas X-ray opaque so they show up more clearly in the scan. Can identify major structures in the brain and detect problems such as brain tumours, bleeding in the brain or swellings of the arteries in the brain (aneurisms). Cannot be used to show how areas of the brain are used or change during different activities. Can be linked to observed changes in behaviour to indicate the importance of certain areas of the brain in particular functions. Magnetic resonance imaging (MRI scan): Uses magnetic fields and radio waves to image the soft tissues (mainly due to amount of water in tissue), so no potentially damaging X-rays. Particularly good for imaging water in the body. The signals are analysed by computer and used to produce an image. Images show much finer detail than CT scans. Edexcel A2 Biology Implementation and Assessment Guide for Teachers and Technicians ©Pearson Education Limited 2009

142


Chapter 8.3 (cont.)

2D scans are usually produced – a computer can create a 3D image from these. Can distinguish regions of the brain. Widely used to diagnose brain injuries, strokes, tumours and infections of the brain or the spine. Can also indicate links between the structures in the brain and patterns of behaviour. Functional magnetic resonance imaging (fMRI): monitors uptake of oxygen in different brain areas, so indicates active areas of the brain. Can be observed in real time, so makes it possible to watch brain response while people carry out tasks. Gives an extremely spatially accurate image of the brain. Has to be carr ied out with patient’s head remaining completely still, which limits tasks that can be done. Used mainly to investigate normal brain structure and function. May soon be used to diagnose diseases such as the early signs of stroke damage and the onset of Alzheimers. 2


valid. Absolutist: Animals should not be used for experiments – in this case for learning more about how the human brain might work. Morally, experimenting on animals that causes pain or death is completely unacceptable. Relativist: It is morally more acceptable to experiment on animals as a model for humans where there is a positive purpose to the research, e.g. it will benefit the treatment of humans with disease or damage to the brain.

pages 228–231 1

[S1C] The answer should include some or all of the following. Other points may be valid. a Absolutist: it is completely unacceptable to experiment on animals causing pain or death.

Relativist: the work of Crowley and Katz could not ethically have been carried out on humans, but it was important research that helps us understand the development of vision in mammals, including humans and therefore justified using primates. b Absolutist: it is completely unacceptable to experiment on animals causing pain or death.

Relativist: the work of Weisel and Hubel could not ethically have been carried out on humans, but it was important research that helps us understand the development of vision in mammals, including humans. It would not be ethical to use primates for this work as it was not directly of medical benefit, but kittens are more readily available experimental animals and therefore it is acceptable to use them. [S1C] 2 •

Animal work with embryos examining the structure of the brain as it is laid down to show that the basic structure develops at a specific stage of fetal development regardless of input.

•

Animal work with kittens stitching eyes shut at various stages of development to show that there is a critical window during which loss of sight becomes permanent, suggesting that this is when the mature visual cortex develops.

•

Animal work with monkeys showing same thing as work with kittens – different critical period.

•

Human observations, made on babies who are blind for some reason during the critical window, e.g. babies who needed surgery on cataracts etc.


143


Chapter 8.3 (cont.)

3 Answers should show awareness that it is unethical to experiment on human infants in

the way work has been done on kittens and monkeys, and that evidence might come from circumstances where babies are born with damage to brain areas or who are temporarily blinded/deprived of sight for some reason. 4 They were the first people to show that the brain is very plastic; that at birth, although

many of the neural areas are present, they need input and experience to develop and work fully. They developed the idea and produced evidence to support the concept of critical windows of development which underpins much of our subsequent understanding of how the brain works.

pages 232–237 1

The period of time during which vital neural connections are made in the brain in response to specific stimuli. Nature: axons from the light sensitive cells in the retina grow synapse in visual cor tex in a very regular way known as ocular dominance columns. Neighbouring columns of cells receive input from the same area of the retina from the left eye and the right eye. Nurture: right stimulation needed during critical window of development for the area of the brain to develop properly. If the necessary input during the critical period of development fails, the opportunity to make the vital neural connection can be lost permanently.

2


Must consider the needs of the baby first and the needs of research less important, e.g. it would be ethically unacceptable to leave a newborn baby with cataracts in order to test a hypothesis when it is known that the cataracts will interfere with the development of vision as the baby grows up. As long as the baby is not harmed or deprived in any way, then research could be acceptable. 3

a 36 b 20 c Each time a critical window of development appears, new neurones and connections

are laid down if the appropriate stimulus is present. This increases the size of the brain. If the child is deprived of the stimulus, the brain does not develop fully, and once the critical window has passed, that damage cannot be undone so the brain ends up smaller. In children who suffer severe neglect, many critical windows are missed and so brain development is severely affected. 4


Damaged brains: show what happens to the person as a result of specific damage or disease; may be difficult to separate the effect on specific areas of the brain if more than one region damaged; some kinds of damage are very rare, so takes a long time to collect sufficient evidence of its effect. Newborn babies: looks at how newborn babies respond to different stimuli; very limited in what can be done as cannot harm the babies; useful because the effect of nurture is minimal at this stage, so easier to separate from the effects of nature. Edexcel A2 Biology Implementation and Assessment Guide for Teachers and Technicians ©Pearson Education Limited 2009

144


Chapter 8.3 (cont.)

Twin studies and face recognition: comparison of responses from fraternal and identical twins; should help to separate out the effects of the environment and the genes. However, this assumes that identical twins develop identically from their genes, and this may not be true in all cases. Cross-cultural studies: looks at the impact of different beliefs and methods on behaviour; can be used to look at distinct differences in nur ture. However, every individual is different and not everyone is affected by their culture to the same extent. The level of these differences between individuals may be difficult to measure, and so difficult to correlate with the results from the study.

pages 238–239 1 Innate behaviour shows there are genetically determined responses to certain stimuli

which occur as a result of specific nerve pathways laid down in the embryo from the instructions of the DNA. The stimulus for a piece of innate behaviour will always elicit the same response and the development of these neural pathways in the brain depends almost entirely on the information in the DNA. 2

Animal behaviour can be used to see the importance of nature on brain development through the study of innate behaviour and the role it plays in animals of different species. Learning behaviour from habituation and conditioned reflexes to exploratory and insight learning all play an important role in developing understanding of nurture in brain development. Which behaviours are completely the result of genetic pathways and which depend on learning (nurture) as well. For example, maternal behaviour might be thought to be instinctive and some aspects are, but animals which are hand-reared with no contact with their own species are often incompetent mothers. Any other valid points

pages 240–241 1 Invertebrates with fast impulses in unmyelinated nerves have relatively large diameter

nerve fibres which make the fibres relatively easy to identify and stimulate ar tificially. Also, as invertebrates, less ethical issues arise from their use as experimental animals. 2


valid. Habituation and accommodation both lead to the reduction in response to permanent or rapidly repeated stimulus. Accommodation is the physical result of depletion of neurotransmitter and synaptic vesicles in the synaptic knob of a presynaptic neurone. After a brief absence of the stimulus (action potential in the presynaptic neurone), the neurotransmitter and vesicles are resynthesised and the response restored. Habituation is the result of loss of response of the calcium ion channels in the presynaptic membrane, resulting in a loss of response to an action potential in the neurone. It is a learning response, and can be retained over a long time, reducing the response to things that happen constantly or very frequently and so leaving the brain better able to focus on occasionally changing stimuli which may be more important for survival.


145


Chapter 8.4

pages 242–247 1 Neurotransmitter synthesis and storage: If a drug blocks this process, synaptic

transmission would lessen and then stop as supplies of neurotransmitter reduced. This would mean nerves using that neurotransmitter would no longer be able to pass impulses between them, causing loss of motor or sensor y skills and processes in the brain. Neurotransmitter release: A drug stopping neurotransmitter release would stop synaptic transmission as the impulse in the presynaptic fibre would not be transmitted to the postsynaptic membrane causing loss of motor or sensory skills and processes in the brain. Neurotransmitter-receptor binding: A drug blocking this would stop the development of the post-synaptic potentials which in turn would prevent the development of an action potential in the post-synaptic fibre. Alternatively the drug may maintain binding so that the stimulus to the post-synaptic fibre was continuous, causing confusion and fatigue further in the system. Neurotransmitter reuptake: a drug blocking this would slowly reduce the intensity of the response as less neurotransmitter would be resynthesised and be ready for release. This would speed up fatigue etc. Neurotransmitter breakdown: a drug blocking this would mean that stimulation would continue as the concentration of neurotransmitter in the synaptic gap would build up steadily so there would be constant stimulation of the post-synaptic membrane and fatigue. 2 Dopamine synapses: produce the neurotransmitter dopamine, the axons from them

spread through the frontal cortex, the brain stem and the spinal cord, so they are closely involved in the control and coordination of movement. Serotonin synapses: produce serotonin in a group of cells in the brain stem with axons that spread throughout the brain into the cortex, the cerebellum and the spinal cord. They have a widespread influence on cells throughout the brain so low levels mean overall brain activity is suppressed. Particularly related to depression. 3

L-dopa

Ecstasy

precursor of dopamine so crosses the blood–brain barrier

crosses blood–brain barrier

enables brain cells to maximise dopamine production

affects serotonin synapses in brain, blocks the serotonin reuptake system so synapses ooded

with serotonin, and may cause release of all the serotonin from presynaptic knob, ooding brain

with serotonin relieves stiffness and slowness of movement

acts as stimulant to brain and psychotropic

therapeutic

improves mood, sense of well-being, energy etc. physiological effects include increased heart rate, change in thermoregulation, loss of thirst sensation, prevention of urine production by kidney


146


Chapter 8.4 (cont.)

4


valid. SSRIs seem to work by blocking the serotonin reuptake proteins and so maintain a relatively high level of serotonin in the synaptic cleft, to bind with the post-synaptic membrane receptors. However the working hypothesis from the genetic evidence is the opposite. The short form of the serotonin reuptake protein allele and its frequency (i.e. in homozygous or heterozygous state) affect whether or not the person is likely to suffer from depression. People with the short form produce less of the reuptake protein so serotonin is taken up more slowly by the presynaptic neurone after its release into the synapse. Logically this would result in more serotonin being left in the synaptic cleft and so less depression – in the same way as people respond to SSRIs. In reality, people with the short form are more likely to experience depression. Perhaps they have less serotonin available in the presynaptic knob to be released with the subsequent impulses? Further investigations would need to look at how much serotonin is in the presynaptic knob, and/ or how much and how long serotonin remains in the synaptic cleft in the different genetic forms and during treatment with SSRIs.

pages 248–251 1

The Human Genome Project has identified all of the human genes and increasingly identifies the genes that are associated with particular diseases and risk groups. This is the information on which pharmacogenomics builds as it develops medicines that work with a particular genome.

2

The ability to prescribe the right drug for an individual patient that would work with their particular genetic makeup. It would increase the ability to prescribe the right and lowest possible dosage with minimal risk of side effects or adverse drug reactions.

3


valid. Identification of genes for treatment: time-consuming and expensive; response to drugs can be complex; should the money be invested in this kind of research that will benefit just groups of people, or would it be better to spend it on something that benefits everyone? Cost: having identified that only a proportion of a population will benefit, is it socially and morally acceptable to produce a dr ug that works for them, but not one that works for others in that population? Knowing your genes: should only you know, or should genomes be stored for analysis by drug companies to help them decide what drugs they should focus on? Should individuals be told about gene variants that may lead to other problems that can’t be tackled, or is it better not to know? Cost of training: to keep doctors and pharmacists up to date; will cost society but would be essential.


147


Chapter 8.4 (cont.)

4


valid. Many possibilities including: kappa-opioids as painkillers for women; screening for cytochrome P450 before drug trials; identifying ‘at risk’ individuals for certain types of cancer to allow treatment before the cancer starts; P-glycoprotein transporter substance affects the ability of people to take up digoxin, a heart drug – one genotype results in a far greater uptake of the drug than another; racial variations, e.g. incidence of heart disease in Asian populations and development of drugs targetted at people with a specific genetic profile.

pages 252–257 1 The diagram given here is a minimum. Give credit for any correct extra details. required gene cut out using restriction endonuclease enzyme

plasmid to be used as vector

plasmid with new gene inserted using DNA ligase host bacterial cell

modified bacterium makes drug as new gene expressed

2

[S1C] The answer should include some or all of the following . Other points may be valid.

(1) Wrong to use genetically engineered organisms under any circumstances. (2) The potential benefits of the use of genetically engineered microorganisms far outweigh any risks. 3 a

Drugs from GM plant

Drugs from GM microorganism

required gene cut from human or other organism and inserted into Ti plasmid of

required gene cut from human or other organism and inserted into plasmid

Agrobacterium tumefaciens

plant cells infected by modified A. tumefaciens which transfers desired gene to the plant genome

plasmid transferred into host bacterial cells

plant cells then cloned on suitable hormonecontaining medium to produced mass of undifferentiated modified plant cells

bacteria identied by marker cultured in fermenters to make new protein drugs

plant cells then transferred to suitable medium

downstream processing required to separate the microorganisms and the desired end-product

to produce huge numbers of GM plantlets that

where it becomes part of bacterial DNA, marker

gene usually added

will mature to produce the desired drug in their leaves/fruit etc. Edexcel A2 Biology Implementation and Assessment Guide for Teachers and Technicians ©Pearson Education Limited 2009

148


Chapter 8.4 (cont.)

b Risks: cost of development may deter companies and main use in less-developed

countries, possibility that GM crops will interbreed with wild species and change them, or that GM plants will lose their ability to produce the vaccine. Benefits: long-term, relatively cheap, can be grown by communities that need them around the world, no problems with needing cool storage or specialised medical personnel to deliver, easy for children or adults to take. 4


valid. Possible examples include: blood-clotting factors Factor VII and IX from goat/sheep/ rabbit milk; alpha-1-antitrypsin from sheep milk; ATryn (for treating hereditary antithrombin deficiency) from goat milk. Evaluation will depend on drug chosen, but should include an assessment of cost of producing the drug, effect on animals used, success of procedure used to create transgenic animals, benefits to people who are treated with the drug compared with previous treatment. 5


valid. Answer needs to include points such as genetically modified microorganisms, plant vaccines, GM crops etc. Benefits include: •

specic drugs made in large quantities in controlled way

•

lose dependence of chemicals extracted from animal cadavers

•

drugs/vaccines produced in a way which makes delivery/cost etc. in developing world easier

•

plants modied to suit growing conditions/nutritional requirements etc.

Risks include: •

risk of release into outside world

•

risk of contamination of natural plants with gene markers, e.g. infertility

•

cost to countries which cannot afford the GM technology for drug or foods.


149

Technician

Practical 8.1

1 of 1

Can snails become habituated to a stimulus?

Purpose •

To investigate habituation of snails to a stimulus.


Notes

Giant African land snail (or large garden snail)

Giant African land snails can be purchased in some pet shops or from Biological suppliers such as Blades Biological. If purchasing giant African land snails they should be bred in this country, to reduce chance of them having parasites. They are easy to keep in a glass or plastic aquarium tank containing several centimetres of compost, organic compost is recommended. There are several publications available about the care of land snails. If giant land snails are not available, a large garden snail can be used. Snails should be kept in moist conditions before the lesson, to encourage activity. If snails are still inactive, they can be paced for a short time in warm water. Safety

Wash your hands thoroughly after handling the snail. Equipment and surfaces should be disinfected with 1% Virkon.

Cotton wool bud Small beaker

Any small container for water to allow cotton wool bud to be dampened.

Board

Clean, firm surface for the snails, for example a plastic chopping board.

Stop watch


157

1 of 1

Teacher/Lecturer

Practical 8.1


Purpose •

Safety

To investigate habituation of

Wash your hands thoroughly after touching the snails once

snails to a stimulus.

all the equipment has been put ready for disinfection. Take care that the stimulus causes no harm to the snails.

Notes on the procedure In this investigation students find out if habituation to a touch stimulus occurs in snails. A giant African land snail is best, but large garden snails can be used. The student sheet that accompanies this activity contains a procedure for this experiment but students could be asked to plan the investigation themselves. The experiment is more reliable if snails are handled little prior to the experiment, so avoiding them becoming habituated to stimuli. The completion of Spearman’s rank correlation provides an extension to this activity. The null hypothesis to be tested would be: there will be no correlation between the number of stimuli the snail receives and the time taken for the snail to re-emerge. Further investigations might include how long the habituation lasts, or how handling prior to the experiments affects the results.

Answers 1

As the number of stimuli increase, the time taken for the snail to re-emerge will decrease – a negative correlation.

2

Students present the results as a scatter graph, with the number of stimuli on the x-axis, and the time taken to re-emerge on the y-axis. Students should make a simple statement, from their results, as to whether there appears to be a positive, a negative, or no, correlation.

3

There is a negative correlation – as the number of stimuli increase the time taken for the snail to re-emerge decreases. Students should make a reference to the data. With repeated stimulation, Ca21 channels in the presynaptic membrane become less responsive. Less Ca21 crosses the membrane into the presynaptic (sensory) neurone. As a result less neurotransmitter is released into the synaptic cleft. This means that an action potential across the postsynaptic membrane is less likely. Fewer action potentials are produced in the postsynaptic motor neurone so less of a response is observed.

4

The snail learns that the stimulus is not causing harm, and so it is withdrawing unnecessarily. This effect may be used in its natural habitat when faced with repetitive stimuli, such as vegetation touching the head/eye stalks.

5

Relevant comments would include: the need for replication using snails of approximately the same size and age; the need to control of the size and position of the stimulation; the need to control other variables that may affect the response, e.g. drying out of the snail; difficulties in determining when the snail has fully extended (measuring the eye stalk length prior to stimulation may overcome this problem); effect of handling the snail prior to the experiment.

6

If the snails have been handled too much prior to the experiment, they may have already become habituated to this type of stimulus so further stimulations will not change the response.


22

Student

Practical 8.1

1 of 2


Purpose •

To investigate habituation of snails to a stimulus.

Touching snails Lots of people, at some time in their childhood, will have touched a snail in the garden and noticed that it withdraws its eye stalks into its body. For such a slow-moving animal this seems a very quick response, this suggests it is an important response for protection and survival. A snail only withdraws into its shell when it is either inactive or threatened. When touched, it withdraws to avoid danger. Do snails become habituated to the stimulus, ceasing to withdraw with repeated stimulation? In this investigation you will collect data to find out if habituation to a touch stimulus does occur in these organisms.

Safety Wash your hands thoroughly after touching the snails once all the equipment has been put ready for disinfection. Take care that the stimulus causes no harm to the snails.

Procedure You will need: • One giant African land snail (or a garden snail if not available) • One dampened cotton wool bud • Suitable clean, rm surface for the snails (e.g. a plastic chopping board) • Stopwatch

1

Collect one giant African land snail, and place it on a clean, firm surface. Allow the snail to get used to its new surroundings for a few minutes until it has fully emerged from its shell.

2

Dampen a cotton wool bud with water.

3

Firmly touch the snail between the eye stalks with the dampened cotton wool bud and immediately start the stopwatch. Measure the length of time between the touch and the snail being fully emerged from its shell once again, with its eye stalks fully extended.

4

Repeat the procedure in step 3 for a total of 10 touches, timing how long the snail takes to re-emerge each time.

5

Record your results in a suitable table.

6

Present your results in an appropriate graph. Edexcel practical materials created by Salters-Nuffield Advanced Biology, ©University of York Science Education Group.

43

2 of 2

Student

Practical 8.1 (cont.) Can snails become habituated to a stimulus?

Questions 1

Write a hypothesis which this experiment will test.

2

Using your graph, state if you think there is a positive, negative, or no, correlation between the number of stimulations and the time for eye stalk withdrawal.

3

Explain any patterns or trends in your data, supporting your ideas with evidence f rom the data and your biological knowledge of habituation. Relate your findings to your hypothesis.

4

Suggest a reason why snails may become habituated to a prodding stimulus in the wild.

5

Evaluate the procedure used for this experiment.

6

This experiment has been shown to be less successful if the snails are handled regularly prior to the experiment. Suggest why handling prior to the experiment could affect the results of the experiment.

Going further 7

Write a null hypothesis that this experiment will test.

8

Complete a Spearman’s rank r s correlation test to determine if there is a statistically significant correlation between the variables. A table with the headings below will help. Number of times the snail has been stimulated

9

Rank stimulation

Time/seconds

Rank time

Difference/D

D2

Use a table of critical values to accept or reject your null hypothesis. If your calculated Spearman rank value (r s) is greater than the critical value, then the null hypothesis is rejected. If your calculated r s value is less than the critical value, then the null hypothesis is accepted.

10

Write a statistical conclusion for your experimental data. Make sure you include: •

your calculated value of r s

•

the number of pairs of data

•

the signicance level

•

the critical value

•

whether the null hypothesis is being accepted or rejected.


44


Unit 5 Topic 8

1

The diagram below shows a section through a human brain. A

C B

Complete the table below by writing in the letter and the name of the region of the brain that carries out each function.

Function

Letter

Name of region

Initiating and controlling voluntary muscle movement Coordinating skeletal muscle movement, balance and posture Controlling heart and breathing rate

2

[Total 4 marks]

Neurones transmit electrical impulses along their cell surface membranes when stimulated to do so. These impulses are called action potentials and involve changes in the electrical potential across the membranes due to movement of positive ions. Direction of impulse

The diagram opposite shows the distribution of charge inside and outside an unmyelinated axon as an impulse passes along.

�� ��

 ��

��

����

D a

C

B

A

Use the most suitable term from the list below to describe the state of the axon in regions B, C and D. Region A has been done for you. depolarising

resting

repolarising

A resting B _________________ C _________________ D _________________ Edexcel A2 Biology Implementation and Assessment Guide for Teachers and Technicians ©Pearson Education Limited 2009

62

(2)



b

The diagram below shows two action potentials recorded using an oscilloscope. 60

40

20

V m / l 0 a i t n e t o �20 p e n a r b m �40 e M X

�60

�80

�100

0

1

2

3

4

5

6

7

8

9

10

Time/ms

(i)

Explain how the change in membrane potential between 0.5 and 2.0 milliseconds is brought about.

(4)

(ii) Calculate the number of action potentials occurring per second. Show your working.

Answer _________________ per second

(2)


63



c

Explain what is taking place during the period marked X on the diagram in

(3)

part b.

[Total 11 marks] 3

Research on visual development in cats has led to knowledge of how information is processed in the visual cortex of the brain. The diagrams below show the growth of neurones in part of the visual cortex after normal visual development and when the left eye has been deprived of sensor y information. Normal visual development

left eye

a

right eye

Sensory deprivation in the left eye

closed left eye

Use the diagrams above to describe differences in the visual cortex after sensory deprivation.

b

(2)

Explain how this type of experiment has provided evidence which shows the need for exposure to sensory information in normal visual development.

c

right eye

(2)

Describe one other piece of evidence which suggests that humans must be exposed to particular stimuli if they are to develop normal vision.

(1)


64



4

Sea slugs are marine invertebrates with gills for gas exchange on their body surface. A sea slug is able to withdraw its gill when touched. In an investigation into this response, the gill was touched and the time taken for the g ill to be exposed again after withdrawal was measured. This was repeated at half-minute intervals. The table below shows the results of this investigation.

Touch First Second Third

a

b

c

Time taken for gill to be exposed again/seconds 23.0 9.0 16.0

Fourth

4.5

Fifth

7.5

Sixth

6.5

Seventh

6.0

Eighth

4.5

Ninth

5.5

Tenth

6.5

Describe the effect of repeated touching on the time taken for the gill to be exposed again.

(3)

Name the type of learning shown by a sea slug in this investigation.

(1)

Explain how this learned response may be of benefit to the sea slug in its natural environment.

(4)


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5

The human brain has many different neurotransmitters which influence behaviour. One of these neurotransmitters is serotonin. A lack of serotonin in the brain can cause depression and an excess can cause anxiety. Drugs that increase levels of serotonin are used as antidepressants. The diagram below shows a synapse in which serotonin acts as a neurotransmitter. presynaptic membrane

post-synaptic membrane

synaptic cleft

serotonin synthesis

vesicle containing serotonin

Key

serotonin post-synaptic receptor serotonin re-uptake mechanism serotonin

a

In sufferers of depression the levels of serotonin are reduced. Using the diagram above, suggest

b

one

explanation for low serotonin levels.

Use the diagram and your own knowledge of synaptic transmission to explain how information is transmitted across a synapse.


66

(1)

(6)

Examzone: Topic Topic tests 6 of 6


ways anti-depressants work. (2)

c

Using the information information given, suggest and explain

d

The drug known as ecstasy (MDMA) can cause anxiety. The diagram below

two

represents an ecstasy molecule. Using your knowledge and the diagrams provided suggest how ecstasy may have this effect.

(3)

[Total 12 marks] (Totall for test 40 marks) (Tota


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Unit 5 Topic 8

1

a (i)

type:

effector / motor / multipolar;

role:

{transmit / eq} impulses to to {effectors {effectors / muscles / glands / named e.g.}; e.g.}; (2)

(1)

(ii) from cell cell body to synaptic synaptic knob; knob; b where:

grey matter;

importance: {link / eq} with {sensory / relay} neurone neurone / reference to summation;

(2)

c 1 gap in the myelin sheath / eq;

2 enables {depolarisation {depolarisation / action potential / eq} (of axon); axon); 3 causes impulse to jump from from node to node / saltatory conduction; conduction;

(3)

4 speeds up (transmission of) impulse;

[Total 8 marks] [S1C] 2 Accept points only in correct con text.

1 absorption of light causes changes changes in / of photosensitive photosensitive / photoreceptiv photoreceptive, e, pigments; pigments; 2 reference to phytochrome (pigments) in, plants / leaves; 3 P660 / PR and P730 / PFR; 4 P660 / PR converted to P 730 / PFR when red light / eq absorbed, P730 / PFR converted to P660 / PR when far-red light absorbed; 5 P730 / PFR high during daytime because more red light present in daylight / white light; 6 reference to slow conversion of, P730 / PFR, back to, P 660 / PR, in darkness; 7 reference to phytochromes involved in, initiation of germinati germination on / flowering flowering / photomorphogenesis; 8 reference to, to, visual visual pigments / iodopsin / rhodopsin / visual purple, in retina (of mammals); 9 rhodopsin / visual purple in rod cells; 10 (rhodopsin consists consists of) opsin combined combined with, retinal / retinene retinene / retinine; 11 cis form of retinal retinal / retinene / retinine retinine changes to trans form in light; light; 12 (this causes rhodopsin) to split; 13 iodopsin (thought to be) in cone cells; cells; 14 three types (of iodopsin) iodopsin) respond to different, waveleng wavelengths ths / colours, / reference reference to trichromatic theory; 15 changes in (retinal) pigments pigments lead to, generator potential potential / hyperpolarisation hyperpolarisation / action potential / changes in membrane potential;

[Total 10 marks] 3 a (i)

as light intensity increases, increases, initially {little / no} change then then pupil size decreases decreases and then levels off / line {s-shaped / sigmoid};

(1)

(ii) circular muscles contract; radial muscles relax; [Allow converse in context] Edexcel A2 Biology Implementation and Assessment Guide for Teachers Teachers and Technicians Technicians ©Pearson Education Limited 2009

92

(2)



b (i)

receptor:{rod / cone cells / retina / photoreceptors} effector: iris muscle / radial and / or circular muscle;

(1) (1)

(ii) involuntary / does not require thought / autonomic; c similar shape to acetylcholine;

attaches / binds to receptors; blocks / prevents acetylcholine from binding; prevents depolarisation of post-synaptic membrane / eq;

(2)

d 1 rhodopsin breaks down into retinal and opsin;

2 closing of {ion / Na1} channels; 3 Na1 influx to outer segment stops; 4 inner segment continues to pump Na1 out; 5 (inside of) cell / membrane {more negative / hyperpolarised}; 6 release of neurotransmitter / glutamate stops / inhibition of bipolar cell lifted;

(5)

7 bipolar Na1 channels open / cell membrane depolarised;


a 1 identical twins are genetically identical / eq;

2 height mainly due to {genes / nature} / not affected much by environment; 3 body mass and intelligence are mainly due to {environment / nurture / eq}; 4 reference to figure(s) to back up argument / valid comparison, e.g. less effect of nurture of those reared together / converse; [max 2 marks for description] 5 reference to body mass, e.g. diet, exercise; 6 reference to intelligence, e.g. schooling, parental encouragement; 7 reference to height being {polygenic / multifactorial}; [max 2 marks for explanation]

(3)

b 1 only a small sample / eq;

2 fewer (MZ) twins reared apart / converse; 3 characteristics not measured using {same / comparable} units; 4 intelligence {difficult to measure / subjective / testing unreliable}; 5 reference to not knowing whether the differences are statistically significant; 6 no comparison with {unrelated people / rest of population / dizygotic twins / other siblings}; 7 reference to two samples being matched for {age / sex / culture / race / eq}; 8 one-off measurements and differences in characteristics may change over time; 9 fears that the data may have been falsified;

(2) [Total 5 marks]


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A2 Biology Notes for edexcel

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