Biology Topic 3 & 4 - Notes

Edexcel AS Biology Revision Notes

Written by Tim Filtness

Edexcel AS Revision Unit 22:: Development, Plants & the Environment Topic 3: The Voice of the Genome 2.3.2 & 2.3.3

Prokaryotic Cell

Prokaryotic Organelles: Ribosomes. Same function as eukaryotic cells (protein synthesis), but are smaller (70s rather than 80s). Nuclear Zone. The region of the cytoplasm that contains DNA. There is no nuclear membrane. DNA. Always circular, and not in chromosome form. Plasmid. Very small circles of DNA, containing non-esential genes. Can be exchanged between different bacterial cells.



Cell membrane. made of phospholipids and proteins, like eukaryotic membranes. Mesosome. Tightly-folded region of the cell membrane containing all the proteins required for respiration and photosynthesis. Cell Wall. DIFFERENT from plant cell wall. Made of murein (a protein). There are two kinds of cell wall, which can be distinguished by a Gram stain:

A: Gram positive bacteria have a thick cell wall and stain purple B: Gram negative bacteria have a thin cell wall with an outer lipid layer and stain pink. Capsule (or Slime Layer). Thick polysaccharide layer outside of the cell wall. Used for:

1. Sticking cells together 2. As a food reserve 3. As protection against desiccation (drying out) and chemicals, and as protection against phagocytosis (being broken down by a white blood cell). Flagellum. A rotating tail used for propulsion. Eukaryotic Cell



Endoplasmic Reticulum. Site of protein folding (see 2.3.4) Ribosome. Site of protein synthesis Nucleus. DNA “store” & site of mRNA synthesis. Golgi apparatus. Site of protein folding and packaging (see 2.3.4) Centriole. Makes spindle protein, which pulls chromosomes apart during cell division Vesicle. “Bubble” of membrane, used to transport materials around a cell and between cells Lysosome. A vesicle filled with digestive enzymes. Protects against bacterial attack and removes cell debris. Cell membrane. Made of phospholipid and protein, controls movement in / out of the cell Mitochondrion. Site of respiration Chloroplast . Site of photosynthesis

Prokaryotic Cells

Eukaryotic cells

Small cells (< 5 mm)

Larger cells (> 10 mm)

Always unicellular

Often multicellular

No nucleus or any membranebound organelles

Always have nucleus and other membrane-bound organelles

DNA is circular, without proteins

DNA is linear and associated with proteins to form chromatin

Ribosomes are small (70S)

Ribosomes are large (80S)

No cytoskeleton

Has a cytoskeleton

Cell division is by binary fission

Cell division is by mitosis or meiosis

Reproduction is always asexual

Reproduction is asexual or sexual



2.3.4 Amino acids are “stuck together” in the correct order during translation . This take place using ribosomes, which are therefore the site of protein synthesis. After synthesis, proteins are put into the rER, which folds primary proteins into their specific secondary and tertiary forms. 20 and 30 proteins are packaged into vesicles and sent to the Golgi In the Golgi, 30 proteins are stuck together to form completed 40 proteins. They are packaged into large secretory vesicles, which bud off the Golgi and go the cell membrane for exocytosis. The Golgi also makes lysosomes.

2.3.5 Tissue: a group of specialized cells, which all carry out the same function. Organ: a group of different tissues.

Although every cell contains the entire library of genes, each tissue only expresses a select few of them. This is because, as cells become specialized, they progressively switch off genes. This is called cell differentiation . The Cell Cycle

2.3.6

G1 Phase:

Growth phase Approximately 40% of cell cycle

S Phase:

DNA replication occurs Approximately 45% of cell cycle

G2 Phase:

Preparation for mitosis Organelles replicate

Mitosis:

Cell divides Approximately 10% of cell cycle

Cytokinesis :

Cell physically splits Approximately 5% of cell cycle


Stage


Explanation Stage is: Prophase 1. 2. 3. 4.

DNA coils up onto chromosomes Centriole divides Centrioles move to cell poles Nuclear envelope disappears

Stage is: Metaphase 1. 2. 3. 4.

Chromosomes move to equator Spindle attaches to centromeres Centrioles split Chromatids separate

Stage is: Anaphase 1. Chromatids separate completely 2. New nuclear envelope grows

Stage is: Telophase 1. Cytokinesis occurs 2. Cells separate

Stage is: Interphase As above (G1, S & G2)

Underlined comments = definition of stage end



2.3.7 Dig up your Mitosis Core Practical notes in the Practical Handbook 2.3.8 Mitosis produces genetically identical daughter cells, whereas Meiosis produces genetically dissimilar gametes. The variation in gametes comes from; 1. Random fusion of gametes

Each individual makes many gametes, each of which is genetically different. This creates a huge number of potentially different embryos as which two gametes are selected for fertilization is largely random. If the number of different gametes made by both parents is n, therefore the total number of possibilities is n2, which is huge! 2. Independent assortment

During meiosis, chromosomes pair up at the equator (they don’t at during mitosis). Whichever way up the pair are aligned will affect the combination of alleles in the gamete. i.e.

AA BB aa bb

→ →

AB ab

aa BB AA bb

→ →

aB Ab

3. Crossing Over When the chromosomes are paired up during metaphase sections of DNA are swapped between chromatids (this is called crossing over). This means that alleles which were previously linked with others (i.e. in set combinations of alleles) become unlinked, thus increasing the potential number of combinations of alleles



2.3.9

A Mammalian Ovum: Follicle cells (from ovary) Zona Pellucida Cytoplasm Nucleus Lysosomes Lipid droplets Cell membrane

Adaptations: Part of Ovum Nucleus

Adapted for… Contains only one copy of each chromosome (haploid)

Follicle cells

Secrete chemicals that secrete the acrosome reaction

Cytoplasm

Very large so fertilised cell can divide immediately

Lipid droplets

Source of energy for future growth and division

Zona pelludica

Hardens once sperm has entered ova, stops further cells entering.

Lysosomes

Cause the zona pellucida to harden once a sperm’s nucleus has entered the ova.



A Mammalian Sperm:

Adaptations: Part of Ovum Nucleus

Adapted for… Contains only one copy of each chromosome (haploid)

Head

Detachable. Contains the nucleus.

Middle

Contains lots of mitochondria, which make ATP

Tail

Made from motor proteins, which use ATP to propel the sperm forwards

Acrosome

An adapted lysosome on the top of the sperm’s head. The acrosome swells and bursts when the sperm embeds in the zona pellucida (zona pellucida releases chemicals that trigger this). The enzymes in the acrosome digest the follicle cells and the zona pellucida and allow the cell membranes to fuse.

Cytoplasm

Very little cytoplasm, which means cells are small and therefore can be released in large numbers.



2.3.10 Fertilization is the successful fusion of two haploid gametes to create a diploid cell (a zygote). The zygote then divides rapidly by mitosis to become an embryo. Mammalian fertilisation: 1. The sperm is attracted to the ovum by hormones released by the follicle cells surrounding the ovum 2. When the sperm reaches the ovum it embeds its head in the zona pellucida, triggering the acrosome reaction 3. The acrosome swells and bursts, releasing proteolytic enzymes 4. The enzymes digest a hole into the ovum 5. Sperm membrane fuses with ovum membrane and the sperm nucleus enters the ovum by endocytosis 6. Lysosomes in the ovum cause the zona pellucida to harden once the sperm’s nucleus has entered the ovum, stopping further sperm from penetrating the ovum. Plant fertilisation: 1. The pollen grain (male gamete) lands on the stigma 2. Pollen grain grows a pollen tube down into the stigma. The pollen nucleus is at the tip of the tube 3. The pollen tube enters the ovule 4. The pollen tube reaches an ovum and the nucleus enters it by endocytosis forming a zygote. 5. Many pollen grains may fertilize many ova



2.3.11 Stem Cell: an undifferentiated cell (i.e. a cell that can grow into more than one type of cell). Totipotent Cell: an undifferentiated cell capable of growing into a new embryo Pluripotent Cell: an undifferentiated cell capable of growing into any cell, but not a new embryo Multipotent Cell: an undifferentiated cell capable of growing into a few types of cell

Stem Cells are very useful because they can be used to grow replacement organs. However, it is not yet possible to get a differentiated cell to revert to being a stem cell. Therefore, stem cells tend to be harvested from embryos, which causes serious ethical problems. You might like to consider;      

Where do the embyos come from? Is an embryo a human? Do embryos have the same rights as adult humans? Can we use animal embryos (or human-animal hybrids) instead? The utilitarian argument The right to life (for both adult and ambryo)

2.3.12 Dig up your Plant Culture Core Practical notes in the Practical Handbook 2.3.13 Cells become specialized (or differentiated) by progressively switching genes off. This is sometimes done by adding methyl groups to the gene, which stop it being opened in transcription. The



gene is then permanently inactivated in the adult cell, but also in any daughter cell produced through mitosis. In addition, some genes may require a transcription factor to activate them i.e. the gene is normally off, but will be transcribed in the presence of a TF. Usually the TF is a hormone (e.g. Steroids think about their effects), but sometimes it can be an environmental factor (e.g, the presence of lactose in E.coli – see text book)

2.3.14 The phenotype is a product of the genotype and the environment. For some genes the environment has minimal effect (e.g. blood group), but for the majority the environment plays a significant role. You need to know 4 examples Animal Hair Colour:

Some animals have fur colour that is a product of the environment e.g. Siamese cats should have black fur all over as their genotype codes for the enzyme tyrosinase that converts tyrosine into melanin (which is a dark protein – remind yourself of Albimism in 1.2.16). However, the enzyme is denatured by body heat, so only the cold parts of the animal are black (tail, ears etc) and teh rest is white. Human Height:

Is controlled by many genes, each with a range of alleles, making it an example of polygenetic inheritance (i.e. controlled by lots of genes). In addition, diet has a huge effect on height. Mono Amine Oxidase A (MAOA):

MAOA enzymes break down neurotransmitters released by nerves in the brain. High levels of MAOA have been linked to risk-taking and aggression , whereas low levels of MAOA can cause depression



and Parkinsons disease. Mutations of MAOA are the genetic component to these conditions, but environmental factors such as stress levels also have a profound effect. Cancer:

A tumour is a ball of cells dividing quicker than they should. Tumours that split apart and spread around the body ( metastasis ) are the most dangerous (malignant) The rate of cell division is controlled by; Oncogenes – speed cell cycle up Tumour Supressor Genes – slow cell cycle down

Mutations in either of these genes can cause tumours. Although mutations occur naturally, the environment can have an effect e.g. radiation, free radicals, carcinogen chemicals all increase the mutation rate.

2.3.15 Discontinuous variation: phenotypes appear in discrete categories (i.e. blood group). Usually controlled by one gene where the environment has little effect. Continuous variation: phenotypes appear in a range of categories (i.e. height). Usually controlled by many gene (polygenes) where the environment has a large effect.

End of Topic 3



Edexcel AS Revision Unit 22:: Development, Plants & the Environment Topic 4: Biodiversity & Natural Resources 2.4.2 Animal and plant cells are both eukaryotic cells, they have common eukaruyotic features. However, plant cells also have some features unique to them.

Cell wall: A structure made from cellulose fibrils and pectin crosslinks. It strengthens the cell and allows it to be turgid without bursting. Amyloplast: A membrane-bound organelle full of starch (starch grain) Chloroplast: Site of photosynthesis. Vacuole: A water-filled membrane-bound organelle that helps a cell maintain turgor pressure



Tonoplast: The vacuolar membrane Plasmodesmata: A junction between adjacent cells where the cytoplasm of one cell joins the cytoplasm of the other. Used for intercellular communication Pit: A thin patch in the cell wall where plasmodesmata can form or have formed previously Middle lamella: A pectin “glue” attaching one cell wall to another

2.4.3 Plant cells are strong because they are wrapped in a protective layer of cellulose. This forms the cell wall. Cellulose is a polysaccharide made from β glucose monomers. Alternate glucose molecules “flip over” in the chain, forming hydrogen bonds between adjacent cellulose chains. Because cellulose has no side branches the chains can be packed closely which increases the strength of the hydrogen bonds further.

Alternate glucose molecules “flipping over” Individual cellulose chains are packaged together into microfibrils . The microfibrils wind around each other forming cellulose fibres. The cell wall is build from layers of these fibres.



2.4.4 Primary cell wall: First to form, cellulose fibres laid down in the same direction Secondary cell wall: Forms later, cellulose fibres laid down at right angles to those in the primary wall. Provides much greater strength. Collenchyma: found around the outside of the stem have their cell walls further strengthened with more cellulose (secondary thickening) to form thick supporting cells. Sclerenchyma : in larger plants strings of collenchyma begin to lay down the protein lignin in their cell walls to form very strong fibres within the stem. These tend to form as a cap to the vascular bundles in the stem. Sometimes the sclerenchyma can be extracted by humans for making into rope (e.g. hessian) or clothes (e.g. flax or jute).

2.4.5

Vessel Xylem Phloem Sclerenchyma Collenchyma

Location in stem Inside of vascular bundle Middle of vascular bundle Cap on vascular bundle Inside epidermis

Purpose Carries water and minerals up the stem Carries sucrose up & down the stem Support for the stem

Lesser support for the stem



2.4.6 Plant materials are used for three main reasons; 1. Sustainable - they are not a limited resource as, although they are used, they can be replanted. 2. Carbon neutral - do not contribute to rising CO2 levels (although they may give off CO2, replanting uses the CO2 up again). 3. Biodegrade. Plant materials are used as fibres (wood, cotton etc) as they have a high tensile strength and can be used in clothing, building industry etc. Oils from plants can be used as biofuels and starch can be used in packaging, glues, absorbants as well as for food.

2.4.7 Xylem

   

Regular rings of lignin Wide lumen No end walls (continuous tube) Location at inside of VB

Sclerenchyma

    

Irregular layers of lignin Tiny lumen End walls with pits Location as cap to VB Presence of sclerids



2.4.8 Dig up your Celery Fibres Core Practical notes in the Practical Handbook 2.4.9 Mineral Nitrate Calcium Magnesium

Function in plant Used to make amino acids, which the plant uses to form proteins Used to make pectin for cell walls Central ion in the chlorophyll molecule.

Plants also need water for; 1. 2. 3. 4. 5. 6.

Photosynthesis (~5%) Cooling via transpiration (~90%) Transport of substances (e.g. sucrose, mineral ions) Maintain turgor Solvent for chemical reactions Gamete distribution

2.4.10 Dig up your Plant Mineral Deficiencies Core Practical notes in the Practical Handbook 2.4.11 Dig up your Mint / Garlic Core Practical notes in the Practical Handbook 2.4.12 William Withering experimented with foxglove extract as a cure for dropsy (oedema caused by congestive heart failure). He gave the drug in increasing amounts until the patient became ill, then he



worked out a dose based on that. He also killed someone. His studies, though important are NOT ethical and do not follow the basis of modern clinical trials Clinical Trial Process: Stage Pre-clinical testing

Clinical Trials – Phase 1



Purpose of stage 1. Proposed drug is tested in a lab with cultured cells to see the general effects of the drug 2. Proposed drug is given to animals to see the effects on a whole animal. Any side effects away from target cells are noted. 1. A small group of healthy volunteers are given different doses of the drug. They are told what the drug does 2. The distribution, absorbance rate, metabolism & excretion profile of the drug are assessed. 3. The effects of the different doses are assessed to try and determine the optimum dose 4. An independent organisation (UK Medicines Control Agency) assesses whether it is appropriate to move to Phase 2 1. A small group of people with the disease are given the drug. 2. Studies are very similar to Phase 1 3. The optimum dose is worked out 1. A large group of people with the disease are given optimum doses of the drug 2. The patients are either given the drug or a placebo in a double-blind test 3. The results are analysed 4. If the drug has had a significant positive effect in the treatment of the disease it is put forward to licensing authority



2.4.13 Biodiversity: The number of species, the number of individuals within those species and the number and variety of alleles those individuals. Endemic: Where a species is found only within a particular niche in a particular ecosystem. Species richness: is a measure of biodiversity where the number and variety of species in an area is recorded. Can be measured in different ways; •

•

•

•

Indicator species i.e. the presence of specific species (usually those least tolerant to pollution / climate change etc) are used to indicate the “health” of the ecosystem.

Population of keystone species i.e. the population of crucial species (usually those providing prey for the rest) are used to measure the “health” of the ecosystem Quantitative sampling – a direct way of sampling the biodiversity using quadrats Capture / recapture – a direct way of working out populations of species.

Genetic diversity: the number of different alleles within the gene pool. The greater the diversity, the more likely the species is to survive environmental change or disease.

2.4.14 A niche is the specific part of the ecosystem in which a species lives and any adaptations the species has that make it successful there. Adaptations can be; Behavioural e.g. Iguana on the Galapagos islands dive for seaweed they are the only lizards to venture into the sea.



Physiological e.g. some Ethiopians have evolved a different shaped haemoglobin molecule that resembles foetal Hb. It loads O2 much more efficiently at altitude (see end of Topic 1) Anatomical e.g. the Fiddler crab has two very different claws. One is huge and is used for “fiddling” i.e. signalling for a mate. The smaller claw is, rather disappointingly, used for feeding.

2.4.15 Darwin made two observations and a conclusion;

O1: More offspring are born than can survive O2: There is variation within a species Conclusion: There is a “struggle for survival” only the “fittest can survive and reproduce.” This is Natural Selection In order for NS to lead to evolution a few extra conditions are required; 1. Isolation (see table below) so no flow of alleles 2. Small population & inbreeding 3. Mutation (generates new “fitter” alleles) 4. Mutations accumulate within population 5. Eventually the isolated population cannot reproduce with the originals. At this point a new species has formed (speciation)


Method of isolation

Ecological isolation Temporal isolation Behavioural isolation Physical incompatibility Hybrid inviability Hybrid sterility


Description

The species occupy different parts of the habitat The species exist in the same area, but reproduce at different times The species exist in the same area, but do not respond to each other’s courtship behaviour Species coexist, but there are physical reasons which stop them from copulating In some species, hybrids are produces but they do not survive long enough to breed Hybrids survive to reproductive age, but cannot reproduce

Allopatric speciation occurs when species are far from each other Sympatric speciation occurs when species are close to each other

2.4.16 The taxonomic classification system follows a hierarchy of groups (the 5 Kingdoms at the top, individual species at the bottom) in which all species are categorised according to their anatomy. However, this is not necessarily the best approach as species with similar anatomies (e.g. dolphin and shark) are not necessarily closely related. A better system is based on molecular phylogeny i.e. comparing the molecules species are comprised of. The best molecule to examine is DNA. A recent proposal along this line (the three domain theory) argues that all organisms evolved into three broad groups; Bacteria – prokaryotes, fundamentally different structures Archaea – those species that exhibit characteristics of both (i.e. “early eukaryotes” and their descendents) Eukaryotes – eukaryotes, fundamentally different structures



2.4.17 Seed banks and Zoos help because they allow us to preserve biodiversity, reintroduce species, set up captive breeding programmes, educate people about ecology and generate money from tourism.

However, be aware that some species (those that have a lot of leaned behaviours e.g. tigers) do not tend to fare well on reintroduction programmes.

Cohesion-Tension Theory of Transpiration This is not mentioned on the syllabus, but it is in the text book. The prudent man learns it anyway… Epidermis: A single layer of cells often with long extensions called root hairs, which increase the surface area enormously. A single plant may have 1010 root hairs. Cortex: A thick layer of packing cells often containing stored starch.

Stele: contains the Xylem, Phloem & Cambium and is protected by a layer of endodermis cells

epidermis cortex endodermis v a pericycle t i s s s c phloem u u l cambium e a r xylem root hairs



Endodermis: A single layer of tightly-packed cells containing a waterproof layer called the casparian strip. This prevents the movement of water between the cells and helps “waterproof” the stele – keeping water in.

casparian cell cell cytoplasm vacuole strip wall membrane

Water moves through the root by two paths: (i) (ii)

Symplast pathway (10%) Apoplast pathway (90%)



Symplast pathway: water moves from cytoplasm to cytoplasm through plasmodesmata (holes in the cell walls, where cell membranes of adjacent cells are joined - so there are no membranes for the polar water molecules to cross). Apoplast pathway: water moves from cell wall to cell wall. The cell walls are quite thick and very open, so water can easily diffuse through cell walls without having to cross any cell membranes.

However the Apoplast pathway stops at the endodermis because of the waterproof casparian strip. At this point water has to cross the cell membrane and enter the symplast pathway. This effectively water-proofs the Stele, which stops water loss higher up the root. Root Pressure:

Water moves from high water potential to low water potential by osmosis. However, most soils are dry (especially in the desert) and the water potential of the soil is low. Plant cells are full of water, so why doesn’t water leave the plant cells and enter the soil? The answer is that plant roots take up lots of ions from the soil, which lowers the water concentration of their cytoplasm enough for water to enter the root by osmosis, even if the soil if very dry. The ions are taken up by active transport, by proteins in the cell membrane of the root hairs. This uses up lots of energy (uses lots of ATP). Because of the low water potential root cells become very turgid. This creates a small pressure, which forces water up the xylem in the stem. This is called Root Pressure. In small plants root pressure is very important for transpiration. In woody plants it does not have a significant effect.


Xylem Tissue


small xylem vessels (tracheids) large xylem vessel

Xylem tissue is composed of dead cells joined together to form long empty tubes. Different kinds of cells form wide and narrow tubes, and the end cells walls are either full of holes, or are absent completely.

thick cell wall empty interior Transverse Section (T.S.)

Longitudinal Section (L.S.)

lignin rings

Before death the cells form thick cell walls containing lignin, which is laid down in rings, giving these cells a very characteristic appearance under the microscope.

remains of end wall

perforated end walls

Lignin makes the xylem vessels very strong, so that they don’t collapse under pressure, and they also make woody stems strong. The xylem vessels form continuous pipes from the roots to the leaves. Water can move up through these pipes at a rate of 8m h-1, and can reach a height of over 100m. Water molecules are polar and bind to each other by hydrogen bonds forming a strong column of water (for it’s diameter the column is stronger than steel!)

H

1. Water evaporates out of the leaves and causes low pressure in the leaves.

H

2. This creates a suction (or tension) force which sucks water up the stem

H

3. Because water molecules are polar they stick to each other and the entire column of water in the Xylem moves upwards.

H

4. This mechanism is called the cohesion-tension theory.

End of Topic 4

O H O H O H O H

Biology Topic 3 & 4 - Notes

Recommend Documents