A2 Biology Notes: Cellular Respiration
Cellular Respiration 1. Outline the need for energy in living organisms, as illustrated by anabolic reactions, active transport, movement and the maintenance of body temperature. Cells use energy for many different purposes. These include a. The synthe synthesis sis of protein proteinss and the other other large molecu molecules les from from smaller smaller ones. ones. These These are examples of anabolic reactions, that is energy-consuming reactions (Anabolic reactions build ne molecules and!or and!or store energy". b. #or active transport of ions and molecules across cell membranes against the concentration gradient. c. #or transm transmiss ission ion of nerve nerve impuls impulse. e. d. #or moveme movement, nt, for exampl examplee muscle muscle contraction contraction (such as heart beat, beat, breathin breathing g movements, al$ing" or movement of cilia. e. %n mammals mammals and and birds, birds, the producti production on of heat heat to maintain maintain body body temperatu temperature re at a steady steady level. f. Activat Activation ion for for glyco glycolysi lysiss of gluco glucose se herei herein n the molec molecule ule is energi&ed by the addition of phosphates. g. 'ight-indepe 'ight-independent ndent reactions reactions of photosynthes photosynthesis is use the the AT and and )A*+ )A*+ synthesi&e synthesi&ed d during a chemical reactions of light-dependent reactions to provide energy for the synthesis of glucose and other organic molecules from inorganic carbon dioxide and ater. . *escribe the structure of AT as a phosphorylated nucleotide. •
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%t consists of adenine (an organic base" and ribose (pentose sugar", hich together ma$e adenosine (a nucleotide". Adenosine is then combined ith phosphate groups to ma$e AT (adenosine triphosphate".
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A2 Biology Notes: Cellular Respiration •
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Adenosine Tri-hosphate or more conveniently AT, is a small molecule made of a carbon ribose sugar, a nitrogenous base Adenine (a purine" and three phosphates. %t is $non as a nucleotide derivative as it is very similar in structure to an /)A nucleotide. The main difference beteen the to is that AT has three phosphates hile the /)A nucleotide only has one phosphate. AT is a source of energy for all living organisms and is referred to as the 0niversal nergy Currency. As AT is used by all cells in an organism, it has a standardi&ed process of releasing or storing energy. AT is therefore a currency of energy that is universally used ithin organisms. AT stores its energy in high energy bonds beteen each of the inorganic phosphates. nergy can be released each time a phosphate is cleaved from the main AT molecule. The energy that is released is done so in small manageable amounts, enough to carry out most metabolic reactions hoever not too much. nergy released in small 2uantities ensures there is no excess energy that ill be asted or that could physically damage cell components. *uring respiration AT can be created by using the energy from respiratory substrates (lipids, carbohydrates and proteins" to add a phosphate to A* to create AT and so creating a high energy bond beteen the to phosphates.
. *escribe the universal role of AT as t he energy currency in all living organisms. •
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AT - A universal energy currency %t acts as immediate donor of energy in the cell3s re2uiring reactions. AT is the universal intermediary molecule beteen energy-yielding reaction and energy re2uiring reactions used in a cell. roperties of AT as energy currency AT is small, ater-soluble molecule. This allos it to be easily transported around the cell. 4hen a phosphate groups is removed from the AT, adenosine diphosphate (A*" is formed and 5.67 mol-1 of energy released. /emoval of a second phosphate produces adenosine monophosphate (A8" and 5. 67 mol-1 of energy released. /emoval of the last phosphate, leaving adenosine, releases only 19.1 67 mol-1. These reactions are all reversible. nergy transfers are inefficient because :ome energy is converted to thermal or heat energy. 8any energy-re2uiring reactions in cells use less energy than that released hich ill change to thermal.
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A2 Biology Notes: Cellular Respiration
Shows the breakdown of ATP releases energy for powering cellular activities in organisms
9. xplain that the synthesis of AT is associated ith the electron transport chain on the membranes of the mitochondrion. Synthesis of ATP nergy for AT synthesis can be become available in to ays; %n respiration energy released by reorgani&ing chemical bond during glycolysis and 6rebs cycle (chemical potential energy" is used to ma$e AT. 8ost AT in cells in general using electrical pot ential energy. •
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Chemiosmosis (electrical potential energy"; %t is the process in hich AT is generated using electrical potential energy. The energy is from the transfer of electrons by electron carriers in the mitochondria. hospholipids membranes in mitochondria are impermeable to hydrogen ions. +ydrogen ions are then alloed to flo don their concentration gradient through protein and part of this protein acts as an en&yme hich synthesi&es AT called AT synthesis. The flo of hydrogen allos the production of one AT molecule provided that A* and inorganic phosphate group ( Pi " are available inside the organelles. AT synthesis has binding sites and a part of the molecules that rotates as hydrogen ion pass, this produce structural changes and allo them to pass se2uentially through phase; a" binding AT and ( Pi ", b" forming tightly bound AT c" releasing AT
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A2 Biology Notes: Cellular Respiration
. Outline glycolysis as phosphorylation of glucose and the subse2uent splitting of hexose phosphate (lycolysis occurs in both pro$aryotic and eu$aryotic cells. A hexose, usually glucose, is split in the process. This splitting actually involves many steps but e can explain it effectively in three stages.
Three Stages of Glycolysis
First: To molecules of AT are used to begin glycolysis. %n the first reaction, the phosphates from the ATs phosphorylate glucose to form fructose-1, <-biphosphate. This process involves phosphorylation.
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A2 Biology Notes: Cellular Respiration
Second: The !"carbon phosphorylated fructose is split into two 3"carbon sugars called glyceraldehydes"3" phosphate #$3P%& This process involves lysis&
Third: Once the to > molecules are formed, they enter an oxidation phase involving AT formation and production of the reduced coen&yme )A*. ach > or triose phosphate molecule undergoes oxidation to form a reduced molecule of )A*?, hich is )A*+. As )A*+ is being formed, released energy is used to add an inorganic phosphate to the remaining -carbon compound. This results in a compound ith to phosphate groups. n&ymes then remove the phosphate groups so they can be added to A* to produce AT. The end result is the formation of 9 molecules of AT, molecules of )A*+, and molecules of pyruvate. yruvate is the ioni&ed form of pyruvic acid.
Summary
A2 Biology Notes: Cellular Respiration
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Summary of glycolysis Two ATPs are used to start the process& A total of 4 ATPs are produced 'a net gain two ATPs& Two molecules of (A)* are produced& hill&com+sites+,,-2,-4-,+student.vie w,+chapter2+animation..how.glycol ysis.works&html
NAD+ is an electron carrier molecule that helps pass energy from glucose to other pathways in a cell by taking high-energy electrons and holding on to them until they can be transferred to other molecules. <. xplain that, hen oxygen is available, pyruvate is converted into acetyl (C" coen&yme A, hich then combines ith oxaloacetate (9C" to form citrate (
@. Outline the 6rebs cycle, explaining that citrate is reconverted to oxaloacetate in a series of small steps in the matrix of the mitochondrion (no further details are re2uired". Once glycolysis has occurred and there is oxygen present, pyruvate enters the matrix of the mitochondrion via active transport. %nside, pyruvate is decarboxylated to form the -carbon acetyl group. This is the lin$ reaction. The removed carbon is released as CO , a aste gas. The acetyl group is then oxidi&ed ith the formation of reduced )A* ?. #inally, the acetyl group combines ith coen&yme A (CoA" to form acetyl CoA. The Link Reaction: Controlled by a system of en&ymes. The greatest significance of this reaction is that it produces acetyl CoA. Acetyl CoA may enter the 6rebs cycle to continue the aerobic respiration process.
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A2 Biology Notes: Cellular Respiration
%f cellular AT levels are lo, the acetyl CoA enters the 6rebs cycle. This cycle is also called the tricarboxylic acid cycle. %t occurs in the matrix of the mitochondrion and is referred to as a cycle because it begins and ends ith the same substance. This is a characteristic of all cyclic pathays in metabolism. ou do not meet to remember the names of all compounds formed in the 6rebs cycle. +oever, it is import ant that you understand the overall process. 'et3s consider the cycle as a series of steps. 1. Acetyl CoA from the lin$ reaction combines ith a 9-carbon compound called oxaloacetate. The result is a <-carbon compound called citrate.
Acetyl /oA combines with o0aloacetate to form citrate
. Citrate (<-carbon compound" is oxidi&ed to form a -carbon compound. %n this process, the carbon is released from the cell (after combining ith oxygen" as carbon dioxide. 4hile the <-carbon compound is oxidi&ed, )A*? is reduced to form )A*+.
Then a "carbon compound is formed
. The -carbon compound is oxidi&ed and decarboxylated to form a 9-carbon compound. Again, the removed carbon combines ith oxygen and is released as carbon dioxide. Another )A*? is reduced to form )A*+
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A2 Biology Notes: Cellular Respiration
(e0t a 4"carbon compound is produced&
9. The 9-carbon compound undergoes various changes resulting in several products. One product is another )A*+. The coen&yme #A* is reduced to form #A*+. There is also a reduction of an A* to form AT. The 9-carbon compound is changed during these steps to re-form the starting compound of the cycle, oxaloacetate. The oxaloacetate may then begin the cycle again.
inally the 4"carbon compound is converted to
A2 Biology Notes: Cellular Respiration
Summary of Kres Cycle
%t is important to remember that the 6rebs cycle ill run tice for each glucose molecule entering cellular respiration. This is because a glucose molecule forms pyruvate molecules. ach pyruvate produces one acetyl CoA hic h enters the cycle. 'oo$ again at the complete 6rebs cycle and note the folloing products hich result from the brea$don of one glucose molecule; a. AT molecules b. < molecules of )A*+ (allo energy storage and transfer" c. molecules of #A*+ d. 9 molecules of carbon dioxide (released" :o far, only 9 ATs have been gained; < are generated (9 from glycolysis and from the 6rebs cycle" but to are used to start the process of glycolysis. ach of these ATs has been produced by substrate-level phosphorylation. 0ltimately, the brea$don of each glucose molecule results in a net gain of < ATs. 'et3s no consider the phase of cellular respiration here most of the ATs are produced. %n this phase, oxidative phosphorylation is the means by hich the ATs are produced. C!"CK T!#S $"%S#T" F&R R"F"R"'C" Animation for Glycolysis( Kres cycle and electron transport chain http;!!highered.mcgrahill.com!sites!55@5@9@5!studentBvie5!chapter!animationBBhoBtheB$rebsBcycleBo r$sBB2ui&B1B.html . xplain that these processes involve decarboxylation and dehydrogenation and describe the role of )A*. *ecarboxylation D removal of carbon from a molecule forming CO *ehydrogenation D removal of hydrogen )A* D can accept hydrogen (reversible" to form reduced )A* ()A*+" )A*? is an electron carrier molecule that helps pass energy from glucose to other pathays in a cell by ta$ing high-energy electrons and holding on to them until they can be transferred to other molecules.
A2 Biology Notes: Cellular Respiration
)icotinamide adenine dinucleotide ( )A*" serves as an electron acceptor in the metabolic pathay $non as glycolysis . 4hen )A* accepts its electrons it also ac2uires a proton (+?"and is converted into )A*+. )A*+ is a reduced electron carrier.
NAD and FAD become NADH and FADH 2respectiely! this is because they become electron carriers. "his happens in the breakdown of Acetyl #oA in the $rebs cycle %aka the citric acid cycle& inside the second membrane of the mitochondria. NADH and FADH 2 will carry and donate the electrons to the 'lectron "ransport #hain on the internal membrane( the transfer of the electron)s energy allows for the proteins crossing the membrane to pump hydrogen ions into the space between the two membranes and build up a gradient for chemiosmosis. "he NADH and FADH 2becomes NAD and FAD again and returns to the $rebs cycle.
)A*+ then becomes oxidi&ed in the first step of electron transport by mitochondrial complex % or )A*+ dehydrogenase. )A*+ contains flavin mononucleotide (#8)" as a bound prosthetic group, hich is responsible for cataly&ing the reaction.
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A2 Biology Notes: Cellular Respiration
E. Outline the process of oxidative phosphorylation, including the role of oxygen (no details of the carriers are re2uired".
Animation5http5++highered&mcgraw" hill&com+sites+,,-2,-4-,+student.view,+chapter2+animation..electron.transport.sys tem.and.atp.synthesis..6ui7.1.&html
a. The hydrogen pic$ed up by )A* and #A* (in glycolysis )A*+, 'in$ reaction )A*+, 6rebs cycle < )A*+ and #A*+" :'%T: by a dehydrogenase en&ymes into electrons and proton. b. The electrons are passed along the TC on the inner membrane of the mitochondrion. c. As the electrons move along the chain, they lose energy. This energy is used to actively transport hydrogen ions (proton" from the matrix of the mitochondrion across the inner membrane (cristae" and into the space beteen the inner and outer membrane. d. The movement of hydrogen ions (proton" in an intermembrane space builds up an electrochemicalgradient or chemiosmosis. e. The hydrogen ions (proton" are alloed to diffuse bac$ into the matrix through ATases (a channel in the cristae that transport protons bac$ to the matrix" hich ill provide energy to ma$e A* combine ith inorganic phosphate to form AT. 11
A2 Biology Notes: Cellular Respiration
f. Then the electrons that are previously split by a dehydrogenase en&yme reunites and combine ith oxygen to produce ater. Oxygen is the final acceptor for the hydrogen removed from the respiratory substrate during glycolysis, lin$ reaction and 6rebs cycle. 15. xplain the production of a small yield of AT from anaerobic respiration and the formation of ethanol in yeast and lactate in mammals, including the concept of oxygen debt.
Anaeroic respiration %f oxygen is not available, oxidative phosphorylation cannot ta$e place, as there is nothing to accept the electrons and protons at the end of the electron transport chain. This means that reduced )A* is not reoxidised, so the mitochondrion 2uic$ly runs out of )A* and #A* that can accept hydrogen from the 6rebs cycle reactions. The 6rebs cycle and the lin$ reaction therefore come to a halt. >lycolysis can still continue so long the pyruvate produced at the end of it can be removed and the reduced )A* can be converted bac$ to )A*. The lactate path)ay
'ocation; cytoplasm :ubstrate; >lucose roduct; lactic acid (lactate" ? AT
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A2 Biology Notes: Cellular Respiration )ote; lactic anaerobic respiration supplements aerobic respiration in the production of AT. Foth aerobic and anaerobic respiration can ta$e place in the human cell at the same time. The lactate that is produced (usually in muscles" diffuses into the blood and is carried in solution in the blood plasma to the liver. +ere, liver cells convert it bac$ to pyruvate. This re2uires oxygen, so extra oxygen is re2uired after exercise has finished. The extra oxygen is $non as the oxygen debt. 'ater hen the exercise has finished and oxygen is available again, some of the pyruvate in the liver cells is oxidi&ed through the lin$ reaction, the 6rebs cycle and the electron transport chain. :ome of the pyruvate is reconverted to glucose in the liver cells. The glucose may be released into the blood or converted to glycogen and stored. The ethanol Path)ay %n yeast and in plants, the pyruvate is removed by converting it to ethanol.
'ocation; cytoplasm :ubstrate; >lucose roduct; thanol ? carbon dioxide ? AT This is the end point for this fermentation reaction. thanol and COare both excreted ith no further metabolism of the energy stored in the ethanol (very inefficient" )ote; The glucose molecule has been hydrolysed further than in human respiration. :ome organisms are totally anaerobic others can sitch beteen anaerobic and aerobic. #ermentation respiration in yeast yields to useful products from a human perspective. The carbon dioxide can be used in a variety industrial processes the best $non of hich is to raise bread. 8any Freers of alcohol ill bottle the CO for use in the GcarbonationG of other drin$ products. The alcohol itself is of course the basis of many industries such as beer breing. %n more recent time the use of fermentation products is being used as an alternative source of fuel such as is the case in fuel for automobiles. ATP yield in aeroic and anaeroic respiration Only small amounts of AT are produced hen one glucose molecule undergoes anaerobic respiration. This is because only glycolysis is completed. The 6rebs cycle and oxidative phosphorylation, hich produce most AT, do not ta$e place. The precise number of molecules of AT produced in aerobic respiration of one glucose varies beteen different organisms and different cells, but is usually beteen 5 and molecules
11. xplain the relative energy values of carbohydrate, lipid and protein as respiratory substrates. The greater number of hydrogen present, the greater the energy value. 'ipids have higher energy density than carbohydrates.Could use a calorimeter to burn substances to compare the rise in temperature. • • •
The more hydrogens, the more AT is produced in the electron transport chain :ome molecules have more hydrogens than others The more hydrogen atoms there are in a respiratory substrate, the more AT is produced
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A2 Biology Notes: Cellular Respiration •
%f there are more hydrogen atoms per mole (fixed amount" of substrate, the more oxygen is needed to be the final acceptor
nergy values of different respiratory substrates /espiratory substrate nergy released!$7g-1 Carbohydrates 1< 'ipid E rotein 1@ 'ipid provides more than tice as much as energy per gram as carbohydrate or protein. This is because of lipid molecule contains relatively more hydrogen atoms (in comparison ith carbon or oxygen atoms" than carbohydrate or protein molecules do. %t is hydrogen that are used to generate AT via the electron transport chain 1. *efine the term (l" respiratory 2uotient (/H". /espiratory 2uotient D is the ratio of the volume of carbon dioxide released to the volume of oxygen consumed by a body tissue or an organism in a given period.
)ote; if /H of animal is 5.E - mainly suggest that the animal is metaboli&ing protein molecule. 'ipid /H; 5.@ >lucose; 1 1. IAJ Carry out investigations, using simple respirometers, to measure /H and the effect of temperature on respiration rate. /espirometer measures volume of oxygen used by organism • • •
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:oda lime absorbs carbon dioxide produced by aerobic respiration. Kolume of oxygen used is measured in manometer capillary tube. 8easure distance travelled by meniscus over time. As area of capillary tube is $non, can calculate mean rate of oxygen upta$e in mmmin-1 %mportant to e2uilibrate e2uipment!organism to temperature 0se a control to ma$e sure differences not due to temperature!pressure %f testing plants, then need to be in dar$ to prevent photosynthesis.
*ehydrogenase - an en&yme that cataly&es the removal of hydrogen from a substrate and the transfer of the hydrogen to an acceptor in an oxidation-reduction reaction.
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A2 Biology Notes: Cellular Respiration
'ab bench on respiration http;!!.phschool.com!science!biologyBplace!labbench!lab!features.html
Practical Acti*ity
8easuring respiratory 2uotient The purpose of this activity is; •
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to use a respirometer to measure the oxygen upta$e of some respiring plant or animal material to use the respirometer again to measure the net volume of gas exchanged by the respiring material and hence calculate the volume of carbon dioxide given out by the material
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to calculate the respiratory 2uotient (/H" for the respiring material
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to use the /H as evidence of the respiratory substrate in use
Procedure otassium hydroxide and soda lime are corrosive. 4ear goggles hen handling and see$ first aid immediately if any gets in your eyes. ush the parts of the e2uipment together firmly but gently to get airtight seals, but reduce the ris$ of brea$ing any glass apparatus and inLuring yourself. Preparation 1. 0se a funnel to pour cm of potassium hydroxide solution (corrosive" into each respirometer vessel. 8a$e sure none of the potassium hydroxide touches the sides of the vessels. . Add small rolls of filter paper to act as ic$s. . #ill the bas$et or cage ith respiring material and put it into vessel F. 8a$e sure that the seeds or invertebrates are not touching the potassium hydroxide or the ic$. Add ater to vessel A to match the volume of respiring material in vessel F (see diagram overleaf".
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A2 Biology Notes: Cellular Respiration
9. #it vessel A ith a bung holding to connecting tubes D one ith a scre clip on flexible tubing. Alternatively fit a bung ith a -ay tap connected to the same items. . #it vessel F ith a bung holding a 1 cm syringe and a connecting tube as shon in the diagram. Alternatively fit a bung ith a -ay tap connected to the syringe and tube. <. *ra some coloured fluid into the manometer 0-tube. The fluid must be free of bubbles and come to about the middle of the scale on each side. @. Open the scre clip and remove the syringe, then connect the manometer 0tube. To chec$ that the apparatus is airtight, move the mar$er fluid in the manometer to one end ith the syringe and leave for a fe minutes. The fluid should not move. . :et the piston of the syringe at about the 5. cm mar$ and insert the syringe as shon. Close the scre clip. 0se the syringe to adLust the manometer so that the fluid levels are the same on both sides.
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A2 Biology Notes: Cellular Respiration E. /ecord the exact position of the syringe piston, the position of the menisci on both sides of the manometer, and the time. #n*estigation 15. /ecord ne positions of the manometer fluid at regular intervals for 5 minutes. 4hen it nears the end of the scale on one side, restore it to its original position and note the ne position of the syringe piston. 11. #ind the amount of oxygen absorbed by germinating seeds in a period of 5 minutes at 5 MC. This is Vol1. 1. /emove the potassium hydroxide solution from both vessels and ash them out ith ater. 1. /eplace the bas$et containing seeds or invertebrates in one vessel, an e2uivalent volume of ater in the other vessel and the bungs in both. :et up the respirometer a 5 MC again and record any increase or decrease in gas volume over the next 5 minutes. This is Vol2. 19. Calculate the volume of carbon dioxide produced. 1. Calculate the respiratory 2uotient.
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0sing - different organisms - They should be the same in mass, conduct the experiment in a temperature-controlled room (air-conditioned room". - %f handling algae or pond eed - $eeping them in a container and in the dar$. Alays include /%:6 A::::8)T in your ritten procedure. ou may include the use of thermostatically controlled ater bath to maintain temperature. A change in temperature ill cause a direct change in volume. Fecause the temperature in the respirometersmay vary during the course of the experiment, you must correct for differences in volume that are due to temperature fluctuation rather than rate of respiration. To do this, subtract any difference in the movement of ater into the vial ith glass beads from the experimental vials held at the same temperature. /ecord the result as the corrected difference. nsure apparatus is airtight 8ention the ord /AT or /'%CAT at least times. 1-
A2 Biology Notes: Cellular Respiration •
ou can also provide control in your experiment by using a dead organism or a glass beads for comparison
+o to calculate the rate of respiration (oxygen upta$e" http;!!.phschool.com!science!biologyBplace!labbench!lab!measure.html After you have collected data for the amount of oxygen consumed over time by germinating and nongerminating peas at to different temperatures, you can compare the rates of respiration. 'etGs revie ho to calculate rate.
/ate N slope of the line, or %n this case, y is the change in volume, and x is the change in time (15 min". Or divide volume by mass or divide O by time. How to calculate rate of respiration or oxygen uptake Collect the date by taking the diameter of the capillary tube and its volume. Divide volume with its diameter then multiply with the distance travelled by the water/ air or dye in the capillary tube in a given time. Or Divide volume of O 2 by mass to give you the unit cm 3/g Or Divide of O2 by time to give you the unit cm3/min Or Volume of O2 divided by time times mass cm 3 s!" g!"
How to calculate the rate of carbon dioxide #eigh the CO2 absorbent before and after the e$periment Calculate the difference Divide the difference in distance over volume by time
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A2 Biology: Cellular Respiration
Anglo Singapore International School
QUESTIONS
%se this table to record your results and guide your calculations. Compare your results with those of others in your class.
&et volume change in 3' minutes with carbon dio$ide absorbed o$ygen taken in
&et volume change in 3' minutes with no CO2 absorbed
(mount of carbon dio$ide absorbed
Vol1
Vol2
Vol1 - Vol2
)espiratory *uotient +carbon dio$ide produced,/ +o$ygen absorbed,
+Vol1 - Vol2 , / Vol1
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#hat respiratory substrate would you e$pect to f ind in seeds
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Does your value for ) support your thinking
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#hat tests can you think of that would allow you to find out what substrates are present
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#hat would you e$pect to be the ) of a growing culture of yeast
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0uggest three different e$planations for respiring material in this test producing an ) of ".'.
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A2 Biology: Cellular Respiration
Anglo Singapore International School
&NS'E(S
Calculations1 this will depend on e$perimental results.
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0eeds such as sunflower contain oils. Other seeds contain more carbohydrates. (ll contain proteins raw materials for the initial growth of seedlings.
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(n ) value around '. is what you would e$pect for oils.
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4ou could carry out standard food tests to confirm the components of seeds. To test 3 for oils: Crush the seeds and shake with about " cm of ethanol. %se a pipette to collect some of the ethanol and drip into water. 5f the water becomes cloudy as an emulsion forms6 this suggests the presence of oils or fats in the seeds. To test for starch: crush a couple of seeds and add to a drop of iodine solution on a dimple tile. ( blue!black colour indicates the presence of starch.
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5f the yeast culture is growing rapidly and respiring anaerobically6 it will take in very little o$ygen and so the ) will be very large and effectively meaningless.
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7hree e$planations for respiring material in this test producing an overall ) of ".' are1 •
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aerobic respiration of a carbohydrate substrate6 aerobic respiration of two substrates together one with an ) above ".' and the other with an ) below ".'6 aerobic respiration of a substrate with a respiratory *uotient less than ".' and anaerobic respiration of another substrate.
http;!!.phschool.com!science!biologyBplace!biocoach!index.html
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