Questions on anaerobic respiration and the respiratory quotient

Question 1

Describe how you would compare the rate of respiration of yeast in aerobic and anaerobic conditions, using respirometers.

Accepted Answer

B1 use the same volume of the same yeast suspension and the same volume and concentration of glucose solution in each, in a water bath at the same temperature, left to equilibrate before readings start B1 anaerobic: boil and cool the glucose solution to remove dissolved oxygen, then cover the suspension with a layer of liquid paraffin or oil B1 aerobic: unboiled solution with a large air space, swirled at intervals so oxygen keeps dissolving B1 run each with soda lime or potassium hydroxide to absorb carbon dioxide, so the movement gives oxygen uptake, and again without it; the difference between the two runs gives the carbon dioxide released B1 measure the distance the fluid moves in a set time, convert to a volume using the capillary's cross-sectional area, express as a rate per minute, and use boiled yeast as a control for each condition

Question 2

Explain why glycolysis can continue in a muscle cell that has run short of oxygen, and explain what the reduction of pyruvate to lactate achieves.

Accepted Answer

B1 no step of glycolysis uses oxygen; what it needs is oxidised NAD, to accept the hydrogen removed when triose phosphate is oxidised B1 without oxygen the electron transport chain cannot oxidise reduced NAD, so oxidised NAD is no longer returned to the cytoplasm and glycolysis would stall within seconds B1 pyruvate accepts the hydrogen from reduced NAD instead, catalysed by lactate dehydrogenase, so pyruvate is reduced to lactate and oxidised NAD is regenerated B1 glycolysis can therefore keep running and keep supplying 2 ATP per glucose, quickly and without waiting for oxygen, so the lactate is a by-product of solving the coenzyme problem rather than the purpose of the pathway

Question 3

Compare the anaerobic pathway followed in an animal muscle cell with the anaerobic pathway followed in yeast.

Accepted Answer

B1 in an animal, pyruvate is reduced to lactate in a single step, whereas in yeast pyruvate is first decarboxylated to ethanal and the ethanal is then reduced to ethanol, which is two steps B1 the animal route keeps all three carbons in the lactate, whereas the yeast route loses one as carbon dioxide, so only the yeast route releases a gas B1 the animal route is reversible, so lactate can be converted back to pyruvate and respired once oxygen returns, whereas the yeast route is irreversible because the carbon has left as gas B1 both routes exist to regenerate oxidised NAD for glycolysis, and both give the same net yield of 2 ATP per molecule of glucose

Question 4

Germinating seeds are placed in a respirometer at 20 °C. With soda lime present the fluid drop moves 50 mm towards the seeds in ten minutes; with the soda lime removed and everything else unchanged it moves 5 mm towards the seeds in ten minutes. The capillary has a cross-sectional area of 1.0 mm². Calculate the respiratory quotient and state which substrate it indicates.

Accepted Answer

M1 with soda lime the carbon dioxide is absorbed as fast as it is made, so the movement measures oxygen uptake alone: 50 × 1.0 = 50 mm³ M1 without soda lime the movement measures oxygen consumed minus carbon dioxide released: 5 × 1.0 = 5 mm³, so the carbon dioxide produced is 50 − 5 = 45 mm³ M1 RQ = volume of carbon dioxide produced ÷ volume of oxygen consumed = 45 ÷ 50 A1 RQ = 0.90, quoted with no units, which indicates protein as the main substrate, or a mixture of substrates

Question 5

A student wants to find the effect of temperature on the rate of respiration in yeast, using methylene blue. Describe how the investigation should be carried out.

Accepted Answer

B1 mix a measured volume of yeast suspension with a measured volume of a substrate such as glucose solution, and leave the tube in a water bath until it has reached the bath temperature B1 add a small measured volume of dilute methylene blue, stopper the tube or layer oil on top, invert once to mix and start timing B1 record the time taken for the blue colour to disappear, judged against the same reference each time, and repeat the whole procedure at a series of temperatures such as 20, 30, 40, 50 and 60 °C B1 keep the volume and concentration of yeast, substrate and methylene blue the same in every tube, and calculate the rate as 1 divided by the time

Question 6

Calculate the respiratory quotient of each of the three organisms in the table, and use the values to identify the respiratory substrate each is mainly using.

Accepted Answer

M1 respiratory quotient = carbon dioxide produced ÷ oxygen used, over the same time A1 wheat 24.0 ÷ 24.0 = 1.00, castor 21.0 ÷ 30.0 = 0.70, larvae 18.0 ÷ 20.0 = 0.90 B1 a quotient of 1.00 means carbohydrate, because glucose respired aerobically gives one carbon dioxide for every oxygen used B1 0.70 means lipid, which is more reduced and so needs more oxygen per carbon dioxide released; 0.90 is between the two and indicates protein, or a mixture

Question 7

Describe what happens to lactate produced by hard-working muscle once it diffuses into the blood, up to and including the point at which glucose is released back into the blood.

Accepted Answer

B1 lactate diffuses from the muscle into the blood and is carried to the liver B1 in the liver, lactate is converted back to pyruvate, the reverse of the reaction that produced it in the muscle B1 the liver then uses ATP to resynthesise glucose from that pyruvate, which needs energy and oxygen B1 the newly made glucose is released into the blood, or stored as glycogen, and can be taken up by the muscle again; this whole route is the Cori cycle, and the extra oxygen the liver uses to carry it out is a large part of why breathing and heart rate stay raised after hard exercise has stopped

Question 8

Rice grows successfully in flooded paddy fields, where the roots and the submerged base of the stem receive little dissolved oxygen. Explain three adaptations that allow rice to survive these conditions.

Accepted Answer

B1 rice develops aerenchyma in its root and stem tissue, loosely packed cells with large interconnected air spaces that form a continuous low-resistance channel down which oxygen can diffuse from the shoot above the water to the submerged cells below B1 where the oxygen supply through aerenchyma still falls short, the affected root cells switch to ethanol fermentation, the same reaction as in yeast, which regenerates the oxidised NAD that keeps glycolysis running without oxygen B1 rice root cells can tolerate concentrations of ethanol produced this way that would injure most other crop plants B1 in deepwater varieties, rising floodwater triggers faster stem elongation, growing quickly enough to keep the upper leaves above the surface so photosynthesis, and the oxygen supply feeding the aerenchyma, can continue

Question 9

Explain how the products of lipid digestion enter the respiratory pathways, and explain why a fatty acid molecule yields far more reduced coenzyme than the glycerol from the same lipid molecule.

Accepted Answer

B1 a triglyceride is hydrolysed to glycerol and fatty acids; the glycerol is phosphorylated and converted to triose phosphate, entering glycolysis part-way down rather than at the start B1 the fatty acids are broken down into two-carbon fragments, and each fragment becomes an acetyl group carried by coenzyme A straight into the Krebs cycle, bypassing glycolysis and the link reaction entirely B1 a single molecule of glycerol is only a three-carbon molecule contributing at one point in glycolysis, whereas a typical fatty acid chain is much longer, such as 16 carbons, giving several acetyl groups, each driving a full turn of the Krebs cycle with its own reduced NAD and reduced FAD B1 so the fatty acid component, being both longer and entering directly into repeated turns of the Krebs cycle, contributes far more reduced coenzyme, and eventually far more ATP, than the single glycerol molecule from the same triglyceride

Question 10

Yeast was kept in two respirometers for 10 minutes, one aerobic and one anaerobic, each run with and without soda lime. The readings in the table were constructed for this question rather than measured. Calculate the rate of carbon dioxide release in each, in cm³ min⁻¹, and the respiratory quotient of the aerobic yeast.

Accepted Answer

M1 aerobic carbon dioxide = oxygen taken in minus the net contraction = 0.50 − 0.02 = 0.48 cm³ in 10 minutes A1 aerobic rate = 0.048 cm³ min⁻¹ and RQ = 0.48 ÷ 0.50 = 0.96 M1 anaerobic: no movement with soda lime means no oxygen is taken in, so the whole expansion without soda lime is carbon dioxide, 0.90 cm³ A1 anaerobic rate = 0.090 cm³ min⁻¹

Question 11

Explain why a gram of lipid releases roughly 39 kJ when it is respired while a gram of carbohydrate releases only about 16 kJ.

Accepted Answer

B1 a fatty acid chain is almost entirely carbon and hydrogen, with very little oxygen already attached, so it is highly reduced and carries many more hydrogen atoms per gram than glucose, which holds an oxygen for every carbon B1 more hydrogen per gram means more reduced NAD and reduced FAD are produced, so more electrons pass along the chain and more protons are pumped into the intermembrane space B1 more protons returning through ATP synthase means more ATP is made per gram; it also means more oxygen is needed, which is why lipid gives a respiratory quotient near 0.70

Question 12

Explain why a respirometer investigation includes a second tube holding glass beads of the same mass, and why the whole apparatus is left to equilibrate in a water bath before the first reading is taken.

Accepted Answer

B1 the tube of glass beads contains nothing that respires, so any movement of fluid in it is caused by changes in room temperature or atmospheric pressure rather than by oxygen uptake B1 its reading is used to correct the reading from the experimental tube, so that a warm afternoon is not recorded as a burst of respiration B1 gas warming up to the temperature of the water bath expands, and that expansion would swamp the small volume change caused by the organisms, so the apparatus is equilibrated before readings begin

Question 13

A sealed flask of yeast, fed on glucose, is found to have a respiratory quotient of 1.6. Suggest what this value shows about the respiration taking place in the flask.

Accepted Answer

B1 carbon dioxide is being released that the oxygen uptake cannot account for, so some of it must come from a reaction that does not consume oxygen B1 the yeast must therefore be respiring anaerobically alongside any aerobic respiration, releasing carbon dioxide when pyruvate is decarboxylated to ethanal B1 aerobic respiration of glucose alone would give a quotient of 1.0, so a value as high as 1.6 suggests the oxygen sealed into the flask is running short

Question 14

In the yeast and methylene blue investigation, a tube of boiled and cooled yeast is set up alongside the others. Explain why methylene blue loses its colour in the untreated tubes, and explain why boiled yeast is the appropriate control.

Accepted Answer

B1 dehydrogenases in the respiring yeast remove hydrogen from the substrate, and the methylene blue accepts it in place of NAD, so the dye is reduced and its blue colour is lost B1 boiling denatures those enzymes, so no hydrogen is transferred and the boiled tube stays blue B1 the boiled tube matches the others in everything else, the same cloudy suspension, substrate, dye volume and temperature, so it differs only in enzyme activity and shows that the colour change was caused by respiration rather than by the medium

Question 15

Compare carbohydrate with protein as a respiratory substrate, referring to where each enters the respiratory pathways and to the respiratory quotient each gives.

Accepted Answer

B1 carbohydrate (glucose) enters at the very start of glycolysis, whereas protein is first hydrolysed to amino acids, which are deaminated in the liver, and what remains of each amino acid enters at various different points, some as pyruvate, some as acetyl CoA, some directly as a Krebs cycle intermediate B1 carbohydrate gives a respiratory quotient of about 1.0, whereas protein gives an intermediate value of about 0.9, reflecting protein's intermediate degree of reduction between carbohydrate and lipid B1 carbohydrate is respired in quantity routinely, whereas protein is respired in quantity mainly when carbohydrate and lipid are in short supply, because the amino acids used are drawn from functional body proteins rather than from a substance stored for the purpose

Question 16

A student connects a flask of yeast in glucose solution to a gas syringe. The flask is aerated and the yeast is respiring aerobically, yet the plunger barely moves in 20 minutes. Explain why.

Accepted Answer

B1 aerobic respiration of glucose takes in one molecule of oxygen for every molecule of carbon dioxide released, an RQ of 1.0 B1 equal numbers of gas molecules occupy equal volumes at the same temperature and pressure, so the oxygen removed is replaced by an equal volume of carbon dioxide B1 so the total volume of gas, which is all a gas syringe measures, stays almost constant; any small fall is because carbon dioxide is more soluble in water than oxygen

Question 17

State the products formed when yeast respires glucose anaerobically, and state the net number of ATP molecules gained per molecule of glucose.

Accepted Answer

B1 ethanol and carbon dioxide are the products of the fermentation itself B1 the net gain is 2 ATP per molecule of glucose, all of it from glycolysis, since neither fermentation step makes any ATP

Question 18

State the approximate energy value of a gram of protein used in A-level data, and state what happens to the amino group of an amino acid before the rest of the molecule can be respired.

Accepted Answer

B1 about 17 kJ per gram B1 it is removed by deamination in the liver and converted to urea, and what is left enters respiration as pyruvate, acetyl CoA or a Krebs cycle intermediate, wherever its carbon skeleton fits

Condition	With soda lime	Without soda lime
Aerobic	contracts 0.50 cm³	contracts 0.02 cm³
Anaerobic	no movement	expands 0.90 cm³

Organism	Oxygen used / cm³ h⁻¹	Carbon dioxide produced / cm³ h⁻¹
germinating wheat grains	24.0	24.0
germinating castor oil seeds	30.0	21.0
blowfly larvae	20.0	18.0