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Anaerobic respiration and the respiratory quotient questions
Why glycolysis continues without oxygen and why NAD must be regenerated; lactate fermentation in animals and the Cori cycle; ethanol fermentation in yeast; the yields compared; lipids and proteins as respiratory substrates and why lipid gives more energy per gram; the respiratory quotient with worked values for carbohydrate, lipid and protein; and the respirometer.
7 original questions · 23 marks · the anaerobic respiration and the respiratory quotient notes · Respiration and cellular energy
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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.
Mark scheme
- 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
Compare the anaerobic pathway followed in an animal muscle cell with the anaerobic pathway followed in yeast.
Mark scheme
- 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
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.
Mark scheme
- 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
Explain why a gram of lipid releases roughly 39 kJ when it is respired while a gram of carbohydrate releases only about 16 kJ.
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.
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.
State the products formed when yeast respires glucose anaerobically, and state the net number of ATP molecules gained per molecule of glucose.
Practise anaerobic respiration and the respiratory quotient one question at a time
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