During intense exercise, the muscles of an athlete may switch from aerobic respiration to anaerobic respiration. Explain why anaerobic respiration is less suitable for sustaining prolonged exercise compared with aerobic respiration, and describe what happens in the body after exercise stops to deal with the products of anaerobic respiration.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
An athlete completes a 400 m sprint and immediately feels muscle fatigue and cramp. After the race, the athlete continues to breathe heavily for several minutes.
Model answer (5 marks)
Anaerobic respiration uses glucose to produce ATP without oxygen, giving only 2 mol ATP per glucose, compared with 36–38 mol from aerobic respiration. Because of this low energy yield, it cannot supply the high ATP demand of prolonged exercise.
During anaerobic glycolysis, pyruvate is reduced to lactic acid. The accumulation of lactic acid lowers pH, inhibiting further glycolysis and causing muscle fatigue and cramps, so the activity cannot be sustained.
After exercise stops, lactic acid is carried in the blood to the liver, where it is oxidised back to glucose via the Cori cycle. This reaction requires oxygen, so the athlete continues to breathe heavily to supply the oxygen needed to repay the oxygen debt incurred during the sprint.
During anaerobic glycolysis, pyruvate is reduced to lactic acid. The accumulation of lactic acid lowers pH, inhibiting further glycolysis and causing muscle fatigue and cramps, so the activity cannot be sustained.
After exercise stops, lactic acid is carried in the blood to the liver, where it is oxidised back to glucose via the Cori cycle. This reaction requires oxygen, so the athlete continues to breathe heavily to supply the oxygen needed to repay the oxygen debt incurred during the sprint.
Examiner tips
- State the energy yield difference first to secure the 2 marks for energy comparison.
- Mention lactic acid production and its inhibitory effect to gain the 2 marks on fatigue.
- Explain the Cori cycle and oxygen debt to earn the final mark.
Common mistakes
- Confusing lactic acid with lactate; examiners expect the term lactic acid.
- Failing to mention the oxygen debt or the Cori cycle after exercise.
- Giving a generic “body cleans up” answer without specifying transport to the liver and oxidation.
Mark scheme (5 marks)
- Anaerobic respiration produces lactic acid (in animal/muscle cells)
- Lactic acid build-up inhibits anaerobic respiration / causes cramp and fatigue, so exercise cannot be sustained
- Anaerobic respiration releases less energy than aerobic respiration (from the same amount of glucose)
- After exercise, lactic acid is transported (in the blood plasma) to the liver
- Oxygen is used to oxidise / break down the lactic acid (in the liver), which is why the athlete continues to breathe heavily / an oxygen debt is repaid
Key terms in this question
anaerobic respiration · aerobic respiration
Related
- All Edexcel A-Level Biology A: Salters-Nuffield (9BN0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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