A sprinter trains daily to improve their performance. During intense sprint sessions, their leg muscles respire anaerobically. After each sprint, the athlete breathes heavily for several minutes before their breathing rate returns to normal. Explain why the athlete's muscles respire anaerobically during a sprint and why the athlete continues to breathe heavily after the sprint has finished.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A sprinter trains daily to improve their performance. During intense sprint sessions, their leg muscles respire anaerobically. After each sprint, the athlete breathes heavily for several minutes before their breathing rate returns to normal.
Model answer (5 marks)
During a sprint the muscles need ATP very rapidly. The oxygen that can be delivered by the blood is too slow to meet the demand, so the oxygen supply is lower than the oxygen demand.
Because of this oxygen deficit the muscles use anaerobic respiration. Anaerobic respiration does not require oxygen, so it can continue to produce ATP while the oxygen supply is insufficient.
In human muscle cells anaerobic respiration produces lactic acid (lactate) as a by‑product.
After the sprint the athlete continues to breathe heavily because the body must take in extra oxygen to oxidise the lactic acid that has accumulated.
The extra oxygen required to break down the lactic acid is called the oxygen debt; it must be repaid before the breathing rate returns to normal.
Because of this oxygen deficit the muscles use anaerobic respiration. Anaerobic respiration does not require oxygen, so it can continue to produce ATP while the oxygen supply is insufficient.
In human muscle cells anaerobic respiration produces lactic acid (lactate) as a by‑product.
After the sprint the athlete continues to breathe heavily because the body must take in extra oxygen to oxidise the lactic acid that has accumulated.
The extra oxygen required to break down the lactic acid is called the oxygen debt; it must be repaid before the breathing rate returns to normal.
Examiner tips
- Use the exact terms: ‘oxygen supply’, ‘oxygen demand’, ‘anaerobic respiration’, ‘lactic acid’, ‘oxygen debt’.
- Show the causal chain: low oxygen → anaerobic → lactic acid → heavy breathing to repay debt.
Common mistakes
- Saying the muscles use oxygen during a sprint – they actually lack sufficient oxygen.
- Confusing aerobic respiration with anaerobic respiration.
Mark scheme (5 marks)
- During a sprint, oxygen is delivered to the muscles too slowly / oxygen demand exceeds oxygen supply
- Anaerobic respiration does not require oxygen and so allows energy / ATP to be released when oxygen is limited
- Anaerobic respiration in (human) muscle cells produces lactic acid
- After the sprint, the athlete must continue to breathe heavily to take in extra oxygen to break down / oxidise the lactic acid
- The oxygen debt / the amount of extra oxygen required must be repaid before breathing rate returns to normal
Related
- All Pearson Edexcel A-Level Biology A (9BI0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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