A sprinter completes a 100-metre race and then continues to breathe heavily for several minutes afterwards. Explain why the sprinter's muscles switched to anaerobic respiration during the race and describe what happens in the muscle cells after the race to recover from this.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
During intense exercise, the cardiovascular system cannot always deliver oxygen to skeletal muscle cells fast enough to meet their energy demands.
Model answer (5 marks)
During the 100‑metre sprint the demand for ATP rises sharply. The heart and lungs cannot supply oxygen fast enough, so the oxygen supply falls below the oxygen demand. Consequently the muscle cells switch to anaerobic respiration to keep producing ATP.
Anaerobic respiration in muscle cells produces lactate (lactic acid) as a waste product.
After the race the lactate must be oxidised back to pyruvate and then to CO₂ and H₂O. This oxidation requires oxygen, creating an oxygen debt. The sprinter therefore breathes heavily after the race to take in the extra oxygen needed to repay this debt and to clear the lactate.
This explains both the switch to anaerobic metabolism during the sprint and the heavy breathing afterwards.
Anaerobic respiration in muscle cells produces lactate (lactic acid) as a waste product.
After the race the lactate must be oxidised back to pyruvate and then to CO₂ and H₂O. This oxidation requires oxygen, creating an oxygen debt. The sprinter therefore breathes heavily after the race to take in the extra oxygen needed to repay this debt and to clear the lactate.
This explains both the switch to anaerobic metabolism during the sprint and the heavy breathing afterwards.
Examiner tips
- Show the cause (oxygen supply < demand) and effect (anaerobic ATP, lactate) in one sentence; then explain the oxygen debt and recovery in a second sentence.
- Use the exact terms ‘anaerobic respiration’, ‘lactate’, ‘oxygen debt’ and ‘breath heavily’ as the mark scheme rewards.
- Keep the answer to 5–6 short sentences – no extra explanation or filler.
Common mistakes
- Writing ‘glycolysis’ instead of ‘anaerobic respiration’ or omitting lactate; forgetting to mention the oxygen debt; using ‘respiration’ without specifying anaerobic; adding irrelevant details such as muscle fatigue or muscle fibre type.
Mark scheme (5 marks)
- During intense exercise, oxygen cannot be delivered to muscle cells fast enough / oxygen demand exceeds supply
- So muscles respire anaerobically to continue producing ATP / energy for contraction
- Anaerobic respiration produces lactic acid (lactate) as a waste product in the muscles
- After the race, lactic acid must be broken down / oxidised, which requires extra oxygen — this is the oxygen debt
- This is why the sprinter continues to breathe heavily after the race — to take in the extra oxygen needed to repay the oxygen debt
Key terms in this question
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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