A student runs a short sprint and then continues jogging slowly for several minutes afterwards. Describe what happens in the student's muscle cells during the sprint and explain why the student continues to breathe heavily after stopping the sprint.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
During the sprint the muscle cells use anaerobic respiration because the demand for ATP exceeds the oxygen supply. Glycolysis proceeds and pyruvate is converted to lactic acid, which accumulates in the muscle.
After the sprint an oxygen debt has been created. The body must repay this debt by taking in more oxygen than it needs for normal activity. Heavy breathing increases the oxygen available, which is used to oxidise the lactic acid (forming CO₂ and H₂O) and to restore the ATP stores, thereby repaying the oxygen debt.
After the sprint an oxygen debt has been created. The body must repay this debt by taking in more oxygen than it needs for normal activity. Heavy breathing increases the oxygen available, which is used to oxidise the lactic acid (forming CO₂ and H₂O) and to restore the ATP stores, thereby repaying the oxygen debt.
Examiner tips
- Use the terms ‘anaerobic respiration’, ‘lactic acid’, ‘oxygen debt’ and ‘oxidise’ – these are the key phrases the mark scheme looks for.
- Show the cause–effect link: sprint → anaerobic → lactic acid → oxygen debt → heavy breathing to repay debt.
- Keep the answer concise – 4 marks, so one or two short sentences per point.
Common mistakes
- Confusing aerobic with anaerobic respiration during the sprint.
- Failing to mention the oxygen debt or the role of heavy breathing in repaying it.
- Using vague terms like ‘more oxygen’ without explaining the repayment of the oxygen debt.
Mark scheme (4 marks)
- During the sprint, muscles respire anaerobically (because oxygen supply cannot meet demand)
- Lactic acid is produced as a product of anaerobic respiration
- An oxygen debt is built up during the sprint that must be repaid
- Continued heavy breathing provides the extra oxygen needed to oxidise / break down the lactic acid (repaying the oxygen debt)
Related
- All WJEC GCSE Biology (Wales) revision notes →
- How to answer a "Describe and Explain" question →
- Decode the mark scheme abbreviations →
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