A marathon runner is able to complete a 26-mile race using aerobic respiration. However, in the final sprint to the finish line, the runner's muscles switch to anaerobic respiration. Explain why the muscles switch to anaerobic respiration during the sprint and describe the consequences for the runner after the race has finished.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
During the sprint the muscles need ATP very rapidly, faster than aerobic respiration can provide.
Oxygen cannot be delivered to the muscle fibres quickly enough, so the oxygen supply is insufficient.
The muscles therefore use anaerobic glycolysis, which produces lactic acid.
Lactic acid accumulation causes muscle fatigue, pain and cramps.
After the race the runner breathes deeply, taking in extra oxygen to oxidise the lactic acid and repay the oxygen debt.
Oxygen cannot be delivered to the muscle fibres quickly enough, so the oxygen supply is insufficient.
The muscles therefore use anaerobic glycolysis, which produces lactic acid.
Lactic acid accumulation causes muscle fatigue, pain and cramps.
After the race the runner breathes deeply, taking in extra oxygen to oxidise the lactic acid and repay the oxygen debt.
Examiner tips
- Use the exact terms ‘aerobic respiration’, ‘anaerobic glycolysis’, ‘lactic acid’, ‘oxygen debt’
- Show the cause–effect chain: rapid ATP demand → insufficient oxygen → anaerobic pathway → lactic acid → fatigue
- Mention the recovery process: deep breathing, oxygen uptake, oxidation of lactic acid
Common mistakes
- Mixing up aerobic and anaerobic respiration terms
- Failing to explain why oxygen supply is insufficient during the sprint
- Not mentioning the post‑race recovery and oxygen debt repayment
Mark scheme (5 marks)
- During the sprint, muscles require energy (ATP) more quickly than aerobic respiration can supply it
- Oxygen cannot be delivered to the muscles fast enough / oxygen supply is insufficient
- Anaerobic respiration in muscles produces lactic acid
- Lactic acid causes muscle fatigue / pain / cramp in the muscles
- After the race, the runner continues to breathe deeply / takes in extra oxygen to oxidise / break down the lactic acid (to repay the oxygen debt)
Key terms in this question
anaerobic respiration · aerobic respiration
Related
- All AQA A-Level Biology (7402) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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