A long-distance cyclist completes a 120 km race. For most of the race the cyclist maintains a steady pace, but during a steep climb near the end, their muscles cannot receive enough oxygen to meet the demand for energy. Explain what happens in the cyclist's muscle cells during the steep climb, and describe what must happen after the race finishes to return the body to its normal state.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
During intense exercise, the cardiovascular system may be unable to deliver oxygen to muscle cells fast enough to sustain aerobic respiration alone.
Model answer (5 marks)
During the steep climb the oxygen supply to the muscle cells falls below the demand for ATP, so the cells switch from aerobic to anaerobic respiration.
In anaerobic glycolysis glucose is cleaved to two molecules of pyruvate and then reduced to lactic acid, producing only 2 ATP per glucose instead of the 30–32 ATP from aerobic respiration.
The lactic acid accumulates in the muscle and bloodstream, lowering pH and producing the burning sensation and fatigue that the cyclist feels.
After the race the body must repay the oxygen debt. The cyclist breathes rapidly and deeply, increasing oxygen delivery to the muscles. The excess lactic acid is then oxidised back to pyruvate and finally to CO₂ and H₂O, restoring normal pH and energy levels.
In anaerobic glycolysis glucose is cleaved to two molecules of pyruvate and then reduced to lactic acid, producing only 2 ATP per glucose instead of the 30–32 ATP from aerobic respiration.
The lactic acid accumulates in the muscle and bloodstream, lowering pH and producing the burning sensation and fatigue that the cyclist feels.
After the race the body must repay the oxygen debt. The cyclist breathes rapidly and deeply, increasing oxygen delivery to the muscles. The excess lactic acid is then oxidised back to pyruvate and finally to CO₂ and H₂O, restoring normal pH and energy levels.
Examiner tips
- Mention the switch to anaerobic respiration and the reduced ATP yield; link lactic acid build‑up to fatigue; explain oxygen debt repayment via increased breathing and lactic acid oxidation.
- Use the exact terms ‘anaerobic respiration’, ‘lactic acid’, ‘oxygen debt’, ‘CO₂ and H₂O’ – these are the words the scheme rewards.
- Show the sequence: oxygen shortage → anaerobic glycolysis → lactic acid → fatigue → post‑race breathing → oxidation of lactic acid back to CO₂/H₂O.
- Keep the answer concise – 5 marks, so one or two sentences per point.
Common mistakes
- Confusing anaerobic respiration with fermentation; students say ‘fermentation’ instead of ‘anaerobic glycolysis’.
- Failing to mention the reduced ATP yield or the oxygen debt repayment; they only describe lactic acid build‑up.
Mark scheme (5 marks)
- During the steep climb, muscle cells switch to anaerobic respiration (because oxygen supply is insufficient)
- Glucose is broken down to lactic acid (rather than carbon dioxide and water)
- Less energy (ATP) is released per molecule of glucose compared with aerobic respiration
- Lactic acid builds up in the muscles / blood, causing muscle fatigue / pain
- After the race, the cyclist must repay the oxygen debt by continuing to breathe deeply / rapidly so that lactic acid is oxidised / broken down (to carbon dioxide and water)
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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