A athlete is training indoors on a stationary bicycle. At the start of the session, the athlete pedals at a gentle pace and breathes comfortably. After 20 minutes, the trainer increases the resistance significantly, and the athlete finds they cannot take in oxygen fast enough to meet the muscles' energy demands. Explain what happens in the athlete's muscle cells to continue releasing energy, and describe one consequence of this process for the athlete.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
During intense exercise, muscle cells may not receive enough oxygen to sustain aerobic respiration alone.
Model answer (4 marks)
During the high‑intensity phase the muscle cells switch to anaerobic respiration because oxygen supply is insufficient. Glucose is broken down by glycolysis to pyruvate, which is then converted to lactic acid to regenerate NAD⁺, allowing ATP production to continue. Lactic acid accumulates in the muscle, lowering pH and causing fatigue, pain or aching. Anaerobic respiration yields less ATP per glucose molecule than aerobic respiration, so the overall energy output is lower.
Examiner tips
- Use the term ‘anaerobic respiration’ and ‘lactic acid’ – these are key words. Explain the biochemical shift (glycolysis → lactic acid) and link it to the consequence. Show the efficiency difference to gain the extra mark.
Common mistakes
- Confusing anaerobic respiration with fermentation. Forgetting to mention the accumulation of lactic acid or the reduced ATP yield. Using vague terms like ‘more energy’ instead of specifying ‘less ATP per glucose’.
Mark scheme (4 marks)
- Anaerobic respiration occurs (in the muscle cells) when oxygen supply is insufficient
- Glucose is broken down to produce lactic acid (releasing energy)
- Lactic acid builds up in the muscles causing muscle fatigue / pain / aching
- Less energy (ATP) is released from glucose compared to aerobic respiration / anaerobic respiration is less efficient
Related
- Revision notes: Respiration →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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