A long-distance cyclist begins a race at a steady pace but, towards the end of the race, their muscles begin to ache and feel fatigued. Explain why the type of respiration occurring in the cyclist's muscle cells changes during the race, and describe what happens as a result of this change.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
During intense exercise, the body's demand for energy increases significantly. Muscle cells must adapt the way they release energy depending on the availability of oxygen.
Model answer (4 marks)
During intense exercise the demand for ATP rises, but oxygen delivery to the muscle cells cannot keep pace, creating an oxygen debt.
Because of this oxygen shortage the cells switch from aerobic to anaerobic respiration.
In anaerobic respiration glucose is broken down to lactic acid (lactate + H⁺) to regenerate NAD⁺.
The accumulation of lactic acid lowers the pH of the muscle, producing fatigue and aching.
Because of this oxygen shortage the cells switch from aerobic to anaerobic respiration.
In anaerobic respiration glucose is broken down to lactic acid (lactate + H⁺) to regenerate NAD⁺.
The accumulation of lactic acid lowers the pH of the muscle, producing fatigue and aching.
Examiner tips
- Show the oxygen debt first, then the switch to anaerobic respiration, then lactic acid formation, finally fatigue.
- Use the exact terms: oxygen debt, anaerobic respiration, lactic acid, muscle fatigue.
- Keep the answer in four short points to match the 4 marks.
Common mistakes
- Confusing aerobic with anaerobic respiration; writing only aerobic. Failing to mention the build‑up of lactic acid as the cause of fatigue.
Mark scheme (4 marks)
- During intense exercise, oxygen is used up faster than it can be supplied / there is an oxygen debt in the muscle cells
- So anaerobic respiration occurs (in addition to or instead of aerobic respiration)
- Glucose is broken down to produce lactic acid (during anaerobic respiration in animals)
- The build-up of lactic acid causes muscle fatigue / aching in the muscles
Related
- Revision notes: What happens in cells (and what do cells need) →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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