# 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.

> OCR GCSE Biology A: Gateway Science (J247) — B1.2 What happens in cells (and what do cells need) · Explain · 4 marks

> 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.

## Mark scheme (4 marks)

1. During intense exercise, oxygen is used up faster than it can be supplied / there is an oxygen debt in the muscle cells
2. So anaerobic respiration occurs (in addition to or instead of aerobic respiration)
3. Glucose is broken down to produce lactic acid (during anaerobic respiration in animals)
4. The build-up of lactic acid causes muscle fatigue / aching in the muscles

## Related

- [Revision notes for OCR GCSE Biology A: Gateway Science (J247)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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