# A student goes for a long run. Towards the end of the run, the student's leg muscles begin to feel tired and painful. Explain why the student's leg muscles feel tired and painful, referring to the process of aerobic respiration in your answer.

> Cambridge International IGCSE Biology (0610) — 12.2 Aerobic respiration · Explain · 4 marks

## Mark scheme (4 marks)

1. During vigorous exercise, not enough oxygen is delivered to the muscle cells for aerobic respiration to continue at the required rate.
2. The muscle cells switch to anaerobic respiration, breaking down glucose to produce lactic acid.
3. Lactic acid builds up in the muscles, causing fatigue and pain / cramp.
4. Aerobic respiration releases energy from glucose using oxygen; without sufficient oxygen, less energy is released (anaerobic respiration is less efficient), contributing to muscle fatigue.

## Key terms

- [aerobic respiration](https://www.gradenine.co.uk/glossary/aerobic-respiration)

## Related

- [Revision notes for Cambridge International IGCSE Biology (0610)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/a-student-goes-for-a-long-efcf0002) · Published by Druglandscape Ltd.