# Explain why muscle cells have large numbers of mitochondria and describe what happens in muscle cells when the supply of oxygen becomes insufficient during vigorous exercise.

> Cambridge International IGCSE Biology (0610) — 12.1 Respiration · Explain and Describe · 4 marks

## Mark scheme (4 marks)

1. Muscle cells require a large amount of energy (for contraction), so they need many mitochondria where aerobic respiration occurs.
2. When oxygen supply is insufficient, muscle cells switch to anaerobic respiration.
3. Glucose is broken down to produce lactic acid (and energy) during anaerobic respiration in muscle cells.
4. Build-up of lactic acid causes muscle fatigue / cramp.

## Key terms

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

## Related

- [Revision notes for Cambridge International IGCSE Biology (0610)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain and Describe" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-muscle-cells-have-large-53630778) · Published by Druglandscape Ltd.