# A student runs a short sprint and then continues jogging slowly for several minutes afterwards. Describe what happens in the student's muscle cells during the sprint and explain why the student continues to breathe heavily after stopping the sprint.

> WJEC GCSE Biology (Wales) — 1.2.1 Aerobic and anaerobic respiration · Describe and Explain · 4 marks

## Mark scheme (4 marks)

1. During the sprint, muscles respire anaerobically (because oxygen supply cannot meet demand)
2. Lactic acid is produced as a product of anaerobic respiration
3. An oxygen debt is built up during the sprint that must be repaid
4. Continued heavy breathing provides the extra oxygen needed to oxidise / break down the lactic acid (repaying the oxygen debt)

## Related

- [Revision notes for WJEC GCSE Biology (Wales)](https://www.gradenine.co.uk/learn)
- [How to answer "Describe and Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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