# A long-distance cyclist completes a 120 km race. For most of the race the cyclist maintains a steady pace, but during a steep climb near the end, their muscles cannot receive enough oxygen to meet the demand for energy. Explain what happens in the cyclist's muscle cells during the steep climb, and describe what must happen after the race finishes to return the body to its normal state.

> Pearson Edexcel A-Level Biology A (9BI0) — 4.1 Cellular respiration · Explain · 5 marks

> During intense exercise, the cardiovascular system may be unable to deliver oxygen to muscle cells fast enough to sustain aerobic respiration alone.

## Mark scheme (5 marks)

1. During the steep climb, muscle cells switch to anaerobic respiration (because oxygen supply is insufficient)
2. Glucose is broken down to lactic acid (rather than carbon dioxide and water)
3. Less energy (ATP) is released per molecule of glucose compared with aerobic respiration
4. Lactic acid builds up in the muscles / blood, causing muscle fatigue / pain
5. After the race, the cyclist must repay the oxygen debt by continuing to breathe deeply / rapidly so that lactic acid is oxidised / broken down (to carbon dioxide and water)

## Related

- [Revision notes for Pearson Edexcel A-Level Biology A (9BI0)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/a-long-distance-cyclist-completes-a-120-a478bb72) · Published by Druglandscape Ltd.