# A long-distance cyclist completes a gruelling race. During the race, the cyclist's muscles work at high intensity for several hours. Explain how the cyclist's muscle cells obtain energy during the race and why the cyclist's breathing rate increases.

> Pearson Edexcel A-Level Biology A (9BI0) — Topic 5 Energy for Life · Explain · 5 marks

> During intense exercise, the human body must rapidly supply energy to contracting muscles. Both aerobic and anaerobic processes may be involved, and the respiratory system responds to meet the body's increased demands.

## Mark scheme (5 marks)

1. Respiration releases energy from glucose (for the muscles to contract / to do work)
2. During high-intensity exercise, aerobic respiration uses oxygen to release energy (from glucose)
3. When oxygen supply is insufficient, anaerobic respiration also occurs in the muscle cells
4. Breathing rate increases to deliver more oxygen to the muscles (for aerobic respiration)
5. Increased breathing rate also removes carbon dioxide (a waste product of respiration) more quickly from the body

## 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)
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