# A cyclist rides at a steady pace for 30 minutes, then suddenly increases to maximum effort for a final 2-minute sprint to the finish line. During the steady pace section, the cyclist's muscles respire aerobically. During the sprint, the muscles switch to anaerobic respiration. After crossing the finish line, the cyclist continues to breathe heavily for several minutes. Explain why the muscles switch to anaerobic respiration during the sprint, and why the cyclist continues to breathe heavily after finishing.

> Edexcel A-Level Biology A: Salters-Nuffield (9BN0) — 4.1 Cellular respiration · Explain · 5 marks

> Aerobic respiration requires a continuous supply of oxygen. During intense exercise, the rate of oxygen delivery to muscles may not keep pace with demand.

## Mark scheme (5 marks)

1. During the sprint, muscles require energy (ATP) at a faster rate than can be supplied by aerobic respiration alone
2. Oxygen cannot be delivered to the muscles fast enough to maintain aerobic respiration
3. Anaerobic respiration in the muscles produces lactic acid (as well as releasing energy/ATP)
4. After exercise, extra oxygen is needed to break down / oxidise the lactic acid that has built up — this is the oxygen debt
5. Continued heavy breathing supplies the extra oxygen needed to repay the oxygen debt / remove lactic acid

## Key terms

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

## Related

- [Revision notes for Edexcel A-Level Biology A: Salters-Nuffield (9BN0)](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)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-cyclist-rides-at-a-steady-91df53ff) · Published by Druglandscape Ltd.