# A weightlifter performs a series of heavy lifts in quick succession. During the first few lifts, the muscles contract using ATP from aerobic respiration. As the intensity increases, the muscles switch to anaerobic respiration and lactic acid begins to accumulate. After the session ends, the weightlifter continues to breathe rapidly for several minutes. Explain why lactic acid accumulates in the muscles during intense exercise and describe what happens to this lactic acid after the exercise has finished.

> Pearson Edexcel A-Level Biology A (9BI0) — 4.2 Muscle contraction · Explain · 5 marks

> Skeletal muscle can respire aerobically and anaerobically. During intense exercise, lactic acid builds up in muscle tissue. After exercise stops, the body works to remove this lactic acid.

## Mark scheme (5 marks)

1. During intense exercise, oxygen supply to the muscles is insufficient / muscles cannot get enough oxygen to meet demand
2. Anaerobic respiration occurs, producing lactic acid from glucose (instead of carbon dioxide and water)
3. Lactic acid accumulates because it is produced faster than it can be removed / broken down
4. After exercise, increased breathing / continued rapid breathing supplies extra oxygen to oxidise / break down the lactic acid (oxygen debt is repaid)
5. Lactic acid is transported (in the blood) to the liver where it is converted back to glucose / broken down

## Key terms

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

## 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)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-weightlifter-performs-a-series-of-131c1861) · Published by Druglandscape Ltd.