# Explain how ATP is used during the cross-bridge cycle in skeletal muscle contraction.

> IB DP Biology Higher Level (2023 syllabus) — B3.3 Muscle and motility (HL only) · Explain · 4 marks

## Mark scheme (4 marks)

1. ATP binds to the myosin head, causing it to detach from the actin filament (breaking the cross-bridge).
2. ATP is hydrolysed (to ADP + Pi) by myosin ATPase, providing energy that cocks/re-energises the myosin head into the high-energy position.
3. The cocked myosin head (with ADP + Pi bound) attaches to actin, forming a new cross-bridge; release of Pi triggers the power stroke.
4. ATP is also required by the sarcoplasmic reticulum Ca²⁺-ATPase pumps to actively transport Ca²⁺ back into the sarcoplasmic reticulum, ending contraction.

## Key terms

- [cross-bridge cycle](https://www.gradenine.co.uk/glossary/cross-bridge-cycle)

## Related

- [Revision notes for IB DP Biology Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-atp-is-used-during-905322fd) · Published by Druglandscape Ltd.