# Explain how the sliding filament mechanism accounts for the shortening of a sarcomere during skeletal muscle contraction, without any change in the length of the thick or thin filaments.

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

## Mark scheme (4 marks)

1. Myosin heads attach to actin monomers on the thin filament, forming cross-bridges.
2. The power stroke causes the myosin heads to pivot/swivel, pulling the thin (actin) filaments towards the centre of the sarcomere (the M-line).
3. The thin filaments from opposite ends of the sarcomere slide over the thick filaments towards each other, so the Z-lines are pulled closer together, shortening the sarcomere.
4. The thick (myosin) and thin (actin) filaments remain unchanged in length throughout; shortening results solely from increased overlap between the two filament types.

## Key terms

- [sarcomere](https://www.gradenine.co.uk/glossary/sarcomere)
- [sliding filament mechanism](https://www.gradenine.co.uk/glossary/sliding-filament-mechanism)
- [thin filament](https://www.gradenine.co.uk/glossary/thin-filament)

## 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)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-the-sliding-filament-mechanism-3cda8f87) · Published by Druglandscape Ltd.