# Explain how the arrangement of actin and myosin filaments within a sarcomere accounts for the changes in band and zone appearance observed during 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. The sarcomere contains thick myosin filaments forming the A band and thin actin filaments forming the I bands / Z lines at each end.
2. During contraction, myosin heads bind to actin and pull the thin filaments towards the centre of the sarcomere (power stroke), causing the actin filaments to slide inward.
3. The I band (containing only actin) narrows / shortens because actin filaments slide further into the A band region, reducing the zone where only thin filaments are present.
4. The H zone (central region of the A band containing only myosin) narrows or disappears because the overlapping actin filaments encroach into this zone as they slide inward; the A band width remains constant throughout.

## Key terms

- [sarcomere](https://www.gradenine.co.uk/glossary/sarcomere)

## Related

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-the-arrangement-of-actin-c4e6da56) · Published by Druglandscape Ltd.