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.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The sarcomere consists of thick myosin filaments that form the A band and thin actin filaments that form the I band and the Z lines at each end. During contraction the myosin heads bind to actin and, via the power stroke, pull the thin filaments toward the centre of the sarcomere. This sliding of actin inward shortens the I band because the zone where only thin filaments are present becomes narrower. The H zone, the central part of the A band that contains only myosin, also narrows or disappears because the overlapping actin filaments encroach into this region. The overall width of the A band remains unchanged because the length of the myosin filaments does not change.
Examiner tips
- Use the term ‘sliding filament theory’ to link actin–myosin interaction with band changes.
- Show that the A band stays the same width – this is a key point in the mark scheme.
- Mention the H zone specifically – many students omit it.
Common mistakes
- Confusing the A band with the I band or saying the A band shortens.
- Failing to explain that the H zone disappears because actin overlaps myosin.
- Using vague language such as ‘filaments move’ without specifying actin sliding inward.
Mark scheme (4 marks)
- The sarcomere contains thick myosin filaments forming the A band and thin actin filaments forming the I bands / Z lines at each end.
- 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.
- 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.
- 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 in this question
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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