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.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Myosin heads bind to actin monomers on the thin filament, forming cross‑bridges. The power stroke pivots the myosin heads, pulling the thin (actin) filaments toward the centre of the sarcomere (the M‑line). Thin filaments from opposite ends slide over the thick filaments, increasing overlap and drawing the Z‑lines closer together, which shortens the sarcomere. The thick (myosin) and thin (actin) filaments themselves do not change length; shortening is due only to increased overlap.
Examiner tips
- Use the terms ‘cross‑bridge’, ‘power stroke’, ‘Z‑lines’, ‘M‑line’ and ‘overlap’ as these are explicitly rewarded.
- Show the sequence: attachment → power stroke → sliding → Z‑line movement.
- Mention that filament lengths remain constant to hit the last point.
Common mistakes
- Confusing thick filament movement with thin filament movement; the thick filament stays fixed.
- Omitting that filament lengths do not change, or saying the filaments shorten instead of overlap increasing.
Mark scheme (4 marks)
- Myosin heads attach to actin monomers on the thin filament, forming cross-bridges.
- The power stroke causes the myosin heads to pivot/swivel, pulling the thin (actin) filaments towards the centre of the sarcomere (the M-line).
- 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.
- 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 in this question
sarcomere · sliding filament mechanism · thin filament
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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