Explain how the sliding filament model accounts for muscle contraction, including the roles of actin, myosin and ATP in this process.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Skeletal muscles are made up of muscle fibres containing two types of protein filaments — actin and myosin. When a muscle contracts, these filaments slide past one another, shortening the muscle.
Model answer (5 marks)
Myosin heads bind to actin filaments, forming cross‑bridges. The myosin head then pivots, pulling the actin filament toward the centre of the sarcomere. ATP is required for the myosin head to detach from actin; ATP is hydrolysed to ADP + Pi, releasing energy that re‑cock the myosin head into its high‑energy state. Repeated attachment–pivot–detachment cycles cause the actin filaments to slide past the myosin filaments, shortening the sarcomere and the muscle.
Examiner tips
- Use the sequence: bind → pivot → detach → re‑cock; include ATP hydrolysis step; mention actin and myosin roles explicitly; keep answer concise to fit 5 marks
Common mistakes
- Forgetting to state that ATP hydrolysis re‑cock the myosin head; confusing actin as the moving filament with myosin as the moving filament; not mentioning the role of ATP in detachment.
Mark scheme (5 marks)
- Myosin heads attach to actin filaments, forming cross-bridges
- Myosin heads pivot / pull actin filaments towards the centre of the sarcomere
- ATP provides the energy needed for the myosin head to detach from actin
- ATP is hydrolysed to ADP + Pi, releasing energy to re-cock / reset the myosin head
- Repeated attachment–pivot–detachment cycles cause actin filaments to slide, shortening the overall muscle / sarcomere
Key terms in this question
actin · myosin · sliding filament model · ATP
Related
- All Edexcel A-Level Biology A: Salters-Nuffield (9BN0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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