Explain how the structure of a myosin head enables it to function as a molecular motor during skeletal muscle contraction.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The myosin head has an actin‑binding site that allows it to attach to actin, forming a cross‑bridge. It also contains an ATPase active site that hydrolyses ATP to ADP and Pi, providing the energy for the power stroke. When ATP is hydrolysed, the myosin head pivots into a cocked, high‑energy conformation, ready to perform the power stroke. Release of Pi triggers the power stroke: the myosin head rotates about 45°, pulling the actin filament toward the centre of the sarcomere, after which ADP is released.
Examiner tips
- Use the term ‘cross‑bridge’ and ‘power stroke’ – these are key words the scheme rewards.
- Show the sequence: actin binding → ATP hydrolysis → cocked state → Pi release → power stroke → ADP release.
- Mention the ~45° rotation to demonstrate understanding of the conformational change.
- Keep the answer concise – 4 marks, so one sentence per point is sufficient.
Common mistakes
- Forgetting to mention the actin‑binding site or the ATPase site.
- Confusing ATP hydrolysis with ADP release – they are separate steps.
- Using vague language such as ‘moves’ instead of ‘rotates 45°’ or ‘pulls actin’.”
Mark scheme (4 marks)
- The myosin head contains an actin-binding site that allows it to attach to actin, forming a cross-bridge.
- The myosin head has an ATPase active site that hydrolyses ATP to ADP and inorganic phosphate (Pi), providing the energy for the power stroke.
- On hydrolysis of ATP, the myosin head pivots into a cocked (high-energy) conformation, ready to perform the power stroke.
- Release of Pi triggers the power stroke: the myosin head changes conformation (rotates ~45°), pulling the actin filament towards the centre of the sarcomere, after which ADP is released.
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 →
More Muscle and motility (HL only) questions
- Explain how the arrangement of actin and myosin filaments within a sarcomere acc…
- Explain how ATP is used during the cross-bridge cycle in skeletal muscle contrac…
- Explain how a skeletal muscle fibre relaxes after stimulation ceases.
- Explain how the sliding filament mechanism accounts for the shortening of a sarc…
- Explain how calcium ions regulate skeletal muscle contraction at the level of th…
- Explain how the length–tension relationship of a sarcomere determines the force …