Explain how calcium ions regulate skeletal muscle contraction at the level of the thin filament.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Calcium ions are released from the sarcoplasmic reticulum after depolarisation of the T‑tubules or motor‑neuron stimulation.
The released Ca²⁺ binds to troponin on the thin filament, causing a conformational change in the troponin–tropomyosin complex.
This change shifts tropomyosin along the actin filament, moving it out of the myosin‑binding sites and exposing those sites.
With the binding sites exposed, myosin heads can attach to actin, forming cross‑bridges and allowing the power stroke to occur, leading to contraction.
The released Ca²⁺ binds to troponin on the thin filament, causing a conformational change in the troponin–tropomyosin complex.
This change shifts tropomyosin along the actin filament, moving it out of the myosin‑binding sites and exposing those sites.
With the binding sites exposed, myosin heads can attach to actin, forming cross‑bridges and allowing the power stroke to occur, leading to contraction.
Examiner tips
- Use the exact sequence of events: release → binding → conformational change → exposure → cross‑bridge formation.
- Show the role of each protein (tropomyosin, troponin, actin, myosin).
- Keep the answer concise – 4 points, one sentence each.
Mark scheme (4 marks)
- Calcium ions are released from the sarcoplasmic reticulum following depolarisation of the T-tubules / motor neuron stimulation.
- Calcium ions bind to troponin (on the thin filament), causing a conformational change in the troponin–tropomyosin complex.
- This conformational change moves tropomyosin away from / shifts tropomyosin along the actin filament, exposing the myosin-binding sites on actin.
- Myosin heads can now attach to the exposed binding sites on actin, forming cross-bridges and allowing contraction to proceed / the power stroke to occur.
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 the structure of a myosin head enables it to function as a molecular…
- Explain how the length–tension relationship of a sarcomere determines the force …