# Explain how calcium ions regulate skeletal muscle contraction at the level of the thin filament.

> IB DP Biology Higher Level (2023 syllabus) — B3.3 Muscle and motility (HL only) · Explain · 4 marks

## Mark scheme (4 marks)

1. Calcium ions are released from the sarcoplasmic reticulum following depolarisation of the T-tubules / motor neuron stimulation.
2. Calcium ions bind to troponin (on the thin filament), causing a conformational change in the troponin–tropomyosin complex.
3. This conformational change moves tropomyosin away from / shifts tropomyosin along the actin filament, exposing the myosin-binding sites on actin.
4. 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.

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