Explain the role of calcium ions in initiating skeletal muscle contraction following the arrival of an action potential at the neuromuscular junction.

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

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Action potential travels along the T‑tubules into the muscle fibre, triggering the sarcoplasmic reticulum to release Ca²⁺ into the sarcoplasm. Ca²⁺ binds to troponin, causing a conformational change that moves tropomyosin away from the myosin‑binding sites on actin. The exposed actin sites allow myosin heads to attach, forming cross‑bridges, which then perform the power stroke and produce contraction.

Examiner tips

  • Use the word ‘binds’ when describing Ca²⁺ and troponin; mention the movement of tropomyosin; link cross‑bridge formation to the power stroke; keep the sequence logical and concise.

Common mistakes

  • Confusing the role of the T‑tubules with the sarcolemma; omitting the troponin–tropomyosin step; writing that Ca²⁺ binds to actin directly instead of troponin.

Mark scheme (4 marks)

  1. Action potential travels along the T-tubules (transverse tubules) into the interior of the muscle fibre.
  2. The action potential in the T-tubules triggers the sarcoplasmic reticulum to release calcium ions into the sarcoplasm.
  3. Calcium ions bind to troponin, causing a conformational change that moves tropomyosin away from the myosin-binding sites on actin.
  4. Exposure of actin binding sites allows myosin heads to attach, forming cross-bridges, enabling the power stroke and muscle contraction.

Key terms in this question

calcium ions

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