Explain how the ultrastructure of a skeletal muscle cell (myocyte) is adapted for its function.

IB DP Biology Higher Level (2023 syllabus) — B2.3 Cell specialisation · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Abundant mitochondria supply ATP for muscle contraction via aerobic respiration.

Extensive sarcoplasmic reticulum stores and releases calcium ions to trigger contraction.

Multiple peripheral nuclei allow a large cytoplasmic volume to be coordinated and serviced, enabling the cell to be very long.

Actin and myosin filaments are organised into sarcomeres/myofibrils, generating force through the sliding‑filament mechanism.

Examiner tips

  • Use the exact terms (mitochondria, sarcoplasmic reticulum, peripheral nuclei, sarcomeres, sliding‑filament mechanism).
  • Show the link between structure and function for each point.
  • Keep each point concise – one sentence per point.

Common mistakes

  • Confusing the role of mitochondria with that of the sarcoplasmic reticulum.
  • Omitting the peripheral nuclei or describing them as central rather than peripheral.

Mark scheme (4 marks)

  1. Abundant mitochondria provide ATP for muscle contraction via aerobic respiration
  2. Extensive sarcoplasmic reticulum stores and releases calcium ions to trigger contraction
  3. Multiple peripheral nuclei allow a large cytoplasmic volume to be coordinated/serviced, enabling the cell to be very long
  4. Organised arrangement of actin and myosin filaments into sarcomeres/myofibrils generates force through the sliding-filament mechanism

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