Explain how the ultrastructure of a cardiac muscle cell (cardiomyocyte) is adapted for its function of continuous, rhythmic contraction.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Abundant mitochondria provide a continuous supply of ATP via aerobic respiration to sustain uninterrupted contraction.
Well‑developed sarcoplasmic reticulum stores and rapidly releases Ca²⁺ ions to trigger contraction of myofibrils.
Intercalated discs, containing gap junctions, allow rapid electrical coupling between adjacent cardiomyocytes so the heart contracts as a functional syncytium.
Highly organised myofibrils, with repeating sarcomeres of actin and myosin filaments, generate the force of contraction, and T‑tubules ensure rapid conduction of the action potential deep into the cell to coordinate myofibril activation.
Well‑developed sarcoplasmic reticulum stores and rapidly releases Ca²⁺ ions to trigger contraction of myofibrils.
Intercalated discs, containing gap junctions, allow rapid electrical coupling between adjacent cardiomyocytes so the heart contracts as a functional syncytium.
Highly organised myofibrils, with repeating sarcomeres of actin and myosin filaments, generate the force of contraction, and T‑tubules ensure rapid conduction of the action potential deep into the cell to coordinate myofibril activation.
Examiner tips
- Use the exact terminology (mitochondria, sarcoplasmic reticulum, intercalated discs, gap junctions, myofibrils, sarcomeres, T‑tubules).
- Show the link between structure and function for each feature.
- Keep each point concise – one sentence per feature.
- Include the word "continuous" to match the question wording.
Common mistakes
- Mixing up the role of the sarcoplasmic reticulum (storage vs release of Ca²⁺).
- Omitting the T‑tubules or describing them incorrectly.
Mark scheme (4 marks)
- Abundant mitochondria provide a continuous supply of ATP via aerobic respiration to sustain uninterrupted contraction
- Well-developed sarcoplasmic reticulum stores and rapidly releases Ca²⁺ ions to trigger contraction of myofibrils
- Intercalated discs (containing gap junctions) allow rapid electrical coupling between adjacent cardiomyocytes so the heart contracts as a functional syncytium
- Highly organised myofibrils (with repeating sarcomeres of actin and myosin filaments) generate the force of contraction, and T-tubules (transverse tubules) ensure rapid conduction of the action potential deep into the cell to coordinate myofibril activation
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 →
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