# Explain how the ultrastructure of a cardiac muscle cell (cardiomyocyte) is adapted for its function of continuous, rhythmic contraction.

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

## Mark scheme (4 marks)

1. Abundant mitochondria provide a continuous supply of ATP via aerobic respiration to sustain uninterrupted contraction
2. Well-developed sarcoplasmic reticulum stores and rapidly releases Ca²⁺ ions to trigger contraction of myofibrils
3. Intercalated discs (containing gap junctions) allow rapid electrical coupling between adjacent cardiomyocytes so the heart contracts as a functional syncytium
4. 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

- [cardiomyocyte](https://www.gradenine.co.uk/glossary/cardiomyocyte)

## Related

- [Revision notes for IB DP Biology Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-the-ultrastructure-of-a-123893b6) · Published by Druglandscape Ltd.