Explain how changes in membrane permeability to sodium and potassium ions bring about depolarisation and repolarisation during an action potential.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Voltage‑gated Na⁺ channels open when the membrane depolarises, raising Na⁺ permeability. Na⁺ rushes into the cell along its electrochemical gradient, making the inside more positive – this is depolarisation.
The Na⁺ channels then inactivate/close and voltage‑gated K⁺ channels open, increasing K⁺ permeability. K⁺ diffuses out down its gradient, returning the inside to a negative charge – repolarisation (and a brief hyperpolarisation). The Na⁺/K⁺ pump later restores the resting potential.
The Na⁺ channels then inactivate/close and voltage‑gated K⁺ channels open, increasing K⁺ permeability. K⁺ diffuses out down its gradient, returning the inside to a negative charge – repolarisation (and a brief hyperpolarisation). The Na⁺/K⁺ pump later restores the resting potential.
Examiner tips
- Use the exact terms ‘voltage‑gated’, ‘inactivate’, ‘electrochemical gradient’, and ‘repolarisation’.
- Show the sequence: Na⁺ influx → depolarisation → Na⁺ channel inactivation → K⁺ efflux → repolarisation.
- Mention the brief hyperpolarisation and the Na⁺/K⁺ pump as part of the full cycle.
Common mistakes
- Confusing depolarisation with hyperpolarisation; forgetting that Na⁺ influx causes depolarisation.
- Not specifying that Na⁺ channels inactivate rather than simply close.
- Omitting the role of the Na⁺/K⁺ pump in restoring the resting potential.
Mark scheme (4 marks)
- Voltage-gated sodium ion channels open in response to membrane depolarisation, increasing membrane permeability to Na⁺.
- Na⁺ ions rush into the axon down their electrochemical gradient, causing the inside of the membrane to become more positive (depolarisation).
- Voltage-gated sodium channels inactivate / close and voltage-gated potassium channels open, increasing membrane permeability to K⁺.
- K⁺ ions diffuse out of the axon down their electrochemical gradient, restoring the negative internal charge (repolarisation), and may briefly cause hyperpolarisation before the resting potential is re-established by the Na⁺/K⁺ pump.
Key terms in this question
depolarisation · repolarisation · membrane permeability
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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