# Explain how changes in membrane permeability to sodium and potassium ions bring about depolarisation and repolarisation during an action potential.

> IB DP Biology Higher Level (2023 syllabus) — C2.2 Neural signalling · Explain · 4 marks

## Mark scheme (4 marks)

1. Voltage-gated sodium ion channels open in response to membrane depolarisation, increasing membrane permeability to Na⁺.
2. Na⁺ ions rush into the axon down their electrochemical gradient, causing the inside of the membrane to become more positive (depolarisation).
3. Voltage-gated sodium channels inactivate / close and voltage-gated potassium channels open, increasing membrane permeability to K⁺.
4. 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

- [depolarisation](https://www.gradenine.co.uk/glossary/depolarisation)
- [repolarisation](https://www.gradenine.co.uk/glossary/repolarisation)
- [membrane permeability](https://www.gradenine.co.uk/glossary/membrane-permeability)

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- [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-changes-in-membrane-permeability-2bd929ef) · Published by Druglandscape Ltd.