Explain resting potential and action potentials
Explain resting potential, depolarisation, repolarisation and restoration during the refractory period.
Section 15.1 · Control and coordination in mammals
A Level; Cambridge International Biology 9700, 2028–2030. Written answers are self-checked; this is not practical-skills certification.
Before you begin
- A control pathway links a stimulus, receptor, coordination system and response.
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Quick readiness check
Which ion movement causes the rapid depolarising phase?
What you will learn
- Describe how ion distributions and selective membrane permeability maintain resting potential
- Explain sodium and potassium movements during an action potential and recovery
Action potential and refractory period
| Feature | Meaning |
|---|---|
| Resting | Ion gradients and selective permeability maintain a negative interior |
| Depolarisation | Voltage-gated sodium channels open; sodium ions enter |
| Repolarisation | Sodium channels inactivate; potassium channels open and potassium ions leave |
| Refractory | The membrane recovers before another action potential can occur |
Build the mechanism
At rest, the neurone membrane has a potential difference with the inside negative relative to the outside. Sodium–potassium pumps maintain ion gradients by active transport, while selective permeability, including potassium leakage, contributes to the resting potential. When threshold is reached, voltage-gated sodium channels open and sodium enters, depolarising the membrane. Sodium channels then inactivate and potassium channels open; potassium leaves and the membrane repolarises, sometimes briefly hyperpolarising. The refractory period includes recovery of channel states and membrane potential.
Keep the sequence precise
The pump maintains ion concentration gradients over time; it does not create the rapid rising phase of each action potential. Depolarisation is a change towards a less negative interior, while repolarisation restores negativity after the peak.
Pause and explain
Explain why the pump and voltage-gated channels have different roles in an action potential.
Worked example
A membrane potential rises sharply after threshold. Identify the main ion movement and channel event.
Show the worked solution
- Voltage-gated sodium channels open.
- Sodium ions enter the neurone.
Answer Voltage-gated sodium channels open and sodium ions diffuse into the neurone, causing depolarisation.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Describe how a neurone returns towards resting potential after the peak of an action potential.
Give me a hint
Name the first event, then follow each link in order.
Compare my reasoning
- Sodium channels inactivate and potassium channels open.
- Potassium leaves; channels reset and ion gradients are maintained.
After the peak, voltage-gated sodium channels inactivate and potassium channels open. Potassium ions leave, repolarising the membrane; channels reset and the sodium–potassium pump helps maintain the ion gradients.
Look for these in your work
- Mechanism stated
- Steps linked in order
A graph shows the membrane potential becoming more negative than its resting level briefly. Name this phase and link it to ion movement.
Give me a hint
Use the terms in the question and state what changes.
Compare my reasoning
- Potassium permeability can remain high after repolarisation begins.
- Continued potassium exit drives the potential below resting level.
This is a brief hyperpolarisation. Potassium channels remain open temporarily, so continued potassium exit makes the inside more negative than the resting level before the membrane recovers.
Look for these in your work
- Evidence interpreted
- Biological direction explained
Explain why the pump and voltage-gated channels have different roles in an action potential.
Give me a hint
Apply the mechanism to the new context without adding unsupported claims.
Compare my reasoning
- Channels allow rapid passive ion movement down gradients.
- The pump uses ATP to maintain gradients over time.
Voltage-gated channels produce rapid ion movements that change membrane potential during an action potential. The sodium–potassium pump uses ATP to maintain the underlying ion gradients over time.
Look for these in your work
- Mechanism applied
- Limits preserved
Common mistakes
- Attributing the action-potential peak directly to the sodium–potassium pump
- Reversing sodium entry and potassium exit
What can you explain now?
Which ion movement causes the rapid depolarising phase?
Compare with the explanation
Sodium ions enter
Voltage-gated sodium channels open and sodium enters the neurone.
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Make it stick
Which channel events drive depolarisation and repolarisation?
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