AQA · A-Level · Biology
Course version: 7402 · Version from Sept 2016
The establishment of a resting potential in terms of differential membrane…
Full specification wording
Nerve impulses (A-level only): The establishment of a resting potential in terms of differential membrane permeability, electrochemical gradients and the movement of sodium ions and potassium ions.
At rest the inside of the axon is about 70 mV negative relative to the outside.
Checked against the AQA 7402 specification (opens in a new tab) on .
- Point 2 of 6
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
At rest the inside of the axon is about 70 mV negative relative to the outside. Two things create this. First, sodium-potassium pumps in the axon membrane actively transport three sodium ions out for every two potassium ions taken in, using ATP; this alone moves net positive charge outwards and builds steep electrochemical gradients, with sodium ions concentrated outside and potassium ions inside. Second, the membrane is differentially permeable: potassium ion channels are open at rest while the voltage-gated sodium ion channels are shut, so the membrane is far more permeable to potassium ions than to sodium ions. Potassium ions therefore diffuse back out down their concentration gradient faster than sodium ions leak in, leaving the inside negative.
What earns the marks
- The membrane being more permeable to potassium ions and less permeable to sodium ions.
- Sodium ions being actively transported out of the axon and potassium ions in.
- Stating that this active transport is carried out by a sodium-potassium pump and requires ATP.
- Potassium ions diffusing out down the concentration gradient, leaving the inside of the axon negative.
- Quoting a resting potential of about minus 70 mV, with the inside negative relative to the outside.
Common mistakes and how to put them right
Writing sodium or potassium rather than sodium ions or potassium ions, which caps the marks available.
Saying the pump moves equal numbers of each ion, which removes the reason for a net charge difference.
Calling the outward movement of potassium ions active transport, when it is facilitated diffusion through open channels.
Stating that the outside of the axon is negative, so the sign of the potential is the wrong way round.
Claiming the membrane is completely impermeable to sodium ions rather than less permeable.
Exam tips
- Always write ions after sodium and potassium; leaving the word out is the single commonest way to lose marks here.
- Give both halves of the explanation, the pump and the differential permeability; one alone rarely scores full marks.
- If asked how the resting potential is maintained rather than established, stress that the pumping is continuous.
Try the lesson
The start of the Nerve impulses (A-level only): The establishment of a resting potential in terms of differential membrane permeability, electrochemical gradients and the movement of sodium ions and potassium ions. lesson, made from checked teaching notes for this specification point.
Have a go first
Which ion is mainly responsible for establishing the resting potential of a neurone?
Listen
At rest the inside of the axon is about 70 mV negative relative to the outside. Two things create this. First, sodium-potassium pumps in the axon membrane actively transport three sodium ions out for every two potassium ions taken in, using ATP; this alone moves net positive charge outwards and builds steep electrochemical gradients, with sodium ions concentrated outside and potassium ions inside. Second, the membrane is differentially permeable: potassium ion channels are open at rest while the voltage-gated sodium ion channels are shut, so the membrane is far more permeable to potassium ions than to sodium ions. Potassium ions therefore diffuse back out down their concentration gradient faster than sodium ions leak in, leaving the inside negative. A strong answer earns marks for these points. the membrane being more permeable to potassium ions and less permeable to sodium ions. sodium ions being actively transported out of the axon and potassium ions in. stating that this active transport is carried out by a sodium-potassium pump and requires ATP. potassium ions diffusing out down the concentration gradient, leaving the inside of the axon negative. Exam tip: Always write ions after sodium and potassium; leaving the word out is the single commonest way to lose marks here.
Key words
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resting potential of minus 70 mV
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sodium-potassium pump
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active transport and ATP
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differential membrane permeability
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See every definition
- resting potential of minus 70 mV
- At rest the inside of the axon is about 70 mV negative relative to the outside.
- sodium-potassium pump
- Pumps in the axon membrane actively transport three sodium ions out for every two potassium ions taken in, using ATP.
- active transport and ATP
- The sodium-potassium pumps actively transport three sodium ions out for every two potassium ions taken in, using ATP.
- differential membrane permeability
- Potassium ion channels are open at rest while the voltage-gated sodium ion channels are shut, so the membrane is far more permeable to potassium ions than to sodium ions.
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