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6.2.2Organisms Respond to Changes in their Environments (A Level only)

Resting Potential — AQA A Level Biology 7402 Revision Notes & Topical Questions

5 min read 8 parts Lesson 17 of 44 1 quiz

📚 AQA A Level Biology 7402 Hub / Topical Revision
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6.2.2 Resting Potential — AQA A Level Biology 7402 notes: Explain how a resting potential is established and maintained across the axon membrane.In a neurone that is not transmitting an impulse there is a difference in electrical charge between the inside and outside of the axon …

  • potential difference
  • sodium–potassium pump
  • active transport
  • three Na+ ions out
  • two K+ ions in
  • potassium channels

Section 1 of 8What is a resting potential?

In a neurone that is not transmitting an impulse there is a difference in electrical charge between the inside and outside of the axon membrane.

The inside is negative relative to the outside.
Potential difference
This potential difference is about -70 mV and is called the resting potential.
Polarised
The membrane is said to be polarised.
It is the starting point for every action potential, so understanding how it is maintained is essential before studying impulses.

The potential exists because ions are unequally distributed across the phospholipid bilayer, which is impermeable to charged particles unless they use channel or carrier proteins.

Section 2 of 8The sodium–potassium pump

The first process is the sodium–potassium pump, a carrier protein in the membrane that uses ATP (active transport).

The sodium–potassium pump
In each cycle it moves three Na+ ions out of the axon and two K+ ions in.
This creates a high concentration of Na+ outside the axon and a high concentration of K+ inside, so both ions have electrochemical gradients.
Because three positive charges leave for every two that enter, a net loss of positive charge from the inside occurs, contributing to its negative charge.

Section 3 of 8Differential membrane permeability

The second process is the different permeability of the membrane to the two ions.

Differential membrane permeability
Potassium channels
At rest the membrane has many open potassium channels, so K+ ions diffuse out of the axon down their concentration gradient.
Sodium channels are mostly closed
By contrast, the sodium channels are mostly closed, so very little Na+ can diffuse back in.
Large negatively charged molecules inside the axon, such as proteins and organic anions, cannot leave.

As a result, positive charge accumulates outside and the inside becomes negative. This negative charge inside attracts K+ back in while the concentration gradient drives it out; at about -70 mV the two effects balance for K+, so the potential is steady.

Differential membrane permeability
Pump: establishes the ion gradients and uses ATP.
K+ channels open: allow outward diffusion of K+.
Na+ channels closed: prevent Na+ diffusing back in.

Section 4 of 8Interpreting data

If an axon is treated with a metabolic poison such as cyanide, ATP production falls and the pump stops.

1
The resting potential does not collapse at once,
  • because the gradients already exist,
  • but over time they decline and the neurone can no longer generate normal action potentials.
2
A drug that blocks potassium channels makes the inside less negative, showing that K+ diffusion contributes to the potential.
3
Raising the external K+ concentration also makes the potential less negative because the gradient driving K+ out is smaller.
4
These ideas link directly to action potentials, in which the permeability changes and the stored gradients are used.

Section 5 of 8Measuring a resting potential

Resting potentials are measured by inserting a very fine microelectrode into an axon (classically the giant axon of a squid) and comparing it with an electrode outside the membrane, using an oscilloscope.

Measuring a resting potential
The trace stays at a constant value of about -70 mV while no stimulus is applied.
Changing the composition of the fluid bathing the axon allows hypotheses to be tested: if the concentration of K+ outside is raised, the potential becomes less negative,
  • while reducing outside Na+ has little effect on the resting value but greatly alters the size of the action potential.
This is good evidence that the resting potential is mainly due to K+ and that Na+ is mainly involved in depolarisation.

Remember that the potential difference is a property of the membrane, not of the whole cell: the cytoplasm and the tissue fluid are each electrically neutral overall.

Measuring a resting potential
The charge difference is carried by a very thin layer of ions on each side of the membrane.
Only a tiny proportion of the ions in the cell has to move to produce the potential,
  • so large changes in total ion concentration do not occur when an impulse passes along the axon.

Section 6 of 8Exam-style questions

  1. Describe how a resting potential is maintained across an axon membrane. [4]4 marks

  2. A student says the resting potential is zero because no impulse is passing. Explain why the student is wrong. [2]2 marks

  3. Explain what would happen to the resting potential if the axon were poisoned with a substance that stops ATP production. [3]3 marks

Section 7 of 8Key terms

TermMeaning
Resting potentialPotential difference across the membrane of an unstimulated neurone, about -70 mV
PolarisedHaving a charge difference across the membrane, inside negative
Sodium–potassium pumpCarrier protein actively moving 3 Na+ out and 2 K+ in
Electrochemical gradientCombined concentration and electrical gradient acting on an ion
Potassium channelProtein channel allowing K+ to diffuse across the membrane
Active transportMovement of substances against a gradient using ATP

Section 8 of 8Exam technique

1
Stating that the pump moves equal numbers of Na+ and K+; the ratio is 3 out to 2 in.
2
Saying that the resting potential is caused only by the pump; K+ diffusion through open channels is also needed.
3
Saying Na+ channels are wide open at rest; they are mostly closed.
4
Describing the pump as facilitated diffusion; it is active transport and uses ATP.
5
Writing that the outside is negative; the inside is negative.

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