A-Level Biology May/June 2025 Q10(a): During the course of an action potential, the potential difference across a neurone mem…

1 answer

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  accepted ✓

  Point X (-70 mV)

  This is the resting potential. It is established and maintained by the sodium-potassium pump, which uses energy from ATP (active transport) to move three Na⁺ ions out of the neurone for every two K⁺ ions it moves in. The membrane is also more permeable to K⁺ than Na⁺ at rest, so more K⁺ ions diffuse out through leak channels than Na⁺ ions diffuse in. This creates a net loss of positive charge from the axon, making the inside negative relative to the outside and establishing the potential difference.

  Point Y (+40 mV)

  This potential is reached during depolarisation. Following a stimulus that reaches the threshold potential, voltage-gated sodium ion channels open. This allows a rapid influx of Na⁺ ions to diffuse into the neurone down their electrochemical gradient. The influx of positive charge causes the inside of the membrane to become positive relative to the outside, reaching a peak of approximately +40 mV.

  Point Z (-90 mV)

  This potential occurs during hyperpolarisation, which follows repolarisation. At this stage, voltage-gated Na⁺ channels are closed. Voltage-gated potassium ion channels, which opened during repolarisation, are slow to close. This allows K⁺ ions to continue diffusing out of the neurone, making the membrane potential temporarily more negative than the resting potential.

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  How the marks are awarded

  • M1 — For point X, identifying the role of the sodium-potassium pump using active transport. The answer states '...maintained by the sodium-potassium pump, which uses energy from ATP (active transport)...'
  • M1 — For point X, correctly stating the ratio of ions moved by the pump. The answer states '...move three Na⁺ ions out of the neurone for every two K⁺ ions it moves in.'
  • M1 — For point X, explaining the differential permeability and diffusion of ions. The answer states '...more K⁺ ions diffuse out... than Na⁺ ions diffuse in.'
  • M1 — For point Y, identifying the process as depolarisation. The answer states 'This potential is reached during depolarisation.'
  • M1 — For point Y, explaining the movement of sodium ions. The answer states '...a rapid influx of Na⁺ ions to diffuse into the neurone...'
  • M1 — For point Z, identifying the process as hyperpolarisation. The answer states 'This potential occurs during hyperpolarisation...'
  • M1 — For point Z, explaining the movement of potassium ions. The answer states '...K⁺ ions to continue diffusing out of the neurone...'

  Common mistakes

  • Confusing the direction of ion movement, for example, stating that K+ moves in during repolarisation or Na+ moves out during depolarisation.
  • Forgetting to mention the role of the sodium-potassium pump in establishing the resting potential (point X), and instead only focusing on the action potential itself.
  • Mixing up which type of channel (voltage-gated vs. leak) is active at each stage, or which ion uses which channel.
  • Stating that the sodium-potassium pump is directly responsible for repolarisation or hyperpolarisation, rather than the movement of K+ through voltage-gated channels.

  Examiner tip: For process-based questions, always link the structure (e.g., ion channels, pumps) to its function (e.g., ion movement) and the resulting outcome (e.g., change in membrane potential).

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