A-Level Biology May/June 2025 Q7(a): An axon membrane is described as being at its resting potential when an action potentia…

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  The resting potential of an axon membrane is maintained by the action of sodium-potassium pumps and the differential permeability of the membrane to sodium and potassium ions.

  1. Sodium-potassium pumps are present in the axon membrane. These pumps use ATP to actively transport ions against their concentration gradients. (MP1, MP3)
  2. Specifically, for every molecule of ATP hydrolysed, the pump moves three sodium ions (Na⁺) out of the axon and two potassium ions (K⁺) in. (MP2, MP4)
  3. This action establishes steep electrochemical gradients, with a high concentration of Na⁺ outside the axon and a high concentration of K⁺ inside the axon. (MP5)
  4. The axon membrane is significantly more permeable to K⁺ ions than to Na⁺ ions because it contains a large number of K⁺ ion channels that are open at rest, whereas most Na⁺ ion channels are closed. (MP7)
  5. Due to the concentration gradient, K⁺ ions diffuse out of the axon, down their gradient, through these open channels. (MP6)
  6. The outward movement of positive K⁺ ions is much greater than the inward leak of positive Na⁺ ions. (MP8)
  7. This net efflux of positive charge, combined with the presence of large, negatively charged proteins and organic anions trapped inside the axon, causes the inside of the membrane to become negative relative to the outside. (MP9, MP10)
  8. This creates a potential difference across the membrane of approximately -70 mV, which is the resting potential. (MP11)

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  How the marks are awarded

  • MP1 — Identifying the role of 'sodium-potassium pumps' in the axon membrane.
  • MP3 — Stating that the process is 'active transport' or 'uses ATP'.
  • MP4 — Correctly stating the ratio of ions moved: 'three sodium ions / Na⁺' out and 'two potassium ions / K⁺' in.
  • MP7 — Explaining that the membrane is 'more permeable to K⁺' or has more K⁺ channels.
  • MP8 — Correctly comparing the relative movement of ions: 'more, potassium ions / K⁺, move out of axon than, sodium ions / Na⁺, move in'.
  • MP10 — Concluding that the 'inside' of the axon becomes 'negative' relative to the outside.

  Common mistakes

  • Confusing the direction of ion movement, for example stating that Na⁺ is pumped in and K⁺ is pumped out.
  • Incorrectly stating the ratio of ions moved by the sodium-potassium pump, or omitting it entirely.
  • Failing to explain the role of differential permeability, i.e., not mentioning that the membrane is more permeable to K⁺ than Na⁺ at rest.
  • Stating that the pump itself creates the -70mV potential, rather than explaining that it sets up the gradients which then lead to the potential via diffusion.

  Examiner tip: For process-based questions, describe the events in a logical sequence, clearly stating the cause and effect at each step to build a complete explanation.

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