Explain how the sodium-potassium pump maintains the resting membrane potential of a neuron.

IB DP Biology Standard Level (2023 syllabus) — B2.1 Membranes and membrane transport · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

The sodium‑potassium pump is an intrinsic carrier protein in the plasma membrane that uses ATP to actively transport ions against their concentration gradients. It extrudes three Na⁺ ions for every two K⁺ ions it imports, creating an unequal charge distribution. This results in a net negative charge inside the cell relative to the outside, establishing the resting membrane potential (≈ −70 mV in neurons). By maintaining a low intracellular Na⁺ concentration and a high intracellular K⁺ concentration, the pump keeps the cell ready to generate an action potential.

Examiner tips

Use the term ‘active transport’ and ‘ATP’ to show energy use.
Mention the 3 Na⁺/2 K⁺ ratio and the resulting negative inside.
State the typical resting potential value.
Explain that the ion gradients are essential for action potential generation.

Common mistakes

Confusing the direction of ion movement (e.g., saying Na⁺ is pumped in).
Omitting the ATP requirement or the 3:2 ratio.
Using vague terms like ‘ion pump’ without specifying sodium‑potassium pump.

Mark scheme (4 marks)

The sodium-potassium pump is a carrier protein / intrinsic protein embedded in the plasma membrane that uses ATP (active transport) to move ions against their concentration gradients.
Three sodium ions are pumped out of the cell for every two potassium ions pumped in, creating an unequal charge distribution.
The inside of the cell becomes negatively charged relative to the outside, establishing the resting membrane potential (approximately −70 mV in neurons).
The low intracellular Na⁺ concentration and high intracellular K⁺ concentration are maintained, which are essential for the cell to be able to generate an action potential.

Key terms in this question

sodium-potassium pump · resting membrane potential