Explain how the resting membrane potential of a neuron is maintained.

IB DP Biology Standard Level (2023 syllabus) — C2.2 Neural signalling · 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 resting membrane potential is about −70 mV, with the inside of the neuron negative relative to the outside.
1. The Na⁺/K⁺‑ATPase actively pumps 3 Na⁺ out and 2 K⁺ in per ATP, creating an ion imbalance.
2. At rest the membrane is far more permeable to K⁺ than to Na⁺ via leak channels.
3. K⁺ therefore diffuses out down its concentration gradient, carrying positive charge with it.
4. The net outward movement of K⁺ and the unequal pump ratio leave excess negative charge inside, maintaining the −70 mV potential.

Examiner tips

State the approximate value of the resting potential first. Explain the pump’s stoichiometry. Mention K⁺ leak channels. Show how the charge imbalance gives a negative inside.

Common mistakes

Saying the membrane is permeable to Na⁺ at rest. Forgetting the 3:2 Na⁺/K⁺ ratio. Claiming the potential is positive inside.

Mark scheme (4 marks)

The resting membrane potential is approximately −70 mV (inside negative relative to outside).
Sodium–potassium (Na⁺/K⁺) pumps actively transport 3 Na⁺ out of the cell and 2 K⁺ into the cell per cycle, using ATP.
The membrane is more permeable to K⁺ than to Na⁺ at rest (through leak channels), so K⁺ diffuses out down its concentration gradient.
The net outward movement of positive charge (K⁺ leaving) and the unequal pump ratio leave an excess of negative charges inside, establishing the negative resting potential.

Key terms in this question

resting membrane potential