Explain how the resting membrane potential of a neuron is established and 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, the inside being negative relative to the outside.
1. The Na⁺/K⁺‑ATPase actively pumps 3 Na⁺ out for every 2 K⁺ in, using ATP, giving a net outward movement of positive charge.
2. At rest the membrane is far more permeable to K⁺ than to Na⁺ because of leak channels.
3. K⁺ therefore diffuses out down its concentration gradient, leaving behind negatively charged organic anions.
4. An electro‑chemical equilibrium is reached when the electrical attraction pulling K⁺ back in balances its outward concentration gradient, stabilising the membrane potential.

Examiner tips

Use the exact values (−70 mV, 3 Na⁺/2 K⁺) and terms (Na⁺/K⁺‑ATPase, leak channels).
Show the sequence: pump → permeability → diffusion → equilibrium.
Mention the net export of positive charge to explain negativity.

Common mistakes

Confusing the direction of Na⁺/K⁺‑ATPase transport; writing 3 K⁺ out/2 Na⁺ in.
Omitting the role of leak channels or the electro‑chemical equilibrium.
Giving the wrong sign for the membrane potential.

Mark scheme (4 marks)

The resting membrane potential is approximately −70 mV (inside negative relative to outside).
The sodium–potassium pump (Na⁺/K⁺-ATPase) actively transports 3 Na⁺ out of the cell for every 2 K⁺ pumped in, using ATP, creating a net export of positive charge.
The membrane is much more permeable to K⁺ than to Na⁺ at rest (via leak channels), so K⁺ diffuses out down its concentration gradient, leaving behind negatively charged organic anions.
An electrochemical equilibrium is reached for K⁺ when the electrical attraction pulling K⁺ back in balances its outward concentration gradient, stabilising the membrane potential.

Key terms in this question

resting membrane potential