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A2 · Practice questions · Resting and action potentials

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Six original Cambridge-style questions on membrane potentials, from explaining the resting potential to reading an action potential trace.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Pump, channel, node.

01
Explain
[4 marks]

Explain how the resting potential of a neurone is maintained.

  • The Na+/K+ pump uses ATP to actively transport 3 Na+ out for every 2 K+ in. ✓
  • This sets up concentration gradients: more Na+ outside, more K+ inside. ✓
  • The membrane is more permeable to K+ (some K+ channels open) than to Na+, so K+ diffuses out. ✓
  • Together with large negative ions (proteins) inside, the inside stays negative, at about -70 mV. ✓
02
Explain
[5 marks]

Describe and explain the changes in membrane potential during an action potential.

  • A stimulus depolarises the membrane to threshold (about -55 mV). ✓
  • Voltage-gated Na+ channels open; Na+ diffuses in, making the inside positive (to about +40 mV); positive feedback opens more channels. ✓
  • Na+ channels close and voltage-gated K+ channels open; K+ diffuses out: repolarisation. ✓
  • K+ channels are slow to close, so the potential overshoots: hyperpolarisation (about -80 mV). ✓
  • K+ channels close; the pump and K+ leak restore the resting potential during the refractory period. ✓
03
Data
[3 marks]

An action potential trace shows the potential rising from -70 mV to +35 mV in 0.5 ms. Calculate the mean rate of depolarisation, and state which ions are responsible.

  • Change = 35 - (-70) = 105 mV. ✓
  • Rate = 105 ÷ 0.5 = 210 mV ms-1. ✓
  • Sodium ions diffusing in through voltage-gated Na+ channels. ✓
04
Explain
[4 marks]

Explain why impulses travel faster along a myelinated axon than along an unmyelinated axon of the same diameter.

  • Myelin (Schwann cell membranes) is an electrical insulator, so ions cannot move across the membrane where it is present. ✓
  • Depolarisation / action potentials happen only at the nodes of Ranvier. ✓
  • Local circuits of current pass from node to node, so the impulse jumps: saltatory conduction. ✓
  • Fewer regions of membrane must depolarise along the axon, so the impulse travels faster. ✓
05
Explain
[3 marks]

Explain the importance of the refractory period in nerve transmission.

  • It ensures action potentials are discrete / separate and do not merge. ✓
  • It ensures impulses travel in one direction only, as the membrane behind the impulse cannot be depolarised. ✓
  • It limits the maximum frequency of impulses, so stimulus strength can be coded by frequency up to that limit. ✓
06
Analysis
[2 marks]

A weak and a strong touch on the skin both produce action potentials of +40 mV in the same sensory neurone. Suggest how the brain can tell the difference.

  • The strong touch produces a higher frequency of action potentials. ✓
  • It may also stimulate more receptors / more sensory neurones (the size of each action potential is the same: all-or-nothing). ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.