C2.2 Neural signalling

The question asks which mechanism is primarily responsible for maintaining the high intracellular K⁺ concentration (~150 mmol dm⁻³ inside vs. 5 mmol dm⁻³ outside) at rest.

K⁺ is at much higher concentration inside the neuron. This is against the K⁺ concentration gradient (which would push K⁺ outward), so passive diffusion alone cannot account for the high intracellular K⁺.

The Na⁺/K⁺ pump uses ATP to actively transport 2 K⁺ ions into the cell for every 3 Na⁺ pumped out, working against the concentration gradient. This is the primary mechanism that builds up intracellular K⁺.

The correct answer is active transport of K⁺ into the neuron by the sodium–potassium pump using ATP, as this directly and continuously maintains the high intracellular K⁺ concentration at rest.

We need the mechanism that maintains high K⁺ inside the resting neuron, working against the outward concentration gradient.

Passive diffusion through the lipid bilayer is incorrect — ions are charged and cannot freely cross the hydrophobic core of the phospholipid bilayer without protein channels.

Voltage-gated K⁺ channels open during repolarisation actually allow K⁺ to flow out of the neuron, which would reduce — not maintain — the intracellular K⁺ concentration.

While the negative interior does attract K⁺ inward (electrical gradient), this force is counteracted by the outward concentration gradient; it cannot alone account for maintaining the concentration difference against leak channels.

Active transport by the sodium–potassium pump is the primary mechanism — it continuously pumps 2 K⁺ in per cycle at the cost of ATP, directly countering the outward K⁺ leak and maintaining the high intracellular concentration.

The membrane potential rising from −70 mV to +30 mV describes depolarisation — the inside of the neuron becomes progressively more positive.

When threshold (−55 mV) is reached, voltage-gated Na⁺ channels open. Na⁺ has a high extracellular concentration and is attracted by the negative interior — both the concentration and electrical gradients drive Na⁺ into the cell.

The rapid influx of positively charged Na⁺ ions makes the interior progressively more positive, shifting the potential from −70 mV up to approximately +30 mV.

Na⁺ flowing into the neuron through voltage-gated channels is the direct cause of depolarisation during an action potential.

We need the ion movement that makes the inside of the neuron change from −70 mV to +30 mV (i.e., become more positive).

K⁺ flowing out removes positive charge from inside the cell, which would make the interior more negative — this describes repolarisation, not depolarisation.

Cl⁻ flowing in would bring negative charge into the cell, making the interior more negative — this would cause hyperpolarisation, not depolarisation.

Na⁺ pumped out by the sodium–potassium pump removes positive charge, contributing to the resting potential — it does not cause the rapid depolarisation of an action potential.

Only Na⁺ flowing into the neuron through voltage-gated channels introduces positive charge rapidly enough to shift the membrane from −70 mV to +30 mV.