Active transport and the sodium-potassium pump

The big idea: Most movement across a membrane is passive — particles drift down their concentration gradient, from where they are crowded to where they are sparse, and no energy is needed.

But cells often need to move a substance the wrong way — against its gradient, from low concentration to high. That cannot happen on its own.

Active transport is the process that does this. A pump protein in the membrane uses energy from ATP to force the particle against its concentration gradient.

Active transport

The movement of a substance across a membrane AGAINST its concentration gradient, using energy from ATP.

Concentration gradient

The difference in concentration of a substance between two regions. 'Down' the gradient means high → low; 'against' it means low → high.

Pump protein

A membrane protein that uses ATP to move a specific particle against its gradient by changing shape.

ATP

The molecule cells use as their energy currency; active transport spends ATP to power the pump.

Passive transport

Movement DOWN a concentration gradient (e.g. diffusion, osmosis) that needs no ATP.

Two words tell you it's active: If a question says a particle moves AGAINST the gradient, or that the process uses ATP, it is active transport.

Either word on its own is the giveaway — passive transport does neither.

The best-known example of active transport is the sodium-potassium pump (the Na⁺/K⁺ pump), found in the membrane of nearly every animal cell.

It keeps the inside of the cell low in sodium (Na⁺) and high in potassium (K⁺) — the opposite of what diffusion alone would produce.

What the pump does: With each cycle the pump uses one ATP to push:

3 sodium ions (Na⁺) OUT of the cell, and

2 potassium ions (K⁺) IN.

Both ions are moved against their concentration gradients — sodium is already low inside, yet more is pumped out; potassium is already high inside, yet more is pumped in. That is why it costs ATP.

Why the gradients don't just even out: Sodium and potassium constantly leak back down their gradients through channels. If nothing opposed this, the inside and outside would slowly equalise.

The pump runs continuously, replacing what leaks away, so the steep ion gradients are maintained.

This is the key data-question idea: a cell can keep a high-K⁺ / low-Na⁺ interior only because the pump uses ATP to top the gradients up faster than they leak away.

Reading an ion-concentration table: Exams often give a table showing K⁺ much higher inside the cell than outside, and Na⁺ much higher outside than inside.

Those differences are maintained by active transport (the Na⁺/K⁺ pump using ATP) — not by diffusion, which would erase them.

If the cell were starved of ATP, the pump would stop and the concentrations inside and outside would gradually become equal.

A memory hook: Active = Against + ATP. Both 'A' words go together.

And for the pump: '3 out, 2 in' — three sodium out, two potassium in, per ATP.

How this is tested: A 1-mark Identify question gives a description — 'the protein that uses ATP to move particles against a concentration gradient' — and wants you to name it: a pump protein (active transport).

A favourite Paper 1B / data question shows an ion-concentration table (K⁺ high inside, Na⁺ high outside) and asks how the cell maintains those gradients — the answer is active transport by the Na⁺/K⁺ pump, using ATP.

On Paper 2 a 3-mark Describe question can ask for the three processes by which molecules cross the bilayer — give simple diffusion, facilitated diffusion and active transport, making the passive-vs-active contrast clear.

✓ Why this scores full marks: Each line is a distinct process and states two scoring facts — what crosses and the gradient/ATP status.

A 3-mark 'describe' needs three separate processes, with the passive-vs-active difference made obvious — not the same idea three times.