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.
What it does: 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: The pump uses ATP to top the gradients up faster than they leak away. This is the key data-question idea.
Leak and pump
Ions leak back
Sodium and potassium constantly leak back down their gradients through channels.
Without the pump
If nothing opposed this, the inside and outside would slowly equalise.
The pump runs continuously
It replaces what leaks away, so the steep ion gradients are maintained.
The result
A cell can keep a high-K⁺ / low-Na⁺ interior only because the pump uses ATP.
Leak back → pump tops up → gradients kept
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.
What to say
- 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
Passive transport
- Moves down the gradient (high → low)
- No ATP used
- Diffusion, osmosis, facilitated diffusion
- Tends to even out concentrations
Active transport
- Moves against the gradient (low → high)
- Uses ATP (a pump protein)
- Sodium-potassium pump; mineral uptake by roots
- Maintains steep concentration differences
| Feature | Passive transport | Active transport |
|---|---|---|
| Direction of movement | DOWN the concentration gradient (high → low) | AGAINST the concentration gradient (low → high) |
| Energy (ATP) | No ATP used | Uses ATP (energy required) |
| Protein needed? | Simple diffusion: none; facilitated: channel/carrier | Always — a pump protein |
| Examples | Simple diffusion, osmosis, facilitated diffusion | Sodium-potassium pump; mineral uptake by root cells |
An easy way to remember: Active = Against + ATP. Both 'A' words go together.
And for the pump: '3 out, 2 in' — three sodium out, two potassium in, per ATP.
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The big idea: The pump swaps sodium for potassium, three for two. That uneven exchange gives the cell its membrane potential.
The points to remember
- It is an exchange transporter: it moves two kinds of ion in opposite directions in one cycle.
- Three Na⁺ bind on the inside; ATP is split and its phosphate changes the pump's shape.
- The pump opens outside and releases the Na⁺; two K⁺ bind and are carried in as it flips back.
- More positive charge leaves than enters, so the inside becomes negative: the resting potential.
- Na⁺ and K⁺ leak back through channels by facilitated diffusion, so the pump never stops.
Remember it as: Three out, two in, one ATP: a negative inside.
Real example: the brain's energy bill: Sodium-potassium pumps are estimated to use well over a third of the ATP made in a resting human brain, keeping neurons ready to fire.
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How it is asked: Paper 1A gives a diagram of ion movements and asks what can be deduced. Paper 2 asks you to explain how the pump generates the membrane potential.
Explain how sodium-potassium pumps generate the resting potential of a neuron.
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