In Channels, Carriers and Pumps you met the sodium-potassium pump. Here is what it is for. Every living cell has a small voltage across its membrane, the inside slightly negative compared with the outside. Nerve and muscle cells use that voltage to send signals, and mitochondria use a similar charged gradient to make ATP.
Key idea
The sodium-potassium pump uses one ATP to move 3 Na⁺ out and 2 K⁺ in, keeping Na⁺ high outside and K⁺ high inside. Potassium leak channels then let K⁺ drift out. The positive charge it carries away leaves the inside negative: about −70 mV in a resting neuron.
Why it works
Why does K⁺ stop leaking out? Each K⁺ that leaves makes the inside a little more negative, and the negative inside pulls on the positive K⁺ ions, drawing them back. Two forces act on K⁺: the concentration gradient pushes it out, the electrical gradient pulls it in. The voltage settles where they balance. The membrane is far less leaky to Na⁺, so Na⁺ barely moves at rest, even though both its gradients point inward. The pump's own 3-for-2 imbalance adds only a small part of the voltage; its main job is to keep the gradients from running down.
Worked examples
Worked example: Counting what the pump moves
A patch of membrane runs its pumps through 200 cycles. How many Na⁺ and K⁺ ions move, how many ATP are used, and what net charge leaves the cell?
- Each cycle: 3 Na⁺ out, 2 K⁺ in, 1 ATP.
- Na⁺ out: $3 \times 200 = 600$. K⁺ in: $2 \times 200 = 400$. ATP: 200.
- Net charge: 600 positive charges out, 400 in: 200 net positive charges leave.
Answer: 600 Na⁺ out, 400 K⁺ in, 200 ATP, and a net 200 positive charges out.
Worked example: Predicting a change
A drug blocks the sodium-potassium pump. The neuron keeps its leak channels. What happens to the resting potential over time?
- Without the pump, nothing rebuilds the Na⁺ and K⁺ gradients.
- K⁺ keeps leaking out and some Na⁺ leaks in, so the gradients slowly run down.
- As the K⁺ gradient fades, there is less to drive the voltage.
Answer: The membrane potential gradually becomes less negative, drifting toward 0 mV, and the neuron loses the ability to fire normally.
Common mistake
Thinking the pump alone makes the negative charge. The pump builds the gradients, but most of the −70 mV comes from K⁺ leaking out through channels. Check with the counting: the pump moves only one net charge per cycle, while the K⁺ channels are open all the time.
A nerve impulse is a brief change in this voltage: Na⁺ channels open and Na⁺ rushes in, making the inside positive for a moment, before K⁺ channels restore it. You met this in Neurons and Signals.