AP Biology · Unit 2 · Topic 2.8
The sodium-potassium pump moves three ions out and two in for every ATP. That mismatch contributes to a cell's membrane potential.
Learning objective 2.8.A Describe the processes that allow ions and other molecules to move across membranes.
Narrated walkthrough of the “Run the pump” tab. Captions are on by default.
Step through the cycle one stage at a time and watch what the pump is holding, which way it opens, and when the ATP is spent. Then run whole cycles and watch the counters. The membrane holds more than the pump: two potassium leak channels are drawn too, and potassium trickles out through them the whole time. The counters on this tab follow only what the pump moves; the leak is counted on the next tab.
A real resting potential (about −70 mV) is set mainly by potassium leaving through the leak channels, down the gradient the pump maintains. The pump's own share, one net charge per cycle, is small. Its real job is to keep the gradients that the leak and every nerve impulse spend.
Check it on the readout. Sodium starts more concentrated outside, and the pump sends more of it out. Potassium starts more concentrated inside, and the pump brings more of it in. Each cycle makes both gradients steeper, which is exactly what active transport means.
Both ions carry a single positive charge, so the arithmetic is simple: three positive charges leave and two come back, leaving the inside one charge more negative each cycle. Run ten cycles and the pump's share should read exactly −10.
That is only the pump's direct share. Most of a resting cell's potential comes from the potassium leak channels: potassium is far more concentrated inside, so it leaks out down its gradient, and every ion that leaves takes a positive charge with it. The pump's job is to keep that gradient topped up.
Add the inside and outside counts for either ion at any point in the run. The total never changes — every ion the inside loses, the outside gains.
Build a gradient with the pump, then stop paying for it. Ions leak back through channels the whole time, so a gradient is not something a cell builds once — it is something a cell has to keep buying.
The pump spends ATP to put ions where they do not want to be. Nearly everything that follows gets its energy by letting them go back.
Answer these in your own words before opening the answers.
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