---
title: "Na+/K+ Pump — AP Bio Definition & Exam Guide"
description: "The Na+/K+ pump uses ATP to push 3 Na+ out and 2 K+ into animal cells against their gradients, a classic AP Bio example of active transport in Unit 2."
canonical: "https://fiveable.me/ap-bio/key-terms/nak-pump"
type: "key-term"
subject: "AP Biology"
---

# Na+/K+ Pump — AP Bio Definition & Exam Guide

## Definition

The Na+/K+ pump is a membrane protein in animal cells that uses ATP to actively transport 3 sodium ions out of the cell and 2 potassium ions in, both against their concentration gradients.

## What It Is

The Na+/K+ pump (also called the sodium-potassium pump) is a [transport protein](/ap-bio/unit-2/plasma-membranes/study-guide/1aW0ZDGzS56ism3BJwTi "fv-autolink") sitting in the [phospholipid bilayer](/ap-bio/key-terms/phospholipid-bilayer "fv-autolink") of animal cells. Its job is to move sodium ions (Na+) out of the cell and potassium ions (K+) in, and here's the catch: it moves both ions *against* their concentration gradients. Na+ is already more concentrated outside, and K+ is already more concentrated inside, so the pump is pushing each ion uphill. Doing work against a gradient costs energy, which is why this is **active transport** and not passive movement.

Each cycle, the pump hydrolyzes one ATP to ship 3 Na+ out and 2 K+ in. That uneven 3-to-2 swap matters. More positive charge leaves than enters, so the inside of the cell ends up slightly negative relative to the outside. The pump keeps Na+ high outside and K+ high inside, the exact opposite of what diffusion alone would do. Note that this contrasts with **[facilitated diffusion](/ap-bio/unit-2/membrane-transport/study-guide/7yV52dtD7Vj7ZadTEd2P "fv-autolink")** (the focus of CED topic 2.7), where channel and carrier proteins let ions slide *down* their gradients for free.

## Why It Matters

This term lives in [Unit 2](/ap-bio/unit-2 "fv-autolink"): Cells, connected to topic 2.7. While topic 2.7 centers on osmoregulation and how concentration gradients drive movement across membranes ([AP Bio](/ap-bio "fv-autolink") 2.7.A and AP Bio 2.7.B), the Na+/K+ pump is the textbook contrast case: it's what cells do when they need a gradient that diffusion would otherwise erase. Understanding it locks in the distinction between active and passive transport, a core idea the AP exam tests in Unit 2. The pump also sets up why cells must constantly spend ATP just to maintain homeostasis, tying transport directly to the energy themes you'll see again in Unit 3.

## Connections

### [Active Transport (Unit 2)](/ap-bio/key-terms/active-transport)

The Na+/K+ pump is the headline example of [active transport](/ap-bio/key-terms/active-transport "fv-autolink"). If you can explain why this pump needs ATP (because it moves ions uphill against their gradients), you understand active transport in general.

### Facilitated Diffusion and Channel Proteins (Unit 2)

Topic 2.7's [channel proteins](/ap-bio/key-terms/channel-proteins "fv-autolink") let ions flow down their gradients for free. The pump does the opposite, spending energy to build the gradient that those channels later use. They're two halves of the same membrane story.

### [Concentration Gradient (Unit 2)](/ap-bio/key-terms/concentration-gradient)

Gradients normally even out on their own. The pump exists to keep the Na+ and K+ gradients steep, which is exactly the stored potential [energy](/ap-bio/unit-3/environmental-impacts-on-enzyme-function/study-guide/Q8PevM3BI76060aoWtit "fv-autolink") that nerve and muscle cells tap into.

### [Osmoregulation (Unit 2, topic 2.7)](/ap-bio/key-terms/osmoregulation)

By controlling internal ion concentrations, the pump helps set the cell's solute potential and water potential, which ties straight into the osmoregulation goals of AP Bio 2.7.B.

## On the AP Exam

Expect the Na+/K+ pump in multiple-choice stems that test whether you can spot active transport. A common setup gives you ion concentrations across a membrane (for example, Na+ is 10x higher outside while K+ is 20x higher inside) and asks what happens when a channel is altered, like a mutation that locks voltage-gated Na+ channels open. To answer, reason that Na+ would rush in down its gradient, the gradient the pump worked to build would collapse, and the cell would lose its membrane charge. No released FRQ uses this term verbatim, but it supports the kind of reasoning FRQs reward: explaining why a process requires energy and predicting what happens when a gradient or transport protein is disrupted.

## Na+/K+ pump vs Facilitated diffusion

Both involve transport proteins in the membrane, so they're easy to mix up. The difference is direction and energy: facilitated diffusion moves molecules *down* their gradient with no ATP, while the Na+/K+ pump moves ions *against* their gradients and must burn ATP to do it. If it costs energy, it's the pump.

## Key Takeaways

- The Na+/K+ pump uses ATP to move 3 Na+ out of the cell and 2 K+ in, both against their concentration gradients.
- Because it works against the gradients, it is active transport, not passive transport or facilitated diffusion.
- The 3-out, 2-in ratio leaves the inside of the cell slightly negative compared to the outside.
- Na+ stays high outside the cell and K+ stays high inside, the reverse of what diffusion alone would produce.
- If the gradients the pump builds are disrupted, ions rush down their gradients and the cell loses its membrane charge.
- The pump's control of internal ion concentrations links it to solute potential and osmoregulation in topic 2.7.

## FAQs

### What is the Na+/K+ pump and what does it do?

It's a membrane protein in animal cells that uses one ATP per cycle to pump 3 sodium ions out and 2 potassium ions in, moving both against their concentration gradients. This keeps Na+ high outside and K+ high inside.

### Is the Na+/K+ pump active or passive transport?

Active. It moves ions uphill against their gradients, which always requires energy, so it hydrolyzes ATP. If a process needs ATP to move something against its gradient, it's active transport.

### How is the Na+/K+ pump different from facilitated diffusion?

Facilitated diffusion uses proteins to let molecules slide *down* their gradient for free, while the Na+/K+ pump spends ATP to push ions *against* their gradients. Same idea of a transport protein, opposite direction and energy cost.

### Why does the Na+/K+ pump need ATP?

Because moving ions against their concentration gradients is uphill work, and uphill movement is never spontaneous. The energy from breaking down ATP powers the protein's shape change that shuttles the ions.

### What happens if the Na+/K+ pump stops or its gradients break down?

The carefully built gradients collapse as Na+ floods in and K+ leaks out down their gradients. The cell loses its membrane charge and can no longer maintain the ion balance it depends on for normal function.

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