---
title: "Depolarization — AP Biology Definition & Exam Guide"
description: "Depolarization is when a cell's membrane potential becomes less negative, usually from sodium flooding in. Here's how it shows up on the AP Bio exam."
canonical: "https://fiveable.me/ap-bio/key-terms/depolarization"
type: "key-term"
subject: "AP Biology"
unit: "Unit 5"
---

# Depolarization — AP Biology Definition & Exam Guide

## Definition

In AP Biology, depolarization is a change in a cell's membrane potential toward a more positive (less negative) value, driven by the influx of sodium ions across the membrane. In muscle and nerve cells, it triggers responses like an action potential or muscle contraction.

## What It Is

Depolarization is what happens when the inside of a cell suddenly becomes less negative compared to the outside. At rest, the inside of a neuron or muscle cell sits at a negative voltage (the resting [membrane potential](/ap-bio/key-terms/membrane-potential "fv-autolink")). When sodium (Na⁺) channels open and positive sodium [ions](/ap-bio/unit-2/plasma-membranes/study-guide/1aW0ZDGzS56ism3BJwTi "fv-autolink") rush in, that voltage shoots upward toward zero and beyond. That upward swing is depolarization.

Think of the membrane like a stretched rubber band held at a baseline tension. Depolarization is the moment something yanks it, the charge flips, and a signal fires. In a neuron this kicks off an action potential; at a [neuromuscular junction](/ap-bio/key-terms/neuromuscular-junction "fv-autolink"), the signal that crosses the synapse can depolarize a muscle cell and start a contraction. The key cause to remember is the movement of positive ions (especially Na⁺) into the cell.

## Why It Matters

Depolarization is a cell-signaling and membrane-transport idea, so it ties into how cells respond to their environment and convert one signal into a physical response. It connects directly to facilitated diffusion and ion channels, since sodium can't cross the membrane on its own and needs protein channels to move down its gradient. On the exam, depolarization is the kind of process you might explain when a question hands you a receptor, a ligand, and a membrane and asks what happens next. The reasoning skill is the same one rewarded across [AP Bio](/ap-bio "fv-autolink"): read a scenario, predict the [cellular response](/ap-bio/key-terms/cellular-response "fv-autolink"), and justify it using how ions and charge move.

## Connections

### Facilitated Diffusion and Ion Channels (Unit 2)

Sodium ions are charged and can't slip through the [lipid bilayer](/ap-bio/key-terms/lipid-bilayer "fv-autolink") alone. They move through protein channels down their concentration gradient, so depolarization is really facilitated diffusion of Na⁺ producing a voltage change.

### Cell Communication and Signal Transduction (Unit 4)

Depolarization is a classic signal-to-response conversion. A ligand like [acetylcholine](/ap-bio/key-terms/acetylcholine "fv-autolink") binds a receptor, the receptor opens an ion channel, and the resulting charge change becomes the cell's response.

### Membrane Potential and the Sodium-Potassium Pump (Unit 2)

Depolarization only matters because the cell starts out negative inside. The sodium-potassium pump builds that resting gradient using ATP, so the pump sets the stage and depolarization is the dramatic flip that follows.

## On the AP Exam

Depolarization shows up most clearly in cell-signaling and membrane-transport scenarios. A 2018 short FRQ used acetylcholine receptor (AChR) proteins at the synapse between neurons and skeletal muscle cells: acetylcholine binds the receptor, which opens a channel, and the cell depolarizes and contracts. That's the move to practice. Given a ligand, a receptor, and a membrane, explain step by step how binding leads to ions crossing and the membrane potential changing. On multiple-choice questions, expect to predict the direction of the voltage change when a sodium channel opens, or to identify what disrupts the response if the receptor is blocked. State the cause (positive ions entering) and the effect (membrane becomes less negative).

## Depolarization vs Repolarization

Depolarization makes the inside of the cell more positive as sodium rushes in. Repolarization is the recovery step that brings the membrane back toward its negative resting value, usually as potassium (K⁺) leaves the cell. Depolarization is the spike up; repolarization is the return down.

## Key Takeaways

- Depolarization is a shift in membrane potential toward a less negative, more positive value.
- It is caused by positive ions, mainly sodium (Na⁺), flowing into the cell through ion channels.
- At a neuromuscular junction, a ligand like acetylcholine can trigger depolarization that starts muscle contraction.
- Depolarization is an example of facilitated diffusion turning a chemical signal into an electrical response.
- Repolarization is the opposite, recovery step that returns the membrane toward its resting potential.

## FAQs

### What is depolarization in AP Biology?

It's a change in a cell's membrane potential toward a less negative (more positive) value, caused by sodium ions flooding into the cell through ion channels. In nerve and muscle cells it triggers signals like action potentials or contractions.

### Does depolarization mean the inside of the cell gets positive?

Yes, that's the direction. The inside starts out negative at rest, and depolarization moves it toward zero and can push it positive as Na⁺ enters. The cell doesn't have to fully flip positive to count as depolarizing; it just has to become less negative.

### How is depolarization different from repolarization?

Depolarization is the membrane becoming more positive as sodium enters. Repolarization is the recovery afterward, where the membrane returns toward its negative resting value, usually as potassium exits. One is the spike, the other is the reset.

### How does acetylcholine cause depolarization?

Acetylcholine, released from a neuron, binds to acetylcholine receptor (AChR) [proteins](/ap-bio/unit-1/proteins/study-guide/UyJypYtavwuCLFlWa8wo "fv-autolink") at the synapse. That binding opens ion channels, sodium flows into the muscle cell, the membrane depolarizes, and the muscle contracts. This exact setup appeared on a 2018 short FRQ.

### Is depolarization active transport?

No. Depolarization itself is sodium moving down its gradient through channels, which is facilitated diffusion and uses no ATP. The active transport happens earlier, when the sodium-potassium pump uses ATP to build the gradient that makes depolarization possible.

## Related Study Guides

- [5.4 Non-Mendelian Genetics](/ap-bio/unit-5/non-mendelian-genetics/study-guide/5oRHoGlMbML8IgtaHaHs)

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