---
title: "Simple Diffusion — AP Bio Definition & Exam Guide"
description: "Simple diffusion is passive transport where small nonpolar molecules cross the membrane down their concentration gradient, no energy or proteins needed. Key to Unit 2."
canonical: "https://fiveable.me/ap-bio/key-terms/simple-diffusion"
type: "key-term"
subject: "AP Biology"
---

# Simple Diffusion — AP Bio Definition & Exam Guide

## Definition

Simple diffusion is a type of passive transport where particles spread out evenly within an available space without needing energy input, moving from areas of high concentration to areas of low concentration until equilibrium is reached.

## Related Study Guides

- [2.6 Facilitated Diffusion](/ap-bio/unit-2/membrane-transport/study-guide/7yV52dtD7Vj7ZadTEd2P)

## Review

### Related Terms

- [Osmosis](/ap-bio/key-terms/osmosis): This is similar to simple diffusion but specifically involves water molecules moving across a semi-permeable membrane towards an area with higher solute concentration.
- [Facilitated Diffusion](/ap-bio/key-terms/facilitated-diffusion): This is like simple diffusion but requires help from transport proteins to move substances across the cell membrane.
- [Concentration Gradient](/ap-bio/key-terms/concentration-gradient): This is the difference in concentration of a substance between two regions. It's what drives simple diffusion, just like how a hill (gradient) would make a ball roll down.