---
title: "Endosymbiosis — AP Bio Definition & Exam Guide"
description: "Endosymbiosis explains how mitochondria and chloroplasts evolved from engulfed bacteria, providing key evidence for the common ancestry of all eukaryotes."
canonical: "https://fiveable.me/ap-bio/key-terms/endosymbiosis"
type: "key-term"
subject: "AP Biology"
---

# Endosymbiosis — AP Bio Definition & Exam Guide

## Definition

Endosymbiosis is a mutually beneficial relationship where one organism lives inside another. This theory explains how complex cells (eukaryotes) evolved from simple cells (prokaryotes).

## Related Study Guides

- [7.7 Common Ancestry](/ap-bio/unit-7/common-ancestry/study-guide/FNiYICtpxNBjLu17IWjK)

## Review

### Related Terms

- [Symbiosis](/ap-bio/key-terms/symbiosis): A close and long-term biological interaction between two different biological organisms.
- [Mitochondria](/ap-bio/key-terms/mitochondria): An organelle found in large numbers in most cells, where biochemical processes of respiration and energy production occur.
- [Chloroplasts](/ap-bio/key-terms/chloroplasts): Organelles that conduct photosynthesis, where the photosynthetic pigment chlorophyll captures the energy from sunlight.