---
title: "Endoplasmic Reticulum (ER) — AP Bio Definition & Exam Guide"
description: "The endoplasmic reticulum is a membrane network that makes proteins and lipids. Learn how rough vs. smooth ER fuels the endomembrane system on the AP Bio exam."
canonical: "https://fiveable.me/ap-bio/key-terms/endoplasmic-reticulum-er"
type: "key-term"
subject: "AP Biology"
---

# Endoplasmic Reticulum (ER) — AP Bio Definition & Exam Guide

## Definition

The endoplasmic reticulum is a network of tubules and sacs that function in protein synthesis, lipid metabolism, and detoxification processes.

## Related Study Guides

- [2.1 Cell Structure and Function](/ap-bio/unit-2/cell-structure-subcellular-components/study-guide/oFM5gT3D8Pj5lZXmTNB9)

## Review

### Related Terms

- Smooth ER: This is a type of endoplasmic reticulum that lacks ribosomes on its surface. It's involved in lipid synthesis and detoxification processes. You can think of it as the part of the factory where non-protein products are made.
- Rough ER: This is another type of endoplasmic reticulum but with ribosomes attached to its surface. It's primarily involved in protein synthesis. Imagine it as the section of the factory where all protein-related assembly happens.
- [Ribosome](/ap-bio/key-terms/ribosome): These are tiny structures located either freely in the cytoplasm or attached to the rough ER. They're responsible for translating mRNA into proteins - like workers on an assembly line turning blueprints into finished goods.