# The Nucleus and the Endomembrane System

> Follow a protein from the gene that codes for it, through the ER and Golgi, to its destination, and meet the lysosome clean-up crew.

Level: High school | Reading time: 12 min | Last reviewed: 2026-10-01

Web page: https://cellnaut.com/learn/nucleus-and-endomembrane

**Quick answer:** The nucleus stores the cell's DNA and controls gene activity, while the endomembrane system builds, folds, sorts and ships proteins and lipids. A secreted protein is made on the rough ER, modified and packaged in the Golgi apparatus, and sent to the cell surface in vesicles. Lysosomes digest worn-out material.

## What you will learn

- Describe the structure of the nucleus and the role of the nuclear pores.
- Distinguish rough ER from smooth ER.
- Trace the path a secreted protein takes through the cell.
- Explain what lysosomes do and why white blood cells rely on them.

## Key takeaways

- The nuclear envelope has pores that control traffic between the nucleus and the cytoplasm.
- Rough ER carries ribosomes and makes proteins; smooth ER makes lipids and helps detoxify.
- Secreted proteins travel ER, then Golgi, then a vesicle, then the plasma membrane.
- Lysosomes hold digestive enzymes, which is why white blood cells use them to destroy engulfed microbes.

## The nucleus: the genome's vault

The nucleus is usually the largest organelle in an animal cell, about 5 to 10 µm across. It holds nearly all of the cell's DNA, keeps it safe, and controls when each gene is used.

Three features are worth knowing:

- The nuclear envelope is a double membrane. It is dotted with thousands of **nuclear pores**, channels that let messenger molecules out and regulatory proteins in, while keeping the DNA itself inside.
- Inside, the DNA is wound around proteins to form chromatin. When a cell is not dividing, chromatin is spread out so genes can be read.
- A dense region called the **nucleolus** assembles the parts of ribosomes.

Interactive 3D model: [Animal Cell](https://cellnaut.com/studio/animal/nucleus) - The nucleus of an animal cell.

For a labelled diagram of these parts, open the topic [The Cell Nucleus](https://cellnaut.com/topics/nucleus).

> **Common misconception**
>
> **"Every cell has a nucleus."** Prokaryotes do not, as you saw in lesson 1. Even within the human body, mature red blood cells push out their nucleus to make more room for oxygen-carrying hemoglobin.

## The endomembrane system

Most proteins and lipids the cell exports, or builds into its membranes, go through a connected network of compartments called the endomembrane system. Think of it as a factory with a post office attached.

### Endoplasmic reticulum: the factory floor

The endoplasmic reticulum (ER) is a maze of flattened sacs and tubes that is directly joined to the nuclear envelope. It comes in two types:

- **Rough ER** is studded with ribosomes. Proteins made here are threaded into the ER interior as they are built. It is especially large in cells that secrete a lot of protein.
- **Smooth ER** has no ribosomes. It makes lipids, helps break down toxins in liver cells, and stores calcium ions.

### Golgi apparatus: sorting and shipping

Proteins leave the ER wrapped in small vesicles and travel to the Golgi apparatus, a stack of flattened membrane sacs. Inside, enzymes trim, tag, and modify them, such as by adding sugar chains. At the far face, the finished products are packaged into new vesicles with a destination label.

Interactive 3D model: [Animal Cell](https://cellnaut.com/studio/animal/golgi) - The Golgi apparatus sits close to the nucleus.

### A protein's journey

Here is the route a protein takes when the cell is going to secrete it, for example an antibody or a digestive enzyme:

1. The gene is copied into messenger RNA in the nucleus.
2. The mRNA leaves through a nuclear pore, and a ribosome on the rough ER builds the protein.
3. A transport vesicle carries the protein to the Golgi.
4. The Golgi modifies and sorts it into a secretory vesicle.
5. The vesicle moves to the plasma membrane and releases its contents by exocytosis.

**Check your understanding:** Where are proteins that will be exported from the cell first made?

- A. Ribosomes on the rough ER
- B. Smooth ER
- C. Lysosomes
- D. The nucleolus

**Answer:** A. Ribosomes on the rough ER

Ribosomes attached to the rough ER build proteins destined for secretion or for the cell membrane.

**Check your understanding:** Which organelle modifies, sorts, and packages proteins into vesicles for delivery?

- A. Mitochondrion
- B. Golgi apparatus
- C. Nucleus
- D. Ribosome

**Answer:** B. Golgi apparatus

The Golgi apparatus receives proteins from the ER, modifies them, and sends them to their destinations.

## Lysosomes: the recycling center

A lysosome is a small, acidic vesicle filled with about 50 different digestive enzymes. It does two jobs: it digests material the cell has taken in, and it recycles the cell's own worn-out parts.

The acid inside is a safety feature. The enzymes only work well at low pH, so if a lysosome leaks into the neutral cytoplasm, they are mostly inactive and the cell is not digested from within.

White blood cells make heavy use of lysosomes. A neutrophil engulfs a bacterium in a vesicle, which then fuses with lysosomes. The invader is broken down inside a sealed compartment, where it cannot harm the cell.

Interactive 3D model: [White Blood Cell](https://cellnaut.com/studio/whiteBlood/lysosome) - A white blood cell, with lysosomes and a lobed nucleus.

**Check your understanding:** Why can lysosomes digest material without harming the rest of the cell?

- A. Their enzymes work best in an acidic compartment enclosed by a membrane
- B. They only digest water
- C. They sit inside the nucleus
- D. They have no membrane

**Answer:** A. Their enzymes work best in an acidic compartment enclosed by a membrane

Lysosomal enzymes need acidic conditions, and the lysosome keeps them sealed away from the cytoplasm.

## Sources

- [Biology 2e, Chapter 4: Cell Structure (OpenStax)](https://openstax.org/books/biology-2e/pages/4-introduction) - CC BY-NC-SA 4.0. Concepts adapted and rewritten; no text reproduced verbatim.
- [NIH 3D: Animal cell model](https://3d.nih.gov/entries/3DPX-015797/2) - See entry page and docs/ASSETS.md