# What Is a Cell?

> Meet the basic unit of life, the three ideas of cell theory, and the big divide between prokaryotic and eukaryotic cells.

Level: Introductory | Reading time: 10 min | Last reviewed: 2026-10-01

Web page: https://cellnaut.com/learn/what-is-a-cell

**Quick answer:** A cell is the smallest unit of life that can take in energy, build and repair itself, respond to its surroundings and reproduce. Cell theory says all living things are made of cells, the cell is the basic unit of structure and function, and every cell comes from an existing cell.

## What you will learn

- State the three parts of cell theory.
- List the structures that every cell has in common.
- Compare prokaryotic and eukaryotic cells.
- Name three organelles found in eukaryotic cells and say what each does.

## Key takeaways

- Cell theory has three parts: living things are made of cells, the cell is the basic unit of life, and all cells come from existing cells.
- Every cell has a plasma membrane, cytoplasm, DNA and ribosomes.
- Prokaryotic cells have no nucleus; eukaryotic cells keep their DNA in a nucleus and have membrane-bound organelles.
- Viruses are not cells.

## The smallest unit of life

Look at your hand, a leaf, or a drop of pond water through a strong enough microscope and you will see the same thing: tiny compartments packed side by side. These are cells. A cell is the smallest unit that can do everything we call living: take in energy, build and repair itself, respond to its surroundings, and reproduce.

The word "cell" was coined by Robert Hooke in 1665, when he looked at a thin slice of cork and saw rows of small box-like chambers that reminded him of monks' rooms. Those were the empty walls of dead plant cells. Within a few decades, Antonie van Leeuwenhoek was watching living cells swim in water drops.

## Cell theory

By the middle of the 1800s, a simple but powerful idea had taken shape. Today we call it the cell theory:

1. All living things are made of one or more cells.
2. The cell is the basic unit of structure and function in living things.
3. All cells come from other cells that already exist.

The third point, usually credited to Rudolf Virchow, ended the old belief that living cells could appear out of non-living matter. Every cell in your body descends from a single fertilized egg, and that cell descends from cells in your parents, all the way back through billions of years.

> **Note**
>
> Viruses are not cells. They cannot make their own energy or reproduce by themselves, which is why most biologists treat them as lying at the edge of the definition of life.

## What every cell has

Bacteria, oak trees, and humans look nothing alike, yet all of their cells share four things:

- A plasma membrane, the thin boundary that separates the inside of the cell from the outside.
- Cytoplasm, the jelly-like interior in which chemical reactions take place.
- DNA, the molecule that stores the instructions for building and running the cell.
- Ribosomes, the molecular machines that build proteins.

Having these four parts in common is one of the strongest pieces of evidence that all life shares a common ancestor.

## Two kinds of cells

Cells fall into two big groups, based on how they store their DNA.

A prokaryote such as a bacterium has no nucleus. Its DNA sits in a region of the cytoplasm called the nucleoid, which is not wrapped in a membrane. Prokaryotic cells are small, typically 1 to 5 µm across, and most have a cell wall made of peptidoglycan.

Interactive 3D model: [Bacteria Cell](https://cellnaut.com/studio/bacteria/nucleoid) - A bacterium. Drag to rotate, then pick a structure below the model.

Interactive 3D model: [Cyanobacterium](https://cellnaut.com/studio/cyanobacteria/nucleoid) - A cyanobacterium is also a prokaryote: its DNA floats free too, but it carries membranes for photosynthesis.

A eukaryote has a true nucleus, plus other internal compartments called organelles, each wrapped in its own membrane. Animals, plants, fungi, and protists are all eukaryotes. Their cells are usually 10 to 100 µm across, around ten times wider than a typical bacterium.

|                           | Prokaryotic cell           | Eukaryotic cell                          |
| ------------------------- | -------------------------- | ---------------------------------------- |
| Nucleus                   | No, DNA is in the nucleoid | Yes, DNA is inside a nuclear envelope    |
| Membrane-bound organelles | None                       | Many (mitochondria, Golgi, ER, and more) |
| Typical size              | 1 to 5 µm                  | 10 to 100 µm                             |
| Examples                  | Bacteria, archaea          | Animals, plants, fungi, protists         |

> **Common misconception**
>
> **"Prokaryotes have no organelles."** Prokaryotes do have ribosomes and often have specialized regions inside the cell. What they lack are **membrane-bound** organelles. That is a more precise, and more accurate, statement.

**Check your understanding:** Which structure is found in both prokaryotic and eukaryotic cells?

- A. Nucleus
- B. Ribosomes
- C. Mitochondria
- D. Golgi apparatus

**Answer:** B. Ribosomes

Ribosomes build proteins in every kind of cell. The other three are membrane-bound organelles, which prokaryotes do not have.

## A tour of a eukaryotic cell

An animal cell is a good place to start exploring. The nucleus stores most of the cell's DNA. Mitochondria convert fuel into usable energy. The Golgi apparatus packages proteins for delivery. Click through the labels in the viewer to see where each sits.

Interactive 3D model: [Animal Cell](https://cellnaut.com/studio/animal/nucleus) - An animal cell, the standard eukaryotic example.

Interactive 3D model: [Yeast Cell](https://cellnaut.com/studio/yeast/nucleus) - Yeast is a single cell, yet it is a eukaryote with a true nucleus.

Want a labelled picture? Compare [animal and plant cells](https://cellnaut.com/topics/animal-vs-plant-cell) or [prokaryotic and eukaryotic cells](https://cellnaut.com/topics/prokaryote-vs-eukaryote) side by side in the illustrated topics.

The rest of this course takes these organelles one at a time. Before that, a question that sounds simple: just how big is a cell? That is the subject of the next lesson.

**Check your understanding:** Where is the DNA of a bacterium located?

- A. In a nucleus
- B. In the nucleoid region of the cytoplasm
- C. In the cell wall
- D. In mitochondria

**Answer:** B. In the nucleoid region of the cytoplasm

Bacteria have no nucleus. Their DNA is concentrated in the nucleoid, a region with no membrane around it.

**Check your understanding:** According to cell theory, where do new cells come from?

- A. From non-living matter
- B. From existing cells
- C. From viruses
- D. From the nucleus only

**Answer:** B. From existing cells

The third part of cell theory states that all cells arise from pre-existing cells.

## 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