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.
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.
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 cellsCellThe smallest unit of life that can carry out all the activities of a living thing: taking in energy, using information stored in DNA, and reproducing.. 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 theoryCell theoryThe foundation of biology: all living things are made of one or more cells, the cell is the basic unit of life, and all cells come from existing cells.:
- All living things are made of one or more cells.
- The cell is the basic unit of structure and function in living things.
- 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.
What every cell has
Bacteria, oak trees, and humans look nothing alike, yet all of their cells share four things:
- A plasma membranePlasma membraneThe thin, flexible boundary that surrounds every cell and controls what enters and leaves., the thin boundary that separates the inside of the cell from the outside.
- CytoplasmCytoplasmEverything inside the plasma membrane except the nucleus: the watery cytosol together with the organelles suspended in it., the jelly-like interior in which chemical reactions take place.
- DNADNA (deoxyribonucleic acid)The molecule that stores genetic information as a sequence of four chemical bases (A, T, G, C) along a double helix., the molecule that stores the instructions for building and running the cell.
- RibosomesRibosomeA molecular machine made of RNA and protein that reads messenger RNA and links amino acids together to build a protein. It is found in all cells., 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 prokaryoteProkaryoteAn organism, such as a bacterium, whose cells have no nucleus or other membrane-bound organelles. such as a bacterium has no nucleusNucleusThe membrane-bound organelle of eukaryotic cells that holds most of the cell's DNA and is the site where genes are copied into RNA.. Its DNA sits in a region of the cytoplasm called the nucleoidNucleoidThe region of a prokaryotic cell where the DNA is concentrated. It is not enclosed by a membrane., which is not wrapped in a membrane. Prokaryotic cells are small, typically 1 to 5 µm across, and most have a cell wallCell wallA rigid layer outside the plasma membrane of plants, fungi, and most bacteria. It protects the cell and resists the pressure of water pushing outward. made of peptidoglycanPeptidoglycanA mesh-like polymer of sugars and short peptides that forms the bacterial cell wall..
A eukaryoteEukaryoteAn organism whose cells have a nucleus and other membrane-bound organelles. Animals, plants, fungi, and protists are eukaryotes. has a true nucleus, plus other internal compartments called organellesOrganelleA specialized structure inside a cell that performs a specific job, such as the nucleus, mitochondria, or Golgi apparatus., 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 |
Which structure is found in both prokaryotic and eukaryotic cells?
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.
Want a labelled picture? Compare animal and plant cells or prokaryotic and eukaryotic cells 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.
Where is the DNA of a bacterium located?
According to cell theory, where do new cells come from?
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.
Sources
- Biology 2e, Chapter 4: Cell Structure (OpenStax) Concepts adapted and rewritten; no text reproduced verbatim.
- NIH 3D: Animal cell model
Last reviewed October 1, 2026