What Is a Cell?

A cell is the smallest possible unit of life. Every living thing on Earth is made of cells. Some, like bacteria, are just one cell. Others, like you, are made of around 37 trillion cells. Cells are tiny: you would need 20 of the smallest ones lined up to span the thickness of a human hair. But each cell is a fully functional living machine, with its own boundary, its own machinery, its own instructions and its own ways of getting energy and making more of itself.

  • Smallest unit of lifeA single cellSmaller than this and nothing is alive
  • Discovered1665By Robert Hooke, in a slice of cork
  • Two main typesProkaryote, eukaryoteSimple vs complex cells
  • Smallest cellMycoplasmaapprox. 0.2 micrometres across
  • Largest cellOstrich egg yolkUp to 8 cm across (technically one cell)
  • Cells in human bodyapprox. 37 trillionIn around 200 different types

How cells were discovered

Cells could not be seen until the microscope was invented in the early 1600s. In 1665, an English scientist called Robert Hooke sliced a thin piece of cork and looked at it under one of the first microscopes. He saw it was made of tiny box-shaped chambers, which he named "cells" because they reminded him of the small rooms (cells) used by monks in a monastery. Hooke had not actually seen living cells (cork is dead plant tissue) but the name stuck.

A few years later, the Dutch lens-maker Antonie van Leeuwenhoek built much better microscopes and saw, for the first time, single-celled bacteria and protists swimming around in pond water. He called them animalcules (little animals). He had become the first person in history to see microbes, the most common form of life on Earth.

Cell theory

It took almost 200 more years for biologists to put together what is now called cell theory, in the 1830s. Cell theory has three main rules:

  • All living things are made of cells.
  • The cell is the smallest unit of life.
  • All cells come from other cells (they do not just appear from nothing).

Cell theory is one of the foundations of all modern biology. It explained why diseases caused by microbes were contagious, why crops could be grown from cuttings, and why life can only come from life. (Before cell theory, many people believed in "spontaneous generation": the idea that maggots came from rotting meat or mice came from old rags. Cell theory put a stop to that.)

The two main types of cell

All cells fall into one of two huge groups:

  • Prokaryotes are simple cells with no nucleus. Their DNA floats around freely inside the cell, and they have very few internal structures. Bacteria and archaea are both prokaryotes. They are tiny (typically 1 to 2 micrometres) and they were the only kind of life on Earth for the first 2 billion years.
  • Eukaryotes are much more complex cells with a nucleus that contains the DNA, plus many other internal structures called organelles. Plants, animals, fungi and protists are all built from eukaryotic cells. Eukaryotes are typically 10 to 100 times bigger than prokaryotes.
Fact If you stretched out all the DNA from a single one of your cells, it would be about 2 metres long. The DNA from all 37 trillion cells in your body, stretched end to end, would reach to the Sun and back over 200 times. Yet inside each cell, the DNA is folded so tightly it fits inside a nucleus only a few micrometres wide.

What cells do

Every cell, no matter what kind, has to do the same basic jobs:

  • Get energy from food (or from sunlight, if it is a plant cell).
  • Make new proteins using the recipes stored in its DNA.
  • Detect and respond to its environment.
  • Get rid of waste.
  • Eventually divide in two to make more cells.

In a multi-cellular creature like you, different cells specialise to do different jobs. Muscle cells contract. Nerve cells carry signals. Red blood cells carry oxygen. Skin cells form barriers. There are around 200 different cell types in a typical human, all descended from a single fertilised egg cell.

Did you know? Most of the cells in your body are not actually "you". You also carry around roughly 38 trillion bacteria, mostly in your gut, almost matching the number of your own cells. These bacteria (collectively called your microbiome) help digest your food, train your immune system and produce some vitamins. Without them, you would not be very healthy.
Deeper dive: how eukaryotic cells got their organelles

One of the most extraordinary stories in biology is how eukaryotic cells came to exist. For the first 2 billion years of Earth's history, the only life was simple prokaryotes (bacteria and archaea). Then, sometime around 1.7 to 2 billion years ago, a strange event happened: one prokaryote swallowed another, and instead of digesting it, the two settled down to live permanently inside one another.

The swallowed cell was a small bacterium that was particularly good at using oxygen to make energy. Sheltered inside its bigger host, it kept doing what it did best, and the host gave it food and protection in return. Over millions of years the two became completely interdependent. That swallowed bacterium is now what we call a mitochondrion, the energy factory inside every animal, plant and fungal cell on Earth. Plant cells later did something similar with a photosynthesising bacterium, which became the chloroplast.

The evidence for this story (called the endosymbiotic theory) is overwhelming. Mitochondria and chloroplasts still have their own DNA, separate from the cell's main DNA in the nucleus. They divide on their own schedule, like bacteria do. And the DNA they carry is more similar to free-living bacteria than to anything else in the cell. So every cell in your body is, in a real sense, two ancient cells living together as one.

For the bits inside a cell, see parts of a cell. For the difference between animal and plant cells, see animal cells vs plant cells. For how cells divide, see mitosis.