All living beings are made of cells

The cell, the basic building block of life

Whether you look at a bacterium, a lettuce leaf, or yourself in a mirror, you are actually observing assemblies of cells. The cell is the smallest unit capable of carrying out, on its own, all the functions of life: feeding, breathing, reproducing, reacting to its environment.

Cell theory

Since the 19th century, biologists have established three fundamental ideas, brought together under the name of cell theory:

• every living being is made up of one or more cells;
• the cell is the unit of structure and function of living things;
• every cell comes from a pre-existing cell, through division.

Unicellular and multicellular organisms

• Unicellular organisms (such as bacteria or certain yeasts) are made up of a single cell, which carries out all vital functions on its own.
• Multicellular organisms (such as animals, plants, fungi) bring together billions of cells, often specialized: muscle cells, nerve cells, skin cells...

A size invisible to the naked eye

A cell generally measures between 10 and 100 micrometers (thousandths of a millimeter): a microscope is therefore needed to observe it. It was in fact thanks to the invention of the microscope, in the 17th century, that the English scientist Robert Hooke first observed structures which he named "cells" (from the Latin cellula, small room), while looking at a piece of cork.

Key takeaway

All life on Earth relies on the same basic unit, the cell. Whether you are a bacterium or a human being made up of several tens of thousands of billions of cells, the logic of functioning remains the same at this microscopic scale.