Cells

A cell is the smallest unit of life. Every single living thing on Earth is made of cells: some, like bacteria, are just one cell; others, like you, are made of tens of trillions. Cells were discovered in 1665 by the English scientist Robert Hooke, who saw little box-shaped chambers in a slice of cork under a microscope and named them after the small rooms (cells) in a monastery. The cell theory, that all living things are made of cells and that all cells come from earlier cells, is one of the foundations of all biology.

• Cells in human bodyapprox. 37 trillionOf around 200 different types
• SmallestMycoplasma bacteriumapprox. 0.2 micrometres across
• LargestOstrich egg yolkA single cell up to 8 cm across
• First cellapprox. 3.7 billion years agoProbably a simple bacterium
• Two main typesProkaryote, eukaryoteSimple cells vs complex cells
• Smallest unit of lifeA single cellBelow this, nothing is alive

What is a cell?

A cell is like a tiny self-contained factory. It has its own boundary (the cell membrane), its own machinery (the organelles), its own instructions (the DNA), its own energy supply, and its own way of making more of itself. Every cell is a balance of chemistry and structure, kept alive by thousands of tiny reactions happening every second.

Two main types of cell

Biologists split all cells into two groups:

• Prokaryotes: simple cells without a nucleus. Their DNA floats freely inside the cell. Bacteria and archaea are prokaryotes. They were the only life on Earth for the first 2 billion years.
• Eukaryotes: more complex cells with a nucleus and other organelles like mitochondria. Plants, animals, fungi and protists are all made of eukaryotic cells.

Eukaryotic cells are much bigger than prokaryotic cells (typically 10 to 100 times wider). Scientists think eukaryotes evolved when one prokaryote engulfed another and they ended up living together permanently. That captured cell became the mitochondrion, the energy factory inside every modern animal and plant cell.

How cells multiply

Cells make more of themselves by dividing. In a process called mitosis, a cell first copies its DNA, then splits cleanly in half, with each half getting one full copy. The new daughter cells are identical to the original. Mitosis is how you grow from a single fertilised egg into trillions of cells, and how your skin keeps replacing itself every few weeks.

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 it is packed so tightly it fits inside a nucleus smaller than the dot on this letter "i".

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