What Is Life? The Cell
Everything alive is made of cells, tiny bags of chemistry that eat, grow, copy themselves and die. You are a cooperative of about 37 trillion of them.
Ask what separates a living thing from a rock and the answers come easily at first. Living things grow, move, eat, reproduce and respond to the world. But fire grows and consumes; crystals grow; a river moves. Biologists have argued about a watertight definition for centuries. What they agree on is this: on Earth, all life is made of cells, and if you want to understand what life is, you start there.
The basic unit
A cell is a tiny compartment, enclosed by a thin oily membrane, in which the chemistry of life takes place protected from the outside. Most are far too small to see; a typical human cell is about a fiftieth of a millimetre across, and a bacterium ten times smaller still. Inside, dissolved in watery fluid, thousands of different molecules are constantly being built, broken down and shuffled about in a controlled chaos that chemists still cannot reproduce in a test tube.
Some organisms are a single cell, and they are by far the most numerous living things on the planet. Others, like oak trees and people, are vast cooperatives. Your body is made of roughly 37 trillion cells of more than 200 kinds, each specialised: muscle cells that contract, nerve cells that carry signals, red blood cells that ferry oxygen, skin cells that form a barrier. All of them descended from a single fertilised egg, and all carry the same set of instructions.
Inside a cell
A cell is not a bag of soup but a busy, organised factory. The membrane is its wall, studded with gates and pumps that admit nutrients and expel waste. The nucleus is the archive, holding the DNA, two metres of it in each human cell, coiled and folded into a space a few millionths of a metre across. The DNA carries the instructions for making proteins, and proteins do almost everything: they are the enzymes that speed up chemical reactions, the fibres that give the cell shape, the motors that move things around, the pumps in the membrane.
Ribosomes, tens of thousands per cell, are the machines that read copies of the DNA instructions and assemble proteins from amino acids. Mitochondria are the power plants, burning sugar with oxygen to produce the energy currency that drives everything else. Plant cells carry an extra structure, chloroplasts, which capture sunlight to make sugar in the first place.
Bacteria are built more simply, with no nucleus and no mitochondria, their DNA floating free in the cell. This simpler design is older. For the first two billion years of life on Earth, it was the only one.
Where mitochondria came from
Mitochondria have their own DNA, separate from the nucleus, and they divide on their own like bacteria. That is because they were bacteria. Around two billion years ago, one cell engulfed another and failed to digest it, and the captive, instead of dying, kept producing energy for its host. The partnership proved so successful that every animal, plant and fungus alive today is a descendant of it. Chloroplasts have the same history: they were once free-living photosynthetic bacteria. Complex life is built on an ancient merger.
Birth, work and death
Cells make more cells by dividing in two. First the cell copies its entire DNA, then it pulls the two copies to opposite ends and pinches itself in half. In your body this happens about four million times a second. Skin cells last a few weeks, the lining of the gut only a few days; red blood cells about four months. Your body replaces roughly one percent of its cells every day. A few, including most of the neurons in your brain, are never replaced and must last a lifetime.
Cells also die on purpose. When a cell is damaged, infected or simply no longer needed, it can trigger a tidy self-destruct programme, shrinking and packaging itself for recycling. The webbing between a developing baby's fingers is removed this way. When this control fails, and damaged cells keep dividing when they should stop, the result is cancer.
What makes it alive
A cell is a system that captures energy from its surroundings and uses it to maintain its own order, to grow and to copy itself, carrying instructions that can change slightly from one generation to the next. Fire captures energy but carries no instructions. A crystal has order but does not maintain it. A virus has instructions but no machinery of its own, which is why biologists argue about whether viruses count as alive. The cell has all three, and so far, on this planet, that is what life is.
What we still don't know
How the first cell came to be, from chemistry alone, is one of science's deepest open questions. Why cells age, and whether they must, is another. And whether life elsewhere would be built from cells at all, or from something we have never imagined, we cannot say until we find it.
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Test yourself
4 questions · answers explained at the end
1.What is the basic unit of all living things?
2.What do mitochondria do?
3.What surprising origin do mitochondria have?
4.Roughly how many cells are in an adult human body?