The cell

For well over a century after Hooke, people saw cells without grasping what they meant. Then in 1838 the German botanist Matthias Schleiden argued that every plant is built from cells, and in 1839 Theodor Schwann showed the same for animals. Their joint claim became the cell theory: every living thing is made of cells, and the cell is the smallest unit that is alive.

Some living things are just one cell. An amoeba feeds, moves and reproduces on its own, so its single cell is a complete organism. Others, like us, are built from trillions.

Schleiden and Schwann got one thing wrong. They thought new cells could assemble themselves out of a shapeless fluid, rather as crystals grow. Robert Remak, and then Rudolf Virchow in 1855, argued the opposite, and the evidence backed them: every cell comes from an existing cell that divides.

Cells are so small that biologists measure them in a unit of their own, the micrometre: a millionth of a metre, or a thousandth of a millimetre. A typical bacterium is only a micrometre or two long. A red blood cell is a flat disc about eight micrometres across, and many of the other cells in your body are ten to thirty micrometres wide.

For a sense of scale, a human hair is roughly fifty to a hundred micrometres thick. A few of your body cells laid side by side would span it, and dozens of bacteria would fit across the same hair. That is why cells stayed invisible until lenses could enlarge things many times over.

A cell cannot simply keep growing, and the reason is geometry. Everything a cell takes in or gets rid of must cross its outer surface, while everything inside that surface needs supplying.

As a cell gets bigger, its surface area grows more slowly than its volume. A cell twice as wide has four times the surface to trade through, but eight times the volume to feed. Each patch of surface must now serve twice as much cell, and the centre sits farther from the edge. Grow much larger, and food and oxygen could not get in, or wastes out, fast enough.

So large living things grow by adding cells, not by making them bigger. A body built this way, from many cells working together, is called multicellular.

Every cell is wrapped in a membrane, a flexible skin of fatty molecules thousands of times thinner than the cell itself. It holds the cell's contents in, but it is not a sealed bag. It is choosy: some substances cross it freely, while others stay out unless the cell lets them through.

Small molecules such as oxygen and carbon dioxide slip straight across, by diffusion. Molecules are always jostling at random, so where they are packed densely they tend to spread into places where there are fewer. Nothing pushes them, and the cell spends no energy. Oxygen keeps flowing in because the cell uses it up, so there is always less of it inside than outside.

Water crosses membranes as well, and its drift has a name of its own: osmosis. Water moves across a membrane toward the side where more is dissolved, because there the water is less concentrated.

That is why a red blood cell dropped into pure water swells and bursts. It is full of dissolved salts, sugars and proteins, so water flows in faster than it flows out. Put the same cell in very salty water and the opposite happens: now more is dissolved outside the cell than inside, so water drains out and the cell shrivels.

Hospitals work with the same rule. A drip fed into a vein is never pure water but a salt solution as concentrated as blood, so water crosses red blood cells equally both ways and they neither swell nor shrivel.

Diffusion only ever runs one way, from more to less. Yet cells often need the reverse. A root cell must gather minerals from soil that holds less of them than the cell already does, and animal cells keep far more potassium inside, and far less sodium, than the fluid around them.

To move a substance against its natural drift, a cell uses protein pumps set in its membrane, and every stroke of a pump costs energy. This is active transport. The first such pump was identified in 1957 by the Danish scientist Jens Skou: it pushes sodium out of the cell and pulls potassium in. That one pump consumes a large share of an animal cell's energy.

Most of the work in a cell is done by proteins, and the recipe for each one is written in DNA. In your cells, that DNA is kept inside the nucleus, a compartment with a membrane of its own. Stretched end to end, the DNA in one human cell would reach about two metres, yet it folds up to fit inside a nucleus around six micrometres wide.

The DNA stays safely put. When the cell needs a protein, it makes a working copy of that one recipe and sends the copy out of the nucleus. There, tiny machines called ribosomes read it and link small building blocks, amino acids, into a chain in the order the recipe spells out. The finished chain folds into a working protein.

Everything inside a cell's membrane apart from the nucleus is the cytoplasm: a gel crowded with organelles, the cell's small working parts, along with ribosomes and dissolved chemicals. It is less like a bag of water with things floating in it and more like a packed workshop.

Most of it, though, is water. Water makes up around seventy per cent of a typical cell's weight, and much of the rest is protein. The water is not filler. Most of a cell's chemistry happens in it: sugars, salts and other small molecules dissolve in water, meet one another and react. A cell that loses too much water cannot carry on that chemistry, which is why every living cell depends on it.

Plant cells have everything described so far, plus parts that animal cells lack. The most obvious is a stiff cell wall made of cellulose, outside the membrane. It gives each cell a boxy shape and lets a plant stand upright without a skeleton.

Inside, most of a mature plant cell is filled by one large vacuole, a sac of watery sap. Water drawn in by osmosis swells the vacuole until it presses the cell out against its wall. The cell wall pushes back against the swelling, so instead of bursting like a red blood cell in pure water, the cell turns firm, like a well-pumped tyre. Millions of firm cells keep a leaf flat and a stem upright. When a plant runs short of water, its vacuoles shrink, the pressure drops, and it wilts.

Many plant cells, especially in leaves, also hold chloroplasts, green organelles that capture sunlight. Inside them, the energy of light is used to build sugar from carbon dioxide and water, giving off oxygen. This is photosynthesis, and the pigment that traps the light is why leaves are green.

Making sugar is not the same as using it. To release the energy stored in sugar, plant and animal cells alike rely on mitochondria. There, sugar combines with oxygen in a process called respiration, which frees the energy the cell needs and gives back carbon dioxide and water. Hard-working cells carry many: a muscle cell is packed with them.

So a leaf cell runs both halves of one loop: chloroplasts turn carbon dioxide and water into sugar, and mitochondria turn it back, releasing energy. The materials go round, but the energy does not; sunlight keeps supplying it.

A cell that has grown cannot keep swelling for ever. As it widens, its surface grows slower than volume, and it would soon starve its own middle. So it divides, and the order of work matters. Before it splits, it must copy DNA, so that two complete sets of instructions sit side by side. Only then does it pull one set to each end and separate into two cells.

This everyday division is called mitosis. It produces two cells with exactly the same DNA as the one they came from, which is how a body grows from a single cell and replaces the hundreds of billions of cells it wears out each day, such as those lining the gut. Checks built into the cycle hold a cell back until its DNA is fully copied. When those checks fail, damaged cells can keep dividing, which is one way cancer begins.

In a body of trillions, most cells stop being all-rounders and become specialised: shaped and equipped for one job. A red blood cell is a striking case. As it matures it pushes out its own nucleus, freeing room for more of the red protein that carries oxygen. It can never divide again, but for about four months it carries oxygen superbly.

Similar cells doing the same job work together as a tissue. Muscle is a tissue, and so is the thin layer of cells lining your gut. Different tissues combine into an organ, like the heart, which joins muscle, nerves and linings into one pump. Organs work together in organ systems, and the systems together make up the whole body.