In 1665 the English scientist Robert Hooke looked at a thin slice of cork through an early microscope and saw rows of tiny empty boxes. They reminded him of the small rooms monks lived in, so he called them cells. He was looking at the dead walls of plant cells, but the name stuck. A few years later the Dutch cloth merchant Antonie van Leeuwenhoek, grinding his own powerful lenses, became the first to see living single-celled organisms swimming in pond water, which he called “animalcules.”

Cell theory

Nearly two centuries passed before scientists pulled these observations together. The German botanist Matthias Schleiden concluded in 1838 that plants are made of cells, and the zoologist Theodor Schwann extended the idea to animals in 1839. In 1855 the physician Rudolf Virchow popularized the Latin phrase omnis cellula e cellula, “every cell from a cell,” building on work by his colleague Robert Remak. Together these ideas form cell theory:

• All living things are made of one or more cells.
• The cell is the basic unit of structure and function in living things.
• All cells come from preexisting cells.

Most cells are microscopic for a good reason. As a cell grows, its volume rises faster than its surface area, and the surface is where food enters and waste leaves. A cell that grew too large could not supply its interior fast enough.

Two basic kinds of cell

Prokaryotic cells, found in bacteria and archaea, are small and simple. They lack a nucleus: their DNA sits in a region of the cell's interior rather than inside a membrane-wrapped compartment. Eukaryotic cells, found in animals, plants, fungi and protists, are usually much larger and contain a nucleus and many organelles, small structures that each do a particular job.

A tour of the eukaryotic cell

• The nucleus holds the cell's DNA and controls which genes are used.
• Ribosomes build proteins by following genetic instructions. They are found in every cell, prokaryotic and eukaryotic alike.
• The endoplasmic reticulum and Golgi apparatus fold, package and ship proteins and fats, like a factory and its post office.
• Mitochondria release energy from food and store it in a molecule called ATP. Busy cells such as muscle cells contain many of them.
• Chloroplasts, found in plants and algae, capture sunlight to make sugar.
• Plant cells also have a rigid cell wall made of cellulose and a large central vacuole that stores water and keeps the plant firm.

The cell membrane

Every cell is wrapped in a membrane made of a double layer of fatty molecules called phospholipids. It is selectively permeable: it lets some substances through and keeps others out. Small molecules such as oxygen slip across by diffusion, moving from where they are concentrated to where they are scarce. Osmosis is the diffusion of water across a membrane, from the side with fewer dissolved particles toward the side with more. When a cell must move a substance against that flow, it uses energy to pump it, a process called active transport.

Where mitochondria came from

In 1967 the American biologist Lynn Margulis argued that mitochondria and chloroplasts were once free-living bacteria that were swallowed by larger cells and, instead of being digested, became permanent partners. This endosymbiotic theory was widely rejected at first. Today it is broadly accepted, because the evidence is strong: mitochondria and chloroplasts have their own DNA, arranged in a loop like bacterial DNA; they are wrapped in two membranes; they divide on their own much as bacteria do; and their ribosomes resemble bacterial ribosomes. Every animal, including you, carries descendants of ancient bacteria in nearly every cell.

From one cell to trillions

Many organisms are a single cell. Others, like humans, are made of trillions; a 2016 estimate put the adult human body at around 30 trillion cells, of a few hundred types. In multicellular organisms, cells specialize: nerve cells carry signals, red blood cells carry oxygen, and skin cells form a barrier. Yet nearly all of them carry the same DNA. What makes them different is which genes each one switches on.

Worked example: lettuce, slugs and seawater

Osmosis explains several everyday puzzles.

Limp lettuce. Lettuce left out wilts as its cells lose water. Put it in a bowl of plain water and it crisps up again. The water in the bowl has fewer dissolved particles than the fluid inside the leaf's cells, so water moves into the cells. Their central vacuoles swell, pressing against the cell walls, and the leaf stiffens.

Salt on a slug. A slug's moist skin acts like a membrane. Salt on its surface creates a very concentrated solution outside the body, so water rushes out of its cells. That is why salt is so harmful to slugs.

Seawater. Shipwrecked sailors who drink seawater get thirstier, not less thirsty. Seawater is saltier than body fluids. The extra salt pulls water out of cells, and the kidneys must use even more water to flush the salt away, leaving the body more dehydrated than before.

In each case, nothing is pumping the water. It simply moves across membranes toward the side with more dissolved particles. The same quiet process is at work in every cell of your body, every second.