What is a cell? The basic unit of life, explained

A cell is the smallest unit of life that can survive on its own: a droplet of water, proteins and other molecules wrapped in a thin membrane, carrying its own DNA and the machinery to read it, make what it needs and copy itself. Every living thing is one cell or many, from a single bacterium to the roughly 30 trillion human cells in an adult body.

In short

• Cell theory says three things: all living things are made of cells, the cell is the basic unit of life, and every cell comes from an earlier cell.
• Every cell starts out with a membrane, cytoplasm, DNA and ribosomes; the cells of plants, animals and fungi also have a nucleus and other compartments.
• Cells are small for a physical reason: the bigger a cell gets, the less surface it has for each unit of volume it must feed.

What does cell theory say?

Cell theory is the idea that cells are the basic unit of all life. It rests on three statements: every organism is made of one or more cells; the cell is the smallest unit that has the properties of life; and every cell arises from a cell that existed before it. No cell has ever been seen to form from non-living chemicals today.

The theory took nearly two centuries to put together.

Year	Step
1665	Robert Hooke's book Micrographia describes thin slices of cork as full of small "pores, or cells," the first use of the word for them
1670s	Antonie van Leeuwenhoek, grinding his own lenses, sees living single-celled organisms in water
1838–1839	Matthias Schleiden argues that plants are made of cells, and Theodor Schwann extends the claim to animals
1852	Robert Remak publishes observations that new cells come from the division of existing ones
1855	Rudolf Virchow popularizes the rule "omnis cellula e cellula," every cell from a cell

Hooke was looking at the empty walls of dead plant cells, which is why he saw a honeycomb, as the University of California Museum of Paleontology notes in its account of his work. The living contents only came into focus with better microscopes.

What is inside a cell?

Every cell that can grow and divide has four things. The cell membrane is a film of fatty molecules two layers thick that separates inside from outside and lets only some substances through. The cytoplasm is everything inside it, a crowded gel of water, salts and proteins. The DNA holds the instructions, as what is DNA? explains. And ribosomes are the small machines that read those instructions and build proteins.

Bacteria and archaea stop there, more or less. The cells of animals, plants and fungi add membrane-wrapped compartments called organelles, which divide the work. This split, between cells with a nucleus and cells without, is the subject of prokaryotic vs eukaryotic cells.

One useful picture is a factory, so long as its limits are kept in mind.

Part	What it does	In a factory
Nucleus	Stores the DNA and copies genes into messages	The archive of blueprints
Ribosomes	Read the messages and join amino acids into proteins	Assembly lines
Endoplasmic reticulum	A folded membrane network where many proteins and fats are made	The production floor
Golgi apparatus	Sorts, modifies and packages proteins for delivery	Packing and shipping
Mitochondria	Turn food into ATP, the cell's usable energy	The power plant
Lysosomes	Break down worn-out parts and swallowed material	Recycling
Cytoskeleton	Protein fibers that give shape and carry cargo	Girders and conveyor tracks
Cell membrane	Controls what enters and leaves	Walls with guarded gates

Plant cells also have a rigid cell wall, chloroplasts that capture sunlight, and usually a large water-filled vacuole. The mitochondria earn a note of their own, since they do far more than make energy.

The factory picture breaks in three places. There is no manager: nothing in a cell decides anything, and every process runs because molecules bump into each other and stick or react. The parts are constantly rebuilt, so a cell is more like a factory that replaces many of its own machines within hours or days. And unlike any factory, a cell builds a complete copy of itself, archive included.

Why are cells so small?

Most cells are too small to see. Bacteria and archaea are typically 0.1 to 5 micrometers (millionths of a meter) across, and eukaryotic cells are 2 to 100 times wider, according to Wikipedia's overview of the cell. The reason is geometry. A cell feeds and cleans itself through its surface, but its needs grow with its volume, and volume grows faster than surface.

Here is the arithmetic, treating a cell as a cube for simplicity.

1. A cube 10 micrometers on a side has six faces of 100 square micrometers each, so a surface of 600 square micrometers. Its volume is 10 × 10 × 10 = 1,000 cubic micrometers. Each cubic micrometer of cell has 0.6 square micrometers of surface serving it.
2. Make the cube 100 micrometers on a side, ten times wider. The surface is 6 × 100 × 100 = 60,000 square micrometers. The volume is 100 × 100 × 100 = 1,000,000 cubic micrometers.
3. The ratio is now 0.06. Making the cell ten times wider gave each unit of its inside one tenth as much surface to feed it.

Distance matters too. Molecules move around a cell mostly by diffusion, random jostling, and the time diffusion takes grows with the square of the distance. A small molecule crosses 10 micrometers of water in a fraction of a second, but needs several minutes to cross a millimeter and a couple of months to cross 10 centimeters. That is why large organisms are built of many small cells with blood vessels or sap to carry supplies, rather than one giant cell.

The exceptions obey the same geometry. Nerve cells can be very long but stay thin. The largest known bacterium, Thiomargarita magnifica, averages about 1 centimeter long, but a water-filled sac takes up 65% to 80% of its volume and pushes the living cytoplasm into a thin layer just under the membrane, according to its Wikipedia entry.

How many cells are in the human body?

The best-known estimate comes from a 2016 study by Ron Sender, Shai Fuchs and Ron Milo in PLOS Biology. For a 70 kilogram "reference man," they counted about 30 trillion human cells, with a calculated uncertainty of about 2% and person-to-person variation of about 14%, and about 38 trillion bacteria. It is an estimate for one standard body, not a count of anyone's. Older figures had ranged widely; the U.S. National Human Genome Research Institute's glossary entry for "cell" still gives 10 to 100 trillion.

Number and mass tell different stories. By count, 84% of human cells are red blood cells, among the smallest cells in the body, with a volume of about 100 cubic micrometers each. By mass, about three quarters of all cell mass is fat cells and muscle cells. The many types of cell in a body, from neurons to skin cells, all descend from one fertilized egg and all carry, with few exceptions, the same DNA. Red blood cells are one of those exceptions: in mammals they lose their nucleus, and with it their DNA, as they mature, so a mature red cell can't divide or make new proteins.

What do people get wrong about cells?

The most common mistake is to picture a cell as a bag of watery liquid with a few floating parts. The inside is packed: proteins take up a large share of the volume, and a protein fiber network holds organelles in place and carries cargo along tracks.

A second is that a cell is a fixed object. A cell is a process. It takes in nutrients, pumps ions, repairs damage, breaks down old proteins and builds new ones all the time, and it stops being a living cell when that turnover stops. That ceaseless chemistry is its metabolism, powered by the energy it takes in.

Cells lead to almost everything else in biology: how they divide, in mitosis and meiosis; how they read their genes; and how, in the case of bacteria, one cell can be a whole organism.

Check yourself

• Without looking back, state the three parts of cell theory and say who first showed that cells come from cells.
• Explain with numbers why a cell ten times wider has a harder time feeding itself.
• Name three places where the factory picture of a cell stops being true.

Sources

• Revised Estimates for the Number of Human and Bacteria Cells in the Body — Sender, Fuchs and Milo, PLOS Biology, 2016
• Cell — National Human Genome Research Institute
• Robert Hooke — University of California Museum of Paleontology
• Cell theory — Wikipedia
• Cell (biology) — Wikipedia