Cell theory

In 1665 Robert Hooke looked at a thin slice of cork and named the tiny boxes he saw “cells”.

What a cell is

A cell is the smallest unit of a living thing. Some organisms are a single cell, like a bacterium. Others, like you, are made of an enormous number of them. The U.S. National Human Genome Research Institute gives an estimate of 10 to 100 trillion cells in an adult human body. Whether there is one cell or trillions, the cell is the basic building block.

In a body with many cells, the cells do not all do the same job. Cells of one kind that join up and share a task form a tissue. Tissues combine into an organ such as the stomach, heart or brain. Several organs that work together make an organ system, like the digestive system, and the systems together make the whole organism.

Why did it take almost two centuries to get from Hooke’s cork to a theory of cells? Part of the answer is the cells themselves. Most living cells are nearly transparent, so under a simple lens they show little more than an outline. The parts inside became clear only as lenses improved and as scientists learned to stain tissues with dyes. Each better tool showed a little more, and the theory grew out of those small steps.

From animalcules to a theory

A few years after Hooke, the Dutch shopkeeper Antony van Leeuwenhoek ground his own small, powerful lenses. In the 1670s he saw single-celled organisms swimming in water and called them animalcules, “little animals”. He was among the first people to see bacteria and protozoa. Better lenses and stains followed, and observers began to see parts inside cells. In 1831 the botanist Robert Brown described the nucleus as a regular feature of plant cells.

The big step came in Germany. In 1838 the botanist Matthias Schleiden proposed that every part of a plant is made of cells or of things cells produce. In 1839 the zoologist Theodor Schwann, after talking with Schleiden, showed that animal tissues are built the same way. Plants and animals, so different on the outside, turned out to share one building plan.

Where do new cells come from?

Schleiden and Schwann got one thing wrong. They thought new cells could form out of non-cellular material, a little like crystals growing in a liquid. In 1852 Robert Remak published strong evidence that cells arise from other cells by division. In 1855 Rudolf Virchow made the idea famous with the Latin phrase omnis cellula e cellula, “every cell comes from a cell”. Virchow did not credit Remak, and historians still discuss who deserves the credit. Virchow went on to argue that diseases begin as changes in cells, which shaped modern medicine.

Put together, these findings make up the cell theory as biology books state it today: all living things are made of one or more cells, the cell is the basic unit of life, and new cells come only from existing cells. Every unit that follows in this course rests on those three sentences.