Chapter I
Little Rooms and Little Animals
In 1665 Robert Hooke published Micrographia, a book of astonishing drawings made through his microscope: a flea, the eye of a fly, the point of a needle. Looking at a thin slice of cork, he saw a honeycomb of tiny empty boxes and called them cells, a word often said to recall monks' rooms. He was looking at the walls of dead plant cells, and saw no reason to think they were important.
A decade later Antonie van Leeuwenhoek, a draper in Delft with no scientific training, saw far more. His tiny single lenses, a few millimetres across and held close to the eye, magnified more than two hundred times. In pond water, rainwater and the plaque from his teeth, he found creatures by the thousand, too small to see: protists and bacteria. The Royal Society in London doubted him until its own members confirmed the observations. He kept his lens-making methods secret, and for 150 years microscopes improved little.
Chapter II
All Life Is Cells
Better lenses changed that. In 1830 Joseph Jackson Lister, a wine merchant, showed how to combine lenses so their colour fringes cancel. Cell interiors became clear, and in 1831 Robert Brown described the nucleus. In 1838 the botanist Matthias Schleiden argued that plants are built entirely of cells. Over dinner he described the nuclei of plant cells to the physiologist Theodor Schwann, who realised he had seen the same structures in animal tissue. His 1839 book proposed that all living things, plant and animal, are made of cells.
They got one thing wrong. They believed new cells crystallise out of a formless fluid. Robert Remak, studying frog embryos, showed in 1852 that cells arise only by division. Rudolf Virchow, a Berlin pathologist, adopted the idea, summed it up as omnis cellula e cellula, and built medicine on it: disease, he argued, is a disorder of cells. In 1882 Walther Flemming, using new dyes, watched the threads in the nucleus split and separate as a cell divided, and called the process mitosis. Those threads, the chromosomes, would become the centre of genetics.
Chapter III
A Closer Look: Why Cells Are Small
Most cells are between 1 and 100 micrometres across, and there is a physical reason. Inside a cell, molecules move mostly by diffusion, random jostling. The time for a molecule to diffuse a distance grows with the square of the distance:
where is the diffusion coefficient. For a small molecule such as glucose in water, m²/s. (In crowded cytoplasm it is a few times slower.)
| Distance | Example | Diffusion time |
|---|---|---|
| 1 µm | across a bacterium | about 0.5 milliseconds |
| 10 µm | across a human cell | about 50 milliseconds |
| 1 mm | a large egg cell | about 8 minutes |
| 1 cm | a fingertip | about 14 hours |
Because the time grows with the square of the distance, making something ten times larger makes diffusion across it a hundred times slower. A cell the size of a bacterium is supplied almost instantly. A cell a centimetre across would starve in its middle. There is a second constraint: a cell takes in food and gets rid of waste through its surface, and as a cell grows, its volume increases faster than its surface. Double the width and the volume grows eightfold while the surface grows only fourfold.
That is why large organisms are made of many small cells rather than a few big ones, and why they need circulation, the blood that carries oxygen within a fraction of a millimetre of almost every cell. The few giant cells that exist, such as bird eggs, are mostly inert food store, or, like nerve cells a metre long, are thin in every direction but one and move material with molecular motors.
Chapter IV
The Unit of Life
Cell theory became the frame for the rest of biology. The organisms Leeuwenhoek saw became the subject of microbiology. The chemistry inside cells became biochemistry, and the machinery that organises them cell biology. The theory also has a deep evolutionary meaning. Since every cell comes from a cell, every living cell today descends in an unbroken line from the first one. How that first cell arose, and how little a cell needs in order to live, are still open.