Chapter I
Below the Limit of Light
By 1900 cell theory was established, but light microscopes had reached their limit. Details smaller than about a fifth of a micrometre blur together, and most of a cell's machinery is smaller. Meanwhile biochemistry ground cells up to study their enzymes, which told what cells do but not where.
The electron microscope broke the limit. Ernst Ruska built the first in 1931, focusing electrons with magnetic coils. Electrons have wavelengths many thousands of times shorter than light, and in 1945 Keith Porter, Albert Claude and Ernest Fullam published the first electron micrograph of a whole cell. It showed a lace of internal membranes that no one had seen, the endoplasmic reticulum.
Chapter II
The Cell's Factories
At the Rockefeller Institute, George Palade combined electron microscopy with Claude's method of separating cell parts by spinning them in a centrifuge. Using radioactive labels, his group followed newly made proteins through the cell: made on ribosomes studding the endoplasmic reticulum, packaged in the Golgi apparatus, shipped out in small vesicles. Christian de Duve found the lysosome, the cell's recycling compartment. The cell was a factory with an assembly line.
In 1972 S. Jonathan Singer and Garth Nicolson described the membranes that bound all these compartments as a fluid mosaic, proteins floating in a double layer of lipids. Mitochondria and chloroplasts, with their own DNA and double membranes, turned out to be descended from bacteria that had moved in, the endosymbiosis of evolutionary biology.
Chapter III
The Cycle
How does a cell decide when to divide? In the 1970s Leland Hartwell collected mutant yeast cells that stalled at particular stages of division, and found the genes that control each step. Paul Nurse found the central controller in fission yeast, a kinase, and in 1987 showed that the human gene could replace the yeast one. Tim Hunt, studying sea urchin eggs at a summer course, found cyclins, proteins that rise and are destroyed with each division. A billion years of evolution had left the machinery almost unchanged, and when it fails, cells divide out of control, which is cancer.
Chapter IV
A Closer Look: What a Microscope Can See
In 1873 Ernst Abbe showed that a light microscope cannot separate two points closer than about
where is the wavelength of the light and NA, the numerical aperture, measures how wide a cone of light the lens collects. The best oil-immersion lenses reach NA ≈ 1.4. With green light, nm:
Compare the sizes of the things inside a cell:
| Structure | Size |
|---|---|
| Human cell | 10,000–30,000 nm |
| Mitochondrion | about 500–1,000 nm wide |
| Ribosome | about 25 nm |
| Microtubule | 25 nm wide |
| Membrane | about 5 nm thick |
A mitochondrion is just visible as a blur. A ribosome, a microtubule and the membrane itself are about seven to forty times smaller than the limit. No improvement in lenses can fix this. It is set by the wave nature of light.
Electrons get round it because their wavelength is tiny. Accelerated through 100,000 volts, an electron has a wavelength of about 0.004 nm, and electron microscopes resolve detail down to a fraction of a nanometre, enough to see individual proteins. The price is that cells must be fixed, sliced thin and placed in a vacuum. They are dead.
Super-resolution light microscopy finds another way round. If only a few fluorescent molecules glow at once, each appears as a blurred spot 200 nm wide, but the centre of a spot can be located far more precisely than its width, to about 20 nm if enough photons are collected. Switching different molecules on and off, and locating each, builds up an image ten times sharper than Abbe's limit, in living cells.
Chapter V
Watching Molecules
The glowing protein of a jellyfish, found by Osamu Shimomura in 1962, became the tool that let biologists watch cells live. Douglas Prasher cloned its gene, Martin Chalfie showed in 1994 that it glows in other organisms, and Roger Tsien made it in many colours. Fused to any protein, it shows where that protein goes. Stefan Hell, Eric Betzig and W. E. Moerner then broke Abbe's limit for fluorescence. Cell biology now watches single molecules move in living cells. How a cell senses its own size, and times its division by it, is still not fully understood.