Cell size and microscopes

A red blood cell is about 8 µm across, so about 250 of them would fit across a 2 mm pinhead.

Measuring something you cannot see

Rulers stop at millimeters, and cells are much smaller than that. Biologists use the micrometer (µm), one thousandth of a millimeter, and the nanometer (nm), one thousandth of a micrometer. So 1 mm = 1,000 µm and 1 µm = 1,000 nm. A typical human red blood cell is about 8 µm across, and a pinhead is about 2 mm. Divide 2,000 µm by 8 µm and you get 250: that many red blood cells could line up across one pinhead.

Cells vary a lot. Bacteria and other prokaryotic cells are about 0.1 to 5.0 µm across. Most eukaryotic cells, the kind that make up plants and animals, are about 10 to 100 µm. Inside them, a nucleus is roughly 5 µm across and a mitochondrion is usually 1 to 2 µm long. A ribosome is only 20 to 30 nm, far below anything a classroom microscope can show. The smallest objects the unaided eye can see are about 0.1 mm, so a few very large cells, such as a human egg cell, sit right at the edge of what you can see without a lens.

Light microscopes

Most school microscopes are light microscopes. Visible light passes through the specimen and then through two sets of lenses. Student microscopes usually magnify up to about 400 times, and with an oil immersion lens about 1,000 times. Magnification alone is not enough, though. What matters as much is resolving power, the ability to show two points close together as two separate points. A good student light microscope resolves about 200 nm. Two structures closer than that blur into one, however large you make the picture.

Light microscopes have a big advantage: you can watch living cells. Most cells are nearly transparent, so their parts are hard to tell apart unless they are stained with dyes, and staining usually kills the cells. The two-lens system also flips the picture. A slide moved to the left seems to move right, and a letter “e” on the slide looks upside down.

Electron microscopes

Electron microscopes use a beam of electrons instead of light. Electrons have a much shorter wavelength than light, so these instruments reach magnifications around 100,000 times and resolve details down to about 50 picometers. The price is that electrons travel well only in a vacuum and the preparation kills the specimen, so you cannot watch a living cell this way. A scanning electron microscope sweeps the beam over a surface and shows its shape. A transmission electron microscope sends the beam through a thin slice and shows the parts inside, which is how the organelles in the next units were first seen in detail.

Why cells stay small

Why not one giant cell? Everything a cell uses comes in through its surface, and everything it throws away leaves the same way. Imagine a cube-shaped cell 1 mm on each side. Its surface is 6 mm² and its volume is 1 mm³, a ratio of 6 to 1. Double the side to 2 mm and the surface becomes 24 mm² while the volume becomes 8 mm³, a ratio of only 3 to 1. As a cell grows, its volume grows faster than its surface, and each bit of membrane has to feed more cell. Past some size the membrane cannot keep up. Cells deal with this by dividing, by becoming flat or long and thin, by folding their membranes, or by splitting work among organelles.