Microscopy

Biotechnology depends on tools that let us see and measure what the eye cannot. Robert Hooke (1665) saw tiny boxes in cork, which he called "cells", with an early microscope; the cell theory of Schleiden and Schwann followed in 1838. Frederick Sanger, profiled in this unit, won two Nobel Prizes: in 1958 (for the structure of insulin) and in 1980 (for DNA sequencing).

Two ideas matter in microscopy. Magnification makes the image bigger: total magnification = objective × eyepiece, so a 10× eyepiece with a 40× objective gives 400×. Resolving power makes it clearer: it is the ability to show two points close together as separate points. Enlarging a blurry image without better resolution only gives a bigger blur.

The compound light (bright field) microscope has an eyepiece, several objectives (4×, 10×, 40×, 100×), a stage to hold the slide, a condenser to focus light, and a light source. Most cells are nearly transparent, so specimens are often stained: with eosin, safranin, methylene blue or Giemsa.

Special light microscopes:

• Dark field: light comes in at a slant, so the specimen shines against a dark background.
• Phase contrast: turns small differences in density inside unstained, transparent cells into visible light-and-dark contrast, so living cells can be seen.
• Fluorescence: uses dyes (fluorophores) that absorb light of one colour and give out light of a longer wavelength (a different colour). It helps identify bacteria and viruses.
• Confocal: gives very sharp images of fluorescently labelled cells and tissues.

Electron microscopes use beams of electrons, whose wavelength is about 1,00,000 times shorter than light's, so they can show far smaller details. The beam is focused by electromagnetic lenses (not glass) inside a vacuum. A TEM (transmission electron microscope) looks through thin stained slices, showing inner structure. An SEM (scanning electron microscope) looks at the surface of coated specimens, showing it in fine 3-D detail.