The transmission electron microscope

T

he TEM is used to study the details of the internal structure of cells. Extremely thin samples of the specimen are needed. To make these the specimen is supported in a resin block to prevent in collapsing during cutting, and is sliced with a diamond or glass knife. The section is then impregnated with a heavy-metal stain, such as osmium tetroxide.

As the beam passes through the specimen, electrons are absorbed by heavily stained parts but pass readily through the lightly stained parts. Electromagnets bend the electron beam to focus an image onto a fluorescent screen or photographic film. Photograph taken through an electron microscope is called an electron micrograph.

The most modern TEMs distinguish objects as small as 0.2nm. This means that they can produce clear images magnified up to 250 000 times. The magnification is varied by changing the strength of the electromagnets.

The scanning electron microscope

The SEM is used to produce three-dimensional images of the surface of specimens. Electron are reflected from the surface of a specimen stained with a heavy metal. This enables the SEM to produce images of whole specimens: cells, tissues, or even organisms.

A

lthough electron microscopes have revolutionized cell biology, they have not completely replaced light microscopes. Light microscopes are used to examine living and unstained specimens. Preparation of specimens for electron microscopy is complicated and time-consuming. Electron microscopes are very expensive and can be used only to study dead specimens stained with heavy metal, which might well produce artifacts.

■ Glossary of essential terms for you to know.

№	English term	Russian equivalent
1	magnified	увеличенный
2	specimen	образец, особь
3	to explore	исследовать
4	to invent	изобретать
5	lens	линза
6	to refract	преломлять
7	magnification	увеличение
8	apparent	видимый
9	compound	составной, сложный
10	blurred	смазанный
11	to distinguish	различать
12	resolution	разрешение
13	wavelength	длина волны
14	clarity	ясность, четкость
15	beam	луч, пучок лучей
16	ray	луч
17	internal	внутренний
18	sample	образец
19	to impregnate	оплодотворять, пропитывать, насыщать
20	stain	пропитка
21	readily	легко
22	to reflect	отражать
23	to enable	позволять
24	tissues	ткани
25	to replace	заменять

■ Your Essential Assignments

I. Quick check

1. How is the magnification varied in:

a) a light microscope

b) an electron microscope?

2. Why is the resolving power of an electron microscope so much better than of a light microscope?

3. What is the approximate size of the smallest structure that can be observed with a light microscope?