Biology
Looking at cells in more detail Learning objectives: • identify the differences in the magnification and resolving power of light and electron microscopes • explain how electron microscopy has increased our understanding of sub-cellular structures.
Key words scanning electron microscope (SEM) transmission electron microscope (TEM)
The transmission electron microscope (TEM) uses an electron beam instead of light rays. Some of the electrons are scattered as they pass through the specimen. Those able to pass through it are focused in TEMs using electromagnetic coils instead of lenses.
Electron microscopes TEMs are used for looking at extremely thin sections of cells. The highest magnification that can be obtained from a transmission electron microscope is around × 1 000 000, but images can also be enlarged photographically. The limit of resolution of the transmission electron microscope is now less than 1 nm.
Figure 1.7 A transmission electron microscope. The electrons are displayed as an image on a fluorescent screen.
The scanning electron microscope (SEM) works by bouncing electrons off the surface of a specimen that has had an ultrathin coating of a heavy metal, usually gold, applied. A narrow electron beam scans the specimen. Images are formed by these scattered electrons. SEMs are used to reveal the surface shape of structures such as small organisms and cells. Because of this, resolution is lower and magnifications used are often lower than for a TEM. Electrons do not have a colour spectrum like the visible light used to illuminate a light microscope. They can only be ‘viewed’ in black and white. Here, false colours have been added.
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1
What is the maximum resolution of an electron microscope?
2
What types of samples would a TEM and an SEM be used to view?
3
How has electron microscopy improved our understanding of cells?
Figure 1.8 A scanning electron micrograph of a cancer cell
AQA GCSE Biology for Combined Science: Trilogy: Student Book
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5/20/16 10:05 AM