The electron microscope is an intense microscope. It can see things that a normal microscope cannot.

Lynsey Mansfield 26th September 2002

Electron Microscopy

The electron microscope is an intense microscope. It can see things that a normal microscope cannot. The electron microscope cannot be used to look at living cells. Increasing the magnification in electron microscopy results in an increase in the amount of visible detail. There is a vacuum inside an electron microscope. Photographs of specimens viewed with an electron microscope are called electronmicrographs.

The electron microscope uses electromagnets to focus the image onto a fluorescent screen. Light areas on an electronmicrograph are produced when electrons have been able to pass through the specimen. Electrons must be able to pass through parts of the specimen. Molecules in the air interrupt the flow of electrons. Electron-dense areas scatter the electrons and produce dark areas on the electronmicrograph. Electron beams cannot pass through glass. Beams of electrons have a very short wavelength. Thin sections are mounted on copper grids, which provide support. The atoms in organic molecules have a low atomic number and so do not scatter electrons.

The resolving power of a transmission electron microscope depends on the wavelength of the electromagnetic radiation used. The transmission electron microscope allows us to see as separate structures, particles that are as close together as two nanometres. The microscope also produces sharp definition at low magnification and can also be used at high magnification. The resolution of the microscope is two nanometres. An image is formed from electron emitted or reflected from the surface of a complete specimen.

The scanning electron microscope gives a three-dimensional effect showing surface detail. It scans electron beams to and fro across the ...

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The scanning electron microscope gives a three-dimensional effect showing surface detail. It scans electron beams to and fro across the surface of a complete specimen. This microscope can take larger specimens than the transmission electron microscope can. The scanning electron microscope does not have such a high resolving power as the transmission electron does.

The specimen has got to be prepared before being examined by the microscope. A specimen must be stained in order to improve contrast. Also the specimen must be very thin. The cells must be cut into very thin sections, about the thickness of the film round a soap bubble. Very small specimens such as viruses and large molecules do not have to be sectioned. The specimen does not have to be cut into thin sections, but the surface is coated in a thin film of gold. The specimen is placed into a vacuum chamber and split along the line of least resistance. The split surface is coated with heavy metal ions. The embedded tissue is cut using a microtome. The sectioned tissue is stained using heavy metals. The tissue is soaked in alcohol to dehydrate it.

The freeze fracture technique prepares the specimen ready to be analysed. The embedded tissue is cut into very thin sections using a knife made of glass. The tissue is embedded in a resin, which becomes hard. It is preserved using substances, which prevent enzyme action. Using liquid nitrogen freezes the specimen. The freeze fracture technique allows surface detail to be seen with a transmission electron microscope.

There are however some disadvantages, for example the treatment may introduce artefacts. Or a high-intensity electron beam can destroy parts of the specimen, producing light-coloured areas on the screen.