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Electron Microscopy.

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Electron Microscopy

An electron microscope can only show dead structures, but will show cell ultrastructure including the fine structure of the cell organelles. When using an electron microscope, the specimen is illuminated in an electron microscope by an electron beam. The electron beam is focused using electromagnets arranged around the path of the electron beam. These electrons then produce an image when focused onto a fluorescent screen. This image is formed from electrons, which have been emitted or reflected from the surface of a complete specimen.

There are two different types of electron microscopes: a transmission electron microscope and a scanning electron microscope. The electrons pass through or past a thin section of the specimen in a TEM on their way to the fluorescent screen or photographic film. In SEMs the electrons are reflected off the prepared surface of the specimen.

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The scanning electron microscope gives a three-dimensional effect, which shows surface detail. It scans electron beams to and fro across the surface of a complete specimen. It doesn’t have a resolving power as high as the transmission electron microscope. Despite this, it can take larger specimens than the transmission electron microscope.

The specimen for both electron microscopes must be very thin. This is because electrons must be able to pass through parts of the specimen. Molecules in air interrupt the flow of electrons. For this reason, the specimen is placed into a vacuum chamber, which is inside an electron microscope, before it is split along a line of weakness. Very small specimens such as viruses and large molecules do not have to be sectioned.

Photographs of specimens viewed with an electron microscope are called electronmicrographs.

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When using a scanning electron microscope, the process is different. First the tissue is frozen rapidly in liquid nitrogen at about –210°c. This technique is known as freeze fracture, which allows surface detail to be seen with a transmission electron microscope. The frozen tissue is then fractured into very thin sections with a sharp blade, made either of metal or glass. The ice is evaporated from the surfaced and the exposed surface is sputter coated with carbon or gold. Then carbon or a gold replica is floated off the tissue and finally it is mounted on a disc. The treatment might introduce artefacts.

The electron microscope is a useful tool to look at the ultrastructure of the cells, including the fine structure of the cell organelles. This is because of its ability to magnify specimens up to 500, 000 times.

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