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Explain how the electron microscope has affected our knowledge of cell form and structure

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Wednesday 9th October 2002.

Explain how the electron microscope has affected our knowledge of cell form and structure.

Electron microscopes use a beam of electrons instead of light to produce images of specimens.

There are two types of electron microscope: -

Transmission electron microscope

Scanning electron microscope

How an electron microscope works.

Electrons are generated in an electron gun, which applies a high voltage of about 100,000 volts through a filament called a tungsten filament. The filament is heated to above 3000 degrees Kelvin, which is roughly 3273 degrees Celsius. Applying an increasing negative voltage to a cathode assembly, which is located just above an anode plate, accelerates the electrons.

The anode plate has a tiny hole in its centre, the electron beam is sent through this hole creating a very concentrated beam of electrons. This beam is focused using magnetic coils that act like the condenser lenses that you find on a light microscope.

        The specimen is on a plate just above a second magnetic coil, which acts as an objective lens. The objective lens resolves the structure and magnifies it slightly.

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Mounting on a copper grid to give support (electrons cannot pass through glass).Staining using heavy metal stains to improve contrast.

Another way of preparing slides is to use the freeze fracture technique. The specimen is frozen using liquid Nitrogen. The specimen is then hit with e chisel, which causes the specimen to break along the line of least resistance. This way allows surface detail to be seen. Not all specimens need to be sectioned, viruses and large molecules are thin enough to be examined without needing to be sectioned in any way.

These stages may induce artefacts to be present in the electronmicrograph. Artefacts are features which can be observed in cells prepared for microscopy which do not appear in real life, they can be caused by disruption in the cell.

The image can be viewed on a fluorescent screen. The image is black and white unless the specimen has been stained to produce a colour picture. Micrographs are prepared by allowing the electrons to fall on photographic paper.

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Without these advances in microscopy, we would not be able to see the ultra structure of cells, or even know if it existed. Nor would we know what purpose they had within the cell.

We would not know that mitochondria have a double membrane, with the inner one folded into cristae. Or that it is the site of ATP synthesis and contains its own circular strands of DNA.

Granted we may have been able to come to the conclusion that the nucleus controlled the cell reactions, but we would not know what carried out those reactions. If we didn’t have electron microscopes we would not know that bacteria and single celled organisms are different from other cells.

Without electron microscopy, we would not know what happened to obsolete cells, they would appear to vanish! Because of this technology we now know a great deal about what happens within a cell, and what role cells play in our lives.

Jacqueline Barrs

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