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. Focusing the specimen can be achieved by altering the electric current through the lens. More magnetic coils act as projector lenses, which enlarge the image. All specimens in electron microscopy are placed in a vacuum, this means that all specimens must be dead. All specimens must be in a vacuum chamber because the electrons would be deflected by particles in the air and so would not give a clear picture. (A sheet has been attached at the back, which shows the structure of an electron microscope).

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The transmission electron microscope takes a section of a specimen and passes electrons through it. But first the specimen must go through 5 stages of preparation.

  1. Fixation and dehydration using alcohol.
  2. Embedding in resin which is hardened in an oven.
  3. Sectioning using an ultramicrotome and a glass knife.
  4. Mounting on a copper grid to give support (electrons cannot pass through glass).
  5. 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 ...

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