Electron Microscopes
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Introduction
Biology - Electron Microscopes
Electron Microscopy is the use of Electron Microscopes. Electron Microscopes have a very high resolving power and a high magnification, and thus are used to view small objects with greater magnification and detail than a light microscope. There are two different types of electron microscopes, each with different abilities and limitations. This essay will analyse and discuss the functions and limitations of each different type individually, and then conclude with comparisons.
Transmission Electron Microscope
This type of electron microscope allows the user to view a 2d image of a cross section of a sample. It has a maximum resolution of 1 nm and a maximum magnification of 250,000 x. The good resolution is due to the short wavelengths of electrons, which is 0.005 nm.
In a TEM the electrons are fired from an electron gun, which is part of the cathode, and are drawn through the microscope by the anode. The electrons pass through the specimen, which must be very thin, and prepared in a certain way for exactly that reason. The specimen preparation process will be explained in detail later in the essay.
There are three electromagnetic lenses in a TEM.
Middle
Dehydrated
↓
Cleared
↓
Embedded
↓
Sectioned
↓
Stained
↓
Mounted
The stages in preparation of the specimen for viewing by a TEM are as above.
The specimen is firstly Fixed to avoid distortion of cell components. The tissue is killed, and then preserved in as natural a state as possible by a chemical fixative. This works by denaturing the cells constituent protein. In electron microscopy, glutaraldehyde is used as the chemical fixative.
The specimen is then Dehydrated. This process is gradual in order to preserve fine detail. This is done by using a series of progressively increasing concentrations of either ethanol or propanone.
The sample is then Cleared. This is done because the alcohol may be immiscible with embedding agents, and is thus replaced by a clearing agent that also has the effect of making the material transparent. The most commonly used cleaning agent is xylol.
The material is then Embedded in plastic or resin, to provide support during sectioning. This prevents deformation of the specimen during the process of sectioning.
Sectioning is the process of cutting the material into extremely thin sections for viewing by the electron microscope.
Conclusion
Freeze Fracturing is a technique whereby a sample is frozen in liquid nitrogen, and then split open. It allows interior detail to be seen with a SEM.
Cells are frozen at -196°C and then split. The exposed surfaces are then coated with carbon and platinum. The organic material is then dissolved with enzymes by a process called freeze etching, leaving a carbon-platinum replica of the original surface, which can be examined with the microscope.
As with the TEM, there is a preparation procedure for the usage of samples with the SEM. However, it is much less complex than the TEM procedure. Samples merely have to be coated with a thin film of carbon or gold.
In conclusion, both types of electron microscope have advantages and disadvantages. The TEM has a high magnification and resolution compared to the SEM, but the SEM can give a true 3d image from one electron micrograph. Thus, the best type of microscope to use for any given sample would depend on the sample, and the sort of image that is required. For completion, both the TEM, and the SEM would have to be used in conjunction with each other. The advantages of one complement the advantages of the other.
This student written piece of work is one of many that can be found in our AS and A Level Microscopes & Lenses section.
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