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Summary Cell Structure and Cell membrane (+microscopes)
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Biology A level revision notes on Cell structure
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Cell StructureElectron microscopes have certain advantages over optical microscopes:
Higher resolution and are therefore also able of a higher
magnification (x500,000). Light microscopes can show a useful
magnification only up to 1000-2000 times. This is a limit because of
the wavelength of the light. Electron microscopes therefore allow
for the visualisation of structures that would normally be not visible
by optical microscopy.
Depending on the type of electron microscope, it is possible to view
the three-dimensional external shape of an object (Scanning
Electron Microscope, SEM).
Optical microscopes cannot show the cell ultrastructure
Electron microscopes have a range of disadvantages as well:
They are extremely expensive
Sample preparation is often much more elaborate. It is often
necessary to coat the specimen with a very thin layer of metal (such
as gold) so it can reflect the electrons
The sample must be completely dry and a vacuum is required. This
makes it impossible to observe living specimens
The energy of the electron beam is very high. The sample is exposed
to high radiation, and therefore not able to live. So we can question
if what we are seeing the exact same as it would have been if the
specimen was alive
It is not possible to observe colour
Maintenance costs are high
Magnification: how far the microscope can zoom into the specimen and
therefore how large it appears when compared to the original size.
Resolution: the minimum distance two objects can be apart and still be
distinguished separately. The higher the resolution of the image, the
greater the detail that can be seen.
The resolution of an optical microscope is better using natural light rather
than artificial light as natural light has a shorter wavelength and therefore
a higher resolving power.
A vacuum is needed in an electron microscope because if there were air
particles, then the electrons would be refracted and obstructed so they
would not be able to reach the specimen.
Magnification = size of image
actual size
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