AS Biology - Cell Structure - Analyses of Cell Components

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Magnification & Resolution

Magnification:

How much bigger the image is than the specimen (the sample you're looking at).
It can be calculated using the formula:
Magnification = length of image/length of specimen

Example: 5mm wide magnified image and 0.05mm specimen
Magnification = 5mm/0.05mm X 100

Resolution:

How detailed the image is. More specifically it's how well a microscope distinguishes between two points that are close together.
-If a microscope lens can't separate two items, then increasing magnification won't help.  

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Light & Electron

Light microscopes:

  • they use light
  • have a lower resolution then electron microscopes

Electron microscopes:

  • use electrons instead of light to form an image
  • have a higher resolution than light microscopes so give a more detailed image
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Electron Microscopes - 'Transmission' or 'Scanning

Transmission Electron Microscopes (TEMs):

  • Use electromagnets to focus on a beam of electrons, which is then transmitted through the specimen
  • Denser parts of the specimen absorb more electrons, which makes them look darker on the image
  • TEMs are good because they give high resolution images.
  • On the other hand, they can only be used on thin specimens

Scanning Electron Microscopes (SEMs):

  • Scan a beam of electrons across the specimen
  • This knocks off electrons from the specimen, which are gathered in a cathode ray tube to form an image
  • The images you end up with show the surface of the specimen and can be 3D
  • SEMs are good because they can be used on thick specimens
  • On the other hand, they only give a low resolution image
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Staining Samples

In light microscopes and TEMs the beam of light/electrons passes through the object being viewed.

An image is produced because some parts of the object absorb more light than others.

Sometimes the object is completely transparent which makes the whole thing look white because the light rays/electrons pass straight through it.

To get around this issue, the object can be stained:

Light Microscopes:

  • use dye - the most common stains are methylene blue or eosin
  • the stain is taken up by some parts of the object more than others - the contrast makes different parts show up

Electron Microscopes:

  • objects are dipped in solution - heavy metals, like lead
  • the metal ions scatter the electrons, again creating contrast
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