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Studying Cells - Microscopes

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Magnification and Resolution

Two parameters that are important in microscopy are magnification and resolution.

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Magnification

  • Magnification is the process of enlarging an object in appearance.
  • The image size is how big the object appears to be in a picture or drawing, which will be in milimeters (mm).
  • The actual size is often given in micrometers (µm) - units must be converted so that they are the same.
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Calculating magnification

  • The equation for magnification is:
    • Magnification = size of image ÷ size of real object
  • E.g. A mitochondrion is 20 µm long. An image of the mitochondrion is measured as 20 mm long. What is the magnification?
    • Magnification = 20,000 µm ÷ 20 µm = 1,000x
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Resolution

  • Resolution is the ability of a microscope to distinguish two adjacent structures as separate.
  • The higher the resolution, the better the clarity and detail of the image.

Microscopes

It is important to understand the principles and limitations of optical microscopes, transmission electron microscopes and scanning electron microscopes.

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Optical (light) microscopes

  • Visible light passes and is bent through the lens system to enable the user to see the specimen.
  • The specimen can be alive.
  • Individual cells are generally transparent and their components are not distinguishable unless they are coloured with special stains.
  • Staining usually kills the cells.
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Uses of light microscopes

  • Most student microscopes are classified as light microscopes.
  • Maximum resolution is 0.2 micrometres.
  • The nucleus and mitochondria can be seen with a light microscope.
  • The maximum magnification is around x1,500.
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Electron microscopes

  • In contrast to light microscopes, electron microscopes use a beam of electrons instead of a beam of light.
  • This allows higher magnification and higher resolving power.
  • This means that more detail can be seen.
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Uses of electron microscopes

  • Electron microscopes have a maximum resolution of 0.0002 micrometres.
  • This is around 1000 times more than light microscopes.
  • The maximum magnification is around x1,500,000.

Types of Electron Microscopes

There are two main types of electron microscopes: transmission (TEM) and scanning (SEM) electron microscopes.

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TEM

  • In a TEM, the electron beam penetrates the cell and provides details of a cell’s internal structures.
    • TEMs use electromagnets to focus the electron beam.
    • TEMs are high resolution microscopes.
    • In thin specimens, you can see the internal structures of organelles such as chloroplasts.
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SEM

  • In a SEM, a beam of electrons moves back and forth across a cell’s surface, creating details of cell surface characteristics.
    • SEMs knock electrons off the specimen and these electrons come together to form an image.
    • SEM images can be three-dimensional.
    • Specimens do NOT have to be thin like when using a TEM.
    • Resolution is lower than that produced by a TEM.

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1Cell Structure

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4Cell Membranes & Transport

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12Energy & Respiration (A2 Only)

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19Genetic Technology (A2 Only)

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