Observing cells

• By the 1840s, it was widely accepted that cells are the base units of life. Cell theory is now widely accepted and states that cells are fundamental units of structure, function and organisation in all living cells.
• The light/optical microscope can magnify up to 1500 times and still give a clear image
• The electron microscope can magnify up to 500000 times.
• The advantages of a light microscope

  • can see living plants and animals or parts of them directly
  • relatively cheap, so are available in schools, universities, hospitals, industrial and research labs
  • relatively light and portable so can be used almost anywhere.
• Disadvantages of a light microscope

  • preserving and staining tissue can produce artefacts
  • limited powers of resolution and magnification
• Artefacts are things that are not naturally occurring and man made in the process of preparing a slide for a microscope. They are not part of the living tissue but can be mistaken for parts of the tissue
• The electron microscope uses a beam of electrons to form an image. The electrons are scattered by the species. Electrostatic or electromagnetic lenses focus the electron beam to form an image. For an electron image to work, the specimens must be in a vacuum so they are always dead.
• Before being interpreted in an electron microscope, specimens are often stained using heavy metal ions such as lead and uranium. This is to improve the scattering of electrons and produces an image with more contrast which is clearer and easier to interpret.
• Advantages of electron microscope

  • huge powers of magnification and resolution
• Disadvantages of electron microscope

  • all species are examined in a vacuum so it is impossible to look at living material
  • specimens undergo treatment that is likely to result in artefacts
  • extremely expensive
  • very large and must be kept at a constant temperature and pressure, and with an internal vacuum.
• There are two types of electron microscopes. Transmission electron micrographs are 2D, and scanning electron microscopes have a lower magnification but are 3D.
• Magnification is how much bigger the object appears compared with the real object, whereas resolution is how far apart two points must be before we see them as separate objects.