Methods of Studying Cells

Electron Microscopes

• Transmission Electron Microscope (TEM):
  • A beam of electrons passes through a thin section of a specimen
  • Areas that absorb electrons appear darker on the electron micrograph
• Scanning Electron Microscope (SEM):
  • A beam of electrons passes across the surface and scatters
  • The pattern of scattering builds up a 3D image depending on the contours of the specimen
• Limitations of Electron Microscopes:
  • The whole system must be in a vacuum, making it impossible to observe living specimens
  • A complex staining process is required, which may introduce artefacts into the image
  • Specimens have to be very thin, particularly for TEM
  • SEM has a lower resolving power than TEM, but both have greater resolving power than a light microscope

Cell Fractionation and Ultracentrifugation

• Cell fractionation is the process of separating different parts and organelles of a cell to study them in detail
• The most common method of cell fractionation is differential centrifugation
• Steps of differential centrifugation:
  • Homogenization: blending cells in an homogeniser to form a homogenate
  • Centrifugation: spinning the homogenate at a slow speed to sediment the heaviest organelle (nuclei)
  • Removing the supernatant and transferring it to another tube
  • Spinning at a slightly faster speed to sediment the next heaviest organelle (mitochondria)
  • Repeating the process to separate out other organelles

Cell Structure

• All living organisms are made up of cells, with various types of cells sharing common features
• Eukaryotic cells, found in humans, contain a nucleus and membrane-bound organelles

Microscopes

• Light Microscopes:
  • Use convex glass lenses to resolve images 0.2um apart
  • Limited by the wavelength of light, restricting resolution
• Electron Microscopes:
  • Can distinguish between items 0.1nm apart
  • Allow for higher resolution than light microscopes