Microscopes

• Magnification:

  • how many times bigger the image is than the specimen
  • magnification = image size / actual size
• Resolution:

  • the minimum distance between the 2 objects where they can still be seen as separate objects
• Optical microscope:

  • uses light to form an image of the specimen
  • magnification: x1500, max resolution: 0.2 um
  • adv: specimen can be living or dead, colourful image, easy sample preparation, cheap and easily accessible
  • disadv: low resolution and magnification
• Electron microscope:

  • uses electrons to form an image of the specimen
  • magnification: x1500000, max resolution: 5nm for TEM, 0.5nm for SEM
• TEM (Transmission electron microscope)

  • adv: 2D image in high resolution and magnification (higher res than SEM), can see internal structure of organelles
  • disadv: thin specimen (to allow electrons to pass through), dead specimen, complex staining process so risk of artefacts, black and white image
• SEM (Scanning electron microscope)

  • adv: 3D in high resolution and magnification, can be used on thick specimen
  • disadv: dead specimen, lower resolution than TEM, complex staining process so risk of artifects, black and white image
• Organelles that cannot be seen with optical microscope:

  • ribosomes
  • lysosomes
  • endoplasmic reticulum
• Optical microscopes have a lower resolution as light has a longer wavelength
• TEM uses electromagnets to transmit a beam of electrons through a specimen. The denser part absorbs more electrons, so appear darker in the image formed.
• SEM scans a beam of electrons across the surface of a specimen. Reflected electrons are then used to form an image.
• Making a mount

  1. Add a drop of water to a glass slide
  2. Obtain a thin piece of specimen and place it on the drop of water on the slide
  3. Stain the specimen with iodine in potassium iodide
  4. Lower the cover slip using a mounted needle
• Accurate counting

  • count whole cells only -> standardise method
  • repeat -> ensure accurate number
  • many cells -> representative sample
• Preparing slide

  • Push down firmly on cover slip: to spread tissues so it is thin enough to allow light through
  • But don't smear/push sideways: avoid overlapping of cells
• Mitosis microscopy

  • cut root tip: where growth occurs / where mitosis is taking place