microscopes

• Define magnification

  how many times larger the image is than the specimen
• Formula for magnification

  magnification = image size / actual size
• define resolution

  How well a microscope distinguishes between two points that are close together
• which specimen need to be stained

  transparent specimen with no contrasting colours so their organelles are invisible
• how do stains work to make cells visible

  the stain is taken up by some parts of the object more than others - the contrast makes the different parts show up
• define a stain

  a dye taken up by some parts more than others
• which parts do positively charged stains stain and why

  negatively charge parts as they combine with
• what is differential staining and what is it used for

  using multiple stains at once to distinguish between/ identify different types of cells or different organelles within the same cell
• steps to prepare a slide for a dry mount

  1) select the thinnest slice so light can get through

  2) use tweezers to place the specimen in the middle

  3) use a mounted needle to slowly lower the cover slip at an angle
• steps to prepare a slide for a wet mount

  1) use a pipette to drop a small drop of water onto the slide

  2) use tweezers to place the specimen on top of the water

  3) use a mounted needle to slowly lower the cover slip at an angle to avoid artefacts e.g. air bubbles

  4) to add a stain: put a drop of stain next to one edge of the cover slip and a bit of paper towel next to the opposite edge, the stain will get drawn under the slip across the specimen
• Steps of using a light microscope

  1) clip the slide onto the stage

  2) select the lowest-powered objective lens

  3) use the coarse-adjustment knob to bring the stage up to just below the objective lens

  4) look down the eyepiece lens and use the coarse adjustment knob to move the stage downwards until roughly in focus

  5) adjust the focus with the fine adjustment knob until clear

  6) if needed, swap to a higher-powered objective lens and refocus
• steps to using an eyepiece graticule

  1) line up the eyepiece graticule and the stage micrometer

  2) work out how many divisions of the eyepiece graticule is in 1 division of the stage micrometer (0.1mm)

  3) 0.1/ number of divisions = ANS

  4) remove the stage micrometer

  5) number of divisions in image x ANS = length
• how do light (optical) microscopes work?

  • light is transmitted through specimen 
  • image is magnified using objective and eyepiece lens
• what are light (optical) microscopes used to view?

  Whole cells, tissues and organisms
• advantages of Light (optical) microscope?

  • can view in colour 
  • specimen can be alive or dead 
  • cheap
• disadvantages of light microscope?

  • only 2D images 
  • can’t view organelles
• how do transmission electron microscopes (TEM) work?

  • electromagnets focus a beam of electrons which is transmitted through specimen 
  • denser parts absorb more electrons = darker
• what are TEMs used to view?

  • very small organelles e.g. ribosomes 
  • internal structure of organelles
  • very small organisms e.g. viruses
• advantages of TEMs?

  • highest resolution and magnification
• disadvantages of TEMs?

  • only works on thin specimens 
  • only 2D images 
  • specimen must be dead
• how do Scanning electron (SEM) work?

  • Scans a beam of electrons across specimen
  • this knocks electrons from the specimen off, which are gathered in a cathode ray tube to form an image
• what are SEMs used to view?

  • very small organelles e.g. ribosomes 
  • very small organisms e.g. prokaryotes
• advantages of SEMs?

  • shows the surface and can be 3D
  • shows external structure of organelles in detail
• disadvantage of SEMs?

  • lower resolution than TEMs 
  • expensive
• what is the order of magnification of the 3 microscopes (start from smallest)?

  light

  SEM

  TEM
• what is the order of resolution of the 3 microscopes (start from smallest)?

  light

  SEM

  TEM