• Microscopes?can be used to analyse?cell components?and observe?organelles
• Magnification?and?resolution?are two scientific terms that are very important to understand and?distinguish?between when answering questions about microscopy (the use of microscopes):
  • Magnification?tells you how many times bigger the?image?produced by the microscope is than the?real-life object?you are viewing
  • Resolution?is the ability to?distinguish between objects?that are close together (i.e. the ability to see two structures that are very close together as two separate structures)

   

• There are different types of microscopes:
  • Optical?microscopes (sometimes known as light microscopes)
  • Electron?microscopes
  • Laser scanning confocal?microscopes

   

Optical (light) microscopes

• Optical microscopes use?light?to form an image
• This?limits the resolution?of optical microscopes
  • Using light, it is impossible to resolve (distinguish between) two objects that are closer than half the wavelength of light
  • The wavelength of visible light is between 500-650 nanometres (nm), so an optical microscope cannot be used to distinguish between objects closer than half of this value

   

• Optical microscopes have a?maximum resolution of around 0.2 micrometres (μm) or 200 nm
  • Therefore optical microscopes?can be used?to observe?eukaryotic cells, their?nuclei?and possibly mitochondria and chloroplasts
  • Optical microscopes?cannot be used?to observe?smaller organelles?such as?ribosomes, the?endoplasmic reticulum?or?lysosomes

   

• The?maximum useful magnification?of optical microscopes is about?×1500

Electron microscopes

• Electron microscopes use?electrons?to form an image
• This?greatly increases the resolution?of electron microscopes compared to optical microscopes, giving a?more detailed image
  • A beam of electrons has a much smaller wavelength than light, so an electron microscope can resolve (distinguish between) two objects that are extremely close together

   

• Electron microscopes have a?maximum resolution of around 0.0002 μm or 0.2 nm?(i.e. around 1000 times greater than that of optical microscopes)
  • This means electron microscopes can be used to observe?small organelles?such as?ribosomes, the?endoplasmic reticulum?or?lysosomes

   

• The?maximum useful magnification?of electron microscopes is about?×1,500,000
• There are two types of electron microscopes:
  • Transmission?electron microscopes (TEMs)
  • Scanning?electron microscopes (SEMs)

   

Transmission electron microscopes (TEMs)

• TEMs use electromagnets to focus a?beam of electrons
• This beam of electrons is?transmitted through?the specimen
• Denser parts of the specimen absorb more electrons
  • This makes these denser parts appear darker on the final image produced (produces contrast between different parts of the object being observed)

   

• Advantages?of TEMs:
  • They give?high-resolution?images (more detail)
  • This allows the?internal structures?within cells (or even within organelles) to be seen

   

• Disadvantages?of TEMs:
  • They can only be used with?very thin specimens?or?thin sections?of the object being observed
  • They?cannot be used to observe live specimens?(as there is a vacuum inside a TEM, all the water must be removed from the specimen and so living cells cannot be observed, meaning that specimens must be dead, unlike optical microscopes that can be used to observe live specimens)
  • The?lengthy treatment required to prepare specimens?means that?artefacts can be introduced?(artefacts look like real structures but are actually the results of preserving and staining)
  • They?do not produce a colour image?(unlike optical microscopes that produce a colour image)

   

Scanning electron microscopes (SEMs)

• SEMs scan a beam of electrons across the specimen
• This beam?bounces off the surface of the specimen?and the electrons are detected, forming an image
  • This means SEMs can produce?three-dimensional images?that show the?surface?of specimens

   

• Advantages?of SEMs:
  • They can be used on?thick?or?3-D?specimens
  • They allow the?external,?3-D?structure?of specimens to be observed

   

• Disadvantages?of SEMs:
  • They give?lower resolution?images (less detail) than TEMs
  • They?cannot be used to observe live specimens?(unlike optical microscopes that can be used to observe live specimens)
  • They?do not produce a colour image?(unlike optical microscopes that produce a colour image)

   

Laser scanning confocal microscopes

• These microscopes are relatively new technology
• The cells being viewed must be stained with?fluorescent dyes
• A thick section of tissue or small living organisms are scanned with a laser beam
  • The laser beam is reflected by the fluorescent dyes

   

• Multiple depths of the tissue section/organisms are scanned to produce an image
  • Think of it like the laser beam is building up the image layer by layer

   

• Advantages:
  • They can be used on?thick?or?3-D?specimens
  • They allow the?external,?3-D?structure?of specimens to be observed
  • Very clear images are produced. The?high resolution?is due to the fact that the laser beam can be focused at a very specific depth
    • You can even see the structure of the cytoskeleton in cells

     

   

• Disadvantages:
  • It is a?slow?process and takes a long time to obtain an image
  • The laser has the potential to cause?photodamage?to the cells