Microscopes

 

Light Microscopes are limited due to the long wavelength of light it limits the ability to view small parts of the cell, for example like the Ribosomes.

Electron Microscopes are 2000x better than light microscopes. They can see objects down to 0.1nm. Electrons are asorbed or deflected in air therefore a vacuum must be used. As a result the specimen must be dead before it can be taken.

There are 2 types of Electron Microscopes:

  • Transmission Electron Microscopes (TEM)
  • Scanning Electron Microscopes (SEM)

The Transmission Electron Microscope is an electron gun that produces a beam that is focussed on to the specimen, by a condenser electromagnet. The electrons passes through a thin section.  Parts of the specimen absorbs electrons and therefore appears dark. An image is produced called a photomicrograph. The resolving power is 0.1nm, although problems with specimen preparations means that this cannot always be achieved. The main limitations of TEMs:

  • The entire system must be in a vacuum, so dead specimens must be used
  • Complex ‘staining’ process is required
  • The resulting image is in Black and White
  • The image may contain artefacts

The specimens must be extremely thin to allow for the electrons to penetrate for it. The images are 2D, however multiple samples can build up a 3D image.

The Scanning Electron Microscope shares many of the same flaws as the SEM

  • The entire system must be in a vacuum, so dead specimens must be used
  • Complex ‘staining’ process is required
  • The resulting image is in Black and White
  • The image may contain artefacts
  • Not as high of a magnification as TEM (highest magnification yields of TEM 0.1nm, compared to SEM 20nm)

The SEM directs its beam of electrons on to the surface of the specimen from above, compared to the TEM which fires electron beam from below the specimen upwards. This difference allows for the electrons to be passed back and forth across a portion of specimen in a regular pattern. The electrons are scattered by the specimen and the pattern of this scattering depends on the countours of the specimen’s surface. We can build 3D images of these images.

Units to remember:

Unit How many cm in each
Centimetre (cm) 1
Millimetre (mm) 10
Micrometre (µm) 10,000
Nanometre (nm) 1,000,000