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Transmission Electron Microscope

The TEM consists of a gun that fires electrons which are focused onto the specimen.

  • The specimen must be extremely thin
  • a complex staining process is required
  • even then the image is only in black and white
  • the process take places in a vacuum, so living specimens cannot be observed

note: some of the electrons are absorbed by the specimen and appear dark on the image. other parts allow the electrons through so appear light. this prouduces an image of the specimen

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Scanning Electron Microscope

all the limitations of the TEM apply to the SEM but the specimen:

  • does not have to be extremely thin as the electrons do not penetrate.
  • the beam of electrons is directed over the surface of the specimen in a regular pattern
  • the electrons bounce on the contours of the specimen and are scattered
  • the scattering of the electrons can be analysed and from this an image can be produced using a computer

note: the SEM has a lower resolving power than the TEM (20nm) but is still ten times better than a light microscope

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electrons have a shorter wavelength than light so they have a greater resolving power.

as electrons are negatively charged, the bean can be focused using an electromagnet

because electrons are absorbed by molecules in the air, a near vacuum must be created within the chamber on an electron microscope for it to work effectively

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Structure of epithelial cells - NUCLEUS

NUCLEUS- controls the cells activities and contains hereditary material
NUCLEAR ENVELOPE- a double membrane that surrounds the nucleus. Its outer membrane is continuous with the endoplasmic reticulum and often has ribosomes on its surface. (it can control the substances entering and leaving)
NUCLEAR PORES- allow the passage of large materials into and out of the nucleus.

NUCLEOPLASM- is granuar jelly like material that makes up the bulk of the nucleus
CHROMATIN- the DNA found within the nuceloplasm. this is the form of dissusion chromosomes take up when the cell is not dividing
NUCLEOLUS- a small spherical body within the nucleoplasm. it manufactures ribosomal RNA and assembles ribosomes

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