The flow of electrons is interrupted by molecules of air so a vacuum is used inside n electron microscope. Electrons must be able to pass through parts of the specimen
Light areas on an electron micrograph are produced when electrons have been able to pass through the specimen. An electron micrographs are photographs of specimens viewed with an electron microscope.
Specimens must be stained in order to improve contrast To do so cells must be cut into very thin sections about the thickness of the film around a soap bubble. Thin sections are mounted on copper grids which provide support. Sectioned tissue is stained by using heavy metals. In other procedures the specimen does not have to be cut into thin sections but the surface is coated with a thin film of gold, The tissue is soaked in alcohol to dehydrate it Tissue is embedded in a resin which becomes hard. The embedded tissue is cut using a microtome or into very thin sections using a knife made of glass. Very small specimens such as viruses and large molecules do not have to be sectioned. Prior to sectioning and cutting tissue is preserved using substances which prevent enzyme action
The general outline for freeze facture technique consists of cells frozen quickly in liquid nitrogen which stops cell components instantly. The Block of frozen cells are fractured. This fracture is irregular and occurs along lines of weakness like the plasma membrane or surfaces of organelles. Consequently the surface ice is removed by a vacuum also known as freeze etching. A thin layer of carbon is evaporated vertically onto the surface to produce a carbon replica. The Surface is shadowed with a platinum vapour. The organic material is digested away by acid, leaving a replica. Carbon-metal replica is put on a grid and examined by a transmission electron microscope. The treatment might introduce artefacts.
Furthermore scanning electron microscope can take larger specimens than the transmission electron microscope can, since it scans electron beams to and fro across the surface of a complete specimen. On the Other hand a transmission electron microscope which allowed us to see as separate structures, particles which are in actual fact as close together as 2 nanometres. To do so it uses electromagnets to focus the image onto a fluorescent screen, the specimen can then be viewed via low and high magnification. The specimen can not be a living cell.
Electron beams can not pass through glass and likewise can not be used to look at living cells, since the atoms in organic molecules have a low atomic number and so do not scatter electrons. In a scanning electron microscopes an image is emitted or reflected from the surface of a complete specimen. The similarity between a scanning electron microscope and a transmission electron microscope are specimens must be stained in order to improve contrast. Important since a high-intensity electron beam can destroy parts of the specimen, producing light-coloured areas on the screen.
