2.4: Electron Microscopy (EM)

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Rosanna Hartline
West Hills College Lemoore

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Electron Microscopes (EM)

In contrast to light microscopes, electron microscopes (see the figure, part (b) above) use a beam of electrons instead of a beam of light. Not only does this allow for higher magnification and, thus, more detail (see the figure above), it also provides higher resolving power. The method used to prepare the specimen for viewing with an electron microscope kills the specimen. Electrons have short wavelengths (shorter than photons of light) that move best in a vacuum, so living cells cannot be viewed with an electron microscope.

The best type of EM to examine a cell's internal structures such as vesicles, storage granules, ribosomes, mitochondria, and nuclei is transmission electron microscopy (TEM) since it is designed to allow electrons to pass through a thin sample. Samples must be prepared for TEM by embedding the sample, such as a cell, in a tiny plastic block, and using either a glass knife or a diamond knife to thin-section the sample using a microtome (laboratory equipment that makes thin sections). As a result, thin slices of cells can be observed with TEM, exposing intracellular details. An example of a thin-sectioned skin cell magnified by TEM is shown in the figure, part (a) below.

Another common type of EM is scanning electron microscopy (SEM). SEM works much differently than TEM since SEM radiates electrons onto a sample’s surface and these electrons are reflected back by the sample to elucidate the surface details of the sample. As a result, SEM is an excellent technique for examining the three-dimensional surface of an un-sectioned specimen and viewing only the external surfaces of cells and tissues. An example of an SEM image of skin cells with yeast infecting them is shown in the figure, part (b), below.

(a) A lung cell that has been thin-sliced and imaged with TEM. Thin-slicing a cell with TEM exposes the internal structures such as the nucleus and mitochondria. (b) Exterior of blood cells imaged with SEM. SEM images only allow surface visualizations of cells and tissues.

Above: (a) A lung cell that has been thin-sliced and imaged with TEM. Thin-slicing a cell with TEM exposes the internal structures such as the nucleus and mitochondria. (b) Exterior of blood cells imaged with SEM. SEM images only allow surface visualizations of cells and tissues.

Attributions

"Biology 2e" by Mary Ann Clark, Matthew Douglas, Jung Choi, OpenStax is licensed under CC BY-NC 4.0
"Clara cell lung - TEM" by Louisa Howard is in the Public Domain
"SEM blood cells" by Bruce Wetzel, Harry Schaefer is in the Public Domain