For an electronic microscope to be effective, it works in vacuum so that the beam of electrons (the main source of illumination) is not scattered by air molecules. The specimen is placed in a vacuum chamber. Cells cannot survive in a vacuum hence only studied when dead in an electron microscope.
The disadvantage of studying dead cells is that they limit the understanding of cellular processes because they cannot be viewed in real-time. Studying live cells allows scientists to understand physiological changes of cells and their biological relevance.
