Electron microscopy is a fascinating field that provides high-resolution images of molecules, viruses and the interior and exterior of cells, tissues and organisms. Resolution is the ability to distinguish two objects as separate from one another. Electron microscopes use a beam of electrons, with a wavelength around 100 000 × shorter than light, so they are capable of much higher resolution than a light microscope. However, this comes with disadvantages for biology. An electron beam travels best in a vacuum. Biological material is largely water, which evaporates immediately in a vacuum. This means that most biological samples must be preserved or fixed so that they are not damaged when inside the microscope. There are two main types of electron microscopy — transmission electron microscopy (TEM) and scanning electron microscopy (SEM).
A TEM is similar to a light microscope in that it focuses a beam (of electrons) through a very thin (approximately 50–70 nm) slice — section — of a sample. Eukaryotic cells are usually 30–50 µm in diameter, so it takes at least 300 sections to get through a single cell. Each time you look at a TEM image (micrograph) only a tiny proportion of the sample is actually shown (see Figure 1). To get an idea of the three-dimensional structure of the components you need the data from sequential images, processed with specialised software or can use electron tomography (see Figure 2).
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