BIOLOGY Volume 1 - A Guide to General Biology - 2004

5. CELLS

5.6. Electron Microscopy

5.6.1. The Electron Microscope

By the early 1900s, further progress in The Study of Cell Structure had ground to a halt because even the most advanced Light Microscope could not provide a magnification greater than 1500. This limitation is dictated by the very nature of light. Light is a form of electromagnetic radiation (Fig. 5.4) that travels in a series of waves. The human eye perceives electromagnetic radiation within a wavelength range from 400 nm (violet) to 700 nm (red). However, this visible light makes up only a small fraction of the full electromagnetic spectrum, which encompasses radiations of varying wavelengths (Fig. 5.4). Radiation of any wavelength travels at the speed of light, but the shorter the wavelength, the greater the energy it carries. It is impossible to resolve an object smaller than half the wavelength of the radiation being used, because the object must be large enough to disrupt the passage of the waves. Therefore, the smallest object that can be seen using visible light must be at least 200 nm in diameter (half the wavelength in the violet region of the spectrum). Given the dimensions of certain Cells and cell Organelles (discussed above), it is easy to understand why, for instance, Mitochondria (1 µm, or 1000 nm) appear under a light microscope merely as tiny granules inside cells, while Ribosomes are not visible at all. Consequently, the light microscope does not allow for a detailed examination of The structure of mitochondria, ribosomes, and other cellular components.

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Fig. 5.4. The electromagnetic spectrum. Visible light constitutes only a small part of it. (Wavelengths are not drawn to scale.)

Aware of these Limitations of the light microscope, scientists attempted to design a microscope that would utilize radiation with a significantly shorter wavelength. Initially, they tried using X-rays for this purpose, but it soon became clear that the best results could be achieved with the Electron microscope. Instead of light radiation, it uses a beam of electrons. Electrons are negatively charged particles that Orbit atomic nuclei. Under certain conditions, they behave like waves. Compared to visible light, they offer two major advantages. First, they have an extremely short wavelength, almost identical to that of X-rays (Fig. 5.4), and second, since they carry negative charges, they can be focused using electromagnetic lenses (electromagnets). Electromagnetic lenses steer a beam of electrons in the exact same way that Glass lenses steer a beam of light.

The electron microscope makes it possible to achieve magnifications of the order of 250,000 for biological specimens. With certain Materials, even greater magnification can be attained, and nowadays images of individual atoms are sometimes obtained.