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Cells are not necessarily sessile. For many microorganisms, moving around is essential. By changing location, they can get access to new energy resources and nutrients or move away from toxins (chemotaxis).

Movement is most commonly achieved in microbes with the use of a flagellum (plural flagella), a remarkable and exquisite cell structure that works by rotation (Figure 5.21), driven by the energy molecule, ATP. Swimming speeds from 2 to 200 μm s⁻¹ in water have been reported in microbes.

Figure 5.21 Microbial movement and flagella. The microbe Salmonella, stained to show the flagella. Each organism is about 1 μm long.

Source: Reproduced with permission of Centers for Disease Control.

Below is shown the structure of a bacterial flagellum in a Gram-negative bacterium. The structure is a motor embedded in the cell membranes and driven by ATP. It comprises a number of protein subunits which are labeled.

Source: Reproduced with permission of wikicommons.

A flagellum can be rotated at 200 revolutions per second. The flagella are typically about 15–20 μm long. The main “whip” of the flagellum is composed of a hollow cylindrical structure, about 20 nm (nanometers) in diameter, which is anchored to a rotating mechanism by a specialized structure called the hook. The chemical motor that rotates the flagellum is situated across the cell membrane and is made up of over 20 different proteins. To build the flagellum, new proteins are passed from the base of the flagellum, up through the hollow core, and added to the end of the filament, just underneath a cap protein at the tip. The flagellum is probably one of the most remarkable structures that have evolved in microorganisms.