In biology, the movement of cells towards or away from the source of a chemical stimulus. The former is described as positive, the latter as negative chemotaxis. Chemotaxis requires cell motility (the ability to move spontaneously and indpendently), a specific receptor to recognize the chemical stimulus, and a signalling pathway linking the receptor to the elements controlling the movement. It is in some ways analogous to other kinds of taxis such as phototaxis (in response to light).
Chemotaxis ist best understood in bacteria which use bundles of flagella to swim, such as Escherichia coli. In each bacterium, the bundle of flagella is attached to a rotary motor located in the cell membrane, which frequently switches between clockwise and anticlockwise rotation. While the motor rotates anticlockwise (viewed from the outside of the bacterium), the flagella will stay together and efficiently drive the cell forward in a straight movement. The opposite rotation causes the bundle to disintegrate and the bacterium tumbles.
The chemotactic movement of bacteria is based on a very simple rule: if the concentration of a desirable chemical (such as a nutrient) increases during a straight swim, they will keep going for longer. If it decreases, they will switch into tumbling mode and thus try a different direction picked at random.
Motile cells of eukaryotes, by contrast, have more sophisticated ways of orienting themselves within a chemical gradient and following a chosen direction. For example, white blood cells are attracted to the site of infection by the release of substances during certain types of immune response.