taxis
In botany and microbiology, the movement of a single cell, such as a bacterium, protozoan, single-celled alga, or gamete, in response to an external stimulus. A movement directed towards the stimulus is described as positive taxis, and away from it as negative taxis. The alga Chlamydomonas, for example, demonstrates positive phototaxis by swimming towards a light source to increase the rate of photosynthesis.
Chemotaxis is a response to a chemical stimulus, as seen in many bacteria that move towards higher concentrations of nutrients.
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