Light provides many organisms with both energy and information about their surroundings, which is why these organisms commonly display motile responses to light. Such movements of motile organisms or free plant parts in response to light stimulation are called phototaxis. Movements directed toward or away from sources of light are further described respectively as positive or negative: A moth's flight toward a shining lamp is an example of positive phototaxis, whereas the tendency of a ciliate cell to swim toward darker waters exhibits negative phototaxis. Organisms can respond not just to the direction of light but also to its intensity. Typically, phototactic movement allows organisms to maintain optimal physiological and growth conditions. Certain defensive behaviors, especially those of insects, are also considered to be forms of phototaxis. See also: Cell motility; Environment; Light; Plant movements; Plant physiology; Taxis

Phototactic responses are mediated by photoreceptors, which are highly specialized, light-sensitive cells containing photopigments. Different species and tissues can vary in their complements of photoreceptors, which is part of why phototactic responses differ as well. Retinals (vitamin A aldehydes that are contained in sensory rhodopsin pigments, for example) are key carotenoid molecules involved in the vision of animals, including vertebrates, arthropods, and molluscs; flavins, including riboflavin (vitamin B₂), allow phototaxis in algae and plants; tetrapyrrole compounds provide photosynthetic bacteria with the means for orienting themselves to most efficiently receive the light that they convert into energy; and phytochromes are another type of protein pigment used by plants and other photosynthetic organisms to detect light, especially in the red and far-red regions of the visible spectrum. See also: Carotenoids; Photochemistry; Photoreception; Photosynthesis; Phytochrome; Pigmentation; Riboflavin; Vision; Vitamin A