Light is involved in several life-processes other than vision. Its innate importance may rest either in its energy content (e.g. photosynthesis) or in its function as a Zeitgeber, its energy content being secondary (e.g. vision). This latter aspect of light is also applicable to photomorphogenesis, photomovement, circadian rhythms and reproductive cycles. In these instances photo-reception, as distinct from the simple absorption of light quanta, is involved. A characteristic of photoreception is that the energy required to process biological activities triggered by the reception of photons are completely out of proportion to the energy contained within the light stimulus itself. Conversely, carbon fixation in photosynthesis, cellular destruction by ultraviolet radiation, and to a lesser extent the degree of tanning of human skin are proportional to the number of quanta in the light which produces these effects. These processes are all mediated by photopigments. In photoreception, however, the pigments are located within specialised cells — photoreceptors.
KeywordsOuter Segment Luminous Flux Pineal Organ Locomotor Activity Pattern Pineal Photoreceptor
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