Photoreceptor proteins are light-sensitiveproteins involved in the sensing and response to light in a variety of organisms. Some examples arerhodopsin in thephotoreceptor cells of the vertebrateretina,phytochrome in plants, andbacteriorhodopsin and bacteriophytochromes in somebacteria. They mediate light responses as varied asvisual perception,phototropism andphototaxis, as well as responses to light-dark cycles such ascircadian rhythm and otherphotoperiodisms including control of flowering times in plants and mating seasons in animals.

Photoreceptor proteins typically consist of aprotein attached to a non-proteinchromophore (sometimes referred asphotopigment, even so photopigment may also refer to the photoreceptor as a whole). The chromophore reacts to light viaphotoisomerization orphotoreduction, thus initiating a change of the receptor protein which triggers asignal transduction cascade. Chromophores found in photoreceptors includeretinal (retinylidene proteins, for examplerhodopsin in animals),^[1]flavin (flavoproteins, for examplecryptochrome in plants and animals)^[2] andbilin (biliproteins, for examplephytochrome in plants).^[3] The plant proteinUVR8 is exceptional amongst photoreceptors in that it contains no external chromophore. Instead, UVR8 absorbs light throughtryptophan residues within its proteincoding sequence.^[4]

• Melanopsin: in vertebrate retina, mediates pupillary reflex, involved in regulation of circadian rhythms
• Photopsin: reception of various colors of light in thecone cells of vertebrate retina
• Rhodopsin: green-blue light reception in therod cells of vertebrate retina
• Protein Kinase C: mediates photoreceptor deactivation, and retinal degeneration^[5]
• OPN5: sensitive toUV-light^[6]

• UVR8: UV-B light reception
• Cryptochrome: blue and UV-A light reception
• Phototropin: blue and UV-A light perception (to mediate phototropism and chloroplast movement)
• Zeitlupe: blue light entrainment of thecircadian clock
• Phytochrome: red and far-red light reception

All the photoreceptors listed above allow plants to sense light with wavelengths range from 280 nm (UV-B) to 750 nm (far-red light). Plants use light of different wavelengths as environmental cues to both alter their position and to trigger important developmental transitions.^[7] The most prominent wavelength responsible for plant mechanisms is blue light, which can trigger cell elongation, plant orientation, and flowering.^[8] One of the most important processes regulated by photoreceptors is known asphotomorphogenesis. When a seed germinates underground in the absence of light, its stem rapidly elongates upwards. When it breaks through the surface of the soil, photoreceptors perceive light. The activated photoreceptors cause a change in developmental program; the plant starts producing chlorophyll and switches to photosynthetic growth.^[9]

(Also see:Eyespot apparatus)

• Channelrhodopsin: in unicellular algae, mediates phototaxis
• Chlamyopsin andvolvoxopsin
• Flavoproteins

• Bacteriophytochrome
• sensorybacteriorhodopsin
• Halorhodopsin
• Proteorhodopsin
• Cyanobacteriochrome

• Visual cycle
• Visual phototransduction

• Visual perception
• Phototropism
• Phototaxis
• Circadian rhythm (body clock)
• Photoperiodism

• Biliproteins
• Photomolecular biology

• Circadian rhythm
• Sensory receptors
• Signal transduction
• Biological pigments
• Integral membrane proteins
• Molecular biology

• Articles with short description
• Short description matches Wikidata