Photoreceptor proteins are light-sensitive

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...

s involved in the sensing and response to light in a variety of organisms. Some examples are rhodopsin in the photoreceptor cells of the vertebrate

retina The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which the ...

, phytochrome in plants, and bacteriorhodopsin and bacteriophytochromes in some

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were am ...

. They mediate light responses as varied as

visual perception Visual perception is the ability to interpret the surrounding environment through photopic vision (daytime vision), color vision, scotopic vision (night vision), and mesopic vision (twilight vision), using light in the visible spectrum ref ...

, phototropism and phototaxis, as well as responses to light-dark cycles such as

circadian rhythm A circadian rhythm (), or circadian cycle, is a natural, internal process that regulates the sleep–wake cycle and repeats roughly every 24 hours. It can refer to any process that originates within an organism (i.e., endogenous) and responds to ...

and other photoperiodisms including control of flowering times in plants and mating seasons in animals.

Structure

Photoreceptor proteins typically consist of a

attached to a non-protein chromophore (sometimes referred as photopigment, even so photopigment may also refer to the photoreceptor as a whole). The chromophore reacts to light via photoisomerization or

photoreduction Light-dependent reactions is jargon for certain photochemical reactions that are involved in photosynthesis, the main process by which plants acquire energy. There are two light dependent reactions, the first occurs at photosystem II (PSII) and ...

, thus initiating a change of the receptor protein which triggers a signal transduction cascade. Chromophores found in photoreceptors include retinal (

retinylidene protein Retinylidene proteins, are proteins that use retinal as a chromophore for light reception. They are the molecular basis for a variety of light-sensing systems from phototaxis in flagellates to eyesight in animals. Retinylidene proteins include ...

s, for example rhodopsin in animals), flavin ( flavoproteins, for example cryptochrome in plants and animals) and bilin ( biliproteins, for example phytochrome in plants). The plant protein

UVR8 UV-B resistance 8 (UVR8) also known as ultraviolet-B receptor UVR8 is an UV-B – sensing protein found in plants and possibly other sources. * It is responsible for sensing ultraviolet light in the range 280-315 nm and initiating the plant ...

is exceptional amongst photoreceptors in that it contains no external chromophore. Instead, UVR8 absorbs light through tryptophan residues within its protein

coding sequence The coding region of a gene, also known as the coding sequence (CDS), is the portion of a gene's DNA or RNA that codes for protein. Studying the length, composition, regulation, splicing, structures, and functions of coding regions compared to ...

Photoreceptors in animals

''(Also see: Photoreceptor cell)'' * Melanopsin: in vertebrate retina, mediates pupillary reflex, involved in regulation of circadian rhythms *

Photopsin Vertebrate visual opsins are a subclass of ciliary opsins and mediate vision in vertebrates. They include the opsins in human rod and cone cells. They are often abbreviated to ''opsin'', as they were the first opsins discovered and are still th ...

: reception of various colors of light in the

cone cell Cone cells, or cones, are photoreceptor cells in the retinas of vertebrate eyes including the human eye. They respond differently to light of different wavelengths, and the combination of their responses is responsible for color vision. Cone ...

s of vertebrate retina * Rhodopsin: green-blue light reception in the rod cells of vertebrate retina * Protein Kinase C: mediates photoreceptor deactivation, and retinal degeneration *

OPN5 Opsin-5, also known as G-protein coupled receptor 136 or neuropsin is a protein that in humans is encoded by the ''OPN5'' gene. Opsin-5 is a member of the opsin subfamily of the G protein-coupled receptors. It is a photoreceptor protein sensitiv ...

: sensitive to

UV-light Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiatio ...

Photoreceptors in plants

: 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 the circadian 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. The most prominent wavelength responsible for plant mechanisms is blue light, which can trigger cell elongation, plant orientation, and flowering. One of the most important processes regulated by photoreceptors is known as photomorphogenesis. 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.

Photoreceptors in phototactic flagellates

''(Also see:

Eyespot apparatus The eyespot apparatus (or '' stigma'') is a photoreceptive organelle found in the flagellate or (motile) cells of green algae and other unicellular photosynthetic organisms such as euglenids. It allows the cells to sense light direction and int ...

)'' * Channelrhodopsin: in unicellular algae, mediates phototaxis * Chlamyopsin and volvoxopsin * Flavoproteins

Photoreceptors in archaea and bacteria

* Bacteriophytochrome * sensory bacteriorhodopsin * Halorhodopsin * Proteorhodopsin * Cyanobacteriochrome

Photoreception and signal transduction

* Visual cycle * Visual phototransduction

Responses to photoreception

Visual perception Visual perception is the ability to interpret the surrounding environment through photopic vision (daytime vision), color vision, scotopic vision (night vision), and mesopic vision (twilight vision), using light in the visible spectrum ref ...

* Phototropism * Phototaxis *

Circadian rhythm A circadian rhythm (), or circadian cycle, is a natural, internal process that regulates the sleep–wake cycle and repeats roughly every 24 hours. It can refer to any process that originates within an organism (i.e., endogenous) and responds to ...

(body clock) * Photoperiodism

References

{{DEFAULTSORT:Photoreceptor Protein Circadian rhythm Sensory receptors Signal transduction Biological pigments Integral membrane proteins Molecular biology