View source for Opsin

'''Opsins''' are the universal photo[[receptor]] molecules of all visual systems in the animal kingdom.<ref>Plachetzki, D.; Fong, C.; Oakley T. 2010. "The evolution of phototransduction from an ancestral cyclic nucleotide gated pathway". ''Proceedings of the Royal Society / Biological sciences'' '''277''' (1690): 1963–1969. doi:10.1098/rspb.2009.1797. PMID 20219739.</ref><ref>Shichida Y.; Matsuyama T. 2009. "Evolution of opsins and phototransduction". [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2781858/?tool=pmcentrez] ''Philosophical transactions of the Royal Society of London. Series B, Biological sciences''. '''364''' (1531): 2881–2895. doi:10.1098/rstb.2009.0051. PMID 19720651</ref>   

They change from a resting state to a signalling state by absorbing light. This activates the G protein, resulting in a [[signal]]ling cascade which produces [[Physiology|physiological]] responses. 

This process of capturing a photon and transforming it into a physiological response is known as photo[[transduction]].

Five groups of opsins are involved in vision. Another opsin found in the mammalian retina, melanopsin, is involved in [[circadian rhythm]]s and [[pupil (eye)|pupillary]] [[reflex]], but not in image-forming.<ref>Lamb T.D; Collin S.P. & Pugh E.N. Jr. 2007. Evolution of the vertebrate eye: opsins, photoreceptors, retina and eye cup. ''Nature Reviews Neuroscience''. '''8''' (12): 960–976. [http://www.nature.com/nrn/journal/v8/n12/full/nrn2283.html]</ref>

Each opsin allows vision in just a short stretch of [[wavelength]]. This is equivalent to seeing in only one color. Two opsins allow vision in two colors, and is usual in mammals. Four opsins allows vision in full color, and is usual in [[teleost]] fish, [[reptile]]s and [[bird]]s. In [[mammal]]s, only [[Old World monkey]]s, [[ape]]s and [[human]]s have [[trichromacy]], called full color vision. 

It is thought – see [[evolution of color vision]] – that mammals lost much of their color vision capability during the long period in the [[Mesozoic]] when they lived as mostly [[nocturnal]] animals.<ref>Kemp T.S. 2005. ''The origin and evolution of mammals''. Oxford University Press.</ref> 
:"...two cone opsin gene families appear in contemporary [[eutherian]] mammals and, with the exception of some [[primate]]s, none of these animals [get] more than a single photopigment type from each of their two gene families".<ref name="Bow">Bowmaker J.K. 1998. Evolution of colour vision in vertebrates. ''Eye'' '''12''' (3b): 541–547. [http://www.nature.com/eye/journal/v12/n3b/abs/eye1998143a.html]</ref><ref name="Jacobs">Jacobs G.H. 2009. ''Phil Trans Roy Soc B''. '''364'''  (1531) 2957-2967. [http://rstb.royalsocietypublishing.org/content/364/1531/2957.full]</ref>

== References ==
{{Reflist}}
{{Eye proteins}}

[[Category:Physiology]]
[[Category:Biochemistry]]
[[Category:Eye]]