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Opsins are the universal photoreceptor molecules of all visual systems in the animal kingdom.[1][2]

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

This process of capturing a photon and transforming it into a physiological response is known as phototransduction.

Five groups of opsins are involved in vision, mediating the conversion of a photon of light into an electrochemical signal, the first step in the visual transduction cascade.

Another opsin found in the mammalian retina, melanopsin, is involved in circadian rhythms and pupillary reflex but not in image-forming.


  1. 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.
  2. Shichida Y.; Matsuyama T. 2009. "Evolution of opsins and phototransduction". [1] Philosophical transactions of the Royal Society of London. Series B, Biological sciences. 364 (1531): 2881–2895. doi:10.1098/rstb.2009.0051. PMID 19720651