/roh dop"sin/, n. Biochem.
a bright-red photosensitive pigment found in the rod-shaped cells of the retina of certain fishes and most higher vertebrates: it is broken down by the action of dim light into retinal and opsin. Also called visual purple.
[1885-90; RHOD- + Gk óps(is) sight, vision + -IN2]

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Light-sensitive, purple-red organic pigment contained in the rod cells of the retina that allows the eye to see in black and white in dim light.

It is composed of opsin, a protein, linked to retinal, a conjugated molecule (see conjugation) formed from vitamin A. Photons of light that enter the eye are absorbed by retinal and cause it to change its configuration, starting a biochemical chain of events that ends with impulses being sent along the optic nerve to the brain. In bright light, to protect rod cells from overstimulation, rhodopsin breaks down into retinal and opsin, both of which are colourless. In dim light or darkness the process is reversed (dark adaptation), and purple-red rhodopsin is reformed. Similar light-sensitive compounds made of retinal and other opsin proteins are the pigments in the retina's cone cells responsible for colour vision in bright light.

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also called  Visual Purple,  

      a chromoprotein (protein linked to a pigment-carrying substance) that is contained in the light-sensitive cells of the rod type in the retina of the eye; it functions in the eye's adaptation to dim light. When the eye is exposed to bright light, the rhodopsin bleaches; after an interval of darkness, it returns to its former purple-red colour.

      The pigment-bearing portion of rhodopsin is retinal, a substance formed by oxidation of vitamin A. The protein portion is opsin. In a bright light rhodopsin breaks down into retinal and opsin; in the dark the process is reversed. See also rod.

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Universalium. 2010.

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