Vision cone cell pigments

Light-absorbing pigments differentiate rod cells for black-white vision and three types of cone cells for color vision 807... 

Some details about cone cells and invertebrate vision. The biochemistry of retinal cones is less well known but is similar to that of rod cells. Cone pigments are present in the plasma membrane rather than in isolated discs (Fig. 23-40C). Different a, P, and y subunits of transducin are formed in rods and cones.522 Many differences are seen among various invertebrate visual systems. Inositol triphosphate (IP3) and Ca2+ often serve as signals of photoexcitation. G proteins also play prominent roles.522... 

Vision begins when light rays (photons) are focused by the eye s lens on to the retina, the layer of cells lining the inside of the eye ball. The retina contains large numbers of photoreceptor cells known as rods and cones. The ends of the rods and cones contain a molecule called rhodopsin which consists of a protein (opsin) covalently bonded to a purple pigment molecule called retinal. Structural changes which occur around a carbon-carbon double bond in the retinal component of the rhodopsin trigger a series of chemical reactions that eventually result in vision. 

In the pigment epithelium of the retina, all-/ra j-retinol is isomerized to 11-cm-retinol and oxidized to 1 l-at-retinaldehyde. This reacts with a lysine residue in opsin, forming the holoprotein rhodopsin. Opsins are cell type specific they shift the absorption of 11-a r-retinaldehyde from the ultraviolet (UV) into what we call, in consequence, the visible range — either a relatively broad spectrum of sensitivity for vision in dim light (in the rods) or more defined spectral peaks for differentiation of colours in bright light (in the cones). 

Goldstein EB (1970) Cone pigment regeneration in the isolated frog retina. Vision Res 10 1943-1951 Bunt-Milan AH, Saari JC (1983) Inununocytochemical localization of two retinoid-binding proteins in vertebrate retina. J Cell Biol 97 703-712... 

The retinal pigment epithelium cells (RPE) of the retina form an epithelial cell layer that takes up retinol from choroid capillaries and stores it as retinyl esters, to be used as substrate for the generation of 11-ds-retinal. In the layer of rod and cone photoreceptor cells adjacent to the RPE, 11-ds-retinal combines covalently with the protein opsin to generate the visual pigment rhodopsin in rods and, similarly, iodopsin in cones. Each rod outer segment is densely packed with some 10 molecules of rhodopsin per cell. The small quantity of vitamin A stored in the retina would be inadequate to maintain vision were it not for the visual cycle, a process in which 11-ds-retinal is regenerated after... 

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