Editing Retinol (section)

===Vision===
{{Main|Visual cycle}}
Retinol is an essential compound in the cycle of light-activated chemical reactions called the "[[visual cycle]]" that underlies vertebrate vision. Retinol is converted by the protein [[RPE65]] within the [[pigment epithelium]] of the [[retina]] into 11-''cis''-retinal. This molecule is then transported into the [[retina]]'s [[photoreceptor cell]]s (the [[rod cell|rod]] or [[cone cell|cone]] cells in mammals) where it binds to an [[opsin]] protein and acts as a light-activated molecular switch.  When 11-''cis''-retinal absorbs light it [[cis-trans isomerism|isomerizes]] into all-''trans''-retinal. The change in the shape of the molecule in turn changes the configuration of the opsin in a cascade that leads to the [[Action potential|neuronal firing]], which signals the detection of light.<ref>{{cite book | vauthors = Purves D, Augustine GJ, Fitzpatrick D, Katz LC, LaMantia AS, McNamara JO, Williams SM |date=2001 | chapter = Phototransduction |chapter-url = https://www.ncbi.nlm.nih.gov/books/NBK10806/ |title = Neuroscience |publisher=Sinauer Associates | edition = 2nd }}</ref> The opsin then splits into the protein component (such [[metarhodopsin]]) and the cofactor all-''trans''-retinal. The regeneration of active opsin requires conversion of all-''trans''-retinal back to 11-''cis''-retinal via retinol. The regeneration of 11-''cis''-retinal occurs in vertebrates via the conversion of all-''trans''-retinol to 11-''cis''-retinol in a sequence of chemical transformations that occurs primarily in the pigment epithelial cells.<ref name=bs>{{cite journal | vauthors = Sahu B, Maeda A | title = Retinol Dehydrogenases Regulate Vitamin A Metabolism for Visual Function | journal = Nutrients | volume = 8 | issue = 11 | pages = 746 | date = November 2016 | pmid = 27879662 | pmc = 5133129 | doi = 10.3390/nu8110746 | doi-access = free }}</ref>

Without adequate amounts of retinol, regeneration of rhodopsin is incomplete and [[night blindness]] occurs. Night blindness, the inability to see well in dim light, is associated with a deficiency of [[vitamin A]], a class of compounds that includes retinol and retinal. In the early stages of [[vitamin A]] deficiency, the more light-sensitive and abundant [[rod cell|rod]]s, which have [[rhodopsin]], have impaired sensitivity, and the [[cone cell]]s are less affected. The cones are less abundant than rods and come in three types, each contains its own type of [[iodopsin]], the opsins of the cones. The cones mediate [[color vision]], and vision in bright light (day vision).