Vitamin A is a group of unsaturated nutritional organic compounds that includes retinol, retinal, retinoic acid, and several provitamin A carotenoids (most notably beta-carotene). Vitamin A has multiple functions: it is important for growth and development, for the maintenance of the immune system and good vision (Fennema, 2018). Vitamin A is needed by the retina of the eye in the form of retinal, which combines with protein opsin to form rhodopsin, the light-absorbing molecule necessary for both low-light (scotopic vision) and color vision (Berdanier, 2017). Vitamin A also functions in a very different role as retinoic acid (an irreversibly oxidized form of retinol), which is an important hormone-like growth factor for epithelial and other cells.
In foods of animal origin, the major form of vitamin A is an ester, primarily retinyl palmitate, which is converted to retinol (chemically an alcohol) in the small intestine. The retinol form functions as a storage form of the vitamin, and can be converted to and from its visually active aldehyde form, retinal.
All forms of vitamin A have a beta-ionone ring to which an isoprenoid chain is attached, called a retinyl group. Both structural features are essential for vitamin activity (The orange pigment of carrots (beta-carotene) can be represented as two connected retinyl groups, which are used in the body to contribute to vitamin A levels. Alpha-carotene and gamma-carotene also have a single retinyl group, which give them some vitamin activity. None of the other carotenes have vitamin activity. The carotenoid beta-cryptoxanthin possesses an ionone group and has vitamin activity in humans.
ABDULMALIK wisdom, A. (2020). Biochemical Role of Vitamin A in Vision. Afribary. Retrieved from https://afribary.com/works/biochemical-role-of-vitamin-a-in-vision
ABDULMALIK wisdom, Adonuja "Biochemical Role of Vitamin A in Vision" Afribary. Afribary, 15 Mar. 2020, https://afribary.com/works/biochemical-role-of-vitamin-a-in-vision. Accessed 31 Jul. 2021.
ABDULMALIK wisdom, Adonuja . "Biochemical Role of Vitamin A in Vision". Afribary, Afribary, 15 Mar. 2020. Web. 31 Jul. 2021. < https://afribary.com/works/biochemical-role-of-vitamin-a-in-vision >.
ABDULMALIK wisdom, Adonuja . "Biochemical Role of Vitamin A in Vision" Afribary (2020). Accessed July 31, 2021. https://afribary.com/works/biochemical-role-of-vitamin-a-in-vision