Vitamin amino acids. Most vitamins are not single molecules, but groups of related molecules called vitamers. For example, vitamin E consists of four tocopherols and four tocotrienols. The thirteen vitamins required by human metabolism are: vitamin A (retinols and carotenoids), vitamin B (thiamine), vitamin B (riboflavin), vitamin B (niacin), vitamin B (pantothenic acid), vitamin B (pyridoxine), vitamin B (biotin), vitamin B (folic acid or folate), vitamin B (cobalamins), vitamin C (ascorbic acid), vitamin D (calciferols), vitamin E (tocopherols and tocotrienols), and vitamin K (quinones). Vitamins have diverse biochemical functions. Some forms of vitamin A function as regulators of cell and tissue
Vitamin D the transformation of previtamin D to vitamin D in an organic solvent takes about 12 days to complete. The conversion of previtamin D to vitamin D in the skin is about 10 times faster than in an organic solvent. The conversion from ergosterol to vitamin D follows a similar procedure, forming previtamin D by photolysis, which isomerizes to vitamin D. The transformation of previtamin D to vitamin D in methanol has a rate comparable to that of previtamin D. The process is faster in white button mushrooms. Vitamin D is produced photochemically from 7-dehydrocholesterol in the skin of most vertebrate