Any compound, essential in trace amounts for the normal functioning of an organism, but not synthesized by some heterotrophs, which thus need to obtain it from plants or microorganisms. All vitamins with the exceptions of vitamins C (*ascorbic acid
), Ε (*tocopherol
), and K, function as coenzymes. They can be divided into two groups, the water-soluble vitamins, which contain all the B-group vitamins (see thiamin
, nicotinic acid
, pantothenic acid
, folic acid
) and vitamin C, and the fat-soluble vitamins, including vitamins A, D, E, and K.