vitamin

Meaning: A vitamin is an organic compound required in small amounts that the body cannot make in sufficient quantity, so it must be supplied by the diet. Water-soluble vitamins (the B vitamins and vitamin C) are generally excreted in urine and stored little. Fat-soluble vitamins (A, D, E, K) are absorbed with dietary fat and can accumulate in tissue. Many B vitamins are precursors of coenzymes: niacin → NAD⁺, riboflavin → FAD, pantothenic acid → coenzyme A, thiamine → TPP. Other roles include vitamin C in hydroxylating proline in collagen, vitamin K in carboxylating glutamate in clotting factors, and vitamin A as retinal in vision.

On the MCAT: A passage describes an enzyme that needs an organic cofactor and asks you to recognize it as vitamin-derived. With a deficiency, less functional enzyme is available, so reaction rate and Vmax typically fall. Other questions ask you to classify a vitamin by solubility and predict which type is more likely to reach toxic levels.

Common confusion: Vitamins are not minerals. Metal ions such as Zn²⁺ or Mg²⁺ are inorganic cofactors, while coenzymes are organic and often vitamin-derived. Vitamins also supply no energy themselves; they help enzymes extract it. "Must come from the diet" has exceptions: skin makes vitamin D with UV light, and niacin can be made from tryptophan.