Coenzymes are small organic molecules that function with thousands of different enzymes in all organisms, assisting in the catalytic processes needed for life. They often contain vitamins as components. Several coenzymes participate in the major oxidation and reduction processes of cells. Some assist in the making and breaking of carbon–carbon bonds. Others are carriers of molecular fragments. Coenzymes participate in virtually every aspect of the chemistry of every living cell whether of bacteria, protozoa, fungi, higher plants, animals, or human beings. Some vitamins are incorporated into the coenzymes in which they function. Others become chemically attached to proteins and act as bound coenzymes, often described as prosthetic groups. Vitamin C (ascorbic acid) functions as a coenzyme without any firm attachment to a protein. Vitamin A, in one of its forms (retinoic acid), acts as a hormone. Vitamin D, which can be formed in the skin by the action of sunlight, is a natural precursor to oxygencontaining derivatives which also act as hormones. Most modern human beings would have difficulty in meeting the body’s needs for vitamin D by sunbathing; hence the designation of this prohormone as vitamin D.