A vitamin is an organic compound needed in small quantities that cannot be made in cells. In human nutrition , most vitamins function as coenzymes after modification; for example, all water-soluble vitamins are phosphorylated or are coupled to nucleotides when they are used in cells.  Nicotinamide adenine dinucleotide (NAD + ), a derivative of vitamin B 3 ( niacin ), is an important coenzyme that acts as a hydrogen acceptor. Hundreds of separate types of dehydrogenases remove electrons from their substrates and reduce NAD + into NADH. This reduced form of the coenzyme is then a substrate for any of the reductases in the cell that need to reduce their substrates.  Nicotinamide adenine dinucleotide exists in two related forms in the cell, NADH and NADPH. The NAD + /NADH form is more important in catabolic reactions, while NADP + /NADPH is used in anabolic reactions.
1650s, from Dutch gas, probably from Greek khaos "empty space" (see chaos ). The sound of Dutch "g" is roughly equivalent to that of Greek "kh." First used by Flemish chemist . van Helmont (1577-1644), probably influenced by Paracelsus, who used khaos in an occult sense of "proper elements of spirits" or "ultra-rarified water," which was van Helmont's definition of gas .
Modern scientific sense began 1779, with later specialization to "combustible mix of vapors" (1794, originally coal gas); "anesthetic" (1894, originally nitrous oxide); and "poison gas" (1900). Meaning "intestinal vapors" is from 1882. "The success of this artificial word is unique" [Weekley]. Slang sense of "empty talk" is from 1847; slang meaning "something exciting or excellent" first attested 1953, from earlier hepster slang gasser in the same sense (1944). Gas also meant "fun, a joke" in Anglo-Irish and was used so by Joyce (1914). As short for gasoline , it is American English, first recorded 1905.