Flavin adenine dinucleotide, or FAD, is a coenzyme found in all living cells. It plays a crucial role in various biological processes, particularly in cellular respiration and metabolism. FAD acts as an electron carrier, helping to transfer electrons during chemical reactions, which is essential for energy production. FAD is derived from the vitamin riboflavin (vitamin B2) and consists of a riboflavin molecule linked to an adenine nucleotide. In its oxidized form, FAD can be reduced to FADH2, which stores energy and is used in the electron transport chain to generate ATP, the primary energy currency of cells.