Adenosine triphosphate (ATP) is the energy currency of the cell and is required in cell metabolism of all living beings including bacteria, plants, and animals. It is utilized for the endergonic reactions and various energy-requiring processes in the cell. Various energy-requiring processes of the cell include transport of ions and biomolecules across plasma membrane against concentration gradient, movement of substances within the cell, cell growth including cell division, anabolic reactions of the cell, and various other cellular processes. ATP synthase catalyzes ATP synthesis during oxidative phosphorylation and photophosphorylation. Structure of ATP synthase and the mechanism of ATP synthesis have remained conserved through evolution. ATP was discovered in 1929 by German chemist Karl Lohmann. In 1941, it was demonstrated by Fritz Albert Lipmann that ATP was the universal energy carrier of the cells. Later on, Lohmann shared Nobel Prize with Hans Krebs in 1953 for his work on citric acid cycle. Lipmann was the first to use the squiggle (~) symbol and the term “high-energy bond” for the compounds having high phosphate group transfer potential. In 1948, Alexander Todd undertook chemical synthesis of ATP and deciphered its structure, for which he was awarded Nobel Prize in 1957. The standard free energy of ATP hydrolysis is 30.5 kJ.mol−1. However, under cellular conditions, the free energy value is around −50 kJ.mol−1. Thus, synthesis of 1 mole of ATP requires around 50 kJ of energy. There are two ways by which ATP can be synthesized in a cell. One of the ways is known as substrate-level phosphorylation, in which high-energy bond of the substrate is hydrolyzed and the energy released is utilized for ATP synthesis. Another mechanism involves coupling of energy of proton gradient with the synthesis of ATP. Proton gradient is created by electron transport chain either during oxidation of NADH/FADH2 or during the light reaction of photosynthesis in mitochondria and chloroplasts, respectively.