There are two distinct methods through that cells synthesize ATP. In oxidative phosphorylation which containing the electron transport chain and the generation of ATP is linked to the oxidation of NADH and FADH2 to NAD+ and FAD respectively, and occurs by the generation of a proton gradient across the inner mitochondrial membrane. In compare, the two ATP synthetic reactions in glycolysis (catalyzed by phosphoglycerate pyruvate and kinase kinase) include the direct transfer of a phosphate from a sugar-phosphate intermediate to ADP; these reactions are instances of substrate-level phosphorylation.