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.