Substrate-level phosphorylation:

There are two distinct techniques through that cells synthesize ATP.  In  oxidative phosphorylation which including  the electron transport  chain and the generation  of ATP is connected to the oxidation of NADH and FADH₂   to NAD⁺ and FAD respectively,  and  occurs  through the generation  of a proton  gradient  across  the inner mitochondrial  membrane.  In compare, the two ATP synthetic reactions in glycolysis (catalyzed through 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.  A third instance of substrate-level phosphorylation is the synthesis of GTP by succinate dehydrogenase in the citric acid cycle. The GTP can be used to phosphorylate ADP to build ATP.