Respiration is the process or mechanism to break down food molecules to release energy in the form of ATP. During the process of aerobic respiration, phosphorylation of ADP occurs forming the energy currency molecule - Adenosine triphosphate. The energy required for the production of proton gradient which is needed for phosphorylation is obtained from the oxidation-reduction of the molecules. Such phosphorylation step is called substrate level phosphorylation. One of the steps of the respiration process is glycolysis where 6C-glucose is broken down to 3C-pyruvate. During glycolysis, substrate level phosphorylation occurs at two steps. Conversion of 1,3-diphosphoglyceric acid to 3-phosphoglyceric acid by losing a phosphate group (used in making ATP) catalyzed by enzyme phosphoglycerokinase. Pyruvate kinase hydrolyses phosphoenol pyruvate to pyruvic acid where PEP loses phosphate group which is used in making ATP.

Also, during Kreb's cycle in the step of hydrating succinyl coenzyme A to succinic acid energy is liberated. This energy is used in formation of GTP from GDP but it is quite unstable. Thus, it is transformed to ATP. This process of ATP formation is by substrate level phosphorylation.