The processes that feed ~P into ATP/ADP cycle fall mainly within four groups:

1. Oxidative phosphorylation: The greatest quantitative source of ~P is from reactions catalysed by the enzyme ATP synthase, which, in fact, reverses the hydrolysis of ATP. The free energy to drive this process is obtained from the respiratory chain within mitochondria, and it is called oxidative phosphorylation.
2. Catabolism of glucose to lactic acid: Energy is captured during catabolism of glucose to lactic acid in a sequence of reactions commonly called glycolysis. In this pathway, there is a net formation of two high-energy phosphate groups, which produce two ATP molecules from two ADP molecules.
3. Oxidation of pyruvic acid: The energy capture occurs at the succinyl thiokinase step of the Krebs cycle, wherein succinyl CoA, one of the intermediates of the cycle, transforms to succinate. Here again, one ATP molecule is formed from one ADP molecule.
4. The muscle compounds: Another group of compounds acting as storage forms of high-energy phosphate within muscles include creatine, occurring in muscle.