The two mechanisms that generate ATP are: oxidative phosphorylation and substrate-level phosphorylation. Substrate-level phosphorylation involves an enzyme that transfers a phosphate group from a substrate to ADP. Substrate-level phosphorylation includes processes such as glycolysis. There are a total of 4 ATP produced per glucose molecule from glycolysis and the citric acid cycle. Oxidative phosphorylation generates most of the ATP in cellular respiration. Enzymes transfer the hydrogen atoms to the carrier molecules NAD and FAD. These carriers deliver the protons and electrons to the mitochondrial membrane. The movement of electrons along the ETC fuels the pumps that transport protons in the region between two mitochondrial membranes. These protons then diffuse back across the membrane using ATP synthase.