The majority of ATP molecules are produced during the stage of cellular respiration called oxidative phosphorylation, also known as the electron transport chain.
During the electron transport chain, high-energy electrons generated by the previous stages of cellular respiration are passed through a series of electron carriers, ultimately to oxygen. As electrons move through the electron transport chain, energy is released and used to pump protons (H+) from the mitochondrial matrix to the intermembrane space. This creates an electrochemical gradient, with a high concentration of H+ in the intermembrane space and a low concentration in the matrix.
The H+ ions then diffuse back into the matrix through ATP synthase channels, driving the phosphorylation of ADP to form ATP. This process is called chemiosmosis and is responsible for producing the majority of the ATP molecules in cellular respiration.
In contrast, glycolysis only produces a small amount of ATP through substrate-level phosphorylation, and the citric acid cycle produces a small amount of ATP through substrate-level phosphorylation and electron transfer to NAD+ and FAD. Pyruvate oxidation produces a small amount of ATP through substrate-level phosphorylation and electron transfer to NAD+. However, these stages generate electrons and other molecules that are essential for the electron transport chain to produce ATP.