During a process called oxidative phosphorylation, the NADre and FADre molecules are oxidized, releasing energy that is then used to phosphorylate ADP into ATP. The energy is released as NADre and FADre give up high-energy electrons and hydrogen atoms to oxygen to form water. In between the points at which the NADre and FADre give up their electrons and oxygen accepts them, the electrons are passed down a chain of electron-transport compounds. As the electrons pass down the chain, they give up their energy to ADP molecules. The electrons from the eight NADre molecules produced during aerobic respiration pass down the whole chain; these electrons release energy at three points, producing three ATP each. The electrons from the two NADre produced during glycolysis and the two FADre pass down only part of the chain; these electrons release energy at only two points along the chain, producing two ATP each. Thus, during oxidative phosphorylation, an additional 32 molecules of ATP are produced (95 percent of the total energy captured in ATP form), bringing the total ATP produced from a single glucose molecule to 36. Anaerobic and aerobic respiration together are able to store about 38 percent of the energy of the glucose molecule as ATP.
In addition to carbohydrates, fats and proteins can also be broken down to release energy. Fats are first hydrolyzed into glycerol and fatty acids. The glycerol, a three-carbon compound, is then fed into the glycolytic pathway at the point where G3P appears. The fatty acids are broken into two-carbon fragments, converted into
