Glycolysis refers to the conversion of glucose to two molecules of pyruvic acid. In the process, two molecules of ATP are consumed and four molecules of ATP are formed. In the steps of glycolysis two pairs of hydrogens are released. When respiration is anaerobic, reduced NAD is oxidized by pyruvic acid, which accepts two hydrogen atoms and is thereby reduced to lactic acid.
Heart muscle respires anaerobically for just a short time, under conditions of ischemia.
The Krebs cycle begins when coenzyme A donates acetic acid to an enzyme that adds it to oxaloacetic acid to form citric acid. The formation of citric acid begins a cyclic pathway that ultimately forms a new molecule of oxaloacetic acid. As the Krebs cycle progresses, one molecule ATP is formed, and three molecules of NAD and one of FAD are reduced by hydrogens from the Krebs cycle. Reduced NAD and FAD donate their electrons to an electron-transport chain of molecules located in the cristae. The last cytochrome becomes oxidized by donating its electron to oxygen, which functions as the final electron acceptor.