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.