Glycolysis takes place in the cytoplasm of cells and begins with the activation of the main respiratory substrate, namely the hexose sugar called glucose. This activation involves the addition of two phosphate molecules provided by two molecules of ATP. The resultant activated molecule is known as phosphorylated glucose and in the next stage of glycolysis it is split into two molecules called triose phosphate. The third stage entails the oxidation of these molecules by the removal of hydrogen, which is transferred to a carrier called NAD. The final stage is the production of the 3-carbon molecule pyruvate, which also results in the formation of two molecules of ATP.