In the presence of oxygen the oxidation of glucose that begins in glycolysis continues, where glycolysis leaves off with pyruvate. The cell harvests pyruvates considerable energy in two steps : first by oxidising pyruvate to form acetyl Co-A and then by oxidising acetyl Co-A in the Krebs cycle. The reactions of Krebs cycle occur in mitochondria. In this cycle the two carbon acetyl Co-A combines with a 4-carbon molecule called oxaloacetate. The resulting 6 C molecule goes through a sequence of electron yielding oxidation reaction to form $ C{{O}_{2}} $ and wafer with releasing sufficient energy. The NADH and $ FAD{{H}_{2}} $ formed during Krebs cycle carry energetic electrons and are employed to drive the synthesis of large amount of ATP by electron transport chain.