 In the mitochondrion , pyruvate is oxidized by the pyruvate dehydrogenase complex to the acetyl group , which is fully oxidized to carbon dioxide by the citric acid cycle ( also known as the Krebs cycle ) . Every " turn " of the citric acid cycle produces two molecules of carbon dioxide , one molecule of the ATP equivalent guanosine triphosphate ( GTP ) through substrate @-@ level phosphorylation catalyzed by succinyl @-@ CoA synthetase , three molecules of the reduced coenzyme NADH , and one molecule of the reduced coenzyme FADH2 . Both of these latter molecules are recycled to their oxidized states ( NAD + and FAD , respectively ) via the electron transport chain , which generates additional ATP by oxidative phosphorylation . The oxidation of an NADH molecule results in the synthesis of 2 – 3 ATP molecules , and the oxidation of one FADH2 yields between 1 – 2 ATP molecules . The majority of cellular ATP is generated by this process . Although the citric acid cycle itself does not involve molecular oxygen , it is an obligately aerobic process because O2 is needed to recycle the reduced NADH and FADH2 to their oxidized states . In the absence of oxygen the citric acid cycle will cease to function due to the lack of available NAD + and FAD . 
