gel. The cellular respiration (complete oxidation of fuels) can be divided into three stages StageI All the fuel molecules are oxidized to generate a common two-carbon unit. acetyl-coA Stage II The acetyl-CoA is completely oxidized into CO2, with electrons collected by NAD and FAD via a cyclic pathway (named as the citric acid cycle Krebs cycle, or tricarboxylic acid cycle) Stage lll Electrons of NADH and FADH, are yo cansferred to o, via a series carr riers, producin g and a H gradient, which will promote ATP formation1. The cellular respiration (complete oxidation of fuels) can be divided into three stages • Stage I All the fuel molecules are oxidized to generate a common two-carbon unit, acetyl-CoA. • Stage II The acetyl-CoA is completely oxidized into CO2 , with electrons collected by NAD and FAD via a cyclic pathway (named as the citric acid cycle, Krebs cycle, or tricarboxylic acid cycle). • Stage III Electrons of NADH and FADH2 are transferred to O2 via a series carriers, producing H2O and a H+ gradient, which will promote ATP formation