<P> In eukaryotes, oxidative phosphorylation occurs in the mitochondrial cristae . It comprises the electron transport chain that establishes a proton gradient (chemiosmotic potential) across the boundary of inner membrane by oxidizing the NADH produced from the Krebs cycle . ATP is synthesized by the ATP synthase enzyme when the chemiosmotic gradient is used to drive the phosphorylation of ADP . The electrons are finally transferred to exogenous oxygen and, with the addition of two protons, water is formed . </P> <P> The table below describes the reactions involved when one glucose molecule is fully oxidized into carbon dioxide . It is assumed that all the reduced coenzymes are oxidized by the electron transport chain and used for oxidative phosphorylation . </P> <Table> <Tr> <Th> Step </Th> <Th> coenzyme yield </Th> <Th> ATP yield </Th> <Th> Source of ATP </Th> </Tr> <Tr> <Td> Glycolysis preparatory phase </Td> <Td> </Td> <Td> − 2 </Td> <Td> Phosphorylation of glucose and fructose 6 - phosphate uses two ATP from the cytoplasm . </Td> </Tr> <Tr> <Td> Glycolysis pay - off phase </Td> <Td> </Td> <Td> </Td> <Td> Substrate - level phosphorylation </Td> </Tr> <Tr> <Td> 2 NADH </Td> <Td> 3 or 5 </Td> <Td> Oxidative phosphorylation: Each NADH produces net 1.5 ATP (instead of usual 2.5) due to NADH transport over the mitochondrial membrane </Td> </Tr> <Tr> <Td> Oxidative decarboxylation of pyruvate </Td> <Td> 2 NADH </Td> <Td> 5 </Td> <Td> Oxidative phosphorylation </Td> </Tr> <Tr> <Td> Krebs cycle </Td> <Td> </Td> <Td> </Td> <Td> Substrate - level phosphorylation </Td> </Tr> <Tr> <Td> 6 NADH </Td> <Td> 15 </Td> <Td> Oxidative phosphorylation </Td> </Tr> <Tr> <Td> 2 FADH </Td> <Td> </Td> <Td> Oxidative phosphorylation </Td> </Tr> <Tr> <Td_colspan="2"> Total yield </Td> <Td> 30 or 32 ATP </Td> <Td> From the complete oxidation of one glucose molecule to carbon dioxide and oxidation of all the reduced coenzymes . </Td> </Tr> </Table> <Tr> <Th> Step </Th> <Th> coenzyme yield </Th> <Th> ATP yield </Th> <Th> Source of ATP </Th> </Tr>

Molecular oxygen (o2) has what role in aerobic cellular respiration