Substrate for ATP synth ase is -

Ans. is 'b' i.e., ADP o ATP synthesis occurs in ETC by enzyme F0F1, ATPase.o F1. component of F0F1, ATPase is ATPase synthase which catalyzes the conversion of ADP to ATP.ATP synthesis in ETCo Free energy of electron transport" is conserved by pumping protons from mitochondrial matrix to the intermembrane space, so as to create an electrochemical proton gradient across the inner mitochondrial membrane, with outer side of membrane is positively charged as compared to inside.o Electrochemical potential of this gradient is used to synthesize ATP by ATP synthase.o Inner mitrochondrial membrane is impermeable to protons; therefore, proton pumps are required to pump protons (hydrogen ions) from mitochondrial matrix to the intermembrane space.o The complex IQ (Site I), complex IIIQ (Site II) and complex IVQ (Site III) act as proton pumps ejecting protons (hydrogen ions) from the mitochondrial matrix to intermembrane space.o Complex I (NADH-CoQ reductaseQ) and complex III (CoQ - cytochrome c reductaseQ) pump 4 protons each and complex IV (cytochrome c oxidase) pumps 2 protons.o The concentration of hydrogen ions (protons) on the outer side of inner membrane becomes higher as compared to inner side.o This results in generation of the electrochemical potential.o Due to this electrochemical potential or proton motive force, the H+ ions ejected out (by proton pumps) flow back into the mitochondrial matrix down its electrochemical gradient through F0F1 ATPase molecule, which is also known as complex V of respiratory chain.o This proton influx causes ATP synthesis. F0 components acts as channel for passage of hydrogen ions (Protons)Q. F1 component possesses ATP synthase activity, which is switched on when the hydrogen ions pass through F0 component.