Where does the energy come from to allow ATPsynthase to phosphorylate ADP?

During oxidative phosphorylation, electrons originating from NADH and FADH2 join with oxygen, and the energy released from these oxidation/reduction reactions is utilized to allow ATPsythase to phosphorylate ADP. The transfer of electrons from NADH to O2 is a high energy-yielding reaction. For the electrons to be in usable form, energy must be produced slowly by going through a series of carriers in the electron transport chain. The carriers are classified into 4 complexes in the inner mitochondrial membrane. A fifth protein complex then acts to couple the high energy-yielding reaction of electron transport to ATP synthesis. 

Energy released in the reactions is harvested as a proton gradient, which is then utilized to generate ATP in chemiosmosis. As protons move down the concentration gradient through the ATP synthase, the resulting energy released causes the rotor and rod of ATP synthase to rotate. Chemiosmosis and the electron transport chain together make up oxidative phosphorylation.