Final answer:
The energy for ATP production via oxidative phosphorylation comes from protons flowing through ATP synthase, powered by electrons moving through the electron transport chain.
Step-by-step explanation:
The source of the energy used to make ATP by oxidative phosphorylation is the flow of protons through ATP synthase. During cellular respiration, electrons move through the electron transport chain and create a proton gradient across the mitochondrial membrane. This gradient is a form of stored energy, and when protons flow back through the membrane-bound enzyme, ATP synthase, ATP is synthesized from ADP and inorganic phosphate. The electron transport chain itself is fueled by electrons from reduced compounds NADH and FADH2, which are generated during glycolysis and the citric acid cycle.