So ultimately, we establish a proton gradient in which we have a high proton concentration here and a low proton concentration here. And this ATP synthase basically uses that established electrochemical gradient of H plus ions, moves these H plus ions down that established electrochemical gradient from a high to a low electrochemical potential. And that allows the ATP synthase to actually phosphorylate ADP to ATP. And that's why we call it oxidative phosphorylation because we actually use an oxygen on the electron transport chain. So, remember, the citric acid cycle doesn't actually require oxygen itself, but the electron transport chain does. And we need the citric acid cycle to generate the majority of the NADH molecules and these Fadh two molecules.