And finally, if we examine protein complex four, this is essentially where we take those electrons and we use them to reduce oxygen molecules into water molecules. So this is where we find the final electron acceptor of the electron transport chain. And this also uses the movement of those electrons to pump those protons out of the matrix and into the space between the two membranes. And so together, protein complex one, three and four are basically proton pumps which help establish a proton gradient. And then, as we'll see in a future lecture, the ATP synthase uses that proton gradient to generate those ATP molecules via oxidative osphorylation. And it's called oxidative because protein complex four uses oxygen as that final electron acceptor.