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. So we see that Coenzyme Q, also known as Ubiquinone, shown here, is basically a small hydrophobic molecule that is dissolved in the inner mitochondrial membrane. And it acts as an electron carrier that shuttles electron from protein complex one or protein complex two, ultimately, to protein complex three. Now, in its oxidized form, we call it Ubiquinone Q.