Well, by breathing in. So when we breathe in, when we inhale, we take an oxygen, and that's the same oxygen that is used by protein complex number four. Now, once we reduce our oxygen, this protein complex number four also transports four H plus ions from the mitochondrial matrix to the intermembrane space. And so we see that if we use one NADH molecule, we basically transport a total of ten four, two and two. So ten H plus ions into the intermembrane space, and that establishes an electrochemical gradient. Now, that means we have many more H plus ions found on the intermembrane space than in the mitochondrial matrix.