This is where the electrical work is done. Now, when this electron or two electrons travel all the way down to this cathode. These electrons react with the oxygen molecule, reducing it. But notice that in order for this build up of H plus ions not to occur, these H plus ions must pass to this side. So this, in a way, acts as a sole bridge because if this membrane wasn't here, we'd have a build up of positive charge here and a lack of positive charge here. And then that means our electrons will stop flowing.