And this diagram that basically describes this transmembrane pump in its closed state. And what that means is the substrate molecules cannot actually enter this molecule. Now, what happens is it basically inter converts from the closed state and the open state. So it moves back and forth relatively quickly. So it's closed here, open here, closed here, open here, and only when it's open can the actual substrate molecules, shown here in red, actually enter this central cavity of the transmembrane domain. So remember, we have the two transmembrane domains that essentially create this internal cavity that can fit that substrate molecule.