 This concept of how liquid a membrane is, is interesting. So I'm going to spend a little bit of time on that. This is an old simulation of why not a pure PODPPC bilayer. A pure bilayer is going to be super flexible and it will even have these undulation motions if I don't subject it to attention. It's literally like a film, very floppy. I'll move this up so that it doesn't perturb my view of you so much. But what if I take this and add some other molecules, in particular cholesterol. Cholesterol occurs in virtually all vertebrate cells but not in prokaryotes. Just adding 10 percent of cholesterol completely changes the properties of the system. The system becomes tighter, it becomes more packed, and as a consequence it also becomes a bit thicker. If we move from, I think that's from 10 to 20 percent. At 20 percent it's an even stronger effect, and at 30 percent of the lipids being cholesterol, it's extreme. This is a completely flat bilayer now, and it's very rigid. It hardly doesn't move at all, and there's almost no diffusion left of the lipids. This is a very typical property of cholesterol. Cholesterol tends to rigidify membranes and cell walls, which is occasionally useful. In some cases we want the structure. In plants it definitely makes sense to have more rigid structures. And in other cases such as my blood vessels, eventually the depositions of cholesterol and everything leads to a very rigid system that I might not want. But note here how the specific composition of lipids, cholesterol itself is a lipid too, how the specific composition of lipids will significantly influence the physiochemical properties of a lipid bilayer. I haven't even started adding proteins yet.