 So in the last few slides you've increasingly seen me talk about not just a small helix But an entire protein the lipids small molecules other types of lipids It's increasingly a large complex of many things interacting, right? This turns out to be important for membranes in general and in fact if we just think about two random particles In a large membrane that should interact if they diffuse around freely according to the single nickels on model Well, if they have to interact in general it would take a very long time before two random particles find each other in a cell It would be much more convenient if I could somehow group them together and force them to be close to each other This is a theoretical concept that physicists and people doing modeling proposed a long time ago essentially called a lipid raft So the idea with the lipid raft is Roughly using Many of the things that I brought up here that what if I had a domain in a membrane that was not quite as Fluid as the fluid mosaic model But maybe having excess cholesterol and others here stiffen it up so that proteins here don't really diffuse so much relative to each other That would mean that these two proteins would stay close to each other And it could facilitate and speed up reactions that were dependent on having multiple partners together The only cavities was very hard to measure this experimentally because you can't really determine a structure of it I would still say that this is not common We might see it now again in cells. There is some advanced light microscopy For instance that appear to see some more well-ordered regions in cells But be aware of the concept of lipid rafts Ten years from now we might tell that it wasn't until the late 2020s that we found all the lipid raft structures But today we don't have that much experimental data on it On the other hand remember my comment about each protein having a layer or two of lipids around the right This agrees kind of well with the idea of having a few lipids that don't really diffuse freely But these lipids stick together with a protein or two and this region would then diffuse freely around in the membrane So it's not an alternative to the fluid mosaic model It's more of a modification that we might have locally more rigid Regions that diffuse together in the large sea of the remaining lipids