 So the key difference between an ideal lipid bilayer that is quite easy to simulate in a computer and a biological membrane is that a biological membrane contains a total of proteins and other stuff. This is just an example. In fact, this structure is so complicated that it took us quite a while before we started having even decent models. One of the best models was developed by Singer and Nicholson, and it was published in 1972. I'm going to write down that one because it's important to remember. It's occasionally called the fluid mosaic model or alternatively the Singer-Nicholson model. So the idea Singer and Nicholson had is that looking at the lipids, the lipids behave kind of like a two-dimensional liquid, not even kind of. You can actually show that the lipid bilayer is a two-dimensional liquid. Each lipid is free to diffuse around in the XY plane, just like water molecules would be free to diffuse around in three dimensions. It's just that the lipids can't escape the lipid bilayer because then they would have to go out in the water. But in addition to the lipids, we have this mosaic of tons of stuff diffusing around. But they're diffusing around quite freely, and that's the fluid components. So you have a fluid where many components forming a mosaic are diffusing around freely. Some of these components you have, do you see the blue big blob there? That's an integral membrane protein. So that's a large protein sitting in the membrane, which is a large component of the protein itself in the membrane. Now the protein has one domain outside of the membrane and possibly another domain part inside the membrane. You have another protein up here. That's a peripheral membrane protein, meaning that a protein that might just be anchored on one side. So it's a protein that sits close to the membrane, but most of the protein itself is not really in the membrane. But we frequently group those with membrane proteins anyway. There are these long chains you see here. There are usually sugars or so. They can be very important for other cells and molecules to recognize a cell. You have some yellow molecules in the bilayer here. Those are cholesterol. Cholesterol is technically lipid, but I suppose for lipid even, but it has slightly different properties. And in particular, it will cause the bilayer to become stiffer, which is very important for certain membranes, not all of them. You might have an individual alpha helix sticking around somewhere. I'm not sure maybe the red one is that. So some membrane proteins are so small that it's literally just one single alpha helix. In fact, most membrane proteins tend to be alpha helical. There are only a handful of beta sheet ones that I won't have time to talk about today. So there is a huge diversity here. There are glycoproteins on the surface. Very important on some viruses. There are oligosaccharides. Those are the sugars. And then you're seeing some sort of virus or something here wanting to recognize the membrane. So we're going to start by having a look at these components, but primarily only focus on the membrane proteins and what they do.