Cell junctions. There are tight junctions, adherence junctions, which we'll talk about by another name in a moment, and desmosomes. Membrane proteins participate in these cell junctions and you'll see in a moment that can't hear and display a major role. Belt desmosomes are the so-called adherence junctions, whereas desmosomes are thought of more like spot rivets. Desmosomes of both sorts are the glue that holds cells together in a tissue. You can see that desmosomes are created when two cells come close to one another and attach to one another by their cat hearings. Tight junctions, if you look at the upper right, are held together by tight junction membrane proteins or TJMPs shown here. They are not among the proteins we've talked about so far. And what these do is create a watertight seal between two cells. And so what you see in the upper right is some dissolved substances above the tight junction membrane protein complexes, but not being able to get through. So you'll see in a moment what good that is. And on the left, the third kind of junction is a gap junction. And these are actually channels which are very small, but will allow their hydrophilic, and so they will allow ions to flow between cells. So adherence junctions, as I said, these are belt desmosomes and they help to create strong sheets of epithelial cells or endothelial cells that line the blood vessels or your gut, for example. The spot desmosomes I told you were rivets, it links cells together tightly, the tight junction made with those TJMPs form a watertight barrier that can control how substances can move. They can either get into the cell and traverse a sheet of cells from one side to the other, or they can't get across the sheet. That's what the tight junction is all about. Finally, the gap junction is really less a junction than a communication channel, and it allows ion flow between the cells. These junctions were originally identified by their structures suggested by this electron micrograph, which shows three different kinds. We know that all membranes, including the cell membrane, are a fluid mosaic, which allows lateral movement of components, including proteins, shown here as green and blue things. But cells can, in fact, restrict the mobility of proteins by synthesizing and putting into their membranes barrier proteins, barrier molecules, and collecting certain membrane proteins on one side of the barrier or another. And so in this cartoon, you see that all of the green proteins are above these black barriers and the blue ones below. Some of these black barrier proteins may in fact be a cell junction protein. Mainly, though, there are barriers to the mobility of proteins between domains. They will move within the domain, but not between them.