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Fluid mosaic model of cell membranes

Fluid mosaic model of cell membranes. Roles of phospholipids, proteins, glycoproteins, cholesterol, and glycolipids.
Video transcript
- [Voiceover] Let's explore the Fluid Mosaic Model of cell membranes. Now, why is it called the Fluid Mosaic Model? Well, if we were to look at a cell membrane and just to be clear what we're looking at, if this is a cell right over here, and this is its membrane, it's kind of what keeps the cell, the inside of the cell, separated from whatever is outside the cell. We're looking at a cross-section of its surface, Where down here, this is inside the cell. If we look at it relative to this diagram, this is inside the cell, and this is outside. And when you zoom in, and this little part right over here, this is actually a phospholipid bilayer that forms it. And so when you hear that you might say, well, what is a phospholipid? And that's a good question. Because when you understand what a phospholipid is, it starts to make sense why it would form a bilayer like this, and why it's the basis for so many membranes in biological systems. So this is indicative of a phospholipid and as its name implies, and let me write that down, this is a phospholipid. It's a lipid that involves a phosphate group. And in general the word lipid, and we have a whole video on lipids, means something that doesn't dissolve so well in water. And that's true, as the case of this phospholipid, you have these hydrocarbon tails that are coming from fatty acids, and so these hydrocarbon tails, they have no obvious charge or no obvious polarity. We know that water's a polar molecule that's what gives it its hydrogen bonds, and it's attracted to itself. But these don't have those, and so they're not going to be attracted to the water and the water's not going to be attracted to it, to them, and so these tails are hydrophobic. So you have hydrophobic tails, and these are really kind of the lipid part of the phospholipids. And then you have the phosphate head right over here, and as you can clearly see, this has some charge. Charged molecules do well in polar substances like water. They're going to dissolve well, and so this part right over here, is going to be hydrophilic. And actually molecules that have a hydrophilic part and a hydrophobic part, there's a special word for them. Amphipathic, a word that I sometimes have trouble saying. So phospholipids are Amphipathic, which means that they have both a hydrophilic end, a part that is attracted to water, and a hydrophobic end, that is not attracted to water. And hopefully that starts to explain why they organize themselves in this way. Because you could image, the hydrophilic heads are going to want to be where the water is, which is going to be either outside the cell or inside the cells. And the tails are hydrophobic, the water's going to go away from them or they're going to go away from the water and so they're just going to face each other and they're going to be on the inside of the membrane. But the really cool thing is, a structure like this, having this Amphipathic molecule, allows things like these bilipid, these lipid bilayers, I should say, to form. And it's actually fascinating. You would think that if you go far back enough, even before life in cellular form, formed, that you might have had phospholipids spontaneously forming these spheres where you have a bilayer, a lipid bilayer. So you could imagine something, let me see, if I drew a cross-section, let me see if I can draw it relatively neatly. So, I think I'll draw half of it, just because you get, well I'll draw the whole thing and hopefully you get the idea. So that would be one layer of the phosphate heads facing the outside. This is the inner layer, and I'm doing a cross-section right over here. And then you have your hydrophobic tails, let me do that in a different color. So your hydrophobic tails, I think you get the picture. We have a bunch of hydrophobic tails on either end and then you could spontaneously form a structure like this which starts to feel like, hey, well maybe there's a protocell forming. And obviously to actually have real life you have to have some form of information that can be passed on, and you have to have some type of metabolism, and the cell is living, and all of the definitions of life. But at least this basic structure of the cellular membrane you could imagine how it forms in a pre-life state even, by virtue of Amphipathic molecules like a phospholipid. So fair enough. We're able to form this phospholipid bilayer, but what are all these other things that I have drawn here? Well, these are proteins and these are examples of, this is a protein right over here, this is a protein, this is a protein, and I just drew some blobs to be indicative of the variety of proteins. But the important thing to realize is, if we think of cells, there's all of this diversity. There's all of this complexity that is on, or embedded, inside of its membrane. So instead of just thinking of it as just kind of as a uniform phospholipid bilayer, there's all sorts of stuff, maybe if we view this as a cross-section, there's all sorts of stuff embedded in it and we see it right over here in this diagram. You could say there's a mosaic of things embedded in it. A mosaic is a picture made up of a bunch of different components of all different colors, and you can see that you have all different components here, different types of proteins. You have proteins like this, that go across the membrane. We call these transmembrane proteins, they're a special class of integral protein. You have integral proteins like this, that might only interact with one part of the bilayer while these kind of go across it. You have things like glycolipids. So this right over here, this is a glycolipid, which is fascinating. It lodges itself in the membrane because it has this lipid end, so that's going to be hydrophobic. It's going to get along with all of the other hydrophobic things, but then it has an end that's really a chain of sugars and that part is going to be hydrophilic, it's going to sit outside of the cell. And these chains of sugars, these are actually key for cell-cell recognition. Your immune system uses these to differentiate between which cells are the ones that are actually from my body, the ones I don't want to mess with, the ones I want to protect and which cells are the ones that are foreign, the ones that I might want to attack. When people talk about blood type, they're talking about, well, what type of specific glycolipids do you have on cells. And there's all sorts of, that's not all we're talking about when we talk about glycolipids as a way for cells to be recognized, or to be tagged in different ways. So it's a fascinating thing that these chains of sugars can lead to such complex behavior, and frankly, such useful behavior, from our point of view. But you don't just want to have sugar chains on lipids, you also have sugar chains on proteins. This, right over here, is an example of a glycoprotein. And as you can see, when you put all this stuff together, you get a mosaic, and I'm actually not even done. You have things like cholesterol embedded. Cholesterol is a lipid, so it's going to sit in the hydrophobic part of the membrane and that actually helps with the fluidity of the membrane, making sure it's not too fluid or not too stiff. So this is cholesterol, right over there. So you see this mosaic of stuff, but what about the fluid part? And I just talked about cholesterol's value in making sure that it's just the right amount of fluidity. What's neat about this, is this isn't a rigid structure. If this thing were to be jostled around a little bit or maybe it would be plucked-out somehow, the phospholipids would just spontaneously re-arrange to fill in the gap. You could imagine these things are all flowing around. That this membrane actually has a consistency of oil or salad dressing. So it isn't like a rubbery texture, like you might imagine, or a membrane, like a balloon. It's actually fluid. These things can move around, but even though it's fluid, it's good enough to separate the two environments. The environment inside the cell from the environment outside of the cell. And that's where the name Fluid Mosaic Model comes from.