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Membrane dynamics

Let's explore some of the ways that the structure of the fluid mosaic model influences its function by focusing on the movement of phospholipids. Learn about the different types of diffusion, including transbilayer (flip-flop) and lateral, and the role of catalysts like flippase, floppase, and scramblase in these processes. By William Tsai.

Created by William Tsai.

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Video transcript

In this video, we're going to explore a little bit about membrane dynamics. So we know that in our fluid mosaic model of our cell, everything in the cell membrane moves around. So our cholesterol moves around, and our phospholipids move around, and our proteins all move around. But in this video, we're actually going to focus in on our phospholipids. So over here, I've pre-drawn a picture of our cell membrane. And you notice that these phospholipids are really tightly packed together. So how do these lipids, these phospholipids, actually move in our cell membrane? Well, before we get into answering that question, we're just going to quickly label our cell membrane. So out here we have our extracellular. This is the outside of our cell. And in here we have our intracellular, or the inside of the cell. And there's an important distinction that we have to make between intra and intercellular. Intracellular is the inside of the cell, while intercellular is between cells. I like to remember this by thinking about the word intercontinental. That means between continents. So intercellular must mean between cells. We sometimes call the phospholipids that border the extracellular environment as the outer leaflet. And we call the phospholipids that border the inside, or the intracellular environment, as the inner leaflet. And you'll notice that this cell membrane that we've drawn is very basic. We've taken out all the cholesterol, the proteins, and all of the other stuff that makes up the cell, because in this video we're actually going to focus in on the phospholipids themselves. So the first type of movement is uncatalyzed. This means that there's no need for a catalyst. So one type is we can actually have a phospholipid on the outer leaflet move onto the inner leaflet. Or we could have it in reverse. We can have something on the inner leaflet move to the outer leaflet. We call this transbilayer diffusion. And this actually has a nickname, which we call flip-flop. And this type of movement is really slow. There's no catalyst. And you're trying to move a phospholipid from one leaflet to the other. So this process is very slow. And it doesn't happen that often. Now, there's another type of movement where we can have a phospholipid like this actually move side to side. And this is what we call lateral diffusion. And just to give a bird's-eye view of what this actually looks like, if we actually have the head of our phospholipid, the movement is not just from side to side, but it goes all around the cell membrane. So if we were to look at this from the top, this phospholipid can move in any direction. It's what we call lateral diffusion. And since we're not actually switching between leaflets in this type of movement, this is actually pretty fast, and it happens a lot in our cell membrane. So if we have uncatalyzed movement, naturally, we will have catalyzed movement. And since this movement is catalyzed, we're going to need a catalyst. And in this case, our catalyst will be a protein. The first one we're going to talk about is we can have something on the outer leaflet, like this, actually flipped to the inside, kind of like our transbilayer diffusion. But this time, it's aided by a protein-- a catalyst. And not only so, this process actually uses ATP, which we call adenosine triphosphate. Just to remind us, ATP breaks down into adenosine diphosphate, ADP, with the phosphorus. And this provides energy for this reaction to happen. This catalyst that we use, or this protein, is actually called flippase. And this is pretty fast compared to our transbilayer diffusion. And the next one we have, again, also uses a catalyst. So we have another protein. And this particular protein is called floppase. And floppase also uses ATP. Now what floppase does is it actually brings a phospholipid on the inner leaflet to the outer leaflet. So it does the opposite of what flippase does. And again, this is also pretty fast, because it uses a catalyst. Now the last one that is catalyzed does something really interesting. It actually brings a phospholipid from the inner leaflet to the outer leaflet and one from the outer leaflet to the inner leaflet. And this is what we call scramblase. And this actually does not need ATP. It doesn't require that extra input of energy. And again, because it is catalyzed, it is pretty fast. So in summary, these are the ways that allow our phospholipids to actually move around our cell membrane and create this amazing, moving, fluid cell membrane structure that we call the fluid mosaic model.