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Cell membrane overview and fluid mosaic model

Video transcript
In this video, we're going to explore a little bit about the cell membrane. So just as a little refresher, let's say this is a picture of our cell with a little tiny nucleus in the middle. Our cell membrane is what's on the outside of our cell, so our cell membrane is what protects our cell from a really harsh outside environment. If it weren't for the cell membrane, we wouldn't be alive today, because there would be nothing to protect us from the outside world. So we're going to talk about the main three things that make up the cell membrane-- the first, phospholipids, the second, cholesterol, and the third, proteins. So the first one we're going to talk about-- and this makes up the majority of what's in our cell membrane-- are phospholipids. And just for the sake of time, I've predrawn a picture of the cell membrane here. And you'll notice that all of these individual pieces are phospholipids, and a phospholipid looks like this. It has that polar head group, that polar phosphate group. And it has two fatty acid tails. And so this is the way that we normally represent what a phospholipid looks like. And in the cell membrane, you can see that these phospholipids are packed pretty closely, pretty tightly together, all throughout the entire membrane. And we're looking at this membrane. This is kind of like a cross-section. You can imagine that we cut the membrane in half. So what we have here is actually what we call our phospholipid bilayer, and sometimes it's also called the lipid bilayer. The second thing that we can find in our membrane is cholesterol. Now, we often hear cholesterol in foods and cholesterol in our blood, and we think it's a bad thing. But in this case cholesterol is actually very important for our cell membrane. And cholesterol looks like this. And again, just for the sake of time, I've predrawn what cholesterol looks like. And you'll notice that cholesterol has a lot of rings, and this gives cholesterol a pretty stable structure. And what cholesterol does is cholesterol kind of inserts itself between phospholipids, kind of like that. And the way I think about it is cholesterol is kind of like a buffer. It maintains the fluidity of our cell membranes. So as temperatures become lower, cholesterol will help increase the fluidity. And as temperatures become higher, cholesterol will help reduce the fluidity of the cell membrane. So cholesterol keeps our cell membrane in kind of a happy middle ground of fluidity. And the third thing that makes up our cell membrane are proteins, and proteins are actually a big one. And depending on the cell, some cells will actually have a significant amount of protein in the membrane. And so proteins can take two major forms. The first is you can have a protein that crosses the entire membrane. We call this an integral protein. We also can call this a transmembrane protein. And this can occur throughout different areas of the cell, like that. And some proteins actually kind of sit on top of the membrane, like this. Or they might sit on another protein, like that. And these are what we call peripheral proteins. There are some very rare proteins that actually can go halfway through the membrane. And even rarer, there are occasionally a few proteins that actually can be found inside the cell membrane, like this, between the two phospholipids inside our bilayer. Now, proteins are a very big player in the function of cell membranes. They actually carry out nearly all of the membrane processes that we can think of. And the two biggest things that proteins do is, the first, they can actually act as receptors. So the proteins can actually tell the cell what's going on in the outside world. They act as communication. And the second thing that proteins can do, which generally occur in transmembrane proteins, is that proteins can actually help transport molecules in and out of the cell. So now that we know the function of proteins, why do you think proteins that are lipid-bound or bound within our lipid bilayer, like this one here, is so rare? Well, it's because if the role of proteins is primarily to act as receptors-- to communicate with our outside world-- or to act as transport-- to allow things to go from the inside to the outside or the outside to the inside-- the proteins that are kind of stuck in between don't really have a big role in our cell membrane. And lastly, there's one very important type of molecule that actually binds to our lipids or our proteins, and these are carbohydrates. And we call these glyco for short. So they would be glycoproteins, or they might be glycolipids. And what these do is they play a big role in communication. So for example, it allows a cell to recognize another cell in our body. If they play a role in communication, in cells recognizing other cells, where do you think these sugars would go? Well, these sugars would mainly occur on the outside of our membrane. So they would kind of stick out on proteins-- these would be glycoproteins-- and they can be on peripheral or integral proteins. Or they might stick out on lipids, like this. And these would be glycolipids. Now, this a little confusing to look at it. What we've just drawn is a cross-section of our cell membrane. But what if we were looking at the cell membrane from the outside, kind of like a top view? What would that look like? Well, again for the sake of time, I've predrawn our phospholipids. So if we were looking at the cell membrane from the outside-- looking onto the top of the cell membrane-- all we would see are these head groups of our phospholipids. We might see some cholesterol in between our cell membranes, like this. And we might see some larger proteins that are on top of our cell membrane, like this, scattered throughout our cell. And lastly, we might actually see some glycoproteins and glycolipids on the outside. And these would attach themselves to our proteins and our phospholipids, like that. So from the top, this is what our cell membrane would look like. And you know something really special about this-- this kind of looks like a piece of art. So if we think back to elementary school, where we had the project where we would put a lot of beans or different macaroni together to create a piece of art, this kind of reminds me of that. So this is actually what we call a mosaic. So scientists kind of thought the same thing. So scientists actually named this model of the cell the fluid mosaic model, and so the mosaic portion of our cell can be described here. Again, you can see that there are a lot of different pieces-- different colorful types of pieces-- put together to create this beautiful cell membrane. But why did we call it fluid? Well, the reason we call the cell membrane fluid is because these pieces in our cell membrane can actually move around. They're not set in stone. So the proteins and phospholipids in our cell membrane can move around, like that. This is why we call it fluid. What would that look like if we look at the cell membrane from the top? Well, the movement is actually not two-dimensional-- just up and down, or just left and right. It can actually go in a lot of different directions. So our proteins can move all around the cell membrane, and so can our phospholipids. So again, this is what we call the fluid mosaic model. And just as a little bit of a fun fact, this was only really discovered in 1972. So it was only 40 years ago that we really figured out that our cell membrane was actually the fluid mosaic model. So in summary, our cell membrane is made up of three major things. The first are phospholipids. These make up the most of the cell membrane, and they're kind of like a basic building block for our cell membrane to exist. The second are cholesterol. Cholesterol is scattered randomly through our cell membrane, and it helps maintain the fluidity of the cell membrane. And the third are protein, and proteins carry out nearly all of the essential cell membrane functions. And together we call this our fluid mosaic model, because our cell membrane is made up of so many different things, and all of these things are always moving around like a fluid.