 Hey everybody, Dr. O. This video I want to cover the cell membrane also known as the plasm membrane. So obviously why is it important? The cell membrane is what keeps things that have to stay out, out keeps things that have to stay in, in, and also is a way to move material. So let's go ahead and dive right in with some of the key terms you probably would have heard about cell or plasm membrane. So let's start here with the fact that they're called a phospholipid bilayer. And that's because they're made of these phospholipids here which have that phosphate head and the lipid tails. Now the key here is that the head of a phospholipid is hydrophilic or water loving. The tails are hydrophobic. So I'm going to go back here to this image. The reason they form a phospholipid bilayer is because the hydrophilic heads are all exposed to water either outside or inside the cell and the hydrophobic tails are avoiding water. They're hiding from it. So if you were to pour these phospholipids into a glass of water, they would actually form a membrane like this. So both sides are happy. So that's where you get the term phospholipid bilayer. The other major way you're going to hear a cell membrane described is as a fluid mosaic model. This one's pretty cool. So let me jump ahead here and show you. So this would be what a typical plasm membrane looks like. This one would be of a bacteria. I'll explain why in a moment. But so the reason it's called a fluid mosaic model is number one. It's fluid because it's not rigid, right? It looks like a rigid cell. Whenever you see it, just a drawing of a plasm membrane, it looks rigid and looks like all these structures are stuck there, anchored there, and they're going to stay there. That's just not the case. The consistency of this cell membrane would be the same as something like olive oil. And the structures that are built into that phospholipid bilayer are actually just free floating through there. So think about things like floating on a pond of water, something like that. So that's the fluid portion of the fluid mosaic model. The reason it's called a mosaic, like when I think of a mosaic, I think of a picture made of a hundred or a thousand other pictures, right? So mosaic just means multiple different pieces. So yes, you do have the phospholipid bilayer, but you also have as you can see here some glycolipids, glycoproteins, and other types of proteins that are built right into it as well. That's why it's a mosaic, a combination of different structures that are put together in a fluid, non-rigid pattern. So how did I know that this would have been a bacterial cell membrane? Because there is one critical ingredient that's missing. Let me show you here. You'll notice you're going to see a lot of similarities here. But look what happened. This here would be a eukaryotic or animal cell like a human cell. Look at those yellow structures. Those are cholesterol. So cholesterol is a sterol. And what cholesterol does is it keeps your cells from tearing open. It keeps them from lysing if water rushes into them. So a reason this is significant is because bacteria don't have these sterols, which is why they need cell walls outside of their cell membrane. So our cells don't need cell walls for that reason. That's why cholesterol is wildly important. I always like to stick up for cholesterol because it's been so demonized, but every cell makes it because every cell needs it. So it is very, very important. So that's where you get the fluid mosaic model of the cell or plasma membrane. So I'll just go ahead and stay here and again, just talk about if in a word, what I would call the plasma membrane is a gatekeeper. And that's because it is a selectively permeable membrane, also known as a semi permeable membrane. So what that means is some things can freely move through the plasma membrane. Other things either can't move through it or have to be funneled through it. So for example, like your oxygen and carbon dioxide, they're both small, they're both lipid soluble. They can just traverse right through the cell membrane. They go wherever they want really, whereas other things are either going to have to be transported in and out or they're just not going to be able to make it through. So that's what it means to be semi permeable or selectively permeable. Reason this is so important is because to be a living cell, you have to have a plasma membrane that is selectively permeable. If you have a freely permeable cell membrane or membrane, then anything goes wherever it wants, which means you can't keep things inside the cell that have to stay in. You can't keep things out that have to stay out either. So that wouldn't work. And then if you had an impermeable membrane, nothing could get in, which means you couldn't get food or resources into this cell and nothing could get out, which means you couldn't get waste products out. So that's not going to work either. So I call it a gatekeeper or a bouncer because the plasma membrane or cell membrane determines what does get to enter and leave and what can't. Now again, it can't control everything, like oxygen is going to go wherever it wants, but that's why I like that term gatekeeper. I also do call the plasma membrane the brain of the cell. I know when you talk about control centers and brains, people use to think of the nucleus, but the plasma membrane is what senses the external environment and responds. Just like your brain receives sensory information and then responds. So I like to think of it as really a much more important piece than just a wall. All right, so that is the fact that your plasma membranes are selectively permeable. We've already mentioned why cholesterol is there in your animal cells and just to go back. So this would be a bacterial one, lots and lots of similarities, but no cholesterol. So outside this cell membrane would be a cell wall. There's different types of cell walls, but I won't go into that right now. What else to cover here? So I just wanted to put the structure of cholesterol here and just so you could see it. But I think that's about it for the plasma membrane itself. Now I do want to just show you, you see a lot of different structures here, these different shades of purple and blue, these proteins. So I have one more image that shows the different types of proteins. There's your phospholipid bilayer with the hydrophilic heads on the inside and outside and the hydrophobic tails on the inside. But notice these are the different types of proteins you're going to see inside these membranes. So an integral membrane protein is going to be one that runs through the entire length and is built into that plasma membrane. And then you have your peripheral proteins, like the little ones there on the top and the bottom. And the last group you can see labeled here would be the channel proteins. They're going to have a pore so things can move through them. In other videos we'll talk about chemically gated channels and voltage gated channels and all this. But for now I just want to kind of walk through a recipe for how to make a plasma membrane and what all these key pieces are. So let's go back and just get one last look here you see. So that fluid mosaic model, all these different pieces come together to form this cell membrane. That is what makes a cell a cell and makes a cell, makes it possible for a cell to be alive. So all right, I hope this helps. Have a wonderful day. Be blessed.