Video Transcript: Cell Membrane

The Cell Membrane

Hi. It's Mr. Andersen and in this podcast I'm going to give you a brief overview of the cell membrane, you probably learned growing up that the function of the cell membrane is to regulate what comes into and what goes out of the cell. That's oversimplifying it a little bit, but it's a pretty good definition. And so you may ask yourself, why do you have a picture of a musk ox here? Well, the reason why is it when I ever talk about the cell membrane, I like to talk about the musk ox and how it's a whole heck of a lot like this over here, this thing over here is called a phospho. And the reason it's phospho is because we have a phosphate group right up here in the head; it's called a phospholipid. And the phospholipid, basically has two parts to it up here, it's going to have a charged part. So it's going to have a negative charge up here. And then the tail end this lipid part, or that's a fat part, since it has so much carbon, carbon with hydrogen around the outside, it's nonpolar, this whole rear end is going to be nonpolar.

What does that mean? Well, this end over here loves water. And this end down here hates water, you know that if you've put oil, which is similar to a lipid in structure, in water, it just doesn't mix. And so basically, the head is going to love water. Tail is not. We call a molecule like this amphipathic.

And so basically, why is this like a musk ox? Well, if you throw a bunch of musk ox together, and then you have a predator show up, the heads of the musk oxen are going to face the predator because the heads of the musk ox love a fight, and they want to fight that predator. Now the tail of a musk ox is going to protect the baby musk oxen that are going to sit on the inside. And so basically, they'll form a ring around whatever they want to protect. And that's because a musk ox is amphipathic.

If we look at those phospholipids, then they're going to be the same way. In other words, the heads are all going to face the water, and then the tails in here are going to be away from the water. And so what you get is a membrane, this would be a single layer of phospholipids; this would be a bilayer right here. And so basically, all the cells that we have, have a cell membrane, and all of those cell membranes do the same thing: they regulate what comes into and out of the cell.

Now the model we use to explain how a cell membrane works is it is a fluid mosaic model. And so let me get a color that you can see. So it's a fluid mosaic model, if we break down this word, it's fluid. And that means that all the material inside the membrane is actually moving around. And if it ever stops moving, then it doesn't function as a membrane. And the other thing is that it is a mosaic. And so it's made up of a lot of different things. What are the major things that it's made up of? Well, you can see, here's those phospholipids, those are forming this membrane, but the big blue things are going to be proteins. And so the two major parts of a membrane are going to be the phospholipids, which are the red things. And then we're going to have proteins. What's the function of the phospholipids? Those basically keep water on either side. And so it makes the structure. What's the function of the proteins? Well, those actually give proteins their specific characteristics. And so it allows material in it connects to material outside.

And so if you to get your head around this, the cytoplasm is going to be down here. So this would be the cytoplasm and this is going to be the cytoskeleton attached to it. And then the extracellular fluid is going to be on the outside. Now there are a few other things that you can see inside here. We've got this folded protein, we've got right up here, I can see this right here is called “Let me get rid of some of that.” This is called a glycoprotein. Glyco means that it's got protein on the inside, but it also has a sugar attached to it, we're gonna have glycolipids, we're gonna have a number of things like here would be cholesterol. But basically, when you're thinking about a membrane, the function of it is to keep material on either side, and it has to maintain its fluidity. And so all of these things are moving, these phospholipids are moving horizontally, they're floating around. And I've seen some neat animations of this. I'll think of it as like a sea, this thing is whole floating. And all of these proteins are like floating around inside it as well. Now it's attached to the cytoskeleton and to the outside to this extracellular matrix, but it's really in flux.

And then the characteristics are going to come from the proteins what it does. So if we talk more about that phospholipid itself, the phospholipid, again, has a hydrophilic head has a hydrophobic tail, and they float back and forth. If they get too close together, that's bad. And so what they lots of times will have is kinky tail and unsaturated lipid tail and that kind of keeps them apart. Sometimes if it gets too cold, the cholesterol will actually keep them from getting too close. And if it gets too warm as they start to drift apart that cholesterol kind of hold on to it and keep it there. But the function of this is to allow the movement of material so oxygen needs to get into a cell it's going to do that through diffusion and carbon dioxide is going to get out.

So what actually moves through a cell? Well, the only things that can really move through a cell are going to be things that are uncharged, or things that are really, really small, but really things that are uncharged. And so how does oxygen get in? It can diffuse because it doesn't have a charge; carbon dioxide goes out the same way. But even things that are water that are really, really small, they can't get in, or glucose can't get in. It actually has to move through proteins. And so what proteins do, proteins allow material into and out of a cell. So we'll talk more about how this actually works. But how does water get into a cell then we used to think that it would just flow in using osmosis. But we now know that you have these proteins called Aquaporins. And basically what they do is they will allow water to move through really, really quickly. But sometimes we'll make proteins that actually have this area on the inside where it's protected. And that allows the material to move through that has any kind of a charge. So that would be like glucose moving through here. And then we have some of them that will move through using the process of active transport where we put energy in to move them apart or facilitated diffusion. So we got a lot of different things going on. But if you can remember that a cell membrane is in all cells, and it's fluid. It's always in movement, and it's really made up of two things. phospholipids and proteins. You've got a pretty good start, and I hope this is helpful.