字幕列表 影片播放 列印英文字幕 >> Hi, everyone, and welcome to the Penguin Prof Channel. Today I want to introduce you to the wonderful world of cell membranes. Okay, so if you ask people to draw a cell, generally what you'll get is a circle, and most people don't really think too much about the circle itself, but it's actually really important. The circle really defines, you know, you versus everything else in the universe that's not you, but it's more complicated than that because inside the cell you're going to produce wastes that need to get out, and there's going to be food and good stuff that needs to get in. How are you going to do it? What happens if you puncture this barrier and goodness from the inside leaks out and bad stuff from the outside leaks in? How are you going to manage all this? So the circle itself is actually worthy of our investigation. So what the heck are these membranes like? Are they sort of like a container or, you know, a bag? I mean, that separates, you know, inside from outside. I mean, oh my God. That's so cute. Is it like a barrier? I mean, if you get a hole in your barrier, that's not good. The truth of the matter is that membranes really aren't like any container or barrier that we're familiar with in our everyday life, so that makes them a little bit harder to imagine and talk about. Membranes have to control what goes in and out of the cell. They have to be free to move and signal and communicate and repair and do all kinds of other things. I mean, so much of what the cells do happens at the cell membrane, but you always have this balance of control versus freedom. So how are you going to build a barrier that can do all of those things? One of the more amazing things I think that membranes do is that they form all by themselves, and if you puncture them, they reseal immediately. It's kind of like magic. How the heck are we going to do that? The magic of cell membranes comes from the fact that they're built from molecules called amphiphiles. Now, that may not be a term that you're familiar with, but it comes from the Greek, and there's a word that you probably do know, and that's amphibian. Amphis means both, so amphibians live both on land and in the water, so that will help you to remember it. In chemistry, we're talking about molecules that have both a hydrophilic -- that is, a polar end -- and a hydrophobic -- that is, a nonpolar end. So these are really cool molecules because they kind of play both sides. There are actually amphiphiles that are present in our regular life in the kitchen and so forth. We use amphiphiles for detergents as well as for soaps. We use them for surfactants. They help to break up fat. They make fats dissolvable. This is a really cool little mnemonic device that you can remember. Amphibians hold amphiphiles. So this little image can help you to remember that word. The secret then is not in the sauce; it's in the amphipathic lipids that build cell membranes. So we have on every molecule of a phospholipid a hydrophilic -- that means water loving -- polar head. This polar head is comprised of a glycerol molecule, a phosphate, and then there's the side chain in our group. We'll see what those are here in a second. And then there is a nonpolar end that's made up of two fatty acids tails, and they are hydrophobic. They are water fearing. So you have this amazing, amphipathic molecule that has one side that is polar and likes water and one side that is nonpolar and fears water. Oh my gosh. We're going to see why that's such a great thing, but before we do that, I said I would show you some of the actual particulars about phospholipids and also glycolipids. If you need to know a little bit more detail about them, probably one of the more common varieties in animal cell membranes is this guy, phosphatidylcholine. So these are the different options, and then the rest of it, the, these are the fatty acids that are all trailing. These are the hydrophobic parts of the beast, okay, if you needed a little bit more detail there. So so what? What you got to remember is that in chemistry, like likes like. So polar molecules stick together and nonpolar molecules stick together, but polar and nonpolar molecules do not mix. You have seen this if you have poured oil in water. Everybody knows that oil and water don't mix. The reason is that water is polar and oil is nonpolar. They hate each other. So you'll get this film of oil on top of water. So what if you have a molecule that plays both sides? And that's the key. We've got these amazing phospholipids that will spontaneously form a bilayer all by themselves. Oh my God. That's so cool. They actually will form other shapes too, but for the purposes of cell membranes, this is the shape that we are most concerned with. The most favorable shape for phospholipids to make is a shape where all the polar ends face the water because they like water and all the nonpolar ends face each other. The fatty acid tails point inward. Isn't that cool? If you really want to see this, I guess the easiest way to see it is if you play with your soup and you have some oil drizzled on the top of your soup and get yourself a fork, okay. Don't let anybody tell you not to play with your soup. Get yourself a fork and experiment with different amounts of pressure and speed and drag the fork through the oil and see if you can get the big globs of oil to break up into smaller globs, and then see if you can get the little globs to join together and congeal into big globs. If you watch the behavior of the oil, that's really kind of the most similar thing that you can see with the naked eye that is similar to what phospholipids do in solution. So in 1972 after a lot of study -- membranes are actually very delicate, so it turns out they're very hard to study -- they were able to freeze and then split a membrane and expose the center of the phospholipid tails, and that's what you're seeing right here. And then they, it's called freeze fracture electron microscopy. They were able to actually verify that this is what cell membranes look like and these are the different components of cell membranes. So what you've got, about 75% -- this is for animal cells, by the way -- about 75% of the membrane is made of these phospholipids, the molecules that we just looked at, which the polar end shown here and the fatty acid, the nonpolar end, shown there. And they insert themselves all by themselves into a bilayer like this. In addition to that, we have glycolipids. They make up about five percent of the membrane. And cholesterol. Check out the cholesterol. Cholesterol, about 20% of most animal cell membranes can be cholesterol. That can be as high as 50%, by the way. Depends on the species of animal. The cholesterol will actually stabilize the structure of the membrane, and by stabilizing the phospholipids, animal cells actually get away with not having to have a cell wall, which is pretty cool. The other thing that you notice throughout are proteins, and some proteins span the cell membrane like these. We call them integral proteins. And some of them are only on one side of the cell membrane or the other. Could be the inside or the outside. We call those peripheral membrane proteins. You might think for a minute about how a cell could anchor proteins in a cell membrane, and how you do it is of course you recall that proteins are made of chains of amino acids, so how you anchor something into a structure that is not a solid is you have to make the membrane-spanning portions nonpolar, so they like to hang out with the fatty acid tails. And then on either side of these integral proteins, so here and here, these amino acids would be polar. So they don't like to hang out with the fatty acid tails. They prefer the polar heads and the water that's surrounding this bilayer on both sides. So that's pretty amazing. All of this allows for the amazing variety of membrane functions that we're going to see as we go through biology and physiology. Membranes really do account for a lot of what cells are able to do. One thing that's really important is membranes allow for compartmentalization, and that's going to allow us to create gradients. Something you're going to hear me say all the time is, you know, this is a gradient-driven process. Well, how do you have a gradient? You have to have a separation. You have to have one side which is different from the other side -- different in concentration, in pH, or whatever the variable is -- but membranes allow for that to be possible. Membranes are going to allow for cells to recognize each other. They're essential for cell-cell recognition, communication, for one neuron to talk to another, for hormones to be received by receptors, and so much more. So you're going to see as you go through your studies of biology, membranes are essential. I hope that that introduction to cell membrane structure was helpful. As always, I ask for your comments and your subscriptions. Please visit on Facebook and follow on Twitter. Good luck.