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Phospholipid structure

Phospholipids are molecules that form the cell membrane. They consist of a polar phosphate head group and two nonpolar fatty acid tails joined by a glycerol backbone. The phosphate group can link with different molecules, such as serine or choline, to generate diverse kinds of phospholipids. The fatty acid tails can have cis or trans double bonds, which influence the membrane fluidity. The molecular details of phospholipids and their variations can be drawn as shown. Created by William Tsai.

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Video transcript

In this video, we're going to actually explore in detail the structure of phospholipids in our cell membrane. Just to briefly remind us, our phospholipid is often drawn like this. It has that polar phosphate head group, and it has two fatty acid chains. And all of this is held together by glycerol backbone. But what does that really mean? What dose is picture actually look like down to a molecule? Well, let's talk about the first one, the fatty acid. And just to remind us, there are actually two fatty acids-- times 2. You can see that there are two tails hanging down here. So our fatty acid is basically a carboxylic acid attached to a very long carbon chain. And so our carboxylic acid is like this. It has a double bonded O and a hydroxyl group. And it has that really long carbon chain which we're just going to call an R-group. The next one is our glycerol backbone, and glycerol is a pretty basic structure. It looks like this. It has three carbons attached to three hydroxyl groups-- three alcohol groups. And there's only one glycerol in each phospholipid. a The last one is our phosphate group, that big polar head group that we talk about. And just like you would think, there's a phosphorus in a phosphate group, and there are four oxygens attached to it. Now, what does this actually look like all put together? Just for the sake of time, I've pre-drawn a picture of all this put together. It looks like this. So you can see that we have our two fatty acid chains attached through an ester bond with our glycerol attached through another ester bond with our phosphate group. Now, you'll notice that one of the negative oxygens is missing, and it's been replaced with a hydroxyl group-- an alcohol group. And that's what this is in the orange. Well, that's because in our cell, a phospholipid actually looks like this. The negative oxygen actually picks up the hydrogen and becomes an alcohol group. Now, a phospholipid molecule that looks like this is actually pretty rare in our cell membrane, and the reason why is because phospholipids can occur. And the reason why is because this molecule could actually bond with several different molecules, giving a really diverse set of phospholipids. And again, for the sake of time, I've pre-drawn these molecules, and unless you're a researcher who really, really loves the cell membrane, you probably won't need to know this by heart, because these structures get a little complicated. But it's still good idea to get acquainted to what they kind of look like. So there's serine, choline, ethanolomine, inositol, and glycerol. And you'll notice that I've also drawn these particular special hydroxyl groups in orange, and the reason why is because these hydroxyl groups in orange, from serine, choline, and so on, can actually bond to our phosphate group through what we call a phosphoester bond. Now, what would this actually look like in a real molecule? What would it look like if serine actually bonded with this phospholipid? Well, we're going to transition briefly into another slide, just because I'm running out of space. And you'll notice that there are five different phospholipids that they can actually occur. There's phosphatidylserine, phosphatidylcholine, phosphatidtylethanolomine, phosphatidylinositol, and diphosphatidylgylcerol, also known as cardiolipin. And you'll notice that in this last one, there are actually two phosphatidyl p groups that actually bond to a middle gylcerol. And again, unless you're someone who really researches the cell membrane, you probably don't need to know these structures by heart, but what we do need to know is that the phospholipids in our cell membrane are actually very, very diverse, and there are several different forms they can take. So if we were to take a look in detail at the phospholipids that make up our cell membrane, we would actually find all of these scattered throughout the membrane. Now, we're just going to go back to our original picture. Just to remind us, this is again our nonpolar side, and this side in the yellow is polar. And so if we were to match up our general picture of a phospholipid to the picture that we've drawn here, it would actually look like this. This is our polar head group, and we have two fatty acids here. And again, you'll notice that the glycerol group isn't really drawn in, because that's what holds everything together. And just to wrap up, we need to talk about one brief thing. So we have our phospholipids like this. Now, this so-called R-group is made up of a really long chain of carbons. Now, in many cases, these carbons can actually form double bonds with each other, like a lot of different carbons do. And remember that double bonds occur in the form of cis and occur in the form of trans. So a cis bond in chemistry is when we have a double bonded carbon and we have the carbons on each side being on the same side, while in the trans bond, our carbons are on opposite sides. Again, these are hydrogens. And if we were to zoom out of this detailed molecule, in the case of trans, our fatty acid would just be pretty straight, like that. But in the case of cis, we can actually create a kink, because this bend from our cis bond actually gives it a kink. And this actually has a lot of significance when we talk about the fluidity of a cell membrane. So in summary, our phospholipids are made up of three major things-- fatty acids, glycerol, and phosphate. And these three things actually looks like this if we were to draw out a detailed molecule. And not only so, but there's an OH group on this polar phosphate group that actually can bond with several different types of molecules, producing a really, really diverse set of phospholipids that make up our cell membrane.