HomeArticlesLipid overview | Macromolecules | Biology | Khan Academy
Lipid overview | Macromolecules | Biology | Khan Academy
September 10, 2019
– [Voiceover] Let’s talk about lipids. Now for those of you who are familiar with the term you might associate it with things like fat molecules, and that would not be incorrect. Fat molecules are a very common form of lipid, in fact this is an example of a fat molecule, or a triglyceride right over here. Fat or triglyceride, tryglyceride molecule right over here. This one in particular is a polyunsaturated tryglyceride and we have, we have a, go into in depth, we go into a lot of depth on this on the video on the molecular structure of fats slash triglycerides or on saturated and unsaturated fats. But you see the tell-tale signs, you see a glycerol backbone right over here and three fatty acids, or what were three fatty acids attached to what was a glycerol molecule. We go into a lot of depth on that. But fats are not the only type of lipids. And so what makes all of these other things that I’m about to show you also lipids? What commonality do they have with fats? Well lipids are just the general term, the general term for a whole class of molecules that tend to not be so soluble in water, that tend to kind of clump up or ball up when placed in water. So not, not so soluble, not so soluble in water. And I’m being a little bit careful with my words, I didn’t say outright hydrophobic. There are definitely lipids that are outright hydrophobic, but there are also some lipids that have some end of their molecule that’s hydrophobic, but then other parts of the molecule actually might be hydrophilic. And there’s actually words for that. So you have some lipids that are just straight out hydrophobic, hydrophobic. They literally try to avoid the water, they’re non-polar molecules, so they just clump together. But then there’s some that have hydrophobic parts and hydrophilic parts. And these are called, and I always have trouble saying this word, amphi, amphipathic, amphipathic molecules. Where some part is hydrophilic and some part is hydrophobic. And we’re gonna see that in a few seconds when we look at phospholipids, which are crucial for the structure of cell membranes. And you’re gonna see that a lot when you, when you go into biology. So what are all these other molecules? And let’s think about what parts of them might be hydrophobic and what parts might be hydrophilic. When you look at fats you have this long hydrocarbon chain, there are any obvious, and there aren’t any obvious, there aren’t any obvious charges over here. In fact, there’s aren’t any, oxygen is more electronegative, so you might have a little bit of a partial negative or partial positive charge. A partial negative at the oxygen or maybe a partial positive end at the carbon, but carbon isn’t, is still more electronegative than say hydrogen, so you’re not going to be able to form the type of polar bonds that, the type of, I should say, hydrogen bonds you would if you had hydroxyl groups here, if this was an alcohol. And these hydrocarbon chains over here, these are very hydrophobic, so that’s what makes fat not be so soluble in water and clump up with you put it into water. This molecule right over here, which we would classify as an ester, and that’s because we have an ester group, we have an ester functional group right over here where you have a carbon double-bonded to an oxygen, and then single-bonded to another oxygen, and then that oxygen’s bonded to a long hydrocarbon chain, and that carbon is bonded to a long hydrocarbon chain right over there. This is clearly going to be hydrophobic. And this particular molecule that we’re looking at right over here, this is a major constituent of beeswax. Of bees, of beeswax. And if you’ve even dealt with beeswax, or really any type of wax, and waxes in general are considered to be lipids. You can see they don’t, they’re not soluble in water, in fact they’re often used to repel water, to keep water from penetrating into something. So wax, and in particular beeswax, and beeswax isn’t made up of only this molecule, this is one of the main constituents, it has other molecules mixed in there, but this is also a lipid. Now what’s this thing over here? This thing I have one six carbon ring, another six carbon, whoops, I have another six carbon ring, another six carbon ring, and then I have a five carbon ring. This is the tell-tale sign, and let me circle these four rings right over here, these rings are the tell-tale signs that we are looking at a steroid, at a steroid. And in popular culture steroids have, you know, you think of steroids as something that bodybuilders might inject illegally to pump up their muscles, but steroids are actually, when you think about it in chemical terms, they’re actually referring to things that have this general, these molecules that have this general structure where they have this six carbon ring, this six carbon ring, and another six carbon ring in this orientation, and then another five carbon ring. And this steroid that we’re looking at, this has an O-H group attached to it, so it’s actually going to be an alcohol, so a steroid that’s an alcohol you would call a sterol, sterol. And this particular sterol is one that you’ve actually dealt with a lot, or at least you’ve heard about, this is cholesterol. So this one in particular is cholesterol. And cholesterol is often viewed as a negative thing, people don’t, people wanna lower their cholesterol, but it’s essential for life. It’s essential to the functioning of your cells, and it is a precursor molecule for your steroid hormones, which make you you. And this right over here is a steroid hormone, it’s a very well known one. This is testosterone. Testos, testosterone, testosterone, testosterone. And you see the tell-tale, you have a six carbon ring right over here, another six carbon ring right over here, the double-bond is in a different place than cholesterol here, you have a double-bond right over there. Then you have another six carbon ring, and then you have the five carbon ring. And instead of an O-H group here you have a double-bond, you have a carbonyl group, you have a carbon double-bonded to an oxygen, but it actually does still have an O-H group up here. But this is a derivative of cholesterol, this is a testosterone, it is a steroid hormone. This is another steroid hormone. This is cortisol, cortisol. Also can be derived from cholesterol. And you see the tell-tale signs. You see the six carbon ring, this has a double-bond right over there. You see another, you see another six carbon ring, six carbon ring like this. It’s actually hard to see the double-bond, so I won’t even refer to them, I’ll just refer to the general shape. So you have a six carbon ring there, six carbon ring there, six carbon ring there, and then five carbon, whoops, and then a five carbon ring just like that. So once again, they all have this steroid base structure, but then they all, they also have other parts that make them different. For example, that is different than that is different than that. And just so you can visualize these things in three dimensions, in an actual molecule their not going to look exactly like this and even talk about what something in an atomic scale looks like is kind of strange because light behaves in weird ways, but you can imagine the molecule would look like this if you actually, this is kind of a ball and stick model, well here you’re thinking about what it actually might look like in space where the white balls are hydrogen, the grey ones are carbon, and the red ones are oxygen. So this is also, this right over here, is what a cortisol molecule would look like if you think in a space filling visualization. Now I talked about amphipathic molecules and phospholipids are probably the most well-known example of it. And phospholipids, they have a lot in common with triglycerides in that you have this three carbon backbone right over here. Three carbon, three carbon backbone, let me do that in a different color. So you have a three carbon backbone right over here, so this is a carbon, that’s a carbon, that’s a carbon there. Each of them are attached to an oxygen, so you can imagine that this could have been derived from glycerol. And then two of the carbons, like in a triglyceride, are bound to a fatty acid like this, but then one of the carbons, the third carbon, instead of being bond to a fatty acid like you have in a triglyceride, it is bounded to a phosphate group. So this right over here, this right over here is a phosphate group. And this R could just be another chain, another chain of, another organic chain, so to speak. But when we talked about amphipathic molecules, I always have to say it slowly, so it’s a bit of a tongue twister for me. We’re talking about having a hydrophobic end and a hydrophilic end. Well what’s the hydrophobic end here? Well this, these, this chains from the fatty acids, especially the hydrocarbon chains right over here, these are gonna be hydrophobic. Hydro, hydrophobic. While the phosphate end right over here it has charge, charged molecules dissolve in water very, very well. And so this one over here is going to be hydrophilic. It’s going to be, it’s going to be, I guess you could say it’s going to be attracted to water. And that’s why phospholipids, and this is just one type of phospholipids, these chains could be different. We have a, we have a unsaturated chain here, and then, a fatty acid chain, and then we have a saturated fatty acid chain on the left, and this just actually just made up in this molecule, but, and they could be different depending on the phospholipid you’re talking about. But this general property of having a hydrophilic head and hydrophobic tails make them very well suited for cellular membranes. Because you could imagine, so hydro, that end is hydro, hydrophilic, and then you have these hydrophobic, hydrophobic tails. Actually, let me see if I can copy and paste this really fast. I actually can just draw it really fast. So let me just draw a bunch of them. Whoops, nope, I’m having trouble. Alright, let me just switch back to my drawing tool. So let me just draw a bunch of the hydrophilic heads and then a bunch of the hydrophobic, whoops, actually I’ll draw a few more hydrophilic heads out here, and then I’ll draw the hydrophobic tails. Hydrophobic, hydrophobic tails, let me draw them, draw them really fast. I’m almost done right over here. Hydrophobic tails right over here. And this configuration that I’ve just drawn where you have a bilayer of phospholipid, of phospholipids, a phospholipid bilayer, this is how cellular membranes are constructed. Because this you have water, or things that are very water based inside the cell, this could be inside, inside the cell, and this could be outside, this is outside the cell. And so the phosphate ends is attracted to the water and then, but the hydrophobic, the hydrocarbon tails, those are going to, they’re going to orient themselves in this way to get away, to get away from the water. Really they’ll just let the phosphate, the phosphate ends interact with the water. And so this forms a nice boundary for the cell. And we’re gonna study that thoroughly as we go more into biology. So hopefully this gives you more appreciation of what a lipid is and the different types of lipids.