 So let's look at fatty acids. Remember that in fats, we have a glycerol with three fatty acids attached. And in a phospholipid, we only have two fatty acids. And in these amazing steroids, we have no fatty acids whatsoever. So what is a fatty acid? Well, fancy that you asked me such a question. Fatty acids, I think I already said this, are basically just crazy long chains of carbons. There's a carbon and hydrogens. They basically have a carbon backbone. And my little hydrogens are chilling off to the side. And notice, this is a chemistry fact. Carbon, you'll learn this when you're brilliant in take chemistry. Carbon forms four chemical bonds. When carbon is happiest, it has four chemical bonds that it has created or shared with other atoms. And remember that an atom, a chemical bond is formed when atoms literally share their electrons. So in this scenario, what you can see is that carbon and hydrogen are sharing electrons. And here, carbon and carbon are sharing. Here's another carbon and carbon sharing. And here's another carbon and hydrogen sharing electrons. And the sharing of the electrons forms that chemical bond. All carbon atoms are going to form four chemical bonds with atoms around it. It's just how it's going to be. And in fact, if a carbon atom was missing a chemical bond, it only had three. That structure that only has three chemical bonds is going to be super unstable. And it won't stay with three chemical bonds. It just won't do it. This is stuff that you'll totally explore when you take chemistry. You'll explore it further. But knowing that carbon forms four chemical bonds will let you distinguish between a saturated fat and an unsaturated fat. And here's the deal. In a saturated fat, every single carbon has formed chemical bonds, four chemical bonds, and as many hydrogens as are possible are attached to those carbons. Now look, you remember my green carbon background? I mean backbone. That ain't a background. That's a backbone. If I form a backbone of carbon, each carbon is going to have enough electrons left over to form two chemical bonds with hydrogens. So we're going to have two hydrogens attached to every carbon until we get to the end of our chain, in which case we're going to add, have three hydrogens bonded to that carbon. We're saturated. We've filled up as many of our bonding potential places with hydrogen atoms. There are no, it's saturated with hydrogen. If a fat is unsaturated, then it could have more hydrogens attached to the backbone, but it doesn't. Why? I just told you, that ain't possible. That ain't possible. It is. It's possible if you have a double bond. In a double bond, there's two double bonds in this little scenario here. We're not just sharing two electrons between these carbons. We're sharing four. We have two chemical bonds between those two carbon atoms. Now think about this. In this carbon, this one right here that I just made green, is attached to two different carbon atoms, right? That's like all the other ones. However, it's only attached to one hydrogen. Look, there's only one hydrogen. But does it have its four chemical bonds? Of course, because it must. One, two, three, four. Boom. Carbon still has four chemical bonds, but we've actually like thrown away a hydrogen. There was a seat for the hydrogen, and the carbon said, I don't want to share my electron with that hydrogen punk. I want to share my electron with this carbon, and I'll share two electrons with this carbon. And still forming the correct number of chemical bonds. Now, who cares? Like, why does this even matter? This is so cool. I'm going to give you an example of a saturated fat. Even better, lard. Lard is an example of a saturated fat. I'm going to give you an example of an unsaturated fat. Oil, any one of them. Olive oil is an unsaturated fat. What's the difference between butter and oil? How about saturated fats are solid and fat room temperature. And unsaturated fats are liquid. And here's the reason why. If you had a pile of saturated fats, and they're not moving around very fast because it's room temperature, they're linear, can you imagine stacking like 500 saturated fats into a little box? They're all going to fit in there nice and clean, right? And they're going to be in such a nice little solid stack that they're going to be solid. Throw in an unsaturated fat. Look at those twangers twanging off in a lotty twanging land. They're twisted and twanging. And if you try to make a stack of those guys, you're going to end up with, like, you can't stack them up nice and tight with each other. Why they're liquid at room temperature? Who cares? Well, the reason why we care is because unsaturated fats supposedly are better for us, although there's current research that's suggesting that we're not exactly sure what's better for us and what's not better for us. Another type of fatty acid that is kind of, like, questionable, like, we're hearing a lot about it in media because we have these trans fats. And this is interesting because trans fats are made by us. We go in and we add hydrogens into an unsaturated fat. We add hydrogens into the mix. And it's this, I don't have any idea how they do this. But when they do it, when they throw in those extra hydrogens and basically try to make something saturated, this configuration is the normal configuration of a unsaturated fat found in nature. But when we make, when we add hydrogens in and try to unsaturate the fat, we actually produce a different form of the exact same molecule. Can you see that all we did was spin this half, and when you spin it, this is the trans configuration. And even though there's an unsaturated chemical bond in there, do you agree that it acts like a saturated fat? So these trans fatty acids actually are solid at room temperature and there's a suggestion that those things are gnarly. Now, trans fatty acids come from processed foods and it's the processing of the food that produces them. Under normal circumstances, they would be in this configuration, which would mean they wouldn't be solid at room temperature. All right, are you cool? Why do we care about fatty acids again? Well, because we're going to use fatty acids in our phospholipids to make up our cell membrane. So let's go back and look at the structure of the cell membrane itself.