Switching gears a little, let's look at some questions related to lipids. If you haven't answered question 16 yet, pause the video here. So, which of the following is true about sterols? The answer is that all sterols share a fused ring structure which actually includes 4 rings. I'll draw it out for you right now. It is made up of 3 six-membered rings and one five-membered ring. And that is a sterol or I should say the backbone of a sterol molecule. Now, the other question or the other statements here are obviously false. So, sterols are not only found in the membranes of living cells. They can be found in the membranes of dead cells. Just kidding. They can be found in many other places. They're used as hormones. So, they have a wide variety of uses in the body. They are found all over the place. They are not soluble in water. I mean, look at that structure. That is screaming hydrophobic. It does have this OH, so it can form H bonds, and actually when it is embedded in the membrane, you know, you'll have sort of the surface of the membrane there. This being the inside and this the outside. So this can interact with water. This obviously can't. Cholesterol is actually the main sterol in humans, whereas ergosterol is the main sterol in fungi. So this is completely backwards basically. Yeah, ergosterol in fungi and cholesterol in humans. Now, the fluidity of the membrane is generally increased by increasing the number of double bonds in fatty acids. If you remember, increasing the number of double bonds in the fatty acid actually lowers its melting point significantly. Additionally, all of these other statements are false. So let's go through them. Increasing temperature will increase the fluidity. Decreasing temperature decreases fluidity. And that's why, the composition of fatty acids in the membrane of an organism really depends on where it lives. If it lives in colder climates, it's going to want more unsaturated fatty acids because they'll stay more fluid at the lower temperatures. If it lives in a warmer climate, it's going to want more unsaturated fatty acids because those will remain more rigid at higher temperatures. It wouldn't, you know, if there were more unsaturated fatty acids at high temperatures, they would become too fluid and the membrane would fall apart more or less. Binding of water to fatty acyl side chains, good luck with that. Water and those fatty acyl side chains want nothing to do with each other. And an increase in the percentage of phosphatidylethanolamine actually decreases fluidity. Not by a huge amount, but it does a little bit. And actually, phosphatidylcholine increases fluidity.
Looking at question 18, sphingosine is not a component of cardiolipin. Sphingosine is an essential component of sphingolipids. It's pretty much where they get their name from, and it's more or less what they're using in place of glycerol, and also fatty acid. If you think of phospholipids as having glycerol and two fatty acids, sphingolipids will sometimes have that fatty acid, but they won't have two fatty acids, that is to say. They'll only possibly have one. The sphingosine molecule sort of replaces the glycerol and one of the fatty acid side chains. Cardiolipin is actually a really kind of strange molecule and it's found in mitochondrial membranes and in bacterial membranes.
Looking at question 19 now, fatty acids are components of cerebrosides which again are sphingolipids. So they're going to have a sphingosine molecule attached to a fatty acid and also some sort of head group. Carotenes, these are isoprenes. And these guys, all sterols. Alright. Moving on. Which of the following statements concerning fatty acids is correct? The answer is that a fatty acid is the precursor of prostaglandins and that fatty acid is arachidonic acid. This is known as an eicosanoid. Just looking at the wrong answer choices, phosphatidic acid is a common fatty acid. No. It's not a fatty acid. Fatty acids all contain one or more double bonds? No. Some are unsaturated and some are saturated, right? So this is wrong because there are some saturated fatty acids. Fatty acids are a constituent of sterols. No, they're not. Sterols are those four fused rings. Fatty acids are strongly hydrophilic. No way. Super hydrophobic. Right? Alright. So let's finish it up here. And diagram this molecule. I'm just going to do it in a skeletal structure to keep it nice and simple. So I'm actually going to start by drawing my esters just to kind of give shape to the rest of my molecule.
So there are my esters. This is my glycerol. And here is my phosphate group. And, again, I apologize that this is not the prettiest picture in the world. So here we have choline. This is, of course, our phosphate right here. Glycerol is our backbone right there. Let's do our fatty acids now. So, we know that it has this fatty acid which we can see contains one double bond at carbon 9 and it has this fatty acid which is completely saturated. It doesn't tell us which one goes where but we should know that if there is only one unsaturated fatty acid, that's going to appear at position 2 on the glycerol. So here's position 1, 2, and this is, we have 3 there. Acid naming convention here. So let's start with dodecanoic acid. It's going to have 12 carbons and again, 0 double bonds. That's what those numbers mean. And now, palmitoleic acid. This is going to have 16 carbons, one double bond, and that double bond is going to be between carbon 9 and 10. And there we go. That is our molecular structure. I'm sorry it's such a hideous picture. Let's try to forget about it as we turn the page.
