Let's turn our attention to lipids as we transition to talking about the membrane. Now, many lipids in biological systems contain systems containing fatty acids, and fatty acids, just like nucleotides, have a sort of unique naming and numbering convention. If we take a look at this example here of cis oleic acid, cis oleic acid being the common name of this molecule. This molecule's formal name would be 18:1(Δ9) and the reason for these numbers are 18 which is the number of carbons in the chain. The number of double bonds, and you put a colon between those 2. And then in parenthesis, Δ9 represents the number of the carbon participating in the double bonds. So if you look at our chain here, carbon number 9 is where the double bond starts. So again, this molecule's formal name would be 18:1(Δ9) and then it would be octadecanoic acid. The 'oic acid' of course is because of the carboxyl group right there.
So, let's take a look at this other molecule which has the common name, EPA. That's sort of generically how it's referred to and let's think about what numbers we would put in here. So why don't you take a minute, maybe pause the video and try to figure this out yourself. And I'm going to explain how we would number this. So if you count out our chain or simply look at the numbers provided, you see that it's 20 carbons long and it contains 1, 2, 3, 4, 5 double bonds. So this would be 20:5(Δ5, 8, 11, 14, 17). So 5, 8, 11, 14, and 17. So this is only one way of naming and numbering fatty acids. You might also notice that in red, we have the letter, the Greek letter omega and numbers starting at the end of the chain. And this is actually referring to a sort of different naming numbering convention for fatty acids which is, mostly used by, you know, nutritionists and biologists not so much biochemists and that is the omega system of naming and numbering fatty acids. So you might see that counting backwards, we have a double bond starting at carbon 3 and also one at carbon 6. And you may have heard, you know, sort of in the media people talking about omega-3 and omega-6 fatty acids. And these are fatty acids that actually have to be acquired through your diet and are essential for us. And we have to acquire them through our diet because we actually can't synthesize fatty acids that are this long.
Anyhow, looking at this molecule we have here, you can see that this is an omega-3 fatty acid because it has this, omega-3 double bond. If it didn't have that and it just had this omega-6 double bond, we would call this an omega-6 fatty acid. And, you know, generally omega-3 fatty acids, you tend to acquire from, vegetables and nuts and fish and omega-6s tend to come more from animal products specifically like red meat type products, so, beef, butter, milk, that sort of stuff. So, you don't need to necessarily, you know, memorize these molecules but you do need to understand how the numbering convention works for fatty acids and also you need to understand how the omega numbering convention works. Moving on, naturally occurring fatty acids are, that are unsaturated are always cis and unsaturated of course refers to the like degree of unsaturation or basically how many double bonds are present in the hydrocarbon. So, whenever you have naturally occurring unsaturated fatty acids, the double bond is always in the cis form like we saw above with, with cisoleic acid and of course, with EPA. Now, in terms of properties of fatty acids, as the chain length increases, the melting point also increases. So you can kind of think of this as a result of the Van Der Waals interactions, meaning the stronger the intermolecular forces between the molecules, meaning the higher the melting point. Additionally, the longer the chain on the fatty acid, the less soluble that fatty acid is. So, smaller chain fatty acids tend to be far more soluble than longer chain fatty acids. So longer chain fatty acids, are conversely less soluble in water. Unsaturation actually greatly reduces the melting point of fatty acids. So, even having just one double bond present in your fatty acid will, plummet the melting point of the molecule. And this does come with a caveat and that caveat is that trans fats, right? So we were saying that unsaturation is, always cis in naturally occurring fatty acids. But you can have fatty acids that have trans bonds in them. And those are the, so-called trans fats that everyone is really worried about and, you know doesn't want to eat. And trans fats actually have higher melting points. And this is because even though they're unsaturated, despite unsaturation, their trans double bonds are linear. So if we were to look at a chain with a cis bond right there, that cis bond creates a kink in the chain like you see up here in oleic acid. It actually makes the chain kinked. Whereas trans fats still actually have a straight chain form even though they have a double bond in it and that means that trans fats can actually, fit together more tightly or those trans fatty acids can fit together more tightly whereas these cis fatty acids aren't going to be able to fit together as tightly. So, again, that's going to increase the strength of the intermolecular forces because they're going to be closer to those dipoles will be closer together. So, trans fatty acids will actually have higher melting points despite being unsaturated. Now, usually, the simplest form of a fat molecule that we'll see in biology that uses a fatty acid are triacylglycerols and these are basically storage lipids. They store energy for later use in the body, generally long term energy storage. And they're made from glycerol, a glycerol backbone combined with 3 fatty acids that are attached by ester bonds. So here is your glycerol backbone. And in red, I'm going to mark those ester bonds, right? Those are the ester bonds between the fatty acids and the glycerols and what's important to note is if only one chain is unsaturated, it's always on carbon 2 of the glycerol. So here we have carbons 1, 2, and 3. So if there's only 1 unsaturated fatty acid, it's always going to be the middle one basically. It's always going to be on carbon 2. If you have more than 1 unsaturated fatty acid in your triacylglycerol, you'll have, you know, you'll have 1 in the 2 position and then one either either in the 1 or 3 position and and it's, less strict there. So, just remember, if only one chain is unsaturated, it's always going to be on carbon 2. And also notice right here that on carbon 3, we have an omega 3 fatty acid. Alright, with that, let's flip the page.
