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Lipids - Structure in cell membranes

Created by Ryan Scott Patton.

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Video transcript

- [Voiceover] Alright so in a previous video I introduced a class of biological molecules called lipids. I broke lipids down into two main categories. We have hydrolyzable lipids, hydrolyzable, and then not hydrolyzable. Essentially what this means and we've kind of talked about this is these lipids can be broken down into smaller sections, and these Lipids is non-hydrolyzable. Lipids cannot be broken down into smaller sections and in this we fit probably the most abundant lipid, triglycerol and if you remember triglycerol has the main function in our bodies at least of energy storage. So we break down triglycerol because it's a hydrolyzable lipid to get energy. I want to continue talking about these hydrolyzable lipids but the other ones that I think are important instead of kind of having a predominant energy storage function, they have a structural function in our cells and kind of in the biological role, so a structural function. So the first one I wanna talk about are phospholipids. Phospholipids are hydrolyzable lipids, that contain a phosphorus atom. So this phosphorus atom usually comes in a form of a phosphodiester bond and I have no expectation of you to know what a phosphodiester bond is because I don't think we talked about it so we start with phosphoric acid, which is H three, P O four, that's phosphoric acid and it has a, kind of has a structure like this. We've got phosphorus double bonded to oxygen and then bonded three other times to O H group. So we have four oxygens in total and three hydrogens and a phosphorus atom. That might look a little bit goofy if you're paying real close attention because this phosphorus is bonded five times but remember that phosphorus is a third row element unlike carbon, so it can bond more than four times. It has, it's capable of sharing more than eight valence electrons. So we've got phosphoric acid right here and if we take these two side O H groups and replace them with O R groups, so let me kind of redraw the base structure here. Still have it double bonded to an oxygen but if we replace those sides with O R groups, say for example in a dehydration reaction this becomes a phosphodiester. And it's an -ester because this phosphorus is double bonded to an oxygen and an O R group and it's a -diester because it happens twice, so this is a phosphodiester. This is the form that phosphorus is gonna come into play for our lipids. So if we start with triglycerol, which is kind of that basic hydrolyzable lipid and I guess I better go ahead and draw out triglycerol here. So this is triglycerol and if we replace this lower fatty acid chain, remember all of these are fatty acids, if we replace that with a phosphodiester it's gonna look like this. So this would be one of the O R groups. This whole rest of the molecule would be the other O R group and I guess in our bodies not to confuse you too much but phosphodiesters had a pH in our body of around 7.4, this hydrogen right here is actually usually deprotonated so usually carries a negative charge on this oxygen so we wouldn't have an H right here instead we'd have a negative charge. So this right here is a phospholipid and one of the cool things about phospholipids is unlike triglycerols now we have a specifically polar section, so this right here is polar and then we have a non-polar section to this molecule. So this is all still non-polar and the purple part principally because of this kind of being negative charge and just this the polar nature of this phosphodiester but that allows it to play a pretty cool role in cell membranes. You've probably heard of a phospholipid bi-layer and what happens is you have a phosphodiester head, so kind of a phosphodiester head here and then you have two kind of fatty acid tails coming off and this is polar, this purple part is polar and the orange part would be non-polar and you get a whole bunch of these and they kind of line up together, the polar heads kind of line up and the non-polar tails kind of line up. And those non-polar tails become attracted to another set of non-polar tails, and another polar head right here, and this forms a two layer membrane to our cells. So this is kind of the thing that separates the inside contents of our cell from the outside contents and this whole middle section right here is not attracted to water because of this kind of non-polar lipid characteristic and it keeps water over here and water over here but separates these two fluid compartments so that's a pretty neat function, a structural function of the phospholipid and I don't think you can really talk about phospholipids without stopping at least briefly to mention sphingolipids, they're another hydrolyzable lipids. So sphingolipid, and you need to talk about sphingolipids because they're another lipid that incorporate this phosphodiester unit, but instead of having it kind of on a base of a triglycerol, it's gonna be on the base of the amino alcohol sphingosine. So let me kind of draw in sphingosine. And so sphingosine is an amino alcohol, it's got these O H groups, but it's also got a lot of lipid characteristics because it's really high molecular weight alcohol. This is a lot of carbon here, 15 carbons on this kind of tail, it would extend out like that if I kind of drew them all out but if we replace this O H group with a phosphodiester just like we did with our triglycerol so kind of replace it. It also develops a structural function within cell membranes but mostly within nerve cells and I'll show you why. So we've got a nerve cell here with a long axon and nerves are surrounded by the insulation of myelin, so we've got myelin right here. The properties of myelin, which allowed to insulate this nerve axon right here are really predominantly due to the sphingolipid concentration in myelin. So sphingolipids kind of have a similar structure function within cell membranes but mostly within their cells and then I guess the last hydrolyzable lipid that I really wanna talk about are waxes, so waxes. Again, I'll kind of do this briefly but waxes are also esters and they're made of a high molecular weight alcohol. So again a long carbon chain alcohol that might look like this with an R group but it'd be really long, so it'd kind of look like this and then they've got a fatty acid so -- A fatty acid, again really high molecular weight. If you form an ester out of these two molecules you end up with a product that looks a little something like this, so a long chain, double bonded to an oxygen with an O R group right here. This would be the ester and so you got an ester right here but it's kind of a unique ester in the sense that it's got two really long carbon chains making it two kind of non-polar sections here. These two non-polar sections make the molecules very hydrophobic, waxes are very hydrophobic and so we see waxes in nature often forming a real barrier against water and this happens in leaves, we put waxes on our cars to protect the surface of our cars from rain and from the kind of the humidity but waxes are another type of hydrolyzable lipid. And their hydrolyzable again, I just kind of wanna beat this point home because I've got this ester group right here, carbon double bonded to oxygen with an O R group that can be hydrolyzed. You can do an ester hydrolysis reaction. So that's with waxes, same thing with sphingolipids they've got this, sorry let me make sure you know that that's an oxygen, they've got this ester group right here that you can break down. You can do the same thing if I go up with phospholipids. Again we have this O, this kind of this ester bond right here that we can hydrolyze and we can do it with these bonds as well, just like we could with triglycerols. So all of these, again are lipids that can be broken down into smaller units through hydrolysis reactions.