If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

Molecular structure of triglycerides (fats)

AP.BIO:
SYI‑1 (EU)
,
SYI‑1.B (LO)
,
SYI‑1.B.2 (EK)
,
SYI‑1.C (LO)
,
SYI‑1.C.1 (EK)
Triglycerides, or fats, are formed from the combination of glycerol and three fatty acid molecules. Triglycerides are formed through dehydration reactions. Another word for triglyceride is triacylglycerol. Fats can be solid (such as coconut oil) or liquid (such as vegetable oil) at room temperature.

Want to join the conversation?

  • piceratops sapling style avatar for user Rose
    At , "Acyl" groups are introduced. But what is the difference between an acyl group and a carbonyl group? Don't they have the same structures?
    (39 votes)
    Default Khan Academy avatar avatar for user
    • old spice man green style avatar for user Matt B
      Great question, very tricky to answer too!
      A carbonyl has the structure of C=O only.
      An acyl has the structure of R-C=O, where R is any hydrocarbon. A carbonyl is present in a acyl group.

      Example:
      (CH3)2-C=O is has an acyl group, which is CH3-C=O. It also has a carbonyl group, C=O.
      (57 votes)
  • leafers sapling style avatar for user Meritt  Wiggin
    At and a little after, why are the oxygens positive? I thought when you have an oxygen-hydrogen bond the oxygen becomes partially negative?
    (12 votes)
    Default Khan Academy avatar avatar for user
    • piceratops ultimate style avatar for user Darmon
      The oxygen in this case is positive because it is sharing 2 of its non-valence electrons with that hydrogen. Its 2 valence electrons are already tied up in the two covalent bonds with the two carbons, and when it gives up some of its rights to the 2 non-valence electrons to the hydrogen, it reduces its negative charge (making it positive). :)
      (19 votes)
  • blobby green style avatar for user Phil Rattazzi
    What happens to the hydrogen atoms originally bonded to glycerol after the fatty acids are added?
    (10 votes)
    Default Khan Academy avatar avatar for user
    • blobby green style avatar for user Alex E
      I think that Sal simply failed to draw the hydrogen atoms on the glycerol carbons. From other sources on biology I have seen the same illustration, but the outer (here, top and bottom) carbons have two hydrogen atoms each, and the middle carbon has one additional hydrogen atom attached.
      (3 votes)
  • starky ultimate style avatar for user alina
    If there are three ester groups, why isn't a triglyceride/triacylglycerol also known as a triesterglycerol?
    (5 votes)
    Default Khan Academy avatar avatar for user
    • old spice man green style avatar for user Matt B
      Because that would be a mouthful and it is clear what you mean when you say triglyceride. In fact, if you really wanted to, you could even incorporate the name of the carbon chains in the glyceride. There are many ways to name large organic molecules :-)
      (7 votes)
  • leafers ultimate style avatar for user Kevin K
    What is he talking about when he mentions oxygen's lone pairs? If its bonded to H and C its valence shell is satisfied, how does it still have lone pairs?
    (6 votes)
    Default Khan Academy avatar avatar for user
    • duskpin tree style avatar for user VLK
      Oxygen has a total of 8 electrons: two are in its first shell, so they don't react, and the other six are in the next shell up, so they are available for bonding. If it is only bonded to two other molecules, there is still one more lone pair available.
      (5 votes)
  • piceratops ultimate style avatar for user Matthew Chen
    At , can a fatty acid have a triple bond?
    (4 votes)
    Default Khan Academy avatar avatar for user
  • purple pi purple style avatar for user Jay
    Around , why cant the double bonded oxygen break on of its bonds to let the glycerol in?
    (5 votes)
    Default Khan Academy avatar avatar for user
    • starky sapling style avatar for user Jm Cresencio
      recall on your organic chemistry that when carboxylic acids lose a proton, they have resonance forms that make the double bond jump between the two oxygens and this stabilizes the entire molecule. Removing the carbonyl oxygen is not only very unfavorable because of losing the stability but also hard to do since it's a double bond which is stronger compared to the O-H bond
      (2 votes)
  • purple pi pink style avatar for user stuffundefined
    if glycerol has an alcohol group, would having fat in your body make you drunk?
    (2 votes)
    Default Khan Academy avatar avatar for user
  • orange juice squid orange style avatar for user Samuel Rex
    But doesn't the Oxygen already have 8 electrons? Why would it want to take some from Carbon?
    (2 votes)
    Default Khan Academy avatar avatar for user
  • duskpin seedling style avatar for user Jess
    It is mentioned at that most fatty acid chains in biological systems have an even number of carbons in them. Why is this case?
    (3 votes)
    Default Khan Academy avatar avatar for user

Video transcript

- [Voiceover] Let's study the molecular structure of triglycerides. And in everyday language we often call these fats. We often call these fats. Which is often kind of a feared word but as we'll see it's essential for life. Fats in a liquid form is sometimes referred to as oils. And a more technical term for triglycerides-- Triglyceride is the word that you might hear when you go to the doctor's office. But it can also be referred to as tri-- And I'm going to do the different parts in different colors. Triacylglycerol. I actually like this second name more because it makes sense when you break down the molecule, triacylglycerol. But triglycerides, this is literally fat. Or where what most people typically refer to as fat, they're talking about triglycerides. If you were to get your cholesterol checked or if your parents get their cholesterol checked, they'll probably get a report on triglycerides which is really a measure of the fat concentration in their blood. But let's think about what a triglyceride is. And so here I have the constituent molecules of a triglyceride. This molecule on the left, this is glycerol. Let me do it in that same blue that I just wrote glycerol in. This is glycerol. We can see we have a three carbon chain right over here. Each of those carbons is attached to a hydroxyl group, an O-H group. If you just get attached to one of them, this makes you an alcohol so this definitely makes it an alcohol. This is sometimes called a triol. It has three hydroxyl groups right over here. And this is considered to be a sugar alcohol. A sugar alcohol. We see why it's an alcohol, it's got its hydroxyl groups. And it's also sweet. If you were to drink a solution of glycerol it will actually be sweet. And you might already be familiar with the word glycerol from the famous Bush song Glycerine. Glycerine is referring to glycerol. Glycerine. ♫ Glycerine ♫ So there you go, that is glycerol right over there. And you form a triglyceride when a glycerol molecule reacts with three fatty acid molecules. So these characters on the right, these are each a fatty acid. These are each a fatty acid. This is one fatty acid, this is another fatty acid, this is another fatty acid. Now why do we call them "fatty" and in particular, why do we call them a "fatty acid"? Well, you have one part that has these big carbon chains here. And I drew these dot dot dots 'cause it can actually have different numbers of carbons. They can have anywhere between-- Based on my research you can have into the high 20's numbers of carbons. In most biological systems you tend to have an even number of carbons. And in most animal biology you tend to have 16 to 20 carbons. And if it's an even number, you're talking about 16, 18, 20 carbons involved in these fatty acids. All the ones I've drawn have all single bonds. We have as many hydrogens bonded to it as possible. But you could also have double bonds there. And we'll talk about that more when we talk about saturated and unsaturated fats. So you have this carbon chain here. That part is hydrophobic, it won't dissolve well in water. And that's why people tend to refer to it as kind of fatty. And then you have on the left-hand, or at least the way I've drawn it on the left-hand side, you have a carboxyl group. So this right over here, this is a carboxyl group. Carboxyl group. And this is acidic. It's very easy for it to donate a hydrogen because if it donates a hydrogen or a hydrogen proton I should say. Because if this oxygen takes those electrons that negative charge can be spread between this oxygen and this oxygen over here. And so it's very willing to give away a hydrogen so that makes it acidic. So that's where the "acid" comes from. That's a fatty acid. And when you have these three molecules they don't have to be identical. This one could have 18 carbons. This one over here could have 16 carbons. This one could have 20 carbons, it might have a couple of double bonds in it. So they could all be different. But let's think about how they actually form together. Like those robot movies from the 80's like Voltron. So what you have happening, and this is gonna be a little bit of organic chemistry and this really just a review of dehydration synthesis. You have a lone pair in each of these oxygens. These oxygens that are part of the hydroxyl group. So let me draw that. So you have a lone pair there or actually you have two lone pairs. You have two lone pairs over there. You have two lone pairs over there. And so you can imagine a world where if they just bump past each other in just the right way this carbon over here is gonna have a partially positive charge. It has three bonds to two oxygens. Oxygens that are very elctro negative. And so this guy, if he just bumps into it in the right way, these electrons could be used to form a bond with this carbon, the one that has that partially positive charge. And just as that is happening this bond can be let go. This oxygen say, "Ok look, you're sharing in another pair? "I've had enough of that. "I'm going to take these electrons for myself "or even better I'm going to take them for myself "or to eventually capture or maybe I capture first, "it can happen in all different ways, a hydrogen proton. "I'm gonna capture a hydrogen proton and then I can become "this collection right over here "could become a water molecule." I'm doing just a very high level overview of the mechanism. You can go to more depth of this when you go into organic chemistry. But this could happen three times. So it can happen again over here. It could happen again over here. This guy takes this-- Let me do that in that same white color. So he takes both of these electrons that he so badly wants to take back and maybe uses it to grab a hydrogen proton. It could happen the other way. Maybe grabs a hydrogen proton and then just as he's leaving, this guy comes back. So there's different ways that all of these could happen but this is the general idea. And then you have it happening a third time down here. One of these lone pairs come and form a bond with this carbon. This carbon in the carbonyl group, part of this carboxyl group. And so once again, this guy can take those two electrons away and maybe share that pair with a hydrogen proton. Once again, this is forming a water molecule, this is forming a water molecule. So three water molecules are going to be produced. This is why we call it dehydration synthesis. We're losing three water molecules in order to form these bonds. So what's it going to look like after this has happened? Well, let me scroll down here. And actually let me just zoom. Actually just let me scroll down here. So this green bond over here is going to now-- So this green bond that's this big long curvy thing is this green bond. And this second green bond is this green bond. And this third green bond is that green bond. The way I've drawn it right now, each of these oxygens haven't let go of its hydrogens. And that could actually happen before or after or all at the same time. Chemistry actually is not a clean thing. But I could, if I want, I could draw the hydrogens here. I could draw the hydrogens. I could draw the hydrogens over here and then these would have a positive charge. These would have a positive charge. But then you could imagine another water molecule coming by and kind of taking one of these hydrogen protons, taking the hydrogen protons away from each of those oxygens. And then you would be left with this molecule right over here. And remember we produced three water molecules. So that's one, two and three water molecules. And now this molecule, if you ignore the water molecules out there, this is a triglyceride. Let me write it again. Actually, let me write the slightly more technical term. Sometimes referred to as triacylglycerol. Well we know where the glycerol comes from. It has this glycerine or this glycerol backbone right over here. Now what is "acyl" mean? Well acyl is a functional group where you have a carbon that's part of a carbonyl group. It can be bound to a kind of an organic chain right over here, I'll just call that R. And then it's bound to something else. And so we have three acyl groups so "triacyl". So this right over here, this right here is an acyl group. This right over here is an acyl group. This right over here is an acyl group. Each of them, they're bound to an oxygen right over here. And actually that gives us more practice with another functional group. When you have a situation-- Let me get another color out. When you have a carbonyl group and let's say you have just an organic chain right over here and then you have an oxygen and then you have another organic chain right over there. This thing is called an ester. And you actually see three esters right over here. So you see this ester. This ester right over here. You see this ester right over here. And then this ester right over here. So this wasn't just practice in learning about fats or triacylglycerol or triglycerides. These are all words for the same thing. It's giving us good practice with these functional groups. But now you see where triacylglycerol comes from. You have glycerol backbone and you have these three acyl groups that allow it to attach to the fatty acids. So that's what fat is when people are referring to it. And this is just pictures of it. This is coconut oil right over here. It's right here, it's below its melting point so it's in its solid form. And this right over here, I'm not sure which type of oil this is but it's past its melting point and so it's in its liquid form. So people would typically refer to this as an oil. But this is made up of a bunch of triglycerides. This is a big mixture of triglycerides here. And if you were to see their molecules, they would all have generally this shape but the fatty acids would have different numbers of carbons and might have different numbers of double bonds.