Triacylglycerols: Study with Video Lessons, Practice Problems & Examples

Triacylglycerols, or triglycerides, consist of a glycerol backbone bonded to three fatty acid chains via ester bonds. These fatty acids can vary in length and saturation. Fats, typically solid at room temperature, have fewer double bonds, while oils, usually liquid, contain more unsaturated fatty acids, resulting in lower melting points due to kinking in their structure. This structural difference affects their physical properties and biological functions, including energy storage in adipose tissue.

Triacylglycerols Concept 1

In this video, we're going to take a look at Triacylglycerols. Now, Glycerolipids are lipids with fatty acid chains attached to a Glycerol backbone. And when we say Triacylglycerols or triglycerides, this is when we have three fatty acids chains attached to the glycerol backbone through ester bonds. Now, these fatty acids can all vary. In this example of our Trisylglycerol or our triglyceride, we have our glycerol backbone and we have connected to it our three fatty acids. We can see that all three fatty acids are not the same; their chain lengths do vary. So, they don't all necessarily need to be the same. Remember, if we're looking at lipids and we're breaking it down, we have lipids that are broken down into fatty acids and steroids, fatty acids are broken down further into waxes or what we have here now, Glycerolipids, as well as two other designations which we'll talk about later on. Now, this Glycerol Lipids, you have our Triacylglycerols here or our Triglycerides. Again, it is a Glycerol backbone with our three fatty acids which could all be the same or be different from one another. Now, here, what's the function of these triglycerides? Well, they have two main functions, and they are an energy source that we could tap into, and we're gonna say here they can act as storage in the form of adipose tissue in animals. So just remember when we're talking about our triglycerides or our Triacylglycerols, we're talking about a glycerol backbone and attached to it are three fatty acids connected by an ester bond, or in this case, three ester bonds.

Triacylglycerols Example 1

It says draw a triglyceride structure composed of palmitoleic acid for the first carbon, myristic acid for carbon 2, and oleic acid for carbon 3. Now, based on our memory tools, we know that myristic acid is a saturated fatty acid. It has 14 carbons and no pi bonds. Palmitoleic acid has 16 carbons and 1 pi bond, and oleic acid has 18 carbons and 1 pi bond. In addition to this, their 1 pi bonds start on carbon 9. This will be important when drawing these structures. Now, here we're going to say, step 1 is to draw the Glycerol Molecule and the 3 Fatty Acids. We're going to place -OH groups next to the carboxyl groups of the fatty acid. So, let's do that part first. We are going to draw our glycerol molecule. We're going to say we have CH₂CH₂CH₂, and then we have our -OH groups. Now we have to draw our fatty acids. So remember, our first one is an unsaturated fatty acid. We are going to say it has 16 carbons, so 2, 4, 6, 8. Remember the first pi bond happens on carbon 9. So 9, 10, 11, 12, 13, 14, 15, 16. Next, we have our myristic acid. It is saturated so it has 14 carbons and no pi bonds. So 2, 4, 6, 8, 10, 12, 14. And then finally, oleic acid has 18 carbons. 2, 4, 6, 8, 9 is where we have our double bond, 10, 11, 12, 13, 14, 15, 16, 17, 18. Now, instead of -OH on glycerol, we are just going to write an -O. So, we are going to take away these H's here, and do not draw -OH on the fatty acids. So now, it's up to us to connect for step 2. We are going to form ester bonds between the glycerol -OH groups and the 3 fatty acids. So, we are going to connect the oxygens of the glycerol to the carbonyl carbons of the fatty acids. Doing this will give us our triglyceride. So, we are going to have our CH₂, and then CH₂, CH₂. Then we have O, O and then O. Here is our ester linkage. So, then we are going to draw. Remember, we need 16 carbons. So 2, 4, 6, 8, 9, 10, 11, 12, 13, 14, 15, 16. Here, we need 14 carbons. 2, 4, 6, 8, 10, 12, 14. And then here, we need 18 carbons. 2, 4, 6, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18 carbons. So, this will represent our Triacylglycerol molecule or our triglyceride structure. This would be our final answer.

Triacylglycerols Concept 2

Now when it comes to fats and oils, we're going to say that they are mixtures of different Triacylglycerols or Triglyceride Molecules. When it comes to fats, we're talking about in relation to animals here, they tend to have melting points that are high. We're going to say that they are solid at room temperature. Now, in terms of their saturation, we're going to say they have a low number of double bonds, and they are low in unsaturated fatty acids. Here we have an example of a Triacylglycerol molecule, we can see that the top 2 fatty acid chains are saturated, they have no pi bonds. And the bottom one is unsaturated, but it only has one double bond.

Next, when we look at oils, we're talking about in relation to vegetables. Here, we're going to say they tend to have low melting points. We're going to say they tend to be liquids at room temperature. We're going to say here that they tend to have a high number of pi bonds or double bonds, and they tend to be high in unsaturated fatty acids. If we take a look here, we can see that the first fatty acid chain is saturated while the next 2 are unsaturated. They have a lot more double bonds or pi bonds involved. This causes kinking and it kind of alters the shape of how the carbon chains align themselves because of these double bonds. Because you can see that these structures are bending, not in the same fashion as the top one which is saturated. This kinking and contorting of these chains causes them not to be able to stack as efficiently which is why they exist more as oils than as solids. Solid fats. So just remember, when we have the implementation or the incorporation of pi bonds here, this is going to cause a drop in your melting point. You'll tend to exist more as oil; you'll have a lower melting point. We'll have these kinks as a result of all this.

Triacylglycerols Example 2

This example question asks, "Which Triacylglycerol would you expect to be liquid at room temperature?" Remember, we have fats which are indicative of animals and oils which are indicative of vegetables and plants. Remember, the more double bonds or pi bonds that we possess, then the lower our melting point will be, and therefore more likely to exist as a liquid at room temperature. If we take a look at option a, we see that we have 1, 2, 3 double bonds or pi bonds involved. And if we look at option b, we only have 1 pi bond involved. The one most likely to be a liquid at room temperature would have to be option a. It possesses more double bonds which will result in more kinking of the long fatty acid chain, which will result in less stacking of these structures on top of each other. Meaning they'll exist more as a liquid at room temperature and have a lower melting point as a result. Alright. B is less likely to be a liquid when compared to a because it has much less pi bonds or double bonds involved. So again, in this particular question, option a is more likely to be a liquid than option b because of the presence of these pi bonds or double bonds.

Draw a skeletal structure of a triglyceride with linolenic acid (C1) and 2 palmitoleic acids. State whether it would have high or low melting point.

Here it says, draw a skeletal structure of a triglyceride with linolenic acid for carbon 1, and for carbons 2-3 we're going to have palmitoleic acids. Here, we need to state whether it would have a high or low melting point. Alright. So, remember to draw this triglyceride, we're going to have our glycerol. We have these three CH2 groups connected to oxygen. Those oxygens will be connected to the carbonyl groups of each of these fatty acids. We're going to say, based on memory tool 2, when we talk about our first fatty acid here, it's going to have 18 carbons and 3 pi bonds. And then here, these other 2 fatty acids, they're going to have 16 carbons and 1 pi bond. Now, if we want to draw this out, we're going to say here, let's draw these two bottom ones first because they're the easier one. Since they only have 1 pi bond, remember, that 1 pi bond starts at carbon number 9. So here we're going to have 2, 4, 6, 8, 9, 10, 11, 12, 13, 14, 15, 16. And this would be the same thing. 2, 4, 6, 8, 9, 10, 11, 12, 13, 14, 15, 16. And then finally, the first carbon here, we have 18 carbons, and still, we're going to have our first pi bond happen at carbon number 9. So, 2, 4, 6, 8, 9, 10, next pi bond, 11, 12, 13, 14, 15, 16, 17, 18. This would represent the top one. So this would be, be the triglyceride or the triacylglycerol molecule when we have our glycerol backbone with these 3 particular fatty acids. Now, here, they're trying to tell us, well, they're asking us, will it have a high or low melting point? We have the presence of 1, 2, 3, 4, 5 double bonds here. There would be more kinking involved in terms of the structure. I'm not showing that. I'm just showing them kind of linear in terms of structure, but there would be some kinking, some bending of these fatty acid chains, in particular the top one, because of the presence of those three double bonds. But here, because we have so many double bonds or pi bonds, we'd expect this Triglyceride to have a low melting point. Remember, the more double bonds you have, the lower your melting point will be, and therefore you're more likely to exist as a liquid at room temperature. So, for this particular one, this is our triglyceride structure and because of the presence of all these double bonds, we should expect it to have a low melting point.

Here’s what students ask on this topic:

What are triacylglycerols and how are they structured?

Triacylglycerols, also known as triglycerides, are a type of lipid molecule composed of a glycerol backbone bonded to three fatty acid chains via ester bonds. The fatty acids attached to the glycerol can vary in length and degree of saturation. This means that the fatty acids can be either saturated (no double bonds) or unsaturated (one or more double bonds). The structure of triacylglycerols is crucial for their function, as they serve as a major form of energy storage in adipose tissue. The ester bonds between the glycerol and fatty acids are formed through a dehydration reaction, where a water molecule is removed for each bond formed.

What is the difference between fats and oils in terms of triacylglycerols?

Fats and oils are both mixtures of different triacylglycerols, but they differ in their physical properties due to the saturation of the fatty acids they contain. Fats, typically found in animals, are solid at room temperature and have higher melting points. This is because they contain a higher proportion of saturated fatty acids, which have no double bonds and can pack closely together. Oils, on the other hand, are usually derived from vegetables and are liquid at room temperature. They have lower melting points due to a higher content of unsaturated fatty acids, which contain double bonds that cause kinks in the fatty acid chains, preventing them from packing tightly.

How do the physical properties of triacylglycerols affect their biological functions?

The physical properties of triacylglycerols, such as their melting points and state at room temperature, significantly influence their biological functions. Fats, which are solid at room temperature due to their high content of saturated fatty acids, are primarily used for long-term energy storage in animals. They are stored in adipose tissue and can be mobilized when energy is needed. Oils, which are liquid at room temperature due to their high content of unsaturated fatty acids, are more commonly found in plants and are used for quick energy release. The kinks in the unsaturated fatty acids prevent tight packing, making oils more fluid and easier to metabolize.

Why do unsaturated fatty acids have lower melting points than saturated fatty acids?

Unsaturated fatty acids have lower melting points than saturated fatty acids due to the presence of double bonds in their hydrocarbon chains. These double bonds introduce kinks or bends in the fatty acid chains, preventing them from packing closely together. This loose packing reduces the intermolecular forces between the molecules, resulting in a lower melting point. In contrast, saturated fatty acids have no double bonds, allowing them to pack tightly and form more stable, solid structures at room temperature. This difference in molecular structure is why unsaturated fatty acids are typically found in oils, which are liquid at room temperature, while saturated fatty acids are found in fats, which are solid.

What role do triacylglycerols play in energy storage?

Triacylglycerols play a crucial role in energy storage, particularly in animals. They are stored in adipose tissue, where they serve as a dense form of energy reserve. When the body requires energy, enzymes called lipases break down the triacylglycerols into glycerol and free fatty acids, which can then be oxidized to produce ATP, the energy currency of the cell. This process is highly efficient, as triacylglycerols provide more than twice the energy per gram compared to carbohydrates or proteins. Additionally, the storage of energy in the form of triacylglycerols is advantageous because it is compact and does not require water, making it an efficient way to store large amounts of energy.