Structure and Function of Lipids

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  • 0:06 Lipids
  • 1:13 Triglycerides
  • 5:23 Other Lipids
  • 7:16 Lesson Summary
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Lesson Transcript
Instructor: Meg Desko

Meg has taught college-level science. She holds a Ph.D. in biochemistry.

Molecules called lipids have long hydrocarbon chains that determine the way they act. They can be fats, oils, or hormones, and even exist in our cell membranes. Learn more about the chemical structure and biological function of various lipids in this lesson.


Lipids fall under the categories of glycerol or steroid
Glycerol and steroids

Lipids are biological molecules that are insoluble in water, but are soluble in non-polar solvents, meaning that they are non-polar molecules. The lipids we're most familiar with are probably dietary fats. So if you get a little hungry, and you decide you want to eat some macaroni and cheese, and you look at the nutrition label, what you'll find is that it contains 12 grams of fat per serving. It also contains 31 grams of carbohydrates and 5 grams of protein.

So you have 12 grams of fat that give you 110 calories, and 36 grams of carbohydrates and protein combined, giving you 140 calories in each serving. So the fat gives you 9.2 calories per gram, while the carbohydrates and proteins give you only 3.9 calories per gram, which means that the fats contain over two times as much energy per gram! So this tells us that lipids and fats are good for storing energy, but what do they look like?


Lipids fall into two categories. One is based on glycerol, and the other is steroids. First, we'll talk about the glycerol kind. Most dietary and storage fats are triglycerides. This means that they are made of glycerol combined with three carboxylic acids, which we call fatty acids. These form esters via dehydration.

Now, to give you an idea of what this means, glycerol is a 3-carbon alcohol that contains three different hydroxyl, or OH, groups on each of the carbon atoms. You may also recall that carboxylic acids are molecules that contain a carbon atom double-bonded to an oxygen atom, the carboxyl group, and that carbon atom is also bonded to a hydroxyl group, forming the carboxylic acid group.

Three types of triglycerides are listed on nutrition labels
Types of dietary fats

In the case of triglycerides, this glycerol combines with the three fatty acids, or carboxylic acids, to form esters. Now, esters are a functional group that consists of a carbon atom that is double-bonded to an oxygen atom, and that same carbon atom is single-bonded to an oxygen atom, bonded to another carbon atom. And these are formed via dehydration, or loss of water.

Now, to get to know our triglycerides a little bit better, we can look back at the nutrition label. On the nutrition label, there are three types of fats. Two of these are listed on the label - saturated fats and trans fats - and the rest are unsaturated fats that are not trans. Saturated and unsaturated fats are the kinds that we find in nature while trans fats are synthetic.

Saturated fats are triglycerides that have no double bonds in their carboxylic acid chains. Saturated fats are found in things like butter. Because their fatty acid chains are long and flexible, they can intertwine with one another, and because they're non-polar, they're attracted to each other, and not to polar things, like water. So this is part of why butter is a solid at room temperature - it's because of the molecular interactions between these different fatty acid chains.

Unsaturated fats are triglycerides that have double bonds in their carboxylic acid chains. These are found in things like olive oil, and can be monounsaturated, which means they contain one double bond, or they can be polyunsaturated, meaning they contain many double bonds. In unsaturated fats, the carbons on either side of the double bond are on the same side of the double bond. These double bonds kink and bend the chains of carbons making it harder for them to interact closely. It's sort of like, if saturated fat chains are spaghetti and can wrap around one another very easily and stick together, these are like trying to fit together puzzle pieces that don't have the same edges, so you can't get them terribly close to each other. This is why substances in which there are many unsaturated fats, such as olive oil, tend to be liquids.

In contrast to spaghetti-like saturated fats, unsaturated fats are like irregular puzzle pieces
Unsaturated fats

Trans fats, the synthetic kind, are triglycerides that have trans double bonds in their carboxylic acid chains. What trans means is that means is that the carbons on either side of the double bond are on opposite sides of the double bond. Trans fats are the byproduct of hydrogenating polyunsaturated fats. The goal of scientists was to make saturated fats, and these happened as a side product. The trans double bonds make the carbon chain very rigid and straight, so you get very close packing between these different carbon chains. It's sort of like, if the saturated fats are like spaghetti and can wrap around each other to interact, there's still some room for movement. And trans fats are a little bit more like, if you put a bunch of rods, and stuck them together - there's not a whole lot of room for them to move.

Because trans fats are able to pack so tightly together, they're able to become solid at higher temperatures, and this is part of why they're bad for us. If these trans fats are able to come together in our arteries, they may form blockages.

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