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
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
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
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.
Now, to change topics a little bit, one of the things we use to get rid of fat on our dishes is soap. And soap works so well because it's made from things that look an awful lot like fatty acids on their own. They have a polar head group, much like the carboxylic acid, that is soluble in water, and long tails that can dissolve in non-polar environments, like fats. This way, the soap molecule can surround a blob of non-polar lipid and help to pull it off of a dirty pan as water is washed over it, because the head group interacts with the polar water.
We've talked about lipids as a storage mechanism, but they are also extremely important because they make up part of our cell membranes. The lipids that make up our cell membranes are phospholipids, and they're awfully similar to soap. They consist of glycerol attached to a phosphate group and to two fatty acids, or carboxylic acids. Basically, the phosphate group is very polar, and can dissolve in water, while the carboxylic acid chains like to hang out with each other, because they're non-polar. What we end up with is a lipid bilayer, because the carboxylic acid chains like to hang out with each other, and both the inside and outside of the cell are polar aqueous, or water-containing, environments.
Lipids that make up cell membranes are called phospholipids
Lipids can also serve as chemical messengers, or hormones. These don't look much like the glycerol-based lipids we've seen so far, but they, too, are important, non-polar biological molecules. Most of them are steroids. Steroids get kind of a bad rep, but they're incredibly important. Sex hormones, like estrogen and testosterone, are steroids, but all that means is that they contain a particular system of carbon rings, as you can see here.
To summarize, lipids are biological molecules that are insoluble in water, but are soluble in non-polar solvents. Lipids are incredibly good for storing energy, and contain twice as much energy as the same weight of carbohydrates or proteins, so they are the most energy-dense food.
Most dietary and storage fats are triglycerides, which consist of a glycerol molecule combined with three carboxylic acids. These can be classified in three ways. Triglycerides can be saturated, which is when the fatty acid chains contain no double bonds in their carboxylic acid chains at all. They can also be unsaturated, which is when the triglycerides contain double bonds in their carboxylic acid chains. They can also be trans, which is where the carbons on either side of the double bond are on opposite sides of the double bond.
We've learned that lipids make up part of the cell membrane in the form of phospholipids. We've also learned that lipids can serve as chemical messengers in the form of steroid hormones.
Once you finish this lesson you'll undertand the basic function of lipids and their various forms.