Lipids - Online Tutor, Practice Problems & Exam Prep

Lipids are diverse hydrophobic biomolecules, including fats, oils, phospholipids, steroids, and waxes. Fatty acids, the building blocks of lipids, can be saturated (solid at room temperature) or unsaturated (liquid at room temperature). Triglycerides consist of three fatty acids linked to glycerol via dehydration synthesis. Phospholipids, essential for cell membranes, are amphipathic, having hydrophilic heads and hydrophobic tails. Steroids, like cholesterol, feature four fused carbon rings and play critical roles in membrane structure. Waxes provide protection and prevent water loss.

Lipids

In this video, we're going to begin our lesson on lipids. Lipids are a very interesting class of molecule because they're highly diverse in both their structure and their function. This diversity makes it tougher to organize the lipids because they differ so much in structure and function. However, one thing that all lipids have in common is that they are hydrophobic biomolecules, which means they are insoluble in water. They do not mix well with water because they are hydrophobic or water-fearing. Although all lipids are hydrophobic, sometimes lipids can also be amphipathic. Amphipathic means they have both hydrophobic groups that are water-fearing and hydrophilic groups that are water-loving within the same molecule. We'll see examples of amphipathic molecules as we move forward in our course. It's important to keep in mind that although they are all hydrophobic, sometimes they can be amphipathic, having hydrophobic and hydrophilic groups.

Lipids are also a unique category of molecule because they differ from other classes of biomolecules, which are carbohydrates, proteins, and nucleic acids. These other biomolecules use consistent monomers to build polymers, but lipids do not have a consistent monomer and do not form long chains of polymers like the other biomolecules. They do not form long chains like we saw in our previous videos.

There are many different types and classes of lipids, but we are only going to discuss some of them. Lipids include fats and oils, phospholipids, steroids, and waxes. Below in our image, we are displaying some types of lipids. Notice that we have a picture of some butter and some vegetable oil, perhaps some olive oil. Fats and oils are important for long-term energy storage in animals and plants. We'll talk more about fats and oils as we move forward in our course. Next, phospholipids, represented in the image as individual structures, are a major component of cell membranes. We'll discuss phospholipids and cell membranes more as we progress in our course. Over in the corner, we have an interesting-looking structure representing a steroid. Steroids are vital components of plasma membranes and function as hormones. For example, testosterone and estrogen are steroids and lipids. We'll explore more about these hormones as we continue. Lastly, waxes, such as beeswax, serve in the protection and prevention of water loss. We'll delve deeper into waxes as our course advances.

This concludes our introduction to lipids and some of the different types of lipids. I'll see you all in our next video.

Fatty Acids

In this video, we're going to introduce fatty acids. Fatty acids are defined as hydrocarbon chains of varying length with a carboxylic acid functional group at the very end. We'll be able to see some examples of fatty acids down below when we get to our image. Now, there are two main types of fatty acids that you all should know. The first main type are the saturated fatty acids, which, as their name implies, are fully saturated with hydrogen atoms. This is because they only contain carbon-to-carbon single bonds. When they contain only carbon-to-carbon single bonds, that means they can be fully saturated with hydrogens. Saturated fatty acids tend to be solids at room temperature. Let's take a look at our example down below at the saturated fatty acids on the far left-hand side. These are saturated fatty acids, because notice that the hydrocarbon chain we have here, made up only of hydrogens and carbon atoms, is fully saturated with hydrogens. They have the maximum amount of hydrogens on them because the carbon-carbon bonds only contain single bonds, as mentioned above. Notice that this hydrocarbon chain at the very end has a carboxylic acid group, which is a carboxyl group, and that is why this is a fatty acid, and it's saturated once again because the hydrocarbon chain is fully saturated with hydrogens. Over here, this is another way to represent the hydrocarbon chain where each of the corners, each of these points, represents a carbon atom, and notice that the hydrogen atoms aren't being shown. It's just another representation of the same exact molecule. That's why we have the equal sign between them. Saturated fatty acids, they tend to be solids at room temperature, just like this butter up here. Butter, of course, is solid at room temperature, and so that is a source of saturated fatty acids.

The second main type of fatty acids that you all should know are the unsaturated fatty acids. The unsaturated fatty acids, as their name implies, are not fully saturated with hydrogens. The reason they're not fully saturated with hydrogens is that they have the presence of at least one carbon-to-carbon double bond. The double bonds in the hydrocarbon chain create a bend or a kink in the chain, and the kink in the chain ultimately makes these unsaturated fatty acids liquids at room temperature instead of being solids. When we take a look at our image down below in the middle, notice that what we're showing you is an example of an unsaturated fatty acid, and notice that the hydrocarbon chain we have here is not fully saturated with hydrogens because there is a carbon-to-carbon double bond. They are missing some hydrogens here at these positions, and that's why they call them unsaturated because they're not fully saturated due to the presence of a double bond. The double bond also creates a bend in the chain. It's not straight or linear like the saturated fatty acids are. It creates a bend or a kink in the chain, and because it has a kink in the chain, the unsaturated fatty acids cannot be tightly packed together, so they remain relatively loose. That loose nature is why they are liquids at room temperature. These oils, vegetable oils, and olive oil, and things like that, are examples of unsaturated fatty acids that are liquids at room temperature. One subset of unsaturated fatty acids that are called trans fatty acids. You may have heard of trans fatty acids or trans fats before, and they are a subtype of unsaturated fatty acids. That's why we have them grouped in the same grouping as unsaturated fatty acids. Trans fats are artificial unsaturated fatty acids. Instead of having a kink, the trans fats do not have a kink in their chain. They have a double bond, but the double bond does not make them kinked, so they remain linear. Let's take a look at the trans fatty acid, which we have down below right here. Notice that the trans fatty acid does have a double bond in its chain. However, the double bond that's in the chain of the trans fatty acid does not create a kink in the chain or a bend in the chain like other unsaturated fatty acids. Trans fatty acids specifically are going to have straight chains despite having a double bond in them. Trans fatty acids, again, are artificially made, and so you'll find them in foods like burgers and french fries and things of that nature. That's why we have this image right here. Trans fatty acids usually are associated with being very unhealthy, and so this is typically the type of food that you would want to try to avoid in your typical everyday diet. This here concludes our introduction to fatty acids and the difference between saturated and unsaturated fatty acids. We'll be able to get some practice moving forward in our course. I'll see you all in our next video.

Which type of fatty acids only contain Carbon-Carbon single bonds?

a) Unsaturated.

b) Saturated.

c) Trans fats.

d) Steroids.

• A. Unsaturated.
• B. Saturated.
• C. Trans fats.
• D. Steroids.

Triglycerides

So now that we know what fatty acids are, in this video, we're going to talk about how fatty acids can be used to build another type of lipid called Triglycerides. And so Triglycerides, as their name implies with the tri prefix here, these are lipids with 3 fatty acid chains. And so the tri- is a prefix that means 3, and that's why they have 3 fatty acid chains. Now these 3 fatty acid chains in a triglyceride are going to be covalently linked to a single glycerol molecule. Now fatty acids are going to be linked to the glycerol molecule via dehydration synthesis reactions. And so we know that dehydration synthesis reactions are used to build up a molecule. So let's take a look at our image down below to get a better understanding of this triglyceride. So notice over here on the far left what we're showing you is a molecule in purple that represents the glycerol molecule, the single glycerol molecule we were discussing up above here. And then also notice over here in yellow, we have these 3 fatty acids, and so we can go ahead and put 3 fatty acids here. And so notice that the fatty acids are all separate from one another here in this image and the glycerol is also separate. And so if we want to combine these separate units together and build a larger structure, then of course we're going to need a dehydration synthesis reaction. And so notice that each of these chains, these fatty acid chains here, can undergo a dehydration synthesis reaction to release 3 water molecules, one for each of the chains that interacts with each of these, components here on the glycerol. And so you can see the OHs here, that react with the Hs here to form water molecules, and there are 3 of them that get formed. And once again, that's why we have the 3 out in front. Now notice over here on the far right, after the dehydration synthesis reactions, we've now combined and connected these, 3 fatty acids covalently to the glycerol molecule. And so, recall that when we have 3 fatty acid chains covalently linked to a single glycerol molecule, we've then created a triglyceride. And so over here on the right, we have the Triglyceride. Now, of course, if we wanted to break down the triglyceride into its separate components like what we see over here, then, of course, we're going to need a hydrolysis reaction. And so the hydrolysis reaction, is going to use water to break apart the Triglyceride into its smaller components. And Triglycerides are going to be a main source of fats. This is how fats are stored in animal cells and the fats that we have are stored as triglycerides. And so this here concludes our introduction to triglycerides. And once again, we'll be able to get some practice applying these concepts as we move forward in our course. So I'll see you all in our next video.

A triglyceride is a form of _______ composed of ___________.

a) Lipid ; fatty acids & Glucose.

b) Lipid ; Fatty acids & Glycerol.

c) Carbohydrate ; Fatty acids only.

d) Lipid ; Ribose.

• A. Lipid ; fatty acids & Glucose.
• B. Lipid ; Fatty acids & Glycerol.
• C. Carbohydrate ; Fatty acids only.
• D. Lipid ; Ribose.

Phospholipids

In this video, we're going to introduce yet another type of lipid called the phospholipid. And so phospholipids, as their name implies, are lipids themselves. In fact, they are a large class of lipids. And also, as their name implies with the "phospho" here, these are a large class of lipids that contain a phosphate group, which you all should be familiar with as one of the functional groups. And so phospholipids are actually a major component of all cell membranes, and we'll be able to see this down below in our image. Now, later in our course, we'll talk more about cell membranes, but in this video, we're focusing specifically on phospholipids. Now phospholipids are what we call amphipathic molecules, and "amphipathic" is just a fancy word that means that it contains both hydrophilic or water-loving and hydrophobic or water-fearing groups. And so phospholipids are amphipathic molecules with a hydrophilic head and hydrophobic tails. And so if we take a look at our image down below at these phospholipids, notice that on the far left we're showing you a cell, and we're zooming in specifically to the cell's membrane, which is this little area that's right here. And when we zoom into the cell's membrane, what we'll see is that the vast majority of the membrane is made up of these little structures, these little units that we see here that we call phospholipids. And so if we zoom into just one of these units and blow it up, it would look like what we see right here. And so it noticed that it really has two regions. It has this head region that's up here, and then it has the two tails that extend down below. And so the head region is a hydrophilic region, a hydrophilic head. So this is the head. And, that means that it is water-loving and interacts with water. Whereas the two tails that we see down below, these are hydrophobic tails, meaning that they are water-fearing. And so, these tails that we see here are trying to get away from water. And so, because we have a single molecule here with both hydrophilic groups and hydrophobic groups. That is why we call this an amphipathic molecule. Now if we were to zoom into the structure of the phospholipid even more, we could see the details of its chemical structure, which we see over here. And so notice that it does contain fatty acids, which are these yellow regions here. It also will have a glycerol molecule, and then it will also have a phosphate molecule at the top. And so this phosphate functional group is why we call them phospholipids. And so, really this here concludes our introduction to phospholipids and we'll be able to talk more about phospholipids later in our course when we're talking about cell membranes. But for now, this here concludes our introduction to phospholipids and we'll be able to get some practice as we move forward in our course. So I'll see you all in our next video.

How do phospholipids interact with water molecules?

a) The polar heads avoid water; the nonpolar tails attract water (because water is polar and opposites attract).

b) Phospholipids do not interact with water because water is polar and lipids are nonpolar.

c) The polar heads interact with water; the nonpolar tails do not.

d) Phospholipids dissolve in water.

• A. The polar heads avoid water; the nonpolar tails attract water (because water is polar and opposites attract).
• B. Phospholipids do not interact with water because water is polar and lipids are nonpolar.
• C. The polar heads interact with water; the nonpolar tails do not.
• D. Phospholipids dissolve in water.

Steroids & Waxes

In this video, we're going to introduce two other classes of lipids, the steroids and waxes. Steroids are a complex class of lipids that look very different from the other lipids that we've talked about so far in our course. This is because steroids are not made of fatty acids. Instead, these steroids are going to be lipids that are made of four fused carbon ring structures. Now, the molecule cholesterol is actually a classic example of a steroid. You may have heard of this molecule cholesterol before because it is a common steroid that is important for the structure of animal cell membranes.

If we take a look at our example image down below, notice that the molecule we're showing you here on the left is cholesterol, which again is a classic example of a steroid. Again, steroids have four fused carbon ring structures, and note that each of the four fused carbon ring structures are numbered in this cholesterol molecule down below 1, 2, 3, and 4. Cholesterol is critical to the structure of animal cell membranes, and on the right side of the image, notice we're showing you an animal cell membrane which we know from our previous lesson videos that cell membranes are predominantly going to be made of phospholipid molecules; so you can see these phospholipids here in this membrane. However, embedded between this phospholipid bilayer, you'll find these cholesterol molecules in animal cell membranes.

Later in our course, we'll talk more about the exact role of cholesterol in the cell membrane but for now, you should just note that steroids are another class of lipid and cholesterol is an example of a steroid. Now, the waxes again are a second class of lipid and they are going to be predominantly used for protection purposes as well as the prevention of water loss.

Notice over here on the right side of our image what we're showing you is a structure of a molecule that represents a wax. This molecule right here is actually a structure that makes up beeswax and that's why we are showing you a bee over here. Notice that this wax molecule is going to be made up of a fatty acid and you can see the fatty acid is right here, and it is also going to be made up of a long chain, a molecule that used to be a long chain alcohol group, and at this end right here, this used to be an alcohol group with a hydroxyl group there. But after combining with this fatty acid, that alcohol group is no longer there. But this here concludes our brief lesson on steroids and waxes, these other two classes of lipids, and we'll be able to get some practice applying these concepts as we move forward. So I'll see you all in our next video.

The molecule shown the figure is a ________.

a) Fatty acid.

b) Wax.

c) Steroid.

d) Triacylglycerol.

e) Phospholipid.

• A. Fatty acid.
• B. Wax.
• C. Steroid.
• D. Triacylglycerol.
• E. Phospholipid.

Choose the correct statement about biological waxes:

a) They contain at least one steroid molecule.

b) They are fatty acids bound to long chain alcohol molecules.

c) They are extremely hydrophilic.

d) They are made of 4 fused carbon ring molecules.

• A. They contain at least one steroid molecule.
• B. They are fatty acids bound to long chain alcohol molecules.
• C. They are extremely hydrophilic.
• D. They are made of 4 fused carbon ring molecules.