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 triglycerides, 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, featuring hydrophilic heads and hydrophobic tails. Steroids, like cholesterol, have a unique structure of four fused carbon rings, while 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 are highly diverse in both their structure and their function. This diversity makes it a bit tougher to organize the lipids because they are so different in their structure and function. However, one thing that all lipids have in common is that they are hydrophobic biomolecules, meaning that they are insoluble in water; they will not mix well with water because they are hydrophobic or water-fearing. Although all lipids are hydrophobic, sometimes lipids can also be what's known as amphipathic.

Amphipathic is just a fancy word that means they have both hydrophobic groups that are water-fearing and hydrophilic groups that are water-loving within the same molecule, and we'll be able to see examples of amphipathic molecules as we move forward in our course. But 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 an interesting category of molecule because they are unlike all of the other classes of biomolecules, which are carbohydrates, proteins, and nucleic acids. Unlike carbohydrates, proteins, and nucleic acids that use consistent monomers to build polymers, lipids do not have a consistent monomer and do not form long chains of polymers like the other biomolecules do.

Unlike other classes of biomolecules, they don't form long chains like what we saw in our previous videos. There are many different types and classes of lipids, and we're not going to talk about all of them, but we are going to discuss some of them. Lipids include fats and oils, phospholipids, steroids, and waxes. Notice in our image below, we're showing you some types of lipids. Here, we have a picture of some butter and perhaps some vegetable oil or olive oil. This represents the fats and oils, which are important for long-term energy storage in animals and plants. This is just a brief introduction to them.

The second type we have here are the phospholipids, each structure depicted is a phospholipid, a major component of cell membranes. We'll discuss phospholipids and cell membranes more as we move forward in our course. Then, in the corner, we have an interesting structure representing a steroid. Steroids are an important component of plasma membranes as well. We'll discuss cholesterol, an example of a steroid, and its function as hormones, like testosterone and estrogen, examples of steroids and lipids.

Lastly, the type of lipid we're going to discuss moving forward are the waxes, which serve in the protection and prevention of water loss. An example of a wax is beeswax. We'll explore this topic more as we course progresses. This concludes our introduction to lipids and some of the different types of lipids, and I'll see you all in our next video.

Fatty Acids

In this video, we're going to introduce fatty acids. And so fatty acids are defined as hydrocarbon chains of varying length with a carboxylic acid functional group at the very end. And so we'll be able to see some examples of fatty acids down below when we get to our image. Now really there are 2 main types of fatty acids that you all should know. And the first main type are the saturated fatty acids, which as their name implies with the saturated, these are fully saturated with hydrogen atoms.

And so this is because they only contain carbon to carbon single bonds. And when they contain only carbon to carbon single bonds, that means that they can be fully saturated with hydrogens. Now, saturated fatty acids tend to be solids at room temperature. And so let's take a look at our example down below at the saturated fatty acids, which we have over here on the far left-hand side. So these are saturated fatty acids, because notice that the hydrocarbon chain that we have here, which is only made up of hydrogens and carbon atoms, they're fully saturated with hydrogens.

They have the maximum amount of hydrogens on them because notice that the carbon-carbon bonds in here only contain single bonds as we mentioned up above. And so, notice that, this hydrocarbon chain here at the very end, it has a carboxylic acid group, which was right here. It's basically a carboxyl group. And so that is why this is a fatty acid, and it's saturated once again because the hydrocarbon chain is fully saturated with hydrogens. Now notice that over here, this is another way to represent the hydrocarbon chain where each of these corners that you see here, 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 in between them. Now once again, saturated fatty acids, they tend to be solids at room temperature just like this butter up here. Butter, is of course solid at room temperature, and so that is a source of saturated fatty acids. Now the second main type of fatty acids that you all should know are the unsaturated fatty acids.

And so the unsaturated fatty acids, as their name implies, are not fully saturated with hydrogens. And the reason that they're not fully saturated with hydrogens is because they have the presence of at least 1 carbon to carbon double bond. And so the double bonds that are in the hydrocarbon chain, they create a bend in the chain 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 at room temperature. And so when we take a look at our image down below over here in the middle, notice what we're showing you is an example of unsaturated fatty acids.

And notice that the hydrocarbon chain that we have here is not fully saturated with hydrogens because there is a carbon to carbon double bond, and so they are missing some hydrogens here at these positions. And so that's why they call them unsaturated because they're not fully saturated due to the presence of a double bond. Now notice that 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 so because it has a kink in the chain, the unsaturated fatty acids, they cannot be tightly packed together, so they're going to remain relatively loose. And that loose nature is why they are liquids at room temperature. So these oils, vegetable oils, and olive oil and things like that, they are examples of unsaturated fatty acids that are liquids at room temperature. Now there is one subset of unsaturated fatty acids that are called trans fatty acids. And so you may have heard of trans fatty acids or trans fats before, and really these are a subtype of unsaturated fatty acids.

That's why we have them grouped in the same grouping as unsaturated fatty acids. So trans fats are artificial unsaturated fatty acids. And instead of having a kink, the trans fats, they actually do not have a kink in their chain. So they have a double bond, but the double bond does not make them kinked, and so they remain linear. So let's take a look at the trans fatty acid, which we have down below right here.

And so notice that the trans fatty acid does have a double bond in its chain. However, notice that 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. Now trans fatty acids, again, they are artificially made, and so you'll find them in foods like burgers and French fries and things of that nature. And so that's why we have this image right here.

And trans fatty acids usually they're associated with being very unhealthy. And so this is typically types of foods that you would want to try to avoid in your typical everyday diet. And so this here concludes our introduction to fatty acids and the difference between saturated and unsaturated fatty acids. And we'll be able to get some practice moving forward in our course. So 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.

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 that 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 the hydrolysis reaction. And so the hydrolysis reaction is of course going to use water to break apart the triglyceride into its smaller components. 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.

Phospholipids

In this video, we're going to introduce yet another type of lipid called the phospholipid. Phospholipids, as their name implies, are lipids themselves. In fact, they are a large class of lipids. 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. Phospholipids are actually a major component of all cell membranes, and we'll be able to see this in our image below.

Now, later in our course, we'll talk more about cell membranes, but in this video, we're focusing specifically on phospholipids. Phospholipids are what we call amphipathic molecules, and "amphipathic," recall, is just a fancy word that means it contains both hydrophilic or water-loving and hydrophobic or water-fearing groups. Phospholipids are amphipathic molecules with a hydrophilic head and hydrophobic tails. If we take a look at our image 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 right here.

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 units that we call phospholipids. If we zoom into just one of these units and blow it up, it would look like what we see right here. Notice 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. The head region is a hydrophilic head, which means that it is water-loving and interacts with water. The two tails that we see down below are hydrophobic tails, meaning that they are water-fearing. These tails are trying to get away from water. Because we have a single molecule here with both hydrophilic groups and hydrophobic groups, that is why we call this an amphipathic molecule.

If we were to zoom into the structure of the phospholipid even more, we could see the details of its chemical structure, which include fatty acids (these yellow regions here), a glycerol molecule, and a phosphate molecule at the top. This phosphate functional group is why we call them phospholipids. This concludes our introduction to phospholipids, and we'll be able to talk more about them later in our course when we're discussing cell membranes. But for now, this 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.

Steroids & Waxes

In this video, we're going to briefly introduce 2 other classes of lipids, the steroids and the waxes. Steroids are a complex class of lipid that look significantly different than the other lipids we've discussed. This difference results from the fact that steroids are not composed of fatty acids. Instead, steroids are lipids made up of 4 fused carbon ring structures. A classic example of a steroid is cholesterol, which you may have heard of before because it is a common steroid important for the structure of animal cell membranes.

If we look at our image down below on the left-hand side, notice this molecule we are showing you is cholesterol, and it is a steroid because it has these 4 fused carbon ring structures, numbered 1, 2, 3, and 4. Cholesterol is a common steroid important for the structure in animal cell membranes. Meanwhile, on the right, we're showing you a zoom-in of an animal cell membrane, where cholesterol molecules are embedded within the phospholipids that are the major components of cell membranes as we learned from our last lesson video.

We will discuss the actual function of cholesterol in cell membranes later in our course when focusing specifically on the cell membrane. This concludes our introduction to steroids. Now, the other class of lipid that we're going to briefly talk about are waxes. Waxes are another class of lipid used for protection and for the prevention of water loss. Looking at the image on the right-hand side, we're showing you a structure that is a classic example of a wax because this structure is beeswax, as indicated by the bee featured here.

Notice that beeswax is made up of a fatty acid but also contains a long-chain alcohol group. Waxes can be used for protection purposes and prevention of water loss. It's essential to know that waxes are a class of lipid. Both steroids and waxes are types of lipids, and this is the main takeaway of this video. This concludes our introduction to steroids and waxes, and we'll be able to get some practice in our next couple of videos.

So I'll see you all there.

The molecule shown the figure is a ________.

• 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.

Here’s what students ask on this topic:

What are the main types of lipids and their functions?

Lipids are a diverse group of hydrophobic biomolecules that include fats, oils, phospholipids, steroids, and waxes. Fats and oils are primarily used for long-term energy storage in animals and plants. Phospholipids are essential components of cell membranes, featuring hydrophilic heads and hydrophobic tails, making them amphipathic. Steroids, such as cholesterol, are important for maintaining the structure of animal cell membranes and can also function as hormones like testosterone and estrogen. Waxes serve protective roles and help prevent water loss, with beeswax being a classic example.

What is the difference between saturated and unsaturated fatty acids?

Saturated fatty acids have hydrocarbon chains fully saturated with hydrogen atoms, containing only single bonds between carbon atoms. This makes them solid at room temperature, like butter. Unsaturated fatty acids have at least one carbon-to-carbon double bond, creating a kink in the chain, which prevents tight packing and makes them liquid at room temperature, like vegetable oils. Trans fatty acids are a type of unsaturated fatty acid with a double bond but no kink, making them linear and often found in unhealthy, artificially produced foods.

How are triglycerides formed and what is their function?

Triglycerides are formed by linking three fatty acid chains to a single glycerol molecule through dehydration synthesis reactions, which release water molecules. The structure consists of three fatty acids covalently bonded to glycerol. Triglycerides are the main form of stored fat in animal cells, providing a significant energy reserve. They are crucial for long-term energy storage and insulation in animals.

What are phospholipids and why are they important for cell membranes?

Phospholipids are a class of lipids that contain a phosphate group. They are amphipathic molecules with hydrophilic heads and hydrophobic tails. This dual nature allows them to form the bilayer structure of cell membranes, with hydrophilic heads facing the aqueous environment and hydrophobic tails facing inward, away from water. This arrangement is essential for the integrity and functionality of cell membranes, enabling selective permeability and fluidity.

What are steroids and how do they differ from other lipids?

Steroids are a class of lipids characterized by a structure of four fused carbon rings, unlike other lipids that are typically composed of fatty acids. Cholesterol is a common steroid that plays a crucial role in maintaining the structure of animal cell membranes. Steroids can also function as hormones, such as testosterone and estrogen, which regulate various physiological processes. Their unique ring structure distinguishes them from other lipid types.