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, containing both hydrophilic heads and hydrophobic tails. Steroids, like cholesterol, feature four fused carbon rings, while waxes provide protection and prevent water loss. Understanding these structures is crucial for grasping cellular functions and lipid metabolism.

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. It makes it a bit tougher to organize the lipids because they are so different in their structure and their function. However, one thing that all lipids have in common is that they are hydrophobic biomolecules, meaning they are insoluble in water and 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 means that they have both hydrophobic groups that are water-fearing and hydrophilic groups that are water-loving within the same molecule. We'll be able to see examples of amphipathic molecules as we move forward in our course. Keep in mind that although they are all hydrophobic, sometimes they can be amphipathic, having hydrophobic and hydrophilic groups.

Lipids are a unique category of molecule compared to other classes of biomolecules, which are carbohydrates, proteins, and nucleic acids. That's because carbohydrates, proteins, and nucleic acids use consistent monomers to build polymers. However, lipids do not have a consistent monomer and so they do not form long chains of polymers like the other biomolecules do. They are distinct from other classes of biomolecules and they don't form long chains like what we saw in our previous videos. There are many different types and classes of lipids, and while we won't talk about all the different types and classes, we will cover some of them. Lipids include fats and oils, phospholipids, steroids, and waxes.

Notice in our image below we show some types of lipids. Here, we have an image of some butter and some vegetable oil, perhaps some olive oil. This represents the fats and oils, which are important for long-term energy storage in animals and plants. We'll discuss more about fats and oils as we move forward in our course. The second type we have here are the phospholipids, which are a major component of cell membranes. We'll discuss more about phospholipids and cell membranes as we move forward in our course. Over here in the corner, we have an interesting-looking structure representing a steroid. Steroids are an important component of plasma membranes as well. Moving forward, we'll talk about cholesterol, an example of a steroid, and how steroids can also function as hormones. Examples include testosterone and estrogen, which are steroids and lipids. Finally, the last type of lipid we'll discuss moving forward are waxes. Waxes serve in the protection and prevention of water loss. An example of a wax is beeswax. We'll talk more about them as we move forward in our course. 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. 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 is 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 that allow them to 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, which we have over here on the far left-hand side. These are saturated fatty acids because notice that the hydrocarbon chain that we have here 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 above. Notice that this hydrocarbon chain here at the very end, it has a carboxylic acid group which is right here. It's basically a carboxyl group. That is why this is a fatty acid, and it's saturated. 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. Once again, saturated fatty acids 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. As their name implies, they 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. 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 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. That's why they call them unsaturated because they're not fully saturated due to the presence of a double bond. 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. Because it has a kink in the chain, the unsaturated fatty acids cannot be tightly packed together, so they're going to remain relatively loose. That loose nature is why they are liquids at room temperature. These oils, such as vegetable oils and olive oil, are examples of unsaturated fatty acids that are liquids at room temperature.

There is one subset of unsaturated fatty acids called trans fatty acids. 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. Trans fats are artificial unsaturated fatty acids. They do not have a kink in their chain despite having a double bond. 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, 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. Again, trans fatty acids are artificially made, and so you'll find them in foods like burgers, french fries, and similar items. That's why we have this image right here. Trans fatty acids usually are associated with being very unhealthy. This is typically the type of food that you would want to try to avoid in your 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, 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. 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.

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 also notice over here in yellow, we have these 3 fatty acids, and so we can go ahead and put 3 fatty acids here. 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. 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.

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. 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 should all 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 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. Phospholipids are what we call amphipathic molecules, and amphipathic, recall, is just a fancy word that means that it contains both hydrophilic or water-loving and hydrophobic or water-fearing groups. Phospholipids are amphipathic molecules with a hydrophilic head and a hydrophobic tail. Hydrophobic tails, I should say.

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 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 little units that we see here 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 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 region, a hydrophilic head. This head is water-loving and interacts with water. The two tails that we see down below, these 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, 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. Notice that it contains fatty acids, which are these yellow regions here. It also has a glycerol molecule, and then it also has a phosphate molecule at the top. This phosphate functional group is why we call them phospholipids. 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 briefly introduce 2 other classes of lipids, the steroids and the waxes. Steroids are a complex class of lipid that look a lot different than the other lipids that we've talked about. That's because steroids are not made up of fatty acids. Instead, steroids are lipids that are made up of 4 fused carbon ring structures. A classic example of a steroid is the molecule cholesterol, which you may have heard of before because cholesterol is a common steroid that's important for the structure of animal cell membranes. If we take a look at our image down below on the left-hand side, notice that this molecule we're showing you is cholesterol, and it is a steroid because it has these 4 fused carbon ring structures that we have numbered 1, 2, 3, and 4. Cholesterol is a common steroid important for the structure in animal cell membranes.

Over here on the right, we're showing you a zoom in of an animal cell membrane. One of the major components of cell membranes, as we learned in our last lesson video, are the phospholipids, which we're showing here. Embedded within the cell membrane, and within these phospholipids, are these cholesterol molecules. You can see the cholesterol in the positions that we see here. We'll talk more about the actual function of cholesterol in the cell membrane later in our course when we focus specifically on the cell membrane. This here concludes our introduction to steroids.

Now the other class of lipid that we're going to briefly talk about are the waxes. Waxes are another class of lipid that can be used for protection purposes and also for the prevention of water loss. If you take a look at the image over here on the right-hand side, what we're showing you here is a structure that is a classic example of a wax, and that's because this structure is beeswax, which is why we're showing you a bee over here. Beeswax is made up of a fatty acid. You can see a fatty acid embedded in here, but it also contains another molecule over here that used to be a long-chain alcohol group. The idea here is that waxes can be used for protection purposes and prevention of water loss, and you need to know that waxes are a class of lipid. Both steroids and waxes are types of lipids, and that's the main takeaway here of this video. This here concludes our introduction to steroids and waxes, and we'll be able to get some practice in our next couple of videos. I'll see you all there.

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.

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, providing structural integrity and facilitating cell signaling. Steroids, such as cholesterol, are involved in cell membrane structure and function as hormones like testosterone and estrogen. Waxes serve protective roles and prevent water loss, as seen in beeswax. Understanding these types and their functions is crucial for comprehending cellular processes and lipid metabolism.

What is the difference between saturated and unsaturated fatty acids?

Saturated fatty acids have no carbon-carbon double bonds, meaning they are fully saturated with hydrogen atoms. This structure allows them to be solid at room temperature, like butter. Unsaturated fatty acids contain one or more carbon-carbon double bonds, creating kinks in their structure. These kinks prevent tight packing, making them liquid at room temperature, like vegetable oil. Trans fatty acids, a type of unsaturated fatty acid, have double bonds but remain linear, often found in processed foods and considered unhealthy.

How are triglycerides formed and what is their role in the body?

Triglycerides are formed by linking three fatty acid chains to a single glycerol molecule through dehydration synthesis reactions, which release water molecules. They are the main form of stored fat in animal cells, providing a significant energy reserve. When energy is needed, triglycerides are broken down through hydrolysis reactions, releasing fatty acids and glycerol for metabolic processes. Understanding triglycerides is essential for studying energy storage and lipid metabolism.

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

Phospholipids are a class of lipids that contain a phosphate group and are amphipathic, meaning they have both hydrophilic (water-loving) heads and hydrophobic (water-fearing) tails. This unique structure allows them to form the bilayer of cell membranes, providing a barrier that separates the cell's interior from its external environment. Phospholipids are crucial for membrane fluidity, cell signaling, and the formation of lipid rafts, which are involved in various cellular processes.

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, they are not made up of fatty acids. A common example is cholesterol, which is vital for maintaining the structure and fluidity of animal cell membranes. Steroids also function as hormones, such as testosterone and estrogen, playing key roles in regulating various physiological processes. Understanding steroids is important for studying cell membrane dynamics and hormone function.