Steroids: Study with Video Lessons, Practice Problems & Examples

Steroids, a class of isoprenoid lipids, feature a unique 17-carbon tetracyclic ring structure known as gonane, derived from isoprene units. Cholesterol, the most abundant steroid in animal cell membranes, contains a hydroxyl group and a hydrocarbon side chain. It regulates membrane fluidity, decreasing it at high temperatures and increasing it at low temperatures, ensuring proper permeability. Additionally, cholesterol serves as a precursor for bile acids, such as cholic acid, essential for fat digestion. Understanding these functions is crucial for grasping lipid metabolism and membrane dynamics.

Steroids

In this video, we're going to introduce our 2nd class of isoprenoid lipids, which are the steroids. Now before we get started, let's first revisit our lipid map to make sure we're all on the same page. And of course, we know that we've already explored our fatty acid based lipids in our previous lesson videos, and so in this flowchart, we're just abbreviating it like this. And so currently, we're starting to explore our isoprenes and isoprenoids, and we've already talked about our terpenes and terpenoids. And so in this video, we're going to introduce the steroids. And so steroids, again, are isoprenoid lipids themselves. And what makes them so unique is that they have a core 17 carbon tetracyclic ring structure called Gonane. And so if we take a look at our image down below over here on the left, what you'll notice is that we're starting here with these isoprene units and by combining these isoprene units, we're able to build this molecule here called Gonane. And so this molecule that you see right here, exactly as it is, is called Gonane. And this Gonane molecule is actually found at the core of our steroids. And so this steroid gonane core, as you can see down below in our image, again has 4 rings that are fused together which is why we call it a tetracyclic ring. Tetra meaning 4 and then of course, cyclic ring, referring to the cyclic ring structures. And so, these four rings that are fused together, you can see that there are 3 six-membered rings that you can see, we can call A, B, and C. These are all six-membered rings here, here, and here. And then we also have one five-membered ring as well and we can call this one ring D. And so, what's important to note is that although it might not be obvious that the gonane structure is actually derived from isoprene units, it's important to remember that gonane and all of our steroids are biosynthetically derived from isoprene units and that is what makes these steroids isoprenoids because they're derived from isoprenes, so that's always important to keep in mind. Now, it's also important to note that sterols are very specific types of steroids that have at least 1 hydroxyl group. And so, of course, hydroxyl groups are just OH groups and if you take a look down below at our image over here, notice that simply by adding a hydroxyl group to the gonane core, we're able to get our sterol. So this is our sterol, if you will, because of the hydroxyl group. Now, in our next lesson video, we're going to talk about a very specific type of sterol called cholesterol. But for now, this here concludes our introduction to the steroids and again, we're going to continue to talk more about some specific steroids as we move forward in our course. So I'll see you guys in that video.

Steroids

In this video, we're going to talk about our most abundant steroid, which is cholesterol. And so cholesterol, as we just mentioned, is a lipid steroid. But more specifically, cholesterol is actually a lipid sterol. And, of course, this "o" at the end of the word indicates that there's a hydroxyl group that's present. And so cholesterol has a C₃ hydroxyl group and a C₁₇ hydrocarbon side chain. Now as we mentioned, cholesterol is the most abundant steroid, but specifically in animals, such as our cells, and they're commonly found in animal cell membranes. Now cholesterol is also derived from cyclization of the terpene lipid molecule called squalene. And so if we take a look at our image down below right here, what you'll note is that we've got these isoprene units over here. Notice we have a total of six different isoprene units here. And these six isoprene units can be combined to create this squalene molecule. And so squalene, as we mentioned up above, is this terpene lipid that we see here, and it can actually cyclize itself to generate cholesterol, whose structure we're showing you right here. And so notice that cholesterol has a hydroxyl group at this position, which would be the C₃ position, And then it also has this hydrocarbon chain that we see up here, which is showing up at the C₁₇ position here. Now again, in most cases, it's not going to be important for you guys to know how to number all of the carbon atoms in cholesterol. However, in many cases, it will be helpful to know that position 1 has the hydroxyl group, and position 17 has the hydrocarbon chain. And so, generally, what we'll see moving forward is that cholesterol is going to be found membrane of animal cells. So you can see here that we've got these cholesterol molecules embedded in this membrane.

Now another important thing to note is that cholesterol is actually a precursor molecule for a lot of other molecules, including molecules known as bile acids, such as cholic acid, which really are important for digestion of fats. And so we'll be able to talk more about this idea of bile lipids and the digestion of fats later in our course when we're talking about lipid metabolism. But for now, what I want you guys to know is that cholesterol is a precursor molecule for a lot of molecules including bile acids like cholic acid. And so what you'll notice is we can take this cholesterol molecule right here, and we can convert it into a bile acid like what we have down below. And so this is a bile acid, specifically cholic acid, and cholic acid is one of the most prevalent bile acids. And so again, the main takeaway here is that cholesterol is a precursor molecule for bile acids.

And so this here concludes our introduction to cholesterol, and as we move forward, we'll be able to talk more about cholesterol's function. So I'll see you guys in our next video.

Which of the following structures is a sterol?

A) A.

B) B.

C) C.

D) D.

• A.
• B.
• C.
• D.

Steroids

So in our previous videos, we talked about how cholesterol is commonly found in animal cell membranes. And so in this video, we're going to talk about cholesterol specific membrane functions. Cholesterol will actually regulate an animal cell membrane's fluidity, but it turns out that cholesterol will have multiple regulation effects that depend on the temperature. Cholesterol's regulation effect on animal cell membranes fluidity is dictated by the temperature. By changing the temperature, we can change cholesterol's regulation effect. There are two different regulation effects that you should know. The good thing is that they're complete opposites of each other. Just by knowing one of these regulation effects, you'll automatically be able to know the other one.

The first one here is under conditions of really high temperatures. When the temperatures are really high, membranes actually risk being way too fluid. Too fluid is not always a good thing for cells. Cholesterol’s job under these specific conditions is to help reduce the membrane fluidity to make sure they're not too fluid and to help increase the membrane's rigidness and viscosity. If we take a look at our image down below here, notice we're showing you a membrane here under high temperatures with no cholesterol. Under these high temperatures without any cholesterol, notice that the membrane is simply way too fluid. Notice that these phospholipid molecules are really spaced apart because they're moving very fast at these high temperatures. When the membrane is too fluid like this, it actually means that it's also going to be relatively more permeable. Things that normally cannot cross the membrane are able to when the membranes are too fluid. That's not always a good thing for the cell. Cholesterol, like this sloth right here, is able to slow down these fast-moving phospholipid molecules so that the membrane becomes less fluid, as we mentioned above, and more rigid and viscous. So, when it's less fluid like this, those molecules that were penetrating might not be able to penetrate any longer, thanks to cholesterol's regulation effect.

The second effect here again is the complete opposite of the first effect. At really low temperatures, the membranes are going to risk being too rigid instead of being too fluid. Under these conditions, cholesterol is actually going to help increase the membrane fluidity to make sure that they're not too rigid, and they're also going to help decrease the rigidness and viscosity. Over here on the far left, notice that we're showing you a membrane here at low temperatures with no cholesterol, and notice that all of these phospholipid molecules here are really tightly packed together, and so this membrane is too rigid. When it's too rigid, molecules that used to be able to penetrate might not be able to penetrate anymore because the membrane is too rigid. Cholesterol's job is to get in between these phospholipid molecules to make sure that they're not too rigid and forming too ordered structures. And so here what we have is an image of Jack Nicholson from the movie "The Shining" where he's saying, "Here's cholesterol," and he's getting in between these phospholipid molecules to make sure that they're not too rigid. Really, this is what you need to know about cholesterol's membrane functions, and we'll be able to get some practice in our next few videos. So, I'll see you guys there.

What is the effect of cholesterol in a membrane?

A) Increases membrane fluidity by preventing acyl chain packing.

B) Reduces membrane fluidity acyl chain movement.

D) Both a & b.

E) Neither a or b.

• A. Increases membrane fluidity by preventing acyl chain packing.
• B. Reduces membrane fluidity acyl chain movement.
• C. Both a & b.
• D. Neither a or b.

Here’s what students ask on this topic:

What is the core structure of steroids and how is it derived?

The core structure of steroids is a 17-carbon tetracyclic ring known as gonane. This structure is derived from isoprene units, which are the building blocks of isoprenoid lipids. Gonane consists of four fused rings: three six-membered rings (labeled A, B, and C) and one five-membered ring (labeled D). Despite its complex appearance, gonane is biosynthetically derived from the simpler isoprene units, making steroids a subclass of isoprenoid lipids. This unique structure is fundamental to the biological functions of steroids.

What is cholesterol and what are its main functions in animal cell membranes?

Cholesterol is the most abundant steroid in animal cell membranes and is classified as a lipid sterol due to its hydroxyl group at the C3 position and a hydrocarbon side chain at the C17 position. Cholesterol plays a crucial role in regulating membrane fluidity. At high temperatures, it decreases membrane fluidity by increasing rigidity and viscosity, preventing the membrane from becoming too permeable. Conversely, at low temperatures, cholesterol increases membrane fluidity by preventing the phospholipids from packing too tightly, ensuring the membrane remains permeable enough for essential molecules to pass through.

How does cholesterol act as a precursor for bile acids?

Cholesterol serves as a precursor for bile acids, which are essential for the digestion of fats. One of the most prevalent bile acids derived from cholesterol is cholic acid. The conversion process involves modifying the cholesterol molecule to produce bile acids, which are then secreted into the digestive tract to emulsify fats, aiding in their breakdown and absorption. This function highlights cholesterol's importance beyond its role in membrane structure, contributing significantly to lipid metabolism.

How does cholesterol affect membrane fluidity at different temperatures?

Cholesterol has a dual role in regulating membrane fluidity based on temperature. At high temperatures, it reduces membrane fluidity by increasing rigidity and viscosity, preventing the membrane from becoming too permeable. This is achieved by slowing down the movement of phospholipid molecules. At low temperatures, cholesterol increases membrane fluidity by preventing the phospholipids from packing too tightly, thus decreasing rigidity and viscosity. This ensures that the membrane remains sufficiently permeable for essential molecules to pass through, maintaining proper cellular function.

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

Sterols are a specific type of steroid that contain at least one hydroxyl group. This hydroxyl group differentiates sterols from other steroids, which may not have this functional group. An example of a sterol is cholesterol, which has a hydroxyl group at the C3 position. The presence of the hydroxyl group in sterols contributes to their unique properties and functions, such as their role in maintaining cell membrane structure and fluidity.