The next group of macromolecules we're going to talk about are the lipids. And if you recall from our overview slide, we said that the lipids are the one group of macromolecules that is not grouped together based on a similar overall structure. There are going to be pieces of some of the lipids that are similar in structure, but that's not their unifying feature like it is for the other groups that have that monomer-polymer structure. Instead, lipids are grouped together based on a like characteristic. And that characteristic is that either a portion or all of the molecule is nonpolar.
And as a result, the molecule is hydrophobic, which is, if you recall, means water-fearing, aka molecules that do not mix well with water. So that's sort of our unifying characteristic is that these are all molecules that don't mix well with water. Now, there are three types of lipids we're going to talk about, three major categories.
We're going to talk about fats, and specifically we're going to talk about three types of fats, saturated fats, unsaturated fats, and trans fats. We're going to talk about sterols and phospholipids. And the slides to come are mainly focused on their structure because right here we have listed a summary of their function. The function of fats is to act as long-term energy storage.
And if you look at sort of this little diagram I have off to the left, this graph, you can see that fats contain more stored energy than proteins or carbohydrates, which can also act as a source of energy for cells. So why? Why can fats store more energy than carbohydrates or proteins? Well, remember where the energy is stored in these molecules. It's between the hydrogen and carbons.
This bond is where a lot of potential energy is at, and when we break that bond, the energy gets released. The reason why fats have so much stored energy is because they have a really large number of carbon-hydrogen bonds, which is where the energy is stored. So there's more potential energy stored there when we break those bonds off to capitalize on.
In addition to acting as long-term energy storage, fats can also provide good insulation, which help with temperature regulation. So essentially the... The insulatory properties of the fats reduce heat loss in organisms. Sterols, the vast majority of these are hormones, which are chemical messengers that enact long-term change in living organisms. Specifically, they're responsible for growth and development of the organisms.
And then we have, finally, phospholipids. Phospholipids are cellular membranes, or you can think of these as cellular barriers. They're going to act as the barrier around the cell that separates one cell from the next, and then also some of the internal features of the cell will be surrounded by these membranes as well. And we're going to hit on this in this chapter, but then in chapter four we're going to come back to phospholipids because chapter four is all about cells, and these are a really important component of our cells.
Now that we have sort of the function outlined here, let's go through and start talking about structure, and let's start with sterols. So although sterols differ in their overall structure, they do have one base piece that's common amongst them, and that is all sterols have four carbon rings fused together. So that's like the base structure of the sterol are these four carbon rings. What the carbon rings are made up of and what's attached to them varies, but they all have that four carbon ring present. We can see that in these three examples.
Remember, we're kind of showing shorthand here When we draw these rings, the apexes represent a carbon atom, and we don't draw in every hydrogen. So if we look at testosterone, for instance, we can see that here's one, two, three, four rings fused together. We can see the same thing in the hormone estradiol, one, two, three, four. And yet another example in cholesterol, one, two, three, four. Sort of an overview shows that similarity in their structure.
Moving on to the fats, we'll get to the three different types of fats and how they differ structurally, but first let's just talk about sort of their continuity, their similarity. So like carbohydrates, fats are made up of primarily three elements, carbon, hydrogen, oxygen, but the ratios of these vary quite a bit different from what we saw with the carbohydrates. There are a lot of carbon-hydrogen bonds, you can see that here because the carbon's illustrated in black and the hydrogen's and white, you can see how many of those bonds exist that hold on to that energy, hence why fats do have such high energy, can store so much energy. So there's two primary pieces to the fat, and just as a heads up, fats are also referred to as triglycerides.
If we look at the structure, one component is what we call the head of the fat, and the head is made up of a glycerol molecule. Glycerol is three carbons, and each of those carbons is attached to a hydroxide group. A hydroxide group is just an oxygen bonded to a hydrogen.
The head is linked to the other major component, which is called the tail of the molecule. The tail is comprised of three fatty acid chains. which kind of going forward, I'm just going to abbreviate fatty acid chains as FAC, so that I'm not taking up a bunch of space writing that over and over again. So three fatty acid chains, and really a fatty acid chain is just a hydrocarbon chain.
Hydrocarbon essentially is in reference to a series of atoms that are just carbons bonded together making up the backbone. and then sort of any remaining bonds are made with hydrogens. And we can see that over here to the right. Let's kind of zoom in on this segment right here and just look at what that would look like in a structural formula. So the black circles represent carbons, right?
So let's draw this over here so we have more room. The top carbon, we'll label it 1, 2, Now we don't need four of them. Let's shrink our box just a little bit.
We're going to show three carbons and what the bonding looks like here. There we go. So one, two, three.
All right, that first carbon is bonded to a carbon above it that we're not going to draw in and bonded to a carbon below it. Now we know that carbon wants to make four bonds. That means that there's still two more bonds that this carbon needs to make, and it's going to make those with hydrogen atoms.
so a hydrogen and a hydrogen. Now this carbon is making four bonds, it's happy. If we look at the next carbon below it, it's kind of the same deal.
It's making a bond with the carbon above it, a bond with the carbon below it, which means there's two additional bonds that it needs, so it's going to bond with two hydrogens. If we look at the third carbon, this is called the terminal carbon because it's at the end of the chain. That terminal carbon is only making one bond with a carbon above it.
There are no carbons below it. So it's going to need to bond with three hydrogens in order to sort of reach its quota of four bonds to fill that valence shell and stabilize it. The thing that's linking these together, so the head that is the glycerol and the three fatty acid chains, is a bond called an ester linkage. And this ester linkage is formed when dehydration synthesis occurs, right? When dehydration synthesis occurs, the hydrogen...
of the hydroxide is removed and bonds with another hydroxide to form water. So this is removed, and when it's removed, that is what links the glycerol to the fatty acid chains. Now, fats are hydrophobic.
They don't mix well with water. And if we look, we can see why using what we learned in Chapter 2. The reason why they're hydrophobic is because of the nonpolar... the nonpolar fatty acid chains. The bonds that form between the atoms that make up the fatty acid chains are all nonpolar covalent bonds. For instance, the bond between hydrogen and carbon.
So here's one. Here's one carbon-hydrogen bond. If we look at an electronegativity chart, we know that carbon has an electronegativity of 2.55 and hydrogen has an electronegativity of 2.20. If we subtract those two, the difference is 0.35, which is less than 0.5, hence it's a nonpolar covalent bond.
Likewise, the only other types of bonds that exist are ones that are between carbon and carbon. And the electronegativity of carbon is 2.55, so if you take 2.55 minus 2.55, the difference is 0. So that's definitely nonpolar. A lot of equal sharing going on there. So the fact that these are all nonpolar covalent bonds means that...
This component of the molecule is completely neutral, and we know from chapter two that things that are neutral don't mix well with water because there's nothing for water to be attracted to. Water has partial charges but can only draw other things that are charged into solution. Since this is all neutral, there's nothing for the water to attach to to pull it into solution. Now with that in mind, let's talk about the different types of fat, starting with the difference between saturated and unsaturated. I'm going to start taking notes on this other slide here in a minute because I'm not going to have enough room.
on that slide. So we're going to kind of create sort of a chart to look at the differences. So here's going to be our saturated fat column and here's going to be our unsaturated fat column. And we'll do a side-by-side comparison. First things first, let's just take what we know about the word saturated.
If I tell you that I have a sponge that is saturated with water, what does that mean? Well, it means that the sponge is holding as much water as it possibly can. And that meaning is the same in terms of fat. We're not talking about water.
Instead, we're talking about the saturation of hydrogen atoms in the fatty acid chain. Okay, that's really what these terms are in reference to. Saturated fats have the maximum number hydrogen atoms in their fatty acid chain whereas unsaturated fats do not have the maximum number. Now how can this be? How can one fat have more hydrogen atoms than another?
Well the difference and what allows this is whether or not there are double bonds. Saturated fats do not have double bonds, so they only have single bonds between the carbons in the fatty acid chain. On the flip side, saturated fats do have double bonds.
So let's go back and look at the picture here for a minute again. Every carbon in a saturated fatty acid chain is making bonds with two hydrogens, except the terminal carbon, which is making three. If we look at unsaturated fats, some of the carbons are not bonded with two hydrogens because they're making a double bond between them.
So if we look at this versus this, here these two carbons are bonded to four hydrogens and here they're only bonded with two because the additional bond they would be making with hydrogen they're using to make a double bond between them. Now this has a consequence with the three-dimensional shape. Because there's only single bonds in a saturated fat, the fatty acid chain, the tails, are straight.
No bends. However, once you start introducing double bonds between carbons, the double bonds cause the fatty acid chains to bend where the double bonds are located. Because of whether or not the chains are straight or bent changes how dense the molecules can be packed together. Fatty acid saturated fats can be more compacted, so they can be more dense. You can fit more molecules in a given area because their fatty acid chains are straight.
so you can pack them closer together. On the flip side, the unsaturated fats cannot be as compacted because of the bends in their fatty acid chains, meaning they are less dense. You can't fit as many molecules in a given area.
The difference in density causes these fats to be at different states at room temperature. Saturated fats are solid at room temperature. Whereas unsaturated fats are liquid at room temperature.
Because remember, the states, one thing that differs between them is their density. For most molecules, generally speaking, they are densest as a solid and then less dense as a liquid. And therefore, since unsaturated fats can't be packed as tightly together, they're liquid at room temp.
An example of some saturated fats, for instance butter, is an example of a saturated fat, whereas a lot of the oils that we find, like for instance in fish, would be examples of unsaturated fats. So the reason why saturated fats are solid at room temperature is because they can be compacted closer together due to their straight chains, whereas the unsaturated fats can't be compacted as tightly because the bent chains keep them further apart. Now let's talk about the third kind, trans fats. And the reason why I've sort of put these on a separate slide is trans fats can be naturally occurring, but the vast majority of them are artificially produced.
And they're produced by a process called hydrogenation. Hydrogenation refers to adding hydrogen atoms to unsaturated fats. And that changes the orientation of the bonds in the fatty acid chains.
Okay, so real quick to understand this, we need to talk about something we haven't talked about yet. In chemistry, there is a word that refers to the arrangement of atoms when a double bond is present between two carbons. Okay, so if we look here for a minute, here are the two differences. We can have something called a cis bond.
In a cis formation... When you have a double bond between a carbon, the two hydrogens that they are bonded to are on the same side as each other, and whatever else they bonded to, like whatever else they're attached to, is on the same side. So this is what a cis formation would look like around a double bond between carbons. In a trans formation, the hydrogens that they are bonded to are on opposite sides and so are whatever else they are bonded to.
Okay, so in a cis formation, they're on the same side. In a trans formation, the hydrogen atoms that they're bonded to are on opposite sides of that double bond. Essentially, what a trans fat is, is it's a changing of the orientation when that double bond is broken and reformed.
So you take an unsaturated fat, naturally occurring unsaturated fats, have cis bonds that form where those hydrogen atoms are on the same side, and then they connect to the chain on the same side. So for instance, it might look like something like this. If we kind of blow up this area right here, it would look like this.
The hydrogen atom that they're still forming with are on the same side, and then where they connect to the rest of the chain, is on the same side as well, creating sort of this bump out like this. And then the bumps that you see going the other way are due to that same cis bond form the other direction. When you add hydrogen atoms via hydrogenation, some of these get broken, and when they reform, they form trans bonds instead of the cis bonds, hence the name trans fat.
So if we were to blow up sort of this area right here, here's what it would look like. you would have the double bond, but the hydrogen atoms are on opposite sides of each other. One minute, let me draw it in the same orientation as our last picture so we don't get confused.
One minute. So for instance, here's how it would look different. One hydrogen would be off this way, one would be this way, and then the direction where it is attached to the rest of the chain would be on opposite sides as well. So it changes, it kind of straightens, so it's partially straightened. Changing the orientation of where the hydrogens are at in the trans fat straightens out the chain a little bit.
It's not completely straight, but it's not as bent as the unsaturated fat that we started with. So here's our unsaturated fat. And here's our trans fat. The trans fat.
is formed when hydrogen atoms are added to the unsaturated fat, breaking some of the bonds, and then some of the bonds, double bonds, are reforming as trans bonds rather than cis bonds, changing the orientation of the side that the hydrogen ions are on, the atoms, and then that changes how bent the chain is. Now, why would someone go through the process of doing this? Why make a trans fat in the first place? Um, commercially speaking, They're sort of desirable from their properties. They tend to have a longer shelf life than some unsaturated fats, so people who market this like that.
They tend to have a really good texture, so the texture that they produce is desirable to consumers. And they also taste really good. And so for those three reasons, the use of these really shot up after their invention. But On the flip side, they have really negative health impacts that we'll talk about kind of in our health connection video here in a minute.
And so recently they've been really diligent about labeling trans fats because they are so detrimental to your health. But a trans fat is a fat whose fatty acid chain has been partially straightened because it's an unsaturated fat that had hydrogen atoms added to it. And that caused the double bonds to break some of them and reform as a trans. formation rather than a cis formation. The last type of lipid that we're going to talk about are phospholipids.
So in terms of phospholipids, if you look at them, you might first think, oh, this looks just like a fat. And there definitely are pieces that look similar. Phospholipids have a head like a fat and they have the tails like the fat.
What's different? Well, the head is still made up of a glycerol. Okay, good deal.
But here's what's different. The glycerol has three carbons, right? In the fats, each one of those carbons was attached to a hydroxide group, which was attached to a fatty acid chain, right? So in the fats, every carbon was attached to a fatty acid chain.
And that's not the case in the phospholipid. In the phospholipid, one of the carbons is attached to a phosphate group. Remember, we talked about a phosphate group. We said it's a phosphorus that's bonded to four oxygens, and then some of those oxygens can be bonded to other things, which is the case here. Now, the tails are still made up of those hydrocarbon chains, so they're still fatty acid chains, right?
But they're only made up of two hydrocarbon chains instead of three. So we don't have three chains here, we only have two, hence we don't call these triglycerides. Triglycerides are in reference to fat because of those three chains.
Phospholipids do have fatty acid chains, but they only have two instead of three because the third carbon in the glycerol is attached to the phosphate group instead. Now another component that's interesting about the phospholipid versus the fats is that they are They are hydrophobic, but they're also hydrophilic, so they're a little bit of each. We have a word for this, we call it amphipathic.
Amphipathic means that one region is water-loving and one region is water-fearing. The portion of the molecule that is hydrophobic is still the fatty acid chains, the tails, and we talked about why. This is nonpolar, so there's nothing for water to be attracted to, it's neutral.
doesn't like water. However, that's not true of the head. The head has a partial negative, has a negative charge off of one of the oxygens. Here's the reason why.
Phosphate groups, when they are phosphate groups on their own, look like this. Let me kind of draw one over here. We saw a picture of one already, but here's kind of an illustration of what they look like.
This is where it would attach to the larger molecule, which we're seeing right here. Now here's what's happened to this phosphate group compared to sort of the base structure. These two oxygens have both lost their hydrogen. The hydrogen has been donated into the water and is gone. The top oxygen bonded to this structure in place of hydrogen, but this Oxygen that lost its hydrogen, it's not replacing it with anything else.
The hydrogen gave up its electron to this oxygen that now has... a negative charge and the hydrogen ion is now floating around in water as a positively charged, just basically a proton floating around. So this oxygen has a negative charge and therefore is attracted to other things that are charged.
So the head is hydrophilic because one of the oxygens gave, well, one of the oxygens lost its hydrogen as a hydrogen ion. The hydrogen ion gave its electron to oxygen, giving it a negative charge. And we know that things that are negatively charged Anything that's charged really can be attracted to water, right? A water molecule has partial positive charges on the hydrogens and a partial negative on the oxygen. So that means that this partial positive on the hydrogens can be attracted and pull into solution the head of the molecule, but the tail doesn't like the water because it's completely nonpolar.
So we say that this molecule is amphipathic. Part hydrophobic, part hydrophilic. A molecule divided.
Now what does that mean? What does the molecule do? If part of it likes water and part of it doesn't, what does it do with that?
Well, the properties of the phospholipid, part of it liking water and part of it not liking water, cause the molecule to make certain formations when in water. One formation that we see is called a micelle. That was what was shown on the previous slide right here, this arrangement.
In a micelle, essentially the head, which is in picture form usually denoted just as a circle, and the tails are usually denoted as squiggly lines. A micelle is basically just a circle of these with the heads facing outwards, and they're compacted closely together. In this formation, everybody's happy. The heads are facing outwards. They like the water, so they want to be touching the water that is surrounding it.
And the tails are facing inward, and they are excluded from water because they're hydrophobic. So that's one formation that makes kind of everyone happy. Another would be something called a liposome.
A liposome is kind of like a myosal, except that there's two layers of these. phospholipids facing each other in a circle. I'm going to draw roughly here and get the idea.
These are the heads. It's a double layer instead of a single layer, but again the tails are facing inwards towards each other so that they are shielded from water. Water would be in this interior space and around the outside.
This is what we see on the boundaries of cells a lot of times. Or we can just talk about these as phospholipid bilayers. Bi means two.
So a phospholipid bilayer is two phospholayers thick. The heads are facing out towards the water. The tails are facing inwards towards each other so they can be excluded from the water.
Because remember the tails are hydrophobic, they don't like water, so they're going to try to orient themselves into a space where water has been excluded and the heads are hydrophilic. They like water, so they're gonna try to be touching the water. And these are different orientations that allow both aspects of the molecule to be happy and be sort of in the state that allows them to either be near or away from water based on the chemical composition of that part of the molecule.