In this video we are going to talk about the type of macromolecules the group called carbohydrates. Alright, so carbohydrates are macromolecules that are pretty much comprised of three different elements, carbon, hydrogen, and oxygen. And usually we find that in a lot of carbohydrates, these elements show up in a certain ratio. So that ratio is one carbon to two hydrogens to one oxygen.
So in essence, the number of carbon and oxygens are roughly equal and there's generally twice as many hydrogens. So a good example would be of a very common carbohydrate we'll see in class called glucose, C6H12O6. Six carbons, six oxygens, and then twice as many hydrogens at 12. Now the carbohydrates can exist at a couple of different levels. The monomers or the building blocks of carbohydrates are called monosaccharides.
And if we look at the prefix and suffix meaning here, it kind of really does a good job of explaining what these are. So mono is a prefix that means one and saccharide is a suffix that means sugar. So monosaccharides are a single sugar.
The polymers can exist in different... Sizes. The two sizes we're going to talk about in class are disaccharides and polysaccharides. So di is a prefix that means two, and again that saccharide is sugar.
So these would be things that are bigger than a monosaccharide, but not much bigger. And then a polysaccharide is something that means, poly means many, so many sugars. So polysaccharides are going to be the largest in size.
We're going to start with the monosaccharides first, and we'll start by talking about their structure. So monosaccharides, and really all of the carbohydrates, are going to have long chains of carbon as sort of the base piece of the structure, and then those oxygens and hydrogens will kind of be sprinkled in in various places. So if we're talking about the structure of these carbon-based molecules, one thing that we can comment on is the length of the carbons.
So a triose, right, that's going to be something that is comprised of three carbons long. Pentose is going to be something that is five carbon structure. And hexose is going to be something that is a six carbon structure.
And if you look, you'll see that this O-S-E is sort of a common suffix. So O-S-E is a suffix that you will see. that is used to represent a carbohydrate. So you'll see it pop up. If you see O-S-C on the end of the name and you don't know what it is, the fact that it's a carbohydrate would probably be a really good guess.
Now these monosaccharides generally contain the functional group, which is called a carbonyl group. A carbonyl group is simply a carbon that is double bonded to an oxygen. And we generally see these somewhere in our monosaccharides.
And their location can be another thing that we use to characterize the monosaccharide. So there's one category of monosaccharides called aldoses, and these are ones where the carbonyl group is on the terminal carbon. So if we look, we can see in each one of these it's been highlighted in a peachish color.
The carbon at the end of the chain is the one that's double bonded to an oxygen. Ketoses are still going to possess a carbonyl group, but the carbonyl group is on a middle carbon in the chain. So again, if you look, they've been highlighted in peach, and it's not one of the carbons on the end, but one of the interior carbons of the chain that has the double bond. Now, down here in this table, you can see, and also right here, you're looking at monosaccharides that appear to be sort of a straight line, or a... linear structure if you will, right?
But many of these can also exist as a ring structure. In the ring structure what happens is that one of the double bonds from the carbonyl group breaks and binds with a carbon at the other end to make a ring instead of a line. And many of these monosac rides can exist both as a linear structure and as a range structure. Not every single one, but a lot of them. kind of one thing to just keep in mind is that most of the monosaccharides when they're in water are going to be in the form of a ring structure.
That's going to be the most energetically favorable form of the monosaccharide when it's dissolved in water. And since we're talking about molecules inside of living organisms, and living organisms are comprised large part of water, we're going to be seeing mainly these carbohydrates in their ring structures. Alright, so function.
Monosaccharides are one of the primary fuel sources that we use to complete cellular reactions. So cellular reactions are the different kinds of work that the cell is going to complete, and that requires an energy source. energy source that we use or where it's stored is they're stored inside of these monosaccharides. They're the most easily accessible stored energy that the cell can can get a hold of. Within these molecules the energy is stored in the many nonpolar covalent bonds.
This is where the majority of the energy is at. If you remember back to when we talked about bonds, nonpolar covalent bonds are the strongest of all of the bonds, meaning they take the greatest amount of energy to break. That also means that they store the greatest amount of energy in them.
So in this molecule, the majority of the usable energy that's going to be converted into what the cell can use is the energy that exists in these many nonpolar covalent bonds that we see. When those bonds... get broken, right, then that allows us to release that energy, and then that puts into a form that the cell can actually use.
All right, let's progress to disaccharides. So remember we said di means two. Structurally, a disaccharide is basically just two monosaccharides that have been bonded together, and the bond that holds them together is called a glycosidic bond.
So we can see over here an example of a disaccharide would be sucrose. It's a type of sugar. And sucrose is made up of two monosaccharides. One of the monosaccharides is glucose, and the other monosaccharide is fructose.
And these can be either ring or linear formations depending on what the disaccharide is. But at the basis of it... A disaccharide is basically two monosaccharides that have been bought together with a glycosidic bond.
Now, if you want to bind two monosaccharides together to make a disaccharide, so if you are increasing, if you will, the length of your polymer, then ultimately you're going to have to remove water, right? Removing water molecules is what binds those two monosaccharides together, those two monomers. So this would be an example of a...
dehydration synthesis. A dehydration synthesis is what's needed to bind the first monomer to the second to make the disaccharide. In terms of function, basically the the end result is that disaccharides are intended to be broken back into monosaccharides and then harvested for fuel, for cellular fuel.
But basically the disaccharide provides a form that might be a little bit easier to transport in some cases. And so some living organisms will transport disaccharides throughout their structure, and then they'll be broken back down into their monosaccharides to act as a fuel source. And then when they're broken down, we're going to need the opposite reaction.
We're going to need hydrolysis to break the disaccharide back into monosaccharides. so that the fuel can be harvested by the cells. All right, that leads us to our polysaccharides, and we're actually going to talk about four major types of polysaccharides, including their structure and their function.
So the four types of polysaccharides are starch, glycogen, cellulose, and chitin. And I actually want to start with where we find, or what types of organisms make these various polysaccharides. Starch is actually...
two different types of molecules. So there's two types of molecules that make up what we call starch, amylose and amylopectin. And this is formed by plants, right?
So you probably are aware of this to some degree based on the things that you eat, like bread and potatoes are known to be really heavy in starch. Other plants produce starch as well. Glycogen is a type of polysaccharide that animals produce.
Cellulose is produced by plants, right? This is acting as a major reinforcement to cell walls, and chitin is produced by animals. Functionally, starch and glycogen have a very similar overall function for the types of organisms that produce it.
These are your storage polysaccharides. These are essentially long chains of monosaccharides, and The function of them is that when we need more energy, we are going to break these long chains, we're going to break the monosaccharides off and then use those as a source of cellular fuel. So these act as entities to store larger quantities of fuel than an individual monosaccharide on its own.
Cellulose and chitin are both similar in that these are structural polysaccharides. We don't intend for these to be used as a fuel source. They're more intended for reinforcement, right?
So again, if you think about cellulose, that is something that is a main piece of cell walls in plants. The cell wall of a plant is what helps give, in part, the plant its really rigid, solid structure and what allows it to help stand upright. Chitin, likewise, is a structural polysaccharide that we find in some animals.
So for instance, you'll find chitin in things like insects and in crustaceans. Think about crustaceans for a minute, like lobsters and crabs. If you've ever eaten that kind of seafood, then you know that there's a shell on the outside that you have to break through to get to the meat that's on the inside.
That shell that you're breaking through is comprised of chitin. Hence, it's really strong. That acts as that organism's skeleton. All right.
So now that we've kind of got where these are found and what their general function is, let's kind of focus a little bit more on the structure, and then we'll tie that back to its function. For starch, glycogen, and cellulose, the building blocks are all exactly the same. It is glucose. Glucose is the building block. that we use to make each one of these structures.
Now, the building block for chitin, it is not glucose. It actually is something else. I'll write the name down. You don't need to memorize this name, but it's acetylaminoglucose.
And I'm going to go to the next slide for a second because I have a picture of it. Acetaminoglucose is obviously not glucose, but if you look at its structure compared to glucose, it looks kind of similar. I mean, one of the main differences is this piece of the molecule.
And so kind of one thing that sets it apart is that it's a molecule that possesses nitrogen, whereas there is no glucose in nitrogen. So you can think of the monomer of chitin as being very similar looking to glucose. but it's not glucose. It looks similar and it's got nitrogen though in addition to some other things. So we have similar types of building blocks.
In the case of starch, glycogen, and cellulose, identical building blocks. And then the building block for chitin is a little bit different. The arrangement of these building blocks and how they're bonded together though are a bit different. Let's start for a minute by comparing. the orientation of the glucose in starch and glycogen relative to the orientation in cellulose.
Look down here for a minute. If you look at these pictures, each one of these little tiny things is a single glucose. If you look at the starch and glycogen pictures relative to the cellulose pictures, you'll notice That one thing that's different is that the glucose does not appear to be facing the same direction in the pictures. Okay, let's go back for a minute.
That's because there's actually two different versions of glucose. There is alpha glucose and there is beta glucose. And really the only difference between alpha glucose and beta glucose has to do with where this hydroxyl group is located.
Okay, the position of that hydroxyl glucose. is what differentiates alpha glucose, which we can use this little symbol right here to signify alpha, and what signifies beta glucose. When you have alpha glucose versus beta glucose, what changes is how the glucose... molecules can be linked together. When you have alpha glucose molecules, you can put them side by side and have them all oriented the same direction.
So if you look back here for a minute, that's what you're seeing in this chain. Every single one of these glucose molecules has its glucose facing the same direction. They might be linked together in slightly different ways between glucose and glycogen, but every single one of the glucose molecules is oriented the same way because these are all alpha glucose and that orientation allows them to face the same direction when they're bonded together. If you look at cellulose, basically it flip-flops. One glucose is facing this direction, and the other glucose is facing the opposite direction.
So the orientation is inverted every other glucose molecule. And that's because these are beta glucose molecules and when you hook beta glucose molecules together they end up being oriented in opposite directions. Now you might be thinking why does it matter?
Like is there a big difference between whether the glucose is faced the same way or not? And actually there is. When the glucose molecules all face the same way versus when they're flip-flopped, what that changes is it changes what kind of molecules you need to break the bonds between the glucose molecules. As we're going to find out later in this module, you need special types of molecules called enzymes to break the bonds between adjacent glucose molecules.
And ultimately... Organisms are going to want to break these bonds when they try to get the energy from them. You have to break them in order to free up the stored energy. So for starch and glycogen, because these are all facing the same way, these all use a similar type of enzyme.
The type of enzyme that would be needed for cellulose to be broken down because the glucoses are facing opposite direction. That would be a completely different type of enzyme needed to break it down. So starch and glycogen, they use similar types of enzymes to break those down. It's not the exact same enzyme, but it's a relatively similar one. And we have those in our body, so we're able to digest them.
As living organisms, we possess the enzymes that are needed to break down starch and glycogen. And again, they're not the exact same enzyme, but they're similar. And we make those enzymes.
So we can break down starch and glucose into the individual building blocks and then use them as a source of energy. Cellulose and chitin, because they are flip-flopped and they use different types of, they're going to need different types of enzymes, we can't digest them. We don't produce the enzymes that we need to break down the bonds when the glucoses are facing opposite directions because it's a different type of bond. And so if you eat cellulose or chitin, you can't use it as a form of energy because you can't break them down into their basic building blocks. So kind of case in point, celery would be a good example of something that you could physically eat that is made up of a ton of cellulose.
There's really not a lot of nutritional value to humans from celery. It does have water in it and maybe some minerals, but for the most part, it's water and a lot of cellulose. And actually, if you eat a bunch of celery, you end up burning more calories than you actually gain. Because even though you eat it, you can't break it down. So you can't get any energy from those molecules if you can't break them into their tiny building blocks.
It's still not a bad thing for you to eat. When we eat cellulose, and we don't generally eat chitin, when we eat cellulose, we say that that kind of acts as a source of fiber for us, which is still good for your digestive health, but doesn't really act as a good source of fuel. On the flip side, starch and glycogen are things that we can eat that do... provide a lot of usable energy. And the reason it provides usable energy is because we can break it down with the enzymes that we have, we can break the bonds and make them into monosaccharides which we can then harvest the fuel from.
Now there are some organisms that can use cellulose as a fuel source. So for instance if you think of things like cows and horses, organisms that we would quote-unquote called ruminants, they live Pretty exclusively off of grass and other plant matter, right? And that plant matter that they're eating is really heavy in cellulose, but not a lot of other types of molecules.
Grass doesn't have tons of starch in it, some, but it's mainly a lot of cellulose. And they're living off of that, and they're pretty big animals, so they're doing pretty good. The thing is, is that some of those organisms that live off of grass, they don't produce the necessary enzymes.
Just like... we don't. But in their stomach they have they house bacteria and those bacterium produce the necessary enzymes. So they end up still having the correct enzymes in their body needed to break down cellulose for as a fuel source.
Their bodies are not the one producing the necessary enzymes. It just so happens that they possess a bacteria that makes those necessary enzymes. So they can use cellulose as a fuel source. But most organisms can't because they don't have the enzymes needed to break the bonds between the glucose molecules. Now, another thing that's different besides whether the glucoses are oriented the same way or not is the overall shape that those glucose molecules end up getting linked together in.
Okay, so if we look at starch, remember, starch has sort of two subcomponents, amylose and amylopectin. Amylois has no branching. Basically it is a linear helix. If you look below, a helix is referring to a corkscrew shape, and you can see that displayed, but it's a sort of straight line comprising that corkscrew. Amyoplectin does not take on, I mean it does have sort of a corkscrew shape still.
but it also has lots of branching. So kind of imagine like this corkscrew shape, but there's some branching to it. And then glycogen does also kind of have a spiral shape in some places, but it's even more heavily branched.
So there's more branching in glycogen than in any of the other molecules. Okay, and you can kind of see that in that little purple image down there. Now, if we look at cellulose and chitin, they are very different than starch and glycogen.
They have no branching whatsoever. They are both linear chains. They're completely straight.
And actually, if you look at them, you can see the straight lines of the glucoses, and then there are bonds between those straight lines that help reinforce the structure. Remember, cellulose and chitin, those are structural polysaccharides that really aren't intended to be used as a source of fuel. They're intended to provide reinforcement for the structure, right, for the organism that they're in. And that's what, and this shape helps with that.
You have these straight lines, but then you have bonds between the chains of glucose that help reinforce it and give it a stronger structure overall. So we can see here that there is definitely a coupling between the function of the molecule and the overall structure of the molecule. The molecules that are used for structural support are long straight chains.
that have bonds between them that help reinforce and provide greater tensile strength. Whereas the structural molecules, starch and glycogen, have helix shapes whose molecules are oriented in a way that they are easily broken apart so they can be accessed for fuel. So remember we said at the beginning, a theme in this class is that form fits function, and we are seeing that here. The structure of these various Polysaccharides fit the function that they serve for the organism that they're in.