Polysaccharide: Study with Video Lessons, Practice Problems & Examples

Polysaccharides are complex carbohydrates formed from monosaccharides linked by glycosidic bonds. Key types include starches, cellulose, glycogen, amylose, and amylopectin. Starches, produced by plants, serve as energy stores, while cellulose provides structural support. Glycogen, the animal equivalent of starch, is crucial for energy storage in humans. The digestibility of these polysaccharides varies; alpha linkages (as in amylose and glycogen) are easier to digest than beta linkages (as in cellulose). Understanding these structures is essential for grasping carbohydrate metabolism and energy storage mechanisms.

Polysaccharide

Hey, guys. In this video, I just want to touch on polysaccharides and highlight a few of the most important polysaccharides that you might need to know. Alright, guys. So you know what monosaccharides are by now, and you should know a little bit about disaccharides. But what about what we call polysaccharides if it's more than 2? Well, there are actually a few different categories that we need to be familiar with. They're very straightforward. This is going to be like the easiest page in your packet, but let's just go through it. Trisaccharides. So guys trisaccharides would be 3 monosaccharides linked through O-glycosidic linkages. Amazing. Oligosaccharides, it's some. So it's anywhere from between 4 to 10, like a few but not only 3. It's a little more than 3. And then polysaccharides, anything that is more than 10 monosaccharides in length would solidly be in polysaccharide territory. So when you hear something's a polysaccharide, it is a very long chain. Usually, it's a very long chain of sugars that are all linked together through those, O-glycosidic linkages. Okay. Now guys, that's pretty much where the definitions end, and now what I want to do for the rest of this video is just highlight a few of the most important polysaccharides that you might come across in your biology major or your chemistry major or maybe even in your homework. But this is more just for just like the more you know. Okay. So polysaccharides that are created by plants specifically for use as an energy store, that's very important, it needs to be used for energy, are known as starches. You've heard of the word starch before. So a starch is just the name for a polysaccharide that's made by a plant, not by an animal, by a plant for energy. It's a very specific category. Okay? And, so I'm going to show you guys some examples of that in a second. Second. And polysaccharides can have straight linkages and branched linkages. That's interesting. \(\alpha(1,4)\) linkages. We've seen these before with disaccharides. Remember that \(\alpha(1,4)\) linkages tend to make these nice straight chains so they tend to be straight polysaccharide chains. Whereas \(\alpha(1,6)\) linkages, I'll show you what an \(\alpha(1,6)\) linkage looks like in a second, but it's way more bent up. It's going to make branched chains. So as I'm showing you some examples of common polysaccharides, it's going to be in the context of straight chains and branch chains. And just so you know, as I've already alluded to, the straighter the chain, the harder it is usually to digest. And the reason is because it's going to be very strong. It's going to be more dense. It's going to be more difficult for enzymes to get in there. The more branched it is, usually the easier it is to digest because it's easier for enzymes and fluids to get in between and hydrolyze. Okay. Cool. So that being said, let's look at some common polysaccharides. And by the way guys, I'm going to show you 3 polysaccharides and notice that all I'm looking at here is polysaccharides of D-glucose. D-glucose, D-glucose. So it's just funny that it's crazy how many different types of polysaccharides there could be because all I'm showing you right now is D-glucose. But if I were to look at galactose or whatever other one that you want to look at, you could have different types of polysaccharides for all the other sugars as well. So just keep in mind, this is just a very limited amount of them, but these are some of the most common. So what happens when a plant makes a long polysaccharide made of D-glucose and beta-\(1,4\) linkages. We've talked about these before. Why do we call it beta-\(1,4\)? Because it is going in the same direction as the stereo descriptor, linking the 1 and the 4. So these are beta-\(1,4\) linkages. We know that this is probably going to be strong linkages. Right? Because they're very straight. Does that make sense? Well guys, this is what we call, if you have a beta-\(1,4\) linkage, this is what we call cellulose. Have you heard of cellulose before? Guys, cellulose is like the fiber of plants. Okay? It is like what like cotton is made out of, what stems are made of, what leaves are made of, what wood is made of, what paper is made out of. It's all made out of cellulose. Does that sound very appetizing? Not really. So guys, why do you think that this would be a good structure for wood and plants and fibers and all that kind of stuff? Because they're very strong linkages. So does this count as a starch? Do you think that cellulose counts as a starch? Let's go through the definition. We said, is it a polysaccharide? Yes. Is it made by plants? Yes. Is it designed as an energy store to be digested? No. Because those linkages are so strong that they're going to be very fibrous and very impossible to digest. So this is not, I'm just going to put here, not a starch. Okay. Instead, because it's not an energy store, cellulose is used for structural support. Okay. It's used for structural support, it's used for cell walls, it's used to give rigidity to the structure, It's not eaten by anything. Well, it could be eaten. If you're like a termite or a cockroach, you actually could eat cellulose, but that's off topic. There are some specialized animals and creatures that can eat cardboard but most animals can't, so that's why this is considered not a starch. Cool? Awesome. So let's go on to the next one. So we see is that here we have, once again, D-glucose linked together by what types of linkages? You guys should be able to recognize this by now. These are \(\alpha(1,4)\) linkages. Right? So what happens if you have \(\alpha(1,4)\) linkages and it's a D-glucose polysaccharide? Well, this is going to be called amylose. Okay. And remember guys that these are essentially longer versions of your disaccharides. So remember that D-glucose \(\alpha(1,4)\) linkage would actually be maltose, right, if it was a disaccharide. But if you keep adding, it's not called maltose anymore, it's called amylose. Okay. So question, would amylose count as a starch? What do you guys think? Let's go through the definition. Is it a polysaccharide? Absolutely. Is it made by a plant? Yes. Could this be used as an energy store? Totally because notice that it's made out of \(\alpha\) linkages. What did we say about \(\alpha\) linkages? You can digest those. The beta ones are kind of impossible, but the \(\alpha\) ones are easy to digest. Notice there's more kinks in it. It's not quite as straight. So it's going to be easier to digest. Amylose is one of the major forms of starch. So I'm going to put here major form of starch. There are other forms of starch as well, but this is one form of starch. Cool? Is that making sense? Awesome. So let's go down to another polysaccharide that might be important at some point. So what happens if you take, D-glucose, right, and you form a bunch of \(\alpha(1,4)\) linkages just like amylose. So this is very similar to amylose, \(\alpha(1,4)\) linkages, right? But every the 10 or so, sugars, you throw in an \(\alpha(1,6)\) linkage. And what am I looking at? That's this guy over here. So notice that this is considered an \(\alpha(1,6)\) linkage because this is the carbon. This is the \(\alpha\). Why? Because it's going trans to this stereo descriptor, so it's \(\alpha\). It's at the one position of this carbon, but notice what carbon is it attached to on the other sugar. 1, 2, 3, 4, 5, 6. So this is this is an \(\alpha(1,4)\) along with some \(\alpha(1,6)\) that includes branching. Okay. And guys, it turns out that this is another really important polysaccharide that you might need to know called glycogen. And guys, glycogen is the main energy store of human physiology. Okay? So whenever you eat a little bit too much and you have too much blood sugar going around, and you have like you ate one too many Dunkin Donuts, your body is going to take that sugar, it's going to take that glucose, and lock it up in the form of glycogen. Glycogen can then later be used when you're starving or when you're hungry or when you need energy. It can then be used to to provide energy later, to be broken down. So is glycogen a starch? What do you guys think? Is it a form of starch? Duh. It's definitely not a starch because it's made by animals. So it's the it's kind of like our it's like the animal version of amylose, of starch. We have almost the same thing, but we just add a few of those \(\alpha(1,6)\) linkages and now we call it glycogen, okay? So I'm just going to put here not starch. Okay. Because it's made by animals and starch can only be made by plants. So this would just be, this isn't. So this fills the other criteria. It's a polysaccharide. It is an energy store but it's not called a starch because that would it's made by animals. Cool? And guys, lastly, amylopectin is like the plant version of glycogen, except instead of having branches every 10, monosaccharides, it has branches more like every 30 monosaccharides. Okay? Isn't it crazy how plants and animals are so different from each other, right? But we have these structures that are so similar in function. For plants, they have amylose which is a starch, amylopectin. Do you think amylopectin is also a starch? Totally, Because it is made by plants. It's a polysaccharide and it's used as an energy store, right? It's like glycogen. It's used as an energy store. It's got \(\alpha\) linkages that you can digest. And then humans and all animal physiology, we have very similar structures except that we decided to put our \(\alpha(1,6)\) linkages every 10 sugars, which is this minor difference, but it means that we use glycogen and plants use amylopectin and amylose. Cool? Awesome guys. So I hope that this video helped polysaccharides come alive a little for you a little bit more. And let's go ahead and move on.

Here’s what students ask on this topic:

What are polysaccharides and how are they formed?

Polysaccharides are complex carbohydrates composed of long chains of monosaccharides linked by glycosidic bonds. They are formed through a series of condensation reactions where monosaccharides, such as glucose, are joined together by the removal of water molecules. The glycosidic bonds can be either alpha or beta, influencing the structure and digestibility of the polysaccharide. Examples include starch, cellulose, and glycogen. Starches are energy stores in plants, cellulose provides structural support, and glycogen serves as an energy reserve in animals.

What is the difference between alpha and beta linkages in polysaccharides?

Alpha and beta linkages refer to the orientation of the glycosidic bonds between monosaccharides in polysaccharides. Alpha linkages (α) occur when the OH group on the carbon-1 of one sugar is below the plane of the ring, making the bond easier to digest. Beta linkages (β) occur when the OH group is above the plane, making the bond more difficult to digest. For example, amylose and glycogen have alpha linkages, making them easier to digest, while cellulose has beta linkages, making it more rigid and harder to break down.

What are the main functions of polysaccharides in plants and animals?

In plants, polysaccharides primarily serve as energy stores and structural components. Starches like amylose and amylopectin store energy, while cellulose provides structural support in cell walls. In animals, polysaccharides like glycogen serve as energy reserves. Glycogen is stored in the liver and muscles and can be broken down into glucose when energy is needed. The structural polysaccharides in animals are less common, but chitin in the exoskeletons of arthropods is an example.

How does the structure of cellulose differ from that of starch?

Cellulose and starch are both polysaccharides composed of glucose units, but they differ in their glycosidic linkages and structure. Cellulose has β(1→4) linkages, forming straight, rigid chains that provide structural support in plant cell walls. These β linkages make cellulose difficult to digest. Starch, on the other hand, consists of α(1→4) linkages in amylose and both α(1→4) and α(1→6) linkages in amylopectin, resulting in a more branched and flexible structure that is easier to digest and serves as an energy store in plants.

Why is glycogen considered the animal equivalent of starch?

Glycogen is considered the animal equivalent of starch because it serves a similar function in energy storage. Like starch, glycogen is a polysaccharide composed of glucose units. However, glycogen is more highly branched, with α(1→4) linkages and α(1→6) linkages occurring approximately every 10 glucose units. This branching allows for rapid release of glucose when energy is needed. Glycogen is stored in the liver and muscles, providing a readily available energy source for animals, similar to how starch functions in plants.