Glycogen: Study with Video Lessons, Practice Problems & Examples

Glycogen is a homopolysaccharide composed of D-glucose units linked primarily by α(1→4) glycosidic bonds, with branching occurring at α(1→6) linkages. It serves as an energy storage molecule in animal cells, featuring more frequent branch points than amylopectin, which is found in plants. Other polysaccharides include cellulose and chitin, both structural and linked by β(1→4) bonds, while starch, consisting of amylose and amylopectin, functions in energy storage with α(1→4) linkages. Understanding these structures and functions is crucial for grasping carbohydrate metabolism and energy storage mechanisms.

Glycogen

In this video, we're going to begin our lesson on the polysaccharide glycogen. And so the polysaccharide glycogen is a homopolysaccharide, which of course we know from our previous lesson videos just means that glycogen is made up of just one single type of repeating sugar unit. And that repeating sugar unit is going to be a D-glucose molecule. Now most of the D-glucose molecules and glycogen structure are going to be covalently linked to each other via these alpha-1,4-glycosidic linkages. And so the alpha configuration of the glycosidic linkage reminds us that glycogen's function is going to be energy storage. And so you might recall that way back in our previous lesson videos when we talked about glycogen phosphorylase, that we said glycogen acts as an energy storage molecule in animal cells. And so the organism here we can say are animals, and this includes our cells. Now in terms of being a branch structure, we can say that glycogen is in fact a branch structure. So we can say yes here. And in fact, there are quite a lot of branches, and each of the branch points is going to be linked via alpha-1,6-glycosidic linkages, just like amylopectin's branch points were linked by alpha-1,6-glycosidic linkages. Now speaking of comparisons to amylopectin, you might notice that this table here resembles the same table from amylopectin quite a lot. And really, the only differences between the two tables are the organism, animals, and then this word over here, lot. Other than that, everything else in this table is practically the same from amylopectin. Recall amylopectin, instead of being found in animals, amylopectin is found in plants. And, other than that, in terms of structure, glycogen has a lot more branching than amylopectin. And so we can say that glycogen's branch points are going to occur on average much more frequently than amylopectin's branch points. And so you might recall from our previous lesson videos that amylopectin's branch points occur every 24 to 30 residues on average. Whereas on average, glycogen's branch points occur every 8 to 12 residues. And so that's much, much more frequently and much more branching. And of course, the branch points are going to occur via alpha-1,6-glycosidic linkages, which is the same as the branch points from amylopectin. And so really what we can say is that glycogen is amylopectin with just a lot more branching. And so if we take a look at our image down below, notice that this image here represents the structure of glycogen. And what you'll note is that all of these red circles that we see down below, including this one up here, represent branch points. And so there are a lot more branch points in glycogen structure than in amylopectin structure. And again, the branch points are going to occur at alpha-1,6-glycosidic linkages, just like this one right here. And so all of these red circles represent alpha-1,6-glycosidic linkages. However, the vast majority of the glucose molecules that we see here, here, and pretty much everywhere else are going to be linked via alpha-1,4-glycosidic linkages. And so what you'll note is over here on the right-hand side, we have another visual to help you distinguish between amylopectin and glycogen. And so, one thing to note is that here what we have is a bunch of glucose molecules. All of these blue balls represent glucose molecules, and so this represents the main chain right here. And notice that there are branches coming off of the main chain. And so this, because it has less branching, represents the structure of amylopectin. Now notice over here on the right, the main chain is right here, and coming off are a lot more branches. And so the branches, because they're occurring more frequently, this is going to represent glycogen's structure. And so you can see glycogen just has a lot more branch points. And so really this here concludes our lesson on glycogen. And as we move forward, we'll be able to get some practice applying these concepts. So I'll see you guys in our next video.

The storage form of carbohydrates in animals is:

• A. Glucose.
• B. Amylopectin.
• C. Lactose.
• D. Glycogen.

In glycogen there are:

• A. Alpha 1-4 glycosidic bonds only.
• B. Alpha 1-5 glycosidic bonds only.
• C. Alpha 1-6 glycosidic bonds only.
• D. Alpha 1-4 and 1-6 glycosidic bonds only.
• E. Alpha 1-4 and beta 1-4 glycosidic bonds.

Glycogen

Alright, so in this video, we're going to review the Polysaccharides that we've covered so far by completing the table down below. And so hopefully, by completing this table just a couple of times, you guys will be familiar with the most important features of each of these polysaccharides. And so in our first row, we have cellulose, which is a homopolysaccharide of D-glucose repeating sugars. And so these D-glucose repeating sugars are going to be covalently linked to each other via beta-1-4-glycosidic linkages. Now although this doesn't apply to every polysaccharide that exists, notice that on our table, all of the glycosidic linkages that have beta configuration are going to have a structural function and so you can think and associate beta with structural here for all of these polysaccharides. And so, of course, we know that cellulose is going to be the primary structure of the plant cell wall. So it's found in plant organisms. And in terms of being branched, cellulose is not branched. And so the next polysaccharide that we have is chitin. And chitin is also a homopolysaccharide, but this time it's actually made up of NAG sugars instead of the glucose sugars. And so recall, NAG just stands for N-acetylglucosamine, and all of these NAG sugars are going to be covalently linked to each other via beta-1-4-glycosidic linkages. And again, because it has a beta configuration, it's going to have a structural function. And we know from our previous lesson videos that chitin makes up the structures of hard exoskeleton shells in specific animals. And those animals include insects and, crustaceans like lobsters or crabs and things of that sort. And in terms of being branched, chitin is also not branched.

Now the next polysaccharide is peptidoglycan and so, of course, hetero means that it's going to be made up of more than one repeating sugar unit. And so notice that it's made of the NAG, which is N-acetylglucosamine just like chitin, but it also contains the NAM sugar, which is N-acetylmuramic acid. And so the NAG and NAM sugar units are going to be covalently linked via beta-1-4-glycosidic linkages. And again, the beta configuration reminds us that it's going to have a structural function. And from our previous lesson videos, we know that peptidoglycan makes up the structures of bacterial cell walls. And so for the organism, we can write bacteria here. And in terms of being branched, peptidoglycan is not branched.

So now down below, what we have is starch and recall from our previous lesson videos that starch has 2 forms, amylose and amylopectin. And so what it helps me remember that amylopectin, the only way that it differs from amylose is, that it's branched, is, the pectin part kind of reminds me of pecking. And so, if it's pecking, then, of course, it's going to be branched because it's going to have a bunch of little pecking spots. And so it's kind of silly but, it helps me. So, hopefully, it'll help you guys too. Now in terms of starch, which applies for amylose and amylopectin, they are both homopolysaccharides of D-glucose residues just like cellulose. However, notice that cellulose has beta-1-4-glycosidic linkages, whereas starch is going to have alpha-1-4-glycosidic linkages. And the alpha configuration here on our chart, is going to be associated with an energy storage function. And so, starch, we know is going to act as an energy storage molecule in plant cells. And, of course, the only difference between amylose and amylopectin, which we already mentioned, is that amylose is not branched just like all of these other ones up above, and amylopectin is actually branched. And the branch points occur at alpha-1-6-glycosidic linkages.

Now last but not least down here what we have is the polysaccharide Glycogen. And notice that glycogen is pretty much exactly the same here as amylopectin, and really the only difference is going to be, here with the plants and then down here with the lots. And so, what you'll note is that glycogen is a homopolysaccharide of D-glucose linked by alpha-1-4-glycosidic linkages. It has an energy storage function, but in animal cells, such as our cells. And glycogen is going to be branched. However, it's going to have a lot more branching than amylopectin. And the branch points are still going to occur at alpha-1-6-glycosidic linkages. And so, really, that's it. Our table is now complete, and there are a couple of patterns here that hopefully will help you guys when studying this chart. And so the first is with the type in terms of being homo or hetero, notice that all of them on this list are homopolysaccharides except for peptidoglycan. It's the only hetero one. And then in terms of the repeating sugar unit, notice that most of our repeating sugar units are going to be D-glucose molecules. And the only 2 that don't have D-glucose molecules are chitin and peptidoglycan. But notice that even with these, they both have NAG molecules, and the only difference here is the NAM. Now notice that, with the blue color coordination, that all of the ones with the blue background have beta-1-4-glycosidic linkages and are therefore structural. Whereas the ones with the yellow background all have alpha-1-4-glycosidic linkages, and they are all energy storage. But something that's really convenient to note here is that they're all 1-4-glycosidic linkages, regardless of their configuration. So, at least we don't need to remember too many other numbers unless it's going to be the branch points, of course, which occur at alpha-1-6. And then, of course, the organisms are going to be helpful, to keep in mind, from our previous lesson videos. And in terms of being branched or not, notice that all of them are not branched. Pretty much all of them are not branched and the only ones that are branched are amylopectin and glycogen. And so again, hopefully, by filling this table out from memory a couple of times, you guys will be able to remember the most important features of these polysaccharides. And so, I will see you guys in our next video.

Here’s what students ask on this topic:

What is glycogen and what is its primary function in animal cells?

Glycogen is a homopolysaccharide composed of D-glucose units linked primarily by α(1→4) glycosidic bonds, with branching occurring at α(1→6) linkages. Its primary function in animal cells is to serve as an energy storage molecule. Glycogen is stored mainly in the liver and muscle tissues, where it can be rapidly mobilized to release glucose when energy is needed. This makes it crucial for maintaining blood sugar levels and providing energy during periods of high demand, such as exercise or fasting.

How does glycogen differ from amylopectin in terms of structure and function?

Glycogen and amylopectin are both branched polysaccharides composed of D-glucose units. However, glycogen has more frequent branch points, occurring every 8-12 residues, compared to amylopectin's 24-30 residues. Both have α(1→4) glycosidic bonds in the main chain and α(1→6) linkages at the branch points. Functionally, glycogen serves as an energy storage molecule in animal cells, while amylopectin is found in plants and also serves as an energy storage molecule. The higher branching in glycogen allows for more rapid mobilization of glucose.

What are the key differences between glycogen and cellulose?

Glycogen and cellulose are both polysaccharides, but they differ significantly in structure and function. Glycogen is a homopolysaccharide of D-glucose with α(1→4) glycosidic bonds and α(1→6) branch points, serving as an energy storage molecule in animals. In contrast, cellulose is also a homopolysaccharide of D-glucose but has β(1→4) glycosidic bonds, making it a structural component in plant cell walls. The β(1→4) linkages in cellulose result in a linear, rigid structure, whereas the α(1→4) and α(1→6) linkages in glycogen create a highly branched, compact form.

Why is glycogen more highly branched than amylopectin?

Glycogen is more highly branched than amylopectin to facilitate rapid mobilization of glucose. The frequent branch points in glycogen, occurring every 8-12 residues, provide multiple sites for enzymatic action, allowing for quicker release of glucose units when energy is needed. This is particularly important in animals, where energy demands can change rapidly. In contrast, amylopectin, with branch points every 24-30 residues, is sufficient for the relatively slower energy needs of plants.

What types of glycosidic linkages are found in glycogen?

Glycogen contains two types of glycosidic linkages: α(1→4) and α(1→6). The α(1→4) glycosidic bonds link the D-glucose units in the main chain, while the α(1→6) glycosidic bonds occur at the branch points. These linkages are crucial for glycogen's structure, allowing it to be highly branched and compact, which facilitates efficient energy storage and rapid mobilization of glucose when needed.