Polymers of Carbohydrates – TRANSCRIPT

Alright, so there’s going to be a pretty big table. It’s going to condense so much information from the textbooks. This is everything you need to know about the polymers of carbohydrates: in the textbook, probably pages and pages. So buckle up, sit in, and get ready.

I’m going to draw a table. We’ll get it sped up, and then we can start filling it in.

Okay, let’s start off with the titles. I’ve left a bit of a funky gap here, but there is a reason for that and again, I’m going left to right in a specific order. So first of all, we’ve got cellulose. Then these two are starch, but starch is actually a mixture of two different things. So I’m going to put starch in at the top. And the two compounds that starch is actually made of are amylose and amylopectin. Again, we’ve got ‘ose’ here: Any O’s or any saccharide is a carbohydrate. So these are all polysaccharides. And then finally, we have glycogen. Basically, amylopectin and glycogen are more or less identical apart from amylopectin is found in plants and glycogen is found in animals, and there’s a bit more branching going on in glycogen, and we’ll touch upon that in a moment. Okay, well, let’s start with their function.

Cellulose is a bit of the black sheep here. It doesn’t really fit the rest of the pattern and we’ll see why, so this provides rigidity and strength to plant cell walls. The rest of these, all three of them, store glucose for respiration. So amylose and amylopectin can do that in plants and glycogen does that in animals.

Okay, so, basic function done. Next row. So what are they made of? Well, again cellulose is a bit of the black sheep, here. This is beta glucose and it’s the only time you’re really going to come across beta glucose is for cellulose. And beta is sometimes abbreviated as the symbol (β). The rest of them are going to be Alpha, and if we’re just talking about glucose without saying Alpha or Beta, it’s nearly always alpha glucose. And the Greek letter here looks a bit like that. It’s a really good revision activity. Obviously, if you’re learning this, you’re not going to know it. If you are coming back to this as revision, can you just print the template note for this page and just fill it in? There should be content knowledge that should be in our brains by the time we do the exam in an ideal world.

Alright, okay, so because of this type of glucose this is going to control its structure and a very common question is linking structure to function. That’s typically, cellulose and starch is the classic, but they can ask it any way around.

We also need to look at the type of bonding that’s going on in here. So I touched upon this a little bit in some of the introduction to carbohydrates videos. The carbon atoms in glucose, we can number them. We know that it’s got six carbon atoms and we can number them starting from the oxygen going around clockwise. One, two, three, four, five, and then six being the little one on the side chain. So when I say this has 1-4 glycosidic bonds, that means it connects carbon atom 1 on one molecule to carbon 4 on the other molecule. In other words, it goes basically in a straight line and as I draw the diagram, that will be much more obvious. In amylose, we also have 1-4 glycosidic bonds. And in these two, we have two types of bonding which basically means that we’re forming a branch or a fork and therefore we have a branched molecule. So we have 1-4, all these have 1-4, and these ones also have 1-6 glycosidic bonds. Same for glycogen, as I said, these two are going to be basically the same. You can write this as 1,4 if you prefer or if you see that and you’re not sure what it means, it’s the same.

Okay, well, this is the majority of the information and we’ve got two other sections, really, we’ve got a diagram and we’ve got the properties linking the structure to its function. So I’m going to divide this up into two boxes. I think I might do the diagram next just so that you’ve got a bit more visualisation to add context.

Let’s start with cellulose and I’m going to draw some of that with beta glucoses. So, I’m generally going to summarise our single glucose molecule like this. We have our oxygen and the extra little side chain. So that’s one glucose monomer. Now, the difference in cellulose is that every other monomer is inverted so we can see we’ve got the oxygen on the top, oxygen on the bottom and then it’s going to just keep repeating like that: oxygen on the top, oxygen on the bottom. And basically, each one of these bond angles is not perfectly straight. There’s a bit of a kink, which means it kinks one way and then it kinks the other way. It kinks one way and it kinks the other way. So, because each of these is going in sort of opposite directions each time, we end up with this straight line. And so we end up with this straight molecule.

And we call this chain, which I’m now going to just simplify as a straight line. We call that the cellulose molecule, and we actually form layers of these cellulose molecules. And between the layers, we get hydrogen bonding. You don’t need to know specifically what’s doing it, but it’s the OH groups: They’ve got sort of polarity, little delta charges, the same as water, and that allows us to form hydrogen bonds. So basically, this is the molecular structure. And if I sort of do this as a zoomed-in version, then if we were to zoom in on that little cellulose molecule, that’s what we’d see at the molecular level.

So let’s label some of this up. Obviously, it’s beta glucose. These would be the cellulose molecules, which we could either label as these ones up here, and that’s signifying the same as this, here. We also have the name for the sort of bundle of a bunch of cellulose molecules together, and that’s a microfibril. And then we obviously have the hydrogen bonds, which hold cellulose molecules together to form a microfibril. Technically, the microfibrils bundle up into macrofibrils, but we don’t need to know that. Microfibril is the term that we need for the bundle of cellulose molecules.

Okay, great. So that is cellulose. Let’s move on to amylose, as we move across. Well, I’m going to start with another simplified structure of glucose. But this time, the oxygen atoms, they’re all the same orientation, the same way up, which means we have this little kink here and we have a little angle going on here. And then the next one joins in the same plane, and we have another little angle, and all these little angles kind of add up.

And you can see that we’re now ending up sort of going around in circles. So if I simplify this, we are simply going to form a giant helix or a coil. But because we’ve only got 1-4, so when I say carbon atoms 1, we start the oxygen. This is carbon atom 1, 2, 3, 4, 5, and 6. Or we can have 1, 2, 3, 4, 5, and 6. So we can see here that it’s carbon atom number 1 of this molecule and carbon atom 4 of this molecule. So we’ve got a 1-4 bond, but only 1-4 bonds, which means we’re going basically in a single chain. Now we’re going to do exactly the same over here for our amylopectin, and we’re gonna try and replicate what I’ve drawn without being too different over here because all I’m drawing right now is exactly the same.

But if we now count, let’s say this atom here we’ve got 1, 2, 3, 4. So that’s our 1-4. 5, 6 is this extra little branch on the top here. So, if we can form a 1-6 bond as well, then we can put a branch in, up here. And then this one is obviously going to form a little bit of an angle. So we’re going to kink off over here. Something like that. And so we’re gonna end up with again this sort of macro spiral, and we’re gonna have maybe a spiral coming off it, and so that is our macrostructure of amylopectin.

And the only difference between amylopectin and glycogen is that the glycogen is more branched. So if I’m just gonna draw the simplified version here, I think you get the picture here with the general. So this spiral is basically exactly the same as this spiral, except we’re going to add some extra branching onto it, and we’re going to have more branching in glycogen. And this is obviously a diagrammatic simplification. I’m sure if you look at it in a crazy atomic structure, it’s a little bit different to this, but you get the idea. We’ve got more branching going on here than we do for amylopectin.

Right, the final few bits of details, we’re gonna say, well, we’ve got 1-4 branching only, so that’s one connecting to one other glucose in one place. So this is unbranched. It’s not possible to have a branch because they’ve only got 1-4 bonding.

It’s also a straight chain because we’ve got that as a bond angle in one direction, bond angle in the other direction, and because the oxygen atoms or because the monomers are inverted, so it is a straight chain or straight chained. I suppose we could say, cellulose is a long polymer, long polysaccharide.

Insoluble, I mean some mark schemes accept this one and some don’t. It’s stupid for me for them to not accept it because if cellulose was soluble then plants would dissolve in the rain. They have a high tensile strength, and it’s better to say that than just to say strong.

They are flexible, so they provide rigidity, but they are flexible. So it’s a bit of a weird thing there, but trust me, this is all based on the mark schemes. So these are the words to use to describe it. And it’s relatively inert or unreactive.

Okay, let’s move on with, well, we’ve got 1-4 bonding. Is it branched or unbranched? It is unbranched. But it is coiled because of the structure and the way that it bonds. This coiling means that it’s compact, which means you can store lots of glucose in a small space. So, sometimes you can say coiled and compact. I’m gonna add compact as its own point, and I’m going to add a detail there to say storage of glucose or ‘stores lots of glucose in a small space’ is what I’d like to write, but I’m going to be quite tight on space.

It is also a large molecule. You could probably get away with saying cellulose…cellulose because it forms a straight line ‘long’ is more appropriate and because amylose forms a more complex structure, large is a better term and the reason why large is important: it can’t cross the cell membrane by simple diffusion. And it is also insoluble. And this is really important because glucose (single molecules of glucose) is soluble. And so if it’s soluble that affects the water potential it would make a cell that was storing lots of starch or lots of glycogen have a very low water potential if it was all stored as single glucose molecules. That would mean loads of water would rush in by osmosis and that would cause the cell problems especially if it’s an animal cell such as with glycogen: if we stored glucose in the form of glucose in animals, then it would basically cause the cells to burst which is obviously not ideal. So this is important because it doesn’t affect the water potential. So it has no effect on osmosis.

Okay, so amylopectin we’ve got the 1-4 and the 1-6, so we’re forming this branch here. So we have a branched molecule. Now the importance of this…if we think about an enzyme coming in and we want to convert our stored glucose into a single molecule of glucose for respiration…then we need to basically break one off from the ends. And that’s more or less how it happens, the enzyme comes in and chops off the end one. So the more ends that we have, the faster we can break down this molecule. If this was a thousand molecules long and we can really only access the two on the end, well that’s only so useful to us, whereas if we’ve got hundreds of ends, because we’ve got lots of branches, then we can access the glucose more quickly. So, more ends for rapid hydrolysis.

It is also compact. It is also large. And it is also insoluble. So, each of these reasons would still apply: so it’s effective/ It’s good for storage because it’s compact; it’s large: It can’t cross the cell membrane by simple diffusion; and it’s insoluble which means it doesn’t affect the water potential of the cell.

And the only difference I’m going to put here between amylopectin and glycogen is that…it I’m going to say that it is very branched. Therefore…there is more/ all of these have 1-4 bonds which makes up the the basic polysaccharide and the 1-6 is the branching. So we’ve got more 1-6 bonding going on in glycogen than we do in amylopectin. Which means it can be hydrolyzed even faster, which means we can access the glucose more quickly and that’s because animals have a faster rate of metabolism: They need to access their glucose because they move around, they need to run away from things. Whereas plants can respond in…generally, they don’t have such a rush when they need their glucose. They can take their own sweet time about it.

It is still compact, it is still a large molecule and it’s still an insoluble molecule. This would be a particular problem for animals because if it was soluble, it would have a massive effect on osmosis: you’d get very very negative water potential – loads of water would rush into the cell and, because animal cells don’t have a plant cell wall, those cells would burst and that term would be osmotic lysis, (but, you don’t really need to go into that in this detail here). So let’s just do a quick summary.

Cellulose: the black sheep of the group. It provides rigidity and strength as plant cell walls. It’s beta glucose: The only one you’re going to come across for beta glucose. It’s unbranched, straight chained, it’s long, insoluble, has a high tensile strength, and it’s flexible and unreactive. Its structure is the beta glucose molecules are inverted or you could say flipped: alternate monosaccharides are in opposite orientations. You can choose whichever language you like, there, and the examples will accept all of them. Basically the oxygen atom on the top, oxygen atom on the bottom. We form the cellulose molecules which are these straight chains that are joined by hydrogen bonds and a few of them joined together forms a microfibril.

Starch is amylose and amylopectin. All these are all alpha glucose and all for storing glucose for respiration: Glycogen in animals, amylopectin and amylose in plants. We have branching depending upon the bonding. They’re compact, they’re large and insoluble and this is their rough structure and a really common question to make sure that you practise is linking this or connecting the structure to the function, which is basically these answers here…which is a great summary of the polymers of carbohydrates. You are most welcome!