In this video, we're going to learn about carbohydrates and specifically polysaccharides. So, we'll cover what polysaccharides are and the structure and functions of starch, glycogen, and cellulose. Let's begin with what polysaccharides are. Polysaccharides are complex carbohydrates and this just means they're made up of many monossaccharides that are linked together with glycosidic bonds. So in this case here it would be this entire molecule that we'd call a polysaccharide. Some common examples of polysaccharides include starch, glycogen and cellulose. Then in terms of useful functions, polysaccharides do a couple of important things for living organisms. They act as energy storage molecules, but they also provide structural support as well. Next, let's go over the structure and function of starch. There are actually two forms of starch, amalos and amalopectin. Both of these are made from alphaglucose monomers joined together by glycosidic bonds. Starting with amalos, its structure is unbranched because of the 14 glycosidic bonds that form between each of the alphaglucose molecules. And a 1/4 glycosidic bond just means that the glycoidic bond forms between carbon 1 and carbon 4. And the angle of these bonds means that the molecule coils into a helical shape, which means it looks a bit like a long spiral. On the other hand, amalopectin is branched and this is because it's got both 14 glycosidic bonds and 16 glycoidic bonds. If we zoom in, you can see this means the bonds are formed between both carbons 1 and four, but also sometimes between carbons 1 and six as well. And this is what gives amalopectin its side branches. The main function of starch is to store glucose in plants. And it has several features that make it ideal for this. Firstly, it's insoluble, so it doesn't affect the water balance of cells, but it's also large, so it won't easily diffuse out of cells either. As we mentioned before, amalo is coiled, making it compact, allowing plants to pack a lot of glucose into a small space. By comparison, amalopectin has a lot of side branches and this makes it easy for enzymes to hydrayze the glycosidic bonds and break off glucose for respiration. Next, let's look at the structure and function of glycogen, which is basically like the animal version of starch. Just like starch, glycogen is also made of alphaglucose monomers. And like amalopect pectin it's branched because the monomers are linked by 1/4 glycosidic bonds and 16 glycosidic bonds. So if you zoom in on glycogen the monomers are joined together a bit like this. The primary function of glycogen is to store glucose in animals. To help it carry out this function, its features are quite similar to starch. It's insoluble and large, but it's even more branched than starches. This extra branching is crucial because animals are more active than plants. So they need to access their energy stores more quickly than plants do. Being more branched provides more ends to the molecule which makes it easier to break off glucose molecules for respiration and then to release that stored energy. So now let's move on to explore the structure and function of cellulose. Unlike starch and glycogen, cellulose is made from betaglucose monomers that form long unbranched chains. So on our diagram here, each of these is a different cellulose chain. The interesting thing about cellulose is that every second betucose monomer is flipped upside down. Looking at a couple of betuc up close, you can see this is because if they were both the same way up, the hydroxal groups on carbon 1 and carbon 4 would be too far apart to react with each other. But if we flip one of these glucose monomers upside down, so we have one inverted betalucose like this, the hydroxal groups on carbon 1 and carbon 4 are now close enough to react and form a 1/4 glycoidic bond. It's also important to note that these straight cellulose chains are all cross- linked by hydrogen bonds as well. And these hydrogen bonds hold the chains together to form a strong bundle called a microfibral. And then several microfibrals group together into a macrofibral. Cellulose's main function is to provide structural support in plant cell walls. Its long unbranched chains provide rigidity whilst the hydrogen bonds and microfibrals collectively provide tensile strength. Then before we finish let's do a quick comparison of starch, glycogen and cellulose in terms of their source. Starch both amaloes and amalopectin is found in plants. Glycogen is found in animals and cellulose is found in plants. Then looking at their monomers, both types of starch and glycogen are made from alphaglucose whereas cellulose is made from betalucose. When it comes to bonds, amalos has only 14 glycosidic bonds whilst amalopectin and glycogen have both 14 and 16 glycoidic bonds. Cellulose has only 1/4 glycosidic bonds but adjacent betalucose molecules are flipped upside down relative to each other. And finally considering their branching amalos is unbranched. Amalopectin and glycogen are branched but cellulose is unbranched with hydrogen bonds forming cross links between its chains. If you haven't heard yet, you can find all of our videos on our website, cognito.org. You'll also find questions, flashcards, exam style questions, and past papers. And we track all of your progress so that you always know what to study next. So, sign up for free by clicking here or browse our playlist here on YouTube.