so far we discussed monosaccharides and disaccharides now let's move on and talk about polysaccharides so what exactly is a polysaccharide and what's the purpose of polysaccharides in nature well a polysaccharide is basically a very large carbohydrate that consists of many many individual monosaccharides which are connected by o glycidic bonds and the organisms in nature including our own cell use polysaccharides for one of two purposes they either use polysaccharides as a form of storing energy so we basically store glucose as we'll see in just a moment in the form we call glycogen which is a polysaccharide and whenever we want to form ATP molecules we can break down glycogen into the individual glucose constituents and then we can feed those glucose molecules into the metabolism cycle glycolysis to basically form ATP molecules now certain organisms such as plants as we'll see in just a moment also use polysaccharides to basically form structure give the cell structure and protection now those polysaccharides that consist entirely of the same identical type of monosaccharide these polysaccharides are known as homopolymers and this is what we're going to focus on in this lecture we're going to begin by discussing glycogen which is the major type of polysaccharide that exists in our own cells and other animal cells and then we're going to move on and discuss starch as well as cellulose so let's begin with glycogen so inside our cells we store glucose in the glycogen form and glycogen is essentially a homopolymer it's a polysaccharide that consists of glucose molecules now there are two types of bonds within glycogen we have have the more common Alpha 14 glycosidic Bond and the less common alpha6 glycosidic Bond now we call this an alpha 14 glycosidic Bond because it's a bond that exists between carbon number one on one glucose molecule and carbon number four on the Jason glucose molecule now if we examine the stereochemistry of carbon number one this will have an alpha arrangement of atoms and what that means is we'll have an alpha 14 glycidic Bond so remember the alpha anom means that this Bond points in the opposite direction downward with respect to this Bond here which points upward so this group points up and this Bond here points downward so I just erase the oxygen so let's redraw that oxygen here okay now as a result of the alpha 14 glycosidic Bond these Alpha 14 glyc acidic bonds basically give the glycogen a helical structure so as a result of the alpha 14 glycosidic bonds even though this looks like a linear molecule glycogen is not actually a linear molecule it looks like a helical structure now notice we also have the alpha 16 glycidic Bond so about every 10 or so sugars we're going to have this alpha6 gly acidic Bond and these will be the branching points these will cause branching along that helical structure now we call this an alpha6 glycosidic Bond because it's between this first carbon and this carbon number six on the adjacent sugar molecule and this just like that one is an alpha anomer and so that means we have the alpha6 glycidic bond so glycogen consists of glucose monomers linked via alpha1 14 glycos Bonds in a helical fashion so these alpha1 14 glycidic bonds this one this one this one this one this one this one and so forth they basically create this helical structure and the helical chain has these branching points every 10 or so units as a result of these Alpha one6 glycidic bonds and together this basically gives glycogen a very branched structure now when we want to break down glycogen we can easily break down glycogen because we have the proteins the enzymes that are able to break down glycogen into the individual constituents the glucose molecules and then we can use the glucose molecules in the process of glycolysis and the crep cycle to basically form the ATP molecules the energy molecules which are used by our cells so even though glycogen is not directly used as the energy source we transform glycogen into something that we can use as an energy source namely the high energy adenosine triphosphate molecules now let's move on to starch so if glycogen is the energy storage in animals and humans starch is the energy storage in plants so the most common polysaccharide in plants used for energy storage is starch and unlike glycogen which comes in one form starch actually comes in two forms we have a starch known as amalo and a starch known as amelotin now amalo essentially consist of only one type of bond the alpha4 glycosidic bond so amalo is an unbranched polysaccharide that consists of glucose monosaccharides connected via alpha1 14 glycidic Bond so the only difference between amalo and glycogen is in amalo we don't have these alpha6 glycosidic bonds we only have these alpha1 14 glycosidic bonds so this is our example of amalo now again because of the presence of these Alpha 14 glycosidic bonds the actual structure of starch will essentially be like a helical structure as a result of these Alpha 14 glycosidic Bond so even though in this diagram it looks like it's a linear molecule it's not actually a linear molec Ule because if we redraw these molecules in their chair confirmations we're going to see this helical structure that is formed now the other type of starch molecule is Amal optin and amlopin is essentially almost the same as glycogen because amalan just like glycogen contains the alpha4 glycosidic bonds and the alpha one six glycosidic bonds the only difference between between amalin and glycogen is that in Amal opcin these alpha6 glycosidic bonds are less common in glycogen these appear every 10 or so units but in starch in amalin they appear every 30 or so units so Amal optin is a branched polysaccharide that consists of glucose monomers linked via Alpha 14 linkages and branches connected via alpha1 16 GL acidic bonds and the branches occur every 30 or so units so amalan is just like glycogen except it contains less of these branching points that we find in glycogen now inside our mouth we have the salivary glands and inside our small intestine at least inside our body we have the pancreas that basically release the alpha amalay so both of these glands release Alpha amalay so salivary glands in our mouth and our pancreas produce the alpha amas which are responsible for basically breaking down these bonds and when we ingest the starch the amose and the amop pcan this is the enzyme that is responsible for breaking down these bonds and forming those individual glucose molecules actually we form maltose and then maltose is broken down by malas at the brush border of our small intestine as we discussed in the pre previous lecture so glycogen is the polysaccharide that is used to store energy in animals while star is the polysaccharide that is used to store energy in Plants now let's move on to cellulose cellulose is actually one of the most common types of organic compounds on Earth and cellulose is basically another very common type of polysaccharide that we'll find in plants so cellul unlike starch and glycogen plays a role in structure and we'll see why that's the case it has to do with the type of bonds that exist in cellulose so we saw that in glycogen as well as starch we have these bonds we call the alpha1 14 glycosidic bonds and we said that these alpha1 14 glycosidic bonds create a helical structure and the helical structure is perfect to store it as energy on the other hand in cellulose we have individual monosaccharides of glucose that are connected via beta 14 glycosidic bonds and as a result of these beta 14 glycosidic bonds cellulose doesn't have a helical structure instead it has a linear structure so the beta 14 linkages allows cellulose to form very long straight chain linear fibers as shown in the following diagram so unlike in this case and in this case where the structure actually looks like a helical structure here it actually looks like a long linear straight chain fiber as a result of the confirmation of the beta 14 glycosidic Bond so why is this 14 glycosidic Bond well because the bond is once again between carbon number one of one glucose and the fourth carbon of the Jason glucose but the arrangement the orientation of this first carbon the anomeric carbon is the beta Arrangement that means this Bond points in the same direction up with respect to this Bond and so this is precisely what gives it that long linear structure now as a result of the long linear structure many of these fibers can basically stack on top of one another and they can interact via hydrogen bonds and that can and that will give cellulose of very very strong nature so essentially many of these individual fibers of cellulose can stack on top of one another via hydrogen bonds and that gives it a very high tensile strength and that's exactly why cellulose is optimal it's used for providing structure protection as well as support to plant cells and we'll talk much more about cellulose in future lectures so we see that polysaccharides are used either to store energy and then convert that into ATP molecules as it happens inside our cells or they can be used in the form or they can be used to give the cell structure and protection as the case is with cellulose in plant cells now inside our body we do not have enzymes that can break down these beta 14 glyc acidic Bonds in cellulose but even though we can break down cellulose these types of polysaccharides are very important constituents of our diet because this is what we call dietary fiber cellulose is an insoluble fiber that we can ingest into our body and what it does is it aids in the process of digestion it speeds up the rate at which the food products basically Move Along our small and large intestine what that means is it decreases the likelihood that going to actually ingest toxins into our body