right so the next thing that we actually have to look at is uh polysaccharides now if you look at the definition always remember that saccharites means sugar and poly means many if you remember monosaccharides means one sugar molecule disaccharides means two sugar molecules linked together and polysaccharides by definition is just many repeating sugar units linked together by glycosidic bonds now so let's look at an example with alpha glucose remember alpha glucose is a monosaccharide right the reason why it's a monosaccharide is because it is a one sugar molecule it's just made up of one sugar and always get used to throwing out the ring structure of alpha glucose remember that for alpha glucose in carbon number one the oh is pointing downwards and also carbon number four the oh is also pointing downwards now so that's one alpha glucose but if we take if we have two of a glucose molecules right now as you can see over there if a condensation reaction happens where you remove a water molecule and you link together the two alpha glucose molecules together it forms something called a glycosidic bond now this is known as a maltose and maltose is referred to as a disaccharide the reason why it has to be referred to as a disaccharide is because well it's just basically made up of two monosaccharides linked together by a glycosidic bond what happens if we keep on doing this chain uh we keep on doing this pattern and when we keep on doing this pattern where many alpha glucose are linked together repeatedly that's when we get something known as a polysaccharide and as you can see the polysaccharide has many glycosidic bonds and many alpha glucose molecules you may be wondering um if mono's one poly is two how much is how many is poly then holy is usually referred to when it's more than 20 alpha glucose linked together that's basically it but we don't have to know that as long as you put the word many repeating units of sugar in the exam you're good so the first polysaccharide that we are looking at is starch now starch is basically an energy storage molecule in plant cell now what do i mean by energy storage molecules in plant cells to understand why starch is referred to as an energy storage we have to look at a situation now in this situation here i'm just drawing out a plant cell now always get used to drawing a plant cell just putting in the cytoplasm and if we zoom in we have a chloroplast like that remember chapter one how do we recognize that that's a chloroplast you can see this structure is known as thylakoid you can see the double membrane organelle chloroplast is a double membrane organelle just blowing out one chloroplast and within the chloroplast those purple colored dots uh we just assume that those purple colored dots are glucose molecules or alpha glucose molecules you see once alpha glucose is synthesized by chloroplasts through the process of photosynthesis the alpha glucose is small enough to diffuse out of the chloroplast membrane and once it diffuses out of the chloroplast it can then be used by the mitochondrion uh for a process known as aerobic respiration so you see sometimes when the chloroplast actually synthesizes a lot of glucose molecules however they don't necessarily have to use all the glucose immediately some of the glucose can be kept for the future but the problem here is because glucose is small enough it may diffuse out of the chloroplast all right and it might also diffuse out of the cell and we don't want that to happen another big problem is glucose as a molecule is soluble in water and because it's soluble in water it can actually lower the water potential of the chloroplast and this causes water to rush in by osmosis and if water rushes in by osmosis into the chloroplast the chloroplast may burst and that's a problem the reason is because glucose is able to affect the water potential within the chloroplast because glucose is a soluble and it's a reactive molecule so these are the two problems it can affect the water potential of the cells and it can also escape or diffuse out of the cells so the solution to this is very simple instead of just basically keeping them or storing them as glucose molecules chloroplasts will basically just link the glucose molecules together and when they link the glucose molecules together we're just learning those dots over there when they link it together that's when they get something known as starch and by linking the glucose molecules together it makes the size of the molecule larger and when it makes the size of the molecules larger this is good for two reasons number one the molecule is too large to diffuse out of the chloroplast therefore it cannot diffuse out of the cell number two it will not affect the water potential of the cell the reason why it doesn't affect the water potential of the cell is because starch as the molecule is insoluble in water so those are the two reasons it does not affect the water potential it's not able to diffuse out of the cells so that's basically why plants convert glucose into starch so coming back to starch which is the energy storage molecule in plant cells there are two types of starch molecules that we have to know the first type of starch is known as amylose and amylose is made up of alpha glucose molecules but they only contain one for glycosidic bonds only so what i'm going to do is i'm just basically going to throw out a few alpha glucose molecules and as you can see when one alpha glucose molecule links with another alpha glucose molecules they they form the glycosidic bonds at a certain angle there's a reason why they form the angle but we don't have to know why they form it at an angle we just have to know that when they form the one for glycosidic bonds which i'm putting it in red over that i hope you can see it they form it at an angle so what happens is the molecule the chain starts to curve a little bit so the reason why this is important is because if you have many repeating units of glucose molecules linked together as you can see here they are just basically starting to coil because of that one for glycosidic bonds which causes the shape of amylose to become a helical or spiral shape that is why amylose is referred to as a helical molecule a coiled molecule or a spiral molecule and that's a polysaccharide the second type of starch molecule that we have to know is something known as amylopectin same as uh amylose amylopectin is actually also made up of alpha glucose molecules uh the difference here is they contain alpha 1 4 and also alpha 1 6 glycosidic bonds but they are not helical so as you can see here the red color bonds are the alpha 1 4 bonds but the green color ones are alpha 1 6 bonds instead so if the molecule is only predominantly made up of alpha one four bonds if it's only made up of uh alpha one four bonds they form a coil but because they have alpha one six however they start to form a kind of branched shape so the green color lines signify the alpha 1 6 bonds and the red color lines signify the alpha 1 4 bonds and you can see in the alpha 1 4 bonds they are curling or they are kind of coiling or they are trying to coil but with the presence of one six bonds it causes amylopectin to become branched that's basically it and of course the second type of polysaccharide that we have to look at is glycogen starch is an energy storage molecule in plant cells glycogen is an energy storage molecule in animal cells in terms of structure glycogen is again made up of alpha glucose molecules contains alpha 1 4 and also alpha 1 6 glycosidic bonds so if you notice it's kind of the same as amylopectin in our plants the main difference is even though it's branched it's just basically more branched than amylopectin if a question asks you uh what is the mean difference between glycogen and amylopectin if you wanted to answer that you can say that glycogen contains more alpha 1 6 bonds because it's the alpha 1 6 bonds that causes it to be branched so that's that so i've already mentioned it over there so as you can see these two cells over here just as a bit of a this is just basically the animal cell if they have too many glucose molecules inside the animal cells it will lower the water potential and what will happen to the animal cells uh the water will start rushing into the animal cells by osmosis uh and it causes your animal cells to burst okay our cells are tubers basically because remember we are at the end of the day we are animals as well so our cells will burst when we have too many glucose too much glucose molecules inside the cytoplasm however if your cell converts it into glycogen it will not affect the water potential inside the cell it will equalize the water potential inside and outside the cell so no osmosis will take place and there you go your cell is fine that is the main reason why animal cells and plant cells convert glucose into polysaccharides such as starch or glycogen these are the main reasons and that's why they're known as energy storage molecules and of course when the plants or animals need extra glucose what they can just do is they can just break down the starch object on the glycogen and use up the glucose when it is actually necessary right so the so the third type of polysaccharide that we are going to be looking at is cellulose we've seen starch we've seen glycogen cellulose is actually the third polysaccharide that you have to know for the a level syllabus the difference between cellulose and starch and glycogen is uh that cellulose is actually made up of beta glucose molecules and they are joined together by beta 1 for glycosidic bond bonds so as a revision let's just do a comparison between alpha glucose and beta glucose molecules if you remember alpha glucose they will have the o h groups for carbon number one and carbon number four facing downwards but for beta glucose uh the oh group in carbon number one is facing upwards so even though it may look like a small difference between alpha glucose and beta glucose the consequence of this difference is quite significant and what do i mean by that let's let's look at this so i told you that to polymerize beta glucose molecules together involves a process known as condensation so what we have right now is we have two beta glucose molecules the problem right here is they cannot form the beta one for glycosidic bonds the reason is because as you can see the oh groups are quite far away so the solution to this is pretty simple the second beta glucose molecule will have to rotate 180 degrees and if you see me rotating it let me just change it up a little bit and once it has been rotated 180 degrees look at the oh group between carbon number one and carbon number four the oh groups are quite close to each other right now can condensation happen yes it can happen because the hydroxyl groups or the oh groups are quite nearby so once condensation happens this is where you have a beta 1 4 glycosidic bond if i were to put another beta glucose molecule next to it does that other beta glucose molecule have to rotate 180 degrees it doesn't have to rotate 180 degrees because the oh groups are quite close to each other so that's okay but the fourth beta glucose molecule will have to rotate again the reason why it has to rotate is because it wants to make the oh close quite close by so what we notice over here is if i were to just basically plot a few beta glucose molecules not every beta glucose molecules has to rotate 180 degrees and this is when we get something known as a cellulose molecule and every other beta glucose molecule rotates 180 degrees and every other beta glucose molecules are the ones i'm highlighting in yellow so only those in yellow had to rotate and once you link many beta glucose molecules together that's when you get something known as cellulose the shape of cellulose is going to be a bit of an interesting one but before we look at cellulose let's look back at amylose if you remember amylose which is a type of starch amylose was basically made up of many alpha glucose molecules and when you have all the alpha glucose molecules linked together they kind of start to curve because remember i said when they form alpha glucose molecules they form it at an angle and when the out when the chain starts to coil it will cause amylose to become a helical coiled or spiral shape that's what we saw but cellulose however is a little bit odd if you notice what's happening here i'm just basically drawing out many beta glucose molecules and many alpha glucose molecules linked together because every other beta glucose molecule rotates 180 degrees the glycosidic bond does not curve you see because the glycosidic bonds are not all facing one direction some glycosidic bonds face upwards some glycosidic bonds face downwards so what happens is this just basically neutralizes the angle it doesn't curve or it doesn't call in any way whatsoever it just basically makes beta glucose become a rather linear shape and as i am highlighting it here notice the glycosidic bonds where i'm highlighting it in green okay it does not coil it just basically forms a linear structure so because the al if you look at the alpha glucose their glycosidic bonds are orientating downwards it causes the shape of the chain to coil or form a helix but in beta glucose molecules some glycosidic bonds oriented upwards or face upwards some glycosidic bonds face downwards therefore the beta glucose molecule does not coil and it forms a linear structure that's what we just have to know you don't have to know why it forms a linear structure you just have to know in the exam that cellulose molecules are just linear and melos are helical even though both of them have one for glycosidic bonds the main difference is amylose has alpha one for glycosidic bonds beta glucose has beta one for glycosidic bonds so why is it a big deal that cellulose is a linear molecule the main reason is because if we were to look at this in detail what i'm doing is i'm just throwing out one cellulose molecule as a green line and i'm just putting it over there and another cellulose molecule down there interestingly between one cellulose molecule and another cellulose molecule they can start to form something called hydrogen bonds so what exactly are hydrogen bonds then hydrogen bonds form between atoms with partially positive charges and partially negative charges i will be talking about hydrogen bond in the next video so you don't have to worry about that but right now we just have to know that if an atom has a partially positive charge and another atom has a partially negative charge they can form something known as a hydrogen bond where i've signified in a purple colored dotted line so let's just draw out one of our glue uh one cellulose chain right here oxygen what we just have to understand here is oxygen has a partially negative charge and hydrogen has a partially positive charge we will talk about the reason why oxygen has a partially negative charge and hydrogen has a partially positive charge i will cover it in the next video hopefully but for now what we just have to know is oxygen has a slightly or partially negative charge and hydrogen is partially positive and because they have this difference in charges what i'm doing here is i have two cellulose molecules one at the top and one at the bottom notice that i've circled the hydrogen at the top and the oxygen at the bottom line and they can link together and form something known as hydrogen bonds so individual hydrogen bonds can actually form between hydrogen and oxygen individually one hydrogen bond is weak but cumulatively they become stronger so the more hydrogen bonds link up between one cellulose and another it makes it stronger so why is this a big deal so as we can see here one cellulose and another cellulose can form many hydrogen bonds with each other so there is because it's linear and because they can form hydrogen bonds many cellulose molecules then can be tightly packed together and they can also bond with each other so what i'm going over here are just each of those lines are just individual cellulose molecules packed tightly and they form a kind of bundle and that bundle over there is known as a cellulose micro fibril which is made up of a bundle of about 60 to 70 cylinders molecules linked together by hydrogen bonds and those hydrogen bonds if you notice i'm throwing it in purple like that so they can be packed into a very small area because their shape is linear you can imagine one cellulose molecule to be like a stick and you can imagine cellulose microfibrils to be a bundle of sticks so if i were to ask you a question which is stronger obviously you're going to say the bundle of sticks is stronger because they're all grouped together it's easy for you to break one stick but if i ask you to break a bundle of sticks it's going to be difficult and what happens is many of the cellulose microfibrils can then also group together by more hydrogen bonds and they can then form something known as cellulose fibers so cellulose fibers are just like many bundles of sticks stacked together so they have a very high tensile strength they do not break easily they do not bend easily they do not stretch easily the reason why this is significant is because cellulose fibers are actually used to make plant cell wall and if you remember plant cell wall are extremely rigid and they can actually withstand a high pressure when water rushes into the cell the plant cell will not burst because the cell wall is strong enough to actually withstand the pressure so this is why cellulose molecules being linear is a big deal because they are linear they can actually be packed together in a small space where you can pack a lot of them together to form something called microfibrils and microfibrils can have hydrogen bonds okay and many microfibrils grouped together to become cellulose fibers which are much stronger and these cellulose fibers are then used to make the plant cell wall so to summarize the whole thing if we wanted to look at the three types of polysaccharides that we have covered the first polysaccharide that we looked at was starch which is basically the energy storage in plant cells and starch is divided into amylose and amylopectin amylose is made up of alpha 1 4 glycosidic bonds and the shape is helical or spiral alpha amylopectin however is made up of alpha 1 4 and also alpha 1 6 glycosidic bonds and their shape the shape is branched the second polysaccharide we looked at was glycogen which is the energy storage in animal cells and the shape glycogen is actually made up of alpha 1 4 and also alpha one six glycosidic bonds and their shape is also branched uh the difference is with amylopectin is a glycogen has more alpha one six glycosidic bonds there therefore they're more branched uh the third polysaccharide we then looked at was cellulose cellulose is made up of beta one for glycosidic bonds which means to say they are made up of beta glucose molecules and the shape of cellulose is linear that is the summary for polysaccharides