so what we're going to do right now is we're going to dive right into the next part of carbohydrates which are disaccharides we've covered monosaccharides earlier so now we're going to look at disaccharides now by definition disaccharides are basically made up of two monosaccharides linked together by condensation and they are forming a glycosidic bond so there are three key points here that we have to know the first key point is basically two monosaccharides linked together through a process known as condensation i'm going to talk about condensation in a while and they form a type of covalent bond known as a glycosidic bond so i've highlighted those keywords and those are the ones that you have to understand over here going back to a bit of revision what exactly are monosaccharides there are three types of monosaccharides that we saw in the previous video we saw trials sugars pentose sugars and hexose sugars hexose sugars are just basically sugars with six carbon and we saw four types of hexose sugars which are alpha glucose beta glucose fructose and galactose now in this video we are going to be focusing on the two monosaccharides which are alpha glucose and fructose i've put like a purple star over there so that that's our main to focus for today one of the first examples of a disaccharide would be a sugar known as maltose maltose is actually a type of disaccharide and if you notice over there maltose is actually also referred to as a sugar maltose is a disaccharide that is made up of two alpha glucose molecules linked together by a glycosidic bond it is extremely important for you to understand this because i've had students make these mistakes what i'm going to do is i'm going to draw out two alpha glucose molecules at the bottom now if you see those two alpha glucose molecules at the bottom can we call that maltose no we can't we cannot call that maltose that is not meltos because even though you see two alpha glucose molecules there they are not linked together so what you see over there are just two separate monosaccharides however when i join when i link a purple line between the two alpha glucose molecules representing the glycosidic bond then you may refer to that molecule as a maltose sugar which is a disaccharide another very important question that i do get asked by students is what is the difference between covalent bond and glycosidic bond because that does come out often a glycosidic bond is a type of covalent bond that is only found in carbohydrates meaning to say you will not be able to find glycosidic bonds in let's say a protein lipids or nucleic acids it is very specific to just carbohydrates now aside from maltose the next disaccharide is known as sucrose which is also a type of sugar instead of being made up of two alpha glucose molecules sucrose is actually made up of one of a glucose molecule and one fructose molecule if you see that fru over there that's just my way of representing the fructose and also remember the alpha glucose and fructose must be linked together by something known as a glycosidic bond which is a type of covalent bond found in carbohydrates if i am repeating myself so don't get too annoyed because that is a very important thing to know uh about these glycosidic bonds linking two monosaccharides together now sucrose if you have not heard of sucrose you might know something known as table sugar like you know the sugar that you used to put in your tea or coffee i mean i i put that in my i put a lot of it in my coffee which i shouldn't but i do um and table sugar is basically just sucrose so table sugar is actually just made up of alpha glucose and fructose linked together by a glycosidic bond there we go so we have a real life example of a carbohydrate like that now that we are acquainted to the two types of disaccharides maltose and sucrose we do have to talk about their formation how are you able to form a maltose sugar and how are you able to form a sucrose sugar from monosaccharides now maltose is actually made up of two alpha glucose undergoing a condensation reaction and for those of you who do not know what a condensation reaction is i know this has nothing to do with the condensation where um you know water vapor becomes liquid this is not that condensation in biological molecules when i say condensation reaction we are referring to a chemical process where water molecules are removed from the molecules so what do i mean by that first and foremost i will draw out two alpha glucose molecules remember alpha glucose when you're drawing it put the oxygen and then link the carbon skeleton together first always remember when you're labeling the carbon it's one two three four five six in a clockwise direction this is universal and then what we do is we we just basically put in the oh groups now you might be thinking hey wait a second i thought alpha glucose also has the hydrogen why aren't you including the hydrogen the hydrogen is not really that important but it's important to include the hydroxyl oh groups because they are important for the condensation reaction now next what i'm going to do is i'm going to zoom in and you can see the two alpha glucose molecules we have to remove a water molecule between the two alpha glucose molecules some students will think oh i have to remove h2o from one alpha glucose molecule and a h2o from another alpha glucose molecule no you just have to remove a total of one h2o what that means is i can remove one hydrogen and one oxygen from one alpha glucose and another hydrogen from the other alpha glucose molecule as i've highlighted for you and we are only going to focus on carbon number one and carbon number four i've already put a line over there for you to refer to we are not going to focus on the other carbons at all there's a reason for this and i'll explain that later now so in carbon number one i want to remove hydrogen and oxygen and for carbon number four i want to remove hydrogen or what i can do is in carbon number one i can just basically remove hydrogen and carbon number four i can remove oxygen and hydrogen it doesn't matter we are going to remove h2o from that okay so that has been removed from the two alpha glucose molecules and that becomes the removal of h2o that is what is meant by condensation reaction like that you can see that there's an oxygen left in one of that alpha glucose molecule what that oxygen does is that oxygen will now link the alpha glucose on the left and the alpha glucose on the right what that becomes now is the glycosidic bond that is how the glycosidic bond is formed between the two alpha glucose molecules lo and behold what do we have here we have one maltose sugar which is the disaccharide and when we are labeling the glycosidic bond this is also very important the name of this glycosidic bond yes there's a specific name to it we will call it the 1 4 glycosidic bond can you guess why it's called one for glycosidic one well that's kind of obvious isn't it it's because it is between carbon number one and carbon number four of the two alpha glucose molecules hence we call it one for glycosidic bonds that's what it means another very important question that we can ask is can other types of glycosidic bonds form between two alpha glucose for example we saw alpha one four glycosidic bonds happen can it form you know i don't know alpha one six glycosidic bonds can it form alpha 2 6 glycosidic bonds or what about even alpha 4 6 glycosidic bonds can all these type of glycosidic bonds form well there is no probable answer for all of them okay what we do have to understand is in nature in in living organisms uh in cells whether animals or plants or proteins or whatever the most common glycosidic bonds that form between alpha glucose is either alpha one four glycosidic bonds and all alpha 1 6 glycosidic bonds now i'm not saying that other types of glycosidic bonds may not be able to be formed however in nature they are not so common and for your syllabus you do not have to know all these other types of glycosidic bonds the ones that you will have to know is alpha 1 for glycosidic bonds or alpha 1 6 glycosidic bonds those are the two common ones that usually form in the natural environment or those are the two common glycosidic bonds that form in living organisms and something very important to be said over here is what you're looking at over here is a disaccharide where two alpha glucose are linked together by alpha 1 6 glycosidic bonds now can we refer to this as maltose no we cannot refer to this as maltose immediately i can hear the collective groan of students you're like oh my god really why is this not maltose i can see two alpha glucose i can see they are linked together by glycosidic bonds why can't i refer to this as maltose the answer is also very simple we cannot refer to this as maltose because montos is only formed when two alpha glucose are linked are linked together by alpha one for glycosidic bonds it is extremely specific when it comes to biological molecule biochemistry is like that when you join the alpha glucose a different way they are referred to as a different name now you might be thinking okay so what's the name of that disaccharide on the right then you don't have to know its name we do not have to cover that in detail you just have to know that alpha 1 6 glycosidic bonds between two glucose molecules you cannot refer to them as maltose maltose is only formed when two alpha glucose molecules are linked together by alpha one for glycosidic bonds that's all we have to know for this going on to the next dissect right the one that we have to see is sucrose now the good news is you just have to recognize sucrose's uh structural formula you are not required to draw out the formation of the disaccharide and remember fructose has a rather odd shape it looks like a pentagon even though it looks like a pentagon it is still a hexose sugar still is six carbon sugar and the formation of sucrose is when you have one alpha glucose linked together to the sucrose molecule if you notice my sucrose molecule it has been rotated 180 degrees you just have to be aware that that happens you don't have to memorize that do not worry about it and what you have right there is a sucrose molecule and it's very important to know that a sucrose is a disaccharide formed by glycosidic bonds between one alpha glucose and one fructose molecule that's about it nothing much to know there in detail some common questions that can be asked especially in paper 1 um will include the molecular formula of maltose and sucrose this is an extremely important question because many students make mistakes over here now the common mistake that students will make is they'll assume that alpha glucose is c6h12o6 another alpha glucose is also c6h12o6 so if they are linked together maltose should definitely be c12 h24 o12 now that is wrong by the way huge mistake you might be thinking why again it's very simple remember if you want to join one alpha glucose to another alpha glucose molecule it's a condensation reaction and in condensation reactions it involves the removal of a water molecule so when you form the maltose you will have to remove two atoms of hydrogen and one atom of oxygen which is water therefore the molecular formula of maltose is actually c12h22o11 that is the correct molecular formula of maltose so please do not make that mistake in the exam it's not c12h24o12 it's xe12h22o11 and the same can also be said about sucrose because alpha glucose c6h12o6 fructose is also c6h12o6 when you join it together sucrose is not c12h24 o12 you will still have to remove one water molecule due to condensation reactions so therefore sucrose is also c12h22o that's about it and towards the final part of this video we do have to talk about something called hydrolysis or the hydrolytic reactions hypolysis is the well you can kind of see it's the opposite of condensation because in condensation two monosaccharides are linked together to become a disaccharide and you need to remove water so hydrolysis is the opposite hypolysis is what happens when a disaccharide is broken down or the glycosidic bond in the discipline is broken down by adding a water molecule so how does that look like so what i have over here is i am showing you a maltose molecule that we saw much earlier and i want to break down that glycosidic bond so if i want to break that glycosidic bond the disaccharide will have to undergo a process known as hydrolysis and in hydrolysis you have to add water so how will the maltose molecule be broken down first i'm just going to throw out the water right there which is required to break the glycosidic bond next what i'm going to do is i'm going to erase the glycosidic bond right there and put the oxygen back to the alpha glucose on the left there we go but this alpha glucose is not complete yet you know it's still a bit empty you know some things are still missing what we do is we split the water into o h and h and the hydrogen will go towards the alpha glucose on the left we forms the hydroxyl group over there and the oh goes to the alpha glucose on the right and reforms the hydroxyl group there low and behold what do we have here we have reformed two alpha glucose molecules right here and both of these are monosaccharides that is how hydrolysis will happen same thing if sucrose undergoes hydrolysis you will have to add a water molecule and it will break down into alpha glucose and filters the reason why it is called hydrolysis is because the word lysis means to break and hydro means using water so hydrolysis basically means to break something using water molecules that's what it means