so far in our discussion on carbohydrate molecules we focus on the individual monomers of sugars we call monosaccharides now we're going to discuss how we can actually connect these individual monosaccharides together to form larger carbonhydrate molecules and to begin in this lecture we're going to focus on disaccharides now generally speaking what exactly is a disaccharide well a disaccharide is a carbohydrate that consist of two individual monosaccharides which are connected by a special type of linkage a special type of bond we call the O glycosidic bond which we introduced in the previous lecture now to demonstrate what types of disaccharides we can have and the types of Oly acidic bonds that we can have in nature let's take a look at the three most common types of disaccharide so these are malatos lactose and sucrose so so let's begin with maltose and this is what a single maltose disaccharide actually looks like so a malose disaccharide consists of an alpha D glucose in its cyclic form bound to another Alpha D glucose also in its cyclic form and The Binding takes place between carbon number one of the first glucose and carbon number four of the second glucose now both of these molecules have the alpha anomeric configuration and more specifically because carbon number one of this first glucose has the alpha anomeric configuration so this oxygen basically points in the opposite direction downward with respect to where this group points upward then this special type of Oly acidic bond in malos is called the alpha 14 glycosidic linkage or glycosidic Bond so Alpha designates the configuration of this first anamary carbon of glucose number one 14 means we have a bond between the first Carbon on the first glucose and the fourth Carbon on the second glucose so in maltose two D glucose monosaccharides are linked via an OG glycosidic bond that exists between the first carbon of the first glucose and the fourth carbon of the second glucose and as I just said since carbon one of the first glucose has the alpha anomeric arrangement we call the bond an alpha1 14 gly acidic Bond now where exactly does maltose actually come from well maltose itself is formed when we actually break down starch and starch is the polysaccharide that is stored in plants and we'll discuss starch in much more detail in the next lecture now when we in so when we ingest starch we break down starch into these malose disaccharides and when malose makes its way into the brush border of our small intestine we have these special digestive enzymes we call maltase molecules and malas is the enzyme that is responsible for digesting and breaking down these alpha1 14 glycosidic bonds within Malto so when maltos arrives of the BR when maltos arrives at the brush border of the small intestine an enzyme called malas hydes maltose disaccharides into their individual glucose constituents and then those glucose molecules can be ingested into the cell now let's move on to the second common type of disaccharide we call lactose and lactose is a disaccharide that is found in milk so lactose is disaccharide that is found milk and which consists of two individual monosaccharides the first monosaccharide is a galactose and more specifically it's the beta de galactose while the second molecule the second monosaccharide is the alpha D glucose so let's take a look at the structure of this lactose molecule so notice the linkage just like in this particular case is also between the first carbon of the first sugar molecule and the fourth carbon of the second sugar molecule but notice that our bond on this animer carbon of the first sugar is or has the beta Arrangement and what that means is this Bond points upward in the same direction as this group which also points upward and that's why we call it the beta animer of D galactose and because of this this linkage is known as the beta 14 glycosidic Bond so we have a bond between carbon 1 and four but because this point up not downward this is the beta Arrangement and so we have the beta 14 glycosidic Bond now in humans we also have an enzyme or at least most people have an enzyme known as lactase that breaks down this lactose into the individual constituents galactose and glucose in bacterial cells the enzyme is called Beta galactosidase And so these two enzymes are responsible for breaking down this same energy Source in different organisms one in humans and the other in bacterial cells and finally let's take a look at sucrose now sucrose is what we call table sugar and sucrose is the mobile form of carbohydrates that is found in plants and we obtain sucrose from cane plants or bead plants so sucrose just like lactose and maltose is a disaccharide and sucrose consists of two individual monosaccharides one of them is the glucose and the other one is fructose and fructose exists as a five membered sugar so sucrose is a disaccharide that consists of an alpha glucose bound to a beta fructose now unlike in this particular case in this particular case where the bond is between carbon one and carbon 4 the bond in in uh sucrose is between the two anomeric carbons on the two different sugar molecules and this happens to be a bond between carbon one of glucose the first sugar and carbon 2 of the second sugar the fructose so this is Alpha D glucose in a cyclic form so we have carbon 1 2 3 4 5 and six and the arrangement here is the alpha Arrangement because this Bond points uh downward in the opposite direction to where this group actually points to now this is our beta D fructose and this is carbon number one of the beta D fructose so this is carbon number two carbon number three four five and six and notice that this entire group points downward which is the same direction as this group points so this group points downward this group points downward and so for the fructose this is the beta animer because of the arrangement of this carbon so this carbon one is the animer carbon of glucose and this Carbon 2 is the animic carbon of fructose and because we have a bond between two anomeric carbons this is called the alpha12 glycosidic linkage and because both of these anomeric carbons essentially have a bond like shown by the blue linkage here this uh sucrose this disaccharide is an example of a nonreducing sugar so what exactly is a non-reducing sugar well as we discussed previously a non-reducing sugar is a sugar that when mixed with some type of oxidizing agent will not react via an oxidation reduction reaction on the other hand if we have a reducing sugar when a reducing sugar is in the presence of some type of oxidizing agent it will be uh it will be oxidized it will react in an oxidation reduction reaction now maltos and lactose are examples of reducing sugars and that's because if we examine this sugar here and this sugar here if it opens up into its open chain form it will have an alahh group and aldhy groups are capable of reacting with oxidizing agents on the other hand in sucrose because both of these animer carbons are essentially occupied as a result of this bonding none of these sugars will be able to open up into their open chain confirmation that contains a free aldah or free Ketone group and that means because there is a lack of this reactivity this will be a nonreducing sugar so sucrose is an example of a non-reducing sugar while maltose and lactose are examples of reducing sugars so again unlike maltose and lactose which are reducing sugars sucrose is a non-reducing sugar because neither glucose nor fructose within this sucrose molecule can be transformed into an open chain confirmation with a free aldah or a free Ketone group and so these cannot be oxidized in the presence of an oxidizing agent now just like we have an enzyme in our small intestine that breaks down maltose and lactose we also have an enzyme in our small intestine that breaks down sucrose and this enzyme is called sucra so sucrose is broken down by an enzyme we call sucrase that is found on the brush border of our small intestine