Hello everybody welcome back to my YouTube channel my name is Iman at snd and today we're going to cover chapter 3 for our MCAT biochemistry playlist and chapter three is going to be about carbohydrate structure and function so let's go ahead and get started carbohydrates are the single most abundant class of organic molecules that are found in nature energy from the Sun is captured by Green Plants algae and even some bacteria during photosynthesis to convert nearly 250 billion kilograms of carbon dioxide into carbohydrates every day on Earth and in turn carbohydrates are the metabolic precursors of virtually all other biomolecules what we see is that the breakdown of carbohydrates provides the energy that system stains animal life and in addition to that carbohydrates are covalently linked with a variety of other molecules these glycoconjugates are important components of things like cell wall and extracellular structure in both plants animals and even bacteria and in addition to that to the structural roles said that molecules play the serve in a variety of processes that involve recognition between cell types or recognitions between cellular structures by other molecules and recognition events are really important in things like normal cell growth fertilization transformation of cells and other processes and all of these processes are made possible by the characteristic chemical feature of carbohydrates right the fact that um in carbohydrates there is the existence of at least one or often two or more asymmetric structures centers I'm sorry um the ability for carbohydrates to exist in either linear or ring structures also the capacity to form polymeric structures through glycosidic bonds and the potential to form multiple hydrogen bonds with water or other molecules in their environment so today in our review we're gonna first start with carbohydrate classification how are carbohydrates named and categorized so that's the first question that we're going to try to address we're going to say that carbohydrates they come in many types they can be classified by the number of sugar moieties that make them up or the number of carbons in each sugar or the functional groups present on the molecule and or the stereochemistry of the sugar now the name carbohydrate arises from the basic molecular formula ch2o n where n can equal three or more carbohydrates are generally generally classified into a couple of groups you can have monosaccharides where you have only one sugar molecule you can have disaccharides this is two sugar molecules you can have oligosaccharides these are from anywhere between two to ten sugar molecules and polysaccharides 10 or more sugar molecules now the monosaccharides one sugar molecules are called your simple um um sugars they cannot be broken down into any smaller sugars under mild conditions and in the simplest monosaccharides we can have um three carbons at most and these are called triosis actually if we scroll down here the simplest contain three carbons and they're called triosis now we already said that oligosaccharides they consist anywhere between two to ten um and polysaccharides are polymers of the sugar simple sugars and their derivatives and they have more than 10 um or more sugar molecules now carbohydrates carbohydrates that contain an aldehyde group all right we'll do aldehyde groups look like right you have a carbonyl a r group on one side and a hydrogen on the other all right so carbohydrates that contain an aldehyde group as their most are oxidized functional groups are called aldoses and those with a ketone are called ketosis what does a ketone look like you have your carbonyl and two R groups on both sides all right so that's a little introduction on how carbohydrates are named and classified with this introduction let's ask our second question what are the structures and chemistry of monosaccharides well monosaccharides are like we said can be classified as aldoses or ketosis and monosaccharides consists typically of three to seven carbon atoms and their and they'll fall into those two categories we talked about um depending on whether they have an aldehyde or a ketone group now we can see the structure and stereochemical relationships of D aldoses with three to six carbons the configuration in each case is pretty much determined by the highest numbered asymmetric carbon all right so what we see here is the family of D aldoises all right and we see that with three carbons four carbons five carbons and six carbons and what we see is that the simplest D aldose is D glyceraldehyde all right fantastic now in the next page in the next page what we see is the family of D ketosis all right with three carbons four five and six now the basic structure of a monosaccharide going back we're going to repeat this right is illustrated by the simple sugar d glyceraldehyde right which is in aldose now um on the next page where we see the the the D ketosis is the simplest ketose is dihydroxy acetone which is this guy right here now on the MCAT a few sugars are tested by referencing to their common name all right you should know the structure of the fructose all right you should know the structure of the fructose the glucose D mannose um D galactose all right these are some of the sugars that you should know their structures and their common names for the MCAT all right and D fructose which is on the next page those are four common sugars that will reappear on the MCAT so please commit some time to remembering that common names and their structures as well all right now as a kind of final summary note to sum up these two to sum up so far all right carbohydrates are organized by their not by their number of carbon atoms and their functional groups all right common groups are also frequently used when referring to sugars like glucose fructose galactose Manos those are ones that you should know all right three carbon sugars are referred to as triosis all right for tetrosis and so on sugars with aldehydes as their most oxidized group or aldoses sugars with ketones as their most oxidized groups are ketosis all right so that's the summary so far all right now as you can tell though based off of looking at these charts of the aldoses and D ketosis from what you can tell stereochemistry is the prominent feature of monosaccharides right Optical isomers also called stereoisomers are compounds that are going to have the same chemical formula but they're going to differ from one another in terms of the spatial Arrangements of their atoms a special type of isomerism exists between stereoisomers that are non-identical non-super-imposable mirror images of each other these molecules are called enantiomers all right a chiral carbon all right a chiral carbon is going to be one that has four different groups attached to it any molecule that contains chiral carbons and no internal planes of symmetry has an inane humor so if we look at deglyceride for example all right this the structure that we show as a Fischer projection all right now in general for monosaccharides with two or more asymmetric carbons what they what is used is the prefix d or L which refers to the configuration of the highest numbered asymmetric carbon all right a monosaccharide that's designated the d letter it is designated that if the hydroxyl group all right if the hydroxyl group is um on the highest number asymmetric carbon that's drawn on your Fischer projection so if we look if we look at this glyceraldehyde right here all right your highest numbered carbon not counting your outer carbons by the way not counting these ends which will be this guy right here in the center that's our chiral carbon right the others are not chiral carbons you can test that yourself this carbon that has a blue circle it has four different groups attached to it it's a chiral carbon where is the hydroxyl group it's to the right of our chiral carbon which means that it's going to be designated D this is a d glycerolhyde if we look at its enantiomer here's that chiral carbon where is the hydroxyl group to the left of the carbo chiral carbound chiral carbon sorry so this gets assigned l glyceraldehyde all right and so note that the designation DRL really relates the configuration of a given molecule all right like glyceraldehyde all right and it does not specify the sign of rotation of plane polarized light it doesn't replace the RS nomenclature that you are acquainted with from organic chemistry all right so then according to the convention the DNL forms of a monosaccharide they're mirror images of each other and their stereoisomers that are called enantiomers for molecules that have two or more chiral centers more stereoisomers can exist and the pairs of isomers that have opposite configuration at one or more of the chiral centers but they're not mirror images of each other those are called diastereomers Okay so we covered the DNL nomenclature all right highest highest numbered chiral carbon all right figure out where the o h group is is it to the right or to the left that helps you figure out if it's D or L respectively all right and D and L forms are the same sugar they're called enantiomers but we can also have not superimposable and not mirror images of each other and those are called diastereomers all right you have to have at least two character centers for you to have diastereomers now subgroups to diastereomers that we should know of are epimers all right so two sugars that different configuration at only one chiral Center are described as epimer so essentially epimers are a sub type of diastereomers that differ at exactly one chiral Center also another subclassification of diastereomers that are animals these are a subtype of ephemers that differ at the anomeric carbon which will Define shortly all right now with that being said we can discuss cyclic sugar molecules so another important thing to mention is that monosaccharides can exist in cyclic and anomeric form an interesting feature of carbohydrate is their ability to form cyclic structures with formation of additional asymmetric centers so aldoses and ketosis with five for more carbons can either form they can form ring structures like a furanose or a pyronose ring um because they have more carbons and they can form that cyclic structure all right now we're going to elaborate on this of course we're going to talk about the cyclic structure of monosaccharides first all right monosaccharides with five or more carbon atoms can form either a furanos or a pyreneos ring all right and monosaccharides with five or more carbon atoms they predominantly occur actually in cyclic form in the in aqueous conditions so for the formation of a cyclic structure what happens is that the carbonyl group forms a covalent bond with the oxygen atom of the hydroxyl group of the chain and the ring formation is due to the reaction of an alcohol group a hydroxyl group and an aldehyde group in aldose sugars or it can be due to the reaction of an alcohol group a hydroxyl group or a keto group ketose sugar all right and so that reaction between an alcohol and an aldehyde to start off with all right that's the first one we're going to look at you can have an aldehyde and an alcohol react to form a cyclic structure what at first causes the formation of is this hemiacetyl group all right and similarly I'm just going to scroll down really quickly you can have a ketone and an alcohol react and first before they form a cyclic structure you're going to have a hemiquito group form now the formation of a cyclic structure in the monosaccharide as a result of a hemiacetyl or hemichito is going to create an additional chiral Center so we can see now the formation of a cyclic structure of glucose um which is going to be an Aldo hexos all right here's your D glucose you can have a reaction with this alcohol group with this aldehyde right here and you can form cyclic structure you can have Alpha D glucopyronose or or beta D glucopyronose and so here you're going to form a pyrin group right you have one two three four five carbons and you have this alcohol this oxygen in the alcohol group form a participate in the ring formation and so you have one two three four five carbons like you saw here in the carbon backbone and that oxygen that forms this ring all right and so with the formation of a new chiral Center the cyclic structure can produce these two stereoisomeric forms around the newly formed chiral Center where is that chiral Center you ask it's going to be right here for example this is a chiral Center all right now these two newly created isomers that are designated Alpha and beta a animals these animers Alpha animals have the hydroxyl group on the anomeric carbon trans to the free ch2h group which I'm going to highlight in blue all right so in the alpha configuration your hydroxyl group at your anomeric carbon is trans to your free ch2oh group in your beta they're going to be CIS to one another notice that that they're on the same side they're both up in your beta configuration one is up one is down in your Alpha configuration all right now in the case of Aldo hexos which is what we're seeing here the six-membered ring compounds are called pyronose since it resembles a six-membered ring that compound called pyrant all right now the five-membered ring structures are called furanos they look like furin since they remember since they resemble this five-membered compound we can see this as an example here formation of a cyclic structure of fructose which is a keto hexos what you notice here is that you can have this alcohol group react with this Ketone right here all right it's going to attack at this carbon that means your carbon backbone is one two three four carbons and you're gonna have this oxygen from this alcohol group b part of that spring structure and so you form if you're in like cyclic structure all right so that's what these are this is a fructo pyrenose um and and Alpha d fructopyranose um and a beta d um one so these two newly created isomers were designating them Alpha and beta all right these animals are going to have right an anomeric carbon with an alcohol group all right Alpha is going to have that alcohol group again trans to the ch2oh group all right that's why it's called Alpha and in your beta you're going to have your alcohol group CIS to your ctoa ch2oh group that's why it's called beta all right fantastic now these cyclic compounds can undergo neutral rotation in which they shift from one anomeric form to another with a straight chain form as an intermediate fantastic now in summary here all right what have we discussed cyclization describes the ring formation of carbohydrates from their straight chain forms to their ring forms when the ring forms the anomeric carbon can take on either an alpha or beta conformation all right the animary carbon is the new chiral Center formed in ring closure all right it's the new chiral Center it has it was the carbon containing the carbonyl in the straight chain form now Alpha animers have the hydroxyl group on the anomaryocarbon trans to the free ch2oh group and beta animers have the oh group on the anomary group CIS to the free ch2oh group all right now in an important definition just really quickly to make here is animals or animeric carpet just in case that that um wasn't clear when Hemi acetones and Hemi ketones are formed the carbon atom that carry the carbonyl function becomes an asymmetric or chiral carbon atom isomers of monosaccharides that differ only in their configuration about that carbon atom are called animers and the carbon carbonyl carbon is called the anomerocarbon which is what we've been pointing to right it's that new chiral Center that's formed in the cyclization of linear sugars all right now in the hydroxyl group at the anomeric carbon is on the same side of the fissure projection as the oxygen atom at the highest number asymmetric carbon the configuration of that is Alpha and the opposite is beta that's just another way of thinking of the same thing that we've said Alpha animals have the hydroxyl group on the anomeric carbon trans to the free ch2oh group beta have that CIS all right so those are that definitions now we are going to talk specifically about monosaccharides and their reactions all right monosaccharides sorry contain alcohols in either aldehydes or ketones and as such these functional groups undergo the same reactions that they do when present in other compounds so this includes things like oxidation and reduction esterification nucleophilic attack that creates glycosids all right the same chemistry can happen in monosaccharides as they did when we learned about aldehydes and ketones in organic chemistry so we're going to start off first with oxidation for oxidation one of the most important biochemical reactions in the human body is the oxidation of carbohydrates in order to yield energy aldoses contain alcohol and aldehyde functional groups which can be oxidized to carboxylic acids now depending on the oxidizing agent we can either selectively oxidize the aldehyde or involve the primary alcohol as well all right if both groups are oxidized to a carboxylic acid an aldaric acid is formed while the selective oxidization of the aldehyde results in an aldonic acid all right so here we have our sugar we can use a mild oxidizing agent only to oxidize one group The aldehyde to a carboxylic acid this forms aldonic acid or we can use a strong oxidizing agent and we can oxidize both both of these groups to carboxylic acids and this is an aldaric acid all right fantastic now both now both aldoses can be oxidized and they're considered reducing agents the two standard reagents that are used to detect the presence of reducing agents are tolerance reagent and Benedict's reagent and an interesting phenomena however is that ketose sugars are also reducing sugars and they will give positive pollen and Benedict tests all right so carbohydrates can be oxidized to aldonic acids with br2 toluene and Benedict reagents all right and so you can have your d-glucose reacting with these reagents and you can oxidize your aldehyde to carboxylic acid and detect using these reagents fantastic now although ketones cannot be oxidized directly to carboxylic acids they can tautomerize to form aldoses under basic conditions via a keto enol shift and while in the Aldos form they can react with pollen or Benedict reagents to form carboxylic acids all right so that's oxidation we can also talk about reduction the carbonyl group in aldoses and ketosis can be reduced by sodium borohydride to either a primary or secondary alcohol respectively and the product of this reaction is a poly alcohol called an aldotol all right the reduction of ketose creates a new chiral Center in both configurations this is because the carbonyl group is transformed into a secondary alcohol while in the case of aldoses it forms a primary alcohol which is not chiral remember also that cyclic hemiacetals are always in equilibrium with the open chain form and therefore they can be reduced to altitles all right so we can see that here right the carbonyl group sorry the carbonyl group in aldoses and ketosis can be reduced by sodium borohydride to a sect of primary and secondary alcohol respectively so we notice that this is reduced to an aldehyde here all right this D fructose which is a ketose is reduced to uh the glucose and aldose all right and you can have um you can have further reaction of this all right the reduction of the ketosis creates a new chiral Center in both configurations this is because the carbonyl group is transformed into a secondary alcohol while in the case of aldoses right it forms a primary alcohol which is not a chiral which is not chiral all right so you can do that with sodium borohydride and remember right that cyclic Amy acetyls are also in equilibrium with the open chain form and therefore they can also be reduced to altitles like you see here all right so this aldehyde form all right this reduced format ketones can be oxidized to aldonic acid in basic conditions all right we form an aldehyde we can deprotonate the aldehyde all right or we can have the aldehyde form and we can react it with like we said sodium borohydride to form an aldotal and this aldotol can can exist um as in in both of these forms essentially fantastic now we can also move on to esterification because carbohydrates have hydroxyl groups they are able to participate in reactions with carboxylic acid and carboxylic acid derivatives to form Esters and in the body esterification is very similar to phosphorylation of glucose in which a phosphate ester is formed so we can see that here all right here we have um are our starting ring structure we can react it um we can react it with some carboxylic acid derivative all right and we can pretty much exchange through esterification all right all the hydroxyl groups to these chains that are attached to the carboxylic acid derivative and so now you've replaced all the alcohols with said group all right it's very similar to phosphorylation and then last but not least we can talk about glycosidic formation hemi-acitals can react with alcohols to form acetyls and the anomeric hydroxyl group is transformed into an alkoxy group yielding a mixture of Alpha and beta acetones with water and and what you see is that the resulting carbon oxygen bomb they're called glycosidic bonds and the acetyls formed are glycosides right see we here we see that example right uh we have this group that attached here and it can attach in both forms up and down um axial or not all right disaccharides and polysaccharides form as a result of these glycosidic bonds between monosaccharides fantastic Now we move on to our next topic what are the structure and chemistry of oligosaccharides and so disaccharides are the simplest um oligosaccharides the complex array of oligosaccharides in higher organisms is formed from relatively few different monosaccharides units particularly we have glucose fructose mannose galactose ribose and even xylose like we said that disaccharides are the simplest oligosaccharides they're a category of oligosaccharides um that consists of two monosaccharides linked by a glycosidic bond all right and the most common disaccharides in nature are sucrose maltose and lactose and you should definitely know those three their names and their structures for the MCAT um all right so this is how you can form a glycosidic bond between two sugars all right you have here um a hydrolysis reaction um where you lose the water from the hemiacetyl side of one and the alcohol of the other side and then you can form a bond between the two sugars now the anomeric carbons of oligosaccharides they can be substituted or unsubstituted disaccharides have disaccharides with free unsubstituted annumeric compounds can reduce oxidizing agents and so they're termed reducing sugars um foreign and what we can see here is that the structure of several important disaccharides found in nature with sucrose maltose and lactose being the most common sucrose maltose and lactose so you should know these structures except for sucrose each of these structures possess one free unsubstituted annumeric carbon atom and thus each of these diosaccharides is a reducing agent all right fantastic and then we can talk about last but not least what is the structure and chemistry of polysaccharides polysaccharides are formed when monosaccharides and there are formed from monosaccharides in their derivatives if a polysaccharide consists of only one kind of monosaccharide it's called the homo polysaccharide and if there's more than one kind of monosaccharide that forms the polysaccharide then it's called the heteropolysaccharide now polysaccharides May function as energy storage materials structural components of organisms or even protective substances some common palile saccharides that we should know are cellulose this is the main component for plant cell walls and it is the main source of fiber in the human diet we also have starch and glycogen which are energy which are storage molecules they're the main energy storage form for plants we also have glycogen glycogen functions as a main energy storage form for animals um and uh chitin and cellulose are structural molecules as well these are important ones to know all right fantastic that's all for review um now we're gonna go ahead and jump into the practice problems so let that's it for this video let me know if you have any questions comments or concerns down below other than that I'll see you in the next video