hello everybody my name is Iman welcome back to my YouTube channel last time we covered and reviewed carbohydrates structure and function today we're going to go ahead and jump into the MCAT practice problems so let's go ahead and get started first problem says when glucose in his is in a straight chain form it blank all right glucose like we saw before is an Aldo hexos all right Aldo hexas meaning that it has one aldehyde group and six carbons one aldehyde group and six carbons all right given this information okay we can obviously eliminate a it's not an Aldo ketose it's an Aldo hexos and we can eliminate pentose because that means five carbons but we know it has six all right now in Aldo's sugars each non-terminal carbon is chiral therefore what that means is that glucose has four chiral centers four chiral centers not five which means now we can eliminate C now the number of stereoisomers possible for a chiral molecule all right this is going to be a major throwback to organic chemistry if you remember the formula to figure out how many stereoisomers a molecule has after you figured out the chiral centers is 2 to the power n where n is the number of chiral centers so we said that glucose has four chiral centers two to the fourth power is 16. so glucose is going to have 16 stereoisomers and so D here is the correct answer fantastic two says all of the following are true of epimer's except blank now epimer's are monosaccharide uh diastereomers that differ in their configuration only at one carbon all right they're diastereomers let's write that down diastereomers but they only differ at one carbon so everything else every other chiral carbon is the same except for one all right so as with all diastereomers epimers are going to have different chemical and physical properties and their Optical activities are going to have no relation to each other now Indian tumors have equal but opposite active Optical activity all right and so what that means is looking at these answer choices all of the following are true of epimer's except the different configuration about only one carbon that's true so it's not a because that's a true statement we're trying to find a false statement B says they usually have slightly different chemical physical properties also true so it can't be that answer see they are diastereomers that's true right epimer's are diastereomers diastereom is that differ only at one carbon all right so this is a true statement we're going to cancel that out all right they have equal but opposite Optical activities this is false this is the definition of enantiomers not diastereomers all right so all of the following are true of ephemers except this all right because epimers are diastereomers right and this is referring to enantiomers that's how you know that this is the false statement fantastic three says aldonic acids are compounds that blink all right Alden aldonic acids form after the aldehyde group aldehyde I can't spell aldehyde group on a reducing sugar reduces another compound all right and it becomes oxidized in the process all right so the answer that says that is c aldonic acid are compounds that have been oxidized and they act as reducing agents all right so the correct answer here is C 4 says the formation of alpha D glucopyronose from beta d-glucopyronose is called blank all right this is fantastic this is a definition we mentioned briefly in the review but let's review Mutual rotation all right I'm going to write that bit muta rotation I have to like word it out to spell it spelling is not my strong suit Mutual rotation is the inter-conversion between animers of a compound now when we drew the cyclic form of glucopyronose d-glucopyronose we saw the alpha form and we saw the beta form all right and we determined that the alpha form is when the hydroxyl group is trans to the ch2oh group all right they're trans whereas the beta group they are going to the alcohol group and the ch2oh group are going to be CIS to each other all right this is why this this single difference makes them animals and this point this carbon point that we talk about here these are an American carbons they're new chiral centers that are formed in the cyclization process okay now Mutual rotation is the interconversion between animers of a compound these are animers and so that means they can convert in between each other Alpha and beta forms due to Mutual rotation all right and that means that the correct answer here is going to be B in in anti-amerization and and this racimization they're related okay and anti-amerization is the formation of a mirror image or an optically inverted form of a compound where racimization is moving a solution toward an equal concentration of both enantiomers both of which have nothing to do with the inter-conversion between Alpha and beta animers and glycosidation glyco sedation I'm gonna hope that's how you pronounce it is the addition of a sugar to another compound which is not what's happening here between the alpha and beta form of d-glucopyronose so that's how you know these are not the correct answers all right correct answer for four is B fantastic five five says keto sugars may have the ability to act as reducing sugars which process explains this all right fantastic keto sugars keto sugars they can undergo tautomerization which is a rearrangement of bonds to undergo keto enol shifts what this does is it forms an aldose which then allows them to act as a reducing sugar all right now a ketone alone a ketone group alone cannot be oxidized all right but ketose sugars can undergo tautomerization all right which is this rearrangement of bonds to undergo a Keto enol shift and this forms an Aldos oops sorry let's go back to that this can form an aldose all right that then allows them to act as a reducing sugar all right and so like we said ketones alone cannot be oxidized but they can go through this process to form all doses that can act as reducing sugars and so when we talk about keto sugars have the ability to act as reducing agents it's because they undergo tautomerization all right and that correct answer is a they cannot be oxidized directly all right they don't undergo animalization all right and um they cannot be reduced directly all right well they can be oxidized directly but um I mean they cannot be oxidized directly they cannot they don't act as reducers reducing sugars directly either after only after they go through tautomerization so the correct answer here is a fantastic six what what is the product of the following reaction all right so when glucose reacts with ethanol all right so this is glucose this is why it's important you know your structures this is glucose all right it's reacting with ethanol all right actually cross that dude this is methanol all right one glucose reacts with methanol under acid catalysis all right the hemiacetyl is going to be converted to an acetyl through the replacement of the anomeric hydroxyl group with an alkoxy group okay and this result is a type of acetyl known as glycoside all right well all that means all right is being able to distinguish that when you're reacting to sugar with some alcohol containing group in the presence of high proton concentration under acidic conditions what is going to happen here is that we're going to replace one of the groups one of the groups on the anameric comp on the n-americ carbon with this group all right so here we have our annumeric carbon all right this is the alpha form of glucose we have alcohol trans to our ch2oh group all right this is going to be replaced with methoxide ch3o minus all right and what it's going to have there instead of alcohol now is och3 all right so everything else stays the same all right this position gets converted and that is going to be B all right notice how this alcohol is converted to methoxide all right fantastic seven says which of the following enzymes Cleaves polysaccharide chains and yields maltose exclusively all right beta amylase beta amylase actually Cleaves amylose at the non-reducing end of the Palm polymer to yield maltose exclusively all right Alpha amylase on the other hand all right Alpha Emily's on the other hand Cleaves amylose anywhere along the chain to actually yield shorter polysaccharides maltose and glucose all right now this question is asking us we want to have the enzyme that yields maltose exclusively all right and so the correct answer here has to be beta all right only the beta version of amylase yields maltose exclusively and only maltose exclusively all right now debranching enzymes which is answer Choice here see here this is going to remove oligosaccharides from a branch in glycogen or starches so it's not what we're looking for and and D here glycogen phosphorylase this yields glucose one phosphate so it has nothing to do with what we're talking about in regards to yielding maltose exclusively all right so the correct answer to 7 is B