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MCAT Org. Chemistry > Biological Molecules > Flashcards

Flashcards in Biological Molecules Deck (44):

What features define a carbohydrate?

carbohydrate is an organic compound with a backbone of at least 3 carbons, typically with a C:H:O ratio of 1:2:1. Carbohydrates consist of either aldehydes or ketones with other carbons in the chain bound to hydroxyl groups.

Exceptions to the 1:2:1 ratio exist for "deoxy" sugars, in which a hydroxyl group has been replaced by a hydrogen. 


Carbohydrate structure can be shown in many ways. What name is given to the method of depiction below?

This is a Fischer projection. Straight-chain carbohydrates are often shown in this form.

On any Fischer projection, horizontal lines are considered to point out of the page ("wedges"), while vertical lines point into the page ("dashes"). Thus, this method can accurately convey stereochemistry.


Carbohydrate structure can be shown in many ways. What name is given to the method of depiction below?

This is a Haworth projection. Carbohydrates in the ring form are often shown in this manner.

The relative positions of substituents as "cis" or "trans" can be determined by whether they point upward or downward from the face of the ring.


What is the difference between an aldose and a ketose?

Aldoses are sugar molecules that contain a single aldehyde group, while ketoses contain a single ketone.

Note that both aldoses and ketoses are monosaccharides.


Briefly describe the steps for naming a straight-chain carbohydrate.

  1. The compound is first classified as either an aldose or a ketose.
  2. Next, the number of carbons is counted (di, tri, quatr, pent) and included after the aldo- or keto- prefix.
  3. Finally, the -ose suffix, used to denote a carbohydrate, is attached to the name.

Note that there should be no spaces between the prefix, numerical identifier, and suffix.


According to simple carbohydrate nomenclature, what general name can be given to this molecule?

This molecule is an aldopentose.

This carbohydrate has an aldehyde group, which should be denoted by the prefix "aldo-." As a five-carbon sugar, it should be named using the Greek term "pent." Finally, the "-ose" suffix identifies the molecule as a carbohydrate.


How are cyclic carbohydrates named?

Cyclic carbohydrates are named based on the number of members in the ring. The two main categories of cyclic carbohydrates are the five-membered furanoses and the six-membered pyranoses.

To denote a carbohydrate in the ring form, remove "-se" from the carbohydrate name and replace it with the suffix "-furanose" or "-pyranose," as is appropriate. 


With regard to its cyclic structure, what general name can be given to this molecule?

This molecule is a pyranose.

The carbohydrate has a six-membered ring form, so it is a pyranose. Five-membered rings are known as furanoses.


What is the common name of this molecule?

This molecule is glucose.

Glucose, a hexose, is the most common carbohydrate tested on the MCAT. Its Fischer projection can be identified by the orientation of its hydroxyl groups. From top to bottom, the -OH groups will be oriented either "right, left, right, right" (D-glucose) or "left, right, left, left" (L-glucose). 


What is the common name of this molecule?

This molecule is fructose.

Fructose, a pentose, is often mentioned on the MCAT. Its Fischer projection can be identified by the orientation of its hydroxyl groups. From top to bottom, the -OH groups will be oriented either "left, right, right" (D-fructose) or "right, left, left" (L-fructose). 


What are the three main structural ways in which carbohydrates are classified?

  1. As aldoses (aldehyde-containing carbohydrates) or ketoses (ketone-containing carbohydrates)
  2. As furanoses (five-membered rings) or pyranoses (six-membered rings)
  3. As regular sugars (those with oxygen atoms bound to each carbon) or deoxy sugars (those missing one oxygen atom)


What is the difference between a monosaccharide and a disaccharide?

A monosaccharide is composed of one cyclic sugar subunit, while a disaccharide is composed of two cyclic sugars connected by a glycosidic linkage.

A polysaccharide, then, consists of more than two sugar subunits bound together.


Name three common monosaccharides.

Three of the most common monosaccharides to appear on the MCAT are glucose, fructose, and galactose. Ribose and deoxyribose are also monosaccharides.

Monosaccharides can be connected by glycosidic linkages to form longer sugar chains.


Name three common disaccharides.

Three of the most common disaccharides to appear on the MCAT are sucrose, lactose, and maltose.

In general, disaccharides consist of two sugar subunits bound by a glycosidic linkage.


Which sugar monomers comprise sucrose, lactose, and maltose, respectively?

  • Sucrose is composed of fructose and glucose
  • Lactose is composed of glucose and galactose
  • Maltose is composed of two glucose monomers


Which carbon is used to determine the absolute configuration of a carbohydrate?

The absolute configuration of a straight-chain carbohydrate is determined by the chirality of the carbon stereocenter that is farthest from the carbonyl carbon.

In other words, the absolute configuration depends on the highest-numbered chiral carbon in the molecule.


What determines whether the absolute configuration of a carbohydrate is D or L?

Any carbohydrate depicted as a Fischer projection:

  • is classified as a D sugar if the -OH on the highest-numbered chiral carbon is pointing to the right.
  • is classified as an L sugar if that same -OH is instead pointing to the left. 


What is the absolute configuration of this carbohydrate?

This carbohydrate has a D configuration.

Look at the -OH on the chiral carbon farthest from the carbonyl group. Since that -OH is pointing to the right on the Fischer projection, this must be a D sugar.


What are epimers?

Epimers are two molecules that differ in configuration at a single chiral carbon.

Epimers are a special type of diastereomer. They are especially important in relation to sugars.


What type(s) of isomer are the carbohydrates below?

These carbohydrates are epimers, since they differ in configuration at only one position: the third carbon from the carbonyl.

Like all epimers, these molecules are also diastereomers.


Describe the stereochemical relationship between carbohydrates 1, 2, and 3.

  • 1 and 2 are epimers differing at the chiral carbon closest to the carbonyl. Like all epimers, they are also diastereomers.
  • 2 and 3 are enantiomers, or nonsuperimposable mirror images of each other. They differ in configuration at all chiral centers.
  • 1 and 3 are diastereomers, since they differ in configuration at 3 of 4 chiral carbons. Diastereomers are stereoisomers which differ at some (but not all) stereocenters.


Describe the stereochemical relationship between carbohydrates 1, 2, and 3.

  • 1 and 2 are epimers differing at the chiral carbon closest to the carbonyl. Like all epimers, 1 and 2 are also diastereomers.
  • 1 and 3 are epimers differing at the chiral carbon second from the carbonyl. Again, 1 and 3 are also diastereomers.
  • 2 and 3 have no formal relationship. 


Two carbohydrates are structurally identical but differ in configuration at three of their four chiral centers. What type(s) of isomer are these molecules?

These molecules are diastereomers, a broad term for stereoisomers that are not enantiomers.

Specifically, these molecules cannot be enantiomers since they do not have opposite configurations at all of their chiral carbons. They also are not epimers, which only differ in configuration at one position.


For a carbohydrate to isomerize between the straight-chain and ring forms, what reaction must take place?

In the straight-chain form, the -OH on the chiral carbon farthest from the carbonyl acts as a nucleophile and attacks the carbonyl carbon. This forms a cyclic structure.

In the proccess, the attacking hydroxyl loses its proton and the carbonyl oxygen becomes protonated.


What name is given to the most oxidized carbon in the ring form of a carbohydrate?

This carbon is called the anomeric carbon. Its stereochemical configuration is another way by which carbohydrates are classified.

Below, the anomeric carbon is bound to a hydroxyl group and to the oxygen member of the ring. It derives from the carbonyl carbon in the straight-chain form.


In the ring form of a carbohydrate, how can the anomeric carbon be identified?

The anomeric carbon is the only carbon that is bound to two oxygen atoms.

For this reason, all anomeric carbons are part of another functional group: acetals and hemiacetals (or ketals and hemiketals).


What is an acetal?

An acetal is a functional group that is derived from a carbonyl compound. It consists of two -OR groups bound to the same carbon.

Conventionally, "acetal" has referred to compounds derived from aldehydes, while "ketal" refers to those derived from ketones. The MCAT will likely use "acetal" to refer to both groups.


What reactants are necessary to form an acetal?

An acetal is formed when an aldehyde is reacted with two equivalents of alcohol compounds. The -OH acts as the nucleophile and attacks the aldehyde carbon, as shown below.


What is the difference between an acetal and a hemiacetal?

The carbon on an acetal is bound to two -OR groups, while the carbon on a hemiacetal is bound to one -OR and one -OH.

"Hemi-" means "half," so a hemiacetal is simply a half-formed acetal. In other words, it is the product of a carbonyl compound and a single alcohol.


What functional group is circled in the diagram below?

This functional group is a hemiacetal. It contains a single carbon (the anomeric carbon) bound to one -OR and one -OH group.

The molecule shown below is cyclic glucose, a pyranose.


What functional group is circled in the diagram below?

This functional group is an acetal. It contains a single carbon (the anomeric carbon) bound to two -OR groups. Often, the hemiacetal functionality on a monosaccharide becomes an acetal to form a glycosidic linkage.

The molecule shown below is maltose, a disaccharide composed of two glucose subunits.


What is the difference between the alpha and beta anomers of a carbohydrate?

In a Haworth projection:

  • the -OH on the alpha anomer is trans to the carbon substituent on the ring
  • the -OH on the beta anomer is cis to the carbon substituent on the ring 



Mutarotation is the equilibration of alpha and beta anomers in solution. During this process, the optical rotation of the mixture changes.

Mutarotation occurs due to the opening and closing of carbohydrate rings. When the carbohydrate opens, its carbonyl group can then be attacked from a different side, forming either the alpha or beta anomer.  


What type of linkage does the following disaccharide contain?

These monomers are connected by an alpha linkage.

The nomenclature is determined by the anomeric carbon participating in the linkage. Here, the left carbohydrate's anomeric carbon is linked to the right carbohydrate via an oxygen atom. Since the configuration of the left anomeric carbon is alpha, this is an alpha linkage.


What reaction is used to break the linkage between carbohydrate monomers?

Glycosidic linkages are broken via hydrolysis.

As a nucleophile, water can attack the anomeric carbon in the linkage; the other carbohydrate will act as the leaving group. This reaction is a specific example of the hydrolysis of an acetal. 


Which groups on a monosaccharide can be altered by a mild oxidation reaction?

In a mild oxidation, the aldehyde group on an aldose can be oxidized to a carboxylic acid, as can the terminal carbon.

Tollen's reagent can only react with the aldose's aldehyde group. In contrast, nitric acid can oxidize both the aldose aldehyde and the terminal carbon to carboxylic acids.


What product is formed by the reduction of a monosaccharide?

The reduction of a monosaccharide will generate a polyalcohol.

The aldehyde or ketone group will be reduced to a hydroxy group. The existing hydroxy groups will not be reduced further.


What do Benedict's and Tollens' test detect?

These tests detect the presence of reducing sugars. These are carbohydrates that can act as reducing agents; in other words, they can oxidize themelves.


What reagents are involved in Benedict's test, and what does a positive result look like?

Benedict's test produces a dark red precipitate. The reagent involved is a mixture of sodium citrate, sodium carbonate, and copper sulfate pentahydrate.


What reagents are involved in Tollens' test, and what does a positive result look like?

Tollens' test yields a silver mirror-like product. The reagent involved is a mixture of ammonia and silver nitrate.


Which components of Benedict's and Tollens' reagents directly facilitate the detection of reducing sugars?

These reagents contain metal ions, which can be reduced by compounds that are prone to oxidation. Reducing sugars are examples of such compounds.

The metal ion in Benedict's test is Cu2+, while that in Tollens' test is Ag+.


In a ring structure, the presence of which functional group categorizes a carbohydrate as a reducing sugar?

Reducing sugars in the ring form contain hemiacetals (or hemiketals). Since they contain an -OH group, they can further oxidize when that -OH is replaced by an -OR.

Note that reducing sugars do not reduce themselves - they oxidize, thus acting as reducing agents! In a straight-chain form, aldehydes are the classic marker of a reducing sugar, as they can easily oxidize to carboxylic acids.


Which of the molecules shown below is classified as a reducing sugar?

Molecule B is a reducing sugar. Unlike Molecule A, it contains a hemiacetal group, and is thus able to further oxidize.

Molecule B is maltose, a disaccharide. Molecule A is sucrose and is also a disaccharide.


Which new functional group is created by the reaction of a carbohydrate with acetic anhydride in pyridine?

Assume the acetic anhydride is present in excess.

An ester is formed by this reaction.

All of the carbohydrate's free hydroxyl groups are able to act as nucleophiles. They attack the carbonyl carbon of the anhydride, resulting in acylation at those positions.