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MCAT Biochemistry Review 2025-26 > Chapter 4: Carbohydrate Structure and Function > Flashcards

Chapter 4: Carbohydrate Structure and Function Flashcards

1
Q

There are four manners of classifying carbohydrates. What are they?

A

Number of sugar moieties that make them up

Number of carbons in each sugar

Functional groups present on the molecule

Stereochemistry of the sugar

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2
Q

What is the most basic carbohydrate unit?

A

Monosaccharide

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3
Q

How many carbons do the simplest monosaccharides have?

A

Three carbon atoms called trioses.

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4
Q

Monosaccharides are categorized by how many carbons they have. Name them.

A

Triose
Tetrose
Pentose
Hexose

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5
Q

Carbohydrates with an aldehyde are called what?

Draw the simplest one, L-glyceraldehyde

A

They are called aldoses

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6
Q

Carbohydrates with an ketone groups are called what?

Draw the simplest one, dihydroxyacetone.

A

They are called ketoses

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7
Q

What is the name for a 6 carbon sugar with an aldehyde group? 5 carbon sugar with a ketone?

A

Aldohexose

Ketopentose

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8
Q

There are four six carbon sugars I need to be able to recognize and identify. What are they?

A

D fructose
D glucose
D galactose
D mannose

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9
Q

What is an enantiomer?

A

An enantiomer is an optical isomer, also called a stereoisomer, that have the same chemical formula but differ in their spatial arrangement of their component atoms such that they are nonidentical nonsuperimposable mirror images of each other.

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10
Q

What is a chiral carbon atom?

How can you tell a chiral carbon?

A

A chiral center has 4 different groups attached to it.

A carbon without 4 different groups is achiral.

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11
Q

How do you calculate the number of stereoisomers with a common backbone?

A
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12
Q

What is the D and L notation based off of? Is this directly related to + and - designations denoting optical rotation?

A

D and L notation is based off of the optical rotation of glyceraldehyde. D made a positive rotation, L made a negative rotation.

This is NOT directly related to + and - designation of denoting optical rotation. + and - need to be experimentally determined.

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13
Q

How do we number the carbon priority for organic molecules?

A

To number carbons in organic molecules, you start by identifying the longest carbon chain and then number it from the end that gives the substituents (groups attached to the chain) the lowest possible numbers.

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14
Q

How do we number carbons of aldoses? Draw glyceraldehyde and number the carbons.

A

Aldoses contain a carbonyl carbon. In an aldose, the aldehyde carbon will always be carbon number 1 (C-1)

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15
Q

What is glycosidic linkage?

Can aldoses do glycosidic linkage?

Can ketoses do glycosidic linkage?

A

A glycosidic bond, also known as a glycosidic linkage, is a covalent chemical bond that connects a sugar molecule, or carbohydrate, to another group.

In an aldose, the aldehyde carbon (carbonyl carbon) can participate in glycosidic linkages.

Ketoses can participate in glycosidic linkage at the carbonyl carbon.

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16
Q

What is a glycosyl residue?

A

Sugars acting as substituents via glycosidic linkage are known as glycosyl residues.

Glycosyl residues are crucial in forming glycosides, glycoproteins, and other complex carbohydrates.

Glucosyl residues are sugar units, specifically glucose, that are linked to other molecules, forming part of larger structures like polysaccharides (e.g., starch, cellulose) or glycoproteins.

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17
Q

What is the simplest ketone sugar (ketose)? Draw it and label the carbons.

A

Dihydroxyacetone

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18
Q

How do we assign priority to ketoses?

A

The lowest number the carbonyl group can be is 2 (C-2).

For most ketoses on MCAT, the carbonyl carbon is C-2 in ketoses.

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19
Q

Aldose or ketose: fructose, glucose, galactose, mannose (all D). Draw them and label the carbons.

A
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20
Q

On every monosaccharide, what group will every carbon other than the carbonyl carbon carry?

A

Every carbon other than the carbonyl carbon in a monosaccharide will carry a hydroxyl group.

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21
Q

On the MCAT, how are all monosaccharides assigned D or L configuration?

A

On the MCAT, all monosaccharides are assigned the D or L configuration based on their relationship to glyceraldehyde.

All D sugars have the hydroxide of the highest number chiral center on the right.

All L sugars have the hydroxide of the highest number chiral center on the left.

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22
Q

Draw a Fischer projection for a generic chiral molecule. This is to familiarize with the notation.

A
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23
Q

What is an enantiomer, regarding sugars.

A

The same sugars in different optical families are enantiomers. Non super impossible mirror images.

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24
Q

What is a diastereomer regarding sugars.

A

Two sugars that family (both are either ketosis or or aldoses, and have the same number of carbons) that are not identical and are not mirror images of each other are diastereomers.

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25
What is an epimer regarding sugars? D ribose and D arabinose are epimers, shown in the image of the answer.
An epimer is a special kind of diastereomer where they differ at exactly one chiral center.
26
Identify the diastereomers, enantiomers, and epimers of the following:
27
How many enantiomers can a compound have? Diatereomers?
A compound can only have one enantiomer. A compound may have many diastereomers depending on how many and which chiral carbons are inverted between the two molecules.
28
MCAT concept check carbohydrate classification 4.1 page 137 question 1 What is the name for a five carbon sugar with an aldehyde group? A six carbon sugar with a ketone group?
Aldopentose Ketohexose
29
MCAT concept check carbohydrate classification 4.1 page 137 question 2 Draw all of the possible D stereoisomers of glucose in Fischer projection form.
This is a good question to look at and understand diasteromers. Interesting, there are 16 possible stereoisomers of glucose as there are 4 chiral carbons and 2^4 is 16. The question is asking how many diastereomers there are of D glucose, and there are 8. This makes sense because every one of them has one enantiomer.
30
MCAT concept check carbohydrate classification 4.1 page 137 question 3 Which of the diasteromers of glucose are epimers of glucose? Enantiomers?
31
Hand drawn Fischer projection, hemiacetyl formation, Hawthorne projection, and anomers of D glucose.
32
Monosaccharides have groups that can act as a nucleophile and an electrophile. Which groups do this? Why is this important to understand?
Carbonyl groups serve as electrophile, hydroxy groups serve as nucleophile. Since monosaccharides have two groups within their molecule that can undergo intramolecular reactions, they form hemiacetals (from aldoses) and hemiketals (from ketoses)
33
What is a hemiacetal, a hemiketal?
Ring formations formed by aldoses and ketoses respectively.
34
What is a pyranose ring? Furanose ring?
Due to steric strain, the only cyclic molecules that are stable in solution are: Six membered pyranose rings Five membered furanose rings
35
When we convert the monosaccharide from its Fischer projection to Hawthorne projection, how do we determine which way the groups face? Give an example using d glucose (alpha d glucopyranose)
When we convert the monosaccharide from its straight Fischer projection to the Hawthorne projection, ANY GROUP ON THE RIGHT IN THE FISCHER WILL POINT DOWN.
36
Draw Fischer and Hawthorne projection for d fructose
37
What is mutarotation? What is an anomer?
The spontaneous change of configuration about C1 is known as mutarotation. Exposing hemiacetal rings to water will cause them to spontaneously cycle between the open and closed form. Because the substituent on the single bond between C1 and C2 can rotate freely, either an alpha or beta anomer can be formed.
38
Draw d glucopyranose converting to its alpha and beta anomer via mutarotation.
39
MCAT concept check cyclic sugar molecules 4.2 page 140 question 1
During hemiacetal or hemiketal, the carbonyl carbon becomes chiral and is termed the anomeric carbon. The orientation of the hydroxyl substituent on this carbon determines if the sugar molecule is the alpha or beta anomer.
40
MCAT concept check cyclic sugar molecules 4.2 page 140 question 2 Draw the less stable anomer of d glucose in Haworth projection.
41
MCAT concept check cyclic sugar molecules 4.2 page 140 question 3 Draw the less stable anomer of d glucose in chair formation.
42
Draw d glucose, work through to the alpha and beta glycopyranose.
43
Monosaccharides contain alcohol and either aldehydes or ketones. As such, these functional groups undergo the same reactions that they do when present in other compounds. What kinds of reactions are these?
Redox reaction Esterification Nucleophilic attack (creating glycosides)
44
What is an aldonic acid?
When aldehydes oxidize to carboxylic acid, they make an aldonic acid. Because aldoses can be oxidized, they are dispersed considered agents.
45
Any monosaccharide with a hemiacetal ring is considered a reducing sugar. Why?
As monosaccharide switch between anomeric configurations, the hemiacetal ring spend a short period of time in the open chain aldehyde form. Those aldehyde groups can be oxidized to carboxylic acids. These oxidized aldoses are called aldonic acid. Because aldoses can be oxidized, they are considered reducing agents. Therefore, any monosaccharide with the hemiacetal ring is considered a reducing sugar.
46
What is lactone and what does it have to do with aldoses?
A lactone is a cyclic ester with a carbonyl group persisting on its anomeric carbon. Vitamin C is a lactone. As monosaccharide switch between anomeric configurations, the hemiacetal ring spend a short period of time in the open chain aldehyde form. Those aldehyde groups can be oxidized to carboxylic acids. These oxidized aldoses are called aldonic acid. Because aldoses can be oxidized, they are considered reducing agents. Therefore, any monosaccharide with the hemiacetal ring is considered a reducing sugar. When the aldose in question is in ring form, oxidation yields a LACTONE.
47
Simple. What is a lactone?
A lactone is a cyclic ester.
48
How do we detect the presence of a reducing sugar?
Two standard reagents are used to detect the presence of reducing sugars: Tollen’s reagent Benedict’s reagent
49
What is Tollens reagent?
Tollen’s reagent is used to detect reducing sugars. It reduces to produce a silvery mirror when aldehydes are present. It’s an alkaline solution of silver nitrate.
50
What is Benedict’s reagent?
Benedict’s reagent is used to detect reducing sugars, like aldehydes. It produces a red precipitate (Cu2O) to indicate their presence. Benedict’s reagent readily oxidizes the aldehyde into a carboxyl group.
51
Will nitric acid oxidize aldehydes and ketones?
Yes. It will oxidize both the aldehyde and primary alcohol (C6) to carbolic acids.
52
How can we use enzymes to specifically test for the presence of glucose?
We can use glucose oxidase to oxidize the glucose as it will only react with glucose. Glucose oxidase reacts with glucose to produce gluconic acid. Draw it.
53
What is keto enol tautomerism? Why is this important regarding reducing sugars and testing for them?
Ketones can shift between ketone form and enol form. Because ketones shift to enols, while in the aldose form, they can react with Tollen’s and Benedict’s reagent. Ketones reduce slower than aldehydes because of this.
54
What happens when the aldehyde group of an aldose is reduced to an alcohol?
An alditol is made.
55
What happens when a hydrogen reduces a hydroxyl group of a sugar? Why is this kind of important?
A deoxy sugar is made. The most well known being d 2 deoxyribose, the carbohydrate found in DNA.
56
What is an ester?
Esters are organic compounds formed when an alcohol group bonds with an organic acid, replacing water molecules. They are derived from acids, usually carboxylic acids. Esters are known for their fruity smell and are often present in essential oils and floral waters.
57
Because carbohydrates have hydroxyl groups, they are able to participate in reactions with carboxylic acids and carboxylic acid derivatives to form esters. Show the complete esterification of beta d glucopyranose.
58
Phosphate esters are formed when glucose is phosphorylated by ATP. Draw this.
59
What does hexokinase do (glucokinase in the liver and pancreatic beta islet cells)?
Hexokinase or glucokinase phosphorylates glucose. Recall that kinase are specific tranferases that phosphorylate target molecules.
60
What is an alkoxy group?
61
Hemiacetals react with alcohols to form acetals. The resulting carbon oxygen bond is called a glycosidic bond, and the acetals formed are called glycosides. Draw what happens to glucose when reacted with ethanol and acid.
62
What happens when we react a hemiacetal (like glucose in its ring form) with an alcohol and acid? Why is this important?
This forms an acetal called a glycoside with an alkoxyl group, forming a glycosidic bond. Disaccharides and polysaccharides form as a result of glycosidic bonds.
63
What are glycosides resulting from furanose rings called? What are glycosides resulting from pyranose rings called?
Furanosides Glucosides
64
What is furanose ring? Pyranose ring?
A furanose ring is a five-membered ring structure found in cyclic monosaccharides, consisting of four carbon atoms and one oxygen atom, resembling the compound furan. In organic chemistry, a pyranose ring is a six-membered cyclic structure found in many carbohydrates, containing five carbon atoms and one oxygen atom, resembling the cyclic ether pyran.
65
MCAT concept check 4.3 monosaccharides page 143 question 1 Explain the difference between esterification and glycoside formation.
Esterification: hydroxyl group reacts with carboxylic acids (or derivative) to form and ester. Glucoside formation: alcohol and hemiacetal (or hemiketal) group to form an alkoxy group. Page 142 and 143
66
MCAT concept check 4.3 monosaccharides page 143 question 2 What purpose do Tollen’s reagent and Benedict’s reagent serve? How do they differ?
Tollen’s: silver nitrate forms a mirror in the presence of reducing sugars. Benedict’s: produces red precipitate in the presence of reducing sugars. All monosaccharides, including aldoses and ketoses, are reducing sugars.
67
Are all alsoses and ketoses reducing sugars?
Yes. All monosaccharides, including aldoses and ketoses, are reducing sugars.
68
MCAT concept check 4.3 monosaccharides page 143 question 3 From a metabolic standpoint, does it make sense for carbohydrates to get oxidized or reduced? What is the purpose of this process?
The process of oxidizing sugars releases their energy for metabolic processes. It makes sense to oxidize sugar in order to reduce others. This is how the process of oxidative phosphorylation works.
69
Monosaccharides react with alcohols to produce acetals with a glycosidic linkage. Why is this relevant to disaccharides?
Glycosidic bonds formed between hydroxyl groups of two monosaccharides result in the formation of a disaccharide.
70
What is a homopolysaccharide? Give examples. What is a heteropolysaccharide?
A homopolysaccharide is a polysaccharide composed of entirely one monosaccharide. A heteropolysaccharide is a polymer made up of more than one monosaccharide. Cellulose is a repeating glucose structure (1,4 linked beta d glucose) Starches are polymers of 1,4 linked alpha d glucose.
71
What is cellulose? What enzyme digests cellulose?
Cellulose is the main structural component of plants. It is a homopolysaccharide chain of beta 1,4 glycosidic bonds. Said another way, cellulose is a polymer of 1,4 linked beta d glucose. The enzyme that digests cellulose is cellulase.
72
What are starches? What are two of the most predominant starch found in plants?
Starches are polysaccharides that are more digestible to humans because they are 1,4 linked alpha d glucose. Amylose (linear) and amylopectin (branched) are two common starches found in plants.
73
What is amylose? Amylopectin? What enzymes digest them?
Amylose is the predominant form of starch found in plants and is a linear glucose polymer linked via alpha 1,4 glycosidic bonds. Amylopectin starts with same linkage as amylose but also contains branches via alpha 1,6 glycosidic bonds (branches about every 25 units)
74
What is beta amylase? What are the products?
Beta amylase cleaves amylose at the non reducing end (the end with the acetal). Yields maltose.
75
What is alpha amylase? That are the products?
Cleaves randomly along the chain of of amylose to yield maltose, glucose, and short polysaccharide chains.
76
What is glycogen?
Glycogen is a carbohydrate storage unit in animals. Similar to starch but has more alpha 1,6 glycosidic bonds (more branches means more storage). Amylopectin has a branch every 25 units, glycogen every 10.
77
What is glycogen phosphorylase? Where is glycogen phosphorylase found?
Glycogen phosphorylase is an enzyme that cleaves glucose from the non reducing end of glycogen branch and phosphorylates it, producing glucose 1-phosphate, which plays annotate role in metabolism. Glycogen phosphorylase is found in the liver.
78
What is glucose 1-phosphate?
Glucose 1- phosphate plays an important role in metabolism. It is the product of glycogen phosphorylase and glycogen.
79
What is Hers disease? Hint: glycogen.
Glycogen storage disease. It is a deficiency in glycogen phosphorylase in the liver. Patients with Hers disease cannot break down glycogen in their livers and therefor have swollen livers (hepatomegaly). They may also have hypoglycemia (low blood sugar) between meals because they cannot use glycogen to maintain blood glucose concentrations.
80
MCAT concept check complex carbohydrates 4.4 page 148 question 1 Which of the two forms of starch is more soluble in solution? Why?
Amylopectin is more soluble in solution because if it’s branched nature. The highly branched structure of amylopectin decreases intermolecular binding between polysaccharide polymers and increases interaction with the surrounding solution.
81
MCAT concept check complex carbohydrates 4.4 page 148 question 2 Regarding amylopectin and glycogen, which of these two polymers should experience a higher rate of enzyme activity from enzymes that cleave side branches? Why?
Glycogen has higher rate of enzymatic branch cleavage because it contains significantly more branching than amylopectin.
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MCAT mastery carbohydrate structure and function page 124 question 1
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MCAT mastery carbohydrate structure and function page 124 question 2
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MCAT mastery carbohydrate structure and function page 124 question 3
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MCAT mastery carbohydrate structure and function page 124 question 4
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MCAT mastery carbohydrate structure and function page 124 question 7
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MCAT mastery carbohydrate structure and function page 124 question 8
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MCAT mastery carbohydrate structure and function page 124 question 9
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MCAT mastery carbohydrate structure and function page 124 question 10
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MCAT mastery carbohydrate structure and function page 124 question 11
93
What is maltose?
Maltose, also known as malt sugar or maltobiose, is a disaccharide formed by two glucose molecules joined by an α-1,4-glycosidic bond.
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MCAT mastery carbohydrate structure and function page 124 question 12
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MCAT mastery carbohydrate structure and function page 124 question 13
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MCAT mastery carbohydrate structure and function page 124 question 14
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MCAT mastery carbohydrate structure and function page 124 question 15
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Couple of things to remember from chapter 4

MCAT Biochemistry Review 2025-26 (12 decks)

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