Carbohydrates

Question 1

What suffix does all carbohydrates end with?

Answer 1

All carbohydrates end with the suffix “-ose”

Question 2

What are the 4 common carbohydrates and their structure that should be memorized for the MCAT?

Answer 2

1. D-Fructose
2. D-Glucose
3. D-Galactose
4. D-Mannose

Question 3

What is considered when naming a carbohydrate?

Answer 3

The number of carbons, the functional group (aldehyde vs. ketone), and the suffix “-ose”.

• Ex: aldohexose
  
  • 6 carbons = hex
  • Aldehyde functional group = aldo prefix
  • suffix “ose” indicating carbohydrate

Question 4

What are two carbohydrates that are mirror images of each other called?

Answer 4

Two carbohydrates that are mirror images of each other are also called enantiomers.

Question 5

What are two carbohydrates that differ at least at one stereocenter but are not mirror images of each other?

Answer 5

Two carbohydrates that differ at at least one stereocenter but are not mirror images of each other are diastereomers.

Question 6

What are two carbohydrates that differ at a single stereoisomer but are not mirror images of each other?

Answer 6

Epimers are two carbohydrates that differ at a single stereoisomer but are not mirror images of each other.

• Epimers are a subtype of diastereomers

Question 7

Answer 7

Question 8

How do hemiacetals form?

Answer 8

Hemiacetals form when an alcohol reacts with an aldehyde.

Question 9

How do hemiketals form?

Answer 9

Hemiketals form when an alcohol reacts with a ketone.

Question 10

What is a pyranose?

Answer 10

A pyranose is a sugar that cyclizes into a 6 carbon ring.

• More stable than furanoses and therefore more common in nature.

Question 11

What is a furanose?

Answer 11

A furanose is a sugar that cyclizes into a 5 carbon ring.

• Less stable than pyranoses and less common in nature

Question 12

What are anomers?

Answer 12

Anomers are a special class of C’ 1 epimers for cyclic carbohydrates. The 1’ carbon is known as the anomeric carbon.

• Alpha’s going down (OH group pointing down)
• Beta (OH group pointing up)

Question 13

What is mutorotation in relation to carbohydrates?

Answer 13

Carbohydrates undergo mutorotation. This means the can interconvert or mutate and rotate between anomeric cyclic sugars (alpha or beta: no preference for either). They can also just exist in the straight chain form, but more likely to be found in the cyclic form.

Question 14

Answer 14

Question 15

How can relative levels of oxidation or reduction be estimated in carbohydrates?

Answer 15

The relative levels of oxidation or reduction in carbohydrates can be estimated by counting the bonds to oxygen.

• If the number of bonds to oxygen is increasing, oxidation is occuring; if the number is decreasing, reduction is occuring.

Question 16

Why are aldoses considered reducing sugars?

Answer 16

Aldoses are considered reducing sugars because they can be oxidized (a property of a reducing agent).

• Reducing sugars can be identified using Benedict’s reagent.

Question 17

What is tautomerization and what type of carbohydrate undergoes this mechanism?

Answer 17

Tautomerization is a process in which a ketose (a non-reducing sugar) can be converted to an enol and then to an aldose (a reducing sugar).

Question 18

In substitution reactions, what do hydroxyls tend to act like?

Answer 18

Hydroxyl groups often act as nucleophiles in substitution reactions.

• This is true during esterification reactions

Question 19

What is a common form of esterification?

Answer 19

Phosphorylation is a common form of esterification.

• This is how glycosidic bonds are formed.

Question 20

What is a glycoside?

Answer 20

A glycoside is a molecule that contains a bond between a functional group and one of the carbons on a sugar..

• The bond between the functional group and the sugar’s carbon is termed a glycosidic linkage.
• Glycosidic linkages are what form disaccharides and polysaccharides.

Question 21

How do glycosidic linkages form?

Answer 21

Question 22

Answer 22

Question 23

True or false: Cellulose and amylose are polysaccharide chains of glucose that only differ in the orientation of their glycosidic linkages?

Answer 23

True:

Question 24

What is the mechanism undergone by disaccharide formation (glycosidic linkage)?

Answer 24

The hydroxyl group acts as a nucleophile attacking the slightly positive carbon on the other sugar.

Question 25

What are three common disaccharides tested on the MCAT?

Answer 25

1. Sucrose (galactose + glucose) 2. Sucrose (glucose + fructose) 3. Maltose (glucose + glucose)

Question 26

What is the difference between amylopectin (amylose chains) and glycogen?

Answer 26

The only difference between amylopectin (polysaccharide storage in some plants) and glycogen (polysaccharide storage in animals) is how often the alpha 1-6 linkages appear. * Glycogen has alpha 1-6 branches every 10 glucoses * Amylopectin has alpha 1-6 branches every 20--30 glucoses

Question 27

What is the protein in the center of glycogen?

Answer 27

Glycogenin is the protein in the middle of glycogen that serves as the scaffold for glycogen.

Question 28

What is a gycosyl residue?

Answer 28

A glycosyl residue is any sugar that participates in a glycosidic linkage.

Question 29

What is the difference between esterification and glycoside formation?

Answer 29

Esterification is the reaction by which a carboxylic acid or carboxylic acid derivative reacts with a hydroxyl group to form an ester. Glycoside formation is the reaction between an alcohol that reacts with a hemiacetal (or hemiketal) group on a sugar to yield an alkoxy group.

Question 30

What purpose do Tollens' reagent and Benedict's reagent serve and how do they differ from each other?

Answer 30

Tollens' reagent and Benedict's reagent are used to detect the presence of reducing sugars. Tollens' reagent is used to produce a silvery mirror when aldehydes are present whereas Benedict's reagent is indicated by a reddish precipitate of Cu₂O.

Question 31

What do cellulose, amylose, amylopectin, and glycogen have in similar and what are their differences?

Answer 31

Cellulose, amylose, amylopectin, and glycogen are all homopolysaccharides (polymers of only 1 type of sugar) composed of glucose. However, they all differ on their linkages and/or branching. * **Cellulose:** Beta - 1,4 linkages * **Amylose:** alpha - 1,4 linkages (no branching) * **Amylopectin:** alpha - 1,4 linkages with alpha - 1,6 branches * **Glycogen:** The same as amylopectin, however alpha - 1,6 branches occur every 10 glucose molecules instead of every 25 glucose molecules like amylopectin

Question 32

Which two forms of starch is more soluble in solution and why?

Answer 32

Amylopectin is more soluble in solution than amylose because of its branched structure. The highly branched structure of amylopectin decreases intermolecular bonding between polysaccharide polymers and increases interaction with the surrounding solution.

Question 33

Regarding glycogen and amylopectin, which of these two polymers should experience a higher rate of enzymatic activity from enzymes that cleave side branches and why?

Answer 33

Glycogen has a higher rate of enzymatic branch cleaving because it contains significantly more branching than amylopectin.