Carbohydrate Structures and Functions

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Biochemistry › Carbohydrate Structures and Functions

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1

During times of glucose deprivation in the human body, the liver is able to supply glucose to the bloodstream by breaking down a large, branched polysaccharide that it holds in reserve until it is needed. Which of the following lists the correct type of glycosidic bonds found in this polysaccharide.

for branch points and for straight chain

for branch points and for straight chain

for branch points and for straight chain

for branch points and for straight chain

for branch points and for straight chain

Explanation

For this question, we're told some background information about the liver's role in providing glucose homeostasis. We're told that when blood glucose levels are lowered, the liver is able to help restore glucose levels by keeping a large polysaccharide of glucose in store. In times of need, the liver can break this compound down to provide glucose to the bloodstream.

Even though the question doesn't explicitly tell us what the polysaccharide is, we should be able to infer that the compound in question is glycogen. Therefore, to answer the question, we need to know which kind of glycosidic bonds are found in glycogen.

First, let's recall that an individual glucose molecule is composed of six carbon atoms. In its ring form, glucose can exist as one of two epimers, depending on how its ring closes when transitioning from its straight chain form to its closed ring form. The anomeric carbon of the glucose molecule can be arranged in one of two ways when its ring closes. The anomeric carbon is the one that goes from being achiral to chiral as the ring closes. In the alpha configuration, the hydroxyl group attached to the anomeric carbon faces down, while in the beta configuration it faces up.

In addition to existing as either an alpha or a beta epimer, glucose also participates in glycosidic linkages using its first, fourth, and sixth carbon atom.

In glycogen, each individual glucose molecule is in the alpha configuration. Thus, we can rule out both answer choices that include beta. Moreover, the fourth carbon atom of each glucose molecule is attached to the first carbon atom (the anomeric carbon) in the next glucose molecule in the straight chain. To make branch points at various points along the straight chain, some of the glucose molecules have their sixth carbon atom attached to the anomeric carbon of other glucose molecules.

In conclusion, glycosidic bonds are responsible for branch points and glycosidic bonds are responsible for the straight chain.

2

During times of glucose deprivation in the human body, the liver is able to supply glucose to the bloodstream by breaking down a large, branched polysaccharide that it holds in reserve until it is needed. Which of the following lists the correct type of glycosidic bonds found in this polysaccharide.

for branch points and for straight chain

for branch points and for straight chain

for branch points and for straight chain

for branch points and for straight chain

for branch points and for straight chain

Explanation

For this question, we're told some background information about the liver's role in providing glucose homeostasis. We're told that when blood glucose levels are lowered, the liver is able to help restore glucose levels by keeping a large polysaccharide of glucose in store. In times of need, the liver can break this compound down to provide glucose to the bloodstream.

Even though the question doesn't explicitly tell us what the polysaccharide is, we should be able to infer that the compound in question is glycogen. Therefore, to answer the question, we need to know which kind of glycosidic bonds are found in glycogen.

First, let's recall that an individual glucose molecule is composed of six carbon atoms. In its ring form, glucose can exist as one of two epimers, depending on how its ring closes when transitioning from its straight chain form to its closed ring form. The anomeric carbon of the glucose molecule can be arranged in one of two ways when its ring closes. The anomeric carbon is the one that goes from being achiral to chiral as the ring closes. In the alpha configuration, the hydroxyl group attached to the anomeric carbon faces down, while in the beta configuration it faces up.

In addition to existing as either an alpha or a beta epimer, glucose also participates in glycosidic linkages using its first, fourth, and sixth carbon atom.

In glycogen, each individual glucose molecule is in the alpha configuration. Thus, we can rule out both answer choices that include beta. Moreover, the fourth carbon atom of each glucose molecule is attached to the first carbon atom (the anomeric carbon) in the next glucose molecule in the straight chain. To make branch points at various points along the straight chain, some of the glucose molecules have their sixth carbon atom attached to the anomeric carbon of other glucose molecules.

In conclusion, glycosidic bonds are responsible for branch points and glycosidic bonds are responsible for the straight chain.

3

What is an aldotriose?

A monosaccharide that contains both an aldehyde and three carbons

A monosaccharide that contains three aldehydes and one carbon

A disaccharide that contains an aldehyde and three carbons

A disaccharide that contains three aldehydes and one carbon

Explanation

Aldotrioses are monosaccharides that contain both an aldehyde (an aldose) and three carbons (a triose). Knowing the definition of the word, and the breakdown of parts of the word, can help you recognize the molecule. The simplest aldotriose is glyceraldehyde.

A related concept involves ketotrioses, which are monosaccharides that contain both a ketone (a ketose) and three carbonds (a triose). Dihydroxyacetone is an example of a ketotriose.

4

What is an aldotriose?

A monosaccharide that contains both an aldehyde and three carbons

A monosaccharide that contains three aldehydes and one carbon

A disaccharide that contains an aldehyde and three carbons

A disaccharide that contains three aldehydes and one carbon

Explanation

Aldotrioses are monosaccharides that contain both an aldehyde (an aldose) and three carbons (a triose). Knowing the definition of the word, and the breakdown of parts of the word, can help you recognize the molecule. The simplest aldotriose is glyceraldehyde.

A related concept involves ketotrioses, which are monosaccharides that contain both a ketone (a ketose) and three carbonds (a triose). Dihydroxyacetone is an example of a ketotriose.

5

Why is it that reducing sugars can be metabolized in humans, but non-reducing sugars cannot?

Because reducing sugars can open their cyclic structure into the straight chain form, whereas non-reducing sugars cannot

Because reducing sugars can bind to the proteins needed for metabolism, whereas non-reducing sugars cannot

Because humans lack the enzyme that degrades beta glycosidic linkages

Because only reducing sugars can traverse the cell membrane in order to enter cells where they can be metabolized, whereas non-reducing sugars cannot

Explanation

When it comes to metabolizing sugars, only reducing sugars are able to undergo breakdown. This is because reducing sugars are able to be converted from their closed chain form into their open chain form. It is only in the open chain form that sugars such as glucose can be metabolized.

Only reducing sugars can be converted into their open chain form. The reason for this is that the anomeric carbon for these sugars is not occupied. In their ring form, such sugars exist as hemiacetals that can readily and reversibly undergo chain opening. Additionally, some hemiketals can be converted into their open chain form, but they need to be able to tautomerize into their aldose form first.

Non-reducing sugars have their anomeric carbon tied up in a bond, and thus are locked in an acetal or ketal form. Consequently, they cannot convert into their open chain form, meaning that they cannot be metabolized.

6

Why is it that reducing sugars can be metabolized in humans, but non-reducing sugars cannot?

Because reducing sugars can open their cyclic structure into the straight chain form, whereas non-reducing sugars cannot

Because reducing sugars can bind to the proteins needed for metabolism, whereas non-reducing sugars cannot

Because humans lack the enzyme that degrades beta glycosidic linkages

Because only reducing sugars can traverse the cell membrane in order to enter cells where they can be metabolized, whereas non-reducing sugars cannot

Explanation

When it comes to metabolizing sugars, only reducing sugars are able to undergo breakdown. This is because reducing sugars are able to be converted from their closed chain form into their open chain form. It is only in the open chain form that sugars such as glucose can be metabolized.

Only reducing sugars can be converted into their open chain form. The reason for this is that the anomeric carbon for these sugars is not occupied. In their ring form, such sugars exist as hemiacetals that can readily and reversibly undergo chain opening. Additionally, some hemiketals can be converted into their open chain form, but they need to be able to tautomerize into their aldose form first.

Non-reducing sugars have their anomeric carbon tied up in a bond, and thus are locked in an acetal or ketal form. Consequently, they cannot convert into their open chain form, meaning that they cannot be metabolized.

7

Which of the following is not an example of a compound that contains carbohydrate in its structure?

Collagen

Cellulose

Peptidoglycan

Chitin

Glycogen

Explanation

To identify the answer choice that is NOT a polysaccharide, let's go delve into each answer choice a bit.

Cellulose is a polysaccharide and also a very important component of the cell wall of plants. It consists of many glucose sugars bound together via beta (1-4) linkages.

Peptidoglycan is also a polysaccharide and a very important component of the cell wall of bacteria. Its structure consists of alternating N-acetylglucosamine and N-acetylmuramic acid connected via beta (1-4) linkages. Both of these are modified sugar molecules.

Chitin is a polysaccharide that plays an important role in the structure of the exoskeleton of many fungi, arthropods, and insects. Its structure consists of a long chain of N-acetylglucosamine, which is a derivative of the sugar glucose.

Collagen is the only answer choice that does not represent a carbohydrate. Rather, collagen is a protein that plays a very important role in the extracellular matrix of various connective tissues found in animals.

Glycogen is a branched form of glucose that is the storage form in animals. It contains beta (1-4) and beta (1-6) linkages.

8

You discover that your patient is lactose intolerant, having a mutation that does not allow them to produce an enzyme that cleaves the disaccharide lactose. If they had the lactase enzyme, which glycosidic bond would it cleave?

Galactose-beta-1,4-glucose

Glucose-alpha-1,4-glucose

Glucose-alpha 1,2-fructose

Galactose-alpha-1,4-glucose

Glucose-beta 1,4-glucose

Explanation

lactose is made up of galactose and glucose and is bound via a beta 1,4 glycosidic bond.

the enzyme lactase cleaves this bond to break down the sugar lactose. Maltose is glucose- alpha 1,4- glucose, and sucrose is glucose- alpha, 1,2- fructose.

9

Which of the following is not an example of a compound that contains carbohydrate in its structure?

Collagen

Cellulose

Peptidoglycan

Chitin

Glycogen

Explanation

To identify the answer choice that is NOT a polysaccharide, let's go delve into each answer choice a bit.

Cellulose is a polysaccharide and also a very important component of the cell wall of plants. It consists of many glucose sugars bound together via beta (1-4) linkages.

Peptidoglycan is also a polysaccharide and a very important component of the cell wall of bacteria. Its structure consists of alternating N-acetylglucosamine and N-acetylmuramic acid connected via beta (1-4) linkages. Both of these are modified sugar molecules.

Chitin is a polysaccharide that plays an important role in the structure of the exoskeleton of many fungi, arthropods, and insects. Its structure consists of a long chain of N-acetylglucosamine, which is a derivative of the sugar glucose.

Collagen is the only answer choice that does not represent a carbohydrate. Rather, collagen is a protein that plays a very important role in the extracellular matrix of various connective tissues found in animals.

Glycogen is a branched form of glucose that is the storage form in animals. It contains beta (1-4) and beta (1-6) linkages.

10

You discover that your patient is lactose intolerant, having a mutation that does not allow them to produce an enzyme that cleaves the disaccharide lactose. If they had the lactase enzyme, which glycosidic bond would it cleave?

Galactose-beta-1,4-glucose

Glucose-alpha-1,4-glucose

Glucose-alpha 1,2-fructose

Galactose-alpha-1,4-glucose

Glucose-beta 1,4-glucose

Explanation

lactose is made up of galactose and glucose and is bound via a beta 1,4 glycosidic bond.

the enzyme lactase cleaves this bond to break down the sugar lactose. Maltose is glucose- alpha 1,4- glucose, and sucrose is glucose- alpha, 1,2- fructose.

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