Biochemistry : Structural Carbohydrates

Study concepts, example questions & explanations for Biochemistry

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Example Questions

Example Question #1 : Structural Carbohydrates

Which of the following statements about carbohydrates is true?

Possible Answers:

Glucose is a sugar with six hydroxyl groups and an aldehyde

Amylose makes up the major component of starch by mass

None of the other statements is true

Polysaccharides have glycosidic bonds

Sucrose is a reducing sugar

Correct answer:

Polysaccharides have glycosidic bonds

Explanation:

Glucose has five (not six) hydroxyl groups. Reducing sugars either have an aldehyde group or can form one through isomerism; sucrose doesn’t fit either description. Although there are more amylose molecules than amylopectin in starch, amylose is a minor component by mass; amylopectin makes up 70-80% of starch by mass. Polysaccharides are indeed joined in the union of two oses, which form glycosidic bonds.

Example Question #10 : Carbohydrate Structures And Functions

What functional groups are present on carbohydrates?

Possible Answers:

Amide

Hydroxyl

Phosphate

Carboxyl

Alcohol

Correct answer:

Carboxyl

Explanation:

Carbohydrate chains contain aldehyde or ketone functional groups, which are types of carboxyl groups. Remember the general formula for a carbohydrate is:  since they are hydrates (water) of carbon.

Example Question #2 : Structural Carbohydrates

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

Possible Answers:

Chitin

Peptidoglycan

Cellulose

Glycogen

Collagen

Correct answer:

Collagen

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.

Example Question #3 : Structural Carbohydrates

Both cellulose and starch are polymers of glucose; why can't and enzyme that breaks down starch also break down cellulose?

Possible Answers:

Cellulose is composed of D-glucose molecules while starch is composed of L-glucose molecules

Cellulose forms a right handed helix, while starch forms a left handed helix

Cellulose and starch are made up of different stereoisomers of glucose

Cellulose is composed of cyclic glucose molecules while starch is composed of linear glucose molecules

Glucose monomers in starch are double bonded together while they are connected by single bonds in cellulose

Correct answer:

Cellulose and starch are made up of different stereoisomers of glucose

Explanation:

Cyclized monosaccharides can exist in two different stereoisomers that depend on the orientation of the hydroxyl group on the anomeric carbon. If this  group is on the opposite side of the ring as the  group, the sugar as the alpha anomer. The opposite orientation is referred to at the beta anomer. Cellulose is a polymer of beta glucose while starch is a polymer of alpha glucose, and the bonds between these glucose monomers differ depending on their anomer. This makes it so an enzyme can recognize one or the other but not both. Note that D-sugars are biologically relevant. 

Example Question #4 : Structural Carbohydrates

Which of the following is not a glycosaminoglycan?

Possible Answers:

Keratan sulfate

Erythropoietin 

Dermatan sulfate

Heparin

Hyaluronate

Correct answer:

Erythropoietin 

Explanation:

A glycosaminoglycan (GAG) is a long, unbranched polysaccharide that consists of many repeating disaccharide units. Dermatan sulfate, keratan sulfate, hyaluronate, and heparin are all examples of glycosaminoglycans. Erythropoietin, however, is a glycoprotein hormone that stimulates production of red blood cells.

Example Question #5 : Structural Carbohydrates

Which of the following is true of the two forms of starch: amylose and amylopectin?

Possible Answers:

Amylose contains alpha 1,6 glycosidic linkages

Most of starch is composed of amylose

Amylopectin is branched while amylose is not

Amylose is composed of D-glucose units while amylopectin is composed of L-glucose units

Amylose and amylopectin are composed of different molecular units

Correct answer:

Amylopectin is branched while amylose is not

Explanation:

Amylose and amylopectin are the two components of starch. Approximately 80% of starch is composed of amylopectin. Both amylose and amylopectin are comprised of glucose units, and both have only D-glucose units. Amylose differs from amylopectin in that amylose is a single unbranched chain, but amylopectin branches. Therefore, amylopectin has both alpha 1,4 glycosidic linkages as well as alpha 1,6 glycosidic linkages.

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