GRE Subject Test: Biology : Understanding Carbohydrates

Study concepts, example questions & explanations for GRE Subject Test: Biology

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Example Questions

Example Question #1 : Understanding Carbohydrates

What factor determines whether humans are capable of digesting a polysaccharide?

Possible Answers:

The number of monomers in the polysaccharide

The number of carbons in the carbohydrate

The linkage between monomers

The branching pattern of the polysaccharide

Correct answer:

The linkage between monomers

Explanation:

Carbohydrate monomers (monosaccharides) can be joined together with either alpha or beta linkages. Humans have the enzymes necessary in order to break down alpha linkages, but not beta linkages. Cellulose is a polysaccharide in which the monomers joined together by beta linkages, so humans cannot digest cellulose. Some bacterial species are capable of cleaving these linkages and have developed symbiotic or mutualistic relationships with animals. These bacterial species inhabit the mammalian digestive tract, cleaving beta linkages and gaining protection from the outside environment.

The size and branching pattern of a carbohydrate do not affect its ability to be digested.

Example Question #2 : Understanding Carbohydrates

Which polysaccharide is the primary form of energy storage in plants?

Possible Answers:

Cellulose

Peptidoglycan

Glycogen

Starch

Correct answer:

Starch

Explanation:

The two most common polysaccharides found in plant cells are starch and cellulose. Starch is the primary source of energy storage, while cellulose is used to construct the plant's cell walls.

Example Question #3 : Understanding Carbohydrates

What is the difference between the alpha and beta forms of a monosaccharide?

Possible Answers:

The direction all hydroxyl groups point in the ring

The number of carbons in the ring

Whether the monosaccharide is an aldose or a ketose

The orientation of the hydroxyl group on the anomeric carbon

Correct answer:

The orientation of the hydroxyl group on the anomeric carbon

Explanation:

When a monosaccharide becomes cyclic in form, the anomeric carbon can have its hydroxyl group pointing in the same direction as the methoxy group, or oriented in the opposite direction. This orientation determines whether the sugar is considered alpha or beta.

Example Question #4 : Understanding Carbohydrates

Two monosaccharides can be linked together to form a disaccharide. This linkage is known as a __________ bond.

Possible Answers:

Ionic

Phosphodiester

Hydrogen bond

Glycosidic

Peptide

Correct answer:

Glycosidic

Explanation:

Carbohydrates are linked together to form disaccharides and other polysaccharides through glycosidic linkages. A glycosidic linkage is one in where two sugar molecules are bridged by an oxygen atom. Peptide linkages are found between amino acids and phosphodiester bonds are found between nucleic acid monomers. Ionic bonds involve the complete transfer of one or more electrons from one species to another. Hydrogen bonds are weak intermolecular and intramolecular forces that contribute to the stability of many substances such as liquid water.

Example Question #5 : Understanding Carbohydrates

Glycogen and starch are polysaccharides connected through __________ linkages, and cellulose is connected via __________ linkages.

Possible Answers:

alpha . . . alpha

beta . . . alpha

alpha . . . beta

alpha . . . gamma

beta . . . gamma

Correct answer:

alpha . . . beta

Explanation:

Glycogen and starch molecules are connected by alpha linkages. Glycogen and starch can be digested by humans because we have an enzyme capable of separating these linkages to produce monosaccharides. Cellulose on the other hand is connected through beta linkages. These beta linkages allow for the polysaccharide to form straight chains which can serve structural purposes such as plant cell walls. Cellulose, however, cannot be digested by humans because we do not have enzymes capable of severing these linkages.

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