MCAT Biology : Properties of Carbohydrates

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

Example Question #1 : Carbohydrates

Which carbon in a carbohydrate determines whether a human is capable of digesting it properly?

Possible Answers:

The first chiral carbon

The anomeric carbon

The carbonyl carbon

All chiral carbons affect its digestion capabilities

Correct answer:

The first chiral carbon

Explanation:

Humans are only capable of digesting the "D" isomer of a carbohydrate. The carbon that determines whether a carbohydrate is "D" or "L" is the first chiral carbon in the sugar. If it points to the right, the sugar can be digested by humans.

Example Question #2 : Carbohydrates

Which type of bond is created between carbohydrates and the sidechain amine of select asparagine residues in proteins?

Possible Answers:

Neither is correct

Both are correct

O-glycosidic

N-glycosidic

Correct answer:

N-glycosidic

Explanation:

A glycosidic bond covalently joins a carbohydrate molecule to another molecule. An O-glycosidic bond is a covalent linkage between a carbohydrate and a protein, joining a serine or threonine hydroxyl side chain and a sugar (oxygen in the bond yields "O"). An N-glycosidic linkage involves bonding of a carbohydrate and a protein, joining an asparagine side chain amide and a sugar (nitrogen in the bond yields "N"). Thus, N-glycosidic is the correct answer.

Example Question #3 : Carbohydrates

Which of the following statements is true concerning glucose polymers?

Possible Answers:

Starch is a glucose polymer held together by an -(1,6) linkage.

Humans do not eat foods that contain cellulose.

Glucose is not favored in the body in ring form.

Humans lack the enzyme that can break down the beta linkages in cellulose.

Correct answer:

Humans lack the enzyme that can break down the beta linkages in cellulose.

Explanation:

Glucose is the primary carbohydrate absorbed by humans, and exists for its majority of time in the body in ring form. Humans are only capable of digesting alpha linked glucose polymers, and are unable to break down beta linked polymers. As a result, cellulose is incapable of being broken down by humans.

Starch is formed by an -1,4 linkage, while glycogen is formed by an -1,6 linkage. Both of these can be digested, as they do not contain beta linkages.

Example Question #4 : Carbohydrates

Cellulose is a type of carbohydrate used in plants to store glucose. Cellulose cannot be broken down by humans, but is able to be digested by ruminants (cows for example). What is the best explanation for why ruminants are able to digest cellulose?

Possible Answers:

Ruminants create an enzyme that allows them to digest cellulose

Ruminants house bacteria in their digestive tracts that are able to break down the cellulose

Ruminants rearrange the molecular configuration of cellulose, allowing it to be digested by other enzymes

Ruminants have a stomach pH of roughly 2.5

Ruminants have four stomachs which allow them to digest food for a much longer time

Correct answer:

Ruminants house bacteria in their digestive tracts that are able to break down the cellulose

Explanation:

Cellulose and other polysaccharides with beta linkages can only be broken down by bacteria. As a result, cows have a symbiotic relationship with bacteria that live in their digestive systems.

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