MCAT Biology : Carbohydrates

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

Example Question #1 : Types Of Carbohydrates

Drain cleaners a common household staple, used to open clogged drains in bathtubs and sinks. Human hair is a common culprit that clogs pipes, and hair is made predominately of protein. Drain cleaners are effective at breaking down proteins that have accumulated in plumbing. Drain cleaners can be either acidic or basic, and are also effective at breaking down fats that have accumulated with proteins.

A typical reaction—reaction 1—which would be expected for a drain cleaner on contact with human hair, would be as follows in an aqueous solution:

Pic_1

Another reaction that may occur, reaction 2, would take place as follows in an aqueous solution:

Pic_2

In contrast to the proteins in Reactions 1 and 2, the body uses carbohydrates __________.

Possible Answers:

to store energy for immediate use

to dissolve nonpolar solutes in blood

to function as a secondary energy source after fatty acids have been oxidized

to create strucutral elements of cells

to store energy for use over many days

Correct answer:

to store energy for immediate use

Explanation:

Carboydrates are the main energy currency of cells. They are typically burned first, before fats and proteins, to generate energy. Proteins, in contrast, are usually the functional biomolecules, serving structural and enzymatic roles.

Example Question #1 : Carbohydrates

Which of the following accurately describes glucose?

Possible Answers:

Aldopentose

Ketopentose

Ketohexose

Aldohexose

Correct answer:

Aldohexose

Explanation:

Glucose is composed of six carbons. When not in ring form, there is an aldehyde at the end of the molecule. As a result, glucose is an example of an aldohexose.

Example Question #2 : Types Of Carbohydrates

Which of the following carbohydrates is created in animals, but not in plants?

Possible Answers:

Starch

Glycogen

Cellulose

Amylose

Correct answer:

Glycogen

Explanation:

Glucose is stored in animals cells in the form of glycogen. Plants store glucose as either starch or cellulose. Amylose is a specialized component of starch, and plays a key role in plant energy storage.

Plants do not form glycogen, similar to the way that humans cannot form (or break down) cellulose.

Example Question #1 : Carbohydrates

Which of the following statements is true concerning glucose?

Possible Answers:

The 6th carbon attacks the carbonyl carbon in order to create the cyclic structure of glucose

Maltose and cellulose differ by the type of linkage between the glucose monomers

-glucopyranose has the anomeric carbon's alcohol group pointing in the opposite direction of the 6th carbon's alcohol group

D-glucose has the highest chiral carbon's hydroxyl group pointing to the left

Correct answer:

Maltose and cellulose differ by the type of linkage between the glucose monomers

Explanation:

Maltose and cellulose are both composed of glucose monomers that are combined in a 1,4 glycosidic linkage, however they differ by the type of 1,4 linkage used. Cellulose uses a beta 1,4 linkage, while maltose uses an alpha 1,4 linkage.

Example Question #2 : Carbohydrates

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

Possible Answers:

All chiral carbons affect its digestion capabilities

The anomeric carbon

The carbonyl carbon

The first chiral carbon

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 #3 : Carbohydrates

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

Possible Answers:

N-glycosidic

O-glycosidic

Both are correct

Neither is correct

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 #4 : Carbohydrates

Which of the following statements is true concerning glucose polymers?

Possible Answers:

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

Glucose is not favored in the body in ring form.

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

Humans do not eat foods that contain 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 #1 : Properties Of 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 house bacteria in their digestive tracts that are able to break down the cellulose

Ruminants create an enzyme that allows them to digest cellulose

Ruminants have a stomach pH of roughly 2.5

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

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

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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