Question 1

Which of the following best describes a carbohydrate?

Accepted Answer

A polyhydroxy aldehyde or ketone, often with the empirical formula $$\mathrm{(CH_2O)_n}.$$

Question 2

How many stereoisomers are possible for an aldohexose, and why?

Accepted Answer

16, because there are 4 chiral centers each with 2 possible configurations: $$2^4 = 16$.

Question 3

Which statement correctly distinguishes between a homopolysaccharide and a heteropolysaccharide?

Accepted Answer

Homopolysaccharides are composed of one type of sugar monomer, while heteropolysaccharides contain two or more different types of sugar monomers.

Question 4

What is the difference between an aldose and a ketose?

Accepted Answer

An aldose contains an aldehyde group ($$-CHO$$), while a ketose contains a ketone group ($$\u003eC=O).$$

Question 5

Why does cyclization of glucose produce a mixture of two anomers?

Accepted Answer

Because the nucleophilic oxygen can attack either side of the electrophilic carbonyl carbon, forming $$\alpha$$ and $$\beta$$ anomers.

Question 6

Which of the following correctly describes the difference between $$\alpha$$ and $$\beta$$ anomers of glucose?

Accepted Answer

$$\alpha$$: the hydroxyl group on the anomeric carbon and carbon 6 are on opposite sides of the ring; $$\beta$$: they are on the same side.

Question 7

What is a glycosidic bond?

Accepted Answer

A covalent bond formed between the anomeric carbon of a sugar and an alcohol or amine group.

Question 8

Why is sucrose considered a non-reducing sugar?

Accepted Answer

Because both anomeric carbons are involved in the glycosidic bond, so neither can be oxidized.

Question 9

Which glycosidic bond is found at the branch points in amylopectin and glycogen?

Accepted Answer

$$\alpha(1 \rightarrow 6)$$

Question 10

Why do animals store glucose in the form of glycogen rather than as free glucose?

Accepted Answer

Because storing glucose as glycogen prevents high osmotic pressure that would result from free glucose.

Question 11

Which of the following statements about cellulose is correct?

Accepted Answer

Cellulose is composed of $$\beta(1 \rightarrow 4)$$ glycosidic bonds, forming strong fibers stabilized by hydrogen bonding.

Question 12

Why can't most animals digest cellulose?

Accepted Answer

Because they lack enzymes that can hydrolyze $$\beta(1 \rightarrow 4)$$ glycosidic bonds.

Question 13

What is the difference between O-linked and N-linked glycoproteins?

Accepted Answer

O-linked glycoproteins have sugars attached to the oxygen atom of serine or threonine; N-linked glycoproteins have sugars attached to the nitrogen atom of asparagine.

Question 14

Which of the following is true about the formation of a hemiacetal in monosaccharides?

Accepted Answer

It involves the reaction of an aldehyde group with an alcohol group within the same molecule.

Question 15

Which of the following best explains why the anomeric carbon of a monosaccharide is more reactive than other carbons?

Accepted Answer

It is connected to two oxygen atoms, which are electron withdrawing, making the carbon more electrophilic.

Question 16

Which of the following is a correct description of a reducing sugar?

Accepted Answer

A sugar that contains a free anomeric carbon capable of being oxidized.

Question 17

Which of the following is true about the structure of 2-deoxyribose compared to ribose?

Accepted Answer

2-deoxyribose has a hydrogen atom instead of a hydroxyl group on carbon 2.

Question 18

Which of the following best explains why antibiotics that inhibit cell wall biosynthesis specifically kill bacteria?

Accepted Answer

Because bacterial cell walls contain unique polysaccharides not found in human cells.

Question 19

Which enzyme is required to synthesize ribonucleotides but not deoxyribonucleotides in cells?

Accepted Answer

Ribofuranose-specific enzymes, due to substrate specificity for ribose in nucleotide synthesis

BIOCHEM Chap. 7: Carbohydrates

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