Quiz: Carbohydrates: Oses and Osides — 25 questions

1. What occurs when the free pseudoaldehydic or pseudoketonic function of an ose reacts with Fehling’s solution in alkaline conditions?

A. The ose hydroxyl groups form phosphate esters

B. The ose carbonyl is reduced to a carboxylic acid

C. Blue Cu2+ is reduced to brick-red Cu2O

D. Brick-red Cu2O is oxidized to blue Cu2+

Answer: (C)

Explanation

The ose’s reactive carbonyl function reduces Cu2+ in Fehling’s solution, producing brick-red Cu2O. The carbonyl function itself is oxidized during this reaction, rather than being converted into an alcohol.

2. What product results when an ose carbonyl group undergoes chemical reduction?

A. An imine linked to a primary amine

B. A carboxylic acid within an oxidized ose

C. An alcohol function within a polyol

D. A phosphate ester linked to ATP

Answer: (C)

Explanation

Reduction converts the aldehyde or ketone function of the ose carbonyl group into an alcohol, yielding a polyol. Formation of a carboxylic acid describes oxidation rather than reduction of the carbonyl group.

3. Which ose functional group participates directly in esterification with an acid?

A. A hemiacetal function

B. A hydroxyl function

C. A carbonyl oxygen

D. A primary amino group

Answer: (B)

Explanation

The hydroxyl functions of oses react with acids to form ester or phosphoester functions. A hemiacetal function is associated with glycosidic bonding, not the esterification described here.

4. During the first step of glycolysis, what are the products when D-glucose is phosphorylated by hexokinase or glucokinase?

A. D-glucose 6-phosphate and ADP

B. D-gluconate and ADP

C. D-glucose 6-phosphate and AMP

D. D-glucose and inorganic phosphate

Answer: (A)

Explanation

Hexokinase or glucokinase uses ATP to phosphorylate D-glucose, producing D-glucose 6-phosphate and ADP. ADP is generated in this reaction, whereas ATP is the phosphate donor that is consumed.

5. How is an osamine formed from an ose?

A. The hydroxyl group at C2 is replaced by an NH2 group

B. A phosphate group is attached to the anomeric carbon

C. The hemiacetal function reacts with another ose

D. The carbonyl group is oxidized into a carboxylic acid

Answer: (A)

Explanation

An osamine forms when the hydroxyl group at C2 of an ose is replaced by an NH2 group, as in glucosamine. Phosphate attachment describes phosphorylation, while reaction of a hemiacetal function is associated with glycosidic bonding.

6. Which process defines glycation in the Maillard reaction?

A. Non-enzymatic imine formation between a reducing ose and a primary amine

B. Esterification of an ose hydroxyl group with phosphoric acid

C. Enzymatic transfer of an ose by a specific glycosyltransferase

D. Oxidation of an ose carbonyl into a carboxylic acid

Answer: (A)

Explanation

Glycation is the non-enzymatic association of a reducing ose with a primary amine from an amino acid or protein, forming an imine. Enzymatic glycosylation uses specific transfer mechanisms and is distinct from this Maillard reaction.

7. What does the formation of HbA1c illustrate in the context of glycation?

A. Phosphorylation of hemoglobin by cellular ATP

B. Esterification of hemoglobin with a fatty acid

C. Glycation of hemoglobin by a reducing ose

D. Oxidation of hemoglobin’s amino groups

Answer: (C)

Explanation

HbA1c is produced when hemoglobin undergoes glycation, and the example uses it to distinguish healthy and diabetic conditions. Phosphorylation and esterification are different chemical modifications and do not describe HbA1c formation here.

8. Which description correctly defines an oside?

A. A carbohydrate that cannot undergo hydrolysis into smaller units

B. A substance whose hydrolysis releases one or more oses

C. A single ose whose hydroxyl groups form several ester bonds

D. A molecule formed when two oses combine without eliminating water

Answer: (B)

Explanation

An oside is defined by its ability to release one or more oses when hydrolyzed. The related term ose refers to a simple sugar and does not itself define a hydrolysable carbohydrate.

9. What chemical event forms an O-glycosidic bond between two oses?

A. Condensation between hydroxyl functions with elimination of one water molecule

B. Reduction of an anomeric function with absorption of one oxygen molecule

C. Oxidation of two hydroxyl functions with release of carbon dioxide

D. Hydrolysis between carbonyl functions with addition of one water molecule

Answer: (A)

Explanation

An O-glycosidic bond forms when hydroxyl functions condense and one water molecule is eliminated. Hydrolysis produces the reverse effect by adding water to break the bond.

10. What distinguishes an ose–oside bond from an oside–oside bond?

A. The ose–oside bond is formed by hydrolysis, whereas the oside–oside bond is formed by oxidation

B. The ose–oside bond leaves a free reducing function, whereas the oside–oside bond does not

C. The ose–oside bond joins two non-hemiacetal hydroxyl functions, whereas the oside–oside bond joins two carbonyl functions

D. The ose–oside bond contains two participating hemiacetal functions, whereas the oside–oside bond contains one

Answer: (B)

Explanation

An ose–oside bond uses one hemiacetal function and one non-hemiacetal hydroxyl function, leaving another reducing function free. An oside–oside bond uses both hemiacetal functions, so no reducing function remains available.

11. What is the principal effect of treating an oside with concentrated hydrochloric acid?

A. It breaks glycosidic bonds and releases the constituent oses

B. It transfers methyl groups to the free hydroxyl functions

C. It identifies reducing behavior through a color change

D. It separates intact osides according to their phase affinities

Answer: (A)

Explanation

Concentrated hydrochloric acid hydrolyzes the glycosidic bonds of an oside, releasing its constituent oses. Fehling’s test assesses reducing behavior, while chromatography separates molecules after hydrolysis.

12. Which method can separate and identify the simple oses released after oside hydrolysis?

A. Fehling’s test, methylation analysis, or enzymatic oxidation

B. Thin-layer chromatography, gas chromatography, or high-performance liquid chromatography

C. Anomeric inversion, glycosidic condensation, or water elimination

D. Concentrated hydrochloric acid treatment, heating, or neutralization

Answer: (B)

Explanation

The released oses can be separated and identified by thin-layer chromatography, gas chromatography, or high-performance liquid chromatography. Hydrolysis must occur first because chromatography separates the released oses rather than breaking the original oside.

13. What does a positive Fehling test indicate about an oside linkage?

A. A methylated oside whose hydroxyl positions cannot be assigned

B. A glycosidic bond whose two anomeric functions are both occupied

C. An oside–oside linkage with no free reducing function

D. An ose–oside linkage with a free reducing function

Answer: (D)

Explanation

A positive Fehling test indicates that a free reducing function is present, as in an ose–oside linkage. An oside–oside linkage lacks a free reducing function and therefore gives a negative result.

14. In Haworth methylation analysis, what do hydroxyl groups that remain unmethylated reveal?

A. They identify positions that were free and chemically unreactive

B. They identify positions involved in glycosidic bonding

C. They reveal the oses that chromatography would separate most rapidly

D. They indicate the anomeric configuration of every residue

Answer: (B)

Explanation

Methylation modifies free hydroxyl functions, so hydroxyl groups left unmethylated must have been involved in glycosidic bonding. The method therefore identifies linkage positions rather than directly separating or naming the released oses.

15. Which combination correctly characterizes sucrose?

A. An oside–oside bond, no free reducing function, and hydrolysis by α-glucosidase and β-fructosidase

B. An ose–oside bond, a free reducing function, and hydrolysis by α-glucosidase

C. An ose–oside bond, a free reducing function, and hydrolysis by β-galactosidase

D. An oside–oside bond, a free reducing function, and hydrolysis by α-glucosidase

Answer: (A)

Explanation

Sucrose contains an oside–oside bond between glucose and fructose, leaving no free reducing function, and it can be hydrolyzed by α-glucosidase and β-fructosidase. Maltose and lactose instead contain ose–oside bonds and are reducing disaccharides.

16. Which description correctly identifies maltose?

A. α-D-glucopyranosyl (1-4) D-glucopyranose, a reducing disaccharide hydrolyzed by α-glucosidase

B. β-D-galactopyranosyl (1-4) D-glucopyranose, a reducing disaccharide hydrolyzed by β-galactosidase

C. α-D-glucopyranosyl (1-2) β-D-fructofuranoside, a non-reducing disaccharide hydrolyzed by two enzymes

D. β-D-glucopyranosyl (1-4) D-glucopyranose, a non-reducing disaccharide hydrolyzed by β-glucosidase

Answer: (A)

Explanation

Maltose consists of α-D-glucopyranosyl linked (1-4) to D-glucopyranose, giving it a reducing ose–oside structure that is hydrolyzed by α-glucosidase. The β-galactosyl (1-4) structure describes lactose, not maltose.

17. Which feature distinguishes lactose from maltose?

A. Lactose contains an α-D-glucopyranosyl (1-4) linkage and is hydrolyzed by α-glucosidase

B. Lactose contains an α-D-glucopyranosyl (1-2) linkage to fructose and is non-reducing

C. Lactose contains a β-D-galactopyranosyl (1-4) linkage and is hydrolyzed by β-galactosidase

D. Lactose contains two participating hemiacetal functions and is hydrolyzed by both glucosidases

Answer: (C)

Explanation

Lactose is β-D-galactopyranosyl (1-4) D-glucopyranose and is hydrolyzed by β-galactosidase. Maltose instead has an α-D-glucopyranosyl (1-4) linkage and is hydrolyzed by α-glucosidase.

18. Which description correctly defines a polyoside?

A. A polymer made exclusively of monosaccharide units

B. A short chain containing one sugar and one amino acid

C. A polymer combining sugars with a non-carbohydrate component

D. A single sugar linked to a lipid or protein

Answer: (A)

Explanation

A polyoside is a polymeric molecule composed exclusively of oses. The presence of a non-carbohydrate aglycone would instead characterize a heteroside.

19. A polysaccharide contains several different types of ose in its chain. How should it be classified?

A. As an aglycone-containing heteroside

B. As a heteropolyoside

C. As a homopolyoside

D. As a glucosane

Answer: (B)

Explanation

A heteropolyoside contains several types of ose, whereas a homopolyoside contains one type. A glucosane is a homopolyoside composed specifically of glucose.

20. Which composition and structure correctly describe starch?

A. It contains equal amounts of amylose and amylopectin with only α(1-6) bonds

B. It contains branched amylose and linear amylopectin linked mainly by β(1-4) bonds

C. It contains about 20–30% linear amylose and 70–80% branched amylopectin

D. It contains about 70–80% linear amylose and 20–30% branched amylopectin

Answer: (C)

Explanation

Starch contains approximately 20–30% linear amylose and 70–80% branched amylopectin. Amylopectin branches through α(1-6) bonds, while both components have α(1-4) glucan chains.

21. Which metabolic response is expected when blood glucose becomes high?

A. Cleavage of starch by pancreatic amylase in the bloodstream

B. Activation of glycogenolysis to release glucose from glycogen

C. Activation of glycogenesis to store glucose as glycogen

D. Conversion of glycogen into dietary starch for intestinal digestion

Answer: (C)

Explanation

High blood glucose activates glycogenesis, which promotes glycogen formation and glucose storage. Glycogenolysis is favored when blood glucose is low or during physical effort.

22. Which pairing correctly identifies the two parts of a heteroside?

A. The glycone is the protein part, and the aglycone is the lipid-linked sugar chain

B. The glycone is the carbohydrate part, and the aglycone is the non-carbohydrate part

C. Both the glycone and aglycone are carbohydrate parts with different chain lengths

D. The glycone is the non-carbohydrate part, and the aglycone is the carbohydrate part

Answer: (B)

Explanation

A heteroside contains a carbohydrate component called the glycone and a non-carbohydrate component called the aglycone. These parts are joined by an N-glycosidic or O-glycosidic bond.

23. What type of functional group does an N-glycosidic bond connect to the hemiacetal function of an ose?

A. A primary amine group of a non-carbohydrate molecule

B. A phosphate group of a non-carbohydrate molecule

C. A carboxyl group of a non-carbohydrate molecule

D. A hydroxyl group of a non-carbohydrate molecule

Answer: (A)

Explanation

An N-glycosidic bond links the ose hemiacetal function to a primary amine. Linking to a hydroxyl group describes an O-glycosidic bond instead.

24. Which statement correctly describes the Asn–X–Ser/Thr motif in N-glycosylation?

A. X must be a charged amino acid such as lysine or glutamate

B. X may be any amino acid except proline

C. X must be glycine because it permits attachment to asparagine

D. X must be proline to position the glycosylation site

Answer: (B)

Explanation

In the Asn–X–Ser/Thr motif, X can be any amino acid except proline. N-glycosylation is particularly associated with membrane or secreted proteins and helps direct their cellular addressing.

25. What is the principal membrane role of a glycosylphosphatidylinositol structure?

A. It links a protein’s hydrophobic domain to a carbohydrate chain

B. It forms a protein channel by replacing the lipid bilayer

C. It anchors a protein lacking a hydrophobic domain to the membrane

D. It transports a soluble sugar directly across the membrane

Answer: (C)

Explanation

A glycosylphosphatidylinositol is an O-glycosidic glycolipid that links a carbohydrate chain to phosphatidylinositol and anchors proteins without hydrophobic domains. It functions as a membrane tether rather than as a sugar transporter or channel.