BIOCHEM 210 MODULE 3 EXAM 2026/2027 | Carbohydrate
Structure & Function | Latest Q&A Verified | Portage Learning
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Q1: Which of the following monosaccharides is a ketose with six carbon atoms?
A. Glucose
B. Fructose [CORRECT]
C. Galactose
D. Ribose
Correct Answer: B
Rationale: Fructose is a ketohexose (six-carbon ketose), with the carbonyl group at C-2.
Glucose (A) and galactose (C) are aldohexoses (aldehyde at C-1). Ribose (D) is an
aldopentose (five-carbon aldose). The distinction between aldoses and ketoses is
functionally significant: aldoses are more easily oxidized and form different cyclic
hemiacetal structures. Fructose is the sweetest natural sugar and is metabolized via
fructolysis, bypassing phosphofructokinase regulation in glycolysis.
Q2: In Fischer projection, the D-configuration of a monosaccharide is determined by the
position of the hydroxyl group on which carbon?
A. The carbon bearing the carbonyl group
B. The highest-numbered chiral carbon (penultimate carbon) [CORRECT]
C. The first carbon of the chain
D. The carbon involved in ring formation
Correct Answer: B
,Rationale: D- and L-configuration refers to the stereochemistry of the highest-numbered
chiral carbon (the carbon farthest from the carbonyl, C-5 in hexoses). In D-sugars, the
-OH is on the right in Fischer projection; L-sugars have it on the left. This convention
derives from D-glyceraldehyde reference. The carbonyl carbon (A) is not chiral in the
open chain. C-1 (C) becomes anomeric in cyclic forms. Ring formation carbon (D) is the
carbonyl carbon. Most biologically relevant sugars are D-configuration.
Q3: Which pair of sugars represents C-2 epimers?
A. D-glucose and D-mannose [CORRECT]
B. D-glucose and D-galactose
C. D-glucose and L-glucose
D. D-glucose and D-fructose
Correct Answer: A
Rationale: Epimers are diastereomers differing at exactly one chiral center. Glucose and
mannose differ only at C-2 (mannose has opposite -OH orientation), making them C-2
epimers. Glucose and galactose (B) are C-4 epimers. Glucose and L-glucose (C) are
enantiomers (mirror images, all chiral centers opposite). Glucose and fructose (D) are
structural isomers (aldose vs ketose), not stereoisomers. Epimer relationships are
important in enzyme specificity and interconversion pathways.
Q4: When D-glucose cyclizes to form a six-membered ring, the resulting cyclic form is
called:
A. A furanose
B. A pyranose [CORRECT]
C. An open-chain aldehyde
D. A glycoside
, Correct Answer: B
Rationale: Six-membered oxygen-containing rings are pyranoses (named after pyran).
The aldehyde at C-1 reacts with the C-5 hydroxyl to form a hemiacetal. Five-membered
rings (C-1 to C-4 hydroxyl) are furanoses. Glucose predominantly forms pyranose
(six-membered) rings (>99% in solution), with trace furanose forms. The cyclic structure
explains mutarotation and reducing sugar properties. Option C describes the linear
form; D describes the acetal formed with another molecule (glycosidic bond).
Q5: In the Haworth projection of α-D-glucopyranose, the orientation of the anomeric
hydroxyl group is:
A. Trans to the CH₂OH group (pointing down) [CORRECT]
B. Cis to the CH₂OH group (pointing up)
C. In the plane of the ring
D. Absent due to glycosidic bond formation
Correct Answer: A
Rationale: In D-sugars, the CH₂OH group at C-6 is always up (cis to C-6 -OH in Fischer
becomes up in Haworth). The α-anomer has the anomeric -OH (C-1) trans to this
(down); the β-anomer has it up (cis). This convention reflects the actual
three-dimensional relationship: α-D-glucose has the anomeric -OH axial (down in
standard Haworth), while β-D-glucose has it equatorial (up). The "up = β, down = α" rule
applies specifically to D-sugars; L-sugars reverse this relationship.
Q6: Mutarotation refers to the phenomenon where:
A. Enzymes catalyze ring opening and closing
B. The specific rotation of a sugar changes as α and β anomers interconvert in solution
[CORRECT]
Structure & Function | Latest Q&A Verified | Portage Learning
| Pass Guaranteed - A+ Graded
Q1: Which of the following monosaccharides is a ketose with six carbon atoms?
A. Glucose
B. Fructose [CORRECT]
C. Galactose
D. Ribose
Correct Answer: B
Rationale: Fructose is a ketohexose (six-carbon ketose), with the carbonyl group at C-2.
Glucose (A) and galactose (C) are aldohexoses (aldehyde at C-1). Ribose (D) is an
aldopentose (five-carbon aldose). The distinction between aldoses and ketoses is
functionally significant: aldoses are more easily oxidized and form different cyclic
hemiacetal structures. Fructose is the sweetest natural sugar and is metabolized via
fructolysis, bypassing phosphofructokinase regulation in glycolysis.
Q2: In Fischer projection, the D-configuration of a monosaccharide is determined by the
position of the hydroxyl group on which carbon?
A. The carbon bearing the carbonyl group
B. The highest-numbered chiral carbon (penultimate carbon) [CORRECT]
C. The first carbon of the chain
D. The carbon involved in ring formation
Correct Answer: B
,Rationale: D- and L-configuration refers to the stereochemistry of the highest-numbered
chiral carbon (the carbon farthest from the carbonyl, C-5 in hexoses). In D-sugars, the
-OH is on the right in Fischer projection; L-sugars have it on the left. This convention
derives from D-glyceraldehyde reference. The carbonyl carbon (A) is not chiral in the
open chain. C-1 (C) becomes anomeric in cyclic forms. Ring formation carbon (D) is the
carbonyl carbon. Most biologically relevant sugars are D-configuration.
Q3: Which pair of sugars represents C-2 epimers?
A. D-glucose and D-mannose [CORRECT]
B. D-glucose and D-galactose
C. D-glucose and L-glucose
D. D-glucose and D-fructose
Correct Answer: A
Rationale: Epimers are diastereomers differing at exactly one chiral center. Glucose and
mannose differ only at C-2 (mannose has opposite -OH orientation), making them C-2
epimers. Glucose and galactose (B) are C-4 epimers. Glucose and L-glucose (C) are
enantiomers (mirror images, all chiral centers opposite). Glucose and fructose (D) are
structural isomers (aldose vs ketose), not stereoisomers. Epimer relationships are
important in enzyme specificity and interconversion pathways.
Q4: When D-glucose cyclizes to form a six-membered ring, the resulting cyclic form is
called:
A. A furanose
B. A pyranose [CORRECT]
C. An open-chain aldehyde
D. A glycoside
, Correct Answer: B
Rationale: Six-membered oxygen-containing rings are pyranoses (named after pyran).
The aldehyde at C-1 reacts with the C-5 hydroxyl to form a hemiacetal. Five-membered
rings (C-1 to C-4 hydroxyl) are furanoses. Glucose predominantly forms pyranose
(six-membered) rings (>99% in solution), with trace furanose forms. The cyclic structure
explains mutarotation and reducing sugar properties. Option C describes the linear
form; D describes the acetal formed with another molecule (glycosidic bond).
Q5: In the Haworth projection of α-D-glucopyranose, the orientation of the anomeric
hydroxyl group is:
A. Trans to the CH₂OH group (pointing down) [CORRECT]
B. Cis to the CH₂OH group (pointing up)
C. In the plane of the ring
D. Absent due to glycosidic bond formation
Correct Answer: A
Rationale: In D-sugars, the CH₂OH group at C-6 is always up (cis to C-6 -OH in Fischer
becomes up in Haworth). The α-anomer has the anomeric -OH (C-1) trans to this
(down); the β-anomer has it up (cis). This convention reflects the actual
three-dimensional relationship: α-D-glucose has the anomeric -OH axial (down in
standard Haworth), while β-D-glucose has it equatorial (up). The "up = β, down = α" rule
applies specifically to D-sugars; L-sugars reverse this relationship.
Q6: Mutarotation refers to the phenomenon where:
A. Enzymes catalyze ring opening and closing
B. The specific rotation of a sugar changes as α and β anomers interconvert in solution
[CORRECT]
