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GATE 2025 BT Q.18: solution and answer

GATE 2025 · BTQ.18 · 1 mark · MCQ

Correctly match the following Monosaccharides with their respective Epimers. Monosaccharide: P. D-mannose; Q. D-allose; R. D-galactose; S. D-talose. Epimer: 1. C-3 epimer of D-glucose; 2. C-4 epimer of D-glucose; 3. C-4 epimer of D-mannose; 4. C-2 epimer of D-glucose; 5. C-5 epimer of D-glucose.

GATE 2025 BT question 18: A two-column table.Tap to open full size
Figure from the GATE 2025 BT paper. It is part of the question.
Figure description

A two-column table. Left column header 'Monosaccharide' with rows P. D-mannose, Q. D-allose, R. D-galactose, S. D-talose and a blank fifth row. Right column header 'Epimer' with rows 1. C-3 epimer of D-glucose, 2. C-4 epimer of D-glucose, 3. C-4 epimer of D-mannose, 4. C-2 epimer of D-glucose, 5. C-5 epimer of D-glucose.

  1. (A)

    P-4; Q-1; R-2; S-3

  2. (B)

    P-5; Q-1; R-2; S-3

  3. (C)

    P-4; Q-3; R-5; S-1

  4. (D)

    P-1; Q-5; R-3; S-2

Answer and solution

ANSWER

The correct matching is P-4, Q-1, R-2, S-3, which is option (A).

Official answer: ASource: Official answer key, GATE 2025 BT (IIT Roorkee)

  1. Epimers are sugars that differ in configuration at exactly one carbon. Compare each sugar with its reference sugar, one stereocentre at a time.

  2. D-mannose differs from D-glucose only at C-2, so it is the C-2 epimer of D-glucose: P matches 4.

  3. D-allose differs from D-glucose only at C-3, so it is the C-3 epimer of D-glucose: Q matches 1.

  4. D-galactose differs from D-glucose only at C-4, so it is the C-4 epimer of D-glucose: R matches 2.

  5. D-talose differs from D-mannose only at C-4 (it is also the C-2 epimer of D-galactose): S matches 3.

  6. Entry 5, the C-5 epimer of D-glucose, would be L-idose, which is not in the list. It is the unused extra entry, so no sugar maps to it.

  7. The matching P-4, Q-1, R-2, S-3 is option (A). Option (B) pairs mannose with 5, which is wrong. Options (C) and (D) pair allose, galactose or talose with the wrong carbon.

KEY IDEA

An epimer differs from its parent sugar at exactly one stereocentre. Among the D-aldohexoses, mannose is the C-2 epimer of glucose, allose the C-3 epimer, and galactose the C-4 epimer. Talose is the C-4 epimer of mannose.

WATCH OUT FOR

Pairing mannose with C-5 or confusing which carbon distinguishes allose and galactose from glucose. Entry 5 is the extra entry and matches none of the four sugars.

From the GATE 2025 BT paper. Every question of this paper · Download the full paper · All 12 Biomolecules questions, with notes

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