In which of the following disaccharides are the reducing groups of both monosaccharide units involved in the glycosidic linkage?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Sucrose
The reducing groups of a monosaccharide are its aldehydic or ketonic groups.
A disaccharide in which these groups are used up in the glycosidic linkage has no free group left and is a non-reducing sugar.
In sucrose the glycosidic linkage joins C1 of α-D-glucose (its aldehydic carbon) to C2 of β-D-fructose (its ketonic carbon), so the reducing groups of both units are bonded.
Why the other options are incorrect
Option (1) Lactose — Incorrect. The linkage is between C1 of galactose and C4 of glucose, so a free aldehyde group can form at C1 of the glucose unit; lactose is a reducing sugar.
Option (2) Maltose — Incorrect. C1 of one glucose is linked to C4 of the other, so C1 of the second glucose can give a free aldehyde group; maltose is a reducing sugar.
Option (3) Both lactose and maltose — Incorrect. In both, C1 of one unit is linked to C4 of a glucose unit, so a free aldehyde group can be produced at C1 of that glucose unit; both are reducing sugars.
NCERT (2026-27 edition):
10.1.3 Disaccharides – “(i) Sucrose: One of the common disaccharides is sucrose which on hydrolysis gives equimolar mixture of D-(+)-glucose and D-(–) fructose. These two monosaccharides are held together by a glycosidic linkage between C1 of α-D-glucose and C2 of β-D-fructose. Since the reducing groups of glucose and fructose are involved in glycosidic bond formation, sucrose is a non reducing sugar.”