What are the two monosaccharides which are obtained by hydrolysis of lactose sugar?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → D-(+)- Glucose and D-(+)-Galactose
A disaccharide is made of two monosaccharide units joined by a glycosidic linkage; hydrolysis by dilute acid or an enzyme breaks this linkage and gives back the two monosaccharides. The three common disaccharides are:
Sucrose: glucose + fructose.
Maltose: two glucose units.
Lactose (milk sugar): galactose + glucose, joined between C-1 of β-D-galactose and C-4 of β-D-glucose.
\( \displaystyle \ce{C12H22O11 + H2O} \xrightarrow{\ce{H+}\ \text{or lactase}} \ce{C6H12O6 + C6H12O6} \)
\( \displaystyle \text{lactose} + \text{water} \rightarrow \text{D-(+)-glucose} + \text{D-(+)-galactose} \)
So lactose gives D-(+)-glucose and D-(+)-galactose.
Why the other options are incorrect
Option (1) D-(+) Fructose and D-(+)-Galactose — Incorrect. Fructose is obtained from sucrose, not lactose; it is also laevorotatory, D-(−)-fructose.
Option (2) D-(+)- Maltose and D-(+)-Galactose — Incorrect. Maltose is itself a disaccharide, not a monosaccharide.
Option (4) D-(+) Maltose and D-(+) - Fructose — Incorrect. Neither is a hydrolysis product of lactose; maltose is a disaccharide and fructose comes from sucrose.
NCERT (2026-27 edition):
Lactose
