Comprehension:
Biomolecules
Sugars are the carbohydrates which are sweet in nature and one such example is sucrose. Carbohydrates are classified based on their behavior towards hydrolysis. Carbohydrates can also be classified as reducing or non-reducing sugars based on their behavior towards Fehling's reagent and Tollens' reagent. Monosaccharides can also be classified based on the number of carbon atoms as well as the functional group present in the structure. The two monosaccharides are joined through a special linkage. Carbohydrates are optically active compounds which contain polyhydroxy aldehyde groups or ketones or the compounds which produce such units on hydrolysis.
The two monosaccharide units in Lactose are joined by
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Glycosidic linkage
Two monosaccharide units are joined through an oxygen atom, with the loss of a water molecule. This C–O–C link is called a glycosidic linkage, and all disaccharides and polysaccharides are built this way.
Lactose (milk sugar) is made of β-D-galactose and β-D-glucose, joined between C1 of galactose and C4 of glucose by a glycosidic linkage.
So the two units in lactose are joined by a glycosidic linkage.
Why the other options are incorrect
Option (2) Peptide linkage — Incorrect. Joins amino acids in proteins, formed between –COOH and –\( \ce{NH2} \) groups.
Option (3) Phosphodiester linkage — Incorrect. Joins nucleotides in DNA and RNA.
Option (4) Hydrogen bonding linkage — Incorrect. Hydrogen bonds are weak attractions, as between base pairs in DNA or in the secondary structure of proteins; they do not join the units of a disaccharide.
NCERT (2026-27 edition):
Disaccharides
