Lactose is a disaccharide formed from \(\beta\)-D-galactose and \(\beta\)-D-glucose. Identify anomeric carbon atoms in monosaccharide units
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → ‘a’ carbon of galactose and ‘a’ carbon of glucose
When an aldose such as glucose or galactose closes into a six-membered ring, the –OH group at C5 adds to the aldehyde group at C1. C1 becomes a new chiral centre, and the two possible arrangements of the –OH group on it give the α- and β-forms. These two forms are called anomers, and the C1 carbon is the anomeric carbon. It is the only ring carbon bonded to two oxygen atoms (the ring oxygen and an –OH or –OR group).
Lactose is made of β-D-galactose and β-D-glucose joined by a glycosidic link between C1 of galactose and C4 of glucose. In the given figure:
In galactose (left), carbon ‘a’ is bonded to the ring oxygen and to the bridging oxygen: it is C1, the anomeric carbon (used in the glycosidic link).
In glucose (right), carbon ‘d’ carries the bridging oxygen: it is C4. Carbon ‘a’ is bonded to the ring oxygen and to a free –OH: it is C1, the anomeric carbon.
So the anomeric carbons are the ‘a’ carbon of galactose and the ‘a’ carbon of glucose. Because the anomeric carbon of glucose is free, lactose is a reducing sugar.
Why the other options are incorrect
Option (1) ‘a’ carbon of galactose and ‘d’ carbon of glucose — Incorrect. ‘d’ of glucose is C4; it carries the glycosidic link but is bonded to only one oxygen, so it is not anomeric.
Option (2) ‘d’ carbon of galactose and ‘a’ carbon of glucose — Incorrect. ‘d’ of galactose is C4, an ordinary ring carbon carrying –OH.
Option (4) ‘e’ carbon of galactose and ‘e’ carbon of glucose — Incorrect. ‘e’ is C5, the carbon that carries the –\( \ce{CH2OH} \) group (‘f’); its oxygen closes the ring, but it was never a carbonyl carbon.
NCERT (2026-27 edition):
Lactose
