Consider the following statements about disaccharides.
(A) A glycosidic linkage is an oxide linkage between two monosaccharide units, formed by the loss of a water molecule.
(B) Maltose is composed of two β-D-glucose units in which C1 of one glucose unit is linked to C4 of the other.
(C) Lactose is a reducing sugar because a free aldehyde group may be produced at C-1 of its glucose unit.
(D) In lactose, the linkage is between C1 of galactose and C6 of glucose.
Which of the statements given above are correct? Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (A) and (C) only
In a disaccharide two monosaccharide units are joined through an oxygen atom. Which carbon atoms are linked, and whether a reducing group is left free, decide the nature of the sugar.
(A) A glycosidic linkage is an oxide linkage between two monosaccharide units, formed by the loss of a water molecule. — True. This is the definition of the glycosidic linkage.
(B) Maltose is composed of two β-D-glucose units in which C1 of one glucose unit is linked to C4 of the other. — False. The C1–C4 link is right, but the two units are α-D-glucose.
(C) Lactose is a reducing sugar because a free aldehyde group may be produced at C-1 of its glucose unit. — True. The linkage uses C1 of galactose and C4 of glucose, so a free aldehyde group may be produced at C-1 of the glucose unit.
(D) In lactose, the linkage is between C1 of galactose and C6 of glucose. — False. The linkage is between C1 of galactose and C4 of glucose.
NCERT (2026-27 edition):
10.1.3 Disaccharides – “…The two monosaccharides are joined together by an oxide linkage formed by the loss of a water molecule. Such a linkage between two monosaccharide units through oxygen atom is called glycosidic linkage.”
“(ii) Maltose: Another disaccharide, maltose is composed of two α-D-glucose units in which C1 of one glucose (I) is linked to C4 of another glucose unit (II).”
“(iii) Lactose: …It is composed of β-D-galactose and β-D-glucose. The linkage is between C1 of galactose and C4 of glucose. Free aldehyde group may be produced at C-1 of glucose unit, hence it is also a reducing sugar.”