Select the correct statements related to carbohydrates :
(A) Glucose is also known as dextrose.
(B) In Glucose, an aldehydic group is present, and it gives Schiff's test.
(C) Glycogen is also known as animal starch.
(D) Lactose is a reducing sugar.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (A), (C) and (D) only
Glucose has an aldehyde group in its open-chain formula, but in solution it exists almost entirely as a cyclic hemiacetal: the –OH at C5 adds to the –CHO group and forms a six-membered ring. A sugar is reducing when it has, or can open up to give, a free aldehyde or ketone group that reduces Tollens’ reagent and Fehling’s solution.
(A) Glucose is also known as dextrose. — Correct. Glucose occurs in nature as the dextrorotatory D-(+) form, hence the name dextrose.
(B) In Glucose, an aldehydic group is present, and it gives Schiff’s test. — Incorrect. The first part is true, but glucose does not give Schiff’s test. Because glucose is almost entirely in the cyclic form, the free –CHO group is not available to restore the pink colour of Schiff’s reagent. For the same reason glucose does not form the hydrogensulphite addition product with \( \ce{NaHSO3} \). These facts led to the cyclic structure of glucose.
(C) Glycogen is also known as animal starch. — Correct. Glycogen is the form in which carbohydrates are stored in the animal body (liver, muscles, brain). Its structure is similar to amylopectin, but more highly branched.
(D) Lactose is a reducing sugar. — Correct. Lactose (milk sugar) is made of β-D-galactose and β-D-glucose joined between C1 of galactose and C4 of glucose. C1 of the glucose unit is not used in the glycosidic bond, so a free aldehyde group can form there and lactose reduces Tollens’ and Fehling’s reagents. (Sucrose, by contrast, is non-reducing, because both reducing groups are used in its glycosidic bond.)
NCERT (2026-27 edition):
Glucose – “Glucose is an aldohexose and is also known as dextrose. It is the monomer of many of the larger carbohydrates, namely starch, cellulose.”
Cyclic Structure of Glucose – “The structure (I) of glucose explained most of its properties but the following reactions and facts could not be explained by this structure.
1. Despite having the aldehyde group, glucose does not give Schiff’s test and it does not form the hydrogensulphite addition product with NaHSO3.”
Disaccharides – “(iii) Lactose: It is more commonly known as milk sugar since this disaccharide is found in milk. 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.”
Polysaccharides – “(iii) Glycogen: The carbohydrates are stored in animal body as glycogen. It is also known as animal starch because its structure is similar to amylopectin and is rather more highly branched. It is present in liver, muscles and brain. When the body needs glucose, enzymes break the glycogen down to glucose. Glycogen is also found in yeast and fungi.”