Read the passage carefully and answer the questions.
Carbohydrates such as cane sugar, glucose, starch etc. belong to a class of naturally occurring organic compounds. Structurally these are polyhydroxy aldehydes or ketones or the compounds which produce such units on hydrolysis. Carbohydrates which are sweet in taste are called sugars. Theses sugars can be reducing or non-reducing. Monosaccharides cannot be hydrolyzed to simpler units whereas Oligosaccharides yield two to ten monosaccharide units on hydrolysis. Polysaccharides yield large number of monosaccharide units on hydrolysis. This class of biomolecules is essential for living systems as they serve as storage molecules in plants and animals. In addition, they provide raw materials for important industries like furniture, textiles, paper, lacquers and breweries. Two aldopentose form important constituents of nucleic acids. Carbohydrates are also called saccharides (Greek: sakcharon which means sugar). Sugar which is used in household and in milk also comes under this class of biomolecules.
Among below given carbohydrates, which one is not a polysaccharides?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Ribose
Carbohydrates are classified by their behaviour on hydrolysis. A monosaccharide cannot be hydrolysed into simpler units; an oligosaccharide gives two to ten monosaccharide units; a polysaccharide gives a large number of monosaccharide units.
Ribose, \( \ce{C5H10O5} \), is a five-carbon sugar with an aldehyde group, an aldopentose. It cannot be broken down further, so it is a monosaccharide, not a polysaccharide. It is one of the two aldopentoses mentioned in the passage that form part of nucleic acids (ribose in RNA, 2-deoxyribose in DNA).
Option (1) Starch — A polysaccharide. Starch is a polysaccharide of α-glucose (amylose and amylopectin), the main storage carbohydrate of plants.
Option (2) Cellulose — A polysaccharide. Cellulose is a straight-chain polysaccharide of β-glucose, the main structural material of plant cell walls.
Option (3) Glycogen — A polysaccharide. Glycogen is a highly branched polysaccharide of α-glucose, the storage carbohydrate of animals (animal starch).
Option (4) Ribose — Not a polysaccharide (a monosaccharide, aldopentose). Ribose cannot be hydrolysed into simpler sugars.
NCERT (2026-27 edition):
Polysaccharides
Importance of Carbohydrates – “… Two aldopentoses viz. D-ribose and 2-deoxy-D-ribose (Section 10.5.1, Class XII) are present in nucleic acids.”