Chapter 10 Solutions — Biomolecules

Monosaccharides are the simplest carbohydrates that cannot be hydrolysed further into smaller polyhydroxy aldehyde or ketone units.

They are the basic (monomeric) building blocks of all carbohydrates.

They are classified based on:

(a) The number of carbon atoms present — trioses (3C), tetroses (4C), pentoses (5C), hexoses (6C), heptoses (7C).

(b) The type of carbonyl group present — aldoses (contain an aldehyde group, –CHO) or ketoses (contain a keto group, C=O).

All monosaccharides are reducing sugars — they reduce Fehling's solution and Tollens' reagent.

Examples: glucose (aldohexose), fructose (ketohexose), ribose (aldopentose), 2-deoxyribose (aldopentose).

Reducing sugars are carbohydrates that reduce Fehling's solution (a blue alkaline solution of cupric ions, Cu²⁺) to a brick-red precipitate of cuprous oxide (Cu₂O), and reduce Tollens' reagent (ammoniacal silver nitrate) to give a silver mirror.

A carbohydrate is a reducing sugar if it has a free aldehydic (–CHO) or ketonic (C=O) group that can act as a reducing agent under alkaline conditions.

All monosaccharides (whether aldoses or ketoses) are reducing sugars.

Among disaccharides, maltose and lactose are reducing sugars because they have a free anomeric –OH group (i.e., a potential free –CHO).

Sucrose is a non-reducing sugar because both the anomeric carbons of glucose and fructose are involved in the glycosidic bond, leaving no free –CHO or reducible C=O group.

Polysaccharides such as starch and cellulose are non-reducing sugars.

Two main functions of carbohydrates in plants are:

1. Storage of energy: Starch is the main storage polysaccharide in plants. It is formed from glucose produced during photosynthesis and stored in seeds, tubers, roots, and other plant parts. It serves as a reserve food material that can be mobilised when required.

2. Structural support (building material): Cellulose is a polysaccharide that forms the cell wall of plant cells. It provides mechanical strength and rigidity to the cell wall, maintaining the shape and structure of plant cells. Plants also use cellulose to build fibres used for structural support.