Which of the following is a straight chain polysaccharide composed only of B-D-glucose units?

Cellulose

The correct answer is Option (4) → Cellulose

Polysaccharides are complex carbohydrates formed by the polymerization of monosaccharide units through glycosidic linkages.

The structure of a polysaccharide depends on:

• Type of glucose unit (α or β)
• Type of glycosidic linkage
• Degree of branching

The question asks for a straight chain polysaccharide composed of β-D-glucose units.

Cellulose

Cellulose is a linear (unbranched) polysaccharide composed exclusively of β-D-glucose units.

The glucose units are connected by β(1→4) glycosidic bonds.

Structure features:

• Long straight chains
• Strong intermolecular hydrogen bonding
• Forms rigid fibers in plant cell walls

Because of this structure, cellulose is insoluble in water and provides structural strength to plants.

Option-wise Explanation

Option 1: Gums

Gums are heteropolysaccharides containing different types of sugars such as arabinose, galactose, and others. They are not composed solely of β-D-glucose units and are not strictly straight chains. Hence this option is incorrect.

Option 2: Starch

Starch is composed of two components:

• Amylose – linear chain with α(1→4) glycosidic bonds
• Amylopectin – branched structure with α(1→4) and α(1→6) bonds

Since starch contains α-D-glucose units, not β-D-glucose, this option is incorrect.

Option 3: Glycogen

Glycogen is a highly branched polysaccharide composed of α-D-glucose units with α(1→4) and α(1→6) linkages. It is not a straight chain polymer. Therefore this option is incorrect.

Option 4: Cellulose

Cellulose is a linear polysaccharide consisting only of β-D-glucose units linked by β(1→4) bonds. Hence it perfectly matches the condition given in the question. Therefore this option is correct.