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Biology Question 116 – NEET-UG 2023

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Cellulose does not form blue colour with Iodine because

The ability of a polysaccharide to form a blue color with iodine is dependent on its specific three-dimensional structure, particularly the presence of helical coils.

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Ninja Strategy•Structural Specificity of Iodine Test

Recognize that the iodine test's color formation depends on a specific helical structure (like amylose in starch) that can trap iodine, which cellulose lacks due to its linear, hydrogen-bonded arrangement.

Step 1: Understand the Starch-Iodine Test Principle✦ Active

The blue-black color observed in the starch-iodine test is due to the trapping of iodine molecules (I2) within the helical coils of amylose, a component of starch. This forms an inclusion complex that absorbs light, resulting in the characteristic color.

Step 2: Compare the Structures of Starch and Cellulose○ Expand

Starch (amylose) has a coiled, helical structure. Cellulose, however, is a linear polymer of β-D-glucose units linked by β-1,4-glycosidic bonds. These linear chains are extensively hydrogen-bonded to each other, forming strong, rigid microfibrils, rather than a helical structure suitable for trapping iodine.

Step 3: Relate Structure to Iodine Interaction○ Expand

Because cellulose does not possess the complex helical structure necessary to trap iodine molecules, it cannot form the characteristic blue color. Option (1) is incorrect as cellulose is a polysaccharide, not a disaccharide. Option (2) is incorrect because while some polysaccharides are helical, cellulose's specific structure is not the type that traps iodine. Option (4) is incorrect as iodine does not cause cellulose to break down; the lack of color is due to structural incompatibility.

💡 Teacher's Secret Hint

Remember that the specific type of helical structure is crucial for the iodine test, not just the presence of any helix.

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