Question

Cellulose does not form blue colour with iodine because

• A. It is a helical molecule
• B. It does not contain complex helices and hence cannot hold iodine molecules
• C. It breaks down when iodine reacts with it
• D. It is a disaccharide

Answer 1

The Correct Option is B

Correct Answer: Option 2 – It does not contain complex helices and hence cannot hold iodine molecules 

Cellulose is a complex carbohydrate and a major structural component of plant cell walls. It is a polysaccharide composed of β-D-glucose units linked by β(1→4) glycosidic bonds, resulting in a linear and rigid structure. This structure is stabilized by extensive hydrogen bonding between adjacent chains, forming microfibrils.

In contrast, when iodine reacts with starch (particularly amylose, a component of starch), it produces a characteristic blue-black colour. This colour is due to the formation of an inclusion complex between iodine and the helical structure of amylose. The iodine molecules get trapped inside the helical coils, leading to the blue colouration.

However, cellulose does not possess such a helical structure. Its straight, fibrous chains lack the complex helical formation necessary to accommodate iodine molecules. As a result, no such inclusion complex is formed, and hence, **cellulose does not produce a blue colour with iodine**.

Explanation of Incorrect Options:

Option 1 – It is a helical molecule: Incorrect. Cellulose is not helical; it has a linear structure. The blue colour with iodine is specific to helical structures like amylose, not cellulose.

Option 3 – It breaks down when iodine reacts with it: Incorrect. There is no breakdown of cellulose during interaction with iodine; the absence of colour is purely due to lack of complexation.

Option 4 – It is a disaccharide: Incorrect. Cellulose is not a disaccharide but a polysaccharide. Disaccharides consist of two sugar units, whereas cellulose has hundreds to thousands of glucose units.

Conclusion: The inability of cellulose to form a blue colour with iodine lies in its structural makeup. Since it lacks a helical configuration required to trap iodine molecules, the colour reaction does not occur. Understanding such biochemical interactions is important for mastering concepts in biomolecules, especially for NEET and other competitive biology or chemistry exams.