Question:

Cellulose does not form blue colour with iodine because

Updated On: May 2, 2025

It is a helical molecule
It does not contain complex helices and hence cannot hold iodine molecules
It breaks down when iodine reacts with it
It is a disaccharide

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The Correct Option is B

Solution and Explanation

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.

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Concepts Used:

Biomolecules

Biomolecules are the most essential organic molecules, which are involved in the maintenance and metabolic processes of living organisms. These non-living molecules are the actual foot-soldiers of the battle of sustenance of life.

There are four major classes of Biomolecules – Carbohydrates, Proteins, Nucleic acids and Lipids.

Carbohydrates are chemically defined as polyhydroxy aldehydes or ketones or compounds which produce them on hydrolysis.
Proteins are another class of indispensable biomolecules, which make up around 50per cent of the cellular dry weight. Proteins are polymers of amino acids arranged in the form of polypeptide chains. The structure of proteins is classified as primary, secondary, tertiary and quaternary in some cases.
Nucleic acids refer to the genetic material found in the cell that carries all the hereditary information from parents to progeny. There are two types of nucleic acids namely, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). The main function of nucleic acid is the transfer of genetic information and synthesis of proteins by processes known as translation and transcription.
Lipids are organic substances that are insoluble in water, soluble in organic solvents, are related to fatty acids and are utilized by the living cell.