Problem 163 - Objective Biology for NEET

Question

Assertion: Cellulose doesn'\operatorname{tg} i v e ~ c o l o u r ~ w i t h ~ \(\mathrm{I}_{2}\) Reason: Cellulose doesn't contain helix which can hold \(\mathrm{I}_{2}\)

Step-by-Step Solution

Answer

The assertion and reason are both correct, and the reason is the correct explanation of the assertion. Cellulose's lack of a helical structure is what prevents it from exhibiting a colour change when reacting with iodine.

Step 1: Confirming the Assertion

Cellulose is a complex carbohydrate or polysaccharide consisting of \( \mathrm{C}_{6}\mathrm{H}_{10}\mathrm{O}_{5} \) units. It doesn't react with iodine (\( \mathrm{I}_{2} \)), hence it doesn't give a colour with it.

Step 2: Analyzing the Reason

The reason mentioned states that cellulose doesn't contain a helix that can hold \( \mathrm{I}_{2} \). This is actually correct. Cellulose is a straight-chain polysaccharide with no helical structure. Iodine typically reacts and gets lodged in helical structures like in amylose starch, thereby causing a change in colour.

Step 3: Connecting the Assertion and Reason

Thus, cellulose doesn't give a colour change with iodine because it lacks a structure (helix) that can hold iodine and cause a reaction. Both the assertion and reason are correct, and the reason is indeed the correct explanation for the assertion.

Key Concepts

Polysaccharides

Polysaccharides are long-chain carbohydrates formed by the condensation of many simple sugar units known as monosaccharides. They serve various crucial functions in living organisms, ranging from structural support to energy storage. Polysaccharides are classified based on their structure and the types of sugars they contain.

For example, in plants, cellulose provides structural integrity to the cell walls, while starch serves as an energy reserve. Contrary to starch, cellulose is insoluble in water and does not coil into a helix, which has implications for its interactions with certain chemicals, such as iodine. Understanding the basic characteristics of polysaccharides helps in grasping why cellulose doesn't react with iodine and doesn't change its color, unlike starch.

Cellulose Structure

The structure of cellulose is unique among polysaccharides. It consists of linear chains of glucose molecules linked by β-1,4-glycosidic bonds. Due to the configuration of these bonds, the glucose units flip relative to each other, resulting in a straight and rigid structure.

What distinguishes cellulose is the way these chains are organized; they lie parallel to each other, forming microfibrils through extensive hydrogen bonding. This tight packing is what makes cellulose so strong and resistant to being broken down, which is essential for its role in plant cell walls. Its straight-chain structure does not form a helix, therefore, it lacks the 'pockets' needed to trap iodine, leading to the absence of a colour change during the iodine reaction, a characteristic that contrasts with the helical structure of starch.

Iodine-Starch Reaction

The iodine-starch reaction is a well-known chemical reaction where iodine (I2) molecules become trapped in the helical amylose chains of starch, producing a deep blue or purple coloration. This reaction is commonly used as a test to identify the presence of starch.

In this reaction, the size and shape of the iodine molecules complement the helical structure of amylose, allowing for a fit that is similar to a key entering a lock. Since cellulose does not have this helical structure, it does not capture iodine in the same manner; thus, no color change is observed when iodine is applied to cellulose. The unique characteristics of the iodine-starch reaction highlight the significance of structural conformation in chemical reactions and its impact on the interaction between different molecules.