Problem 102 - A Complete Resource Book in Chemistry for JEE Main

Question

Cellulose is a straight chain polysaccharide composed of only (a) D-glucose units joined by \(\alpha\)-glucosidic linkage (b) D-glucose units joined by \(\beta\)-glucosidic linkage (c) D-galactose units joined by \(\alpha\)-glucosidic linkage (d) D-galactose units joined by \(\beta\)-glucosidic linkage

Step-by-Step Solution

Answer

Cellulose is composed of D-glucose units joined by -glucosidic linkage.

Step 1: Identify the Composition of Cellulose

Cellulose is a polysaccharide found primarily in the cell walls of plants. It consists exclusively of glucose units. Therefore, we need to focus on the options that involve glucose units.

Step 2: Determine the Type of Glucosidic Linkage in Cellulose

Cellulose is made up of D-glucose units. The type of linkage between these D-glucose molecules in cellulose is a -glucosidic linkage. This linkage is critical as it allows cellulose to form strong fibers.

Step 3: Select the Correct Answer Based on the Analysis

Given that cellulose consists of D-glucose units connected by -glucosidic linkages, the correct option is (b). D-glucose units joined by -glucosidic linkage.

Key Concepts

Cellulose Structure

Cellulose is one of the most abundant organic molecules on Earth. It forms the primary structural component of plant cell walls, providing rigidity and resistance to mechanical stresses. Unlike starch, which is a storage form of glucose used for energy, cellulose is tailored for structural purposes. Its configuration is linear, composed of long unbranched chains containing numerous glucose molecules.

These chains align parallel to each other, held together by hydrogen bonds. This extensive hydrogen bonding network not only strengthens the overall structure but also contributes to its insolubility in water. Because of this, cellulose gives plants the strength and stability needed to grow upright and withstand various environmental factors.

Cellulose's structure is like a robust scaffold, demonstrating nature's engineering excellence. It is this unique configuration that allows cellulose to serve such vital roles in plant biology.

Glucosidic Linkage

Glucosidic linkages are a form of covalent bond that connects glucose molecules in polysaccharides. In cellulose, the glucose molecules are bonded together specifically by \( \beta \)-glucosidic linkages. These are different from \( \alpha \)-glucosidic linkages found in other polysaccharides like starch and glycogen.

Understanding \( \beta \)-Glucosidic Linkages:

• In \( \beta \)-glucosidic linkages, the hydroxyl group (-OH) on the first carbon of one glucose unit connects to the fourth carbon of the next glucose unit.
• The orientation of glucose units is such that each successive unit is flipped concerning the previous one.
• This alternating pattern helps in forming straight, fibrous structures.

This linkage is crucial because it determines the physical properties of cellulose. The orientations and types of linkages dictate how easily enzymes can break down the polysaccharide. Due to these \( \beta \) linkages, cellulose is resistant to many enzymes, making it indigestible for humans while acting as dietary fiber.

Glucose Units

Glucose is a simple sugar that is crucial to energy production in many organisms. In the context of cellulose, glucose serves a different purpose – forming the building blocks of this significant polysaccharide.

Key Characteristics of Glucose in Cellulose:

• The glucose units in cellulose are the D-isomer, meaning they have a particular spatial configuration.
• When linked together, these glucose units form a long chain essential for the structure of cellulose.
• In cellulose, thousands of these units are strung together.

The D-glucose units aligned in cellulose contribute to its rigidity and strength. Unlike other polysaccharides, the arrangement and the specific linkage types play a pivotal role in defining the properties and functions of cellulose. This unique use of glucose exhibits nature's versatility in employing simple molecules for creating complex and essential structures.