Problem 16 - Objective Biology for NEET

Question

Select the incorrect statement from the following: (a) Cellulose does not contain complex helices. (b) Cellulose does not give colour with \(\mathrm{I}_{2}\). (c) Cotton fibre, plant cell wall are made up of cellulose. (d) Cellulose is heteropolysaccharide.

Step-by-Step Solution

Answer

The incorrect statements from the given options are (a) 'Cellulose does not contain complex helices' and (d) 'Cellulose is heteropolysaccharide'.

Step 1: Analyzing Statement (a)

Cellulose is made up of glucose units which are linked end-to-end forming complex helices, so option (a) states that 'Cellulose does not contain complex helices' is incorrect.

Step 2: Analyzing Statement (b)

Cellulose does not give a colour with \(\mathrm{I}_{2}\) (Iodine). This statement is correct because cellulose does not have the ability to trap iodine and hence does not give any colour.

Step 3: Analyzing Statement (c)

The statement 'Cotton fibre, plant cell wall are made up of cellulose' is correct. As the main constituent of plant cell walls, cellulose provides strength and rigidity to plants including cotton fibers.

Step 4: Analyzing Statement (d)

Finally, the statement 'Cellulose is heteropolysaccharide' is incorrect. Cellulose is a homopolysaccharide, not a heteropolysaccharide. It is composed of only one type of monomer, which is glucose.

Key Concepts

Polysaccharides

Polysaccharides are large, complex carbohydrates composed of long chains of monosaccharide units linked by glycosidic bonds. These carbohydrates can range from being very simple to highly branched structures. They play critical roles in both biological structures and energy storage.

• Starch: Acts as an energy storage in plants.
• Glycogen: Serves a similar energy storage role in animals.
• Cellulose: Provides structural support in plant cell walls.

Polysaccharides can be classified into two types based on their composition:

• **Homopolysaccharides**: Made up of only one kind of monosaccharide, like cellulose which is composed solely of glucose units.
• **Heteropolysaccharides**: Contain more than one type of monosaccharide.

Glucose units

Glucose units are the building blocks of many significant carbohydrates. As monosaccharides, they can join together to form more complex sugars and polysaccharides like cellulose. Glucose is a simple sugar with the formula \( C_6H_{12}O_6 \). In cellulose, glucose units are linked by \( \beta(1 \rightarrow 4) \) glycosidic bonds which provide the molecule with unique properties.

• These \( \beta \) bonds create straight, fibrous chains perfectly suited for providing strength and rigidity.
• Cellulose fibers from these chains are insoluble in water, adding to their supportive role in plant structures.

Understanding how glucose units form larger polysaccharides helps explain how plants achieve their robust structure.

Plant cell wall

The plant cell wall is an essential structure that defines plant cells, giving them strength, protection, and rigidity. Cellulose is a critical component of this structure. As a network of cellulose fibers, it forms the backbone of the plant cell wall ensuring structural integrity.

• Cellulose provides tensile strength which allows plants to withstand various environmental stressors.
• The fibers are interwoven with other polysaccharides and proteins for additional support and flexibility.
• This structure plays a crucial role in maintaining plant cell turgidity, necessary for normal function and growth.

The design of plant cell walls ensures that plants remain upright, healthy, and capable of growth. The study and understanding of cell walls are crucial in botany and agriculture.

Homopolysaccharide

A homopolysaccharide is a type of polysaccharide that consists entirely of one kind of monosaccharide unit. Cellulose, a homopolysaccharide, is made exclusively of glucose units. Each glucose unit in cellulose is linked in such a way that they form long, linear chains.

• These chains support plants by forming a rigid structure that makes up the plant cell wall.
• The regular pattern of \( \beta(1 \rightarrow 4) \) linkage results in a fiber that plays a key role in plant stability.
• Since cellulose does not include any other monosaccharides, it is termed a homopolysaccharide.

Understanding the nature of cellulose as a homopolysaccharide helps differentiate it from other polysaccharides with mixed compositions.