Problem 42 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one is a protein fibre? (a) rayon (b) cotton (c) polyester (d) silk

Step-by-Step Solution

Answer

Silk is a protein fibre.

Step 1: Understanding Protein Fibres

Protein fibres are natural fibres composed of proteins, mostly derived from animal sources. These include fibres like wool from sheep and silk from silkworms.

Step 2: Identifying Each Option

Let's break down each option to identify its composition: - Rayon is a semi-synthetic fibre derived from cellulose, which comes from plants. - Cotton is a natural fibre made from the cellulose of the cotton plant. - Polyester is a synthetic fibre made from petroleum-derived products. - Silk is a natural fibre produced by silkworms, and it is composed of protein molecules.

Step 3: Analyzing the Options

From the options provided, only silk aligns with the definition of a protein fibre because it is produced by silkworms and composed of protein, specifically fibroin.

Step 4: Selecting the Correct Answer

The only option among the provided choices that is a protein fibre is silk.

Key Concepts

Natural Fibres

Natural fibres are a fascinating category of materials that occur naturally and have been used by humans for thousands of years. These fibres are primarily derived from two sources: plants and animals. Plant-based natural fibres include cotton and linen, which are made from cellulose.
Animal-based natural fibres, on the other hand, include wool and silk, which are made from proteins.

• Plant fibres: These are generally composed of cellulose, giving them certain properties such as absorbency and strength. Common examples include cotton, commonly used in clothing, and flax, used to make linen.
• Animal fibres: These include fibres like wool and silk. Wool is obtained from sheep and is known for its warmth and elasticity, while silk comes from silkworms, known for its lustrous and smooth texture.

Understanding the basics of natural fibres can help us appreciate the natural world's diversity and the different materials it provides.

Silk

Silk is a highly prized natural fibre known for its sheen and strength. It is a protein fibre, meaning it is derived from animal proteins. Specifically, silk is produced by the larvae of the silkworm, which belongs to the moth species *Bombyx mori*. The process of producing silk is fascinating. Silkworms spin this fibre to form cocoons, which are later harvested to extract the silk. The central protein in silk is fibroin, which gives silk its unique properties:

• Luxurious feel: Silk has a soft, smooth texture that stands out from other natural fibres.
• High tensile strength: Despite its delicate feel, silk is incredibly strong.
• Hypoallergenic properties: Due to its structure and production process, silk is resistant to dust mites and mold.

Silk's aesthetic appeal and functional properties make it a beloved material in fashion and home décor.

Composition of Fibres

The composition of fibres plays a crucial role in determining their properties and uses. Fibres can be broadly classified based on their main components: protein, cellulose, or synthetic materials. Each type has unique characteristics. **Protein fibres:** These are made up of proteins from animal sources, such as:

• Silk: Contains fibroin, which provides strength and a glossy finish.
• Wool: Made of keratin, providing elasticity and warmth.

**Cellulose fibres:** These are derived from plants. Cotton, for example, is composed of cellulose, making it breathable and absorbent. **Synthetic fibres:** These are human-made from chemical compounds. Polyester and nylon are examples and are prized for their durability and resistance to wear. A thorough understanding of these compositions allows manufacturers and consumers to choose the right fibre for the right purpose, aligning characteristics like absorbency, strength, or elasticity with the intended application.