Which one of the following is a protein fibre?

1. Classification of Fibres: Natural vs. Synthetic (basic)

To understand the chemistry of what we wear, we must first look at the origin of the fibres. Fibres are broadly classified into two categories: Natural and Synthetic. Natural fibres are sourced directly from nature—either from plants or animals—while synthetic fibres are man-made through complex chemical processes, typically involving petroleum-based products Certificate Physical and Human Geography, Manufacturing Industry and The Iron and Steel Industry, p.279.

The fundamental difference between these fibres lies in their chemical building blocks. Plant-based fibres, such as cotton (often called the 'King of fibres'), jute, and hemp, are primarily composed of cellulose, a complex carbohydrate that provides structural strength to plants Certificate Physical and Human Geography, Agriculture, p.257. On the other hand, animal-derived fibres like silk and wool are made of proteins. Silk, for instance, is produced by silkworms in a process known as sericulture and is composed of two main proteins: fibroin (the structural filament) and sericin (the binding glue) NCERT (2022) Contemporary India II, The Age of Industrialisation, p.87.

Synthetic fibres, such as Nylon and Polyester, do not exist in nature. They are produced in factories through polymerization, where small chemical units are linked into long chains. For example, Nylon is a synthetic polyamide, while Polyester is a synthetic polymer derived from coal and petroleum Certificate Physical and Human Geography, Manufacturing Industry and The Iron and Steel Industry, p.279. While natural fibres are breathable and biodegradable, synthetic fibres are valued for their durability and resistance to moisture.

Category	Source	Chemical Nature	Examples
Plant Natural	Crops (Soil)	Cellulosic (Carbohydrate)	Cotton, Jute, Hemp, Flax
Animal Natural	Insects/Animals	Protein-based	Silk, Wool
Synthetic	Chemical Industry	Polymer (Petroleum-based)	Nylon, Polyester, Rayon

Sources: Certificate Physical and Human Geography, Manufacturing Industry and The Iron and Steel Industry, p.279; Certificate Physical and Human Geography, Agriculture, p.257; NCERT (2022) Contemporary India II, The Age of Industrialisation, p.87

2. Plant-derived Fibres and Cellulose (basic)

To understand plant-derived fibres, we must first look at their chemical 'skeleton.' Plants are built from cellulose, a complex carbohydrate (polysaccharide) that provides structural support to cell walls. Because of this, all natural fibres obtained from plants—such as cotton, jute, flax, and hemp—are chemically classified as cellulosic fibres. This is a fundamental distinction in chemistry: while plants give us carbohydrates (cellulose), animal-derived fibres like silk and wool are composed of proteins.

Cotton is perhaps the most vital plant fibre. It is a 'seed fibre,' meaning the soft fluff protects the seeds of the cotton plant. From a geographical and chemical perspective, cotton thrives in the drier parts of the black cotton soil of the Deccan plateau, requiring high temperatures and exactly 210 frost-free days to mature successfully NCERT, Contemporary India II, Chapter 4, p. 87. Unlike synthetic fibres like Nylon or Polyester, which are polymers derived from petroleum, cotton is a natural polymer that is breathable and biodegradable.

Another heavy-hitter in the plant world is Jute, often called the 'Golden Fibre' for its colour and high value. Unlike cotton, jute is a bast fibre, meaning it is collected from the stem of the plant rather than the seed. It requires a hot, humid climate and well-drained fertile soils, which is why it is concentrated in the flood plains of West Bengal and Assam Majid Husain, Geography of India, Chapter 9, p. 19. Other notable plant fibres include Flax (used to make high-quality linen) and Abaca (Manila hemp), which is highly resistant to seawater and used extensively for marine ropes GC Leong, Certificate Physical and Human Geography, Chapter 28, p. 258.

Fibre Type	Source Example	Chemical Base
Plant Fibre	Cotton, Jute, Flax	Cellulose (Carbohydrate)
Animal Fibre	Silk, Wool	Protein
Synthetic Fibre	Nylon, Polyester	Petroleum-based Polymers

Sources: NCERT, Contemporary India II, Chapter 4: The Age of Industrialisation, p.87; Geography of India, Majid Husain, Chapter 9: Agriculture, p.19; Certificate Physical and Human Geography, GC Leong, Chapter 28: Manufacturing Industry, p.258

3. Synthetic Polymers: Nylon and Polyester (intermediate)

To understand synthetic polymers like Nylon and Polyester, we must first look at the concept of polymerization. A polymer is a large molecule (macromolecule) formed by joining many small repeating units called monomers. While natural fibers like cotton (cellulose) and silk (protein) are bio-polymers, Nylon and Polyester are synthetic polymers derived primarily from petroleum by-products. As these molecules grow in size, their physical properties like melting point and solubility change, though their chemical identity remains tied to their functional groups Science class X (NCERT 2025 ed.), Carbon and its Compounds, p.67.

Nylon was the first fully synthetic fiber, famously known as a polyamide. It is created by the reaction of diamines with dicarboxylic acids, resulting in a chain linked by amide bonds (similar to the peptide bonds found in proteins, but synthetic). Nylon is prized for its high strength, elasticity, and resistance to abrasion. However, its durability is a double-edged sword; nylon net fragments are negatively buoyant, meaning they sink and impact benthic (bottom-dwelling) marine species, persisting in the environment for decades because they are non-biodegradable Environment, Shankar IAS Academy (ed 10th), Environmental Pollution, p.97.

Polyester, on the other hand, is a category of polymers that contain the ester functional group in their main chain. The most common variety is PET (Polyethylene terephthalate). Polyester fibers are celebrated for being crease-resistant and easy to wash, making them a staple in the textile industry. A critical vulnerability of synthetic polymers like polyester is their sensitivity to solar radiation. Over time, UV rays can break the polymer chains, leading to brittleness and degradation, which is why many outdoor plastics require chemical light-stabilizers for protection Environment, Shankar IAS Academy (ed 10th), Ozone Depletion, p.272.

Feature	Nylon	Polyester
Chemical Class	Polyamide	Polyester
Key Property	Exceptional strength and elasticity	Wrinkle resistance and shape retention
Common Uses	Fishing nets, ropes, stockings	Clothing, plastic bottles (PET), sails

Because these materials are non-biodegradable, they pose significant waste management challenges. While new biodegradable plastics are being developed to mitigate environmental harm, the vast majority of current synthetic fibers remain in the environment for hundreds of years Science class X (NCERT 2025 ed.), Our Environment, p.214.

Sources: Science class X (NCERT 2025 ed.), Carbon and its Compounds, p.67; Environment, Shankar IAS Academy (ed 10th), Environmental Pollution, p.97; Environment, Shankar IAS Academy (ed 10th), Ozone Depletion, p.272; Science class X (NCERT 2025 ed.), Our Environment, p.214

4. Biochemistry of Proteins as Biopolymers (intermediate)

To understand proteins as biopolymers, we must first look at their basic chemistry. A polymer is a large molecule made of repeating structural units; in the case of proteins, these units are amino acids. While many organic molecules contain carbon, hydrogen, and oxygen, proteins are unique because Nitrogen is an essential constituent, making up approximately 16% of their total weight Environment, Shankar IAS Academy, Chapter 3, p.19. This nitrogen is vital for building living tissue and is often the limiting factor in plant growth, requiring 'fixation' from the atmosphere into a usable form like ammonia or nitrates. In nature, the blueprint for these protein polymers is stored in our DNA. A specific section of DNA, known as a gene, provides the information required to string amino acids together in a precise sequence Science, Class X (NCERT 2025 ed.), Heredity, p.131. Once formed, these chains fold into complex shapes to perform specific jobs. For example, they can become enzymes (biological catalysts) that trigger growth or structural fibers like those found in silk or wool. Some of these amino acids even contain Sulphur, which helps create strong chemical bonds that maintain the protein's 3D structure Environment, Shankar IAS Academy, Agriculture, p.363. It is helpful to distinguish protein-based polymers from other common materials we encounter daily. While silk (composed of the proteins fibroin and sericin) and wool are natural protein fibers derived from animals, cotton is a carbohydrate-based polymer called cellulose. On the other hand, materials like Nylon are synthetic polyamides, designed in labs to mimic some properties of natural proteins but derived from petroleum products rather than biological processes.

Fiber Type	Chemical Nature	Primary Source
Silk / Wool	Protein (Biopolymer)	Animal (Silkworm / Sheep)
Cotton / Jute	Cellulose (Carbohydrate)	Plant (Seed / Stem)
Nylon / Polyester	Synthetic Polymer	Chemical Synthesis

Sources: Environment, Shankar IAS Academy, Functions of an Ecosystem, p.19; Science, Class X (NCERT 2025 ed.), Heredity, p.131; Environment, Shankar IAS Academy, Agriculture, p.363

5. Animal Fibres: Sericulture and Wool (intermediate)

To understand the chemistry of everyday fabrics, we must first distinguish between their biological origins. While plant-based fibres like cotton and jute are composed of cellulose (a complex carbohydrate), animal fibres like silk and wool are essentially proteins. This chemical difference explains why silk and wool behave differently under heat or chemicals compared to cotton. Silk is a natural protein fibre produced by the larvae of the silkworm, primarily Bombyx mori. Chemically, it is composed of two main proteins: fibroin, which provides the structural strength of the filament, and sericin, a gummy substance that binds the filaments together. The process of rearing these silkworms on mulberry leaves to harvest their cocoons is known as Sericulture NCERT Contemporary India II, The Age of Industrialisation, p.87.

India holds a prestigious position in the global silk industry, ranking as the world's second-largest producer. Interestingly, India has a global monopoly on Muga silk, a golden-hued variety primarily produced in Assam Geography of India, Industries, p.25. On the other hand, wool is the primary animal fibre of temperate regions, harvested not just from sheep, but also from Cashmere goats, Angora goats (producing mohair), and even camels Certificate Physical and Human Geography, Agriculture, p.258. Wool is prized for its insulating properties and its ability to absorb moisture without feeling damp, which is due to the complex protein structure of its follicles.

While India produces significant quantities of wool, it is not entirely self-sufficient in high-quality varieties. For instance, the finest Merino wool — which originated in Spain and is known for its incredible softness and durability — is often imported from Australia to feed India's textile exports, such as Kashmiri shawls and high-end carpets Geography of India, Industries, p.24.

Feature	Plant Fibres (e.g., Cotton)	Animal Fibres (Silk & Wool)
Chemical Basis	Cellulose (Carbohydrate)	Protein (Fibroin/Keratin)
Primary Source	Seeds, Stems, Leaves	Cocoons, Animal Hair
Key Property	Highly Breathable, Absorbent	Insulating, Resilient to Wrinkling

Sources: NCERT Contemporary India II, The Age of Industrialisation, p.87; Geography of India, Industries, p.25; Certificate Physical and Human Geography, Agriculture, p.258; Geography of India, Industries, p.24

6. Chemical Composition of Silk: Fibroin and Sericin (exam-level)

Silk is a natural protein-based animal fiber, setting it apart chemically from plant fibers like cotton, which are composed of cellulose (a carbohydrate). While many people think of silk simply as a fabric, from a chemical perspective, it is a sophisticated dual-protein system secreted by the salivary glands of silkworm larvae, most notably the Bombyx mori species Geography of India, Agriculture, p.95. This protein nature is what gives silk its unique affinity for dyes and its characteristic sheen. The chemical architecture of a silk thread consists of two primary proteins: Fibroin and Sericin. Fibroin is the structural center—it is an insoluble protein that forms the long, continuous filament. To hold these filaments together into a sturdy cocoon, the silkworm secretes Sericin, a sticky, gummy protein that acts as a natural adhesive Geography of India, Agriculture, p.95. In industrial processing, the cocoons are placed in hot water; this heat dissolves the Sericin (a process called degumming), allowing the Fibroin threads to be unwound or 'reeled' for textile production. Understanding silk's composition is essential for distinguishing it from modern alternatives. While Nylon is often called a 'synthetic silk' because it is also a polyamide (containing similar chemical bonds to proteins), it is derived from petroleum by-products rather than biological amino acids Certificate Physical and Human Geography, Manufacturing Industry, p.279. This chemical distinction is why natural silk is biodegradable and has superior moisture-wicking properties compared to most synthetic counterparts.

Component	Type	Primary Function
Fibroin	Structural Protein	Forms the core filament; provides strength and luster.
Sericin	Gum/Adhesive Protein	Cements the filaments together to form the cocoon.

Sources: Geography of India, Agriculture, p.95; Certificate Physical and Human Geography, Manufacturing Industry, p.279

7. Solving the Original PYQ (exam-level)

To solve this question, we must apply the fundamental classification of fibers you just mastered. Fibers are broadly divided into natural and synthetic categories. Among natural fibers, the biochemical composition is the key differentiator: plant-based fibers like cotton, jute, and hemp are cellulosic (made of carbohydrates), while animal-based fibers like silk and wool are protein-based. Since the question specifically asks for a protein fibre, your first step should be to identify which source is animal-derived.

Among the given options, Silk is the only animal-derived fiber, harvested from the cocoons of the Bombyx mori silkworm. As detailed in Geography of India, Majid Husain, silk consists of two primary proteins: fibroin, which forms the structural filament, and sericin, a natural gum that cements those filaments together. This unique biochemical signature confirms that (C) Silk is the correct answer. A helpful rule of thumb for UPSC is to remember that if a natural fiber comes from an animal (hair or secretion), its building blocks are amino acids (proteins).

It is crucial not to fall for common distractors. Cotton is a natural fiber, but it is cellulosic because it is a plant-based seed fiber, as noted in NCERT Contemporary India II. On the other hand, Nylon and Polyester are entirely synthetic polymers created through chemical polymerization of petroleum by-products. While Nylon is technically a polyamide, it is man-made and does not qualify as a natural protein fiber. UPSC frequently tests the distinction between biological origins (cellulose vs. protein) and industrial origins (synthetic), so always categorize the source first.