UPPSC 2019 Prelims GS Paper-I · Question 14 of 150

Correct — A, Cotton. Cotton is the only plant product in the list and it is not a protein at all — it is a carbohydrate. Each cotton fibre is a single elongated epidermal cell growing out of the cotton seed coat, and the dry mature fibre is roughly 90 per cent cellulose, a long chain of glucose units joined by beta-1,4 links. Cellulose is built only from carbon, hydrogen and oxygen: there is no nitrogen in it, no amino acids and no peptide bonds, which are the defining features of a protein. The other three are all animal products and all protein fibres — hair and hoof are keratin, and spider silk is a spun protein — so cotton is the odd one out.

• (b)Hair — Hair IS a protein. Mammalian hair is made of keratin, a fibrous structural protein unusually rich in the sulphur-containing amino acid cysteine; the disulphide bridges between cysteine units give hair its strength and elasticity, and it is these bonds that hair-straightening and perming chemicals break and re-form.
• (c)Spider silk — Spider silk IS a protein. It is spun from spidroins, proteins stored as a concentrated liquid in the spider's silk glands and converted into a solid fibre as the spider draws it out through the spinnerets. Weight for weight it has exceptional tensile strength and toughness — but chemically it is simply a protein fibre, like the fibroin of silkworm silk.
• (d)Hoof — Hoof IS a protein. A hoof is hardened keratin — the same protein family as hair, horn, nail, claw and feather. Its toughness comes from densely packed, heavily cross-linked keratin, not from any mineral.

Fibres of biological origin fall into two chemical camps. Plant fibres — cotton, jute, flax, hemp, coir — are cellulosic: polysaccharides of glucose containing only C, H and O. Animal fibres — wool, hair, silk, hoof, horn, nail, feather — are proteins: polymers of amino acids linked by peptide bonds, always containing nitrogen and usually sulphur as well. The single fastest test in an exam is the nitrogen test: proteins contain nitrogen, carbohydrates do not.

Do not be misled by appearance. All four options are thread-like, tough and fibrous, so texture tells you nothing. Sort by origin instead: hair, hoof and spider silk are made by animals, cotton is made by a plant. In an 'odd one out' among biological fibres, the plant fibre is the carbohydrate. The same logic separates natural silk (a protein) from rayon, the so-called 'artificial silk' that is really cellulose.

• Cotton fibre is almost pure cellulose — a carbohydrate of C, H and O only, with no nitrogen
• Hair, hoof, horn, nail, claw and feather are all keratin, a fibrous protein cross-linked by cysteine disulphide bonds
• Spider silk (spidroin) and silkworm silk (fibroin, coated with sericin) are both protein fibres
• Every protein contains carbon, hydrogen, oxygen and nitrogen; most also contain sulphur
• Plant fibres (cotton, jute, flax, hemp, coir) are cellulosic; animal fibres (wool, silk, hair) are proteinaceous

• Assuming anything strong and thread-like must be a protein — cotton and jute are plant carbohydrates
• Confusing natural silk (a protein) with rayon, marketed as 'artificial silk' but actually regenerated cellulose
• Thinking hooves and horns are mineral like bone — they are keratin, a protein

UPPSC and UPSC both ask this as 'which is/is not a protein', 'which fibre is of plant origin', or by naming a protein (keratin, fibroin, collagen, myoglobin) and asking where it is found — one clean plant-versus-animal fibre list answers all of them.

• practice — not a real PYQ

  Which one of the following fibres is NOT of animal origin?

  • (a)Wool
  • (b)Mulberry silk
  • (c)Jute
  • (d)Pashmina

  Answer(c) Jute — a bast fibre obtained from the stem of the jute plant and made of cellulose. Wool, mulberry silk and pashmina are all animal protein fibres.

• practice — not a real PYQ

  The protein that makes up human hair and nails and the hooves and horns of animals is:

  • (a)Collagen
  • (b)Keratin
  • (c)Elastin
  • (d)Albumin

  Answer(b) Keratin — a fibrous, sulphur-rich structural protein. Collagen is the protein of connective tissue and skin, elastin gives tissues their elasticity, and albumin is a soluble protein of blood plasma and egg white.