Which of the following substances is NOT a protein?

The correct answer is A) Cotton. Cotton fibre is almost pure cellulose, a polysaccharide made of glucose units joined by beta-1,4 glycosidic bonds. It is a carbohydrate, not a protein, and it contains no amino acid chains.

• B) Hair is a protein. Its main component is keratin, a fibrous structural protein rich in cysteine, whose disulfide bonds give hair its strength.
• C) Spider silk is a protein. It is made of fibroin, built from glycine and alanine-rich repeats that form beta-sheet structures, which is why it is tough yet elastic.
• D) Hoof is also keratin, the same family of protein as hair, nails and horn.

The core concept is the classification of biomolecules: carbohydrates, proteins, lipids and nucleic acids. Proteins are polymers of amino acids joined by peptide bonds. For Mains, spider silk is a standard example in biomimetic materials research, because it is strong by weight and is being studied for medical and textile uses.

Trap: Many aspirants group all natural fibres together as protein because silk and wool are protein fibres. Cotton is a plant fibre, so it is cellulose.