Previous Year Question

Natural Silk is a protein-based natural fibre whose principal chemical constituents are Carbon (C), Hydrogen (H), Oxygen (O), and Nitrogen (N). The presence of nitrogen in silk is primarily due to its protein structure. Proteins are composed of Amino Acids, which contain nitrogen-bearing amino groups.

Natural silk mainly contains two proteins—Fibroin and Sericin. Fibroin provides strength and structural stability to silk fibres, whereas Sericin acts as a glue-like substance that holds the fibres together. Raw silk generally contains approximately 70–80% Fibroin and 20–30% Sericin.

Silk is produced by silkworms. The primary source of mulberry silk is the silkworm species Bombyx mori. Before entering the pupal stage, it secretes protein-rich fibres to construct a Cocoon. During silk processing, most of the Sericin is removed through a process called Degumming, producing soft and lustrous Fibroin fibres.

The chemical distinction between natural fibres is also important. Silk and Wool are protein-based fibres and therefore contain nitrogen. In contrast, Cotton and Linen are plant-based fibres composed primarily of Cellulose, whose principal elements are Carbon, Hydrogen, and Oxygen. Thus, among the given options, nitrogen is a constituent of the fundamental protein structure of natural silk.