Understanding the Concept:
Natural fibers are classified according to their origin and chemical composition. Some fibers are primarily composed of cellulose, while others are composed of proteins.
Silk is a natural protein fiber produced by silkworms, especially Bombyx mori. The principal structural protein present in silk is fibroin, which provides strength, flexibility, and characteristic luster to silk fibers.
Understanding the chemical composition of fibers is important in pharmacognosy because several natural fibers are used in surgical dressings, pharmaceutical preparations, and biomedical applications.
Step 1: Understand the composition of silk fibre.
Silk consists mainly of two proteins:
• Fibroin
• Sericin
Fibroin forms the structural core of the silk thread, whereas sericin acts as a gummy coating surrounding fibroin fibers.
Fibroin contributes the majority of the mechanical strength of silk.
Step 2: Examine cotton fibre.
Cotton is obtained from the seed hairs of species of Gossypium.
Chemically, cotton is composed predominantly of cellulose.
Since fibroin is not a constituent of cellulose fibers, cotton is incorrect.
Step 3: Examine wool fibre.
Wool is an animal fiber obtained from sheep and other animals.
Its major protein constituent is keratin.
Because wool contains keratin rather than fibroin, this option is incorrect.
Step 4: Examine nylon fibre.
Nylon is a synthetic polymer manufactured chemically.
It is not a natural protein fiber and does not contain fibroin.
Therefore, this option is also incorrect.
Step 5: Identify the fibre containing fibroin.
Among all the options, silk is the only fiber whose principal structural protein is fibroin.
Hence, silk is the correct answer.
Conclusion:
Fibroin is the characteristic structural protein of silk fibre. Therefore, the correct answer is Option (C).