How does the silkworm make silk?
By Chelsie Vandaveer
December 5, 2001
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The production of fabric from the cocoons of the silkworm (Bombyx mori Linnaeus) goes back almost 5000 years. The manufacture of Chinese silk was kept secret for 2700 years. Silk from China was such a profitable export that the trade routes from China to the Mediterranean were known as the Silk Road.
The white mulberry (Morus alba) is native to eastern Asia and was the first mulberry cultivated for the rearing of silkworms. Mulberry leaves contain a number of chemicals necessary to the silkworm. White mulberry leaf contains five chemicals that attract the caterpillar: citral, linalyl acetate, linalol, terpinyl acetate, and hexenol. The caterpillar ignores leaves that lack these chemicals.
The lipid, beta-sitosterol, encourages the caterpillar to eat, but the concentration of beta-sitosterol also controls the amount of leaves eaten. Fifteen amino acids (protein building-blocks) are present in the leaf. Three of these amino acids, glycine, alanine, and serine are the primary constituents of the protein polymer, fibroin (fi bro' in). Fibroin molecules make up the silk fibers.
Silk is secreted from two modified salivary glands near the silkworm's mouth. The two strands are bound together with sericin, an amorphous protein. Each cell of the salivary glands has a single gene that codes for fibroin. When the gene is activated, it makes up to 10,000 copies of messenger RNA (mRNA codes the sequence of amino acids).
Each mRNA can direct the synthesis of 100,000 molecules of fibroin. The cocoon is completed within 48 hours, each cell of the salivary glands contributing up to one billion molecules of fibroin. Depending on the race of Bombyx mori, each cocoon yields one silk strand 300 to 900 meters long. It takes up to 10 silk strands (cocoons) to make one silk thread.
The mulberry silkworm is completely domesticated and believed extinct in the wild. The moth can no longer fly; its wings have been 'bred-down'. It no longer chooses its own mate; specific genetic lines are kept pure. Chopped mulberry leaves are brought to the caterpillar. Sericulture (cultivation of the silk caterpillar) is entirely in the hands of humans.
Dr. Anthony Serianni of the University of Notre Dame has posted a diagram of the fibroin molecule. To view the diagram, click on the link:
http://www3.nd.edu/~aseriann/silk.html
The Sericulum Company has an excellent page on the lifecycle of the Bombyx mori. To learn more about the moth, click on the link:
http://www.sericulum.com/lifecycle.html
killerPlants Tendrils: ~~1~~2~~3~~4~~5~~
Suggested Reading:
Mulberries, caterpillars, and the conquest of Asia Plants that Changed History - December 4, 2001
How does the silkworm make silk? Renfield's Garden - December 5, 2001
King James, Shakespeare, and a moron What's in a Name? - December 7, 2001
How did the mulberry push Westward Expansion? Plants that Changed History - December 11, 2001
What caterpillar spins golden silk? Renfield's Garden - December 12, 2001
What experiment turned into a yearly disaster? Plants that Changed History - December 25, 2001
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