For its size, spider silk is stronger than steel, yet it is extraordinarily lightweight and stretchy. Now scientists are learning how spiders produce it.

It was already known that spiders store silk proteins, called spidroins, in their glands. But how they converted these soluble, gel-like proteins into a solid state has been a mystery.

Researchers from Sweden report that the change to solid is brought about by a change in pH that occurs as the proteins travel through the glands. The findings appear in the journal PLOS Biology.

The gradual change in pH, from the neutral 7.6 to an acidic 5.7, is set off by an enzyme called carbonic anhydrase. The enzyme creates an acidic environment by converting carbon dioxide and water to bicarbonate and hydrogen ions.