Wool is almost entirely made up of protein molecules.
Proteins are long molecular chains composed of many smaller units called amino acids.
The alpha-keratin proteins of wool form closely connected pairs of helices. Each pair is composed of two types of keratin chain, that are twisted in parallel into a left-handed coil.
So the molecule looks like a piece of cord that has been twisted, then twisted again.
Some of the amino acids (eg Cysteine) found in wool contain sulphur, and if two sulphur atoms from different protein strands are close together they can form a sulphur-to-sulphur bond which acts as a bridge between different protein strands.
These bonds are known as ’cross-links’ and help maintain the shape of the fibres by locking together adjacent molecules, preventing the chains from being completely mobile.
The sulphur bridges survive any stretching of the fibre and ensure that it reverts to its original shape once the tension has been released, making the wool springy, and able to retain its form.