Gluten

Gluten is an amorphous mixture of ergastic (i.e. non-living) proteins found combined with starch in the endosperm of some cereals, notably wheat, rye, and barley. It constitutes about 80% of the protein contained in wheat, and is a mixture of gliadin and glutenin. Gluten gives kneaded dough its elasticity, allowing leavening and contributing "chewiness" to baked products like bagels. The elasticity of gluten (whence its quality) is proportional to its content of glutenins with low molecular weights.