Gluten consists of water and the amino acids glutenin and gliadin. When flour combined with water during the fast mixing gluten will start to formed in as development. Gluten provides the structure of the bread and traps the yeast’s fermented gases, carbon dioxide, in the cells of the crumb allowing the dough to rise. During baking the gluten matrix is coagulated and fixed into a very strong structure.
It can hold twice its own weight in water, which leads to substantial water absorption that increases the yield of the dough and the shelf life of the bread.
Extra gluten is often added when the dough has added fruit or grains, because the grain will cut off the gluten while mixing the dough.