Yeast is responsible for making bread rise.
This exposes all the starch stored inside the grain. When water is added to make a dough, this activates natural enzymes, like amylases, stored in the flour. These enzymes turn the starch into glucose. The yeast added to the dough uses the glucose to produce carbon dioxide and alcohol. If yeast has no oxygen it will produce more alcohol than you want - which is why bread is always kneaded to trap air inside it, and why dough often smells like barley hops.