Why does bread rise when you add yeast?

Yeast are living microorganisms that consume sugars in the dough through a process called fermentation. As they digest the sugars, they produce carbon dioxide gas and alcohol as byproducts. The carbon dioxide gets trapped within the gluten network that forms when flour proteins are mixed with water, creating tiny gas bubbles throughout the dough. These bubbles expand as more gas is produced, causing the dough to rise and develop a light, airy texture. When the bread is baked, the heat kills the yeast, stopping fermentation, and the alcohol evaporates. The heat also sets the gluten structure permanently, locking in those air pockets and giving the finished bread its characteristic soft, open crumb.