Look a little closer
Bread dough rises not because yeast cells inflate like balloons, but because they ferment sugars into carbon dioxide while an elastic flour-protein network traps the gas in small bubbles.
Mixing flour with water links gliadin and glutenin proteins into gluten. Kneading and folding organize this network so it can stretch without tearing easily.
Yeast consumes simple sugars in the dough. Under oxygen-limited conditions it produces carbon dioxide and ethanol through fermentation, along with flavor compounds. Gas enters small air pockets already introduced during mixing and enlarges them.
Weak gluten leaks gas, while an overly rigid network cannot expand enough. Water, dough temperature, salt, fermentation time and flour protein jointly control the rate and final structure.
In the oven, warming gas and water vapor expand, and yeast briefly becomes more active before heat stops it, producing oven spring. Proteins then set and starch absorbs water, fixing the expanded crumb in place.
Overproofed dough may collapse not simply because yeast died, but because the gluten network weakened and can no longer hold enlarged bubbles. Good bread balances gas production with a structure capable of retaining it.
