During bread making, metabolism of simple sugars by yeast produces carbon dioxide which makes the bread rise.
The essential ingredients of bread dough are flour, water and yeast. As soon as these ingredients are stirred together, enzymes in the yeast and the flour cause large starch molecules to break down into simple sugars.
The yeast metabolizes these simple sugars and exudes a liquid that releases carbon dioxide and ethyl alcohol into existing air bubbles in the dough. If the dough has a strong and elastic gluten network, the carbon dioxide is held within the bubble and begins to inflate it, just like someone blowing up bubble gum.
As more and more tiny air cells fill with carbon dioxide, the dough rises, and so we see bread rising.
