How does yeast rise?

Yeast makes bread rise, and gives it the air bubbles inside.

The term yeast refers to a group of a hundred or so single celled organisms collectively known as Saccharomyces, which are a type of fungi.

When you mix flour and yeast and dampen it, various things start to happen. In addition to the wheat's own enzymes, which begin to convert sugar into starch, the yeast cells produce several enzymes of their own, which convert the various sugars in flour into forms the yeast can absorb. This is how yeast feeds.

Most enzymes inside the yeast cell convert these sugars into carbon dioxide and alcohol, which are excreted. The carbon dioxide forms into bubbles in the dough and causes it to rise. This is fermentation (from the Latin fermo meaning 'to boil') This enzyme activity helps to make the tough gluten in the flour more digestible. Not surprisingly, bread that ferments for longer is better for you.

After feeding heartily and passing gas accordingly, the yeast's cells turn to reproduction, so it splits in two. From then on 2 becomes 4, 4 becomes 8 etc etc... meanwhile some of the excreted alcohol becomes acetic acid, which makes the dough slightly more acidic. Yeast likes that, and the activity increases because of it. All this noticeably raises the temperature of the dough.