Yeast is a single cell organism that exists all around us. They feed on carbohydrates and as they do so they release CO2. Because of that, yeast is used to leaven bread and convers carbs to alcohol. In baking, yeast is used as an leavening agent where it converts the fermentable sugars from the grains in the dough into CO2. This causes the dough to expand or rise as gas forms pockets of bubbles.