a particular species of yeast, saccharomyces cerevisiae, has been used for thousands of years. it is used for baking bread because the yeast makes carbon dioxide as a by-product of it eating sugar, which helps the bread rise (leaven). yeast is also used for making beer and other alcoholic drinks by a process known as fermentation. this process produces both ethyl alcohol and carbon dioxide. ethanol is also used as fuel, and to make other organic chemicals.