Yeast metabolism

Baker's and brewer's yeast is Saccharomyces Cerevisiae, also referred to as the "sugar-eating fungus."

Yeasts feast on glucose and fructose from sugar, and on maltose from the broken-down starch granules in the flour.
Yeast metabolize sugars for energy and produce ethyl alcohol and carbon dioxide following this equation:

C6H12O6  ----->  2(CH3CH2OH) + 2(CO2) + ATP

In making beer and wine, the carbon dioxide escapes the liquid and concentrates the alcohol. In making bread, carbon dioxide and alcohol become trapped in the dough. The flexibility of the dough accomodates the expanding gas by inflating or "rising." The ethyl alcohol, along with other by-products of fermentation, give yeast-leavened breads their typical aroma. At the time of baking, the heat expells both carbon dioxide and alcohol from the dough, leaving a flavorful network of empty air pockets.
A small amount of added table sugar increases yeast activity, whereas a large amount decreases it. Too much sugar dehydrates the yeast. To compensate, bakers of sweet breads add more yeast than ordinary, and allow longer times for the bread to rise.

Yeast are also sensitive to salt, and are greatly affected by temperature. Cells grow and produce gas most rapidly at around 95 degrees F/35 degrees C.