Yeast is a living, microscopic, single-cell organism that, as it grows, converts its food (through a process known as fermentation) carbon dioxide. This trait is what endears yeast to bread bakers. The art of bread making needs the carbon dioxide produced by yeast in order for certain doughs to rise. To multiply and grow, all yeast needs is the right environment, which includes moisture, food (in the form of sugar or starch) and a warm, nurturing temperature (70Â° to 85Â°F is best).
The breakdown of the yeast causes effervescence, the giving off of gasses which get trapped in the dough, and the lump of dough expands. As the bread is usually in a tin, the only way it can expand is upwards. A lump of dough not in a tin will expand sideways as well as upwards.
Yeast holds a chemical [not dangerous to mankind] and when heated it rises just like smoke when cooked by its self it can catch fire at just 35 degrees.
Yeast is a living organism. It feeds off the sugar in the bread and respires, meaning that it takes in oxygen and gives out carbon dioxide. The carbon dioxide given out is the 'air' inside bread.