Alternatively, the gas can be produced by enzymes and chemicals. This may sound unattractive, but the production of gas by baking powder is completely harmless. Yeast has a faster metabolism at higher temperatures but would die when it becomes too hot. So by the time your bread is baked it no longer contains alive cells. The chemical processes caused by baking powder are also faster at higher temperatures.
Enzymes are proteins that have a specific function.
This can be practically anything. The function of an enzyme is determined by its composition and structure. Yeast ferment glucose to alcohol and carbon dioxide. It's the CO2 gas that expands in the dough and causes it to rise. The process is stopped when the oven is turned up.