Many [[animal]]s depend upon oxygen as a highly efficient ingredient for breaking down [[food]] for [[energy]], due to its reactivity. Those that do not are called "[[anaerobic]]", meaning "without air", and include many important [[bacteria]]. | Many [[animal]]s depend upon oxygen as a highly efficient ingredient for breaking down [[food]] for [[energy]], due to its reactivity. Those that do not are called "[[anaerobic]]", meaning "without air", and include many important [[bacteria]]. |