Mitochondrion:

Because they make energy for the cell, mitochondria are basically the "power plant" of a cell. The mitochondrion makes energy by 'digesting' sugar molecules. The mitochondria also requires oxygen to make the energy and gives off carbon dioxide as a waste. Therefore, when an animal breathes in oxygen, the oxygen is often used for the mitochondrion of the animal cells and the carbon dioxide an animal breathes out is the result of the carbon dioxide waste produced by mitochondria.