Respiration/Cellular respiration:

When we burn fuel, it utilizes oxygen and yields energy in the form of light, heat, etc. In the burning process, oxygen is used to break the C-H bonds present in fuel molecules. In the same way, organisms utilize oxygen for the breakdown of C-H bonds present in foods in their cells. This breakdown yields energy which is transformed into ATR. During this process, C-H bonds are broken by oxidation-reduction reactions and so carbon dioxide and water are also produced. The cellular energy-yielding process is also called cellular respiration.