The constant need for oxygen is one of the most essential features of animal life, because the energy for animal is provided by the oxidation of food. All organisms obtain their energy by respiration.
Respiration is a characteristic of all living organisms. It is the release (not ‘production’) of energy from food substances in all living cells. The food substance involved is usually glucose. However, lipids and proteins are also used for respiration during starvation. This energy released is available in the form of ATP (Adenosine Tri Phosphate).