Mammals and birds need energy to maintain a constant body temperature. Energy is also needed for the following life processes:
Respiration involves chemical reactions that break down nutrient molecules in living cells to release energy.
Aerobic respiration needs oxygen. It is the release of a relatively large amount of energy in cells by the breakdown of food substances in the presence of oxygen:
glucose + oxygen → carbon dioxide + water
C6H12O6 + 6O2 → 6CO2 + 6H2O
Aerobic respiration happens all the time in animals and plants. Note that respiration is different to breathing (ventilation). Most of the reactions in aerobic respiration happen inside mitochondria in cells.