Aerobic respiration

Mammals and birds need energy to maintain a constant body temperature. Energy is also needed for the following life processes:

Swimmer doing butterfly stroke in a pool.
Energy is needed for muscle contraction and the passage of nerve impulses

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

A rounded, roughly rectangular structureMitochondria are tiny organelles found in the cell cytoplasm

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.