The word aerobic means that oxygen is needed for a chemical reaction. The food molecules are combined with oxygen. The process is called oxidation and the food is said to be oxidised. All food molecules contain carbon, hydrogen, and oxygen atoms. The process of oxidation converts the carbon to carbon dioxide (CO₂) and the hydrogen to water (H₂O) and, at the same time, sets free energy, which the cell can use to drive other reactions.

The amount of energy you would get by completely oxidising 180 grams (g) of glucose to carbon dioxide and water is 2830 kilojoules (kJ). In the cells, the energy is not released all at once. The oxidation takes place in a series of small steps and not in one jump as the equation suggests. Each small step needs its own enzyme and at each stage a little energy is released. Although the energy is used for the processes mentioned above, some of it always appears as heat. In "warm-blooded" animals (birds and mammals) some of this heat is stored to help maintain body temperature at the correct level.

In "cold-blooded" animals (e.g., reptiles and fish) the heat may build up for a time in the body and allow the animal to move about more quickly. In plants the heat is lost to the surroundings (by conduction, convection, and evaporation) as fast as it is produced.