During combustion of substances,
heat and light are released. Combustion usually takes place in the presence of oxygen.
During cellular respiration,
the energy contained in the food is released in the presence of oxygen. Unlike the combustion process, it is a gradual process, in which energy is stored and then it is utilized to perform various life activities.
Exhaled air contains
less oxygen and more carbon dioxide and water vapor than inhaled air.
The composition of inhaled and exhaled air