Cellular respiration is the chemical process that releases energy from glucose in living cells. It happens continuously, day and night, in every living cell, in plants and fungi as well as in animals. It never stops while the cell is alive.
Respiration is an exothermic reaction. This means it transfers energy to the surroundings, and some of that energy is released as heat. Mammals and birds use this heat to keep their bodies warm.
The energy released is used for metabolic processes, which are the chemical reactions in the cell. Examples are building large molecules such as proteins from amino acids, and muscle contraction.
There are two types of respiration. Aerobic respiration uses oxygen. Anaerobic respiration takes place without oxygen.