Cellular respiration is an exothermic reaction, which means it transfers energy to the surroundings. It is continuously occurring in living cells. The energy transferred supplies all the energy needed for living processes: chemical reactions to build larger molecules, movement and keeping warm.
Respiration can take place aerobically (using oxygen) or anaerobically (without oxygen). Aerobic respiration is represented by the equation: glucose + oxygen → carbon dioxide + water. In symbols this is C6H12O6 + 6O2 → 6CO2 + 6H2O.
Anaerobic respiration in muscles is represented by the equation: glucose → lactic acid. The oxidation of glucose is incomplete in anaerobic respiration, so much less energy is transferred than in aerobic respiration.
Anaerobic respiration in plant and yeast cells makes ethanol and carbon dioxide instead: glucose → ethanol + carbon dioxide. In yeast cells this is called fermentation. It is economically important in making bread, where the carbon dioxide makes the dough rise, and in making alcoholic drinks.