Cells in all organisms need to utilise glucose and oxygen to release the energy they need to perform any function. The chemical equation of respiration is:
glucose + oxygen carbon dioxide + water + energy
Aerobic respiration uses oxygen. Anaerobic respiration releases energy from glucose taken from food, without the presence of oxygen. In this case, the equation looks like this:
glucose lactic acid + energy
The energy released through anaerobic respiration is much less than aerobic, but the body does need to use it when oxygen supply is not quick enough. This is called oxygen debt and it happens during intensive exercise. Lactic acid causes temporary damage to muscles, causing them to become painful. At the end of intensive exercise, our breathing rate increases to replace oxygen and pulse rate also increases to carry lactic acid away from the muscles to be broken down in the liver.
Plants, like animals, respire 24 hours a day to stay alive. They use the glucose and the oxygen produced during photosynthesis to release energy. The equation for respiration is the reverse of photosynthesis.