Respiration happens in cells; it is an exothermic process where energy is released. There are two types of respiration, aerobic respiration and anaerobic respiration.
Aerobic respiration - this process requires oxygen and releases energy, water and carbon dioxide by the breaking down of food and other substances (glucose) in the presence of oxygen. Most of the reactions that happen in aerobic respiration happen in the mitochondria.
This is the equation:
Glucose + Oxygen -> Carbon Dioxide + Water + Energy
Anaerobic respiration - this process happens in the absence of oxygen and is the involves chemical reactions that break down nutrient molecules in living cells in order to release energy.