Respiration is the break down of molecules in living cells to release energy. Most commonly glucose (a sugar molecule) is broken down to release energy. Aerobic respiration occurs in animal cells and plant cells when there is oxygen available, the oxygen and glucose react together to produce carbon dioxide, water and energy. This process completely breaks down the glucose molecule. Anaerobic respiration occurs when oxygen is not available and occurs differently in animal and plant cells. In animal cells anaerobic respiration often occurs during exercise. The glucose does not get fully broken down in this process, so it does not release its full potential energy. Instead of carbon dioxide and water it breaks down to form lactic acid and a small amount of energy. The lactic acid builds up, and must be oxidized (given oxygen) once exercise has ceased. In plant cells and some micro-organisms, such as yeast, anaerobic respiration occurs in a different way. It still occurs without oxygen but the glucose molecule is broken down into ethanol, carbon dioxide and a small amount of energy.