The major food molecule in living organisms is a sugar called glucose. Most carbohydrates (sugars and starches) are converted into glucose before they are broken down in the cytosol to release energy. The series of steps where glucose is broken down to release energy begins with a metabolic pathway called glycolysis. Glycolysis is the "lysing" or cutting of glucose to release energy. The six carbon sugar, glucose, is cut in half and converted into two three carbon sugars called pyruvate. What happens next depends on the presence or absence of oxygen.
If oxygen is present, then glucose can be broken all the way down into carbon dioxide and water. This process is called aerobic respiration because it requires air (oxygen). In the absence of oxygen, the cell uses a process called anaerobic fermentation. or simply fermentation. Fermentation doesn't break the sugar down any further, it simply helps reset the system so that more sugar can be broken down.
Because aerobic respiration breaks the sugar all the way down, it releases much more energy than fermentation.
