The amount of energy produced by aerobic respiration depends in part on the quantity of glucose ingested. Glucose is a type of sugar that comes from other sugars, such as sucrose. The amount of energy in foods is defined in terms of calories. Foods high levels of sugar will be high in calories.
Aerobic respiration comprises three phases: glycosis, the Krebs cycle and the electron transport system. Glycolysis entails the break down of glucose molecules to give pyruvate molecules. In the Krebs cycle, pyruvate is oxidized to form carbon dioxide. This process is highly exergonic, meaning it releases energy that can be used elsewhere in the body. The Krebs cycle also generates hydrogen, which is channeled to the electron transport system. The hydrogen leads to the formation of adenosine triphosphate, the main carrier of energy in cells.
Proteins, in the form of enzymes, perform crucial roles in the Krebs cycle. These enzymes are formed from amino acids that either come from food or are synthesized naturally in the body's cellular processes. Other biochemical processes produce respiratory enzymes from ingested from fats and glycerides that are present in food. Enzymes are responsible for rearranging and oxidizing the compounds generated in the Krebs cycle. The first of these compounds is citric acid, which is why the Krebs cycle is sometimes called the citric acid cycle. The Krebs cycle is a cycle because one of its reactions produces a chemical called coenzyme-A. Coenzyme-A facilitates the operation of the the oxidative enzymes, meaning that process is self-perpetuating as long as the supply of pyruvate is maintained.
Without oxygen, cells can produce energy using anaerobic respiration. Anaerobic respiration completely bypasses the Krebs cycle by breaking down glucose to form lactic acid and energy. This lactic acid accumulates in muscle tissues to cause cramping. Anaerobic respiration is essentially an emergency process employed when cells need energy very quickly. Animals attempting to either evade predators or catch prey use anaerobic respiration. More prosaically, humans use anaerobic respiration when sprinting, where the burning sensation in the lungs and muscles is the result of lactic acid generation.