Organisms need to carry out respiration to convert glucose to usable energy. When organisms consume food, the body targets glucose molecules for conversion to energy. However, respiration must first occur to transform these molecules into adenosine triphosphate, or ATP.
The most efficient form of respiration occurs when oxygen is available to oxidize the glucose. Respiration in the presence of oxygen is called aerobic respiration. Cells use this oxygen to change molecules of glucose into carbon dioxide, water and ATP. The carbon dioxide and water are waste products that are flushed from the system, whereas the ATP is the primary energy source used by the organism. This process is why animals need to breathe in oxygen and exhale carbon dioxide.
Aerobic respiration consists of four distinct stages: glycolysis, in which the initial decomposition of glucose occurs and a small amount of ATP is generated; the formation of acetyl CoA, when acetyl groups are formed from the decomposed glucose and positively charged NAD+ ions are generated; the Krebs cycle or citric acid cycle, when acetyl CoA is oxidized to produce carbon dioxide and the NAD is converted to NADH; and electron transport, when the final output of respiration coalesces, resulting in large energy output and waste products.
Anaerobic respiration occurs when no oxygen is available. This form of respiration conducts glycolysis to produce small quantities of energy and lactic acid.