Aerobic respiration is one process by which organisms utilize food sources to produce usable energy. In this case, organic compounds are oxidized through a series of reactions to produce an energy source called adenosine triphosphate, or ATP. The processing of ATP, in turn, drives the metabolic activity of the body, and thus must be in continuous supply for healthy functioning. Aerobic respiration can be thought of as consisting of roughly three stages, and following a compound like glucose can illustrate the journey.
The first step of aerobic respiration is glycolysis -- which can also be the first step of anaerobic respiration, as oxygen is not expressly required. Here, glucose is converted into pyruvic acid through several enzyme-driven reactions that use the energy of two ATP molecules per one glucose molecule. Glycolysis creates four ATP molecules, however, so there is a net gain of two ATP molecules by the end of the reactions. Glycolysis transpires in a cell’s cytoplasm, the liquid surrounding membrane-enclosed organelles.
The Krebs citric acid cycle transforms the pyruvic acid generated in glycolysis into molecules of two coenzymes, NADH2 and FADH2, and produces two molecules of ATP for every one molecule of original glucose. In addition, the Krebs citric acid cycle creates carbon dioxide -- six molecules of it per one glucose. All this occurs within the “power-house” organelles called mitochondria.
Two more reactions, often married together because of their interconnected nature, finish off aerobic respiration: the electron-transport chain and oxidative phosphorylation. These steps are the ones relying directly on oxygen, which is used as an electron acceptor during the electron-transport chain, which takes place in the interior mitochondrial membranes.
Oxygen is indirectly important in aerobic respiration for glycolysis and the Krebs cycle, because NADH2 and FADH2 are transformed into more basic coenzymes used to drive some of the reactions in those earlier steps.
Electrons are juggled from one compound to another, finally being transferred to oxygen, and this produces water. The electron-transport chain and oxidative phosphorylation transform adenosine diphosphate, ADP, into ATP: three molecules, conceivably, from the passage of each pair of electrons through the cycle. All things considered, aerobic respiration could theoretically generate roughly 34 ATP molecules from every one of glucose.
Aerobic respiration creates a number of other products besides ATP. Some of these cycle back into the process, like the NAD and FAD coenzymes recreated from NADH2 and FADH2 during the electron-transport chain. But the carbon dioxide generated during the Krebs citric acid cycle and the water generated from the electron-transport chain are waste products that must be removed from the body.
- "Animal Physiology";R.W. Hill et al.; 2004