Cellular respiration is the process by which cells convert various forms of chemical energy into adenosine triphosphate, or ATP, a molecule that is used to "transport" usable cellular energy. Cellular respiration can proceed by different mechanisms based on the conditions of the cellular environment during the respiration process. Oxygen, for example, is a major factor — respiration proceeds by different mechanisms in aerobic conditions, or conditions in which oxygen is present, than in anaerobic conditions, or conditions in which oxygen is not present. Some organisms are able to switch between aerobic and anaerobic modes of respiration based on the environmental conditions, though in such cases anaerobic respiration is generally not sustainable for a long period of time.
Aerobic respiration is cellular respiration that requires oxygen. Humans and many other organisms make use of this form of respiration because oxygen's chemical properties allow for very efficient energy transfer. The oxygen for aerobic cellular respiration is provided through breathing; when one's oxygen intake is insufficient to provide enough oxygen, short-term anaerobic mechanisms must be used to supplement the aerobic mechanisms.
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The raw materials used during the energy-transferring respiration processes are provided through the consumption of food, which is broken down into a chemical form that can be used during the reactions that occur during respiration. It should be noted that cellular respiration itself requires energy, as some ATP is used in order that more may be produced.
In some cases, aerobic cellular respiration is not sufficient to provide all of the energy required for the body's cellular needs. This is common when one engages in athletic practices that heavily tax one's muscles. Aerobic mechanisms are not capable of keeping up with the energy needs of the muscles, so a process referred to as fermentation is activated to supplement the energy production. This process is much less efficient than its aerobic counterpart, producing only two ATP molecules per glucose molecule used, compared to the 38 produced through the aerobic mechanisms. It also produces a great deal of energy-rich waste that cannot be broken down into a usable form through fermentation.
Some organisms live in anaerobic conditions and, accordingly, have cellular respiration processes that do not include oxygen. Some of these organisms will actually die in the presence of oxygen; such organisms are referred to as obligate anaerobes. Most of the organisms that primarily use such cellular respiration mechanisms are small and non-complex, and tend to lack energy needs such as muscular movement and complex digestive processes.