Aerobic and anaerobic respiration are types of cellular respiration, the process by which living cells use up the free electrons produced during normal cellular metabolism to produce ATP, or adenosine triphosphate, the chemical compound used by cells for energy. Aerobic and anaerobic respiration differ in their requirement of oxygen and the amount of energy they produce, as well as which organisms they are utilized most often in and why.
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Aerobic respiration can only occur in the presence of oxygen, while anaerobic respiration doesn't require or utilize oxygen in its energy production. In aerobic respiration, glucose molecules are passed through a chemical reaction chain that produces ATP, carbon dioxide and water and ends with oxygen accepting the free electrons that have been generated. In contrast, anaerobic respiration is cellular respiration that occurs using other molecules as the terminal electron acceptor since oxygen isn't present.
Both aerobic and anaerobic respiration use electron-transport chain-reaction pathways in energy production. However, aerobic respiration produces much more energy, or ATP molecules, than anaerobic respiration. For every molecule of glucose that is metabolized in the presence of oxygen by aerobic respiration, 38 ATP molecules are produced. By contrast, anaerobic respiration produces only two ATP molecules by dividing one glucose molecule into two pyruvic acid molecules in a process known as glycolysis.
The only chemical by-products produced by aerobic respiration -- carbon dioxide and water -- are simple for the human body to deal with. However, anaerobic respiration that occurs in muscle cells during times of extremely intense physical activity causes glucose molecules to break down into ATP and lactic acid. The build-up of lactic acid in muscles causes a burning sensation and is responsible for the cramps you may get during exercise.
Organisms that depend primarily on anaerobic respiration for their energy needs, such as bacteria, tend to live in anoxic surroundings, or habitats where only a slight amount of oxygen is available to use in energy production. Since the earth's atmosphere is oxygen-rich, some scientists suggest that this is why aerobic organisms were more resilient than microbes to evolve into higher-order organisms.