Anaerobic respiration is a type of cellular metabolism that produces energy without oxygen. It is facilitated by glycolysis and is important for short bursts of muscle activity. Anaerobic metabolism also plays a role in the nitrogen cycle and can produce methane gas, which is a greenhouse gas.
In biology, anaerobic respiration is the metabolic process by which cells produce chemical energy without using oxygen. It is a type of cellular respiration, an essential part of metabolism and the source of all growth, reproduction and repair that takes place in cells. Glycolysis, which converts sugar into energy molecules, is the main type of anaerobic pathway found in most organisms. Anaerobic metabolism facilitates important chemical exchanges between organisms and the environment, such as the nitrogen cycle.
All living organisms use energy to fuel their growth and reproduction. Cellular respiration is a type of cellular metabolism that extracts biochemical energy from various nutrient chemicals and converts it into ATP, the universal energy transfer molecule. Anaerobic respiration is the type of cellular respiration that occurs in cells that do not use oxygen molecules in their metabolism. The primary chemical pathway of anaerobic respiration—aside from bacteria that consume extreme metals—is glycolysis, which splits one molecule of the simple sugar glucose into two molecules of pyruvic acid, producing two molecules of ATP in the process.
Aerobic respiration, which uses oxygen, is more common than anaerobic respiration in most higher organisms, which rely effectively on oxygen for respiration. It produces far more ATP – 38 molecules – than glycolysis, which produces only two. In the early evolution of life on earth, however, there was not enough oxygen in the atmosphere for cells to metabolise, so they relied on various other molecules for energy anaerobically. Primitive organisms, including the oldest bacteria and surviving creatures that live in hot springs and on the ocean floor, still rely on this; many of these will die when exposed to oxygen.
Humans and other animals use both aerobic and anaerobic respiration. The latter is important during short bursts of muscle activity, which fuels movement using glycolysis and produces lactic acid. If this acid builds up, muscle aches and cramps occur. Some yeasts incorporate a special type of anaerobic respiration, fermentation, into their metabolism. Fermentation digests sugars and produces ethyl alcohol and some gases as a byproduct; this is why bread rises, as the biochemical processes in the yeast change the chemical reactions in the dough.
The elemental exchanges of nitrogen, sulfur and carbon on the earth’s surface and in the atmosphere are moderated by anaerobic respiration. For example, proteins and other biologically essential compounds contain a large amount of nitrogen, which is released into the atmosphere by bacterial anaerobic metabolism. When some bacteria metabolize oxygen-free fuels, they also release methane gas. While this feature is useful in microbiology and the oil industry, excessive methane production is a problem because it is as much a greenhouse gas as carbon dioxide, linked to climate change.
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