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'Anaerobic respiration' refers to the oxidation of molecules in the absence of oxygen to produce energy, in opposition to aerobic respiration which does use oxygen. Anaerobic respiration processes require another electron acceptor to replace oxygen. Anaerobic respiration is often used interchangeably with fermentation, especially when the glycolytic pathway is used for energy production in the cell. They are not synonymous terms, however, since certain anaerobic prokaryotes can generate all of their ATP using an electron transport system and ATP synthase.
General word and symbol equations for the anaerobic respiration of glucose can be shown as
:''glucose $o$ lactic acid + energy (ATP);''
:''C₆H₁₂O₆ $o$ 2C₃H₆O₃ + 2 ATP.''
The energy released is about 120 kJ per mole glucose.

Contents

Obligate anaerobes

Facultative anaerobes and obligate aerobes

Fermentation in other organisms

Anaerobic respiration in prokaryotes

Examples of anaerobic respiration

Commercial applications of anaerobic respiration

Obligate anaerobes

In some organisms called ''obligate (strict) anaerobes'' (ex: ''Clostridium tetani'' (causes tetanus), ''Clostridium perfringens'' (causes gangrene)), the presence of oxygen is lethal. This is because the presence of oxygen is processed by the organisms into the extremely toxic molecules of singlet oxygen (¹O₂), superoxide ion (O₂^-), hydrogen peroxide (H₂O₂), hydroxyl ion (OH^-), and other toxic molecules.

Facultative anaerobes and obligate aerobes

Facultative anaerobic organisms can survive in either oxygenated or deoxygenated environments and can switch between cellular respiration or fermentation, respectively) and ''obligate (strict) aerobes'' (organisms that can survive only with oxygen) have special enzymes (superoxide dismutase and catalase) that can safely handle these products and transform them into harmless water and diatomic oxygen in the following reactions:
:''2O₂^- + 2H⁺ –superoxide dismutase–> H₂O₂ (hydrogen peroxide) + O₂.''
The hydrogen peroxide produced is then transferred to a second reaction:
:''2H₂O₂ –catalase–> 2H₂O + O₂.''
The oxidative powers of the superoxide ion have now been neutralized. Only facultative anaerobes and obligate aerobes possess the two enzymes necessary to reduce the superoxide.
In organisms which use glycolysis, the absence of oxygen prevents pyruvate from being metabolised to CO₂ and water via the citric acid cycle and the electron transport chain (which relies on O₂) does not function. Fermentation does not yield more energy than that already obtained from glycolysis (2 ATPs) but serves to regenerate NAD⁺ so glycolysis can continue. Various end products can also be created, such as lactate or ethanol.
Fermentation in animals is essential to human life.
In lactic acid fermentation, the following reaction occurs:
1. ''Glycolysis''
:''C₆H₁₂O₆ (glucose) + 2 NAD⁺ $o$ 2 C₃H₄O₃ (pyruvic acid) + 2 NADH''
2. ''Lactic acid creation''
:'' 2 C₃H₄O₃ (pyruvic acid) + 2 NADH $o$ 2 C₃H₆O₃ (lactic acid) + 2 NAD⁺''
''Net reaction'':
:''C₆H₁₂O₆ (glucose) $o$ 2 C₃H₆O₃ (lactic acid)''

Fermentation in other organisms

In some plant cells and yeasts, fermentation produces CO₂ and ethanol. The conversion of pyruvate to acetaldehyde generates CO₂ and the conversion of acetaldehyde to ethanol regenerates NAD⁺.

Anaerobic respiration in prokaryotes

In the field of prokaryotic metabolism, anaerobic respiration has a more specific meaning. In this case, anaerobic respiration is defined as a membrane-bound biological process coupling the oxidation of electron donating substrates (e.g. sugars and other organic compounds, but also inorganic molecules like hydrogen, sulfide/sulfur, ammonia, metals or metal ions) to the reduction of suitable ''external'' electron acceptors other than molecular oxygen. In contrast, in [fFermentation (biochemistry)|fermentation]] the oxidation of molecules is coupled to the reduction of an ''internally''-generated electron acceptor, usually pyruvate. Hence, scientists who study prokaryotic physiology view anaerobic respiration and fermentation as distinct processes and therefore do not use the terms interchangeably.
In anaerobic respiration, as the electrons from the electron donor are transported down the electron transport chain to the terminal electron acceptor, protons are translocated over the cell membrane from "inside" to "outside", establishing a concentration gradient across the membrane which temporarily stores the energy released in the chemical reactions. This potential energy is then converted into ATP by the same enzyme used during aerobic respiration, ATP synthase. Possible electron acceptors for anaerobic respiration are nitrate, nitrite, nitrous oxide, oxidised amines and nitro-compounds, fumarate, oxidised metal ions, sulfate, sulfur, sulfoxo-compounds, halogenated organic compounds, selenate, arsenate, bicarbonate or carbon dioxide (in acetogenesis and methanogenesis). All these types of anaerobic respiration are restricted to prokaryotic organisms.

Examples of anaerobic respiration

:''glucose + 3NO₃^- + 3H₂O $o$ 6HCO₃^- + 3NH₄⁺, ΔG⁰' = -1796 kJ''
:''glucose + 3SO₄^2- + 3H⁺ $o$ 6HCO₃^- + 3NH^-, ΔG⁰' = -453 kJ''
:''glucose + 12S + 12H₂O $o$ 6HCO₃^- + 12HS^- + 18H⁺, ΔG⁰' = -333 kJ''
All of these terminal electron acceptors are further upstream in the electron transport chain, compared to O₂. Consequently, anaerobic respiration is less effective than aerobic respiration. The ΔG⁰' of aerobic respiration is -2844 kJ.

Commercial applications of anaerobic respiration

★ Anaerobic digestion

★ Mechanical biological treatment