Anaerobic respiration (anaerobiosis) refers to the
oxidationof molecules in the absence of oxygento produce energy, in opposition to aerobic respirationwhich does use oxygen. Anaerobic respiration processes require another electronacceptor 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. Definition of anaerobic respiration: the breakdown of food substances in the absence of oxygen with a small amount of energy. General word and symbol equations for the anaerobic respiration of glucose can be shown as
glucose lactic acid(+ energy) (ATP);"
"C6H12O6 2C3H6O3 + 2 ATP."
The energy released is about 120 kJ per mole of glucose, but under higher temperatures can be increased to 540 kJ per mole of glucose.
Obligate (strict) 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 dismutaseand 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 pyruvatefrom being metabolised to CO2 and watervia the citric acid cycleand 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.
lactic acid fermentation, the following reaction occurs:
"C6H12O6 (glucose) + 2 NAD+ 2 C3H4O3 (
pyruvic acid) + 2 NADH"
" 2 C3H4O3 (pyruvic acid) + 2 NADH 2 C3H6O3 (lactic acid) + 2 NAD+"
"C6H12O6 (glucose) 2 C3H6O3 (lactic acid)"
Fermentation in other organisms
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 fermentation the oxidation of molecules is coupled to the reduction of an "internally"-generated electron acceptor, usually pyruvate. Hence, scientists who study prokaryotic physiologyview 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 chainto the terminal electron acceptor, protons are translocated over the cell membranefrom "inside" to "outside", establishing a concentration gradientacross 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 6HCO3- + 3NH4+, ΔG0' = -1796 kJ"
"glucose + 3SO42- + 3H+ 6HCO3- + 3SH-, ΔG0' = -453 kJ"
"glucose + 12S - 12H2O 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
Mechanical biological treatment
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