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See also: Fermentation (biochemistry)

Anaerobic respiration refers to the oxidation of molecules in the absence of oxygen to produce energy. These 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. The word & symbol equation for the anaerobic respiration of glucose is:

Glucose ---> Lactic acid + Energy (ATP)

C₆H₁₂O₆ ---> 2C₃H₆O₃ + 2 ATP

The energy released is about 120kJ per mole Glucose.

Contents 1 Obligate Anaerobes 2 Faculative Anaerobes and Obligate Aerobes 3 Fermentation in other organisms 4 Anaerobic respiration in prokaryotes 5 Commercial applications of anaerobic respiration

In some organisms called obligate (strict) anaerobes (ex: C. tetani (causes tetanus), C. 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 dangerous byproducts

Faculative anaerobes (organisms that 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 dimutase and catalase) that can safely handle these products and transform them into harmless water and diatomic oxygen in the following reactions:

1. 2O₂^- + 2H⁺ ---Superoxide Dismutase--> H₂O₂ (hydrogen peroxide) + O₂

The hydrogen peroxide produced is then transferred to a second reaction...

2. 2H₂O₂ ---Catalase--> 2H₂O + O₂

The oxidative powers of the superoxide ion have now been neutralized. Only faculative 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⁺ ---> 2 C₃H₄O₃ (pyruvic acid) + 2 NADH

2. Lactic Acid Creation 2 C₃H₄O₃ (pyruvic acid) + 2 NADH ---> 2 C₃H₆O₃ (lactic acid) + 2 NAD⁺

Net Reaction: C₆H₁₂O₆ (glucose) ---> 2 C₃H₆O₃ (lactic acid)

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⁺.

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 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 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 or carbon dioxide (in acetogenesis and methanogenesis). All these types of anaerobic respiration are restricted to prokaryotic organisms.

• Anaerobic digestion
• Mechanical biological treatment

v • d • e Cellular Respiration
Aerobic Respiration Glycolysis → Pyruvate Decarboxylation → Citric Acid Cycle → Oxidative Phosphorylation (Electron Transport Chain + ATP synthase) Anaerobic Respiration Glycolysis → Lactic Acid Formation or Ethanol Formation

v • d • e Metabolism
Metabolism | Catabolism | Anabolism Metabolic pathway | Metabolic network | Cellular respiration (Anaerobic/Aerobic) Protein metabolism | Carbohydrate metabolism | Lipid metabolism

v • d • e Metabolism map

Glucuronate metabolism Pentose interconversion Inositol metabolism Cellulose and sucrose metabolism Starch and glycogen metabolism Other sugar metabolism Pentose phosphate pathway Glycolysis and Gluconeogenesis Amino sugars metabolism Small amino acid synthesis Branched amino acid synthesis Purine biosynthesis Histidine metabolism Aromatic amino acid synthesis Pyruvate decarboxylation Anaerobic respiration Fatty acid metabolism Urea cycle Aspartate amino acid group synthesis Porphyrins and corrinoids metabolism Citric acid cycle Glutamate amino acid group synthesis Pyrimidine biosynthesis  v • d • e  All pathway labels on this image are links, simply click to access the article. A high resolution labeled version of this image is available here.