Anaerobic respiration
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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 exists in the cell. However, certain anaerobic prokaryotes 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.
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[edit] When glycolysis is used
Oxygen is not necessary for glycolysis to occur in all organisms.
[edit] Obligate Anaerobes
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.
[edit] Faculative Anaerobes and Obligate Aerobes
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 Dimutase---> 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)
The lactic acid can then oxidize by losing hydrogen ions (H+) and turning into lactase ions. The introduction of ions into the solutions lowers the pH, acidifying the environment. When the pH reaches a critical threshold, enzymes essential to respiration fail to function, cutting off either aerobic respiration or anaerobic respiration.
[edit] 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+.
[edit] 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 physiology view anaerobic respiration and fermentation as distinct processes and therefore do not use the terms interchangeably.
In anaerobic respiration, as the elctrons 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.
[edit] Commercial applications of anaerobic respiration
[edit] See also
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| Cell metabolism/Metabolism | Catabolism | Anabolism | |
| Protein | | Protein metabolism (Protein synthesis/Amino acid synthesis/Catabolism) |
| Carbohydrate | | Carbohydrate metabolism (Anabolism/Catabolism) |
| Lipid | | Lipid metabolism (Synthesis/Anabolism/Catabolism) |
| Metabolic pathway | Metabolic network | |
| Cellular respiration (Anaerobic/Aerobic) | |
