Anaerobic respiration is
respiration
Respiration may refer to:
Biology
* Cellular respiration, the process in which nutrients are converted into useful energy in a cell
** Anaerobic respiration, cellular respiration without oxygen
** Maintenance respiration, the amount of cellul ...
using
electron acceptors other than
molecular oxygen (O
2). Although oxygen is not the final electron acceptor, the process still uses a respiratory electron transport chain.
In
aerobic organisms
Aerobic means "requiring air," in which "air" usually means oxygen.
Aerobic may also refer to
* Aerobic exercise, prolonged exercise of moderate intensity
* Aerobics, a form of aerobic exercise
* Aerobic respiration, the aerobic process of cell ...
undergoing respiration, electrons are shuttled to an
electron transport chain, and the final electron acceptor is
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...
. Molecular oxygen is an excellent electron acceptor.
Anaerobes instead use less-oxidizing substances such as
nitrate (),
fumarate (),
sulfate
The sulfate or sulphate ion is a polyatomic anion with the empirical formula . Salts, acid derivatives, and peroxides of sulfate are widely used in industry. Sulfates occur widely in everyday life. Sulfates are salts of sulfuric acid and many ...
(), or elemental
sulfur (S). These terminal electron acceptors have smaller
reduction potential
Redox potential (also known as oxidation / reduction potential, ''ORP'', ''pe'', ''E_'', or E_) is a measure of the tendency of a chemical species to acquire electrons from or lose electrons to an electrode and thereby be reduced or oxidised respe ...
s than O
2. Less energy per oxidized molecule is released. Therefore, anaerobic respiration is less efficient than aerobic.
As compared with fermentation
Anaerobic cellular respiration and
fermentation generate ATP in very different ways, and the terms should not be treated as synonyms. Cellular respiration (both
aerobic
Aerobic means "requiring air," in which "air" usually means oxygen.
Aerobic may also refer to
* Aerobic exercise, prolonged exercise of moderate intensity
* Aerobics, a form of aerobic exercise
* Aerobic respiration, the aerobic process of cel ...
and anaerobic) uses highly reduced chemical compounds such as
NADH
Nicotinamide adenine dinucleotide (NAD) is a coenzyme central to metabolism. Found in all living cells, NAD is called a dinucleotide because it consists of two nucleotides joined through their phosphate groups. One nucleotide contains an aden ...
and
FADH2 (for example produced during
glycolysis and the
citric acid cycle) to establish an
electrochemical gradient
An electrochemical gradient is a gradient of electrochemical potential, usually for an ion that can move across a membrane. The gradient consists of two parts, the chemical gradient, or difference in solute concentration across a membrane, and ...
(often a proton gradient) across a membrane. This results in an
electrical potential
The electric potential (also called the ''electric field potential'', potential drop, the electrostatic potential) is defined as the amount of work energy needed to move a unit of electric charge from a reference point to the specific point in ...
or ion
concentration difference across the membrane. The reduced chemical compounds are oxidized by a series of respiratory
integral membrane proteins
An integral, or intrinsic, membrane protein (IMP) is a type of membrane protein that is permanently attached to the biological membrane. All ''transmembrane proteins'' are IMPs, but not all IMPs are transmembrane proteins. IMPs comprise a signif ...
with sequentially increasing reduction potentials, with the final electron acceptor being oxygen (in
aerobic respiration) or another chemical substance (in anaerobic respiration). A
proton motive force
Chemiosmosis is the movement of ions across a semipermeable membrane bound structure, down their electrochemical gradient. An important example is the formation of adenosine triphosphate (ATP) by the movement of hydrogen ions (H+) across a membra ...
drives
protons down the gradient (across the membrane) through the proton channel of
ATP synthase. The resulting current drives ATP synthesis from
ADP and inorganic phosphate.
Fermentation, in contrast, does not use an electrochemical gradient but instead uses only
substrate-level phosphorylation to produce ATP. The electron acceptor
NAD+ is regenerated from
NADH
Nicotinamide adenine dinucleotide (NAD) is a coenzyme central to metabolism. Found in all living cells, NAD is called a dinucleotide because it consists of two nucleotides joined through their phosphate groups. One nucleotide contains an aden ...
formed in oxidative steps of the fermentation pathway by the reduction of oxidized compounds. These oxidized compounds are often formed during the fermentation pathway itself, but may also be external. For example, in homofermentative lactic acid bacteria, NADH formed during the oxidation of
glyceraldehyde-3-phosphate is oxidized back to NAD
+ by the reduction of
pyruvate to
lactic acid at a later stage in the pathway. In
yeast,
acetaldehyde is reduced to
ethanol
Ethanol (abbr. EtOH; also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound. It is an alcohol with the chemical formula . Its formula can be also written as or (an ethyl group linked to a ...
to regenerate NAD
+.
There are two important anaerobic microbial methane formation pathways, through
carbon dioxide
Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...
/
bicarbonate () reduction (respiration) or acetate fermentation.
Ecological importance
Anaerobic respiration is a critical component of the global
nitrogen
Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
,
iron
Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 element, group 8 of the periodic table. It is, Abundanc ...
,
sulfur, and
carbon
Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon mak ...
cycles through the reduction of the oxyanions of nitrogen, sulfur, and carbon to more-reduced compounds. The
biogeochemical cycling of these compounds, which depends upon anaerobic respiration, significantly impacts the
carbon cycle
The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth. Carbon is the main component of biological compounds as well as a major componen ...
and
global warming
In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to E ...
. Anaerobic respiration occurs in many environments, including freshwater and marine sediments, soil, subsurface aquifers, deep subsurface environments, and biofilms. Even environments, such as soil, that contain oxygen also have micro-environments that lack oxygen due to the slow diffusion characteristics of
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...
gas.
An example of the ecological importance of anaerobic respiration is the use of nitrate as a terminal electron acceptor, or dissimilatory
denitrification, which is the main route by which fixed
nitrogen
Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
is returned to the atmosphere as molecular nitrogen gas. The denitrification process is also very important in host-microbe interactions. Similar to mitochondria in oxygen-respiring microorganisms, some single-cellular anaerobic ciliates use denitrifying endosymbionts to gain energy. Another example is
methanogenesis, a form of carbon-dioxide respiration, that is used to produce
methane
Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Ea ...
gas by
anaerobic digestion. Biogenic methane is used as a sustainable alternative to fossil fuels. On the negative side, uncontrolled methanogenesis in landfill sites releases large volumes of methane into the atmosphere, where it acts as a powerful
greenhouse gas.
Sulfate respiration produces
hydrogen sulfide, which is responsible for the characteristic 'rotten egg' smell of coastal wetlands and has the capacity to precipitate heavy metal ions from solution, leading to the deposition of
sulfidic metal ores.
Economic relevance
Dissimilatory
denitrification is widely used in the removal of
nitrate and
nitrite from municipal wastewater. An excess of nitrate can lead to
eutrophication of waterways into which treated water is released. Elevated nitrite levels in drinking water can lead to problems due to its toxicity. Denitrification converts both compounds into harmless nitrogen gas.
Specific types of anaerobic respiration are also critical in
bioremediation
Bioremediation broadly refers to any process wherein a biological system (typically bacteria, microalgae, fungi, and plants), living or dead, is employed for removing environmental pollutants from air, water, soil, flue gasses, industrial effluent ...
, which uses microorganisms to convert toxic chemicals into less-harmful molecules to clean up contaminated beaches, aquifers, lakes, and oceans. For example, toxic
arsenate
The arsenate ion is .
An arsenate (compound) is any compound that contains this ion. Arsenates are salts or esters of arsenic acid.
The arsenic atom in arsenate has a valency of 5 and is also known as pentavalent arsenic or As(V).
Arsenate res ...
or
selenate
The selenate ion is .
Selenates are analogous to sulfates and have similar chemistry. They are highly soluble in aqueous solutions at ambient temperatures.
Unlike sulfate, selenate is a somewhat good oxidizer; it can be reduced to selenite o ...
can be reduced to less toxic compounds by various anaerobic bacteria via anaerobic respiration. The reduction of
chlorinated chemical pollutants, such as
vinyl chloride
Vinyl chloride is an organochloride with the formula H2C=CHCl. It is also called vinyl chloride monomer (VCM) or chloroethene. This colorless compound is an important industrial chemical chiefly used to produce the polymer polyvinyl chloride (PVC ...
and
carbon tetrachloride, also occurs through anaerobic respiration.
Anaerobic respiration is useful in generating electricity in
microbial fuel cell Microbial fuel cell (MFC) is a type of bioelectrochemical fuel cell system that
generates electric current by diverting electrons produced from the microbial oxidation of reduced compounds (also known as fuel or electron donor) on the anode to oxid ...
s, which employ bacteria that respire solid electron acceptors (such as oxidized iron) to transfer electrons from reduced compounds to an electrode. This process can simultaneously degrade organic carbon waste and generate electricity.
Examples of electron acceptors in respiration
See also
*
Hydrogenosome
A hydrogenosome is a membrane-enclosed organelle found in some anaerobic ciliates, flagellates, and fungi. Hydrogenosomes are highly variable organelles that have presumably evolved from protomitochondria to produce molecular hydrogen and ATP i ...
s and
mitosome
A mitosome is an organelle found in some unicellular eukaryotic organisms, like in members of the supergroup Excavata. The mitosome was found and named in 1999, and its function has not yet been well characterized. It was termed a ''crypton'' by ...
s
*
Anaerobic digestion
*
Microbial fuel cell Microbial fuel cell (MFC) is a type of bioelectrochemical fuel cell system that
generates electric current by diverting electrons produced from the microbial oxidation of reduced compounds (also known as fuel or electron donor) on the anode to oxid ...
*
Standard electrode potential (data page)
*
Table of standard reduction potentials for half-reactions important in biochemistry
*
Lithotroph
Lithotrophs are a diverse group of organisms using an inorganic substrate (usually of mineral origin) to obtain reducing equivalents for use in biosynthesis (e.g., carbon dioxide fixation) or energy conservation (i.e., ATP production) via aerobi ...
s
Further reading
*
References
{{DEFAULTSORT:Anaerobic Respiration
Anaerobic digestion
Biodegradation
Cellular respiration
Anaerobic respiration