Anoxygenic photosynthesis
Encyclopedia
Phototrophy is the process by which organisms trap light energy (photons) and store it as chemical energy in the form of ATP. There are three major types of phototrophy: Oxygenic photosynthesis
, Anoxygenic photosynthesis, and Rhodopsin-based phototrophy
. Oxygenic and anoxygenic photosynthesis undergo different reactions in the presence and absence of light (called Light reactions and Dark reactions, respectively).
, phototrophic purple bacteria
, and heliobacteria
.
Anoxygenic phototrophs have photosynthetic pigments called bacteriochlorophylls (similar to chlorophyll
found in eukaryotes). Bacteriochlorophyll
a and b have maxima wavelength absorption at 775 nm and 790 nm, respectively in ether. In vivo however, these pigments were found to absorb longer wavelengths (many in the infrared spectrum). Unlike oxygenic phototrophs, anoxygenic photosynthesis only functions using a single photosystem. This restricts them to cyclic electron flow only, and they are therefore unable to produce O2 from the oxidization of H2O.
The electron transport chain of purple non-sulfur bacteria begins when the reaction centre
bacteriochlorophyll pair, P870, becomes excited from the absorption of light. Excited P870 will then donate an electron to Bacteriopheophytin, which then passes it on to a series of electron carriers down the electron chain. In the process, it will generate a proton motor force (PMF) which can then be used to synthesize ATP by oxidative phosphorylation. The electron returns to P870 at the end of the chain so it can be used again once light excites the reaction-center.
Green sulfur bacteria
The electron transport chain of green sulfur bacteria uses the reaction centre
bacteriochlorophyll pair, P840. When light is absorbed by the reaction center, P840 enters an excited state with a large negative reduction potential, and so readily donates the electron to bacteriochlorophyll 663 which passes it on down the electron chain. The electron is transferred through a series of electron carriers and complexes until it returns to P840 or is used to reduce NAD+. If the electron leaves the chain to reduce NAD+, P840 must be reduced for the ETC to function again. This is accomplished with the oxidation of hydrogen sulfide (or other inorganic sulfur compound) by cytochrom c555.
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
, Anoxygenic photosynthesis, and Rhodopsin-based phototrophy
Bacterial rhodopsins
Bacterial rhodopsins are a family of bacterial opsins. They are retinal-binding proteins that provide light-dependent ion transport and sensory functions to a family of halophilic and other bacteria...
. Oxygenic and anoxygenic photosynthesis undergo different reactions in the presence and absence of light (called Light reactions and Dark reactions, respectively).
Overview
Anoxygenic photosynthesis is the phototrophic process where light energy is captured and stored as ATP, without the production of oxygen. This means water is not used as primary electron donor. There are three groups of bacteria that undergo anoxygenic photosynthesis: phototrophic green bacteriaGreen sulfur bacteria
The green sulfur bacteria are a family of obligately anaerobic photoautotrophic bacteria. Most closely related to the distant Bacteroidetes, they are accordingly assigned their own phylum....
, phototrophic purple bacteria
Purple bacteria
Purple bacteria or purple photosynthetic bacteria are proteobacteria that are phototrophic, that is capable of producing energy through photosynthesis...
, and heliobacteria
Heliobacteria
The heliobacteria are phototrophic, that is to say, they convert light energy into chemical energy by photosynthesis; they use a PSI type reaction center. The primary pigment involved is bacteriochlorophyll g, which is unique to the group and has a unique absorption spectrum; this gives the...
.
Anoxygenic phototrophs have photosynthetic pigments called bacteriochlorophylls (similar to chlorophyll
Chlorophyll
Chlorophyll is a green pigment found in almost all plants, algae, and cyanobacteria. Its name is derived from the Greek words χλωρος, chloros and φύλλον, phyllon . Chlorophyll is an extremely important biomolecule, critical in photosynthesis, which allows plants to obtain energy from light...
found in eukaryotes). Bacteriochlorophyll
Bacteriochlorophyll
Bacteriochlorophylls are photosynthetic pigments that occur in various phototrophic bacteria. They were discovered by Von Neil in 1932 . They are related to chlorophylls, which are the primary pigments in plants, algae, and cyanobacteria. Groups that contain bacteriochlorophyll conduct...
a and b have maxima wavelength absorption at 775 nm and 790 nm, respectively in ether. In vivo however, these pigments were found to absorb longer wavelengths (many in the infrared spectrum). Unlike oxygenic phototrophs, anoxygenic photosynthesis only functions using a single photosystem. This restricts them to cyclic electron flow only, and they are therefore unable to produce O2 from the oxidization of H2O.
Photosynthetic Electron Transport Chain
Purple non-sulfur bacteriaThe electron transport chain of purple non-sulfur bacteria begins when the reaction centre
Photosynthetic reaction centre
A photosynthetic reaction center is a complex of several proteins, pigments and other co-factors assembled together to execute the primary energy conversion reactions of photosynthesis...
bacteriochlorophyll pair, P870, becomes excited from the absorption of light. Excited P870 will then donate an electron to Bacteriopheophytin, which then passes it on to a series of electron carriers down the electron chain. In the process, it will generate a proton motor force (PMF) which can then be used to synthesize ATP by oxidative phosphorylation. The electron returns to P870 at the end of the chain so it can be used again once light excites the reaction-center.
Green sulfur bacteria
The electron transport chain of green sulfur bacteria uses the reaction centre
Photosynthetic reaction centre
A photosynthetic reaction center is a complex of several proteins, pigments and other co-factors assembled together to execute the primary energy conversion reactions of photosynthesis...
bacteriochlorophyll pair, P840. When light is absorbed by the reaction center, P840 enters an excited state with a large negative reduction potential, and so readily donates the electron to bacteriochlorophyll 663 which passes it on down the electron chain. The electron is transferred through a series of electron carriers and complexes until it returns to P840 or is used to reduce NAD+. If the electron leaves the chain to reduce NAD+, P840 must be reduced for the ETC to function again. This is accomplished with the oxidation of hydrogen sulfide (or other inorganic sulfur compound) by cytochrom c555.