Photosynthetic reactions can be anoxygenic, thus they do not produce
oxygen.
LEARNING OBJECTIVE [ edit ]
Discuss the characteristics that classify a specific type of chlorophototrophy as anoxygenic
photosynthesis
KEY POINTS [ edit ]
Anoxygenic photosynthesis produces cellular energy (ATP), without oxygen as a by-product.
As opposed to eukaryotic organisms, which rely on chlorophylls for photosynthesis, anoxygenic
organisms rely on bacteriochlorophylls.
The electron transport chain of anoxygenic phototrophs is cyclic, meaning the electrons used
during photosynthesis are fed back into the system, therefore no electrons are left over to oxidize
water into oxygen.
TERMS [ edit ]
electron transport chain
An electron transport chain (ETC) couples electron transfer between an electron donor (such as
NADH) and an electron acceptor (such as O2) with the transfer of H+ ions (protons) across a
membrane. The resulting electrochemical proton gradient is used to generate chemical energy in
the form of adenosine triphosphate (ATP). Electron transport chains are the cellular mechanisms
used for extracting energy from sunlight in photosynthesis and also from redox reactions, such as
the oxidation of sugars (respiration).
anoxygenic
That does not involve the production of oxygen
electron donor
An electron donor is a chemical entity that donates electrons to another compound. It is a
reducing agent that, by virtue of its donating electrons, is itself oxidized in the process.
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Phototrophyis the process by which
organisms trap light energy (photons) and
store it as chemical energy in the form of
ATP and/or reducing power in NADPH.
There are two major types of phototrophy:
chlorophyll-based chlorophototrophy and
rhodopsin-based retinalophototrophy.
Chlorophototrophy can further be divided
into oxygenicphotosynthesis and
anoxygenic phototrophy. Oxygenic and
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anoxygenic photosynthesizing organisms undergo differentreactions either in the presence of
light or with no direct contribution of light to the chemical reaction (colloquially called "light
reactions" and "dark reactions", respectively).
Anoxygenic photosynthesis is the phototrophic process where light energy is captured and
converted to ATP, without the production of oxygen. Water is therefore not used as
an electron donor. There are several groups of bacteria that undergo anoxygenic
photosynthesis: Green sulfur bacteria , green and red filamentous anoxygenic phototrophs
(FAPs), phototrophic purple bacteria, phototrophic Acidobacteria, and phototrophic
heliobacteria. Anoxygenic phototrophs have photosynthetic pigments called
bacteriochlorophylls (similar to chlorophyll found in eukaryotes). Bacteriochlorophyll a and b
have wavelengths of maximum absorption at 775 nm and 790 nm, respectively in ether. In
vivo however, due to shared extended resonance structures, these pigments were found to
maximally absorb wavelengths out further into the near-infrared. Bacteriochlorophylls c-g
have the corresponding "peak" absorbance at more blue wavelengths when dissolved in
anorganic solvent, but are similarly red-shifted within their natural environment (with the
exception of bacteriochlorophyll f, which has not been naturally observed).Unlike oxygenic
phototrophs, anoxygenic photosynthesis only functions using (by phylum) either one of two
possible types of photosystem. This restricts them to cyclic electron flow and are therefore
unable to produce O2 from the oxidization of H2O.
Green d winogradsky
A column containing green sulfur bacteria which uses anoxygenic photosynthesis.
The cyclic nature of the electron flow is typified in purple non-sulfur bacteria. 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 tosynthesize 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 reactioncenter. Therefore electrons are not left over to oxidize H2O into O2.