Light-dependent reaction
181674
225889431
2008-07-15T21:45:06Z
Ron Ritzman
154046
Reverted edits by [[Special:Contributions/213.207.249.84|213.207.249.84]] to last version by DOI bot (using [[WP:HG|Huggle]])
{{Refimprove|date=May 2008}}
[[Image:Thylakoid membrane.png|thumb|300px|Light-dependent reactions of photosynthesis at the thylakoid membrane]]
The initial stage of the [[photosynthesis|photosynthetic system]] is the '''light-dependent reaction''', which converts [[solar power|solar energy]] into [[potential energy]].
The light dependent reaction produces [[oxygen]] gas and converts ADP and NADP+ into
the energy carriers ATP and NADPH.
The chlorophyll's electron can follow either of two different pathways, cyclic or non-cyclic.
===History===
The first ideas about light being used in photosynthesis were proposed by [[Jan Ingenhousz]] in [[1779]]<ref>{{cite book|first=J|last=Ingenhousz|title=Experiments Upon Vegetables|publisher=Elmsly and Payne|location=London|year=1779}}</ref>who recognised it was sunlight falling on plants that was required, although [[Joseph Priestly]] had noted the production of oxygen without the association with light in [[1772]]<ref>{{cite book|first=J|last=Priestley|publisher=Phil. Trans. Roy. Soc. |location=London|volume=62|pages=147|title=Observations on Different Kinds of Air|year=1772}}</ref>.
[[Cornelius Van Niel]] proposed in [[1931]] that photosynthesis is a case of general mechanism where a photon of light is used to photodecompose a hydrogen donor and the hydrogen being used to reduce CO<sub>2</sub><ref>{{cite journal
|last=van Niel|first=C. B.|year=1931.|title=On the morphology and physiology of the purple and green sulphur bacteria|journal=Arch. Mikrobiol|volume=3|pages=1–114|doi=10.1007/BF00454965}}</ref>.
Then in [[1939]] [[Robin Hill (biochemist)|Robin Hill]] showed that isolated chloroplasts would make oxygen, but not fix CO<sub>2</sub> showing the light and dark reactions occurred in different places<ref>{{cite journal|title=Oxygen Produced by Isolated Chloroplasts
|first=R.|last=Hill
|journal=Proceedings of the Royal Society of London. Series B, Biological Sciences
|volume=127|issue=847|month=May|year=1939|pages=192–210}}</ref>. This lead later to the discovery of [[photosystem]] 1 and 2.
===Cyclic photophosphorylation===
{{seealso|photophosphorylation}}
In '''cyclic electron flow''', the electron begins in a pigment complex called photosystem I, passes from the primary acceptor to [[ferredoxin]], then to a complex of two [[cytochrome]]s (similar to those found in [[mitochondria]]), and then to [[plastoquinone]] before returning to chlorophyll. This transport chain produces a proton-motive force, pumping H<sup>+</sup> ions across the membrane; this produces a concentration gradient which can be used to power [[ATP synthase]] during [[chemiosmosis]]. This pathway is known as cyclic photophosphorylation, and it produces neither O<sub>2</sub> nor NADPH.
In bacterial photosynthesis, a single photosystem is used, and therefore is involved in cyclic photophosphorylation.
===Noncyclic photophosphorylation===
The other pathway, noncyclic photophosphorylation, is a two-stage process involving two different chlorophyll photosystems. First, a water molecule is broken down into 2H<sup>+</sup> + 1/2O<sub>2</sub> + 2e<sup>-</sup> by a process called [[photolysis]] (or ''light-splitting''). The two electrons from the water molecule are kept in photosystem II, while the 2H<sup>+</sup> and 1/2O<sub>2</sub> are left out for further use. Then a photon is absorbed by the chlorophyll core of photosystem II, exciting the two electrons which are transferred to the acceptor molecule. The deficit of electrons is replenished by taking electrons from another molecule of water. The electrons transfer from the primary acceptor to plastoquinone, then to plastocyanin, producing proton-motive force as with cyclic electron flow and driving ATP synthesis.
The photosystem II complex replaced its lost electrons from an external source, however, the two other electrons are not returned to photosystem II as they would in the analogous cyclic pathway. Instead, the still-excited electrons are transferred to a photosystem I complex, which boosts their energy level to a higher level using a second solar photon. The highly excited electrons are transferred to the acceptor molecule, but this time are passed on to an enzyme called Ferredoxin- NADP reductase|NADP<sup>+</sup> reductase, for short FNR, which uses them to catalyst the reaction (as shown):
:NADP<sup>+</sup> + 2H<sup>+</sup> + 2e<sup>-</sup> → NADPH + H<sup>+</sup>
This consumes the H<sup>+</sup> ions produced by the splitting of water, leading to a net production of 1/2O<sub>2</sub>, ATP, and NADPH+H<sup>+</sup> with the consumption of solar photons and water.
The concentration of NADPH in the chloroplast may help regulate which pathway electrons take through the light reactions. When the chloroplast runs low on ATP for the Calvin cycle, NADPH will accumulate and the plant may shift from noncyclic to cyclic electron flow.
==Steps==
It is important to note that both photosystems are almost simultaneously excited; thus, both photosystems begin functioning at almost the same time.
#The excited electron is passed along until it reaches P680 chlorophyll.
#The excited electron is passed to the primary electron acceptor. [[Photolysis]] in the thylakoid takes the electrons from water and replaces the P680 electrons that were passed to the primary electron acceptor. (O<sub>2</sub> is released into the air as a waste product)
#The electrons are passed to photosystem I via the electron transport chain (ETC) and in the process used to pump protons across the thylakoid membrane into the [[Lumen (anatomy)|lumen]].
#The stored energy in the proton gradient is used to produce ATP which is used later in the [[Calvin Cycle|Calvin-Benson Cycle]].
# P700 chlorophyll then uses light to excite the electron to its second primary acceptor.
#The electron is sent down another ETC and used to reduce NADP<sup>+</sup> to NADPH.
#The NADPH is then used later in the Calvin-Benson Cycle to remove PGA that is produced from [[RuBisCO]] reaction and releases enzyme for continuation of steady state reaction [http://www.farazdaghi.com/papers/html/photosynthesis.htm]
==See also==
*[[Light-independent reaction]]
*[[Reaction center]]
==References==
<references/>
[[Category:Light reactions| ]]
[[ca:Fase lluminosa]]
[[de:Lichtreaktion]]
[[es:Fase clara]]
[[ko:광 의존적 반응]]
[[it:Fase luce dipendente]]
[[nl:Lichtreacties]]
[[ja:光化学反応]]
[[sr:Светла фаза фотосинтезе]]
[[th:ปฏิกิริยาที่ต้องใช้แสง]]
[[tr:Işıklı devre reaksiyonları]]