Photosystem 1126109 213229688 2008-05-18T11:55:01Z SieBot 4005189 robot Adding: [[pl:Fotosystem]] '''Photosystems''' (ancient [[Greek language|Greek]]: ''phos'' = light and ''systema'' = assembly) are [[protein complex]]es involved in [[photosynthesis]]. They are found in the [[thylakoid membrane]]s of plants, algae and cyanobacteria (in plants and algae these are located in the [[chloroplast]]s), or in the cytoplasmic membrane of photosynthetic bacteria. A photosystem (or [[photosynthetic reaction centre|Reaction Center]]) is an [[enzyme]] which uses light to [[redox|reduce]] molecules. This [[membrane protein]] complex is made of several subunits and contains numerous [[cofactor (biochemistry)|cofactors]]. In the photosynthetic membranes, reaction centers provide the driving force for the bioenergetic electron and proton transfer chain. When [[light]] is absorbed by a reaction center (either directly or passed by neighbouring pigment-antennae), a series of oxido-reduction reactions is initiated, leading to the reduction of a terminal acceptor. Two families of photosystems exist: type I reaction centers (like '''photosystem I ([[P700]])''' in chloroplasts and in green-sulphur bacteria) and type II reaction centers (like '''photosystem II ([[P680]])''' in chloroplasts and in non-sulphur purple bacteria). Each photosystem can be identified by the [[wavelength]] of light to which it is most reactive (700 and 680 [[nanometer]]s, respectively for PSI and PSII in chloroplasts), and the type of terminal electron acceptor. Type I photosystems use [[ferredoxin]]-like iron-sulfur cluster proteins as terminal electron acceptors, while type II photosystems ultimately shuttle electrons to a [[quinone]] terminal electron acceptor. One has to note that both reaction center types are present in chloroplasts and cyanobacteria, working together to form a unique photosynthetic chain able to extract electrons from water, creating oxygen as a byproduct. ==Structure== A reaction center comprises several (>10 or >11) protein subunits, providing a scaffold for a series of cofactors. The latter can be pigments (like [[chlorophyll]], [[pheophytin]], [[carotenoids]]), quinones or iron-sulfur clusters. Because chlorophyll ''a'' can only absorb light of a narrow [[wavelength]], it works with the antenna pigments to gain energy from a larger part of the spectrum. The pigments absorb light of various wavelengths and pass along their gained energy to the reaction center chlorophyll. When the energy reaches the chlorophyll ''a'', it releases two [[electron]]s into an [[electron transport chain]]. Though chlorophyll ''a'' normally has an optimal absorption wavelength of 660 [[nanometer]]s, it associates with different proteins in each type of photosystem to slightly shift its optimal wavelength, producing two distinct photosystem types. Other proteins serve to support the structure and electron pathways in the photosystem. ==Relationship between Photosystems I and II== [[Image:P 700.jpg|frame|thumb|100 pixel|Schematic drawing of photosystem I from higher plants]] Historically photosystem I was named "I" since it was discovered before photosystem II, but this does not represent the order of the electron flow, although it does represent the order in which the two photosystems evolved; photosystem I evolved very early; photosystem II evolved later.<ref>[http://faculty.clintoncc.suny.edu/faculty/Michael.Gregory/files/Bio%20101/Bio%20101%20Lectures/Photosynthesis/photosyn.htm Photosystem I evolved first] </ref><ref>[http://www2.mcdaniel.edu/Biology/botf99/photo/l4ightrx.html Photosystem I and II and the Light Reaction]</ref> When photosystem II absorbs light, electrons in the reaction-center chlorophyll are excited to a higher energy level and are trapped by the primary electron acceptors. To replenish the deficit of electrons, electrons are extracted from water (either through photolysis or enzymatic means) and supplied to the chlorophyll. Photoexcited electrons travel though the [[cytochrome b6f complex]] to photosystem I via an electron transport chain set in the [[thylakoid membrane]]. This energy fall is harnessed, (the whole process termed [[chemiosmosis]]), to transport hydrogen (H<sup>+</sup>) through the membrane to provide a proton-motive force to generate ATP. If electrons only pass through once, the process is termed noncyclic photophosphorylation. When the electron reaches photosystem I, it fills the electron deficit of the reaction-center chlorophyll of photosystem I. The deficit is due to photo-excitation of electrons which are again trapped in an electron acceptor molecule, this time that of photosystem I. These electrons may either continue to go through cyclic electron transport around PS I, or pass, via ferredoxin, to the enzyme NADP<sup>+</sup> reductase. Electrons and hydrogen ions are added to NADP<sup>+</sup> to form NADPH. This reducing agent is transported to the Calvin cycle to react with [[glycerate 3-phosphate]], along with ATP to form [[glyceraldehyde 3-phosphate]], the basic building block from which plants can make a variety of substances. ==See also== *[[Photosynthetic reaction centre]] *[[photosynthesis]] *[[chlorophyll]] *[[light reaction]] *[[photoinhibition]] *[[photosystem 2]] ==External links== *[http://www.bio.ic.ac.uk/research/barber Photosystems I + II: Imperial College, Barber Group] *[http://nist.rcsb.org/pdb/molecules/pdb22_1.html Photosystem I: Molecule of the Month in the Protein Data Bank] *[http://nist.rcsb.org/pdb/molecules/pdb59_1.html Photosystem II: Molecule of the Month in the Protein Data Bank] *[http://www.rsbs.anu.edu.au/o2/O2_3_PhotosystemII.htm Photosystem II: ANU] * {{UMichOPM|families|superfamily|2}} - Calculated spatial positions of photosynthetic reaction centers and photosystems in membrane * [http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1693113 Photosystem II: evolutionary perspectives.] ==References== <References/>. {{Multienzyme complexes}} [[Category:Photosynthesis]] [[Category:Light reactions]] [[Category:Metalloproteins]] [[Category:Integral membrane proteins]] [[de:Photosystem]] [[es:Fotosistema]] [[fr:Photosystème]] [[gl:Fotosistema]] [[it:Fotosistema]] [[nl:Fotosysteem]] [[pl:Fotosystem]] [[pt:Fotossistema]]