Chloroplast
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edit=autoconfirmed:move=autoconfirmed
221726387
2008-06-25T20:33:52Z
Nuvitauy07
6762195
/* Evolutionary origin */
<!-- Unsourced image removed: [[Image:Chloroplastsfigure1.jpg|thumb|right|The inside of a chloroplast]] -->
[[Image:Chloroplasten.jpg|frame|Plant cells with visible chloroplasts.]]
'''Chloroplasts''' are [[organelle]]s found in [[plant cell]]s and [[eukaryote|eukaryotic]] [[alga]]e that conduct [[photosynthesis]]. Chloroplasts absorb light and use it in conjunction with water and carbon dioxide to produce sugars, the raw material for energy and [[Biomass (ecology)|biomass]] production in all green plants and the animals that depend on them, directly or indirectly, for food. Chloroplasts capture [[light]] [[energy]] to conserve [[Thermodynamic free energy|free energy]] in the form of [[Adenosine triphosphate|ATP]] and reduce [[NADP]] to [[NADPH]] through a complex set of processes called photosynthesis. It is derived from the Greek words ''chloros'' which means green and ''plast'' which means form or entity. Chloroplasts are members of a class of organelles known as [[plastid]]s.
==Evolutionary origin==
Chloroplasts are one of the many unique organelles in the plant cell. They are generally considered to have originated as [[endosymbiotic theory|endosymbiotic]] [[cyanobacteria]] (i.e. blue-green algae). This was first suggested by Mereschkowsky in 1905 <ref>{{cite journal | author= Mereschkowsky C | title= Über Natur und Ursprung der Chromatophoren im Pflanzenreiche | journal= Biol Centralbl | date=1905 | volume=25 | pages=593–604}} </ref> after an observation by Schimper in 1883 that chloroplasts closely resemble cyanobacteria. <ref>{{cite journal | author= Schimper AFW | title= Über die Entwicklung der Chlorophyllkörner und Farbkörper | journal= Bot. Zeitung | date=1883 | volume=41 | pages=105–14, 121–31, 137–46, 153–62}} </ref> All eukaryote chloroplasts are thought to derive directly or indirectly from a single endosymbiotic event (in the [[Archaeplastida]]), except for ''[[Paulinella]] chromatophora'', which has recently acquired a photosynthetic cyanobacterial endosymbiont which is not closely related to chloroplasts of other eukaryotes.<ref>{{cite journal | title = Diversity and evolutionary history of plastids and their hosts | author = Patrick J. Keeling | url = http://www.amjbot.org/cgi/content/full/91/10/1481 | journal = American Journal of Botany | year = 2004 | volume = 91 | pages = 1481–1493 | doi = 10.3732/ajb.91.10.1481}}</ref> In that they derive from an endosymbiotic event, chloroplasts are similar to [[mitochondrion|mitochondria]] but chloroplasts are found only in [[plant]]s and [[protist]]a. The chloroplast is surrounded by a double-layered composite membrane with an intermembrane space; it has its own [[DNA]] and is involved in energy metabolism. Further, it has reticulations, or many infoldings, filling the inner spaces.
In green plants, chloroplasts are surrounded by two [[cell membrane|lipid-bilayer membrane]]s. The inner membrane is now believed to correspond to the outer membrane of the ancestral cyanobacterium. Chloroplasts have their own genome, which is considerably [[genome reduction|reduced]] compared to that of free-living cyanobacteria, but the parts that are still present show clear similarities with the cyanobacterial genome. Plastids may contain 60-100 genes whereas cyanobacteria often contain more than 1500 genes.<ref>{{ cite journal | author= Martin W, Rujan T, Richly E, Hansen A, Cornelson S, Lins T, Leister D, Stoebe B, Hasegawa M, Penny D | title= Evolutionary analysis of Arabidopsis, cyanobacterial, and chloroplast genomes reveals plastid phylogeny and thousands of cyanobacterial genes in the nucleus | journal = Proc Natl Acad Sci | date = 2002 | volume=99 | pages=12246–12251 | doi= 10.1073/pnas.182432999 | pmid= 12218172}}</ref> Many of the missing genes are encoded in the nuclear genome of the host. The transfer of nuclear information has been estimated in [[tobacco]] plants at one [[gene]] for every 16000 pollen grains.<ref>{{cite journal | author=Huang CY, Ayliffe MA, Timmis JN | title=Direct measurement of the transfer rate of chloroplast DNA into the nucleus | journal=Nature | date =2003 Mar 6 | volume=422 | issue=6927 | pages=72–6 | doi=10.1038/nature01435}}</ref>
In some algae (such as the [[heterokont]]s and other protists such as [[Euglenozoa]] and [[Cercozoa]]), chloroplasts seem to have evolved through a secondary event of endosymbiosis, in which a eukaryotic cell engulfed a second eukaryotic cell containing chloroplasts, forming chloroplasts with three or four membrane layers. In some cases, such secondary [[endosymbiont]]s may have themselves been engulfed by still other eukaryotes, thus forming tertiary endosymbionts. In the alga Chlorella, there is only one chloroplast, which is bell shaped.
==Structure==
[[Image:Chloroplast-new.jpg|thumb|right|The internal structure of a chloroplast, with a granal stack of thylakoids circled.]]
Chloroplasts are observable morphologically as flat discs usually 2 to 10 micrometer in diameter and 1 micrometer thick. In land plants they are generally 5 μm in diameter and 2.3 μm thick. The chloroplast is contained by an envelope that consists of an inner and an outer phospholipid membrane. Between these two layers is the intermembrane space. A typical [parenchyma] cell contains about 10 to 100 chloroplasts.
The material within the chloroplast is called the stroma, corresponding to the [[cytosol]] of the original bacterium, and contains one or more molecules of small circular DNA. It also contains [[ribosome]]s, although most of its proteins are encoded by genes contained in the host cell nucleus, with the protein products transported to the chloroplast.
Within the stroma are stacks of [[thylakoid]]s, the sub-organelles which are the site of photosynthesis. The thylakoids are arranged in stacks called grana (singular: granum). A thylakoid has a flattened disk shape. Inside it is an empty area called the thylakoid space or lumen. Photosynthesis takes place on the thylakoid membrane; as in mitochondrial oxidative phosphorylation, it involves the coupling of cross-membrane [[flux]]es with [[biochemistry|biosynthesis]] via the dissipation of a proton electrochemical gradient.
In the electron microscope, thylakoid membranes appear as alternating light-and-dark bands, each 0.01 μm thick. Embedded in the thylakoid membrane is the antenna complex, which consists of the light-absorbing pigments, including [[chlorophyll]] and [[carotenoids]], and proteins (which bind the chlorophyll). This complex both increases the surface area for light capture, and allows capture of photons with a wider range of wavelengths. The energy of the incident photons is absorbed by the pigments and funneled to the reaction centre of this complex through [[resonance energy transfer]]. Two chlorophyll molecules are then ionised, producing an excited electron which then passes onto the photochemical reaction centre.
==Transplastomic plants==
Recently, chloroplasts have caught attention by developers of [[genetically modified plant]]s. In certain plant species, such as tobacco, chloroplasts are not inherited from the male, and therefore, [[transgene]]s in these plastids cannot be disseminated by [[pollen]]. This makes [[plastid transformation]] a valuable tool for the creation and cultivation of genetically modified plants that are biologically contained, thus posing significantly lower environmental risks. This [[biological containment]] strategy is therefore suitable for establishing the [[Co-existence of genetically modified and conventional crops and derived food and feed|coexistence of conventional and organic agriculture]]. The reliability of this mechanism has not yet been studied for all relevant crop species. However, the research programme [[Co-Extra]] recently published results for tobacco plants, demonstrating that the containment of transplastomic plants is highly reliable with a tiny failure rate of 3 in 1,000,000.<ref>{{cite journal | author=Ruf S, Karcher D, Bock R | title=Determining the transgene containment level provided by chloroplast transformation | journal=PNAS | date=2007 Apr 24 | volume=104 | issue=17 | pages=6998–7002 | doi=10.1073/pnas.0700008104 | pmid=17420459}}</ref>
==See also==
*[[Chloroplast membrane]]
:*[[Inner membrane]]
:*[[Outer membrane]]
*[[Calvin cycle]]
*[[Light-dependent reaction]]
==References==
*{{NCBI-scienceprimer}}
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==External links==
*[http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/Chloroplasts.html Chloroplasts] and [http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/L/LightReactions.html Photosynthesis: The Role of Light] from [http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/ Kimball's Biology Pages]
*[http://reference.allrefer.com/encyclopedia/C/chloropl.html Chloroplast, Botany]
*[http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=8041699 Use of chloroplast DNA in studying plant phylogeny and evolution]
*[http://opm.phar.umich.edu/localization.php?localization=Thylakoid%20membrane 3D structures of proteins associated with thylakoid membrane]
*[http://www.coextra.eu/projects/project199.html Co-Extra research on chloroplast transformation]
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{{organelles}}
{{Botany}}
[[Category:Organelles]]
[[Category:Photosynthesis]]
[[ar:صانعات يخضورية]]
[[bn:ক্লোরোপ্লাস্ট]]
[[bg:Хлоропласт]]
[[ca:Cloroplast]]
[[da:Grønkorn]]
[[de:Chloroplast]]
[[et:Kloroplast]]
[[es:Cloroplasto]]
[[eo:Kloroplasto]]
[[fa:سبزدیسه]]
[[fr:Chloroplaste]]
[[gl:Cloroplasto]]
[[ko:엽록체]]
[[hr:Kloroplast]]
[[id:Kloroplas]]
[[is:Grænukorn]]
[[it:Cloroplasto]]
[[he:כלורופלסט]]
[[lt:Chloroplastas]]
[[hu:Kloroplasztisz]]
[[mk:Хлоропласт]]
[[ms:Kloroplas]]
[[nl:Bladgroenkorrel]]
[[ja:葉緑体]]
[[no:Kloroplast]]
[[oc:Cloroplast]]
[[nds:Chloroplast]]
[[pl:Chloroplast]]
[[pt:Cloroplasto]]
[[ro:Cloroplast]]
[[ru:Хлоропласт]]
[[simple:Chloroplast]]
[[sk:Chloroplast]]
[[sl:Kloroplast]]
[[sr:Хлоропласт]]
[[sh:Hloroplast]]
[[su:Kloroplas]]
[[fi:Viherhiukkanen]]
[[sv:Kloroplast]]
[[vi:Lục lạp]]
[[tr:Kloroplast]]
[[uk:Хлоропласт]]
[[zh:叶绿体]]