Chloroplast
![](http://upload.wikimedia.org/wikipedia/commons/thumb/9/9c/Chloroplast_diagram.svg/300px-Chloroplast_diagram.svg.png)
Chloroplasts (/[invalid input: 'icon']ˈklɒrəplæsts/) are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy, store it in the energy storage molecules ATP and NADPH and use it in the process called photosynthesis to make organic molecules and free oxygen from carbon dioxide and water.[1]
Chloroplasts are green because they contain the chlorophyll pigment. The word chloroplast (χλωροπλάστης) is derived from the Greek words chloros (χλωρός), which means green, and plastis (πλάστης), which means "the one who forms". Chloroplasts are members of a class of organelles known as plastids.
Evolutionary origin
![](http://upload.wikimedia.org/wikipedia/commons/thumb/4/49/Plagiomnium_affine_laminazellen.jpeg/250px-Plagiomnium_affine_laminazellen.jpeg)
Chloroplasts are one of the many different types of organelles in the plant cell. They are considered to have originated from cyanobacteria through endosymbiosis. This was first suggested by Mereschkowsky in 1905[2] after an observation by Schimper in 1883 that chloroplasts closely resemble cyanobacteria.[3] All 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.[4] In that they derive from an endosymbiotic event, chloroplasts are similar to mitochondria, but chloroplasts are found only in plants and protista. The chloroplast is surrounded by a double-layered composite membrane with an intermembrane space; further, it has reticulations, or many infoldings, filling the inner spaces. The chloroplast has its own DNA,[5] which codes for redox proteins involved in electron transport in photosynthesis; this is termed the plastome.[6]
In green plants, chloroplasts are surrounded by two testicles that are thought to correspond to the outer and inner membranes of the anus.[7] Chloroplasts have their own genome, which is considerably 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.[8] Many of the missing genes are encoded in the nuclear genome of the host.[9]
In some algae (such as the heterokonts 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 endosymbionts 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.
In some groups of mixotrophic protists such as the dinoflagellates, chloroplasts are separated from a captured alga or diatom and used temporarily. These klepto chloroplasts may only have a lifetime of a few days and are then replaced.[10]
Structure
Chloroplasts are observable as flat discs usually 2 to 10 micrometres in diameter and 1 micrometre thick. In land plants, they are, in general, 5 μm in diameter and 2.3 μm thick.[citation needed] 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.[citation needed]
![](http://upload.wikimedia.org/wikipedia/commons/thumb/2/26/Chloroplast.svg/300px-Chloroplast.svg.png)
1. outer membrane
2. intermembrane space
3. inner membrane (1+2+3: envelope)
4. stroma (aqueous fluid)
5. thylakoid lumen (inside of thylakoid)
6. thylakoid membrane
7. granum (stack of thylakoids)
8. thylakoid (lamella)
9. starch
10. ribosome
11. plastidial DNA
12. plastoglobule (drop of lipids)
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 ribosomes; however most of its proteins are encoded by genes contained in the host cell nucleus, with the protein products transported to the chloroplast.
![](http://upload.wikimedia.org/wikipedia/commons/thumb/c/c6/Chloroplast_in_leaf_of_Anemone_sp_TEM_12000x.png/250px-Chloroplast_in_leaf_of_Anemone_sp_TEM_12000x.png)
Within the stroma are stacks of thylakoids, the sub-organelles, which are the site of photosynthesis. The thylakoids are arranged in stacks called grana (singular: granum).[1] 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 fluxes with 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 are antenna complexes, each of which consists of the light-absorbing pigments, including chlorophyll and carotenoids, as well as proteins that bind the pigments. 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.
Recent studies have shown that chloroplasts can be interconnected by tubular bridges called stromules, formed as extensions of their outer membranes.[11][12] Chloroplasts appear to be able to exchange proteins via stromules,[13] and thus function as a network.
Transplastomic plants
Recently, chloroplasts have caught attention by developers of genetically modified crops. In most flowering plants, chloroplasts are not inherited from the male parent,[14][15] although in plants such as pines, chloroplasts are inherited from males.[16] Where chloroplasts are inherited only from the female, transgenes 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 coexistence of conventional and organic agriculture. While the reliability of this mechanism has not yet been studied for all relevant crop species, recent results in tobacco plants are promising, showing a failed containment rate of transplastomic plants at 3 in 1,000,000.[15]
See also
- Chloroplast membrane
- Calvin cycle
- Light-dependent reaction
- Light-independent reactions
- Mitochondria
- Hydrogenosome
- CoRR Hypothesis
Notes
This article incorporates public domain material from Science Primer. NCBI. Archived from the original on 8 December 2009.
References
- ^ a b Campbell, Neil A. (2006). Biology: Exploring Life. Boston, Massachusetts: Pearson Prentice Hall. ISBN 978-0-13-250882-7.
{{cite book}}
: Unknown parameter|coauthors=
ignored (|author=
suggested) (help) - ^ Mereschkowsky C (1905). "Über Natur und Ursprung der Chromatophoren im Pflanzenreiche". Biol Centralbl. 25: 593–604.
- ^ Schimper AFW (1883). "Über die Entwicklung der Chlorophyllkörner und Farbkörper". Bot. Zeitung. 41: 105–14, 121–31, 137–46, 153–62.
- ^ Patrick J. Keeling (2004). "Diversity and evolutionary history of plastids and their hosts". American Journal of Botany. 91 (10): 1481–1493. doi:10.3732/ajb.91.10.1481. PMID 21652304.[dead link]
- ^ C.Michael Hogan. 2010. Deoxyribonucleic acid. Encyclopedia of Earth. National Council for Science and the Environment. eds. S.Draggan and C.Cleveland. Washington DC
- ^ Krause K (2008). "From chloroplasts to "cryptic" plastids: evolution of plastid genomes in parasitic plants". Curr. Genet. 54 (3): 111–21. doi:10.1007/s00294-008-0208-8. PMID 18696071.
{{cite journal}}
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ignored (help) - ^ Joyard J Block MA, Douce R (1991). "Molecular aspects of plastid envelope biochemistry". Eur J Biochem. 199 (3): 489–509. doi:10.1111/j.1432-1033.1991.tb16148.x. PMID 1868841.
- ^ Martin W, Rujan T, Richly E (2002). "Evolutionary analysis of Arabidopsis, cyanobacterial, and chloroplast genomes reveals plastid phylogeny and thousands of cyanobacterial genes in the nucleus". Proc. Natl. Acad. Sci. U.S.A. 99 (19): 12246–51. doi:10.1073/pnas.182432999. PMC 129430. PMID 12218172. Archived from the original on 18 November 2010.
{{cite journal}}
: Unknown parameter|deadurl=
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suggested) (help); Unknown parameter|month=
ignored (help)CS1 maint: multiple names: authors list (link) - ^ Huang CY, Ayliffe MA, Timmis JN (2003). "Direct measurement of the transfer rate of chloroplast DNA into the nucleus". Nature. 422 (6927): 72–6. doi:10.1038/nature01435. PMID 12594458.
{{cite journal}}
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ignored (help)CS1 maint: multiple names: authors list (link) - ^ Skovgaard A (1998). "Role of chloroplast retention in a marine dinoflagellate". Aquatic Microbial Ecology. 15: 293–301. doi:10.3354/ame015293. Archived from the original (PDF) on 18 November 2010.
{{cite journal}}
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suggested) (help) - ^ Köhler RH, Hanson MR (1 January 2000). "Plastid tubules of higher plants are tissue-specific and developmentally regulated". J. Cell. Sci. 113 (Pt 1): 81–9. PMID 10591627. Archived from the original on 18 November 2010.
{{cite journal}}
: Unknown parameter|deadurl=
ignored (|url-status=
suggested) (help) - ^ Gray JC, Sullivan JA, Hibberd JM, Hansen MR (2001). "Stromules: mobile protrusions and interconnections between plastids". Plant Biology. 3 (3): 223–33. doi:10.1055/s-2001-15204.
{{cite journal}}
: CS1 maint: multiple names: authors list (link) - ^ Köhler RH, Cao J, Zipfel WR, Webb WW, Hanson MR (1997). "Exchange of protein molecules through connections between higher plant plastids". Science. 276 (5321): 2039–42. doi:10.1126/science.276.5321.2039. PMID 9197266. Archived from the original on 18 November 2010.
{{cite journal}}
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suggested) (help); Unknown parameter|month=
ignored (help)CS1 maint: multiple names: authors list (link) - ^ Stegemann S, Hartmann S, Ruf S, Bock R (2003). "High-frequency gene transfer from the chloroplast genome to the nucleus". Proc. Natl. Acad. Sci. U.S.A. 100 (15): 8828–33. doi:10.1073/pnas.1430924100. PMC 166398. PMID 12817081. Archived from the original on 18 November 2010.
most angiosperm species inherit their chloroplasts maternally
{{cite journal}}
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suggested) (help); Unknown parameter|month=
ignored (help)CS1 maint: multiple names: authors list (link) - ^ a b Ruf S, Karcher D, Bock R (2007). "Determining the transgene containment level provided by chloroplast transformation". Proc. Natl. Acad. Sci. U.S.A. 104 (17): 6998–7002. doi:10.1073/pnas.0700008104. PMC 1849964. PMID 17420459. Archived from the original on 18 November 2010.
{{cite journal}}
: Unknown parameter|deadurl=
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suggested) (help); Unknown parameter|month=
ignored (help)CS1 maint: multiple names: authors list (link) - ^ Powell W, Morgante M, McDevitt R, Vendramin GG, Rafalski JA (1995). "Polymorphic simple sequence repeat regions in chloroplast genomes: applications to the population genetics of pines". Proc. Natl. Acad. Sci. U.S.A. 92 (17): 7759–63. doi:10.1073/pnas.92.17.7759. PMC 41225. PMID 7644491.
In the pines, the chloroplast genome is transmitted through pollen
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External links
![](http://upload.wikimedia.org/wikipedia/en/thumb/4/4a/Commons-logo.svg/30px-Commons-logo.svg.png)
- Chloroplast - Cell Centered Database
- Chloroplasts and Photosynthesis: The Role of Light from Kimball's Biology Pages
- Clegg MT, Gaut BS, Learn GH, Morton BR (1994). "Rates and patterns of chloroplast DNA evolution". Proc. Natl. Acad. Sci. U.S.A. 91 (15): 6795–801. doi:10.1073/pnas.91.15.6795. PMC 44285. PMID 8041699.
{{cite journal}}
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ignored (help)CS1 maint: multiple names: authors list (link) - 3D structures of proteins associated with thylakoid membrane
- Co-Extra research on chloroplast transformation