Simplified structure of a chloroplast

Chloroplasts ( /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[link]

Chloroplasts visible in the cells of Plagiomnium affine — Many-fruited Thyme-moss

A model chloroplast

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 lipid-bilayer membranes that are thought to correspond to the outer and inner membranes of the ancestral cyanobacterium.^[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[link]

Chloroplasts are observable as flat discs usually 2 to 10 micrometers in diameter and 1 micrometer 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]}

Chloroplast ultrastructure:
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.

TEM image of a chloroplast

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[link]

Recently, chloroplasts have caught attention by developers of genetically modified plants. 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[link]

• Chloroplast membrane
• Calvin cycle
• Light-dependent reaction
• Light-independent reactions
• Mitochondria
• Hydrogenosome
• CoRR Hypothesis

Notes[link]

•  This article incorporates public domain material from the NCBI document "Science Primer".

References[link]

External links[link]

Wikimedia Commons has media related to: Chloroplasts

• Chloroplast - Cell Centered Database
• Chloroplasts and Photosynthesis: The Role of Light from Kimball's Biology Pages
• Clegg MT, Gaut BS, Learn GH, Morton BR (July 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. //www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=44285. 
• 3D structures of proteins associated with thylakoid membrane
• Co-Extra research on chloroplast transformation

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