The chloroplasts (from ancient Greek χλωρός chlōrós "green" and πλαστός plastós "shaped") are organelles of the cells of green algae and land plants that carry out photosynthesis . In higher plants , chromoplasts , leucoplasts ( amyloplasts , elioplasts ), etioplasts and gerontoplasts (collectively referred to as plastids ) can develop from photosynthetically active chloroplasts through differentiation .
Build up of chloroplasts
The chloroplasts of land plants have a diameter of about 4 to 8 µm . With many algae with only a single chloroplast per cell, however, this can take up a large part of the cell.
The structure of the chloroplasts is similar to that of the cyanobacteria (blue-green bacteria, formerly called blue-green algae). Chloroplasts (almost always) have their own plastid DNA ( chloroplast DNA , abbreviated cpDNA or ctDNA) together with their own ribosomes (plastid ribosomes or plastoribosomes) and are similar to mitochondria . The genome of the chloroplasts and the other plastids is also known as the plastome .
Chloroplasts are encased in two biomembranes , inside of which the stroma is located as a plasmatic phase . The stroma in turn is traversed by thylakoid membranes , descendants of the inner membrane. With the exception of many phototrophic protists , in the chloroplasts of the higher phototrophs, flat, round protuberances of these membranes are superimposed “like a roll of money” in several places - such a thylakoid stack is called granum (pl. Grana). Various pigments are embedded in the membranes of the thylakoids, especially the green pigment chlorophyll . A lot of it is found in the grana's membranes, which is why they appear intensely green in color.
The pigments can absorb light of certain wavelengths and the energy absorbed is used to produce ATP from ADP and phosphate (see phototrophy ). ATP serves as an energy carrier to build up glucose or starch from CO 2 and water.
Development of a chemiosmotic membrane potential
The biogenesis of these three membrane systems explains the fact that the membrane potential is built up by a proton gradient in chloroplasts across the thylakoid membrane (the thylakoid interior has an acidic environment), while in mitochondria the intermembrane space (area between the inner and outer membrane ) is chemiosmotic with H. + Ions is loaded. Similarly, in chloroplasts, ATP synthase (alias F o F 1 - ATPase ) is an enzyme embedded in the thylakoid membrane (CF 1 part protrudes into the stroma), in mitochondria it is part of the inner membrane (F 1 part facing the matrix). In both systems, ATP is released to the matrix / stroma. In exchange for ADP, it can get into the cytosol of the cell.
Today we know that bacteria also have a cytoskeleton whose proteins show evolutionary relationship to those of the eukaryotic cytoskeleton. Experiments on moss Physcomitrella patens (including with knockout mosses ) is known that the FtsZ proteins, the tubulin - homologues , not only the division of chloroplasts cause, but also a complex network can form in the chloroplasts. Since these networks are strongly reminiscent of the cytoskeleton, Ralf Reski coined the term “plastoskeleton” in 2000 for these complex structures and postulated that they fulfill similar functions in the plastids as the cytoskeleton does for the entire cell .
Chloroplast DNA was detected for the first time in 1962, and a plastome was sequenced for the first time in 1986. Since then, hundreds of chloroplast DNAs from different species have been sequenced. Most of the time, however, these are chloroplastida , i. H. Land plants or green algae. Glaucophytes (Glaucophyta), red algae (Rhodophyta syn. Rhodophyceae) and other groups of algae are strongly underrepresented. The DNA of the chloroplasts - at least in the case of the chloroplastida (green algae and land plants) - is usually structured like most bacteria in a ring. The chloroplast genome (plastome) has portions as opposing copies ( English inverted repeats are present), similar to the genome of cyanobacteria . In mitochondria and plastids such as chloroplasts, as in bacteria, the DNA is usually condensed into nuceloids (nuclear equivalents). This is ensured by the so-called histone-like proteins (HLPs, after English h iStone l ike p roteins ). These are homologous to one another , but only functionally similar ( analogous ) to the real histones in the nucleus of the eukaryotic cells ( eucytes ). The protein- coding genes are transcribed into messenger RNA , which is used as a template for protein synthesis ( translation ) on one's own ribosomes (plastoribosomes). These plastid ribosomes are similar in structure and structure to those of bacteria and mitochondria, but are significantly smaller and simpler than those of the euzytes surrounding them.
Origin of chloroplasts - endosymbiotic theory
The German botanist Andreas Franz Wilhelm Schimper had already established in 1883 that chloroplasts are very similar to cyanobacteria and thus already indicated a symbiotic origin of chloroplasts, so that he can be regarded as a pioneer of the later formulated endosymbiont theory . The Russian biologist Konstantin Sergejewitsch Mereschkowski took up this idea in 1905 and Ivan Wallin in 1922 and made it more concrete.
For a long time it was unknown how chloroplasts divide and change shape. Since it was not possible to cultivate isolated mitochondria or chloroplasts in vitro (in the laboratory on nutrient media), the theory could not be established until the early 1970s ( Lynn Margulis ), when DNA was detected in both types of organelle. The independent “ multiplication ” of the chloroplasts, similar to bacteria, without a structural coupling to the cell division of the surrounding eukaryotic cell, is another argument for the endosymbiont theory.
The exact examination of the chloroplasts and their "host cells" has shown that this process has taken place in several stages (nested). At the beginning, about 600 million to 2 billion years ago, a free-living photosynthetic cyanobacterium entered an early eukaryotic cell (euzyte), either as food ( phagocytosis ) or as an internal parasite ( endoparasite ). However, it was able to escape the phagocytic ( phagosomal ) vacuole in which it was initially, without being dissolved by the euzyte , and became a permanent resident of this "host cell".
Endosymbiotic gene transfer
In the course of the evolution and integration of the endosymbiont, various adjustments were made. This includes, among other things, the adaptation of the chloroplast genome. The size of the genome decreased from approximately 3.5 million bases to 120–160 thousand. This corresponds to a reduction of often more than 1500 genes in cyanobacteria to around 60–100 genes in chloroplasts. The shrinking of the genome has been accompanied by the loss of genetic information and transfer into the cell nucleus , a process known as ' endosymbiotic gene transfer ' (EGT).
Import of proteins into the chloroplast
At the same time, a complex machinery for importing proteins from the cytosol into the chloroplasts developed. So you can find about 2000 proteins in the chloroplast despite the only 100 remaining genes. These leftover protein-coding genes can be roughly divided into two categories: maintenance of the genetic apparatus ( DNA polymerase , tRNAs and rRNAs ) and maintenance of photosynthetic capacity ( photosystem components and other proteins). It has not yet been fully clarified how the expression between the nucleus and the chloroplasts is synchronized . This is necessary because in all protein complexes in the chloroplast, plastid and nuclear-coded products are combined.
In 2014 a plastid without a genome was even found in the non-photosynthetically active green alga Polytomella ( Chlamydomonadales , syn. Volvocales). This shows that chloroplasts can lose their entire genome through endosymbiotic gene transfer. The situation is thus analogous to that of mitochondria , where a loss of the original function is also accompanied by a strong reduction or complete loss of the genome (see hydrogenosome and mitosome ).
The first primary endosymbiosis (main line)
It has long been debated whether the resulting primary chloroplasts originated from a single endosymbiotic event or from several independent events in different eukaryotic lineages. It is now widely believed that virtually all organisms with primary chloroplasts have a single common ancestor. The time taken cyanobacterium was apparently near the present species Gloeomargarita lithophora , this is basal in the family tree of cyanobacteria close to the genus Synechococcus . The alga Cyanophora , a glaucophyte , is one of the most primitive organisms that contain a chloroplast.
Another primary endosymbiosis ( Paulinella )
The exception is the aforementioned amoeboid Paulinella chromatophora ( Euglyphida , see below). This apparently comes from an ancestor who, independently and much later - about 90 to 500 million years ago - took in a cyanobacterium of the genus Prochlorococcus (or Synechococcus ).
Lineage and basic types of chloroplasts
Overall, all primary chloroplasts belong to one of the following four lineages (the first three with a common origin):
- the glaucophyte line:
the unicellular algae of the Glaucophyta (syn. Glaucocystaceae) have plastids that are in many respects still very similar to the cyanobacterium and are therefore often referred to as " cyanelles " or "cyanoplasts", sometimes also as "muroplasts".
- the rhodophyte line:
red algae (wiss. Rhodophyta) have plastids called “ rhodoplasts ”, which still carry the antenna structure ( phycobilisomes ) of the cyanobacteria.
- the chloroplastid line:
The plastids of the viridiplantae (syn. chloroplastida, green algae and higher plants ) represent the most developed plastids and carry a large variety of antenna complexes. The green plastids of algae and higher plants are called chloroplasts.
- of the Paulinella line:
The plastids of the amoeboid Paulinella chromatophora ( Euglyphida ) are called chromatophores .
Secondary and other endosymbioses
Although all chloroplasts, with one exception, can probably be traced back to a single such primary endosymbiosis event, chloroplasts are still found today in an extremely wide range of organisms, some of which are not even directly related to one another. This is explained as a consequence of many secondary and even tertiary endosymbiotic events in which photosynthetically active algae (including their chloroplasts) were ingested instead of cyanobacteria. These originally eukaryotic complex plastids or chloroplasts are called secondary plastids (chloroplasts).
Number of membranes and nucleomorph
While primary chloroplasts only have a double membrane that is derived from their cyanobacterial ancestor, secondary chloroplasts have additional membranes outside of these two original ones. This is interpreted as a consequence of the secondary endosymbiotic events. As a result of the mostly extensive degradation of the incorporated algae, often only their (primary) chloroplast and sometimes their cell membrane and / or even a remnant of their cell nucleus , called nucleomorph , remained. The resulting complex (secondary) chloroplasts can therefore have three or four membranes: the two cyanobacterial membranes, sometimes the cell membrane of the incorporated algae and the phagosome vacuole from the host's cell membrane.
The complex (secondary and tertiary) chloroplasts were often transformed in many ways. In other cases as well - such as the apicomplexa (see below) - such additional functions make even photosynthetically no longer active plastids indispensable for the cell.
The apicomplexa are a subgroup of the alveolata. The malaria parasite Plasmodium belongs to the Apicomplexa . Many apicomplexes have a red algae-derived chloroplast called an apicoplast , which they inherited from their ancestors. The apicoplasts have lost all photosynthetic functions and do not contain photosynthetic pigments or true thylakoids.
The fact that Apicomplexa still kept their non-photosynthetic chloroplasts testifies that the chloroplasts perform other important functions in addition to photosynthesis, as mentioned above - plant chloroplasts supply plant cells with many important substances in addition to sugar, and apicoplasts do not differ in this - they synthesize Fatty acids , isopentenyl pyrophosphate and iron-sulfur clusters . In addition, they perform part of the heme synthesis ( English heme pathway ).
In some cases, existing chloroplasts were replaced by new (complex) ones, in other cases new ones were added to existing plastids if the tasks differed greatly. In the case of dinoflagellates , the chloroplasts are mostly derived from red algae and are of the peridinin type, characterized by the carotenoid pigment peridinin together with chlorophyll a and chlorophyll c 2 . Peridinin is not found in any other group of chloroplasts and has three (occasionally only two) membranes, i.e. H. they have lost the original cell membrane of the red algae endosymbiont.
Some dinoflagellates such as Kryptoperidinium and Durinskia (both Peridiniaceae , also English dinotoms ) have a tertiary chloroplast derived from diatoms (syn. Diatoms, Heterokontophyta ). These chloroplasts are surrounded by up to five membranes, depending on whether the entire diatom endosymbiont is seen as the chloroplast or only the red alga it contains is counted as the chloroplast. The diatom endosymbiont is relatively little reduced - it still contains its original mitochondria and has an endoplasmic reticulum , eukaryotic ribosomes , a cell nucleus and of course the secondary chloroplasts derived from red algae - practically a complete cell - everything inside the host. The original three-membrane peridinin chloroplast was either lost or remodeled into an eye-spot , as is believed to be the case with Durinskia .
Despite the reduction of the endosymbiont genome through gene transfer to the host, including the reduction of nucleomorphs (up to the point of disappearance) and of membrane envelopes in complex plastids, there is still a symbiosis as long as the endosymbiont remains capable of reproduction (which, however, in extreme cases - complete loss of the organelle genome - can end in complete assimilation). In contrast to this, kleptoplastidy describes the 'robbery' of chloroplasts, i. H. the removal of the shell and core of the recorded phototrophs (`green` eucytes), so that only the chloroplasts remain, but are no longer capable of reproduction. These so-called kleptoplasts then have to be replaced again and again by new ones. It even happens that such predators become victims again themselves. Kleptoplastidy has been observed in certain dinoflagellates , ciliates and some marine snails , but some of the genes from the cell nuclei of the food are transferred to the snails in members of the Elysia genus , which is why the chloroplasts can be supplied with proteins that are essential for them. In the case of the green hydra , however, endosymbiosis or an intermediate form is assumed.
- Chemiosmotic coupling
- Calvin cycle
- C3 plant , C4 plant (with “dimorphogenic” chloroplasts) and CAM plant
- Endosymbiont theory (on the origin of chloroplasts)
- Complex plastids
- Eye spot (especially in Chlamydomonas ) and ocelloid
- Cell compartment
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