Chloroplasts /ˈklɔːrəˌplæsts, -plɑːsts/ are organelles that conduct photosynthesis, where the photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in the energy-storage molecules ATP and NADPH while freeing oxygen from water in plant and algal cells.
Chloroplasts are highly dynamic—they circulate and are moved around within plant cells, and occasionally pinch in two to reproduce.
Their behavior is strongly influenced by environmental factors like light color and intensity.
Chloroplasts cannot be made by the plant cell and must be inherited by each daughter cell during cell division.
The word chloroplast is derived from the Greek words chloros (χλωρός), which means green, and plastes (πλάστης), which means "the one who forms".
The first definitive description of a chloroplast (Chlorophyllkörnen, "grain of chlorophyll") was given by Hugo von Mohl in 1837 as discrete bodies within the green plant cell.
In 1883, Andreas Franz Wilhelm Schimper would name these bodies as "chloroplastids" (Chloroplastiden).
In 1884, Eduard Strasburger adopted the term "chloroplasts" (Chloroplasten).
Lineages and evolution
Chloroplasts are one of many types of organelles in the plant cell.
This origin of chloroplasts was first suggested by the Russian biologist Konstantin Mereschkowski in 1905 after Andreas Franz Wilhelm Schimper observed in 1883 that chloroplasts closely resemble cyanobacteria.
Parent group: Cyanobacteria
Main article: Cyanobacteria
Chloroplasts are considered endosymbiotic Cyanobacteria.
Cyanobacteria also contain a peptidoglycan cell wall, which is thicker than in other gram-negative bacteria, and which is located between their two cell membranes.
Like chloroplasts, they have thylakoids within.
Phycobilins are also common cyanobacterial pigments, usually organized into hemispherical phycobilisomes attached to the outside of the thylakoid membranes (phycobilins are not shared with all chloroplasts though).
Somewhere around 1 to 2 billion years ago, a free-living cyanobacterium entered an early eukaryotic cell, either as food or as an internal parasite, but managed to escape the phagocytic vacuole it was contained in.
The two innermost lipid-bilayer membranes that surround all chloroplasts correspond to the outer and inner membranes of the ancestral cyanobacterium's gram negative cell wall, and not the phagosomal membrane from the host, which was probably lost.
The new cellular resident quickly became an advantage, providing food for the eukaryotic host, which allowed it to live within it.
Over time, the cyanobacterium was assimilated, and many of its genes were lost or transferred to the nucleus of the host.
From genomes that probably originally contained over 3000 genes only about 130 genes remain in the chloroplasts of contemporary plants.
Some of its proteins were then synthesized in the cytoplasm of the host cell, and imported back into the chloroplast (formerly the cyanobacterium).
This event is called endosymbiosis, or "cell living inside another cell with a mutual benefit for both".
The external cell is commonly referred to as the host while the internal cell is called the endosymbiont.
This is called serial endosymbiosis—an early eukaryote engulfing the mitochondrion ancestor, and some descendants of it then engulfing the chloroplast ancestor, creating a cell with both chloroplasts and mitochondria.
Whether or not primary chloroplasts came from a single endosymbiotic event, or many independent engulfments across various eukaryotic lineages, has long been debated.
It has been proposed this the closest living relative of this bacterium is Gloeomargarita lithophora.
All primary chloroplasts belong to one of four chloroplast lineages—the glaucophyte chloroplast lineage, the amoeboid Paulinella chromatophora lineage, the rhodophyte (red algal) chloroplast lineage, or the chloroplastidan (green) chloroplast lineage.
The glaucophyte chloroplast group is the smallest of the three primary chloroplast lineages, being found in only 13 species, and is thought to be the one that branched off the earliest.
For this reason, glaucophyte chloroplasts are also known as 'muroplasts' (besides 'cyanoplasts' or 'cyanelles').
Glaucophyte chloroplasts also contain concentric unstacked thylakoids, which surround a carboxysome – an icosahedral structure that glaucophyte chloroplasts and cyanobacteria keep their carbon fixation enzyme RuBisCO in.
The starch that they synthesize collects outside the chloroplast.
Like cyanobacteria, glaucophyte and rhodophyte chloroplast thylakoids are studded with light collecting structures called phycobilisomes.
Rhodophyceae (red algae)
Rhodophyte chloroplasts are also called rhodoplasts, literally "red chloroplasts".
Some contain pyrenoids.
However, since they also contain the blue-green chlorophyll a and other pigments, many are reddish to purple from the combination.
The red phycoerytherin pigment is an adaptation to help red algae catch more sunlight in deep water—as such, some red algae that live in shallow water have less phycoerythrin in their rhodoplasts, and can appear more greenish.
Rhodoplasts synthesize a form of starch called floridean starch, which collects into granules outside the rhodoplast, in the cytoplasm of the red alga.
Chloroplastida (green algae and plants)
Chloroplastida chloroplasts have lost the peptidoglycan wall between their double membrane, leaving an intermembrane space.
Most of the chloroplasts depicted in this article are green chloroplasts.
Green algae and plants keep their starch inside their chloroplasts, and in plants and some algae, the chloroplast thylakoids are arranged in grana stacks.
Some green algal chloroplasts contain a structure called a pyrenoid, which is functionally similar to the glaucophyte carboxysome in that it is where RuBisCO and CO2 are concentrated in the chloroplast.
Helicosporidium is a genus of nonphotosynthetic parasitic green algae that is thought to contain a vestigial chloroplast.
Genes from a chloroplast and nuclear genes indicating the presence of a chloroplast have been found in Helicosporidium even if nobody's seen the chloroplast itself.
While most chloroplasts originate from that first set of endosymbiotic events, Paulinella chromatophora is an exception that acquired a photosynthetic cyanobacterial endosymbiont more recently.
It is not clear whether that symbiont is closely related to the ancestral chloroplast of other eukaryotes.
Being in the early stages of endosymbiosis, Paulinella chromatophora can offer some insights into how chloroplasts evolved.
Paulinella cells contain one or two sausage shaped blue-green photosynthesizing structures called chromatophores, descended from the cyanobacterium Synechococcus.
Chromatophores cannot survive outside their host.
Chromatophore DNA is about a million base pairs long, containing around 850 protein encoding genes—far less than the three million base pair Synechococcus genome, but much larger than the approximately 150,000 base pair genome of the more assimilated chloroplast.
Chromatophores have transferred much less of their DNA to the nucleus of their host.
About 0.3–0.8% of the nuclear DNA in Paulinella is from the chromatophore, compared with 11–14% from the chloroplast in plants.
Secondary and tertiary endosymbiosis
Many other organisms obtained chloroplasts from the primary chloroplast lineages through secondary endosymbiosis—engulfing a red or green alga that contained a chloroplast.
These chloroplasts are known as secondary plastids.
While primary chloroplasts have a double membrane from their cyanobacterial ancestor, secondary chloroplasts have additional membranes outside of the original two, as a result of the secondary endosymbiotic event, when a nonphotosynthetic eukaryote engulfed a chloroplast-containing alga but failed to digest it—much like the cyanobacterium at the beginning of this story.
The engulfed alga was broken down, leaving only its chloroplast, and sometimes its cell membrane and nucleus, forming a chloroplast with three or four membranes—the two cyanobacterial membranes, sometimes the eaten alga's cell membrane, and the phagosomal vacuole from the host's cell membrane.
The genes in the phagocytosed eukaryote's nucleus are often transferred to the secondary host's nucleus.
All secondary chloroplasts come from green and red algae—no secondary chloroplasts from glaucophytes have been observed, probably because glaucophytes are relatively rare in nature, making them less likely to have been taken up by another eukaryote.
Green algal derived chloroplasts
Green algae have been taken up by the euglenids, chlorarachniophytes, a lineage of dinoflagellates, and possibly the ancestor of the CASH lineage (cryptomonads, alveolates, stramenopiles and haptophytes) in three or four separate engulfments.
Many green algal derived chloroplasts contain pyrenoids, but unlike chloroplasts in their green algal ancestors, storage product collects in granules outside the chloroplast.
Euglenophyte chloroplasts have three membranes—it is thought that the membrane of the primary endosymbiont was lost, leaving the cyanobacterial membranes, and the secondary host's phagosomal membrane.
Photosynthetic product is stored in the form of paramylon, which is contained in membrane-bound granules in the cytoplasm of the euglenophyte.
Chlorarachniophytes /ˌklɔːrəˈræknioʊˌfaɪts/ are a rare group of organisms that also contain chloroplasts derived from green algae, though their story is more complicated than that of the euglenophytes.
The ancestor of chlorarachniophytes is thought to have been a eukaryote with a red algal derived chloroplast.
It is then thought to have lost its first red algal chloroplast, and later engulfed a green alga, giving it its second, green algal derived chloroplast.
Chlorarachniophyte chloroplasts are bounded by four membranes, except near the cell membrane, where the chloroplast membranes fuse into a double membrane.
Their thylakoids are arranged in loose stacks of three.
Chlorarachniophyte chloroplasts are notable because the green alga they are derived from has not been completely broken down—its nucleus still persists as a nucleomorph found between the second and third chloroplast membranes—the periplastid space, which corresponds to the green alga's cytoplasm.
Prasinophyte-derived dinophyte chloroplast
Lepidodinium viride and its close relatives are dinophytes (see below) that lost their original peridinin chloroplast and replaced it with a green algal derived chloroplast (more specifically, a prasinophyte).
The chloroplast is surrounded by two membranes and has no nucleomorph—all the nucleomorph genes have been transferred to the dinophyte nucleus.
The endosymbiotic event that led to this chloroplast was serial secondary endosymbiosis rather than tertiary endosymbiosis—the endosymbiont was a green alga containing a primary chloroplast (making a secondary chloroplast).
Red algal derived chloroplasts
Cryptophytes, or cryptomonads are a group of algae that contain a red-algal derived chloroplast.
Cryptophyte chloroplasts contain a nucleomorph that superficially resembles that of the chlorarachniophytes.
Cryptophyte chloroplasts have four membranes, the outermost of which is continuous with the rough endoplasmic reticulum.
Cryptophytes may have played a key role in the spreading of red algal based chloroplasts.
Haptophytes are similar and closely related to cryptophytes or heterokontophytes.
Their chloroplasts lack a nucleomorph, their thylakoids are in stacks of three, and they synthesize chrysolaminarin sugar, which they store completely outside of the chloroplast, in the cytoplasm of the haptophyte.
The heterokontophytes, also known as the stramenopiles, are a very large and diverse group of eukaryotes.
Heterokont chloroplasts are very similar to haptophyte chloroplasts, containing a pyrenoid, triplet thylakoids, and with some exceptions, having four layer plastidic envelope, the outermost epiplastid membrane connected to the endoplasmic reticulum.
Like haptophytes, heterokontophytes store sugar in chrysolaminarin granules in the cytoplasm.
Apicomplexans, chromerids, and dinophytes
The alveolates are a major clade of unicellular eukaryotes of both autotrophic and heterotrophic members.
The most notable shared characteristic is the presence of cortical (outer-region) alveoli (sacs).
These are flattened vesicles (sacs) packed into a continuous layer just under the membrane and supporting it, typically forming a flexible pellicle (thin skin).
In dinoflagellates they often form armor plates.
Many members contain a red-algal derived plastid.
One notable characteristic of this diverse group is the frequent loss of photosynthesis.
However, a majority of these heterotrophs continue to process a non-photosynthetic plastid.
Apicomplexans are a group of alveolates.
Like the helicosproidia, they're parasitic, and have a nonphotosynthetic chloroplast.
They were once thought to be related to the helicosproidia, but it is now known that the helicosproida are green algae rather than part of the CASH lineage.
Other apicomplexans like Cryptosporidium have lost the chloroplast completely.
Apicomplexans store their energy in amylopectin granules that are located in their cytoplasm, even though they are nonphotosynthetic.
Apicoplasts have lost all photosynthetic function, and contain no photosynthetic pigments or true thylakoids.
They are bounded by four membranes, but the membranes are not connected to the endoplasmic reticulum.
The fact that apicomplexans still keep their nonphotosynthetic chloroplast around demonstrates how the chloroplast carries out important functions other than photosynthesis.
Plant chloroplasts provide plant cells with many important things besides sugar, and apicoplasts are no different—they synthesize fatty acids, isopentenyl pyrophosphate, iron-sulfur clusters, and carry out part of the heme pathway.
This makes the apicoplast an attractive target for drugs to cure apicomplexan-related diseases.
The most important apicoplast function is isopentenyl pyrophosphate synthesis—in fact, apicomplexans die when something interferes with this apicoplast function, and when apicomplexans are grown in an isopentenyl pyrophosphate-rich medium, they dump the organelle.
The Chromerida is a newly discovered group of algae from Australian corals which comprises some close photosynthetic relatives of the apicomplexans.
The first member, Chromera velia, was discovered and first isolated in 2001.
The discovery of Chromera velia with similar structure to the apicomplexanss, provides an important link in the evolutionary history of the apicomplexans and dinophytes.
Their plastids have four membranes, lack chlorophyll c and use the type II form of RuBisCO obtained from a horizontal transfer event.
Most dinophyte chloroplasts are secondary red algal derived chloroplasts.
Many other dinophytes have lost the chloroplast (becoming the nonphotosynthetic kind of dinoflagellate), or replaced it though tertiary endosymbiosis—the engulfment of another eukaryotic algae containing a red algal derived chloroplast.
Others replaced their original chloroplast with a green algal derived one.
Most dinophyte chloroplasts contain form II RuBisCO, at least the photosynthetic pigments chlorophyll a, chlorophyll c2, beta-carotene, and at least one dinophyte-unique xanthophyll (peridinin, dinoxanthin, or diadinoxanthin), giving many a golden-brown color.
All dinophytes store starch in their cytoplasm, and most have chloroplasts with thylakoids arranged in stacks of three.
Peridinin is not found in any other group of chloroplasts.
The peridinin chloroplast is bounded by three membranes (occasionally two), having lost the red algal endosymbiont's original cell membrane.
The outermost membrane is not connected to the endoplasmic reticulum.
They contain a pyrenoid, and have triplet-stacked thylakoids.
Starch is found outside the chloroplast.
Most of the genome has migrated to the nucleus, and only critical photosynthesis-related genes remain in the chloroplast.
The peridinin chloroplast is thought to be the dinophytes' "original" chloroplast, which has been lost, reduced, replaced, or has company in several other dinophyte lineages.
Fucoxanthin-containing (haptophyte-derived) dinophyte chloroplasts
The fucoxanthin dinophyte lineages (including Karlodinium and Karenia) lost their original red algal derived chloroplast, and replaced it with a new chloroplast derived from a haptophyte endosymbiont.
Because the haptophyte chloroplast has four membranes, tertiary endosymbiosis would be expected to create a six membraned chloroplast, adding the haptophyte's cell membrane and the dinophyte's phagosomal vacuole.
However, the haptophyte was heavily reduced, stripped of a few membranes and its nucleus, leaving only its chloroplast (with its original double membrane), and possibly one or two additional membranes around it.
Fucoxanthin is also found in haptophyte chloroplasts, providing evidence of ancestry.
Diatom-derived dinophyte chloroplasts
These chloroplasts are bounded by up to five membranes, (depending on whether the entire diatom endosymbiont is counted as the chloroplast, or just the red algal derived chloroplast inside it).
The diatom endosymbiont has been reduced relatively little—it still retains its original mitochondria, and has endoplasmic reticulum, ribosomes, a nucleus, and of course, red algal derived chloroplasts—practically a complete cell, all inside the host's endoplasmic reticulum lumen.
However the diatom endosymbiont can't store its own food—its storage polysaccharide is found in granules in the dinophyte host's cytoplasm instead.
Diatoms have been engulfed by dinoflagellates at least three times.
The diatom endosymbiont is bounded by a single membrane, inside it are chloroplasts with four membranes.
Like the diatom endosymbiont's diatom ancestor, the chloroplasts have triplet thylakoids and pyrenoids.
In some of these genera, the diatom endosymbiont's chloroplasts aren't the only chloroplasts in the dinophyte.
The original three-membraned peridinin chloroplast is still around, converted to an eyespot.
Main article: Kleptoplastidy
These klepto chloroplasts may only have a lifetime of a few days and are then replaced.
Cryptophyte-derived dinophyte chloroplast
However, the cryptophyte is not an endosymbiont—only the chloroplast seems to have been taken, and the chloroplast has been stripped of its nucleomorph and outermost two membranes, leaving just a two-membraned chloroplast.
Cryptophyte chloroplasts require their nucleomorph to maintain themselves, and Dinophysis species grown in cell culture alone cannot survive, so it is possible (but not confirmed) that the Dinophysis chloroplast is a kleptoplast—if so, Dinophysis chloroplasts wear out and Dinophysis species must continually engulf cryptophytes to obtain new chloroplasts to replace the old ones.
Main article: Chloroplast DNA
See also: List of sequenced plastomes
Chloroplasts have their own DNA, often abbreviated as ctDNA, or cpDNA.
It is also known as the plastome.
Since then, hundreds of chloroplast DNAs from various species have been sequenced, but they are mostly those of land plants and green algae—glaucophytes, red algae, and other algal groups are extremely underrepresented, potentially introducing some bias in views of "typical" chloroplast DNA structure and content.
With few exceptions, most chloroplasts have their entire chloroplast genome combined into a single large circular DNA molecule, typically 120,000–170,000 base pairs long.
They can have a contour length of around 30–60 micrometers, and have a mass of about 80–130 million daltons.
While usually thought of as a circular molecule, there is some evidence that chloroplast DNA molecules more often take on a linear shape.
Many chloroplast DNAs contain two inverted repeats, which separate a long single copy section (LSC) from a short single copy section (SSC).
While a given pair of inverted repeats are rarely completely identical, they are always very similar to each other, apparently resulting from concerted evolution.
The inverted repeats vary wildly in length, ranging from 4,000 to 25,000 base pairs long each and containing as few as four or as many as over 150 genes.
Inverted repeats in plants tend to be at the upper end of this range, each being 20,000–25,000 base pairs long.
The inverted repeat regions are highly conserved among land plants, and accumulate few mutations.
Similar inverted repeats exist in the genomes of cyanobacteria and the other two chloroplast lineages (glaucophyta and rhodophyceae), suggesting that they predate the chloroplast, though some chloroplast DNAs have since lost or flipped the inverted repeats (making them direct repeats).
It is possible that the inverted repeats help stabilize the rest of the chloroplast genome, as chloroplast DNAs which have lost some of the inverted repeat segments tend to get rearranged more.
New chloroplasts may contain up to 100 copies of their DNA, though the number of chloroplast DNA copies decreases to about 15–20 as the chloroplasts age.
They are usually packed into nucleoids, which can contain several identical chloroplast DNA rings.
Many nucleoids can be found in each chloroplast.
The leading model of cpDNA replication
The mechanism for chloroplast DNA (cpDNA) replication has not been conclusively determined, but two main models have been proposed.
Scientists have attempted to observe chloroplast replication via electron microscopy since the 1970s.
The results of the microscopy experiments led to the idea that chloroplast DNA replicates using a double displacement loop (D-loop).
As the D-loop moves through the circular DNA, it adopts a theta intermediary form, also known as a Cairns replication intermediate, and completes replication with a rolling circle mechanism.
Transcription starts at specific points of origin.
Multiple replication forks open up, allowing replication machinery to transcribe the DNA.
As replication continues, the forks grow and eventually converge.
The new cpDNA structures separate, creating daughter cpDNA chromosomes.
In addition to the early microscopy experiments, this model is also supported by the amounts of deamination seen in cpDNA.
Deamination occurs when an amino group is lost and is a mutation that often results in base changes.
When adenine is deaminated, it becomes hypoxanthine.
Hypoxanthine can bind to cytosine, and when the XC base pair is replicated, it becomes a GC (thus, an A → G base change).
In cpDNA, there are several A → G deamination gradients.
DNA becomes susceptible to deamination events when it is single stranded.
When replication forks form, the strand not being copied is single stranded, and thus at risk for A → G deamination.
Therefore, gradients in deamination indicate that replication forks were most likely present and the direction that they initially opened (the highest gradient is most likely nearest the start site because it was single stranded for the longest amount of time).
This mechanism is still the leading theory today; however, a second theory suggests that most cpDNA is actually linear and replicates through homologous recombination.
It further contends that only a minority of the genetic material is kept in circular chromosomes while the rest is in branched, linear, or other complex structures.
Alternative model of replication
One of competing model for cpDNA replication asserts that most cpDNA is linear and participates in homologous recombination and replication structures similar to the linear and circular DNA structures of bacteriophage T4.
It has been established that some plants have linear cpDNA, such as maize, and that more species still contain complex structures that scientists do not yet understand.
When the original experiments on cpDNA were performed, scientists did notice linear structures; however, they attributed these linear forms to broken circles.
If the branched and complex structures seen in cpDNA experiments are real and not artifacts of concatenated circular DNA or broken circles, then a D-loop mechanism of replication is insufficient to explain how those structures would replicate.
At the same time, homologous recombination does not expand the multiple A --> G gradients seen in plastomes.
Because of the failure to explain the deamination gradient as well as the numerous plant species that have been shown to have circular cpDNA, the predominant theory continues to hold that most cpDNA is circular and most likely replicates via a D loop mechanism.
Gene content and protein synthesis
Among land plants, the contents of the chloroplast genome are fairly similar.
Chloroplast genome reduction and gene transfer
As a result, the chloroplast genome is heavily reduced compared to that of free-living cyanobacteria.
Chloroplasts may contain 60–100 genes whereas cyanobacteria often have more than 1500 genes in their genome.
Recently, a plastid without a genome was found, demonstrating chloroplasts can lose their genome during endosymbiotic the gene transfer process.
Endosymbiotic gene transfer is how we know about the lost chloroplasts in many CASH lineages.
Even if a chloroplast is eventually lost, the genes it donated to the former host's nucleus persist, providing evidence for the lost chloroplast's existence.
For example, while diatoms (a heterokontophyte) now have a red algal derived chloroplast, the presence of many green algal genes in the diatom nucleus provide evidence that the diatom ancestor had a green algal derived chloroplast at some point, which was subsequently replaced by the red chloroplast.
In land plants, some 11–14% of the DNA in their nuclei can be traced back to the chloroplast, up to 18% in Arabidopsis, corresponding to about 4,500 protein-coding genes.
There have been a few recent transfers of genes from the chloroplast DNA to the nuclear genome in land plants.
Of the approximately 3000 proteins found in chloroplasts, some 95% of them are encoded by nuclear genes.
Many of the chloroplast's protein complexes consist of subunits from both the chloroplast genome and the host's nuclear genome.
As a result, protein synthesis must be coordinated between the chloroplast and the nucleus.
Protein synthesis within chloroplasts relies on two RNA polymerases.
One is coded by the chloroplast DNA, the other is of nuclear origin.
The two RNA polymerases may recognize and bind to different kinds of promoters within the chloroplast genome.
The ribosomes in chloroplasts are similar to bacterial ribosomes.
Protein targeting and import
See also: Translation
These proteins must be directed back to the chloroplast, and imported through at least two chloroplast membranes.
Curiously, around half of the protein products of transferred genes aren't even targeted back to the chloroplast.
Some transferred chloroplast DNA protein products get directed to the secretory pathway, though many secondary plastids are bounded by an outermost membrane derived from the host's cell membrane, and therefore topologically outside of the cell because to reach the chloroplast from the cytosol, the cell membrane must be crossed, which signifies entrance into the extracellular space.
In those cases, chloroplast-targeted proteins do initially travel along the secretory pathway.
Because the cell acquiring a chloroplast already had mitochondria (and peroxisomes, and a cell membrane for secretion), the new chloroplast host had to develop a unique protein targeting system to avoid having chloroplast proteins being sent to the wrong organelle.
Sometimes the transit sequence is found on the C-terminus of the protein, or within the functional part of the protein.
Transport proteins and membrane translocons
After a chloroplast polypeptide is synthesized on a ribosome in the cytosol, an enzyme specific to chloroplast proteins phosphorylates, or adds a phosphate group to many (but not all) of them in their transit sequences.
Phosphorylation helps many proteins bind the polypeptide, keeping it from folding prematurely.
This is important because it prevents chloroplast proteins from assuming their active form and carrying out their chloroplast functions in the wrong place—the cytosol.
At the same time, they have to keep just enough shape so that they can be recognized by the chloroplast.
These proteins also help the polypeptide get imported into the chloroplast.
From here, chloroplast proteins bound for the stroma must pass through two protein complexes—the TOC complex, or translocon on the outer chloroplast membrane, and the TIC translocon, or translocon on the inner chloroplast membrane translocon.
Chloroplast polypeptide chains probably often travel through the two complexes at the same time, but the TIC complex can also retrieve preproteins lost in the intermembrane space.
In land plants, chloroplasts are generally lens-shaped, 3–10 μm in diameter and 1–3 μm thick.
Corn seedling chloroplasts are ≈20 µm in volume.
Greater diversity in chloroplast shapes exists among the algae, which often contain a single chloroplast that can be shaped like a net (e.g., Oedogonium), a cup (e.g., Chlamydomonas), a ribbon-like spiral around the edges of the cell (e.g., Spirogyra), or slightly twisted bands at the cell edges (e.g., Sirogonium).
In some algae, the chloroplast takes up most of the cell, with pockets for the nucleus and other organelles, for example, some species of Chlorella have a cup-shaped chloroplast that occupies much of the cell.
All chloroplasts have at least three membrane systems—the outer chloroplast membrane, the inner chloroplast membrane, and the thylakoid system.
Chloroplasts that are the product of secondary endosymbiosis may have additional membranes surrounding these three.
Inside the outer and inner chloroplast membranes is the chloroplast stroma, a semi-gel-like fluid that makes up much of a chloroplast's volume, and in which the thylakoid system floats.
See also: Chloroplast membrane
There are some common misconceptions about the outer and inner chloroplast membranes.
The fact that chloroplasts are surrounded by a double membrane is often cited as evidence that they are the descendants of endosymbiotic cyanobacteria.
This is often interpreted as meaning the outer chloroplast membrane is the product of the host's cell membrane infolding to form a vesicle to surround the ancestral cyanobacterium—which is not true—both chloroplast membranes are homologous to the cyanobacterium's original double membranes.
The chloroplast double membrane is also often compared to the mitochondrial double membrane.
The only chloroplast structure that can considered analogous to it is the internal thylakoid system.
Even so, in terms of "in-out", the direction of chloroplast H ion flow is in the opposite direction compared to oxidative phosphorylation in mitochondria.
In addition, in terms of function, the inner chloroplast membrane, which regulates metabolite passage and synthesizes some materials, has no counterpart in the mitochondrion.
Outer chloroplast membrane
Main article: Chloroplast membrane
The outer chloroplast membrane is a semi-porous membrane that small molecules and ions can easily diffuse across.
However, it is not permeable to larger proteins, so chloroplast polypeptides being synthesized in the cell cytoplasm must be transported across the outer chloroplast membrane by the TOC complex, or translocon on the outer chloroplast membrane.
When they were first observed in 1962, some plant biologists dismissed the structures as artifactual, claiming that stromules were just oddly shaped chloroplasts with constricted regions or dividing chloroplasts.
However, there is a growing body of evidence that stromules are functional, integral features of plant cell plastids, not merely artifacts.
Intermembrane space and peptidoglycan wall
Usually, a thin intermembrane space about 10–20 nanometers thick exists between the outer and inner chloroplast membranes.
These chloroplasts are called muroplasts (from Latin "mura", meaning "wall").
Other chloroplasts have lost the cyanobacterial wall, leaving an intermembrane space between the two chloroplast envelope membranes.
Inner chloroplast membrane
Main article: Chloroplast membrane
The inner chloroplast membrane borders the stroma and regulates passage of materials in and out of the chloroplast.
After passing through the TOC complex in the outer chloroplast membrane, polypeptides must pass through the TIC complex (translocon on the inner chloroplast membrane) which is located in the inner chloroplast membrane.
Some chloroplasts contain a structure called the chloroplast peripheral reticulum.
The chloroplast peripheral reticulum consists of a maze of membranous tubes and vesicles continuous with the inner chloroplast membrane that extends into the internal stromal fluid of the chloroplast.
Main article: Stroma
The protein-rich, alkaline, aqueous fluid within the inner chloroplast membrane and outside of the thylakoid space is called the stroma, which corresponds to the cytosol of the original cyanobacterium.
Chloroplasts have their own ribosomes, which they use to synthesize a small fraction of their proteins.
While similar to bacterial ribosomes, chloroplast translation is more complex than in bacteria, so chloroplast ribosomes include some chloroplast-unique features.
Small subunit ribosomal RNAs in several Chlorophyta and euglenid chloroplasts lack motifs for shine-dalgarno sequence recognition, which is considered essential for translation initiation in most chloroplasts and prokaryotes.
Such loss is also rarely observed in other plastids and prokaryotes.
They are surrounded by a lipid monolayer.
Plastoglobuli also exhibit a greater size variation under these conditions.
They are also common in etioplasts, but decrease in number as the etioplasts mature into chloroplasts.
Plastoglobuli were once thought to be free-floating in the stroma, but it is now thought that they are permanently attached either to a thylakoid or to another plastoglobulus attached to a thylakoid, a configuration that allows a plastoglobulus to exchange its contents with the thylakoid network.
In normal green chloroplasts, the vast majority of plastoglobuli occur singularly, attached directly to their parent thylakoid.
In old or stressed chloroplasts, plastoglobuli tend to occur in linked groups or chains, still always anchored to a thylakoid.
Plastoglobuli form when a bubble appears between the layers of the lipid bilayer of the thylakoid membrane, or bud from existing plastoglubuli—though they never detach and float off into the stroma.
Practically all plastoglobuli form on or near the highly curved edges of the thylakoid disks or sheets.
They are also more common on stromal thylakoids than on granal ones.
Starch granules are very common in chloroplasts, typically taking up 15% of the organelle's volume, though in some other plastids like amyloplasts, they can be big enough to distort the shape of the organelle.
Starch granules are simply accumulations of starch in the stroma, and are not bounded by a membrane.
Starch granules appear and grow throughout the day, as the chloroplast synthesizes sugars, and are consumed at night to fuel respiration and continue sugar export into the phloem, though in mature chloroplasts, it is rare for a starch granule to be completely consumed or for a new granule to accumulate.
Starch granules vary in composition and location across different chloroplast lineages.
Main article: RuBisCO
The chloroplast stroma contains many proteins, though the most common and important is RuBisCO, which is probably also the most abundant protein on the planet.
Main article: Pyrenoid
They are not found in higher plants.
Pyrenoids are roughly spherical and highly refractive bodies which are a site of starch accumulation in plants that contain them.
They consist of a matrix opaque to electrons, surrounded by two hemispherical starch plates.
The starch is accumulated as the pyrenoids mature.
Starch can also accumulate around the pyrenoids when CO2 is scarce.
Pyrenoids can divide to form new pyrenoids, or be produced "de novo".
Main article: Thylakoid
Thylakoids (sometimes spelled thylakoïds), are small interconnected sacks which contain the membranes that the light reactions of photosynthesis take place on.
The word thylakoid comes from the Greek word thylakos which means "sack".
Suspended within the chloroplast stroma is the thylakoid system, a highly dynamic collection of membranous sacks called thylakoids where chlorophyll is found and the light reactions of photosynthesis happen.
With electron microscopy, it became possible to see the thylakoid system in more detail, revealing it to consist of stacks of flat thylakoids which made up the grana, and long interconnecting stromal thylakoids which linked different grana.
In the transmission electron microscope, thylakoid membranes appear as alternating light-and-dark bands, 8.5 nanometers thick.
For a long time, the three-dimensional structure of the thylakoid membrane system had been unknown or disputed.
Many models have been proposed, the most prevalent being the helical model, in which granum stacks of thylakoids are wrapped by helical stromal thylakoids.
Another model known as the 'bifurcation model', which was based on the first electron tomography study of plant thylakoid membranes, depicts the stromal membranes as wide lamellar sheets perpendicular to the grana columns which bifurcates into multiple parallel discs forming the granum-stroma assembly.
The helical model was supported by several additional works, but ultimately it was determined in 2019 that features from both the helical and bifurcation models are consolidated by newly-discovered left-handed helical membrane junctions.
Likely for ease, the thylakoid system is still commonly depicted by older "hub and spoke" models where the grana are connected to each other by tubes of stromal thylakoids.
Grana consist of a stacks of flattened circular granal thylakoids that resemble pancakes.
Each granum can contain anywhere from two to a hundred thylakoids, though grana with 10–20 thylakoids are most common.
Wrapped around the grana are multiple parallel right-handed helical stromal thylakoids, also known as frets or lamellar thylakoids.
The helices ascend at an angle of ~20°, connecting to each granal thylakoid at a bridge-like slit junction.
The stroma lamellae extend as large sheets perpendicular to the grana columns.
These sheets are connected to the right-handed helices either directly or through bifurcations that form left-handed helical membrane surfaces.
The left-handed helical surfaces have a similar tilt angle to the right-handed helices (~20°), but ¼ the pitch.
Approximately 4 left-handed helical junctions are present per granum, resulting in a pitch-balanced array of right- and left-handed helical membrane surfaces of different radii and pitch that consolidate the network with minimal surface and bending energies.
While different parts of the thylakoid system contain different membrane proteins, the thylakoid membranes are continuous and the thylakoid space they enclose form a single continuous labyrinth.
ATP synthase is a large protein complex that harnesses the concentration gradient of the hydrogen ions in the thylakoid space to generate ATP energy as the hydrogen ions flow back out into the stroma—much like a dam turbine.
There are two types of thylakoids—granal thylakoids, which are arranged in grana, and stromal thylakoids, which are in contact with the stroma.
Granal thylakoids are pancake-shaped circular disks about 300–600 nanometers in diameter.
Stromal thylakoids are helicoid sheets that spiral around grana.
The flat tops and bottoms of granal thylakoids contain only the relatively flat photosystem II protein complex.
This allows them to stack tightly, forming grana with many layers of tightly appressed membrane, called granal membrane, increasing stability and surface area for light capture.
They can't fit in the appressed granal membranes, and so are found in the stromal thylakoid membrane—the edges of the granal thylakoid disks and the stromal thylakoids.
These large protein complexes may act as spacers between the sheets of stromal thylakoids.
The number of thylakoids and the total thylakoid area of a chloroplast is influenced by light exposure.
Shaded chloroplasts contain larger and more grana with more thylakoid membrane area than chloroplasts exposed to bright light, which have smaller and fewer grana and less thylakoid area.
Thylakoid extent can change within minutes of light exposure or removal.
Pigments and chloroplast colors
The types of pigments found are different in various groups of chloroplasts, and are responsible for a wide variety of chloroplast colorations.
Chlorophyll a is a blue-green pigment partially responsible for giving most cyanobacteria and chloroplasts their color.
Chlorophyll b is an olive green pigment found only in the chloroplasts of plants, green algae, any secondary chloroplasts obtained through the secondary endosymbiosis of a green alga, and a few cyanobacteria.
It is the chlorophylls a and b together that make most plant and green algal chloroplasts green.
There are about thirty photosynthetic carotenoids.
Many other forms of carotenoids exist that are only found in certain groups of chloroplasts.
Phycobilins come in all colors, though phycoerytherin is one of the pigments that makes many red algae red.
Phycobilins often organize into relatively large protein complexes about 40 nanometers across called phycobilisomes.
Cryptophyte chloroplasts and some cyanobacteria don't have their phycobilin pigments organized into phycobilisomes, and keep them in their thylakoid space instead.
Specialized chloroplasts in C4 plants
This has the end result of ATP energy being wasted and CO 2 being released, all with no sugar being produced.
C4 plants evolved a way to solve this—by spatially separating the light reactions and the Calvin cycle.
As a result, chloroplasts in C4 mesophyll cells and bundle sheath cells are specialized for each stage of photosynthesis.
They store CO 2 in a four-carbon compound, which is why the process is called C4 photosynthesis.
The four-carbon compound is then transported to the bundle sheath chloroplasts, where it drops off CO 2 and returns to the mesophyll.
Bundle sheath chloroplasts do not carry out the light reactions, preventing oxygen from building up in them and disrupting RuBisCO activity.
Because of this, they lack thylakoids organized into grana stacks—though bundle sheath chloroplasts still have free-floating thylakoids in the stroma where they still carry out cyclic electron flow, a light-driven method of synthesizing ATP to power the Calvin cycle without generating oxygen.
Because the job of bundle sheath chloroplasts is to carry out the Calvin cycle and make sugar, they often contain large starch grains.
Mesophyll chloroplasts have a little more peripheral reticulum than bundle sheath chloroplasts.
Distribution in a plant
Not all cells in a multicellular plant contain chloroplasts.
All green parts of a plant contain chloroplasts—the chloroplasts, or more specifically, the chlorophyll in them are what make the photosynthetic parts of a plant green.
A plant cell which contains chloroplasts is known as a chlorenchyma cell.
A typical chlorenchyma cell of a land plant contains about 10 to 100 chloroplasts.
One square millimeter of leaf tissue can contain half a million chloroplasts.
They are often absent from the epidermis of a leaf.
See also: Cytoplasmic streaming
The chloroplasts of plant and algal cells can orient themselves to best suit the available light.
In low-light conditions, they will spread out in a sheet—maximizing the surface area to absorb light.
Under intense light, they will seek shelter by aligning in vertical columns along the plant cell's cell wall or turning sideways so that light strikes them edge-on.
This reduces exposure and protects them from photooxidative damage.
This ability to distribute chloroplasts so that they can take shelter behind each other or spread out may be the reason why land plants evolved to have many small chloroplasts instead of a few big ones.
Chloroplast movement is considered one of the most closely regulated stimulus-response systems that can be found in plants.
Mitochondria have also been observed to follow chloroplasts as they move.
In some algae, mosses, ferns, and flowering plants, chloroplast movement is influenced by red light in addition to blue light, though very long red wavelengths inhibit movement rather than speeding it up.
Blue light generally causes chloroplasts to seek shelter, while red light draws them out to maximize light absorption.
Studies of Vallisneria gigantea, an aquatic flowering plant, have shown that chloroplasts can get moving within five minutes of light exposure, though they don't initially show any net directionality.
They may move along microfilament tracks, and the fact that the microfilament mesh changes shape to form a honeycomb structure surrounding the chloroplasts after they have moved suggests that microfilaments may help to anchor chloroplasts in place.
Function and chemistry
Guard cell chloroplasts
However, exactly what they do is controversial.
Plant innate immunity
Due to its role in a plant cell's immune response, pathogens frequently target the chloroplast.
Plants have two main immune responses—the hypersensitive response, in which infected cells seal themselves off and undergo programmed cell death, and systemic acquired resistance, where infected cells release signals warning the rest of the plant of a pathogen's presence.
Chloroplasts stimulate both responses by purposely damaging their photosynthetic system, producing reactive oxygen species.
High levels of reactive oxygen species will cause the hypersensitive response.
The reactive oxygen species also directly kill any pathogens within the cell.
Lower levels of reactive oxygen species initiate systemic acquired resistance, triggering defense-molecule production in the rest of the plant.
In some plants, chloroplasts are known to move closer to the infection site and the nucleus during an infection.
Chloroplasts can serve as cellular sensors.
After detecting stress in a cell, which might be due to a pathogen, chloroplasts begin producing molecules like salicylic acid, jasmonic acid, nitric oxide and reactive oxygen species which can serve as defense-signals.
As cellular signals, reactive oxygen species are unstable molecules, so they probably don't leave the chloroplast, but instead pass on their signal to an unknown second messenger molecule.
Main article: Photosynthesis
Main article: Light reactions
The light reactions take place on the thylakoid membranes.
ATP is the phosphorylated version of adenosine diphosphate (ADP), which stores energy in a cell and powers most cellular activities.
ATP is the energized form, while ADP is the (partially) depleted form.
NADP is an electron carrier which ferries high energy electrons.
Main article: Photophosphorylation
The molecules between the photosystems harness the electrons' energy to pump hydrogen ions into the thylakoid space, creating a concentration gradient, with more hydrogen ions (up to a thousand times as many) inside the thylakoid system than in the stroma.
Because chloroplast ATP synthase projects out into the stroma, the ATP is synthesized there, in position to be used in the dark reactions.
See also: Redox reaction
Like ATP synthase, ferredoxin-NADP reductase, the enzyme that reduces NADP, releases the NADPH it makes into the stroma, right where it is needed for the dark reactions.
Because NADP reduction removes electrons from the electron transport chains, they must be replaced—the job of photosystem II, which splits water molecules (H2O) to obtain the electrons from its hydrogen atoms.
Main article: Cyclic photophosphorylation
Normally, the reenergized electrons are taken by NADP, though sometimes they can flow back down more H-pumping electron transport chains to transport more hydrogen ions into the thylakoid space to generate more ATP.
This is termed cyclic photophosphorylation because the electrons are recycled.
Main article: Dark reactions
The Calvin cycle, also known as the dark reactions, is a series of biochemical reactions that fixes CO2 into G3P sugar molecules and uses the energy and electrons from the ATP and NADPH made in the light reactions.
The Calvin cycle takes place in the stroma of the chloroplast.
While named "the dark reactions", in most plants, they take place in the light, since the dark reactions are dependent on the products of the light reactions.
Carbon fixation and G3P synthesis
The result is unstable six-carbon molecules that immediately break down into three-carbon molecules called 3-phosphoglyceric acid, or 3-PGA.
Most of the G3P molecules are recycled back into RuBP using energy from more ATP, but one out of every six produced leaves the cycle—the end product of the dark reactions.
Sugars and starches
These molecules are processed, and from them, the still larger sucrose, a disaccharide commonly known as table sugar, is made, though this process takes place outside of the chloroplast, in the cytoplasm.
Under conditions such as high atmospheric CO2 concentrations, these starch grains may grow very large, distorting the grana and thylakoids.
The starch granules displace the thylakoids, but leave them intact.
This depletes a plant's free phosphate supply, which indirectly stimulates chloroplast starch synthesis.
While linked to low photosynthesis rates, the starch grains themselves may not necessarily interfere significantly with the efficiency of photosynthesis, and might simply be a side effect of another photosynthesis-depressing factor.
Photorespiration can occur when the oxygen concentration is too high.
RuBisCO cannot distinguish between oxygen and carbon dioxide very well, so it can accidentally add O2 instead of CO2 to RuBP.
This process reduces the efficiency of photosynthesis—it consumes ATP and oxygen, releases CO2, and produces no sugar.
It can waste up to half the carbon fixed by the Calvin cycle.
Several mechanisms have evolved in different lineages that raise the carbon dioxide concentration relative to oxygen within the chloroplast, increasing the efficiency of photosynthesis.
These mechanisms are called carbon dioxide concentrating mechanisms, or CCMs.
Chloroplasts in C4 plants are notable as they exhibit a distinct chloroplast dimorphism.
The optimal stroma pH for the Calvin cycle is 8.1, with the reaction nearly stopping when the pH falls below 7.3.
However, chloroplasts in living plant cells are not affected by this as much.
Chloroplasts can pump K and H ions in and out of themselves using a poorly understood light-driven transport system.
In the presence of light, the pH of the thylakoid lumen can drop up to 1.5 pH units, while the pH of the stroma can rise by nearly one pH unit.
Amino acid synthesis
The chloroplast is known to make the precursors to methionine but it is unclear whether the organelle carries out the last leg of the pathway or if it happens in the cytosol.
Other nitrogen compounds
Other chemical products
The plastid is the site of diverse and complex lipid synthesis in plants.
Pyruvate may enter the plastid from the cytosol by passive diffusion through the membrane after production in glycolysis.
Pyruvate is also made in the plastid from phosphoenolpyruvate, a metabolite made in the cytosol from pyruvate or PGA.
Acetate in the cytosol is unavailable for lipid biosynthesis in the plastid.
The typical length of fatty acids produced in the plastid are 16 or 18 carbons, with 0-3 cis double bonds.
The biosynthesis of fatty acids from acetyl-CoA primarily requires two enzymes.
Acetyl-CoA carboxylase creates malonyl-CoA, used in both the first step and the extension steps of synthesis.
Fatty acid synthase (FAS) is a large complex of enzymes and cofactors including acyl carrier protein (ACP) which holds the acyl chain as it is synthesized.
The initiation of synthesis begins with the condensation of malonyl-ACP with acetyl-CoA to produce ketobutyryl-ACP.
2 reductions involving the use of NADPH and one dehydration creates butyryl-ACP.
Extension of the fatty acid comes from repeated cycles of malonyl-ACP condensation, reduction, and dehydration.
Differentiation, replication, and inheritance
Main article: Plastid
Chloroplasts are a special type of a plant cell organelle called a plastid, though the two terms are sometimes used interchangeably.
There are many other types of plastids, which carry out various functions.
All chloroplasts in a plant are descended from undifferentiated proplastids found in the zygote, or fertilized egg.
Proplastids are commonly found in an adult plant's apical meristems.
This process involves invaginations of the inner plastid membrane, forming sheets of membrane that project into the internal stroma.
While etioplasts lack chlorophyll, they have a yellow chlorophyll precursor stocked.
Within a few minutes of light exposure, the prolamellar body begins to reorganize into stacks of thylakoids, and chlorophyll starts to be produced.
This process, where the etioplast becomes a chloroplast, takes several hours.
Gymnosperms do not require light to form chloroplasts.
Light, however, does not guarantee that a proplastid will develop into a chloroplast.
Whether a proplastid develops into a chloroplast some other kind of plastid is mostly controlled by the nucleus and is largely influenced by the kind of cell it resides in.
Plastid differentiation is not permanent, in fact many interconversions are possible.
Starch storing amyloplasts can also be converted to chromoplasts, and it is possible for proplastids to develop straight into chromoplasts.
If a plant is injured, or something else causes a plant cell to revert to a meristematic state, chloroplasts and other plastids can turn back into proplastids.
Chloroplast, amyloplast, chromoplast, proplast, etc., are not absolute states—intermediate forms are common.
Most chloroplasts in a photosynthetic cell do not develop directly from proplastids or etioplasts.
These proplastids differentiate into chloroplasts, which divide to create the 30–70 chloroplasts found in a mature photosynthetic plant cell.
If the cell divides, chloroplast division provides the additional chloroplasts to partition between the two daughter cells.
In single-celled algae, chloroplast division is the only way new chloroplasts are formed.
There is no proplastid differentiation—when an algal cell divides, its chloroplast divides along with it, and each daughter cell receives a mature chloroplast.
Almost all chloroplasts in a cell divide, rather than a small group of rapidly dividing chloroplasts.
Chloroplasts have no definite S-phase—their DNA replication is not synchronized or limited to that of their host cells.
The Min system manages the placement of the Z-ring, ensuring that the chloroplast is cleaved more or less evenly.
The protein MinD prevents FtsZ from linking up and forming filaments.
Another protein ARC3 may also be involved, but it is not very well understood.
These proteins are active at the poles of the chloroplast, preventing Z-ring formation there, but near the center of the chloroplast, MinE inhibits them, allowing the Z-ring to form.
Next, the two plastid-dividing rings, or PD rings form.
The inner plastid-dividing ring is located in the inner side of the chloroplast's inner membrane, and is formed first.
The outer plastid-dividing ring is found wrapped around the outer chloroplast membrane.
It consists of filaments about 5 nanometers across, arranged in rows 6.4 nanometers apart, and shrinks to squeeze the chloroplast.
This is when chloroplast constriction begins.
In a few species like Cyanidioschyzon merolæ, chloroplasts have a third plastid-dividing ring located in the chloroplast's intermembrane space.
Late into the constriction phase, dynamin proteins assemble around the outer plastid-dividing ring, helping provide force to squeeze the chloroplast.
Meanwhile, the Z-ring and the inner plastid-dividing ring break down.
During this stage, the many chloroplast DNA plasmids floating around in the stroma are partitioned and distributed to the two forming daughter chloroplasts.
Later, the dynamins migrate under the outer plastid dividing ring, into direct contact with the chloroplast's outer membrane, to cleave the chloroplast in two daughter chloroplasts.
A remnant of the outer plastid dividing ring remains floating between the two daughter chloroplasts, and a remnant of the dynamin ring remains attached to one of the daughter chloroplasts.
Of the five or six rings involved in chloroplast division, only the outer plastid-dividing ring is present for the entire constriction and division phase—while the Z-ring forms first, constriction does not begin until the outer plastid-dividing ring forms.
In species of algae that contain a single chloroplast, regulation of chloroplast division is extremely important to ensure that each daughter cell receives a chloroplast—chloroplasts can't be made from scratch.
In organisms like plants, whose cells contain multiple chloroplasts, coordination is looser and less important.
It is likely that chloroplast and cell division are somewhat synchronized, though the mechanisms for it are mostly unknown.
Light has been shown to be a requirement for chloroplast division.
Chloroplasts can grow and progress through some of the constriction stages under poor quality green light, but are slow to complete division—they require exposure to bright white light to complete division.
Spinach leaves grown under green light have been observed to contain many large dumbbell-shaped chloroplasts.
Exposure to white light can stimulate these chloroplasts to divide and reduce the population of dumbbell-shaped chloroplasts.
Like mitochondria, chloroplasts are usually inherited from a single parent.
Biparental chloroplast inheritance—where plastid genes are inherited from both parent plants—occurs in very low levels in some flowering plants.
Many mechanisms prevent biparental chloroplast DNA inheritance, including selective destruction of chloroplasts or their genes within the gamete or zygote, and chloroplasts from one parent being excluded from the embryo.
Parental chloroplasts can be sorted so that only one type is present in each offspring.
Flowering plants were once thought to only inherit chloroplasts maternally.
However, there are now many documented cases of angiosperms inheriting chloroplasts paternally.
Angiosperms, which pass on chloroplasts maternally, have many ways to prevent paternal inheritance.
Most of them produce sperm cells that do not contain any plastids.
There are many other documented mechanisms that prevent paternal inheritance in these flowering plants, such as different rates of chloroplast replication within the embryo.
Among angiosperms, paternal chloroplast inheritance is observed more often in hybrids than in offspring from parents of the same species.
This suggests that incompatible hybrid genes might interfere with the mechanisms that prevent paternal inheritance.
Recently, chloroplasts have caught attention by developers of genetically modified crops.
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.
Credits to the contents of this page go to the authors of the corresponding Wikipedia page: en.wikipedia.org/wiki/Chloroplast.