Chloroplast

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Structure of a typical higher-plant chloroplast
Chloroplasts visible in the cells of Plagiomnium affine,
the many-fruited thyme moss

Chloroplasts (pron.: /ˈklɔrəplæsts/) are organelles found in plant cells and some other eukaryotic organisms. As well as conducting photosynthesis, they carry out almost all fatty acid synthesis in plants, and are involved in a plant's immune response. A chloroplast is a type of plastid which specializes in photosynthesis. During photosynthesis, chloroplasts capture the sun's light energy, and store it in the energy storage molecules ATP and NADPH while freeing oxygen from water. They then use the ATP and NADPH to make organic molecules from carbon dioxide in a process known as the Calvin cycle.[1]

The word chloroplast (χλωροπλάστης) is derived from the Greek words chloros (χλωρός), which means green, and plastis (πλάστης), which means "the one who forms".

Contents

Evolutionary origin [edit]

Chloroplasts are one of the many different types of organelles in the plant cell. They are considered to have originated from cyanobacteria through endosymbiosis—when a eukaryotic cell engulfed a photosynthesizing cyanobacterium which remained and became a permanent resident in the cell. This was first suggested by Mereschkowsky in 1905[2] after an observation by Schimper in 1883 that chloroplasts closely resemble cyanobacteria.[3]

Chloroplasts are similar to mitochondria in that they both originate from an endosymbiotic event, but chloroplasts are found only in plants and protista.

Cyanobacterial ancestor [edit]

Main article: Cyanobacteria

Cyanobacteria are considered the ancestors of chloroplasts. They are sometimes called blue-green algae even though they are prokaryotes. They are a diverse phylum of bacteria capable of carrying out photosynthesis, and are gram-negative, meaning they have two cell membranes. They also contain a thick peptidoglycan cell wall which is located between its two cell membranes.[4] Like chloroplasts, they have thylakoids inside of them.

Both chloroplasts and cyanobacteria have a double membrane, DNA, ribosomes, and thylakoids.
Both chloroplasts and cyanobacteria have a double membrane, DNA, ribosomes, and thylakoids.

Endosymbiosis [edit]

Primary endosymbiosis of chloroplasts.

According to the serial endosymbiosis theory, chloroplasts are believed to have arisen after mitochondria, since all eukaryotes contain mitochondria, but not all have chloroplasts.[5][6]

Whether or not chloroplasts came from a single endosymbiotic event, or many independent engulfments across various eukaryotic lineages has been long debated, but it is now generally held that all organisms with chloroplasts share a single ancestor that took in a cyanobacterium 600–1600 million years ago.[7] The alga Cyanophora is thought to be one of the first organisms to contain a chloroplast, which was enclosed by a peptidoglycan wall,[8] like its cyanobacterial parent.[4] Over time, the chloroplast was assimilated, and many of its genes were lost or transferred to the nucleus of the host.[9] Some of its proteins were then synthesized in the cytoplasm of the host cell, and imported back into the chloroplast.[8][9] In green plants, chloroplasts are surrounded by two smooth lipid-bilayer membranes[10] that are thought to correspond to the outer and inner membranes of the ancestral cyanobacterium's gram negative cell wall.[11][12][13] The phagosomal membrane from the host was probably lost, along with the cyanobacterium's peptidoglycan wall between its cell membranes.[13]

Diagram of a four membraned chloroplast containing a nucleomorph.
Diagram of a four membraned chloroplast containing a nucleomorph.

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 engulfs a second eukaryotic cell containing chloroplasts, forming chloroplasts with three or four membrane layers.[14] The eaten eukaryote is broken down, usually leaving only the chloroplast and the cell membrane, forming a chloroplast with four membranes—the two cyanobacterial membranes, the eaten eukaryote's cell membrane, and the phagosomal membrane of the secondary host. The genes in the phagocytosed eukaryote's nucleus are transferred to the secondary host's nucleus.[13] Cryptomonads and chlorarachniophytes retain the phagocytosed eukaryote's nucleus, an object called a nucleomorph,[13] located between the second and third membranes of the chloroplast.[8]

Euglenids and dinoflagellates have chloroplasts with three membranes—it's thought that the membrane of the primary endosymbiont was lost, leaving the cyanobacterial membranes, and the secondary host's phagosomal membrane.[13]

Secondary endosymbiosis of chloroplasts
Possible cladogram for chloroplast evolution[7]

In some cases, such secondary endosymbionts may have themselves been engulfed by still other eukaryotes,[15] thus forming tertiary endosymbionts.

Chromatophores [edit]

While most chloroplasts originate from that first set of endosymbiotic events, Paulinella chromatophora is an exception, which has acquired a photosynthetic cyanobacterial endosymbiont more recently. It is not closely related to chloroplasts of other eukaryotes.[16] Being in the early stages of endosymbiosis, Paulinella chromatophora can offer some insights into how chloroplasts evolved.[9][17] Paulinella cells contain one or two sausage shaped blue-green photosynthesizing structures called chromatophores,[9][17] descended from the cyanobacterium Synechococcus. Chromatophores cannot survive outside their host.[9] 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,[9] but much larger than the ~150,000 base pair genome of the more assimilated chloroplast.[18][19][20] 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.[17]

Kleptoplastidy [edit]

In some groups of mixotrophic protists, like some 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.[21]

Chloroplast DNA [edit]

See also: List of sequenced plastomes

Chloroplasts have their own DNA,[22] often abbreviated as ctDNA,[23] or cpDNA.[24] It is also known as the plastome. Its existence was first proved in 1962,[18] and first sequenced in 1986—when two Japanese research teams sequenced the chloroplast DNA of liverwort and tobacco.[25] Since then, hundreds of chloroplast DNAs from various species have been sequenced—mostly those of land plants and green algæ.[26]

Molecular structure [edit]

Chloroplast DNA blank.svg
Interactive gene map of chloroplast DNA from Nicotiana tabacum. Segments with labels on the inside reside on the B strand of DNA, segments with labels on the outside are on the A strand. Notches indicate introns.

Chloroplast DNAs are circular, and are typically 120,000–170,000 base pairs long.[18][19][20] They can have a contour length of around 30–60 micrometers, and have a mass of about 80–130 million daltons.[27]

Most plants have their entire chloroplast genome combined into a single large ring, though dinoflagellate algæ are a notable exception—their chloroplasts contain around forty small plasmids which each consist of one to three genes.[26][8] Many chloroplast DNAs contain two large inverted repeats about 20,000–25,000 base pairs long each,[20][28] which separate a long single copy section (LSC) from a short single copy section (SSC).[20] The inverted repeat regions are highly conserved among various plants, and accumulate few mutations.[20][28] Similar inverted repeats exist in the genomes of cyanobacteria, suggesting that they predate the chloroplast,[26] though some plants like peas have since lost the inverted repeats.[28][29] 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.[29] Chloroplast genomes as a whole are also generally well conserved among land plants.[20]

New chloroplasts may contain up to 100 copies of their DNA,[18] though the number of chloroplast DNA copies decreases to about 15–20 as the chloroplasts age.[30] They are usually packed into nucleoids which can contain several identical chloroplast DNA rings. Many nucleoids can be found in each chloroplast.[27]

Though chloroplast DNA is not associated with true histones,[5] in red algæ, a histone-like chloroplast protein (HC) coded by the chloroplast DNA that tightly packs each chloroplast DNA ring into a nucleoid has been found.[31]

In primitive red algæ, the chloroplast DNA nucleoids are clustered in the center of the chloroplast, while in green plants and green algæ, the nucleoids are dispersed throughout the stroma.[31]

Genes [edit]

The chloroplast genome includes around 100 genes[8][19] which code for a variety of things, mostly to do with the protein pipeline and photosynthesis. As in prokaryotes, genes in chloroplast DNA are organized into operons.[8]

Among land plants, the contents of the chloroplast genome are fairly similar—they code for four ribosomal RNAs, 30–31 tRNAs, 21 ribosomal proteins, and four RNA polymerase subunits,[32][33] involved in protein synthesis. For photosynthesis, the chloroplast DNA includes genes for 28 thylakoid proteins and the large Rubisco subunit.[32] In addition, its genes encode eleven subunits of a protein complex which mediates redox reactions to recycle electrons,[34] which is similar to the NADH dehydrogenase found in mitochondria.[32][35]

Chloroplast genome reduction and protein synthesis [edit]

The chloroplast genome 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.[36] Over time, many parts of the chloroplast genome were transferred to the nuclear genome of the host,[18][19][37] a process called endosymbiotic gene transfer. In land plants, some 11–14% of the DNA in their nuclei can be traced back to the chloroplast.[17] There have been a few recent transfers of genes from the chloroplast DNA to the nuclear genome in land plants.[19]

Of the approximately three-thousand 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. The chloroplast is mostly under nuclear control, though chloroplasts can also give out signals regulating gene expression in the nucleus.[38]

Protein synthesis within chloroplasts relies on an RNA polymerase coded by the chloroplast's own genome, which is related to RNA polymerases found in bacteria. Chloroplasts also contain a mysterious second RNA polymerase that is encoded by the plant's nuclear genome. The two RNA polymerases may recognize and bind to different kinds of promoters within the chloroplast genome.[39] The ribosomes in chloroplasts are similar to bacterial ribosomes.[32]

[icon] This section requires expansion with: Genome size differences between algæ and land plants, chloroplast stuff coded by the nucleus, DNA replication, NADPH redox, special tRNA synthetases, etc.. (January 2013)

Structure [edit]

Chloroplast ultrastructure Chloroplasts have three distinct membrane systems, and a variety of things can be found in their stroma.
Chloroplast ultrastructure Chloroplasts have three distinct membrane systems, and a variety of things can be found in their stroma.
See also: Chloroplast membrane

In land plants, chloroplasts are generally lens-shaped, 5–8 μm in diameter and 1–3 μm thick.[40] They are larger than mitochondria due to the fact that their internal membranes are not folded up into cristae.[41] The chloroplast is contained by an envelope that consists of an inner and an outer membrane. Between these two layers is the intermembrane space.

Recent studies have shown that chloroplasts can be interconnected by tubular bridges called stromules, formed as extensions of their outer membranes.[42][43] Chloroplasts appear to be able to exchange proteins via stromules,[44] and thus function as a network.

Stroma [edit]

Main article: Stroma

The fluid within the chloroplast is called the stroma, corresponding to the cytosol of the original cyanobacterium. Nucleoids of chloroplast DNA and chloroplast ribosomes can be found floating around in it. The Calvin cycle, which fixes CO2 into sugar takes place in the stroma.

Thylakoids [edit]

Grana structure The prevailing model for granal structure is a stack of granal thylakoids linked by helical stromal thylakoids that wrap around the grana stacks and form large sheets that connect different grana.[45]
Helical granum.png
The prevailing model for granal structure.
Main article: Thylakoid

Within the stroma are stacks of thylakoids, which are the site of the light reactions of photosynthesis. The thylakoids are arranged in stacks called grana (singular: granum).[1] A thylakoid has a flattened disk shape. Inside it is an cavity called the thylakoid space or thylakoid lumen.

Transmission electron microscope image of a chloroplast. Grana of thylakoids and their connecting lamellæ are clearly visible.

In the transmission 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 center of this complex through resonance energy transfer. Two chlorophyll molecules are then ionized, producing an excited electron, which then passes onto the photosynthetic reaction center.

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.[46]

Pyrenoids [edit]

Main article: Pyrenoid

The chloroplasts of some hornworts[47] and algae contain structures called pyrenoids. They are not found in higher plants.[48] Pyrenoids are roughly spherical and highly refractive bodies which are a site of starch accumulation in plants that contain them. They consist of an matrix opaque to electrons, surrounded by two hemispherical starch plates. The starch is accumulated as the pyrenoids mature.[49] In algae with carbon concentrating mechanisms, the enzyme rubisco is found in the pyrenoids. Starch can also accumulate around the pyrenoids when CO2 is scarce.[48] Pyrenoids can divide to form new pyrenoids, or be produced "de novo".[49][50]

Peripheral reticulum [edit]

Some chloroplasts contain a structure called the chloroplast peripheral reticulum. It is often found in the chloroplasts of C4 plants, though it's also been found in some C3 angiosperms,[46] and even some gymnosperms.[51] 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. Its purpose is thought to be to increase the chloroplast's surface area for cross-membrane transport between its stroma and the cell cytoplasm. The small vesicles sometimes observed may serve as transport vesicles to shuttle stuff between the thylakoids and the outside of the chloroplast.[46]

Chloroplast shapes [edit]

Diatoms, unicellular algae, have brownish-yellow chloroplasts

The chloroplasts of most higher plants are usually lens-shaped.[40] Chloroplasts with quite different shapes occur in algae, such as 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). Some algae have two chloroplasts in each cell; they are star-shaped in Zygnema, or may follow the shape of half the cell in order Desmidiales.

Chloroplast colors [edit]

All chloroplasts contain the green pigment chlorophyll a, but not all chloroplasts are green because accessory pigments may be present that can change or override the green color.

Specialized chloroplasts in C4 plants [edit]

See also: Photosynthesis and C4 photosynthesis

To fix carbon dioxide into sugar molecules in the process of photosynthesis, chloroplasts use an enzyme called rubisco. Rubisco has a problem—it has trouble distinguishing between carbon dioxide and oxygen, so at high oxygen concentrations, rubisco starts accidentally adding oxygen to sugar precursors. This has the end result of ATP energy being wasted and CO2 being released, all with no sugar being produced. This is a big problem, since O2 is produced by the initial light reactions of photosynthesis, causing issues down the line in the Calvin cycle which uses rubisco.[52]

C4 plants evolved a way to solve this—by spatially separating the light reactions and the Calvin cycle. The light reactions, which store light energy in ATP and NADPH, are done in the mesophyll cells of a C4 leaf. The Calvin cycle, which uses the stored energy to make sugar using rubisco, is done in the bundle sheath cells, a layer of cells surrounding a vein in a leaf.[52]

As a result, chloroplasts in C4 mesophyll cells and bundle sheath cells are specialized for each stage of photosynthesis. In mesophyll cells, chloroplasts are specialized for the light reactions, so they lack rubisco, and have normal grana and thylakoids, [53] which they use to make ATP and NADPH, as well as oxygen. They store CO2 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 CO2 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.[52] 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. They lack photosystem II, and only have photosystem I—the only protein complex needed for cyclic electron flow.[53][52] Because the job of bundle sheath chloroplasts is to carry out the Calvin cycle and make sugar, they often contain large starch grains.[53]

Both types of chloroplast contain large amounts of chloroplast peripheral reticulum,[53] which they use to get more surface area to transport stuff in and out of them.[51][46] Mesophyll chloroplasts have a little more peripheral reticulum than bundle sheath chloroplasts.[54]

Distribution in a plant [edit]

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.[55] The plant cells which contain chloroplasts are usually parenchyma cells, though chloroplasts can also be found in collenchyma tissue.[56] 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.

A cross section of a leaf, showing chloroplasts in its mesophyll cells. Stomal guard cells also have chloroplasts, though much fewer than mesophyll cells.
A cross section of a leaf, showing chloroplasts in its mesophyll cells. Stomal guard cells also have chloroplasts, though much fewer than mesophyll cells.

In some plants such as cacti, chloroplasts are found in the stems,[57] though in most plants, chloroplasts are concentrated in the leaves. One square millimeter of leaf tissue can contain half a million chloroplasts.[55] Within a leaf, chloroplasts are mainly found in the mesophyll layers of a leaf, and the guard cells of stomata. Palisade mesophyll cells can contain 30–70 chloroplasts per cell, while stomatal guard cells contain only around 8–15 per cell, as well as much less chlorophyll. Chloroplasts can also be found in the bundle sheath cells of a leaf, especially in C4 plants, which carry out the Calvin cycle in their bundle sheath cells. They are often absent from the epidermis of a leaf.[58]

Algal chloroplasts [edit]

[icon] This section requires expansion. (May 2013)

In the cells of many alga there is only one chloroplast[59] (for example in Chlorella, it fills much of the cell and is bell-shaped).

Cellular location [edit]

Chloroplast movement [edit]

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.[60] 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.[61] Chloroplast movement is considered one of the most closely regulated stimulus-response systems that can be found in plants.[62] Mitochondria have also been observed to follow chloroplasts as they move.[63]

In higher plants, chloroplast movement is run by phototropins, blue light photoreceptors also responsible for plant phototropism. In some algæ, mosses, ferns, and flowering plants, chloroplast movement is influenced by red light in addition to blue light,[60] 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.[63]

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.[62][63]

Role in plant immunity [edit]

Plants lack specialized immune cells—all plant cells participate in the plant immune response. Chloroplasts, along with the nucleus, cell membrane, and endoplasmic reticulum,[64] are key players in pathogen defense. Due to its role in a plant cell's immune response, pathogens frequently target the chloroplast.[64]

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.[64]

In some plants, chloroplasts are known to move closer to the infection site and the nucleus during an infection.[64]

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. All these molecules initiate retrograde signaling—signals from the chloroplast that regulate gene expression in the nucleus.[64]

In addition to defense signaling, chloroplasts, with the help of the peroxisomes,[65] help synthesize an important defense molecule, jasmonate. Chloroplasts synthesize all the fatty acids in a plant cell[64][66]—linoleic acid, a fatty acid, is a precursor to jasmonate.[64]

Chemistry [edit]

Main article: Photosynthesis

Light reactions [edit]

Main article: Light reactions

The light reactions take place on the thylakoid membranes. As in mitochondrial oxidative phosphorylation, it utilizes the potential energy stored in an H+, or hydrogen ion gradient. The two photosystems and the molecules between them pump hydrogen ions into the thylakoid space, which flow back out into the stroma through ATP synthase. ATP synthase uses the energy from the flowing hydrogen ions to phosphorylate adenosine diphosphate into adenosine triphosphate, or ATP. The light reactions also charge NADP+ with electrons, reducing it to NADPH.[67]

Calvin cycle [edit]

Main article: Calvin cycle

The Calvin cycle, also known as the dark reactions, is a series of biochemical reactions that fixes CO2 into G3P sugar molecules. These reactions take place in the stroma of the chloroplast. It works using the enzyme Rubisco to fix CO2 into Ribulose bisphosphate (RuBP) molecules. The energy stored in the ATP and NADPH made by the light reactions is then used to produce G3P molecules from RuBP. G3P can double up to form glucose.[67]

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.[67] 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. These include Crassulacean acid metabolism, C4 carbon fixation,[67] and pyrenoids.

pH [edit]

Because of the H+ gradient across the thylakoid membrane, the interior of the thylakoid is acidic, with a pH around 4,[68] while the stroma is slightly basic, with a pH of around 8.[69] The optimal stroma pH for the Calvin cycle is 8.1, with the reaction nearly stopping when the pH falls below 7.3.[70]

CO2 in water can form carbonic acid, which can disturb the pH of isolated chloroplasts, interfering with photosynthesis, even though CO2 is used in photosynthesis. However, chloroplasts in living plant cells are not affected by this as much.[69]

Chloroplasts can pump K+ and H+ ions in and out of themselves using a poorly understood light-driven transport system.[69]

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.[70]

Starch accumulation [edit]

Under conditions such as high atmospheric CO2 concentrations, starch grains may accumulate in the chloroplasts, distorting the grana and thylakoids. Waterlogged roots can also cause starch buildup in the chloroplasts, possibly due to less sugar being exported through the phloem. This depletes a plant's free phosphate supply, which indirectly stimulates chloroplast starch synthesis. The starch granules displace the thylakoids, but leave them intact.[71] While linked to low photosynthesis rates, the starch grains themselves may not necessarily interfere significantly with the efficiency of photosynthesis,[72] and might simply be a side effect of another photosynthesis-depressing factor.[71]

Other chemical products [edit]

[icon] This section requires expansion with: needs more about lipids, also paramylon. (March 2013)

Chloroplasts are the site of complex lipid metabolism.[73]

Development and differentiation [edit]

Main article: Plastid
Plants contain many different kinds of plastids in their cells.
Plants contain many different kinds of plastids in their cells.

Chloroplasts are a special type of 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,[74] or fertilized egg. Proplastids are commonly found in an adult plant's apical meristems. Chloroplasts do not normally develop from proplastids in root tip meristems[75]—instead, the formation of starch-storing amyloplasts is more common.[74]

In shoots, proplastids from shoot apical meristems gradually develop into chloroplasts in photosynthetic leaf tissues as the leaf matures, if exposed to the required light. This process involves invaginations of the inner plastid membrane, forming sheets of membrane that project into the internal stroma. These membrane sheets then fold to form thylakoids and grana.[76]

If angiosperm shoots are not exposed to the required light for chloroplast formation, proplastids may develop into an etioplast stage before becoming chloroplasts. An etioplast is a plastid that lacks chlorophyll, and has inner membrane invaginations that form a lattice of tubes in their stroma, called a prolamellar body. 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.[76] Gymnosperms do not require light to form chloroplasts.[76]

Light, however, does not guarantee that a proplastid will develop into a chloroplast—what type of plastid a proplastid becomes is largely influenced by the kind of cell it resides in.[74]

Many plastid interconversions are possible.
Many plastid interconversions are possible.

Plastid interconversion [edit]

Plastid differentiation is not permanent, in fact many interconversions are possible. Chloroplasts may be converted to chromoplasts, which are pigment-filled plastids responsible for the bright colors you see in flowers and ripe fruit. Starch storing amyloplasts can also be converted to chromoplasts, and it's possible for proplastids to develop straight into chromoplasts. Chromoplasts and amyloplasts can also become chloroplasts, like what happens when you illuminate a carrot or a potato. 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.[74]

Chloroplast division [edit]

[icon] This section requires expansion with: functions, Z-ring dynamic assembly, regulators such as Giant Chloroplast 1. (February 2013)

Most chloroplasts in a photosynthetic cell do not develop directly from proplastids or etioplasts. In fact, a typical shoot meristematic plant cell contains only 7–20 proplastids. 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.[77]

In single-celled algæ, 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.[76]

Almost all chloroplasts in a cell divide, rather than a small group of rapidly dividing chloroplasts.[78] Chloroplasts have no definite S-phase—their DNA replication is not synchronized or limited to that of their host cells.[79] Much of what we know about chloroplast division comes from studying the alga Cyanidioschyzon merolæ.[61]

Most chloroplasts in plant cells, and all chloroplasts in algæ arise from chloroplast division.[76]
Most chloroplasts in plant cells, and all chloroplasts in algæ arise from chloroplast division.[76]

The division process starts when the proteins FtsZ1 and FtsZ2 assemble into filaments, and with the help of a protein ARC6, form a structure called a Z-ring within the chloroplast's stroma.[61][80] 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.[61]

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.[61] The outer plastid-dividing ring is found wrapped around the outer chloroplast membrane. It consists of filaments about 5 nanometers across,[61] arranged in rows 6.4 nanometers apart, and shrinks to squeeze the chloroplast. This is when chloroplast constriction begins.[80]
In a few species like Cyanidioschyzon merolæ, chloroplasts have a third plastid-dividing ring located in the chloroplast's intermembrane space.[61][80]

Late into the constriction phase, dynamin proteins assemble around the outer plastid-dividing ring,[80] helping provide force to squeeze the chloroplast.[61] Meanwhile, the Z-ring and the inner plastid-dividing ring break down.[80] During this stage, the many chloroplast DNA plasmids floating around in the stroma are partitioned and distributed to the two forming daughter chloroplasts.[81]

Later, the dynamins migrate under the outer plastid dividing ring, into direct contact with the chloroplast's outer membrane,[80] to cleave the chloroplast in two daughter chloroplasts.[61]

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.[80]

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.[80]

Regulation [edit]

In species of algæ which contain a single chloroplast, regulation of chloroplast division is extremely important to ensure that each daughter cell receives a chloroplast. In organisms like plants, whose cells contain multiple chloroplasts, coordination is looser and less important. It's likely that chloroplast and cell division are somewhat synchronized, though the mechanisms for it are mostly unknown.[61]

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.[78][81]

Chloroplast inheritance [edit]

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.[82]

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.[83]

Gymnosperms, such as pine trees, mostly pass on chloroplasts paternally,[84] while flowering plants often inherit chloroplasts maternally.[85][86] Flowering plants were once thought to only inherit chloroplasts maternally. However, there are now many documented cases of angiosperms inheriting chloroplasts paternally.[82]

Angiosperms which pass on chloroplasts maternally have many ways to prevent paternal inheritance. Most of them produce sperm cells which 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.[82]

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.[82]

Transplastomic plants [edit]

Recently, chloroplasts have caught attention by developers of genetically modified crops. Since in most flowering plants, chloroplasts are not inherited from the male parent, 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.[86]

See also [edit]

Notes [edit]


References [edit]

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