Chloroplast

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 named these bodies as "chloroplastids" (Chloroplastida). In 1884, Eduard Strasburger adopted the term "chloroplasts" (Chloroplasten). The word chloroplast is derived from the Greek words chloros (χλωρός), which means green, and plastes (πλάστης), which means "the one who forms". == Endosymbiotic origin of chloroplasts ==

Chloroplasts are one of many types of organelles in photosynthetic eukaryotic cells. They evolved from cyanobacteria through a process called organellogenesis. Cyanobacteria are a diverse phylum of gram-negative bacteria capable of carrying out oxygenic photosynthesis. Like chloroplasts, they have thylakoids. The thylakoid membranes contain photosynthetic pigments, including chlorophyll a. 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. and some species of the amoeboid Paulinella. Mitochondria are thought to have come from a similar endosymbiosis event, where an aerobic prokaryote was engulfed. Primary endosymbiosis Approximately twobillion years ago, a free-living cyanobacterium entered an early eukaryotic cell, either as food or as an internal parasite, Some of the cyanobacterial proteins were then synthesized by the host cell and imported back into the chloroplast (formerly the cyanobacterium), allowing the host to control the chloroplast. Separately, somewhere about 90–140 million years ago, this process happened again in the amoeboid Paulinella with a cyanobacterium in the genus Prochlorococcus. This independently evolved chloroplast is often called a chromatophore instead of a chloroplast. Chloroplasts are believed to have arisen after mitochondria, since all eukaryotes contain mitochondria, but not all have chloroplasts. This is called serial endosymbiosis—where an early eukaryote engulfed the mitochondrion ancestor, and then descendants of it then engulfed the chloroplast ancestor, creating a cell with both chloroplasts and mitochondria. In secondary plastids, typically only the chloroplast, and sometimes its cell membrane and nucleus remain, 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. Circles represent endosymbiotic events. For clarity, dinophyte tertiary endosymbioses and many nonphotosynthetic lineages have been omitted. ----a It is now established that Chromalveolata is paraphyletic to Rhizaria. == Primary chloroplast lineages ==

All primary chloroplasts belong to one of four chloroplast lineages—the glaucophyte chloroplast lineage, the rhodophyte ("red") chloroplast lineage, the chloroplastida ("green") chloroplast lineage, and the amoeboid Paulinella chromatophora lineage. Glaucophytes diverged first before the red and green chloroplast lineages diverged. Because of this, they are sometimes considered intermediates between cyanobacteria and the red and green chloroplasts. This early divergence is supported by both phylogenetic studies and physical features present in glaucophyte chloroplasts and cyanobacteria, but not the red and green chloroplasts. First, glaucophyte chloroplasts have a peptidoglycan wall, a type of cell wall otherwise only in bacteria (including cyanobacteria). Second, glaucophyte chloroplasts contain concentric unstacked thylakoids which surround a carboxysome – an icosahedral structure that contains the enzyme RuBisCO responsible for carbon fixation. Third, starch created by the chloroplast is collected outside the chloroplast. Additionally, like cyanobacteria, both glaucophyte and rhodophyte thylakoids are studded with light collecting structures called phycobilisomes. Rhodophyta (red chloroplasts) File:Bornetia secundiflora herbarium item.jpg|Bornetia secundiflora File:AMP Capo Gallo 030 Peyssonnelia squamaria.JPG|Peyssonnelia squamaria File:Callophyllis laciniata 1 Crouan.jpg|Cyanidium File:Laurencia.jpg|Laurencia File:Cyanidium O5A.jpg|Callophyllis laciniata The rhodophyte, or red algae, group is a large and diverse lineage. Rhodoplasts have a double membrane with an intermembrane space and phycobilin pigments organized into phycobilisomes on the thylakoid membranes, preventing their thylakoids from stacking. However, since they also contain the blue-green chlorophyll a and other pigments, many are reddish to purple from the combination. This group is also called Viridiplantae, which includes two core clades—Chlorophyta and Streptophyta. Most green chloroplasts are green in color, though some aren't due to accessory pigments that override the green from chlorophylls, such as in the resting cells of Haematococcus pluvialis. Green chloroplasts differ from glaucophyte and red algal chloroplasts in that they have lost their phycobilisomes, and contain chlorophyll b. Chloroplastida lineages also keep their starch inside their chloroplasts. There are some lineages of non-photosynthetic parasitic green algae that have lost their chloroplasts entirely, such as Prototheca, Morphological and physiological similarities, as well as phylogenetics, confirm that these are lineages that ancestrally had chloroplasts but have since lost them. Paulinella chromatophora The photosynthetic amoeboids in the genus Paulinella—P. chromatophora, P. micropora, and marine P. longichromatophora—have the only known independently evolved chloroplast, often called a chromatophore. they were first described in 1894 by German biologist Robert Lauterborn. The chromatophore is highly reduced compared to its free-living cyanobacterial relatives and has limited functions. For example, it has a genome of about 1 million base pairs, one third the size of Synechococcus genomes, and only encodes around 850 proteins. Because chromatophores are much younger compared to the canonical chloroplasts, Paulinella chromatophora is studied to understand how early chloroplasts evolved. == Secondary and tertiary chloroplast lineages ==

Green algal derived chloroplasts Green algae have been taken up by many groups in three or four separate events. Primarily, secondary chloroplasts derived from green algae are in the euglenoids and chlorarachniophytes. They are also found in one lineage of dinoflagellates and possibly the ancestor of the CASH lineage (cryptomonads, alveolates, stramenopiles and haptophytes) 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 host was lost (e.g. the green algal membrane), leaving the two cyanobacterial membranes and the secondary host's phagosomal membrane. There is also a variant of Pseudoblepharisma tenue that only contains chloroplasts from green algae and no endosymbiotic purple bacteria. Red algal derived chloroplasts Secondary chloroplasts derived from red algae appear to have only been taken up only once, which then diversified into a large group called chromists or chromalveolates. Today they are found in the haptophytes, cryptomonads, heterokonts, dinoflagellates and apicomplexans (the CASH lineage). Cryptophytes . 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. Haptophytes of Gephyrocapsa oceanica, a haptophyte. Haptophytes are similar and closely related to cryptophytes or heterokontophytes. Apicomplexans Apicomplexans are a group of alveolates. Like the helicosproidia, they're parasitic, and have a nonphotosynthetic chloroplast. Dinophytes '', a peridinin-containing dinophyte. The dinoflagellates are yet another very large and diverse group, around half of which are at least partially photosynthetic (i.e. mixotrophic). Dinoflagellate chloroplasts have relatively complex history. Most dinoflagellate chloroplasts are secondary red algal derived chloroplasts. Many dinoflagellates have lost the chloroplast (becoming nonphotosynthetic), some of these have replaced it though tertiary endosymbiosis. Others replaced their original chloroplast with a green algal derived chloroplast. Fucoxanthin-containing chloroplasts are characterized by having the pigment fucoxanthin (actually 19′-hexanoyloxy-fucoxanthin and/or 19′-butanoyloxy-fucoxanthin) and no peridinin. Fucoxanthin is also found in haptophyte chloroplasts, providing evidence of ancestry. Some dinophytes, like Kryptoperidinium and Durinskia, all inside the host's endoplasmic reticulum lumen. Cryptophyte-derived dinophyte chloroplast '' has chloroplasts taken from a cryptophyte. Members of the genus Dinophysis have a phycobilin-containing chloroplast taken from a cryptophyte. 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. == Chloroplast DNA ==

Chloroplasts, like other endosymbiotic organelles, contain a genome separate from that in the cell nucleus. The existence of chloroplast DNA (cpDNA) was identified biochemically in 1959, Since then, hundreds of chloroplast genomes 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. Molecular structure With few exceptions, most chloroplasts have their entire chloroplast genome combined into a single large circular DNA molecule, and a mass of about 80–130 million daltons. While chloroplast genomes can almost always be assembled into a circular map, the physical DNA molecules inside cells take on a variety of linear and branching forms. New chloroplasts may contain up to 100 copies of their genome, Chloroplast DNA is usually condensed into nucleoids, which can contain multiple copies of the chloroplast genome. Many nucleoids can be found in each chloroplast. Many chloroplast genomes contain two inverted repeats, which separate a long single copy section (LSC) from a short single copy section (SSC). A given pair of inverted repeats are rarely identical, but they are always very similar to each other, apparently resulting from concerted evolution. Similar inverted repeats exist in the genomes of cyanobacteria and the other two chloroplast lineages (glaucophyta and rhodophyceae), suggesting that they predate the chloroplast. DNA repair and replication In chloroplasts of the moss Physcomitrella patens, the DNA mismatch repair protein Msh1 interacts with the recombinational repair proteins RecA and RecG to maintain chloroplast genome stability. In chloroplasts of the plant Arabidopsis thaliana the RecA protein maintains the integrity of the chloroplast's DNA by a process that likely involves the recombinational repair of DNA damage. mechanisms. Adapted from Krishnan NM, Rao BJ's paper "A comparative approach to elucidate chloroplast genome 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. 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). 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. These genes 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. a process called endosymbiotic 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. There have been a few recent transfers of genes from the chloroplast DNA to the nuclear genome in land plants. Recent research indicates that parts of the retrograde signaling network once considered characteristic for land plants emerged already in an algal progenitor, integrating into co-expressed cohorts of genes in the closest algal relatives of land plants. Protein synthesis 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 Because so many chloroplast genes have been moved to the nucleus, many proteins that would originally have been translated in the chloroplast are now synthesized in the cytoplasm of the plant cell. 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. Many became exaptations, taking on new functions like participating in cell division, protein routing, and even disease resistance. A few chloroplast genes found new homes in the mitochondrial genome—most became nonfunctional pseudogenes, though a few tRNA genes still work in the mitochondrion. or within the functional part of the protein. phosphorylates, or adds a phosphate group to many (but not all) of them in their transit sequences. 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. == Structure ==

image of a chloroplast. Grana of thylakoids and their connecting lamellae are clearly visible. In land plants, chloroplasts are generally lens-shaped, 3–10 μm in diameter and 1–3 μm thick. 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. 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. 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. that makes up much of a chloroplast's volume, and in which the thylakoid system floats. 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. In addition, in terms of function, the inner chloroplast membrane, which regulates metabolite passage and synthesizes some materials, has no counterpart in the mitochondrion. 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. They may exist to increase the chloroplast's surface area for cross-membrane transport, because they are often branched and tangled with the endoplasmic reticulum. 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 have a peptidoglycan wall between their inner and outer chloroplast membranes. Usually, a thin intermembrane space about 10–20 nanometers thick exists between the outer and inner chloroplast membranes. Glaucophyte algal chloroplasts have a peptidoglycan layer between the chloroplast membranes. It corresponds to the peptidoglycan cell wall of their cyanobacterial ancestors, which is located between their two cell membranes. These chloroplasts are called muroplasts (from Latin "mura", meaning "wall"). Other chloroplasts were assumed to have lost the cyanobacterial wall, leaving an intermembrane space between the two chloroplast envelope membranes, Inner 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. 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 intermembrane space. 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. An additional 4.5S rRNA with homology to the 3' tail of 23S is found in "higher" plants. or when it ages and transitions into a gerontoplast. 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. In red algae, starch granules are found in the cytoplasm rather than in the chloroplast. In plants, mesophyll chloroplasts, which do not synthesize sugars, lack starch granules. and algae contain structures called pyrenoids. 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. Thylakoid system 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. 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, partially responsible for giving most cyanobacteria and chloroplasts their color. Other forms of chlorophyll exist, such as the accessory pigments chlorophyll b, chlorophyll c, chlorophyll d, Carotenoids In addition to chlorophylls, another group of yellow–orange They help transfer and dissipate excess energy, β-carotene is a bright red-orange carotenoid found in nearly all chloroplasts, like chlorophyll a. 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. 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 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. which they use to make ATP and NADPH, as well as oxygen. They store in a four-carbon compound, which is why the process is called photosynthesis. The four-carbon compound is then transported to the bundle sheath chloroplasts, where it drops off 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. Mesophyll chloroplasts have a little more peripheral reticulum than bundle sheath chloroplasts. == Function and chemistry ==

Guard cell chloroplasts Unlike most epidermal cells, the guard cells of plant stomata contain relatively well-developed chloroplasts. However, exactly what they do is controversial. Plant innate immunity Plants lack specialized immune cells—all plant cells participate in the plant immune response. Chloroplasts, along with the nucleus, cell membrane, and endoplasmic reticulum, are key players in pathogen defense. Due to its role in a plant cell's immune response, pathogens frequently target the chloroplast. help synthesize an important defense molecule, jasmonate. Chloroplasts synthesize all the fatty acids in a plant cell—linoleic acid, a fatty acid, is a precursor to jasmonate. Light reactions The light reactions take place on the thylakoid membranes. They take light energy and store it in NADPH, a form of NADP+, and ATP to fuel the dark reactions. Energy carriers 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. In the light reactions, it gets reduced, meaning it picks up electrons, becoming NADPH. Photophosphorylation Like mitochondria, chloroplasts use the potential energy stored in an H+, or hydrogen ion, gradient to generate ATP energy. The two photosystems capture light energy to energize electrons taken from water, and release them down an electron transport chain. 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) Because chloroplast ATP synthase projects out into the stroma, the ATP is synthesized there, in position to be used in the dark reactions. NADP+ reduction Electrons are often removed from the electron transport chains to charge NADP+ with electrons, reducing it to NADPH. 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. Cyclic photophosphorylation While photosystem II photolyzes water to obtain and energize new electrons, photosystem I simply reenergizes depleted electrons at the end of an electron transport chain. 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. Cyclic photophosphorylation is common in plants, which need more ATP than NADPH. Alternatively, glucose monomers in the chloroplast can be linked together to make starch, which accumulates into the starch grains found in the chloroplast. 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. while the stroma is slightly basic, with a pH of around 8. The optimal stroma pH for the Calvin cycle is 8.1, with the reaction nearly stopping when the pH falls below 7.3. Amino acid synthesis Chloroplasts alone make almost all of a plant cell's amino acids in their stroma 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 Chloroplasts make all of a cell's purines and pyrimidines—the nitrogenous bases found in DNA and RNA. The carbon used to form the majority of the lipid is from acetyl-CoA, which is the decarboxylation product of pyruvate. Pyruvate is also made in the plastid from phosphoenolpyruvate, a metabolite made in the cytosol from pyruvate or PGA. 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. Other lipids are derived from the methyl-erythritol phosphate (MEP) pathway and consist of gibberelins, sterols, abscisic acid, phytol, and innumerable secondary metabolites. == Location ==

Distribution in a plant Not all cells in a multicellular plant contain chloroplasts. All green parts of a plant contain chloroplasts as the color comes from the chlorophyll. 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. In some plants such as cacti, chloroplasts are found in the stems, though in most plants, chloroplasts are concentrated in the leaves. One square millimeter of leaf tissue can contain half a million chloroplasts. 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. Mitochondria have also been observed to follow chloroplasts as they move. In higher plants, chloroplast movement is run by phototropins, blue light photoreceptors also responsible for plant phototropism. 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. == Differentiation, replication, and inheritance ==

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. Chloroplasts do not normally develop from proplastids in root tip meristems—instead, the formation of starch-storing amyloplasts is more common. 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. While etioplasts lack chlorophyll, they have a yellow chlorophyll precursor stocked. 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. Chloroplasts have no definite S-phase—their DNA replication is not synchronized or limited to that of their host cells. Much of what we know about chloroplast division comes from studying organisms like Arabidopsis and the red alga Cyanidioschyzon merolæ. 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. Later, the dynamins migrate under the outer plastid dividing ring, into direct contact with the chloroplast's outer membrane, Gymnosperms, such as pine trees, mostly pass on chloroplasts paternally, while flowering plants often inherit chloroplasts maternally. 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. == Footnotes ==