Structure of a typical higher-plant chloroplast
, unicellular algae, have brownish-yellow chloroplasts
Chloroplasts (pron.: /ˈklɒrəplæsts/) are organelles found in plant cells and some other eukaryotic organisms. Their functions include conducting photosynthesis, and in some algae, lipid synthesis. Photosynthesis is their main function, where chloroplasts capture the sun's light energy, and store it in the energy storage molecules ATP and NADPH while breaking down water molecules. They then use the ATP and NADPH to make organic molecules from carbon dioxide and free oxygen from water, in a process known as the Calvin cycle.
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 colour. Chloroplasts are members of a class of organelles known as plastids.
The word chloroplast (χλωροπλάστης) is derived from the Greek words chloros (χλωρός), which means green, and plastis (πλάστης), which means "the one who forms".
Chloroplasts are one of the many different types of organelles in the plant cell. They are considered to have originated from cyanobacteria through endosymbiosis. This was first suggested by Mereschkowsky in 1905 after an observation by Schimper in 1883 that chloroplasts closely resemble cyanobacteria. All chloroplasts are thought to derive directly or indirectly from a single endosymbiotic event (in the Archaeplastida), except for Paulinella chromatophora, which has recently acquired a photosynthetic cyanobacterial endosymbiont which is not closely related to chloroplasts of other eukaryotes. Chloroplasts are similar to mitochondria in that they both originate from an endosymbiotic event, but chloroplasts are found only in plants and protista. In green plants, chloroplasts are surrounded by two smooth lipid-bilayer membranes that are thought to correspond to the outer and inner membranes of the ancestral cyanobacterium.
In some algae (such as the heterokonts and other protists such as Euglenozoa and Cercozoa), chloroplasts seem to have evolved through a secondary event of endosymbiosis, in which a eukaryotic cell engulfed a second eukaryotic cell containing chloroplasts, forming chloroplasts with three or four membrane layers. In some cases, such secondary endosymbionts may have themselves been engulfed by still other eukaryotes, thus forming tertiary endosymbionts.
In some groups of mixotrophic protists such as the dinoflagellates, chloroplasts are separated from a captured alga or diatom and used temporarily. These klepto chloroplasts may only have a lifetime of a few days and are then replaced.
Gene map of tobacco chloroplast DNA. Segments with labels on the outside are located on the A strand of the DNA and transcribed counterclockwise, segments with labels on the inside are located on the B strand and transcribed clockwise. Segments narrower than the surrounding ones (the notches) indicate introns
. Unlabeled segments represent open reading frames
Chloroplasts have their own DNA, often abbreviated as ctDNA, or cpDNA. It is also known as the plastome. Its existence was first proved in 1962, and first sequenced in 1986—when two Japanese research teams sequenced the chloroplast DNA of liverwort and tobacco. Chloroplast DNAs are circular, and are typically 120,000–170,000 base pairs long. This DNA makes up some 10–20% of all the DNA in a plant. Chloroplast genomes are generally well conserved among land plants. They code for about 100 genetic functions including redox proteins involved in electron transport in photosynthesis, transcription enzymes such as RNA polymerase, various tRNAs, and ribosomal RNAs and proteins.
New chloroplasts may contain up to 100 copies of their DNA, though the number of chloroplast DNA molecules decreases to about 15–20 as the chloroplasts age.
Many chloroplast DNAs contain two large inverted repeats about 20,000–25,000 base pairs long each, which separate a long single copy section(LSC) from a short single copy section(SSC). The inverted repeat regions are highly conserved among various plants, and accumulate few mutations. It is possible that they 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. The chloroplast DNAs of some plants such as peas lack inverted repeats.
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. Many of the missing genes, such as the ones for the enzymes needed for DNA replication, are encoded in the nuclear genome of the host. There have been a few recent transfers of genes from the chloroplast DNA to the nuclear genome in land plants.
In land plants, chloroplasts are generally flat discs, 5 μm in diameter and 2.3 μm thick. They are larger than mitochondria due to the fact that their internal membranes are not folded up into cristae. The chloroplast is contained by an envelope that consists of an inner and an outer phospholipid membrane. Between these two layers is the intermembrane space.
A typical parenchyma cell of a land plant contains about 10 to 100 chloroplasts. In the cells of many alga there is only one chloroplast (for example in Chlorella, it fills much of the cell and is bell-shaped).
image of a chloroplast
The material within the chloroplast is called the stroma, corresponding to the cytosol of the original bacterium, and contains one or more molecules of small circular DNA. It also contains ribosomes; however most of its proteins are encoded by genes contained in the host cell nucleus, with the protein products transported to the chloroplast.
Within the stroma are stacks of thylakoids, the sub-organelles, which are the site of photosynthesis. The thylakoids are arranged in stacks called grana (singular: granum). A thylakoid has a flattened disk shape. Inside it is an empty area called the thylakoid space or lumen.
In the electron microscope, thylakoid membranes appear as alternating light-and-dark bands, each 0.01 μm thick. Embedded in the thylakoid membrane are antenna complexes, each of which consists of the light-absorbing pigments, including chlorophyll and carotenoids, as well as proteins that bind the pigments. This complex both increases the surface area for light capture, and allows capture of photons with a wider range of wavelengths. The energy of the incident photons is absorbed by the pigments and funneled to the reaction centre of this complex through resonance energy transfer. Two chlorophyll molecules are then ionised, producing an excited electron, which then passes onto the photochemical reaction centre.
The chloroplasts of some hornworts and algae contain structures called pyrenoids. They are not found in higher plants. Pyrenoids are roughly spherical and highly refractive bodies which are the site of starch accumulation in a chloroplast. They consist of an matrix opaque to electrons, surrounded by two hemispherical starch plates. The starch is accumulated as the pyrenoids mature. 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. Pyrenoids can divide to form new pyrenoids, or be produced "de novo".
Recent studies have shown that chloroplasts can be interconnected by tubular bridges called stromules, formed as extensions of their outer membranes. Chloroplasts appear to be able to exchange proteins via stromules, and thus function as a network.
Diagram of the unicellular alga Chlamydomonas
which has a cup-shaped chloroplast with a pyrenoid
The chloroplasts of most higher plants are oval to disc-shaped in cross-section. 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.
|This section requires expansion. (October 2012)|
Main article: Photosynthesis
Photosynthesis takes place on the thylakoid membranes. As in mitochondrial oxidative phosphorylation, it involves the coupling of cross-membrane fluxes with biosynthesis via the dissipation of a proton electrochemical gradient. Because of this H+ gradient, the interior of the thylakoid is acidic, with a pH around 4, while the stroma is slightly basic. 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..
Main article: Calvin cycle
The Calvin cycle, also known as the dark reactions, is a series of biochemical reactions that fixes CO2 into G3P molecules, or glucose. These reactions take place in the stroma of the chloroplast. The optimal stroma pH for the Calvin cycle is 8.1, with the reaction nearly stopping when the pH falls below 7.3.
Photorespiration can occur when the oxygen concentration is too high. Several mechanisms have evolved in different lineages that raise the carbon dioxide concentration relative to oxygen within the chloroplast (carbon dioxide concentrating mechanisms, CCMs), and thus increase the efficiency of photosynthesis; these include Crassulacean acid metabolism, C4 carbon fixation, and pyrenoids.
Recently, chloroplasts have caught attention by developers of genetically modified crops. In most flowering plants, chloroplasts are not inherited from the male parent, although in plants such as pines, chloroplasts are inherited from males. Where chloroplasts are inherited only from the female, transgenes in these plastids cannot be disseminated by pollen. This makes plastid transformation a valuable tool for the creation and cultivation of genetically modified plants that are biologically contained, thus posing significantly lower environmental risks. This biological containment strategy is therefore suitable for establishing the coexistence of conventional and organic agriculture. While the reliability of this mechanism has not yet been studied for all relevant crop species, recent results in tobacco plants are promising, showing a failed containment rate of transplastomic plants at 3 in 1,000,000.
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