Chloroplast is the organelle of specialized subunits in eukaryotic autotrophic cells such as green plants and algae.Its main function is tophotosynthesis, which containsPhotosynthetic pigmentchlorophyllCapture energy from sunlight and store it in energy storage moleculesATPandNADPH, simultaneously released from wateroxygen。Then, they use ATP and NADPHCalvin cycleFromcarbon dioxideMake organic molecules.Chloroplasts perform many other functions, including plantfatty acidSynthesis, manyamino acidSynthesis of, andimmune reaction。
There are three types of chloroplastsChromosome(plastid), which is characterized by its high concentration of chlorophyll (the other two plastid types areWhite bodyandChromophore, containing a small amount of chlorophyll and unable to carry out photosynthesis).Chloroplasts are highly dynamic. They circulate and move around in plant cells, and occasionally divide into two to reproduce.Their behavior is strongly influenced by environmental factors such as the color and intensity of light.Like mitochondria, chloroplasts have their own genetic material DNA, but because of their limited genome size, they are semi autonomous organelles.Its DNA is believed to have been inherited from the photosynthetic Cyanobacteria ancestors that were engulfed by the cells of ancient eukaryotes.Chloroplasts cannot be produced by plant cells and must be inherited by each daughter cell during plant cell division.[1][36-37]
Chloroplast is an important organelle that converts light energy into chemical energy in plants.The number, size and shape of chloroplasts directly affect the color and photosynthetic intensity of leaves.The division and proliferation of chloroplasts are important factors to maintain the chloroplast population.
size
Chloroplasts visible in cells of Bryum capillare
The size of chloroplasts varies greatly. The chloroplasts of higher plants are usually 2-5 μ m wide and 5-10 μ m longoptical microscopeSee below.With the growth of cotyledons of Arabidopsis thaliana, the chloroplasts gradually become larger[55]。For a specific cell type, the size of chloroplasts is relatively stable, but will be affected by heredity or environment.for examplepolyploidThe chloroplast in the cellhaploidThe cells are larger, and the chloroplasts of plants growing in the shadow are larger than those growing in the sun[50], but there are exceptions[48-49]。Therefore, the chloroplast size of the same plant growing in different environments is not necessarily the same.
shape
Chloroplasts in higher plants are spherical, elliptical or ovoid, with double concave surfaces.Some chloroplasts are rod shaped, with smaller central area and enlarged ends.
The plant body is under stress, and the chloroplast shape changes generally[52][54]。Normalized Velvet Grass（Leontopodium leontopodioides）The chloroplast shape of[51]。"Phnom Penh" Liriodendron chinense（Liriodendron tulipifera'Aureo marginatum')[53]。
number
Chloroplast under electron microscope[2]
The number of chloroplasts in different plants is relatively stableMesophyll cell(Mesophyll cell) contains dozens to hundreds of chloroplasts, accounting for 40% of the cytoplasmic volume.The number of chloroplasts varies with species, cell types and physiological conditions.[58]
AlgaeUsually, there is only one giant chloroplast, and its shape is spiral or star shaped, depending on the shape of the cell.For example, filamentous green algae contains a giant chloroplast of spiral band type, whileporphyridium （Porphyridium purpureum）The cell contains a huge spherical chloroplast, and its inner membrane vesicles are arranged in parallel without accumulation.Loquat（Eriobotrya japonica）polyploidstomaThe number of chloroplasts increased significantly compared with diploid, but the difference between polyploids was not significant, and the number of chloroplasts among varieties was significantly different[56]。The number of chloroplasts in stomatal guard cells of different types of rape parents and their interspecific hybrids have obvious boundaries[57]。
distribution
Chloroplasts are sometimes evenly distributed in the cytoplasm, but sometimes they also gather near the nucleus or near the cell wall.The distribution and arrangement of chloroplasts in cells vary with light.Chloroplast may accompany plant cellsCytoplasmic circulation(cystoplasmic streaming).[62]
Corn（Zea mays）Under water stress, its chloroplasts drift from the cell edge to the center[52]。The chloroplasts of Erigeron normalizum will drift from the distribution along the cell wall to the center of the cell with the increase of altitude[51]。
form
The chemical components of chloroplasts mainly includewater、proteinandlipid, in addition, it also contains a small amount ofcoenzymeIsomolecules andnucleic acid。[59]
PsammophyteBubble prickleAnd those with strong drought toleranceCalligonum mongolicum）Seasonal changes of membrane fatty acid composition and membrane lipid antioxidant system enzymes in chloroplasts[60]。Evergreen juniper（sabina przewalskii andYanbai）The fatty acid composition and ATPase activity of chloroplast membrane also have seasonal changes[61]。
structure
From outside to inside, chloroplasts can be divided into outer chloroplast membrane (OCM), inner chloroplast membrane (IMM)Chloroplast matrix(Stroma), chloroplast thylakoid membrane (CTM) and chloroplast cystoid lumen.
Chloroplasts use pigments in them tosolar energyadoptphotosynthesisConvert tochemical energyStored inAdenosine triphosphate(Adenosine triphase, ATP).Organisms capable of photosynthesis may not have chloroplasts, such as cyanobacteriaprokaryoteThere is no chloroplast, but it contains chlorophyll and cyanophytin, which can also carry out photosynthesis.[3]
Chloroplasts inplant cellThe discovery within is usually attributed to the "father of plant physiology"Julius von Sacks（Julius von Sachs，1832-1897）。
In 1837, Hugo von Mohl, a German botanist, first gave a clear description of chloroplasts (Chlorophyllk ö rnen, or "chlorophyll particles") as a dispersion in green plant cells.[11]In 1883, the German scholar Andreas Franz Wilhelm Schimper named these dispersions "chloroplastids".In 1884,Strasberg(Eduard Strasburger) adopted the term "chloroplasts" (chloroplasten).[4]
The word "chloroplast" in English comes from the Greek word "χλ ω ρ o πλ ≤ σ τη ∨", which is derived from the word "green" (Greek: χλ ωρ ό, romanization:chloros）And "shaping" (Greek: πλ ≤ σ τη, Romanization:plastis）Combined.[33]
Fossil records show that most ancient vascular plants have only axes but no leaves, which indicates that chloroplasts appear much earlier than leaves.[68]Chloroplast is considered as a plastid that is engulfed and transformed by endosymbiotic cyanobacteria.The origin of this chloroplast was first proposed by the Russian biologist Konstantin Mereschkowski in 1905.Previously, Andreas Franz Wilhelm Schimper observed that chloroplasts were very similar to cyanobacteria in 1883.[12]Chloroplasts only exist in plants, algae and three kinds of amoebas -- color carrying bollworm in fresh water（Paulinella chromatophora）And Paulina micropolaris（Paulinella micropora）[13]And the color carrying bollworm in seawater（Paulinella longichromatophora）[14-15]。
In green plants, chloroplasts are surrounded by phospholipid bilayer, which is believed to have evolved from the inner and outer membranes of ancient cyanobacteria.The chloroplast genome has been significantly degraded compared with the independent blue-green bacteria, but the remaining parts still show their relationship through sequence alignment analysis.
Interestingly, in some algae, chloroplasts seem to be formed by secondary endogenesis: one eukaryotic cell enters another eukaryotic cell with chloroplasts, thus forming chloroplasts with four layers of membranes.At present, in addition to pan plants and Paulinia（Paulinella）The chloroplasts of all living things in our country enter our bodies through processes like this.
Cyanobacteria is considered as the ancestor of chloroplasts.[16]They are sometimes called blue green algae, even if they areprokaryote。They are diverse bacteria capable of photosynthesis, and areGram negative bacteria, which means they have two cell membranes.They also contain aPeptidoglycanThe cell wall is thicker than that of other gram-negative bacteria, and it is located between their two membranes.[17]Like chloroplasts, they have their insidesthylakoid。Photosynthetic pigments on thylakoid membrane, includingChlorophyll-a。[18]PhycobilinPhycobilins are also common cyanobacterial pigments, which usually attach to the outside of thylakoid cell membrane to form hemispherical phycobilisomes, but phycobilins are not distributed in any chloroplast.[18-19]
Primary endosymbiosis
Between 1 billion and 2 billion years ago[20-21], a free livingCyanobacteria(cyanobacterium) entered the early eukaryotic cells, either as food or as an internal parasite, and escaped from the phagocytic vacuole it contained[18]。The two innermost lipid bilayer membranes of chloroplasts[22]The outer membrane and inner membrane corresponding to the cell wall of ancestral cyanobacteria[23-25]Instead of the host's phagocytic membrane, the host's phagocytic membrane may have been lost[23]。New cells soon became an advantage, and cyanobacteria provided food for eukaryotic hosts, enabling them to live in them.Over time, cyanobacteria are assimilated, and many of their genes are lost or transferred to the nucleus of the host[26](The genome of cyanobacteria may initially contain more than 3000 genes, but there are only about 130 genes left in the chloroplasts of plants).Some proteins are synthesized in the cytoplasm of the host cell and imported into the chloroplast (formerly cyanobacteria)[23][27]。
At some time between 90 million and 140 million years ago, this phenomenon happened again. The participants were the color carrying bollworm（P. chromatophora）。[21]This phenomenon that cells living in another cell are good for both sides is called endosymbiosis. The external cells are usually called hosts, while the internal cells are called endosymbios or endobionts.
Chloroplasts are thought to bemitochondrionLater, because all eukaryotes contain mitochondria, but not all organisms have chloroplasts.This is calledSymbiosis theory（symbiogenesis、serial endosymbiosis），That is, an early eukaryote engulfs the mitochondrial ancestors, and then some of its descendants engulf the chloroplast ancestors to form a cell with both chloroplasts and mitochondria.
Whether primary chloroplasts come from a single endosymbiosis event or from an independent phagocytosis event of different eukaryotic lineages has always been controversial.It is generally believed that organisms with primary chloroplasts have a common ancestor, which was infected with a cyanobacterium 600 million to 2 billion years ago[21]。Some scholars propose that the close relative of this bacterium is grouper（Gloeomargarita lithophora）[28-30]。But the exception is the color carrying bollworm（P. chromatophora）Its ancestors were infected with a kind of protochlorella cyanobacteria 90 million to 500 million years ago（Prochlorococcus cyanobacterium）[30-31]。
Primary endosymbiosis
These chloroplasts can be directly traced back to the ancestors of cyanobacteria and are called primary plastids (the meaning of "plastids" is almost the same as that of "chloroplasts").All primary chloroplasts belong to one of the four chloroplast lineages, namely, the chloroplast lineage of green algae and the chromophore lineage of amoeboidPaulinella chromatophoralineage）、Rhodophyte chloroplast lineage or chloroplast lineage of green plants.[32]The red alga pedigree and the green plant pedigree are the largest. The so-called green plant pedigree includes the pedigree of land plants.[23]
It is generally believed that endosymbiosis occurs inPrimitive pigment organism(Archaeplastida), in which the chlorophyta may be the earliest differentiation lineage[16][27]。The chloroplast group of green algae is the smallest of the three primary chloroplast lineages. It is only found in 13 species and is considered to be the earliest branch[19-20][38]。Like the parent of cyanobacteria, the chloroplasts of Glaucophytes retain peptidoglycan walls between their bilayer membranes[17]。Therefore, the chloroplasts of green algae are also called "murblasts".Chloroplasts of green algae also contain concentric unstacked thylakoids, which surroundcarboxysome(carboxysome) - an icosahedral structure in which the chloroplasts of green algae and blue algae maintain their carbon fixing enzyme RuBisCO.The starch they synthesize gathers outside the chloroplast[18]。Like blue algae, green algae and red algae are covered with thylakoids calledPhycobiliid(phobilisomes).Therefore, chloroplasts of green algae are considered as the original intermediate between cyanobacteria and red algae and the more evolved chloroplasts in plants.[18]
Schematic Diagram of Blue Algae Structure[39]
Rhodophyceae
Various red algae (the chloroplasts of red algae are characterized by phycobilirubin, which usually makes them red)
Rhodophyte, or red algae chloroplast group is another large and diverse chloroplast lineage[23]。The chloroplasts of red algae are also calledPhycoerythroid(rhodoplast), which literally means "red chloroplasts"[40]。
Phycoerythroid has a double-layer membrane with membrane gap,Phycobilirubin(phycobilin segments) on thylakoid membranePhycobiliidTo prevent its thylakoid accumulation[18]。Some containAmyloid nucleus（pyrenoids）。Phycoerythroid hasChlorophyll-aandPhycobiliprotein（phycobilins）[38]As a photosynthetic pigmentPhycoerythrin(phytoerythrin) makes many red algae present a unique red color.However, because it also contains blue-green chlorophyll a and other pigments, many are red to purple after combination.The red phycoerythrin pigment helps red algae capture more sunlight in deep water. Therefore, some red algae living in shallow water have less phycoerythrin in their algal erythroids and look greener.Phycoerythrin can be synthesizedRed algal starch(floridean starch), which forms and accumulates in the particles outside the phycoerythrin in the cytoplasm of red algae[18]。
Form and structure
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executive summary
Schematic diagram of chloroplast structure
In terrestrial plants, chloroplasts generally show lens shaped shape, with a diameter of 3-10 μ m, a short diameter of 2-4 μ m, and a thickness of 1-3 μ m.The chloroplast volume of maize seedlings is about 20 μ m ³.Higher botanicalMesophyll cellIt generally contains 50~200 chloroplasts, accounting for 40% of the cytoplasm. The number of chloroplasts varies depending on the cell type, ecological environment and physiological status of the species.[5]
In contrast, chloroplasts in algae show more abundant morphological diversity.Algal cells usually contain a chloroplast with different shapes, such as reticular structure (such as Oedogonium)[72]Cup shape (such asChlamydomonas ）[73][82-83]Ribbon spiral formed around the cell edge (such asSpirogyra ）[74], and slightly twisted ribbon structure at the cell edge (such as Alternaria)[75][84-85]。Some algal cells contain two chloroplasts, such asDipteropsis(Scientific name:Zygnema）The two chloroplasts are arranged in stars[76]Or may follow the specific morphology of half cells in the order Desmidiales[77]。In some algae, chloroplasts occupy most of the space of cells, while nuclei and other organelles only occupy a small part of the area.For example, Chlorella (scientific name:Chlorella）The chloroplasts of some species of[79][86-87]。
Chloroplasts of heterotrophic algae, dinoflagellates and red algae as secondary endoproducts[78]
Chloroplasts are composed ofchloroplast envelope （chloroplast envelope）、thylakoid(thylakoid) andstroma(stroma) It is a kind of organelle containing chlorophyll and capable of photosynthesis.All chloroplasts have at least three membrane systems chloroplastsadventitia(outer chloroplast membrane), chloroplastintima(inner chloroplast membrane) andThylakoid membraneThylakoid system.Chloroplasts as secondary endoproducts may have additional membranes around the three[78]。Inside the chloroplast outer membrane and inner membrane is the chloroplast matrix, which is a semi gelatinous liquid, accounting for most of the chloroplast volume, in which the thylakoid system floats.The membrane system divides chloroplasts into three kinds of cavities: membrane gap, matrix and cystoid cavity.[5]
There are some common misunderstandings about the outer membrane and inner membrane of chloroplasts.Because chloroplasts are surrounded by double membranes, people often think that they are the descendants of symbiotic cyanobacteria.This is often interpreted as the outer membrane of chloroplasts is the vesicle formed by the invagination of the host cell membrane to wrap the original cyanobacteria - but this is not the case,Both membranes of chloroplasts are homologous to the original bilayer membrane of cyanobacteria。[80]
The double membrane of chloroplasts is often compared with that of mitochondria.But this comparison is not accurate - the inner membrane of mitochondria is used for operationproton pump, and viaOxidative phosphorylationGenerate ATP energy.In chloroplasts, the only comparable structure is the internal thylakoid membrane system.However, in the so-called internal and external directions, the ion flow direction of chloroplast H+is opposite to that of oxidative phosphorylation in mitochondria.In addition, functionally, the inner membrane of chloroplasts is responsible for regulating the passage of metabolites and the synthesis of certain substances, while there is no corresponding structure in mitochondria.[81]
External cover
Submicroscopic structure of chloroplasts
The chloroplasts are surrounded by two smooth unit membranes.The two films are separated by a bright space with low electron density.These two unit membranes are called chloroplast membrane or outer envelope.The chloroplast membrane is filled with flowing stroma, in which there are many lamellar structures.[6]
thylakoid
Each layer is composed of two closed membranes, which are in the shape of flat sacs, calledthylakoid。[6]
Structure of Chloroplast
The thylakoid body is an aqueous solution.Small thylakoids stack together to form grana. Such thylakoids are calledGrana thylakoid。 The lamellae that comprise the grana are called grana lamellae.Large thylakoids cross the matrix between two or more grana.Such lamellae are called stroma lamellae, and such thylakoids are calledStroma thylakoid。[6]
stroma
Chloroplast structure
It is the fluid in the space between the intima and the thylakoid, and its main components include enzymes related to carbon assimilation, such as 1,5-Ribulose diphosphateCarboxylase occupies matrixsoluble protein 60% of the total.In additionChloroplast DNA、protein synthesizing systemSome granular components, such as various RNA, ribosome and other proteins.[7]
Chloroplast DNA (cpDNA) is the DNA in the chloroplast of eukaryotic cells.Chloroplast and other plasmid bodies have anucleusAnd containsribosome[43]Translatable and synthetic proteins[44], semi autonomous organelle, chloroplast DNA was first discovered through biochemical experiments in 1959[41]And confirmed by actual observation through electron microscope in 1962[42]。1986tobacco（Nicotiana tabacum）[45]AndLocal money（Marchantia polymorpha）[46]The chloroplast DNA of has been sequenced and published, which is the earliest chloroplast genome that has been sequenced. At present, a large number of cpDNA of terrestrial plants and algae have been sequenced and published.[47]
molecular structure
majorityChloroplast genomeIt combines into a large circular DNA molecule, usually 120000~170000 base pairs, with an outline length of about 30~60 microns and a mass of about 8~130 million daltons.[65-66]
Although it is generally considered as a ring molecule, there is some evidence that chloroplast DNA molecules are generally linear.[67]
photosynthesisChlorophyll absorbs light energy, converts it into chemical energy, and usescarbon dioxideAnd water to produce organic matter and release oxygen.This process can be used as followsChemical equationIndicate: 6COtwo+6HtwoO (light, enzyme, chloroplast) → CsixHtwelveOsix（CHtwoO）+6Otwo。It includes many complicated steps, which are generally divided into two stages: light reaction and dark reaction.[8]
Photoreaction: This is a process in which pigment molecules such as chlorophyll absorb, transfer light energy, convert light energy into chemical energy, and form ATP and NADPH.In this process, water molecules are decomposed to release oxygen.[8]
Starch grains in chloroplasts (white)[10]
Dark reaction: the next step of photosynthesis is in the dark (or under light).It uses ATP formed by light reaction to provide energy, NADPHtwoRestore COtwo, fixing the intermediate products formed, manufacturing glucose, etccarbohydrateProcess.Through this process, ATP and NADPHtwoThe active chemical energy in is converted into stable chemical energy stored in carbohydrates.It is also called carbon dioxide assimilation or carbon assimilation process.This is a process involving many enzymes.[8]
Chloroplast is the place where plant cells carry out photosynthesis, and is also the sensitive site of stress factors.Under stress, signals in plant chloroplasts reverse regulate gene expression in the nucleus, forming a reverse signaling pathway.[69-71]
motion
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Chloroplast is the organelle for photosynthesis of plants and the power plant for plant growth and development.Chloroplasts will make corresponding position adjustment with the change of light intensity in the external environment.Under dark environment or weak light conditions, chloroplasts will be scattered around the cells, or will gather along the cell plane wall toward the light source, so as to maximize the area of chloroplasts that absorb light energy to obtain the maximum photosynthetic efficiency. This phenomenon is called accumulation movement.Under strong light, chloroplasts will move away from the strong light, arrange along the vertical wall of the cell, parallel to the light source, and face the light with a small area of side to minimize the light damage. This phenomenon is called avoidance movement.The movement of plant chloroplasts with light intensity is mainly induced by blue light.Although scholars have known this phenomenon for more than a century, the research on its molecular mechanism is still limited.[35]
Blue light receptor phototropin is involved in signal transduction of chloroplast movement.[34]
Chloroplast movement
frontier research
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In October 2020,Center for Excellence and Innovation in Molecular Plant Science, Chinese Academy of SciencesThe research team of Shanghai Plant Stress Biology Research Center has revealed an important signaling pathway connecting cell membrane and chloroplast.The study revealed someVegetable proteinHow to associate with the cell membrane, and how to transfer pathogens from the cell membrane to the inside of the chloroplast when they perceive the presence of pathogens, "warning" the existence of chloroplasts.Chloroplasts transmit these information to the nucleus through the "retrograde signal transmission" process, thus regulating disease resistancegene expression, activate defense against intruders.[9]
