Units of living things: Chloroplasts. Chloroplasts, their structure, chemical composition and functions 1 structure and functions of chloroplasts
(Greek “chloros” - green) - double-membrane organelles of a rather complex structure, containing chlorophyll and carrying out photosynthesis. Characteristic only of plant cells (Fig. 1). In algae, the carriers of chlorophyll are chromatophores - the precursors of plastids; they are also found in the animal - green euglena (various forms). The chloroplasts of higher plants have the shape of a biconvex lens, which most efficiently captures light. There are on average 10-30 (up to 1000) chloroplasts in a cell. The length of the plastid is 5-10 microns, thickness - 1-3, width - 2-4 microns. Chloroplasts are covered with an outer smooth membrane, while the inner membrane forms structures called thylakoids (sacs) in the cavity of the plastid. Disc-shaped thylakoids form grana, and tube-shaped thylakoids form stroma thylakoids, connecting all grana into a single system. One grana contains from several to 50 thylakoids, and the number of grana in a chloroplast reaches 40-60. The space between the stromal thylakoids and the grana is filled with the “ground substance” - the stroma. consisting of proteins, lipids, carbohydrates, enzymes, ATP. In addition, the stroma contains plastid DNA. RNA, ribosomes. Thylakoid membranes have a typical structure, but unlike other organelles they contain coloring substances - chlorophyll (green) pigments and carotenoids (red-orange-yellow). Chlorophyll- the main pigment, associated with globular proteins in protein-pigment complexes located on the outer side of the grana thylakoid membrane. Carotenoids- additional pigments are located in the lipid layer of the membrane, where they are not visible, as they are dissolved in fats. But their location exactly corresponds to the protein-pigment complex, so the pigments in the membranes do not form a continuous layer, but are distributed mosaically. The structure of chloroplasts is closely related to their function. Photosynthesis occurs in them; Light reactions take place on the granal thylakoid membranes, and carbon fixation occurs in the stroma (dark reactions). Chloroplasts- semi-autonomous organelles in which their own proteins are synthesized, but they cannot live outside the cell for a long time, since they are under the general control of the cell nucleus. They reproduce by fission in half or can be formed from proplastids or leucoplasts. Proplastids are transmitted through the zygote in the form of very small bodies, their diameter is 0.4-1.0 microns, they are colorless and covered with a double membrane. Proplastids are found in the cells of the growth cone of the stem and root, and in the primordia of leaves. In green organs - leaves, stems - they turn into chloroplasts. At the end of the life cycle, chlorophyll is destroyed (usually by a change in daylight hours and a decrease in temperature), some of the chloroplasts turn into chromoplasts - green leaves and fruits turn red or yellow, and then fall off.
Rice. 1. Structure:a - chloroplast, b - leucoplast, c - chromoplast; 1 - outer membrane, 2 - internal membrane, 3 - metrics (stroma), 4 - stromal thylakoids (lamellas), 5 - grana, c - thylakoid grana, 7 - starch grain, 8 - carotenoids in lipid droplets, 9 - DNA, 10 - ribosomes, 11 - collapsing membrane structures
photosynthesis occurs in specialized cell organelles - chloroplasts. Chloroplasts of higher plants have a biconvex shape lenses(disk), which is most convenient for absorbing sunlight. Their size, quantity, and location fully correspond to their purpose: to absorb solar energy as efficiently as possible and to assimilate carbon as completely as possible. It has been established that the number of chloroplasts in a cell is measured in tens. This ensures a high content of these organelles per unit of leaf surface. Yes, on 1 mm 2 leaves of beans accounted for 283 thousand chloroplasts, in sunflower - 465 thousand. Diameter chloroplasts on average 0.5-2 microns.
Chloroplast structure very complex. Like the nucleus and mitochondria, the chloroplast is surrounded by a shell consisting of two lipoprotein membranes. The internal environment is represented by a relatively homogeneous substance - matrix or stroma , which is penetrated by membranes - lamellae (rice.). Lamelae connected to each other form bubbles - thylakoids . Tightly adjacent to each other, thylakoids form grains , which can be distinguished even under a light microscope. In turn, the grana in one or several places are united with each other using intergranal strands - stromal thylakoids.
Properties of chloroplasts: capable of changing orientation and move around. For example, under the influence of bright light, chloroplasts turn the narrow side of the disk towards the incident rays and move to the side walls of the cells. Chloroplasts move towards higher concentrations of CO 2 in the cell. During the day they usually line up along the walls, and at night they sink to the bottom of the cage.
Chemical composition chloroplasts: water - 75%; 75-80% of the total amount of dry matter is org. compounds, 20-25% mineral.
The structural basis of chloroplasts is squirrels (50-55 % dry mass), half of them are water-soluble proteins. Such a high protein content is explained by their diverse functions within chloroplasts (structural membrane proteins, enzyme proteins, transport proteins, contractile proteins, receptor proteins).
The most important components of chloroplasts are lipids , (30-40% dry m.). Chloroplast lipids are represented by three groups of compounds.
Structural components of membranes, which are represented by amphipathic lipoids and are characterized by a high content (more than 50%) of galactolipids and sulfolipids. The phospholipid composition is characterized absence of phosphatidylethanolamine and high content phosphatidylglycerol(more than 20%). Over 60 % composition of the liquid crystals accounts for linoleic acid.
Photosynthetic pigments chloroplasts - hydrophobic substances related to lipoids(water-soluble pigments in cell sap). Higher plants contain 2 forms green pigments: chlorophyll a And chlorophyllb and 2 forms of yellow pigments: carotenes And xanthophylls(carotenoids). Chlorophyll plays a role photosensitizers, other pigments expand the spectrum of photosynthesis due to more complete absorption of PAR. Carotenoids protect chlorophyll from photo-oxidation, participate in hydrogen transport, formed during photolysis of water.
Fat-soluble vitamins - ergosterol(provitamin D), vitamins E, TO- concentrated almost entirely in chloroplasts, where they participate in the conversion of light energy into chemical energy. The cytosol of leaf cells contains mainly water-soluble vitamins. Thus, in spinach, the content of ascorbic acid in chloroplasts is 4-5 times less than in the leaves.
The chloroplasts of leaves contain a significant amount RNA and DNA . NCs make up approximately 1% of the dry weight of chloroplasts (RNA - 0.75%, DNA - 0.01-0.02%). The chloroplast genome is represented by a circular DNA molecule 40 µm long with a molecular weight of 108, encoding 100-150 medium-sized proteins. Chloroplast ribosomes make up 20 to 50% of the total population of ribosomes in the cell. Thus, chloroplasts have their own protein synthesizing system. However, for the normal functioning of chloroplasts, interaction between the nuclear and chloroplast genomes is necessary. The key enzyme of photosynthesis, RDP carboxylase, is synthesized under dual control - DNA of the nucleus and chloroplast.
Carbohydrates are not constitutional substances of the chloroplast. They are represented by phosphorus esters of sugars and products of photosynthesis. Therefore, the carbohydrate content in chloroplasts varies significantly (from 5 to 50%). In actively functioning chloroplasts, carbohydrates usually do not accumulate; their rapid outflow occurs. With a decrease in the need for photosynthetic products, large starch grains are formed in the chloroplasts. In this case, the starch content may increase to 50 % dry mass and chloroplast activity will decrease.
Minerals. The chloroplasts themselves make up 25-30% of the leaf mass, but they contain up to 80 % Fe, 70-72 - MgAndZn, 50 - Cu, 60 % Ca contained in leaf tissues. This is explained by the high and diverse enzymatic activity of chloroplasts (including prosthetic groups and cofactors). Mg is part of chlorophyll. Ca stabilizes the membrane structures of chloroplasts.
The emergence and development of chloroplasts . Chloroplasts are formed in meristematic cells from initial particles or rudimentary plastids (Fig.). The initial particle consists of an amoeboid strema surrounded by a double-membrane shell. As the cell grows, the initial particles increase in size and take on the shape of a biconvex lens, and small starch grains appear in the stapes. At the same time, the inner membrane begins to grow, forming folds (invaginations), from which vesicles and tubes emerge. Such formations are called proplastids . For their further development, light is needed. In the dark they form etioplasts , in which a membrane lattice structure is formed - the prolamellar body. In the light, the internal membranes of proplastids and etioplasts form cutting system. At the same time, newly formed molecules of chlorophyll and other pigments are also built into the grana in the light. Thus, structures that are prepared to function in light appear and develop only in its presence.
Along with chloroplasts, there are a number of other plastids, which are formed either directly from proplastids, or from one another through mutual transformations ( rice.). These include starch-accumulating amyloplasts ( leucoplasts) And chromoplasts containing carotenoids. In flowers and fruits, chromoplasts arise early in proplastid development. Chromoplasts of autumn foliage are products of chloroplast degradation, in which plastoglobules act as carotnoid carrier structures.
Pigments chloroplasts involved in capturing light energy, as well as enzymes required for the light phase photosynthesis, built into membranes thylakoids.
Enzymes , which catalyze numerous reactions of the reduction cycle of carbohydrates (tempo phase of photosynthesis), as well as various biosyntheses, including the biosynthesis of proteins, lipids, starch, are present mainly in the stroma, some of them are peripheral lamella proteins.
The structure of mature chloroplasts is the same in all higher plants, as well as in the cells of different organs of the same plant (leaves, green roots, bark, fruits). Depending on the functional load of cells, the physiological state of chloroplasts, and their age, the degree of their internal structure is distinguished: size, number of grains, connection between them. So, in the closing stomatal cells the main function of chloroplasts is photoregulation stomatal movements. Chloroplasts do not have a strict granular structure; they contain large starch grains, swollen thylakoids, and lipophilic globules. All this indicates their low energy load (this function is performed by mitochondria). A different picture is observed when studying the chloroplasts of green tomato fruits. Availability well developed granular system indicates a high functional load of these organelles and, probably, a significant contribution of photosynthesis during fruit formation.
Age-related changes: Young ones are characterized by a lamellar structure; in this state, chloroplasts are able to reproduce by division. In mature ones, the gran system is well expressed. In aging individuals, the stromal thylakoids rupture, the connection between grana decreases, and subsequently the disintegration of chlorophyll and destruction of grana are observed. In autumn foliage, degradation of chloroplasts leads to the formation chromoplasts .
Chloroplast structure labile and dynamic , it reflects all the living conditions of the plant. The mineral nutrition regime of plants has a great influence. If there is a shortage N chloroplasts become 1.5-2 times smaller, deficiency P And S disrupts the normal structure of lamellae and granae, simultaneous deficiency N And Ca leads to an overflow of chloroplasts with starch due to disruption of the normal outflow of assimilates. If there is a shortage Ca the structure of the outer membrane of the chloroplast is disrupted. To maintain the structure of the chloroplast, light is also necessary; in the dark, the granal and stremal thylakoids are gradually destroyed.
They are grouped into grana, which are stacks of disc-shaped thylakoids flattened and closely pressed together. The granae are connected using lamellae. The space between the chloroplast membrane and the thylakoids is called stroma. The stroma contains chloroplast molecules RNA, plastid DNA, ribosomes , starchy grains and enzymes Calvin cycle.
Origin
The origin of chloroplasts by symbiogenesis is now generally accepted. It is believed that chloroplasts arose from cyanobacteria, since they are a double-membrane organelle, have their own closed circular DNA and RNA, a full-fledged protein synthesis apparatus (and ribosomes of the prokaryotic type - 70S), multiply binary fission, and the thylakoid membranes are similar to the membranes of prokaryotes (by the presence of acidic lipids) and resemble the corresponding organelles in cyanobacteria. U glaucophytes algae, instead of typical chloroplasts, the cells contain cyanella- cyanobacteria that, as a result of endosymbiosis, have lost the ability to exist independently, but have partially retained the cyanobacterial cell wall.
The duration of this event is estimated at 1 - 1.5 billion years.
Some groups of organisms received chloroplasts as a result of endosymbiosis not with prokaryotic cells, but with other eukaryotes that already had chloroplasts. This explains the presence of more than two membranes in the chloroplast membrane of some organisms. The innermost of these membranes is interpreted as having lost cell wall the shell of a cyanobacterium, the outer one is like the wall of the host symbiontophoric vacuole. Intermediate membranes belong to a reduced eukaryotic organism that has entered into symbiosis. In some groups, in the periplastid space between the second and third membranes there is a nucleomorph, a highly reduced eukaryotic nucleus.
Chloroplast model
Structure
In different groups of organisms, chloroplasts vary significantly in size, structure and number in the cell. The structural features of chloroplasts have great taxonomic meaning .
Chloroplast shell
In different groups of organisms, the chloroplast membrane differs in structure.
In glaucocystophytes, red and green algae and in higher plants, the shell consists of two membranes. In other eukaryotic algae, the chloroplast is additionally surrounded by one or two membranes. In algae that have four-membrane chloroplasts, the outer membrane usually merges into the outer membrane of the nucleus.
Periplastid space
Lamella and thylakoids
Lamellae connect the thylakoid cavities
Pyrenoids
Pyrenoids are centers of polysaccharide synthesis in chloroplasts. The structure of pyrenoids is varied, and they are not always morphologically expressed. They can be intraplastidal or stalk-like, protruding into the cytoplasm. In green algae and plants, pyrenoids are located inside the chloroplast, which is associated with the intraplastid storage of starch.
Stigma
Stigmas or ocelli are found in the chloroplasts of motile algae cells. Located near the base of the flagellum. Stigmas contain carotenoids and are able to work as photoreceptors.
see also
Notes
Comments
Notes
Literature
- Belyakova G. A. Algae and mushrooms // Botany: in 4 volumes / Belyakova G. A., Dyakov Yu. T., Tarasov K. L. - M.: Publishing center "Academy", 2006. - T. 1. - 320 p. - 3000 copies. - ISBN 5-7695-2731-5
- Karpov S.A. The structure of a protist cell. - St. Petersburg. : TESSA, 2001. - 384 p. - 1000 copies. - ISBN 5-94086-010-9
- Lee, R. E. Phycology, 4th edition. - Cambridge: Cambridge University Press, 2008. - 547 p. - ISBN 9780521682770
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See what “Chloroplasts” are in other dictionaries:
CHLOROPLASTS- (from the Greek chloros green and plastos fashioned), intracellular organelles (plastids) of plants, in which photosynthesis occurs; Thanks to chlorophyll, they are colored green. Found in various cells. tissues of above-ground plant organs,... ... Biological encyclopedic dictionary
CHLOROPLASTS- (from the Greek chloros green and plastos sculpted formed), intracellular organelles of a plant cell in which photosynthesis occurs; colored green (they contain chlorophyll). Own genetic apparatus and... ... Big Encyclopedic Dictionary
Chloroplasts- bodies contained in plant cells, colored green and containing chlorophyll. In higher plants, chlorophylls have a very definite shape and are called chlorophyll grains; Algae have a varied form and they are called chromatophores or... Encyclopedia of Brockhaus and Efron
Chloroplasts- (from the Greek chloros green and plastos fashioned, formed), intracellular structures of a plant cell in which photosynthesis occurs. They contain the pigment chlorophyll, which colors them green. In the cell of higher plants there are from 10 to ... Illustrated Encyclopedic Dictionary
chloroplasts- (gr. chloros green + lastes forming) green plastids of a plant cell containing chlorophyll, carotene, xanthophyll and involved in the process of photosynthesis cf. chromoplasts). New dictionary of foreign words. by EdwART, 2009. chloroplasts [gr.... ... Dictionary of foreign words of the Russian language
Chloroplasts- (from the Greek chlorós green and plastós fashioned, formed) intracellular organelles of a plant cell Plastids in which photosynthesis occurs. They are colored green due to the presence of the main pigment of photosynthesis... Great Soviet Encyclopedia
chloroplasts- ov; pl. (unit chloroplast, a; m.). [from Greek chlōros pale green and plastos sculpted] Botan. Bodies in the protoplasm of plant cells containing chlorophyll and participating in the process of photosynthesis. Chlorophyll concentration in chloroplasts. * * *… … encyclopedic Dictionary
Chloroplasts- bodies contained in plant cells, colored green and containing chlorophyll. In higher plants, X. have a very definite shape and are called chlorophyll grains (see); Algae have a variety of shapes and they are called... ... Encyclopedic Dictionary F.A. Brockhaus and I.A. Efron
Chloroplasts- pl. Green plastids of a plant cell containing chlorophyll, carotene and participating in the process of photosynthesis. Ephraim's explanatory dictionary. T. F. Efremova. 2000... Modern explanatory dictionary of the Russian language by Efremova
CHLOROPLASTS- (from the Greek chloros green and plastоs sculpted, formed), grows intracellular organelles. cells in which photosynthesis occurs; colored green (they contain chlorophyll). Own genetic apparatus and protein synthesizing... ... Natural science. encyclopedic Dictionary
Chloroplasts are structures in which photosynthetic processes occur, ultimately leading to the binding of carbon dioxide, the release of oxygen and the synthesis of sugars. elongated structures with a width of 2-4 microns and a length of 5-10 microns. Green algae have giant chloroplasts (chromatophores) reaching a length of 50 microns.
green algae may have one chloroplast per cell. Typically, there are an average of 10-30 chloroplasts per cell of higher plants. There are cells with a huge number of chloroplasts. For example, about 1000 chloroplasts were found in the giant cells of the palisade tissue of shag.
Chloroplasts are structures bounded by two membranes - internal and external. The outer membrane, like the inner one, has a thickness of about 7 microns; they are separated from each other by an intermembrane space of about 20-30 nm. The inner membrane of chloroplasts separates the plastid stroma, which is similar to the mitochondrial matrix. In the stroma of the mature chloroplast of higher plants, two types of internal membranes are visible. These are membranes that form flat, extended stromal lamellae, and membranes of thylakoids, flat disc-shaped vacuoles or sacs.
The stromal lamellae (about 20 µm thick) are flat hollow sacs or have the appearance of a network of branched and interconnected channels located in the same plane. Typically, the stromal lamellae inside the chloroplast lie parallel to each other and do not form connections with each other.
In addition to stromal membranes, membrane thylakoids are found in chloroplasts. These are flat, closed, disc-shaped membrane bags. The size of their intermembrane space is also about 20-30 nm. These thylakoids form coin-like stacks called grana. 
The number of thylakoids per grana varies greatly: from a few to 50 or more. The size of such stacks can reach 0.5 microns, so the grains are visible in some objects in a light microscope. The number of grains in the chloroplasts of higher plants can reach 40-60. The thylakoids in the grana are close to each other so that the outer layers of their membranes are closely connected; at the junction of the thylakoid membranes, a dense layer about 2 nm thick is formed. In addition to the closed chambers of the thylakoids, the grana usually also includes sections of lamellae, which also form dense 2-nm layers at the points of contact of their membranes with the thylakoid membranes. The stromal lamellae thus seem to connect the individual grana of the chloroplast with each other. However, the cavities of the thylakoid chambers are always closed and do not pass into the chambers of the intermembrane space of the stromal lamellae. The stromal lamellae and thylakoid membranes are formed by separation from the inner membrane during the initial stages of plastid development.
DNA molecules and ribosomes are found in the matrix (stroma) of chloroplasts; This is also where the primary deposition of the reserve polysaccharide, starch, occurs in the form of starch grains.
A characteristic feature of chloroplasts is the presence of pigments, chlorophylls, which give color to green plants. With the help of chlorophyll, green plants absorb energy from sunlight and convert it into chemical energy. 
Functions of chloroplasts
Plastid genome
Like mitochondria, chloroplasts have their own genetic system that ensures the synthesis of a number of proteins within the plastids themselves. DNA, various RNAs and ribosomes are found in the chloroplast matrix. It turned out that the DNA of chloroplasts differs sharply from the DNA of the nucleus. It is represented by cyclic molecules up to 40-60 microns in length, with a molecular weight of 0.8-1.3x108 daltons. There can be many copies of DNA in one chloroplast. Thus, in an individual corn chloroplast there are 20-40 copies of DNA molecules. The duration of the cycle and the rate of replication of nuclear and chloroplast DNA, as has been shown in green algae cells, do not coincide. Chloroplast DNA is not complexed with histones. All these characteristics of chloroplast DNA are close to the characteristics of the DNA of prokaryotic cells. Moreover, the similarity of the DNA of chloroplasts and bacteria is also reinforced by the fact that the main transcription regulatory sequences (promoters, terminators) are the same. All types of RNA (messenger, transfer, ribosomal) are synthesized on chloroplast DNA. Chloroplast DNA encodes rRNA, which is part of the ribosomes of these plastids, which belong to the prokaryotic 70S type (contain 16S and 23S rRNA). Chloroplast ribosomes are sensitive to the antibiotic chloramphenicol, which inhibits protein synthesis in prokaryotic cells.
Just as in the case of chloroplasts, we are again faced with the existence of a special protein synthesis system, different from that in the cell.
These discoveries renewed interest in the theory of the symbiotic origin of chloroplasts. The idea that chloroplasts arose by combining heterotrophic cells with prokaryotic blue-green algae, expressed at the turn of the 19th and 20th centuries. (A.S. Fomintsin, K.S. Merezhkovsky) again finds its confirmation. This theory is supported by the amazing similarity in the structure of chloroplasts and blue-green algae, the similarity with their main functional features, and primarily with the ability for photosynthetic processes.
There are numerous known facts of true endosymbiosis of blue-green algae with cells of lower plants and protozoa, where they function and supply the host cell with photosynthetic products. It turned out that isolated chloroplasts can also be selected by some cells and used by them as endosymbionts. In many invertebrates (rotifers, mollusks) that feed on higher algae, which they digest, intact chloroplasts end up inside the cells of the digestive glands. Thus, in some herbivorous mollusks, intact chloroplasts with functioning photosynthetic systems were found in the cells, the activity of which was monitored by the incorporation of C14O2.
As it turned out, chloroplasts can be introduced into the cytoplasm of mouse fibroblast culture cells by pinocytosis. However, they were not attacked by hydrolases. Such cells, which included green chloroplasts, could divide for five generations, while the chloroplasts remained intact and carried out photosynthetic reactions. Attempts were made to cultivate chloroplasts in artificial media: chloroplasts could photosynthesize, RNA synthesis took place in them, they remained intact for 100 hours, and divisions were observed even within 24 hours. But then there was a drop in the activity of chloroplasts, and they died.
These observations and a number of biochemical works showed that those features of autonomy that chloroplasts possess are still insufficient for long-term maintenance of their functions, much less for their reproduction.
Recently, it was possible to completely decipher the entire sequence of nucleotides in the cyclic DNA molecule of chloroplasts of higher plants. This DNA can encode up to 120 genes, among them: genes of 4 ribosomal RNAs, 20 ribosomal proteins of chloroplasts, genes of some subunits of chloroplast RNA polymerase, several proteins of photosystems I and II, 9 of 12 subunits of ATP synthetase, parts of proteins of the electron transport chain complexes , one of the subunits of ribulose diphosphate carboxylase (the key enzyme for CO2 binding), 30 tRNA molecules and another 40 as yet unknown proteins. Interestingly, a similar set of genes in chloroplast DNA was found in such distant representatives of higher plants as tobacco and liver moss.
The bulk of chloroplast proteins is controlled by the nuclear genome. It turned out that a number of the most important proteins, enzymes, and, accordingly, the metabolic processes of chloroplasts are under the genetic control of the nucleus. Thus, the cell nucleus controls individual stages of the synthesis of chlorophyll, carotenoids, lipids, and starch. Many dark stage enzymes and other enzymes, including some components of the electron transport chain, are under nuclear control. Nuclear genes encode DNA polymerase and aminoacyl-tRNA synthetase of chloroplasts. Most ribosomal proteins are under the control of nuclear genes. All these data make us talk about chloroplasts, as well as mitochondria, as structures with limited autonomy.
The transport of proteins from the cytoplasm to plastids occurs in principle similar to that of mitochondria. Here, too, at the points of convergence of the outer and inner membranes of the chloroplast, channel-forming integral proteins are located, which recognize the signal sequences of chloroplast proteins synthesized in the cytoplasm and transport them to the matrix-stroma. From the stroma, imported proteins, according to additional signal sequences, can be included in plastid membranes (thylakoids, stromal lamellae, outer and inner membranes) or localized in the stroma, being part of ribosomes, enzyme complexes of the Calvin cycle, etc.
The amazing similarity of the structure and energy processes in bacteria and mitochondria, on the one hand, and in blue-green algae and chloroplasts, on the other, serves as a strong argument in favor of the theory of the symbiotic origin of these organelles. According to this theory, the emergence of a eukaryotic cell went through several stages of symbiosis with other cells. At the first stage, cells such as anaerobic heterotrophic bacteria included aerobic bacteria, which turned into mitochondria. In parallel, in the host cell, the prokaryotic genophore is formed into a nucleus isolated from the cytoplasm. This is how heterotrophic eukaryotic cells could arise. Repeated endosymbiotic relationships between primary eukaryotic cells and blue-green algae led to the appearance of chloroplast-type structures in them, allowing the cells to carry out autosynthetic processes and not depend on the presence of organic substrates (Fig. 236). During the formation of such a composite living system, part of the genetic information of mitochondria and plastids could change and be transferred to the nucleus. For example, two thirds of the 60 ribosomal proteins of chloroplasts are encoded in the nucleus and synthesized in the cytoplasm, and then integrated into chloroplast ribosomes, which have all the properties of prokaryotic ribosomes. This movement of a large part of prokaryotic genes into the nucleus led to the fact that these cellular organelles, retaining part of their former autonomy, came under the control of the cell nucleus, which largely determines all the main cellular functions.
Proplastids
Under normal lighting, proplastids turn into chloroplasts. First, they grow, with the formation of longitudinally located membrane folds from the inner membrane. Some of them extend along the entire length of the plastid and form stromal lamellae; others form thylakoid lamellae, which are stacked to form the grana of mature chloroplasts. Plastid development occurs somewhat differently in the dark. In etiolated seedlings, the volume of plastids, etioplasts, initially increases, but the system of internal membranes does not build lamellar structures, but forms a mass of small vesicles that accumulate in separate zones and can even form complex lattice structures (prolamellar bodies). The membranes of etioplasts contain protochlorophyll, a yellow precursor of chlorophyll. Under the influence of light, chloroplasts are formed from etioplasts, protochlorophyll is converted into chlorophyll, new membranes, photosynthetic enzymes and components of the electron transport chain are synthesized.
When cells are illuminated, membrane vesicles and tubes quickly reorganize, and from them a complete system of lamellae and thylakoids, characteristic of a normal chloroplast, develops.
Leukoplasts differ from chloroplasts in the absence of a developed lamellar system (Fig. 226 b). They are found in the cells of storage tissues. Due to their indeterminate morphology, leucoplasts are difficult to distinguish from proplastids and sometimes from mitochondria. They, like proplastids, are poor in lamellae, but nevertheless are capable of forming normal thylakoid structures under the influence of light and acquiring a green color. In the dark, leucoplasts can accumulate various reserve substances in the prolamellar bodies, and grains of secondary starch are deposited in the stroma of leucoplasts. If the so-called transient starch is deposited in chloroplasts, which is present here only during CO2 assimilation, then true starch storage can occur in leucoplasts. In some tissues (endosperm of cereals, rhizomes and tubers), the accumulation of starch in leucoplasts leads to the formation of amyloplasts, completely filled with reserve starch granules located in the stroma of the plastid (Fig. 226c).
Another form of plastid in higher plants is the chromoplast, which usually turns yellow as a result of the accumulation of carotenoids in it (Fig. 226d). Chromoplasts are formed from chloroplasts and much less frequently from their leucoplasts (for example, in carrot roots). The process of bleaching and changes in chloroplasts is easily observed during the development of petals or during ripening of fruits. In this case, yellow-colored droplets (globules) may accumulate in the plastids, or bodies in the form of crystals may appear in them. These processes are associated with a gradual decrease in the number of membranes in the plastid, with the disappearance of chlorophyll and starch. The process of formation of colored globules is explained by the fact that when the lamellae of chloroplasts are destroyed, lipid droplets are released in which various pigments (for example, carotenoids) are well dissolved. Thus, chromoplasts are degenerating forms of plastids, subject to lipophanerosis - the disintegration of lipoprotein complexes.
