The internal structure of the chloroplast. Functions of chloroplasts. Differences of plants storing starch
/. Chloroplasts
2. Thylakoids
3. Thylakoid membranes
4. Protein complexes
5. Biochemical synthesis in the stroma of chloroplasts
1. Embryonic cells contain colorless proplastids. Depending on the type of fabric they develop: into green chloroplasts;
other forms of plastids are derived from chloroplasts (phylogenetically later):
Yellow or red chromoplasts;
Colorless leucoplasts.
Structure and composition chloroplasts. IN cells higher plants, like some algae, there are about 10-200 lenticular chloroplasts, only 3-10 microns in size.
Chloroplasts- plastids of cells of organs of higher plants, in the world, such as:
Non-lignified stem (outer tissues);
Young fruits;
Less commonly in the epidermis and in the corolla of the flower.
The chloroplast envelope, consisting of two membranes, surrounds a colorless stroma, which is penetrated by many flat closed membrane pockets (cistern) - thylakoids, stained in green color. Therefore, cells with chloroplasts are green.
Sometimes the green color is masked by other pigments of chloroplasts (in red and brown algae) or cell sap (in forest beech). Algae cells contain one or more different forms of chloroplasts.
The chloroplasts contain the following various pigments(depending on plant type):
Chlorophyll:
Chlorophyll A (blue-green) - 70% (in higher plants and
green algae); . chlorophyll B (yellow-green) - 30% (ibid.);
Chlorophyll C, D and E is less common in other groups of algae;
Carotenoids:
Orange-red carotenes (hydrocarbons);
Yellow (rarely red) xanthophylls (oxidized carotenes). Thanks to the xanthophyll phycoxanthin, brown algae chloroplasts (pheoplasts) are colored brown;
Phycobiliproteins contained in rhodoplasts (chloroplasts of red and blue-green algae):
Blue phycocyanin;
Red phycoerythrin.
Function of chloroplasts: chloroplast pigment absorbs light to implement photosynthesis - the process of converting light energy into chemical energy of organic substances, primarily carbohydrates, which are synthesized in chloroplasts from substances poor in energy - CO2 and H2O
2. Prokaryotes do not have chloroplasts, but they have there are numerous thylakoids,limited by the plasma membrane:
In photosynthetic bacteria:
Tubular or lamellar;
Either in the form of bubbles or lobules;
In blue-green algae, thylakoids are flattened cisterns:
Forming a spherical system;
Or parallel to each other;
Or randomly placed.
In eukaryotic plants In cells, thylakoids are formed from the folds of the inner membrane of the chloroplast. Chloroplasts from edge to edge are penetrated by long stroma thylakoids, around which densely packed and short thylakoids gran. Stacks of such thylakoid grana are visible under a light microscope as green grana 0.3–0.5 µm in size.
3. Between the grana, the thylakoids of the stroma are reticularly intertwined. Thylakoid granae are formed from superimposed outgrowths of stromal thylakoids. At the same time, internal (in-tracisternal) the spaces of many or all thylakoids remain interconnected.
Thylakoid membranes 7-12 nm thick are very rich in protein (protein content - about 50%, in total over 40 different proteins).
Thylakodda membranes carry out that part of the photosynthesis reactions that is associated with energy conversion - the so-called light reactions. These processes involve two chlorophyll-containing photosystems I and II, connected by an electron transport chain, and an ATP-producing membrane ATPase. Using method freezing-chipping, it is possible to split thylakoid membranes into two layers along the border passing between the two layers of lipids. In this case, using an electron microscope, you can see four surfaces:
The membrane from the side of the stroma;
Membrane from the side of the inner space of the thylakoid;
The inner side of the lipid monolayer adjacent To stroma;
The inner side of the monolayer adjacent to the inner space.
In all four cases, a dense packing of protein particles is visible, which normally penetrate the membrane through and through, and when the membrane is stratified, they break out of one or another lipid layer.
4. Using detergents(for example, digitonin) can be isolated from thylakoid membranes six different protein complexes:
Large FSN-CCK particles, which are a hydrophobic integral membrane protein. The FSN-SSC complex is located mainly in those places where the membranes are in contact with the adjacent thylakoid. It can be divided:
On the FSP particle;
And several identical chlorophyll-rich CCK particles. This is a complex of particles that "collect" light quanta and transfer their energy to the PSF particle;
PS1 particles, hydrophobic integral membrane proteins;
Particles with electron transport chain components (cytochromes) that are optically indistinguishable from PS1. Hydrophobic integral membrane proteins;
CF0 - part of the membrane ATPase fixed in the membrane, 2-8 nm in size; is a hydrophobic integral membrane protein;
CF1 is a peripheral and easily detachable hydrophilic "head" of membrane ATPase. The CF0-CF1 complex acts in the same way as F0-F1 in mitochondria. The CF0-CF1 complex is located mainly in those places where the membranes do not touch;
Peripheral, hydrophilic, a very weakly bound enzyme ribulose bisphosphate carboxylase, functionally belonging to the stroma.
Chlorophyll molecules are contained in the particles of PS1, FSP, and SSC. They are amphipathic and contain:
Hydrophilic disc-shaped porphyrin ring that lies on the surface of the membrane (in the stroma, in the interior of the thylakoid, or on both sides);
Hydrophobic residue of phytol. Phytol residues lie in hydrophobic protein particles.
5. In the stroma of chloroplasts, processes biochemical synthesis(photosynthesis), as a result of which:
Starch grains (a product of photosynthesis);
Plastoglobuli, which are composed of lipids (mainly glycolipids) and accumulate quinones:
Plastoquinone;
Phylloquinone (vitamin K1);
Tocopherylquinone (vitamin E);
Crystals of the iron-containing protein phytoferritin (iron accumulation).
(membrane formations in which the electron transport chain of chloroplasts is located). Thylakoids of higher plants are grouped into grana, which are stacks of flattened and closely pressed to each other disc-shaped thylakoids. The grana are connected with the help of lamellae. The space between the chloroplast membrane and the thylakoids is called the stroma. The stroma contains chloroplast RNA molecules, plastid DNA, ribosomes, starch grains, and Calvin cycle enzymes.
Origin
At present, the origin of chloroplasts by symbiogenesis is generally recognized. It is assumed that chloroplasts originated from cyanobacteria, as they are a two-membrane organoid, have their own closed circular DNA and RNA, a complete protein synthesis apparatus (moreover, ribosomes of the prokaryotic type - 70S), multiply by binary fission, and thylakoid membranes are similar to prokaryotic membranes (the presence of acidic lipids ) and resemble the corresponding organelles in cyanobacteria. In glaucophyte algae, instead of typical chloroplasts, the cells contain cyanella - cyanobacteria that have lost the ability to exist independently as a result of endosymbiosis, but partly retained the cyanobacterial cell wall.
The age 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 have 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 a shell of a cyanobacterium that has lost its cell wall, while the outer one is interpreted as the wall of the host's symbiontophore 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.
Structure
In different groups of organisms, chloroplasts differ significantly in size, structure and number in the cell. Features of the structure of chloroplasts are of great taxonomic importance. Basically, chloroplasts have the shape of a biconvex lens, their size is about 4-6 microns.
Shell of chloroplasts
In different groups of organisms, the shell of chloroplasts differs in structure.
In glaucocystophytes, red, 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 with four-membrane chloroplasts, the outer membrane usually extends into the outer membrane of the nucleus.
Periplastid space
Lamella and thylakoids
The lamellae connect the cavities of the thylakoids.
Pyrenoids
see also
Notes
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Notes
Literature
- Belyakova G. A. Algae and fungi // 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 the protist cell. - St. Petersburg. : TESSA, 2001. - 384 p. - 1000 copies. - ISBN 5-94086-010-9.
- Lee, R.E. Physiology, 4th edition. - Cambridge: Cambridge University Press, 2008. - 547 p. - ISBN 9780521682770.
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An excerpt characterizing Chloroplasts
“This is how they danced in our time, ma chere,” said the count.- Oh yes Danila Kupor! ' said Marya Dmitrievna, letting out her breath heavily and continuously, and rolling up her sleeves.
While the sixth anglaise was being danced in the hall at the Rostovs' to the sounds of tired musicians who were out of tune, and the tired waiters and cooks were preparing dinner, the sixth stroke took place with Count Bezukhim. The doctors announced that there was no hope of recovery; the patient was given a deaf confession and communion; preparations were made for the unction, and the house was full of fuss and anxiety of expectation, common at such moments. Outside the house, behind the gates, undertakers crowded, hiding from the approaching carriages, waiting for a rich order for the count's funeral. The Commander-in-Chief of Moscow, who constantly sent adjutants to learn about the position of the count, that evening he himself came to say goodbye to the famous Catherine's nobleman, Count Bezukhim.
The magnificent reception room was full. Everyone stood up respectfully when the commander-in-chief, having been alone with the patient for about half an hour, left there, slightly answering the bows and trying as soon as possible to get past the eyes of doctors, clerics and relatives fixed on him. Prince Vasily, who had grown thinner and paler these days, saw off the commander-in-chief and quietly repeated something to him several times.
After seeing off the commander-in-chief, Prince Vasily sat alone in the hall on a chair, throwing his legs high over his legs, resting his elbow on his knee and closing his eyes with his hand. After sitting like this for some time, he got up and with unusually hasty steps, looking around with frightened eyes, went through a long corridor to the back half of the house, to the elder princess.
Those who were in the dimly lit room spoke in an uneven whisper among themselves and fell silent each time, and with eyes full of question and expectation looked back at the door that led to the chambers of the dying man and made a faint sound when someone left it or entered it.
“The human limit,” the old man, a clergyman, said to the lady who sat down next to him and listened naively to him, “the limit is set, but you can’t pass it.”
– I think it’s not too late to unction? - adding a spiritual title, the lady asked, as if she did not have any opinion on this matter.
“A sacrament, mother, great,” the clergyman answered, running his hand over his bald head, along which lay several strands of combed half-gray hair.
- Who is this? Was he the commander in chief? asked at the other end of the room. - What a youthful! ...
- And the seventh ten! What, they say, the count does not know? Wanted to congregate?
- I knew one thing: I took unction seven times.
The second princess had just left the patient's room with tearful eyes and sat down beside Dr. Lorrain, who was sitting in a graceful pose under the portrait of Catherine, leaning on the table.
“Tres beau,” said the doctor, answering a question about the weather, “tres beau, princesse, et puis, a Moscou on se croit a la campagne.” [beautiful weather, princess, and then Moscow looks so much like a village.]
- N "est ce pas? [Isn't it?] - said the princess, sighing. - So can he drink?
Lorren considered.
Did he take medicine?
- Yes.
The doctor looked at the breguet.
- Take a glass of boiled water and put une pincee (he showed with his thin fingers what une pincee means) de cremortartari ... [a pinch of cremortartar ...]
- Do not drink, listen, - the German doctor said to the adjutant, - that the shiv remained from the third blow.
And what a fresh man he was! the adjutant said. And who will this wealth go to? he added in a whisper.
“The farmer will be found,” the German replied, smiling.
Everyone again looked at the door: it creaked, and the second princess, having made the drink shown by Lorrain, carried it to the patient. The German doctor approached Lorrain.
"Maybe it'll make it to tomorrow morning, too?" the German asked, speaking badly in French.
Lorren, pursing his lips, sternly and negatively waved his finger in front of his nose.
“Tonight, not later,” he said quietly, with a decent smile of self-satisfaction in that he clearly knows how to understand and express the situation of the patient, and walked away.
Meanwhile, Prince Vasily opened the door to the princess's room.
The room was semi-dark; only two lamps were burning in front of the images, and there was a good smell of smoke and flowers. The whole room was set with small furniture of chiffonieres, cupboards, tables. From behind the screens one could see the white bedspreads of a high feather bed. The dog barked.
“Ah, is that you, mon cousin?”
She got up and straightened her hair, which she always, even now, was so unusually smooth, as if it had been made from one piece with her head and covered with varnish.
- What, something happened? she asked. - I'm already so scared.
- Nothing, everything is the same; I just came to talk to you, Katish, about business, - the prince said, wearily sitting down on the chair from which she got up. “How hot you are, however,” he said, “well, sit down here, causons. [talk.]
“I thought, did something happen? - said the princess, and with her unchanging, stonyly stern expression, sat down opposite the prince, preparing to listen.
“I wanted to sleep, mon cousin, but I can’t.
- Well, what, my dear? - said Prince Vasily, taking the hand of the princess and bending it down according to his habit.
It was evident that this "well, what" referred to many things that, without naming, they understood both.
The princess, with her incongruously long legs, dry and straight waist, looked directly and impassively at the prince with convex gray eyes. She shook her head and sighed as she looked at the icons. Her gesture could be explained both as an expression of sadness and devotion, and as an expression of fatigue and hope for a quick rest. Prince Vasily explained this gesture as an expression of fatigue.
“But for me,” he said, “do you think it’s easier?” Je suis ereinte, comme un cheval de poste; [I'm mortified like a mail horse;] but still I need to talk to you, Katish, and very seriously.
Prince Vasily fell silent, and his cheeks began to twitch nervously, first to one side, then to the other, giving his face an unpleasant expression, which was never shown on the face of Prince Vasily when he was in drawing rooms. His eyes, too, were not the same as always: now they looked insolently jokingly, now they looked around in fright.
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. structures of an elongated shape 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 can have one chloroplast per cell. Usually, there are on average 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 the shag.
Chloroplasts are structures bounded by two membranes - inner and outer. The outer membrane, like the inner one, has a thickness of about 7 µm; they are separated from each other by an intermembrane space of about 20–30 nm. The inner membrane of chloroplasts separates the plastid stroma, similar to the mitochondrial matrix. In the stroma of a mature chloroplast of higher plants, two types of internal membranes are visible. These are membranes that form flat, extended stroma lamellae, and thylakoid membranes, flat disc-shaped vacuoles or sacs.
Stroma lamellae (about 20 μm thick) are flat hollow sacs or they look like a network of branched and interconnected channels located in the same plane. Usually, the lamellae of the stroma inside the chloroplast lie parallel to each other and do not form connections with each other.
In addition to stromal membranes, membranous thylakoids are found in chloroplasts. These are flat closed membrane bags having the shape of a disk. The size of the intermembrane space is also about 20-30 nm. Such thylakoids form stacks like a column of coins, called grana.
The number of thylakoids per grain varies greatly, from a few to 50 or more. The size of such stacks can reach 0.5 μm, 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 so close to each other 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 thylakoids, the grana usually also includes sections of lamellae, which also form dense 2-nm layers at the points of contact between their membranes and thylakoid membranes. Stroma lamellae, thus, seem to connect the individual grains of the chloroplast. However, the cavities of the thylakoid chambers are always closed and do not pass into the chambers of the intermembrane space of the stroma lamellae. Stroma lamellae and thylakoid membranes are formed by separation from the inner membrane during the initial stages of plastid development.
In the matrix (stroma) of chloroplasts, DNA molecules and ribosomes are found; there is also the primary deposition of the reserve polysaccharide, starch, in the form of starch grains.
Characteristic of chloroplasts is the presence in them of pigments, chlorophylls, which give color to green plants. Green plants use chlorophyll to absorb energy. sunlight and turn it into a chemical.
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. In the matrix of chloroplasts, DNA, various RNA and ribosomes are found. 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, having a molecular weight of 0.8-1.3x108 daltons. There can be many copies of DNA in one chloroplast. So, 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 shown in green algae cells, do not match. Chloroplast DNA is not complexed with histones. All these characteristics of chloroplast DNA are close to those of prokaryotic cell DNA. Moreover, the similarity of DNA between chloroplasts and bacteria is also supported by the fact that the main transcriptional regulatory sequences (promoters, terminators) are the same. All types of RNA (messenger, transfer, ribosomal) are synthesized on the DNA of chloroplasts. 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 reawakened 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 to photosynthetic processes.
Numerous facts of true endosymbiosis of blue-green algae with cells of lower plants and protozoa are known, where they function and supply the host cell with photosynthesis 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 are inside the cells of the digestive glands. Thus, intact chloroplasts with functioning photosynthetic systems were found in cells of some herbivorous mollusks, 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 cells by pinocytosis. However, they were not attacked by hydrolases. Such cells, which included green chloroplasts, could divide within 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 studies have shown that the features of autonomy possessed by chloroplasts are still insufficient for the long-term maintenance of their functions, and even more so for their reproduction.
Recently, it has been possible to completely decipher the entire sequence of nucleotides in the cyclic DNA molecule of higher plant chloroplasts. This DNA can encode up to 120 genes, among them: genes for 4 ribosomal RNAs, 20 ribosomal proteins of chloroplasts, genes for some subunits of chloroplast RNA polymerase, several proteins of I and II photosystems, 9 of 12 subunits of ATP synthetase, parts of proteins of electron transport chain complexes , one of the subunits of ribulose diphosphate carboxylase (the key enzyme for CO2 binding), 30 tRNA molecules, and another 40 yet unknown proteins. Interestingly, a similar set of genes in the DNA of chloroplasts was found in such far distant representatives of higher plants as tobacco and liver moss.
The main mass 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. So, the cell nucleus controls the individual stages of the synthesis of chlorophyll, carotenoids, lipids, starch. Many dark-stage enzymes and other enzymes are under nuclear control, including some components of the electron transport chain. Nuclear genes encode DNA polymerase and aminoacyl-tRNA synthetase of chloroplasts. Most of the ribosomal proteins are under the control of nuclear genes. All these data make us speak of 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 in mitochondria. Here, in places where the outer and inner membranes of the chloroplast converge, there are channel-forming integral proteins that recognize the signal sequences of chloroplast proteins synthesized in the cytoplasm and transport them to the matrix stroma. According to additional signal sequences, proteins imported from the stroma can be incorporated into 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 surprising 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 the eukaryotic cell went through several stages of symbiosis with other cells. At the first stage, cells of the type of anaerobic heterotrophic bacteria included aerobic bacteria that turned into mitochondria. In parallel, in the host cell, the prokaryotic genophore is formed into a nucleus isolated from the cytoplasm. So heterotrophic eukaryotic cells could have arisen. Repeated endosymbiotic relationships between primary eukaryotic cells and blue-green algae led to the appearance in them of chloroplast-type structures that allow 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, be transferred to the nucleus. So, 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 the chloroplast ribosomes, which have all the properties of prokaryotic ribosomes. Such a transfer of a large part of prokaryotic genes to 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 determines to a greater extent all the main cellular functions.
proplastids
Under normal light, 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 stroma lamellae; others form thylakoid lamellae, which stack up and form grana of mature chloroplasts. A somewhat different development of plastids occurs in the dark. In etiolated seedlings, at the beginning, an increase in the volume of plastids, etioplasts occurs, but the system of internal membranes does not build lamellar structures, but forms a mass of small bubbles that accumulate in separate zones and can even form complex lattice structures (prolamellar bodies). The membranes of etioplasts contain protochlorophyll, a precursor of yellow chlorophyll. Under the action of light, chloroplasts are formed from etioplasts, protochlorophyll turns into chlorophyll, new membranes, photosynthetic enzymes and components of the electron transport chain are synthesized.
When cells are illuminated, membrane vesicles and tubules quickly reorganize, from which a complete system of lamellae and thylakoids develops, which is characteristic of a normal chloroplast.
Leukoplasts differ from chloroplasts in the absence of a developed lamellar system (Fig. 226 b). They are found in cells of storage tissues. Due to their uncertain morphology, leucoplasts are difficult to distinguish from proplastids and sometimes from mitochondria. They, like proplastids, are poor in lamellae, but nevertheless capable of forming normal thylakoid structures under the influence of light and of acquiring a green color. In the dark, leukoplasts can accumulate various reserve substances in the prolamellar bodies, and grains of secondary starch are deposited in the stroma of leukoplasts. If so-called transient starch is deposited in chloroplasts, which is present here only during the assimilation of CO2, then true storage of starch can occur in leukoplasts. In some tissues (cereal endosperm, rhizomes and tubers), the accumulation of starch in leukoplasts leads to the formation of amyloplasts completely filled with storage starch granules located in the plastid stroma (Fig. 226c).
Another form of plastids 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 often from their leukoplasts (for example, in the root of a carrot). The process of discoloration and changes in chloroplasts is easy to observe during the development of petals or when fruits ripen. At the same time, yellow-colored droplets (globules) can accumulate in plastids, or bodies in the form of crystals 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 drops are released, in which various pigments (for example, carotenoids) dissolve well. Thus, chromoplasts are degenerating forms of plastids subjected to lipophanerosis, the breakdown of lipoprotein complexes.
Its shell consists of two membranes - external and internal, between which there is an intermembrane space. Inside the chloroplast, by lacing off from the inner membrane, a complex thylakoid structure is formed. The gel-like contents of the chloroplast are called the stroma.
Each thylakoid is separated from the stroma by a single membrane. The interior of the thylakoid is called the lumen. thylakoids stacked in the chloroplast grains. The number of grains is different. They are connected to each other by special elongated thylakoids - lamellae. The usual thylakoid is similar to a rounded disk.
The stroma contains its own DNA of chloroplasts in the form of a circular molecule, RNA and prokaryotic-type ribosomes. Thus, it is a semi-autonomous organelle capable of independently synthesizing some of its proteins. It is believed that in the process of evolution, chloroplasts originated from cyanobacteria that began to live inside another cell.
The structure of the chloroplast is due to the function of photosynthesis performed. Related reactions occur in the stroma and on thylakoid membranes. In the stroma - reactions of the dark phase of photosynthesis, on membranes - light. Therefore, they contain various enzymatic systems. The stroma contains soluble enzymes involved in the Calvin cycle.
Thylakoid membranes contain pigments chlorophylls and carotenoids. All of them are involved in capturing solar radiation. However, they catch different spectra. The predominance of one or another type of chlorophyll in a certain group of plants determines their shade - from green to brown and red (in a number of algae). Most plants contain chlorophyll a.
In the structure of the chlorophyll molecule, a head and a tail are distinguished. The carbohydrate tail is immersed in the thylakoid membrane, and the head faces the stroma and is located in it. The energy of sunlight is absorbed by the head, leads to the excitation of the electron, which is picked up by the carriers. A chain of redox reactions is launched, eventually leading to the synthesis of a glucose molecule. Thus, the energy of light radiation is converted into energy chemical bonds organic compounds.
synthesized organic matter can accumulate in chloroplasts in the form of starch grains, and is also excreted from it through the membrane. There are also fat droplets in the stroma. However, they are formed from lipids of destroyed thylakoid membranes.
In the cells of autumn leaves, chloroplasts lose their typical structure, turning into chromoplasts, in which the internal membrane system is simpler. In addition, chlorophyll is destroyed, which makes carotenoids visible, giving the foliage a yellow-red hue.
The green cells of most plants usually contain many chloroplasts in shape similar to a ball slightly elongated in one direction (volumetric ellipse). However, in a number of algae, the cell may contain one huge chloroplast of a bizarre shape: in the form of a ribbon, stellate, etc.
The entire process of photosynthesis takes place in green plastids - chloroplasts. There are three types of plastids: leukoplasts are colorless, chromoplasts are orange, and chloroplasts are green. It is in the chloroplasts that the green pigment chlorophyll is concentrated. Non-green plants, such as mushrooms, lack plastids. These plants do not have the ability to photosynthesize. In the process of evolution, plastid differentiation occurred very early. True, photosynthetic bacteria and blue-green algae do not yet have plastids; their role is played by the colored part of the protoplasm adjacent to the shell. This is the most primitive organization of the photosynthetic apparatus. However, algae already have special formations (chromatophores), in which pigments are concentrated, they are diverse in shape (spiral, ribbon, in the form of plates or stars). Higher plants are characterized by a fully formed type of plastids in the form of a disk or a biconvex lens. Having taken the form of a disk, chloroplasts become universal device photosynthesis.
The chemical composition of chloroplasts is quite complex and is characterized by a high (75%) water content. About 75-80% of the total amount of dry matter falls on the share of various organic compounds, 20-25% - on the share of minerals. The structural basis of chloroplasts are proteins, the content of which reaches 50-55% of dry weight, about half of them are water-soluble. Such a high content of proteins is explained by their diverse functions in the composition of chloroplasts. These are structural proteins that are the basis of membranes, enzyme proteins, transport proteins that maintain a certain ionic composition that differs from the cytosol, contractile proteins, like muscle actomyosin, that provide motor activity chloroplasts. Proteins also perform a receptor function, taking part in the regulation of the intensity of photosynthesis in changing conditions of the internal and external environment.
the most important integral part chloroplasts are lipids, the content of which ranges from 30 to 40% of the dry mass. Chloroplast lipids are represented by three groups of compounds.
Carbohydrates are not constitutional substances of the chloroplast. In very small quantities, phosphoric esters of sugars are involved in the carbon reduction cycle, but mainly they are products of photosynthesis. Therefore, the content of carbohydrates 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 photosynthesis products, large starch grains are formed in chloroplasts. In this case, the starch content may increase to 50% dry weight and the activity of chloroplasts will decrease.
Chloroplasts have a high mineral content. Chloroplasts themselves make up 25-30% of the mass of the leaf, but they contain up to 80% of iron, 70-72% of magnesium and zinc, about 50% of copper, 60% of calcium contained in leaf tissues. These data are in good agreement with the high and varied enzymatic activity of chloroplasts. Mineral elements act as prosthetic groups and cofactors of enzyme activity. Magnesium is part of chlorophyll. An important role of calcium is to stabilize the membrane structures of chloroplasts.
The structure of the chloroplast, observed with an electron microscope, is very complex. Like the nucleus and mitochondria, the chloroplast is surrounded by shell, composed of two lipoprotein membranes. The internal environment is a relatively homogeneous substance - matrix, or stroma, which is pierced by membranes lamellae. Lamellas connected to each other form bubbles - thylakoids. Tightly adjacent to each other, thylakoids form grains, which can be seen even under a light microscope. In turn, the grains in one or more places are connected to each other with the help of interfacial strands - stroma thylakoids. Chloroplast pigments involved in capturing light energy, as well as enzymes necessary for the light phase of photosynthesis, are embedded in thylakoid membranes.
Fig.1. The structure of the chloroplast
1 - outer membrane; 2 - inner membrane; 3 - starch grain; 4 - DNA; 5 - stroma thylakoids (frets); 6 - grana thylakoid; 7 - matrix (stroma)
The structure of mature chloroplasts is the same in all higher plants, as well as in the cells of different organs of one plant (leaves, greening roots, bark, fruits). Depending on the functional load of cells, the physiological state of chloroplasts, their age, the degree of their internal structure is distinguished: size, number of grains, and the relationship between them. Thus, in the guard cells of stomata, the main function of chloroplasts is photoregulation of stomatal movements. This process is provided with energy by highly structured mitochondria. Chloroplasts contain large starch grains, swollen thylakoids, and lipophilic globules, which indicates their low energy load.
With age, the structure of chloroplasts changes significantly. Young chloroplasts are characterized by a lamellar structure; in this state, chloroplasts are able to multiply by division. In mature chloroplasts, the gran system is well expressed. In senescent chloroplasts, the stromal thylakoids break, the connection between the grana decreases, and subsequently, chlorophyll decay and destruction of the grana are observed. In autumn foliage, degradation of chloroplasts leads to the formation of chromoplasts, in which carotenoids are concentrated in plastoglobuli.
Physiological features of chloroplasts
An important property of chloroplasts is their ability to move. Chloroplasts move not only with the cytoplasm, but are also capable of spontaneously changing their position in the cell. The speed of movement of chlorolasts is about 0.12 µm/s. Chloroplasts can be evenly distributed in the cell, but more often they accumulate near the nucleus and near cell walls. Of great importance for the location of chloroplasts in the cell are the direction and intensity of illumination. At low light intensity, the chloroplasts become perpendicular to the incident rays, which is an adaptation for better catching them. At high illumination, chloroplasts move to the side walls and turn edge to the incident rays. Depending on the lighting, the shape of chloroplasts can also change. At higher light intensity, their shape becomes closer to spherical.
The main function of chloroplasts is the process of photosynthesis. In 1955, D. Arnon showed that the entire process of photosynthesis can be carried out in isolated chloroplasts. It is important to note that chloroplasts are found not only in leaf cells. They are found in cells of organs that do not specialize in photosynthesis: in stems, glumes and awns of ears, root crops, potato tubers, etc. In some cases, green plastids are found in tissues located not in the outer, illuminated parts of plants, but in layers, distant from light, in the tissues of the central cylinder of the stem, in the middle part of the lily bulb, as well as in the cells of the seed germ of many angiosperms. The latter phenomenon (the chlorophyll content of the embryo) attracts the attention of plant taxonomists. There are proposals to divide all angiosperms into two large groups: chloroombryophytes and leucoembryophytes, that is, those containing and not containing chloroplasts in the embryo (Yakovlev). Studies have shown that the structure of chloroplasts located in other plant organs, as well as the composition of pigments, are similar to leaf chloroplasts. This gives reason to believe that they are capable of photosynthesis.
In the event that they are exposed to light, apparently, photosynthesis does occur in them. Thus, the photosynthesis of chloroplasts located in the awns of the ear can be about 30% of the total photosynthesis of the plant. Roots that turn green in the light are capable of photosynthesis. Photosynthesis can also take place in the chloroplasts that are in the skin of the fetus until a certain stage of its development. According to the assumption of A. L. Kursanov, chloroplasts located near the conductive pathways, releasing oxygen, contribute to an increase in the intensity of the metabolism of sieve tubes. However, the role of chloroplasts is not limited to their ability to photosynthesize. In certain cases, they can serve as a source of nutrients (E. R. Gubbenet). Chloroplasts contain more vitamins, enzymes, and even phytohormones (in particular, gibberellin). Under conditions where assimilation is excluded, green plastids can play an active role in metabolic processes.