Plastids are membrane organelles found in photosynthetic eukaryotic organisms (higher plants, lower algae, some unicellular organisms). Like mitochondria, plastids are surrounded by two membranes; their matrix has its own genomic system; the functions of plastids are related to the energy supply of the cell, which is used for the needs of photosynthesis. A whole set of different plastids (chloroplast, leucoplast, amyloplast, chromoplast) have been found in higher plants, representing a series of mutual transformations of one type of plastid into another. The main structure that carries out photosynthetic processes is the chloroplast (Fig. 226a).
Chloroplast
As already indicated, the structure of the chloroplast is, in principle, reminiscent of the structure of the mitochondrion. Typically these are 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. The number of chloroplasts in the cells of different plants is not standard. So, green algae can 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 stroma lamellae, and membranes thylakoids, flat disc-shaped vacuoles or bags.
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, chloroplasts contain membranes thylakoids. These are flat, closed, disc-shaped membrane bags. The size of their intermembrane space is also about 20-30 nm. These thylakoids form stacks like a column of coins, called grains(Fig. 227). 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.
Functions of chloroplasts
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.
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. The absorption of light with a certain wavelength leads to a change in the structure of the chlorophyll molecule; it passes into an excited, activated state. The released energy of activated chlorophyll is transferred through a series of intermediate stages to certain synthetic processes leading to the synthesis of ATP and the reduction of the electron acceptor NADP (nicotinamide adenine dinucleotide) to NADP-H, which are spent on CO 2 binding reactions and the synthesis of sugars.
The overall reaction of photosynthesis can be expressed as follows:
nCO 2 + nH 2 O light (CH 2 O) n + nO 2 (I)
chlorophyll
Thus, the main final process here is the binding of carbon dioxide, the use of water to form various carbohydrates and the release of oxygen. Oxygen molecules, which are released during photosynthesis in plants, are formed due to the hydrolysis of a water molecule. Consequently, the process of photosynthesis includes the process of hydrolysis of water, which serves as one of the sources of electrons or hydrogen atoms. Biochemical studies have shown that the process of photosynthesis is a complex chain of events consisting of two phases: light and dark. The first, which occurs only in light, is associated with the absorption of light by chlorophylls and the conduct of a photochemical reaction (Hill reaction). In the second phase, which can occur in the dark, CO 2 is fixed and reduced, leading to the synthesis of carbohydrates.
As a result of the light phase, photophosphorylation occurs, the synthesis of ATP from ADP and phosphate using the electron transport chain, as well as the reduction of the coenzyme NADP (nicotinamide adenine dinucleotide phosphate) into NADP-H, which occurs during the hydrolysis and ionization of water. During this phase of photosynthesis, the energy from sunlight excites electrons in chlorophyll molecules, which are located in the thylakoid membranes. These excited electrons are transported along the oxidative chain components in the thylakoid membrane, just as electrons are transported along the respiratory chain in the mitochondrial membrane. The energy released by this electron transfer is used to pump protons across the thylakoid membrane into the thylakoids, which increases the potential difference between the stroma and the space inside the thylakoid. Just like in the mitochondrial cristae membranes, the thylakoid membranes contain molecular complexes of ATP synthetase, which then begin to transport protons back into the chloroplast matrix, or stroma, and in parallel phosphorylate ADP, i.e. synthesize ATP (Fig. 228, 229).
Thus, as a result of the light phase, ATP is synthesized and NADP is reduced, which are then used in the reduction of CO 2 in the synthesis of carbohydrates already in the dark phase of photosynthesis.
In the dark (independent of the photon flux) stage of photosynthesis, due to reduced NADP and ATP energy, atmospheric CO 2 is bound, which leads to the formation of carbohydrates. This process of fixation of CO 2 and formation of carbohydrates consists of many stages in which a large number of enzymes are involved (Calvin cycle). Biochemical studies have shown that enzymes involved in dark reactions are contained in the water-soluble fraction of chloroplasts, which contains components of the matrix-stroma of these plastids.
The process of CO 2 reduction begins with its addition to ribulose diphosphate, a carbohydrate consisting of 5 carbon atoms, to form a short-lived C 6 compound, which immediately breaks down into two C 3 compounds, into two molecules of glyceride-3-phosphate.
It is at this stage, during the carboxylation of ribulose diphosphate, that CO 2 is bound. Further reactions of conversion of glyceride-3-phosphate lead to the synthesis of various hexoses and pentoses, to the regeneration of ribulose diphosphate and to its new involvement in the cycle of CO 2 binding reactions. Ultimately, in the chloroplast, one molecule of hexose is formed from six molecules of CO 2; this process requires 12 molecules of NADPH and 18 molecules of ATP coming from the light reactions of photosynthesis. Fructose-6-phosphate formed as a result of the dark reaction gives rise to sugars, polysaccharides (starch) and galactolipids. In the stroma of chloroplasts, fatty acids, amino acids and starch are also formed from part of the glyceride-3-phosphate. Sucrose synthesis is completed in the cytoplasm.
In the stroma of chloroplasts, nitrites are reduced to ammonia due to the energy of electrons activated by light; in plants, this ammonia serves as a source of nitrogen during the synthesis of amino acids and nucleotides.
Ontogenesis and functional rearrangements of plastids
Many researchers were interested in the question of the origin of plastids and the ways of their formation.
At the end of the century before last, it was found that in the filamentous green alga Spirogyra, cell division during vegetative reproduction is accompanied by the division of their chromatophore by constriction. The fate of the chloroplast in the green alga Chlamydomonas has been studied in detail (Fig. 230). It turned out that during asexual, vegetative reproduction, immediately after the division of the nucleus, the giant chromatophore is laced into two parts, each of which ends up in one of the daughter cells, where it grows to its original size. The same equal division of the chloroplast occurs during the formation of zoospores. When a zygote is formed after the fusion of gametes, each of which contained a chloroplast, after the nuclei are united, the chloroplasts are first connected by a thin bridge, and then their contents merge into one large plastid.
In higher plants, division of mature chloroplasts also occurs, but very rarely. An increase in the number of chloroplasts and the formation of other forms of plastids (leukoplasts and chromoplasts) should be considered as a way of converting precursor structures, proplastid. The entire process of development of various plastids can be represented as a monotropic (going in one direction) series of changes in forms:
Proplastid leucoplast chloroplast chromoplast
amyloplast
Many studies have established the irreversible nature of the ontogenetic transitions of plastids. In higher plants, the emergence and development of chloroplasts occurs through changes in proplastids (Fig. 231).
Proplastids are small (0.4-1 μm) double-membrane vesicles that have no distinctive features of their internal structure. They differ from cytoplasmic vacuoles in their denser content and the presence of two delimiting membranes, external and internal (like promitochondria in yeast cells). The inner membrane may fold slightly or form small vacuoles. Proplastids are most often found in dividing plant tissues (meristem cells of roots, leaves, growth points of stems, etc.). In all likelihood, an increase in their number occurs through division or budding, the separation of small double-membrane vesicles from the body of the proplastid.
The fate of such proplastids will depend on plant development conditions. 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 the assimilation of CO 2, then true storage of starch can occur in leucoplasts. In some tissues (endosperm of cereals, rhizomes and tubers), the accumulation of starch in leucoplasts leads to the formation amyloplasts, completely filled with reserve starch granules located in the stroma of the plastid (Fig. 226c).
Another form of plastids in higher plants is chromoplast, usually colored 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.
Photosynthetic structures of lower eukaryotic and
prokaryotic cells
The structure of plastids in lower photosynthetic plants (green, brown and red algae) is in general similar to the chloroplasts of cells of higher plants. Their membrane systems also contain photosensitive pigments. Chloroplasts of green and brown algae (sometimes called chromatophores) also have outer and inner membranes; the latter forms flat bags arranged in parallel layers; granae are not found in these forms (Fig. 232). In green algae, the chromatophore includes pyrenoids, representing a zone surrounded by small vacuoles around which starch is deposited (Fig. 233).
The shape of chloroplasts in green algae is very diverse - they are either long spiral ribbons (Spirogira), networks (Oedogonium), or small round ones, similar to the chloroplasts of higher plants (Fig. 234).
Among prokaryotic organisms, many groups have photosynthetic apparatuses and therefore have a special structure. It is characteristic of photosynthetic microorganisms (blue-green algae and many bacteria) that their photosensitive pigments are localized in the plasma membrane or in its outgrowths directed deep into the cell.
In addition to chlorophyll, the membranes of blue-green algae contain phycobilin pigments. The photosynthetic membranes of blue-green algae form flat bags (lamellae) that are arranged parallel to each other, sometimes forming stacks or spirals. All of these membrane structures are formed by invaginations of the plasma membrane.
In photosynthetic bacteria (Chromatium), the membranes form small vesicles, the number of which is so large that they fill almost most of the cytoplasm. These vesicles can be seen to form by invagination and subsequent growth of the plasma membrane. These membrane vesicles (also called chromatophores) contain the photosensitive pigment bacteriochlorophyll, carotenoids, components of the photosynthetic electron transport system and photophosphorylation. Some purple bacteria contain a system of membranes that form regular stacks, like the thylakoids in the grana of chloroplasts (Fig. 235).
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, having a molecular weight of 0.8-1.3x10 8 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 further reinforced by the fact that the main transcriptional 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 C 14 O 2.
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 CO 2 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.
Plastids(from ancient Greek πλαστός - fashioned) - semi-autonomous organelles of higher plants, algae and some photosynthetic protozoa. Plastids have from two to four membranes, their own genome and a protein synthesizing apparatus.
One of the evidence for the origin of plastids from ancient cyanobacteria is the similarity of their genomes, although the plastid genome (plastome) is much smaller.
Plastids are formed by dividing already existing plastids. The most common divisions are proplastids, etioplasts and young chloroplasts. In meristematic tissues, plastid division correlates with cell division, so the number of plastids in mother and daughter cells is approximately the same. The division mechanism is close to the division of prokaryotic cells. The division of plastids begins with compression in the center, which, as it deepens, forms a constriction between two daughter plastids, after which complete separation occurs. At the constriction stage, a ring of a protein similar to the bacterial contractile protein FtsZ is formed on the outer membrane.
In most flowering plants, plastid inheritance occurs through the maternal line, since plastids either do not enter the sperm or are degraded during the development of the male gametophyte or double fertilization. In some plants (geranium, geranium, aspen grass), biparental inheritance of plastids was discovered. Some gymnosperms (ginkgo, cycads) are characterized by paternal inheritance of plastids.
Plastids of higher plants are capable of differentiation, dedifferentiation and redifferentiation; the set of plastids in a cell depends on its type. Plastids of higher plants are diverse in structure and perform a wide range of functions:
1. photosynthesis;
2. reduction of inorganic ions (nitrite, sulfate);
3. synthesis of many key metabolites (porphyrins, purines, pyrimidines, many amino acids, fatty acids, isoprenoids, phenolic compounds, etc.), with some synthetic pathways duplicating already existing cytosolic pathways;
4. synthesis of regulatory molecules (gibberellins, cytokinins, ABA, etc.);
5. storage of iron, lipids, starch.
Based on color and function, the following types of plastids are distinguished:
Plant cells of leaf-stem moss Plagiomnium affine with visible chloroplasts (highly enlarged)
· Proplastids- precursors of other types of plastids, present in meristematic cells. Proplastids range in size from 0.2 to 1 µm, which is significantly smaller than the sizes of differentiated plastids. The internal membrane system is poorly developed, contains fewer ribosomes than differentiated plastids, and may contain deposits of the protein phytoferritin, the main function of which is the storage of iron ions.
· Leukoplasts- uncolored plastids involved in the synthesis of essential oil isoprenoids (usually mono- and sesquiterpenes). A characteristic feature of leucoplasts is the presence of a reticular sheath - a network of smooth endoplasmic reticulum membranes surrounding the plastid. Sometimes the term “leucoplasts” refers to any unstained plastids, and the following types are distinguished: amyloplasts, elaioplasts, proteinoplasts.
· Amyloplasts- externally similar to proplastids, but the stroma contains starch granules. Amyloplasts are usually present in the storage organs of plants, in particular in potato tubers. In the gravisensory cells of the root, amyloplasts play the role of statoliths. Amyloplasts of higher plants can transform into chloroplasts or chromoplasts.
· Elaioplasts- serve to store fats.
Since school. The botany course says that in plant cells plastids can be of different shapes, sizes and perform different functions in the cell. This article will remind those who graduated from school a long time ago about the structure of plastids, their types and functions, and will be useful to everyone who is interested in biology.
Structure
The picture below schematically shows the structure of plastids in a cell. Regardless of its type, it has an outer and inner membrane that perform a protective function, stroma is an analogue of cytoplasm, ribosomes, a DNA molecule, and enzymes.
Chloroplasts contain special structures - grana. Grana are formed from thylakoids, disc-like structures. Thylakoids take part in and oxygen.
In chloroplasts, starch grains are formed as a result of photosynthesis.
Leucoplasts are not pigmented. They do not contain thylakoids and do not take part in photosynthesis. Most of the leucoplasts are concentrated in the stem and root of the plant.
Chromoplasts contain lipid droplets - structures containing lipids necessary to supply the plastid structure with additional energy.
Plastids can come in different colors, sizes and shapes. Their sizes range from 5-10 microns. The shape is usually oval or round, but can be any other.
Types of plastids
Plastids can be colorless (leucoplasts), green (chloroplasts), yellow or orange (chromoplasts). It is chloroplasts that give plant leaves their green color.
Another variety is responsible for the yellow, red or orange coloration.
Colorless plastids in the cell serve as a storage facility for nutrients. Leucoplasts contain fats, starch, proteins and enzymes. When the plant needs additional energy, starch is broken down into monomers - glucose.
Leucoplasts under certain conditions (under the influence of sunlight or when adding chemicals) can turn into chloroplasts, chloroplasts are transformed into chromoplasts when chlorophyll is destroyed, and the coloring pigments of chromoplasts - carotene, anthocyanin or xanthophyll - begin to predominate in color. This transformation is noticeable in autumn, when leaves and many fruits change color due to the destruction of chlorophyll and the appearance of chromoplast pigments.
Functions
As discussed above, plastids come in many different forms, and their functions in a plant cell depend on the variety.
Leukoplasts serve mainly to store nutrients and maintain the life of the plant due to the ability to store and synthesize proteins, lipids, and enzymes.
Chloroplasts play a key role in the process of photosynthesis. With the participation of the chlorophyll pigment concentrated in plastids, carbon dioxide and water molecules are converted into glucose and oxygen molecules.
Chromoplasts, due to their bright color, attract insects to pollinate plants. Research into the functions of these plastids is still ongoing.
Plastids are the main cytoplasmic organelles of autotrophic plant cells. The name comes from the Greek word “plastos”, which means “fashioned”.
The main function of plastids is the synthesis of organic substances, due to the presence of their own DNA and RNA and protein synthesis structures. Plastids also contain pigments that give them color. All types of these organelles have a complex internal structure. The outside of the plastid is covered by two elementary membranes; there is a system of internal membranes immersed in the stroma or matrix.
Classification of plastids by color and function involves dividing these organelles into three types: chloroplasts, leucoplasts and chromoplasts. Algae plastids are called chromatophores.
Chloroplasts are green plastids of higher plants containing chlorophyll, a photosynthetic pigment. They are round bodies measuring from 4 to 10 microns. Chemical composition of the chloroplast: approximately 50% protein, 35% fat, 7% pigments, a small amount of DNA and RNA. Representatives of different groups of plants have a different complex of pigments that determine color and take part in photosynthesis. These are subtypes of chlorophyll and carotenoids (xanthophyll and carotene). When viewed under a light microscope, the granular structure of plastids is visible - these are grana. Under an electron microscope, small transparent flattened sacs (cisterns, or grana) are observed, formed by a protein-lipid membrane and located directly in the stroma. Moreover, some of them are grouped in packs similar to columns of coins (gran thylakoids), others, larger ones, are located between the thylakoids. Thanks to this structure, the active synthesizing surface of the lipid-protein-pigment gran complex, in which photosynthesis occurs in the light, increases.
Chromoplasts- plastids, the color of which is yellow, orange or red, which is due to the accumulation of carotenoids in them. Due to the presence of chromoplasts, autumn leaves, flower petals, and ripe fruits (tomatoes, apples) have a characteristic color. These organelles can be of various shapes - round, polygonal, sometimes needle-shaped.
Leukoplasts They are colorless plastids whose main function is usually storage. The sizes of these organelles are relatively small. They are round or slightly oblong in shape and are characteristic of all living plant cells. In leucoplasts, the synthesis from simple compounds of more complex ones is carried out - starch, fats, proteins, which are stored in reserve in tubers, roots, seeds, fruits. Under an electron microscope, it is noticeable that each leucoplast is covered with a two-layer membrane, in the stroma there is only one or a small number of membrane outgrowths, the main space is filled with organic substances. Depending on what substances accumulate in the stroma, leukoplasts are divided into amyloplasts, proteinoplasts and eleoplasts.
74. What is the structure of the nucleus and its role in the cell? What structures of the nucleus determine its functions? What is chromatin?
The nucleus is the main component of the cell that carries genetic information. The nucleus is located in the center. The shape varies, but is always round or oval. Sizes vary. The contents of the kernel are liquid in consistency. There are membrane, chromatin, karyolymph (nuclear juice), and nucleolus. The nuclear envelope consists of 2 membranes separated by a perinuclear space. The shell is equipped with pores through which large molecules of various substances are exchanged. It can be in 2 states: rest - interphase and division - mitosis or meiosis.
The nucleus carries out two groups of general functions: one associated with the storage of genetic information itself, the other with its implementation, ensuring protein synthesis.
The first group includes processes associated with maintaining hereditary information in the form of an unchanged DNA structure. These processes are associated with the presence of so-called repair enzymes that eliminate spontaneous damage to the DNA molecule (break of one of the DNA chains, part of the radiation damage), which preserves the structure of DNA molecules practically unchanged over generations of cells or organisms. Further, reproduction or reduplication of DNA molecules occurs in the nucleus, which makes it possible for two cells to receive exactly the same volumes of genetic information, both qualitatively and quantitatively. Processes of change and recombination of genetic material occur in the nuclei, which is observed during meiosis (crossing over). Finally, nuclei are directly involved in the distribution of DNA molecules during cell division.
Another group of cellular processes ensured by the activity of the nucleus is the creation of the protein synthesis apparatus itself. This is not only the synthesis, transcription on DNA molecules of various messenger RNAs and ribosomal RNAs. In the nucleus of eukaryotes, the formation of ribosomal subunits also occurs by complexing ribosomal RNA synthesized in the nucleolus with ribosomal proteins, which are synthesized in the cytoplasm and transferred to the nucleus.
Thus, the nucleus is not only the reservoir of genetic material, but also the place where this material functions and reproduces. Therefore, loss of hair and disruption of any of the above functions is detrimental to the cell as a whole. Thus, disruption of repair processes will lead to a change in the primary structure of DNA and automatically to a change in the structure of proteins, which will certainly affect their specific activity, which may simply disappear or change in such a way that it will not provide cellular functions, as a result of which the cell dies. Disturbances in DNA replication will lead to a stop in cell reproduction or to the appearance of cells with an incomplete set of genetic information, which is also detrimental to cells. A disruption in the distribution of genetic material (DNA molecules) during cell division will lead to the same result. Loss as a result of damage to the nucleus or in the event of violations of any regulatory processes in the synthesis of any form of RNA will automatically lead to a stop in protein synthesis in the cell or to its gross disturbances.
Chromatin(Greek χρώματα - colors, paints) - this is the substance of chromosomes - a complex of DNA, RNA and proteins. Chromatin is found inside the nucleus of eukaryotic cells and is part of the nucleoid in prokaryotes. It is in the composition of chromatin that genetic information is realized, as well as DNA replication and repair.
75. What is the structure and types of chromosomes? What is a karyotype, autosomes, heterosomes, diploid and haploid sets of chromosomes?
Chromosomes are organelles of the cell nucleus, the totality of which determines the basic hereditary properties of cells and organisms. The complete set of chromosomes in a cell, characteristic of a given organism, is called a karyotype. In any cell of the body of most animals and plants, each chromosome is represented twice: one of them is received from the father, the other from the mother during the fusion of the nuclei of germ cells during the process of fertilization. Such chromosomes are called homologous, and a set of homologous chromosomes is called diploid. In the chromosome set of cells of dioecious organisms there is a pair (or several pairs) of sex chromosomes, which, as a rule, differ in different sexes in morphological characteristics; the remaining chromosomes are called autosomes. In mammals, genes that determine the sex of the organism are located on the sex chromosomes.
The significance of chromosomes as cellular organelles responsible for the storage, reproduction and implementation of hereditary information is determined by the properties of the biopolymers that make up them.
Autosomes In living organisms with chromosomal sex determination, paired chromosomes are called identical in male and female organisms. In other words, except for sex chromosomes, all other chromosomes in dioecious organisms will be autosomes.
Autosomes are designated by serial numbers. Thus, a person has 46 chromosomes in the diploid set, of which 44 autosomes (22 pairs, designated by numbers 1 to 22) and one pair of sex chromosomes (XX in women and XY in men).
Haploid set of chromosomes Let's start with the haploid set. It is a collection of completely different chromosomes, i.e. in a haploid organism there are several of these nucleoprotein structures, unlike each other (photo). The haploid set of chromosomes is characteristic of plants, algae and fungi. Diploid set of chromosomes This set is a collection of chromosomes in which each of them has a double, i.e. these nucleoprotein structures are arranged in pairs (photo). A diploid set of chromosomes is characteristic of all animals, including humans. By the way, about the last one. A healthy person has 46 of them, i.e. 23 pairs. However, its gender is determined by only two, called sexual, - X and Y. Read more on SYL.ru:
76. Define the cell cycle and characterize its phases. What functions of life are provided by cell division?
Cell cycle- this is the period of existence of a cell from the moment of its formation by dividing the mother cell until its own division or death.
The eukaryotic cell cycle consists of two periods:
1The period of cell growth, called “interphase,” during which DNA and proteins are synthesized and preparation for cell division occurs.
2The period of cell division, called “phase M” (from the word mitosis - mitosis).
Cell division. The growth of an organism occurs through the division of its cells. The ability to divide is the most important property of cellular life. When a cell divides, it doubles all its structural components, resulting in two new cells. The most common method of cell division is mitosis - indirect cell division.
A cell is a complex structure made up of many components called organelles. Moreover, the composition plant cell slightly different from animals, and the main difference lies in the presence plastids.
In contact with
Description of cellular elements
What cell components are called plastids. These are structural cell organelles that have a complex structure and functions that are important for the life of plant organisms.
Important! Plastids are formed from proplastids, which are located inside meristem or educational cells and are much smaller in size than the mature organelle. They are also divided, like bacteria, into two halves by constriction.
Which ones do they have? plastids structure It is difficult to see under a microscope; thanks to the dense shell, they are not translucent.
However, scientists were able to find out that this organoid has two membranes, inside it is filled with stroma, a liquid similar to cytoplasm.
Folds of the inner membrane, stacked, form granules that can be connected to each other.
Also present inside are ribosomes, lipid droplets, and starch grains. Plastids, especially chloroplasts, also have their own molecules.
Classification
They are divided into three groups according to color and functions:
- chloroplasts,
- chromoplasts,
- leukoplasts.
Chloroplasts
The most deeply studied ones are green in color. Contained in plant leaves, sometimes in stems, fruits and even roots. In appearance they look like rounded grains 4-10 micrometers in size. Small size and large quantity significantly increases the working surface area.
They may vary in color, depending on the type and concentration of pigment they contain. Basic pigment - chlorophyll, xanthophyll and carotene are also present. In nature, there are 4 types of chlorophyll, designated by Latin letters: a, b, c, e. The first two types contain cells of higher plants and green algae; diatoms have only varieties - a and c.
Attention! Like other organelles, chloroplasts are capable of aging and destruction. The young structure is capable of division and active work. Over time, their grains break down and the chlorophyll disintegrates.
Chloroplasts perform an important function: inside them the process of photosynthesis occurs— conversion of sunlight into the energy of chemical bonds of forming carbohydrates. At the same time, they can move along with the flow of cytoplasm or actively move on their own. So, in low light they accumulate near the walls of the cell with a large amount of light and turn towards it with a larger area, and in very active light, on the contrary, they stand edge-on.
Chromoplasts
They replace destroyed chloroplasts and come in yellow, red and orange shades. The color is formed due to the content of carotenoids.
These organelles are found in the leaves, flowers and fruits of plants. The shape can be round, rectangular or even needle-shaped. The structure is similar to chloroplasts.
Main function – coloring flowers and fruits, which helps attract pollinating insects and animals that eat the fruits and thereby contribute to the spread of plant seeds.
Important! Scientists speculate about the role chromoplasts in the redox processes of the cell as a light filter. The possibility of their influence on the growth and reproduction of plants is considered.
Leukoplasts
Data plastids have differences in structure and functions. The main task is to store nutrients for future use, so they are found mainly in the fruits, but can also be in the thickened and fleshy parts of the plant:
- tubers,
- rhizomes,
- root vegetables,
- bulbs and others.
Colorless color does not allow you to select them in the structure of the cell, however, leukoplasts are easy to see when a small amount of iodine is added, which, interacting with starch, turns them blue.
The shape is close to round, while the membrane system inside is poorly developed. The absence of membrane folds helps the organelle in storing substances.
Starch grains increase in size and easily destroy the internal membranes of the plastid, as if stretching it. This allows you to store more carbohydrates.
Unlike other plastids, they contain a DNA molecule in a shaped form. At the same time, accumulating chlorophyll, leucoplasts can transform into chloroplasts.
When determining what function leucoplasts perform, it is necessary to note their specialization, since there are several types that store certain types of organic matter:
- amyloplasts accumulate starch;
- oleoplasts produce and store fats, while the latter can be stored in other parts of the cells;
- proteinoplasts “protect” proteins.
In addition to accumulation, they can perform the function of breaking down substances, for which there are enzymes that are activated when there is a shortage of energy or building material.
In such a situation, enzymes begin to break down stored fats and carbohydrates into monomers so that the cell receives the necessary energy.
All varieties of plastids, despite structural features, have the ability to transform into each other. Thus, leucoplasts can transform into chloroplasts; we see this process when potato tubers turn green.
At the same time, in autumn, chloroplasts turn into chromoplasts, as a result of which the leaves turn yellow. Each cell contains only one type of plastid.
Origin
There are many theories of origin, the most substantiated among them are two:
- symbiosis,
- absorption.
The first considers cell formation as a process of symbiosis occurring in several stages. During this process, heterotrophic and autotrophic bacteria unite, receiving mutual benefits.
The second theory considers the formation of cells through the absorption of smaller ones by larger organisms. However, they are not digested; they are integrated into the structure of the bacterium, performing their function within it. This structure turned out to be convenient and gave the organisms an advantage over others.
Types of plastids in a plant cell
Plastids - their functions in the cell and types
Conclusion
Plastids in plant cells are a kind of “factory” where production associated with toxic intermediates, high energy and free radical transformation processes takes place.