Chloroplasts- These are green plastids of higher plants containing chlorophyll - a photosynthetic pigment. They are rounded bodies with sizes from 4 to 10 microns. The chemical composition of the chloroplast: approximately 50% protein, 35% fat, 7% pigments, a small amount of DNA and RNA. In representatives of different groups of plants, the complex of pigments that determine color and take part in photosynthesis is different. 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 (cistern, or grana) are observed, formed by a protein-lipid membrane and located directly in the stroma. Moreover, some of them are grouped into packs that look like columns of coins (thylakoid grana), others, larger ones, are located between the thylakoids. Due to this structure, the active synthesizing surface of the lipid-protein-pigment complex of grana increases, in which photosynthesis occurs in the light.
Chromoplasts
Leucoplasts are colorless plastids, the main function of which is usually storage. These organelles are relatively small. They are round or slightly oblong, characteristic of all living plant cells. In leukoplasts, synthesis is carried out from simple compounds of more complex ones - starch, fats, proteins, which are stored in reserve in tubers, roots, seeds, fruits. Under an electron microscope, it is noticeable that each leukoplast is covered with a two-layer membrane, there is only one or a small number of membrane outgrowths in the stroma, 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, 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 is different, but always round or oval. The sizes are various. The content of the core is a liquid consistency. There are membrane, chromatin, karyolymph (nuclear juice), nucleolus. The nuclear envelope consists of 2 membranes separated by a perinuclear space. The shell is equipped with pores through which the exchange of large molecules of various substances takes place. It can be in 2 states: rest - interphase and division - mitosis or meiosis.
The nucleus performs two groups of general functions: one associated with the actual storage of genetic information, the other with its implementation, with the provision of protein synthesis.
The first group includes processes associated with the maintenance of hereditary information in the form of an unchanged DNA structure. These processes are associated with the presence of so-called repair enzymes, which eliminate spontaneous damage to the DNA molecule (a break in one of the DNA chains, part of radiation damage), which keeps the structure of DNA molecules practically unchanged in a number of generations of cells or organisms. Further, reproduction or reduplication of DNA molecules takes place in the nucleus, which makes it possible for two cells to obtain exactly the same amounts of genetic information, both qualitatively and quantitatively. In the nuclei, the processes of change and recombination of genetic material occur, 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 provided by the activity of the nucleus is the creation of the actual apparatus of protein synthesis. This is not only synthesis, transcription on DNA molecules of various messenger RNA and ribosomal RNA. In the nucleus of eukaryotes, the formation of ribosome subunits also occurs by complexing ribosomal RNA synthesized in the nucleolus with ribosomal proteins that are synthesized in the cytoplasm and transferred to the nucleus.
Thus, the nucleus is not only a container of genetic material, but also a place where this material functions and reproduces. Therefore, the loss of lil, a violation of any of the functions listed above, is detrimental to the cell as a whole. Thus, a violation 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. Violations of DNA replication will lead to a stop in cell reproduction or to the appearance of cells with an inferior set of genetic information, which is also detrimental to cells. The same result will lead to a violation of the distribution of genetic material (DNA molecules) during cell division. Loss as a result of damage to the nucleus or in the event of violations of any regulatory processes for the synthesis of any form of RNA will automatically lead to a halt in protein synthesis in the cell or to its gross violations.
Chromatin(Greek χρώματα - colors, paints) - this is the substance of chromosomes - a complex of DNA, RNA and proteins. Chromatin is located inside the nucleus of eukaryotic cells and is part of the nucleoid in prokaryotes. It is in the composition of chromatin that the realization of genetic information, as well as DNA replication and repair, takes place.
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 main 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 was received from the father, the other from the mother during the fusion of the nuclei of germ cells during fertilization. Such chromosomes are called homologous, the 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 morphological characteristics in different sexes; the rest of the chromosomes are called autosomes. In mammals, sex chromosomes contain genes that determine the sex of an organism.
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, which are the same 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. So, a person in the diploid set has 46 chromosomes, of which 44 autosomes (22 pairs, denoted by numbers from 1 to 22) and one pair of sex chromosomes (XX for women and XY for men).
Haploid set of chromosomes Let's start with haploid. It is a cluster of completely different chromosomes, i.e. in a haploid organism there are several of these nucleoprotein structures that are unlike each other (photo). The haploid set of chromosomes is characteristic of plants, algae and fungi. Diploid set of chromosomes This set is such a collection of chromosomes, in which each of them has a twin, i.e. these nucleoprotein structures are arranged in pairs (photo). The 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 couples. However, his sex is determined by only two, called sex, - X and Y. Read more on SYL.ru:
76. Define the cell cycle, describe 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 to its own division or death.
The eukaryotic cell cycle consists of two periods:
1 A period of cell growth called "interphase" during which DNA and proteins are synthesized and preparations are made for cell division.
2Period of cell division, called "phase M" (from the word mitosis - mitosis).
Cell division. An organism grows by dividing its cells. The ability to divide is the most important property of cellular life. By dividing, the cell doubles all its structural components, and as a result, two new cells appear. The most common way of cell division is mitosis - indirect cell division.
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plastids
Plastids are the main cytoplasmic organelles of autotrophic plant cells. The name comes from the Greek word "plastos", which means "sculpted".
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 determine their color. All types of these organelles have a complex internal structure. Outside, 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 the division of these organelles into three types: chloroplasts, leukoplasts and chromoplasts. The plastids of algae are called chromatophores.
Chloroplasts- These are green plastids of higher plants containing chlorophyll - a photosynthetic pigment. They are rounded bodies with sizes from 4 to 10 microns. The chemical composition of the chloroplast: approximately 50% protein, 35% fat, 7% pigments, a small amount of DNA and RNA. In representatives of different groups of plants, the complex of pigments that determine color and take part in photosynthesis is different. 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 (cistern, or grana) are observed, formed by a protein-lipid membrane and located directly in the stroma.
Moreover, some of them are grouped into packs that look like columns of coins (thylakoid grana), others, larger ones, are located between the thylakoids. Due to this structure, the active synthesizing surface of the lipid-protein-pigment complex of grana increases, in which photosynthesis occurs in the light.
Chromoplasts- plastids, the color of which is yellow, orange or red, due to the accumulation of carotenoids in them. Due to the presence of chromoplasts, autumn leaves, flower petals, ripened fruits (tomatoes, apples) have a characteristic color. These organelles can be of various shapes - round, polygonal, sometimes needle-shaped.
Leucoplasts are colorless plastids, the main function of which is usually storage. These organelles are relatively small.
They are round or slightly oblong, characteristic of all living plant cells. In leukoplasts, synthesis is carried out from simple compounds of more complex ones - starch, fats, proteins, which are stored in reserve in tubers, roots, seeds, fruits. Under an electron microscope, it is noticeable that each leukoplast is covered with a two-layer membrane, there is only one or a small number of membrane outgrowths in the stroma, the main space is filled with organic substances. Depending on what substances accumulate in the stroma, leukoplasts are divided into amyloplasts, proteinoplasts and eleoplasts.
All types of plastids have a common origin and are able to move from one type to another. Thus, the transformation of leukoplasts into chloroplasts is observed when potato tubers turn green in the light, and in autumn, chlorophyll is destroyed in the chloroplasts of green leaves, and they are transformed into chromoplasts, which is manifested by yellowing of the leaves. In each specific cell of a plant, there can be only one type of plastid.
Plastids are organelles of plant cells and some photosynthetic protozoa. Animals and fungi do not have plastids.
Plastids are divided into several types. The most important and well-known is the chloroplast, which contains the green pigment chlorophyll, which ensures the process of photosynthesis.
Other types of plastids are multi-colored chromoplasts and colorless leucoplasts. Also isolated are amyloplasts, lipidoplasts, proteinoplasts, which are often considered varieties of leukoplasts.
All types of plastids are interconnected by a common origin or possible interconversion. Plastids develop from proplastids - smaller organelles of meristematic cells.
The structure of plastids
Most plastids are two-membrane organelles, they have an outer and an inner membrane.
However, there are organisms whose plastids have four membranes, which is associated with the peculiarities of their origin.
In many plastids, especially in chloroplasts, the internal membrane system is well developed, forming structures such as thylakoids, grana (stacks of thylakoids), lamellae - elongated thylakoids connecting neighboring grana. The internal contents of plastids are commonly referred to as the stroma. Among other things, it contains starch grains.
It is believed that in the process of evolution, plastids appeared similarly to mitochondria - by introducing another prokaryotic cell into the host cell, capable of photosynthesis in this case. Therefore, plastids are considered semi-autonomous organelles. They can divide independently of cell divisions, they have their own DNA, RNA, prokaryotic-type ribosomes, that is, their own protein-synthesizing apparatus. This does not mean that proteins and RNA from the cytoplasm do not enter plastids. Part of the genes that control their functioning is located just in the nucleus.
Functions of plastids
The functions of plastids depend on their type. Chloroplasts perform a photosynthetic function. Spare nutrients accumulate in leukoplasts: starch in amyloplasts, fats in elaioplasts (lipidoplasts), proteins in proteinoplasts.
Chromoplasts, due to the carotenoid pigments they contain, color various parts of plants - flowers, fruits, roots, autumn leaves, etc. Bright color often serves as a kind of signal for pollinating animals and fruit and seed distributors.
In the degenerating green parts of plants, chloroplasts turn into chromoplasts. The chlorophyll pigment is destroyed, so the rest of the pigments, despite the small amount, become noticeable in the plastids and color the same foliage in yellow-red shades.
Plastids are the organelles of plant cells. One of the types of plastids are photosynthetic chloroplasts. Other common varieties are chromoplasts and leucoplasts. All of them are united by the unity of origin and the general plan of the structure. Distinguishes - the predominance of certain pigments and the functions performed.
Plastids develop from proplastids, which are present in the cells of the educational tissue and are significantly smaller in size than the mature organoid. In addition, plastids are capable of dividing in two by constriction, which is similar to the division of bacteria.
In the structure of plastids, the outer and inner membranes are isolated, the inner contents are the stroma, the inner membrane system, which is especially developed in chloroplasts, where it forms thylakoids, grana and lamellae.
The stroma contains DNA, ribosomes, various types of RNA. Thus, like mitochondria, plastids are capable of independent synthesis of some of the necessary protein molecules. It is believed that in the process of evolution, plastids and mitochondria appeared as a result of a symbiosis of various prokaryotic organisms, one of which became the host cell, while others became its organelles.
The functions of plastids depend on their type:
- chloroplasts→ photosynthesis,
- chromoplasts→ coloring of plant parts,
- leucoplasts→ supply of nutrients.
Plant cells contain predominantly one type of plastid. The pigment chlorophyll predominates in chloroplasts, therefore the cells containing them are green. Chromoplasts contain carotenoid pigments that give color from yellow through orange to red.
Leucoplasts are colorless.
Coloring of flowers and fruits of the plant in bright colors by chromoplasts attracts pollinating insects and seed dispersal animals. In autumn leaves, the destruction of chlorophyll occurs, as a result, the color is determined by carotenoids. Because of this, the foliage acquires the appropriate color. In this case, chloroplasts turn into chromoplasts, which are often considered as the final stage of plastid development.
Leukoplasts, when illuminated, are able to turn into chloroplasts. This can be observed in potato tubers when they begin to turn green in the light.
There are several types of leukoplasts depending on the type of substances accumulated in them:
- proteinoplasts→ squirrels,
- elaioplasts, or lipidoplasts, → fats,
- amyloplasts→ carbohydrates, usually in the form of starch.
From the school bench. In the course of botany, it is said that in plant cells plastids can be of different shapes, sizes and perform various functions in the cell. This article will remind about the structure of plastids, their types and functions to those who graduated from school long ago, 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 performs a protective function, a stroma is an analogue of the cytoplasm, ribosomes, a DNA molecule, and enzymes.
In chloroplasts there are 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, they do not take part in photosynthesis. Most of the leukoplasts are concentrated in the stem and root of the plant.
Chromoplasts have in their composition lipid drops - structures containing lipids necessary to supply the structure of plastids with additional energy.
Plastids can be of different colors, sizes and shapes. Their sizes fluctuate within 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). Chloroplasts are what give plant leaves their green color.
Another variety is responsible for the yellow, red or orange coloration.
Colorless plastids in the cell act as a storehouse of nutrients. Leukoplasts contain fats, starch, proteins and enzymes. When the plant needs additional energy, starch is broken down into monomers - glucose.
Leukoplasts under certain conditions (under the influence of sunlight or with the addition of chemicals) can turn into chloroplasts, chloroplasts are converted 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 manifestation of chromoplast pigments.
Functions
As mentioned above, plastids can be different, and their functions in the plant cell depend on the variety.
Leucoplasts serve mainly to store nutrients and maintain the vital activity 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 colors, attract insects for pollination of plants. The study of the functions of these plastids is still ongoing.
Plastids are organelles of plant cells and some photosynthetic protozoa. Animals and fungi do not have plastids.
Plastids are divided into several types. The most important and well-known is the chloroplast, which contains the green pigment chlorophyll, which ensures the process of photosynthesis.
Other types of plastids are multi-colored chromoplasts and colorless leucoplasts. Also isolated are amyloplasts, lipidoplasts, proteinoplasts, which are often considered varieties of leukoplasts.
Types of plastids: chloroplasts, chromoplasts, leukoplasts
All types of plastids are interconnected by a common origin or possible interconversion. Plastids develop from proplastids - smaller organelles of meristematic cells.
The structure of plastids
Most plastids are two-membrane organelles, they have an outer and an inner membrane. However, there are organisms whose plastids have four membranes, which is associated with the peculiarities of their origin.
In many plastids, especially in chloroplasts, the internal membrane system is well developed, forming structures such as thylakoids, grana (stacks of thylakoids), lamellae - elongated thylakoids connecting neighboring grana. The internal contents of plastids are commonly referred to as the stroma. Among other things, it contains starch grains.
It is believed that in the process of evolution, plastids appeared similarly to mitochondria - by introducing another prokaryotic cell into the host cell, capable of photosynthesis in this case. Therefore, plastids are considered semi-autonomous organelles. They can divide independently of cell divisions, they have their own DNA, RNA, prokaryotic-type ribosomes, that is, their own protein-synthesizing apparatus. This does not mean that proteins and RNA from the cytoplasm do not enter plastids. Part of the genes that control their functioning is located just in the nucleus.
Functions of plastids
The functions of plastids depend on their type. Chloroplasts perform a photosynthetic function. Spare nutrients accumulate in leukoplasts: starch in amyloplasts, fats in elaioplasts (lipidoplasts), proteins in proteinoplasts.
Chromoplasts, due to the carotenoid pigments they contain, color various parts of plants - flowers, fruits, roots, autumn leaves, etc. Bright color often serves as a kind of signal for pollinating animals and fruit and seed distributors.
In the degenerating green parts of plants, chloroplasts turn into chromoplasts. The chlorophyll pigment is destroyed, so the rest of the pigments, despite the small amount, become noticeable in the plastids and color the same foliage in yellow-red shades.
Chloroplast
Plastids are membrane organelles found in photosynthetic eukaryotic organisms (higher plants, lower algae, some unicellular organisms). Plastids are surrounded by two membranes, their matrix has its own genomic system, the functions of plastids are associated with the energy supply of the cell, which goes to the needs of photosynthesis.
In higher plants, a whole set of different plastids was found (chloroplast, leukoplast, amyloplast, chromoplast), which are a series of mutual transformations of one type of plastid into another. The main structure that carries out photosynthetic processes is the chloroplast.
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 (leucoplasts and chromoplasts) should be considered as a pathway for the transformation of precursor structures, proplastids. The whole process of development of various plastids can be represented as a monotropic (going in one direction) series of changes in forms:
Many studies have established the irreversible nature of the ontogenetic transitions of plastids. In higher plants, the emergence and development of chloroplasts occur through changes in proplastids. Proplastids are small (0.4-1 μm) two-membrane vesicles that do not have 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. The inner membrane may give small folds or form small vacuoles. Proplastids are most often found in dividing plant tissues (cells of the meristem of the root, leaves, at the growth points of stems, etc.). In all likelihood, an increase in their number occurs through division or budding, separation of small two-membrane vesicles from the proplastid body.
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. 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 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. With the help of chlorophyll, green plants absorb the energy of sunlight and turn 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. 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.
Plastids are membrane organelles found in photosynthetic eukaryotic organisms (higher plants, lower algae, some unicellular organisms). In higher plants, a whole set of different plastids was found (chloroplast, leukoplast, amyloplast, chromoplast), which are 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 mentioned, the structure of the chloroplast, in principle, resembles the structure of the mitochondria. Usually these are elongated structures with a width of 2-4 µm and a length of 5-10 µm. Green algae have giant chloroplasts (chromatophores), reaching a length of 50 microns. The number of chloroplasts in plant cells is different. So, green algae can have one chloroplast each, higher plants have an average of 10-30, and about 1000 chloroplasts per cell were found in the giant cells of the palisade tissue of the shag.
The outer membrane of chloroplasts, as well as the inner one, have 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, 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, chloroplasts contain membranous thylakoids. 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 piles like a column of coins, called grana (Fig. 227). 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 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.
The matrix (stroma) of chloroplasts contains DNA molecules, ribosomes; there is also the primary deposition of the reserve polysaccharide, starch, in the form of starch grains.
Functions of chloroplasts. In chloroplasts, photosynthetic processes occur, leading to the binding of carbon dioxide, to the release of oxygen and the synthesis of sugars.
Characteristic of chloroplasts is the presence in them of pigments, chlorophylls, which give color to green plants. With the help of chlorophyll, green plants absorb the energy of sunlight and turn it into chemical energy.
The main final process here is the binding of carbon dioxide with 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. The process of photosynthesis is a complex chain consisting of two phases: light and dark. The first, proceeding only in the light, is associated with the absorption of light by chlorophylls and with the carrying out of a photochemical reaction (the Hill reaction). In the second phase, which takes place in the dark, CO2 is fixed and reduced, leading to the synthesis of carbohydrates.
As a result of the light phase, ATP synthesis and the reduction of NADP (nicotinamide adenine dinucleotide phosphate) occur, which are then used in the reduction of CO2, in the synthesis of carbohydrates already in the dark phase of photosynthesis.
In the dark stage of photosynthesis, due to the reduced NADP and the energy of ATP, atmospheric CO2 is bound, which leads to the formation of carbohydrates. This process of CO2 fixation and carbohydrate formation consists of many steps involving a large number of enzymes (the Calvin cycle).
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 in the synthesis of amino acids and nucleotides.
Ontogeny and functional rearrangements of plastids. An increase in the number of chloroplasts and the formation of other forms of plastids (leukoplasts and chromoplasts) is considered as a pathway for the transformation of precursor structures, proplastid. The whole process of development of various plastids is represented by a series of changes of forms going in one direction:
Proplastida ® leukoplast ® chloroplast ® chromoplast
¯ amyloplast¾¾¾¾¾¾¾¾¾¾
The irreversible nature of the ontogenetic transitions of plastids has been established. In higher plants, the emergence and development of chloroplasts occur through changes in proplastids (Fig. 231).
Proplastids are small (0.4-1 μm) two-membrane vesicles that differ from cytoplasmic vacuoles in denser content and the presence of two delimiting membranes, external and internal (like promitochondria in yeast cells). The inner membrane may give small folds or form small vacuoles. Proplastids are most often found in dividing plant tissues (cells of the meristem of the root, leaves, at the growth points of stems, etc.). An increase in their number occurs by division or budding, separation of small two-membrane vesicles from the proplastid body.
The fate of such proplastids depends on the conditions of plant development. 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.
In the dark, seedlings initially increase in the volume of plastids, etioplasts, 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. 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, unlike chloroplasts, do not have 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 chromoplast, usually colored in yellow light 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 due to 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.
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 sacs arranged in parallel layers; these forms do not have facets (Fig. 232). In green algae, the chromatophore contains pyrenoids, which are a zone surrounded by small vacuoles, around which starch is deposited (Fig. 233).
The shape of chloroplasts in green algae is very diverse - these are either long spiral ribbons (Spirogira), networks (Oedogonium), or small round ones, similar to chloroplasts of higher plants (Fig. 234).
Among prokaryotic organisms, many groups possess photosynthetic apparatuses and, in connection with this, have a special structure. For photosynthetic microorganisms (blue-green algae and many bacteria), it is characteristic that their photosensitive pigments are associated with the plasma membrane or with its outgrowths directed deep into the cell.
In the membranes of blue-green algae, in addition to chlorophyll, there are phycobilin pigments. The photosynthetic membranes of blue-green algae form flat sacs (lamellae) that are arranged parallel to each other, sometimes forming stacks or spirals. All 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 great that they fill almost most of the cytoplasm.
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.
The entire sequence of nucleotides in the cyclic DNA molecule of chloroplasts of higher plants has been completely deciphered. 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. A number of the most important proteins, enzymes, and, accordingly, the metabolic processes of chloroplasts are under the genetic control of the nucleus. Most of the ribosomal proteins are under the control of nuclear genes. All these data speak of chloroplasts as structures with limited autonomy.
4.6. Cytoplasm: Musculoskeletal system (cytoskeleton)
At the heart of all the numerous motor reactions of the cell are common molecular mechanisms. In addition, the presence of motor apparatuses is combined and structurally associated with the existence of supporting, frame or skeletal intracellular formations. Therefore, they talk about the musculoskeletal system of cells.
Cytoskeletal components include filamentous, non-branching protein complexes or filaments (thin filaments).
There are three groups of filaments that differ both in chemical composition and ultrastructure, and in functional properties. The thinnest threads are microfilaments; their diameter is about 8 nm and they consist mainly of actin protein. Another group of filamentous structures are microtubules, which have a diameter of 25 nm and consist mainly of the tubulin protein, and finally intermediate filaments with a diameter of about 10 nm (intermediate compared to 6 nm and 25 nm), formed from different but related proteins. (Fig. 238, 239).
All these fibrillar structures are involved in the processes of physical movement of cellular components or even whole cells, in some cases they play a purely skeleton skeletal role. Elements of the cytoskeleton are found in all eukaryotic cells without exception; analogues of these fibrillar structures are also found in prokaryotes.
The common properties of the elements of the cytoskeleton is that they are proteinaceous, non-branching fibrillar polymers, unstable, capable of polymerization and depolymerization, which lead to cellular mobility, for example, to a change in the shape of the cell. The components of the cytoskeleton, with the participation of special additional proteins, can be stabilized or form complex fibrillar ensembles, and play only a scaffolding role. When interacting with other special translocator proteins (or motor proteins), they are involved in a variety of cellular movements.
According to their properties and functions, the elements of the cytoskeleton are divided into two groups: only scaffold fibrils - intermediate filaments, and musculoskeletal - actin microfilaments interacting with motor proteins - myosins, and tubulin microtubules interacting with motor proteins dyneins and kinesins.
The second group of fibrils of the cytoskeleton (microfilaments and microtubules) provide two fundamentally different modes of movement. The first of them is based on the ability of the main microfilament protein, actin, and the main microtubule protein, tubulin, to polymerize and depolymerize. When these proteins are associated with the plasma membrane, its morphological changes are observed in the form of the formation of outgrowths (pseudopodia and lamellopodia) at the edge of the cell.
With another method of movement, actin fibrils (microfilaments) or tubulin (microtubules) are guiding structures along which special mobile proteins - motors - move. The latter can bind to the membrane or fibrillar components of the cell and thereby participate in their movement.