The cell membrane is a typical component of a plant cell and is a product of the vital activity of the protoplast.
Functions:
1. Strong and rigid cell membranes serve as a mechanical support for plant organs.
2. The membrane limits the extension of the protoplast by the vacuole, and the size and shape of the mature cell cease to change.
3. In the outer tissues, cell membranes protect deeper cells from drying out.
4. Various substances and water can move along cell walls adjacent to each other from cell to cell (path through the apoplast).
5. They influence absorption, transpiration and secretion.
Cell walls are usually colorless and easily transmit sunlight. The walls of neighboring cells are held together by pectin median plate. The median lamina is a single layer common to two neighboring cells. It is a slightly modified cell plate that arose during the process of cytokinesis. The middle plate is less watered and may contain lignin molecules. As a result of intracellular pressure, the angles of cell walls can be rounded, and intercellular spaces are formed between neighboring cells. All plant cell walls, connected to one another and adjacent to water-filled intercellular spaces, provide the existence of a continuous watered environment in which water-soluble substances move freely.
Structure and chemical composition.
Primary cell wall.
Initially, outward from the plasmalemma appears primary cell wall.
Compound: cellulose, hemicellulose, pectin and water.
The primary cell walls of adjacent cells are connected by a protopectin median lamina. In the cell wall, linear, very long (several microns) cellulose molecules consisting of glucose are collected into bundles - micelles, which, in turn, are combined into microfibrils - the thinnest (1.5...4 nm) fibers of indefinite length, and then into macrofibrils . Cellulose forms a multidimensional framework, which is immersed in an amorphous, highly watered matrix of non-cellulosic carbohydrates: pectins, hemicelluloses, etc. It is cellulose that provides the strength of the cell wall. Microfibrils are elastic and have tensile strength similar to steel. Matrix polysaccharides determine wall properties such as high permeability to water, dissolved small molecules and ions, and strong swelling. Thanks to the matrix, water and substances can move along the walls adjacent to each other from cell to cell (the path through the apoplast along the “free space”). Some hemicelluloses can be deposited in the cell walls of seeds as storage substances.
Wall growth.
When cells divide, only the cell plate is created anew. Both daughter cells lay their own walls on it, consisting mainly of hemicellulose. In this case, the formation of a wall also occurs on the inner surface of the remaining walls belonging to the mother cell. The cell plate is transformed into a median plate; it is usually thin and almost indistinguishable. After division, the cell enters an elongation phase due to the absorption of water by the cell and the growth of the central vacuole. Turgor pressure stretches the wall into which cellulose micelles and matrix substances are embedded. This method of growth is called intussusception, implementation. The membranes of dividing and growing cells are called primary. They contain up to 90% water, the dry matter is dominated by matrix polysaccharides: in dicotyledons, pectins and hemicelluloses are in equal proportions, in monocotyledons - mainly hemicelluloses; cellulose content does not exceed 30%. The thickness of the primary wall is no more than 0.1...0.5 microns.
By the time cell growth ends, the cell wall can continue to grow, but in thickness. This process is called secondary thickening. In this case, a secondary cell wall is deposited on the inner surface of the primary cell wall. The growth of the secondary cell wall occurs as a result of apposition, the application of new cellulose micelles to the inner surface of the cell wall. Thus, the youngest layers of the cell wall are closest to the plasmalemma.
For some types of cells (many fibers, tracheids, vascular segments), the formation of a secondary wall is the main function of the protoplast; after the completion of secondary thickening, it dies. However, this is not necessary. The secondary wall performs mainly mechanical, supporting functions. Its composition contains significantly less water and cellulose microfibrils predominate (40...50% of dry matter). In the secondary walls of flax fibers and cotton hairs, the cellulose content can reach 95%.
Mechanism of cell wall construction. The cell wall is formed as a result of the activity of the protoplast. In accordance with this, substances enter the wall from the inside, from the protoplast side. Building materials - cellulose molecules of pectin, lignin and other substances - accumulate and are partially synthesized in the tanks of the Golgi apparatus. Packed in vesicles of the Golgi apparatus, they are transported to the plasmalemma. Having broken it, the bubble bursts, and its contents appear outside the plasmalemma. The vesicle membrane restores the integrity of the plasmalemma. Thanks to the enzymatic activity of the plasmalemma, cellulose fibrils are assembled to form the cell wall. The fibrils formed by the plasmalemma are superimposed from the inside without intertwining. In their orientation, a major role belongs to microtubules, located under the plasmalemma parallel to the forming fibrils.
2. Pores. Modifications of the cell wall.
Pores. When the primary cell wall is formed, thinner areas are distinguished in it, where cellulose fibrils lie more loosely. The tubules of the endoplasmic chain pass through the cell walls here, connecting neighboring cells. These areas are called primary pore fields , and the tubules of the endoplasmic reticulum passing through them are plasmodesmata .
Growth in thickness occurs unevenly at the cell wall; small sections of the primary cell wall remain unthickened at the locations of the primary pore fields (pore channels). The pore canals of two neighboring cells are usually located opposite each other and are separated by a closing film of the pore - two primary cell walls with intercellular substance between them. The film retains submicroscopic openings through which plasmodesmata pass. Thus, time is two pore canals and a closing film between them.
Plasmodesmata penetrate the closing films of the pores. Each cell contains from several hundred to tens of thousands of plasmodesmata. Plasmodesmata are found only in plant cells, where there are solid cell walls. Plasmodesmata are formed from ER tubules that remain in the cell plate between two daughter cells. When the ER of both cells is recreated, they are connected through plasmodesmata.
Plasmodesma passes through the plasmodesmal canal in the closing film of the pore. The plasmalemma lining the canal and the hyaloplasm between it and the plasmodesmata are continuous with the plasmalemmas and hyaloplasms of adjacent cells. Thus, the protoplasts of neighboring cells are connected to each other by plasmodesmata channels and plasmodesmata. They carry intercellular transport of ions and molecules, as well as hormones. The protoplasts of cells in the plant united by plasmodesmata form a single whole - the symplast. The transport of substances through plasmodesmata is called symplastic, in contrast to apoplastic transport along cell walls and intercellular spaces.
During the life of a cell, the cellulose cell wall can undergo modifications.
Poorly studied. Cellulose microfibrils are thought to be synthesized at the cell surface by an enzyme complex associated with the plasma membrane, and the orientation of the microfibrils is controlled by microtubules located at the inner surface of the plasma membrane. Pectins, hemicelluloses and glycoproteins are probably formed in the Golgi complex and are transported to the wall in vesicles released from dictyosomes.
Pores form in the walls of neighboring cells, usually one opposite the other.
They are most often formed where there are primary pore fields. Pores are the openings in the secondary membrane, where the cells are separated only by the primary membrane and the median lamina (Fig. 22). The areas of the primary membrane and the middle plate separating the adjacent pores of adjacent cells are called pore membrane, or the closing film of the pore. The closing film of the pore is pierced by plasmodesmal tubules, but a through hole is usually not formed in the pores.
The contents of neighboring cells are connected to each other through special cytoplasmic strands - plasmodesmata. Plasmodesmata are located in the plasmodesminal tubules of the pore membrane. Through plasmodesmata, the transmission of irritations and the active movement of certain substances from cell to cell occurs.
Mature cells are usually multilayered; in the layers, cellulose fibrils are oriented differently, and their number can also fluctuate significantly. Primary, secondary and tertiary cell walls are usually described. When plant cells divide, after the divergence of chromosomes, a cluster of small membrane vesicles appears in the equatorial plane of the cells, which begin to merge with each other in the central part of the cells. This process of fusion of small vacuoles occurs from the center of the cell to the periphery and continues until the membrane vesicles merge with each other and with the plasma membrane of the lateral surface of the cell. This is how it is formed cell plate. In its central part there is an amorphous matrix substance that filled the merging bubbles. It has been proven that these primary vacuoles originate from the membranes of the Golgi apparatus. Along the periphery of the cell plate, when observed in polarized light, noticeable birefringence is detected, caused by the fact that oriented cellulose fibrils are located in this place. Thus, the growing cell plate already consists of three layers: the central one - the middle plate, consisting only of an amorphous matrix, and two peripheral ones - the primary membrane containing hemicellulose and cellulose fibrils. If the middle plate is a product of the activity of the original cell, then the primary shell is formed due to the release of hemicellulose and cellulose fibrils by two new cell bodies. And all further increase in the thickness of the cell (or rather, intercellular) wall will occur due to the activity of two daughter cells, which from opposite sides will secrete substances of the cell membrane, which thickens by layering more and more new layers. Just as from the very beginning, the release of matrix substances occurs due to the approach of the vesicles of the Golgi apparatus to the plasma membrane, their fusion with the membrane and the release of their contents beyond the cytoplasm. Here, outside the cell, on its plasma membrane, the synthesis and polymerization of cellulose fibrils occurs. This is how it gradually forms secondary cellular shell. It is difficult to determine and be able to distinguish the primary shell from the secondary one with sufficient accuracy, since they are connected to each other by several intermediate layers. The main mass of the cell wall that has completed its formation is the secondary membrane. It gives the cell its final shape. After the cell divides into two daughter cells, new cells grow, their volume increases and their shape changes; cells often elongate. At the same time, there is an increase in the thickness of the cell membrane and a restructuring of its internal structure. When the primary cell membrane is formed, there are still few cellulose fibrils in its composition, and they are located more or less perpendicular to the future longitudinal axis of the cell; later, during the period of elongation (elongation of the cell due to the growth of vacuoles in the cytoplasm), the orientation of these transversely directed fibrils undergoes passive changes: fibrils begin to be placed at right angles to each other and ultimately end up extended more or less parallel to the longitudinal axis of the cell. The process is constantly ongoing: in the old layers (closer to the center of the shell) fibrils undergo passive shifts, and the deposition of new fibrils in the inner layers (closest to the cell membrane) continues in accordance with the original design of the shell. This process creates the possibility of fibrils sliding relative to each other, and the restructuring of the cell membrane reinforcement is possible due to the gelatinous state of the components of its matrix. Subsequently, when hemicellulose is replaced by lignin in the matrix, the mobility of fibrils sharply decreases, the shell becomes dense, and lignification occurs. Often found under the secondary shell tertiary shell, which can be considered as a dried remnant of the degenerated layer of the cytoplasm itself. It should be noted that when plant cells divide, the formation of the primary membrane is not in all cases preceded by the formation of a cell plate.
42. Structure and properties of cell walls of plant cells and bacteria
The plant cell wall is formed with the participation of the plasma membrane and is an extracellular (extracellular) multilayered formation that protects the cell surface, serving as the outer skeleton of the plant cell. The cell wall consists of two components: an amorphous plastic gel-like matrix (base) with a high water content and a supporting fibrillar system. Often, to impart properties of rigidity, non-wetting, etc., additional polymeric substances and salts are included in the composition of the shells. Chemically, the main components of plant shells belong to structural polysaccharides. The shell matrix contains polysaccharides that dissolve in concentrated alkalis, hemicelluloses and pectin substances. Hemicelluloses are polymer chains consisting of various hexoses (glucose, mannose, galactose, etc.), pentoses (xylose, arabinose) and uronic acids (glucuronic and galacturonic acids). These components of hemicelluloses are combined with each other in different quantitative ratios and form various combinations. Chains of hemicellulose molecules do not crystallize and do not form elementary fibrils. Due to the presence of polar groups of uronic acids, they are highly hydrated. Pectin substances are polymers of methyl-D-glucuronate. The matrix is a soft plastic mass reinforced with fibrils. The fibrous components of cell walls usually consist of cellulose, a linear, non-branching polymer of glucose. The molecular weight of cellulose varies from 5*104 to 5*105, which corresponds to 300 – 3000 glucose residues. Such linear cellulose molecules can be combined into bundles or fibers. In the cell wall, cellulose forms fibrils, which consist of submicroscopic microfibrils up to 25 nm thick, which in turn consist of many parallel chains of cellulose molecules. The quantitative ratios of cellulose to matrix substances (hemicelluloses) can be very different for different objects. Over 60% of the dry weight of the primary shells is their matrix and about 30% is the skeletal substance - cellulose. In raw cell membranes, almost all the water is associated with hemicelluloses, so the weight of the main substance in the swollen state reaches 80% of the wet weight of the entire membrane, while the content of fibrous substances is reduced to only 12%. In the case of another example, cotton hairs, the cellulose component makes up 90%; in wood, cellulose makes up 50% of the cell wall components. In addition to cellulose, hemicellulose and pectins, cell membranes contain additional components that give them special properties. Thus, incrustation (incorporation inside) of the shells with lignin (polymer of coniferyl alcohol) leads to lignification of the cell walls, increasing their strength. Lignin replaces the plastic substances of the matrix in such shells and plays the role of the main substance with high strength. Often the matrix is strengthened with minerals (SiO2, CaCO3, etc.). Various adcrusting substances, such as cutin and suberin, can accumulate on the surfaces of the cell membrane, leading to suberization of cells. In epidermal cells, wax is deposited on the surface of the cell membranes, which forms a waterproof layer that prevents the cell from losing water.
44. Skeletal-motor apparatus of the cell.
There is a lot of movement in the cell: chromosomes move to the cell poles during mitosis, vacuoles of cell organelles move, and the cell surface moves. In addition, cytoplasmic currents are observed in plant and animal cells (for example, in plant cells or in amoeba). Moreover, individual cells (free-living unicellular organisms or specific types of cells in multicellular animal organisms) have the ability to actively move and crawl. Some cells have specialized structures, cilia and flagella, that allow them to either move themselves or move the fluid around them. Finally, multicellular animal organisms have specialized cells, the muscular work of which allows for various movements of organs, individual parts and the entire organism. It was found that all these numerous motor reactions are based on common molecular mechanisms. In addition, it was shown that the presence of any motor apparatus should be combined and structurally associated with the existence of supporting, framework or skeletal intracellular formations. Therefore, we can talk (describe and study) about the musculoskeletal system of cells. The actual motor components of cells include various microfilaments and proteins associated with microtubules. Supporting or skeletal intracellular structures include microfibrils and microtubules.
See tickets 45-55.
51. Intermediate filaments.
Intermediate filaments, or microfibrils, have a thickness of about 10 nm, so they are also called 10 nm (or 100 A0) filaments. They are usually collected in bundles, located mainly along the periphery of the cell, but are also found in the central regions of the cell around the nucleus (endoplasm). By chemical nature, this is a variegated class of proteins. Thus, in epithelial cells, 10-nm filaments are represented by keratan proteins (tonofilaments) with a mol. weighing 42 - 70 thousand, in mesenchymal cells (connective tissue cells, including fibroblasts) - vimentin (mol. weight 52 thousand), in muscle cells - desmin (mol. weight 50 thousand), involved in the structuring of a-actinin Z-discs; in nerve cells these are proteins of neurofibrils (molecular weight 210, 160, 68 thousand). Glial intermediate filaments have also been described. These proteins can copolymerize. Thus, fibroblast intermediate filaments contain vimentin and desmin, epithelial filaments contain keratin and vimentin. It was found that two types of intermediate filaments can coexist in one type of cell. For example, in some tissue culture cells there may be a vimentin network around the nucleus and, at the same time, keratin filaments located on the side adjacent to the substrate.
Inhibitors of the polymerization of these proteins are not known, which makes it difficult to elucidate their functional role. It is believed that intermediate filaments have a mainly mechanical, skeletal function, being frame structures inside cells. This idea is confirmed by the fact that in many epithelial cells, especially the integumentary one, intermediate filaments form thick bundles of tonofibrils (or tonofilaments). Tonofibrils give such cells greater elasticity and rigidity. They associate with numerous desmosomes on the surface of the plasma membrane and, indeed, are frame structures that provide mechanical strength to cells that are constantly exposed to large deforming loads. The intermediate filament system is as dynamically mobile as microfilaments and microtubules. Thus, when fibroblasts are spread out on glass, they are initially collected in the perinuclear zone, but then soon appear on the periphery of the cells. When cells are exposed to colchicine, which causes the disappearance of microtubules, intermediate fibrils are collected into thick strands that surround the cell nucleus in a ring. Rings or baskets of 30 nm filaments are often observed under normal conditions. When cells divide, it splits into two horseshoe-shaped structures, which in daughter cells again surround the nucleus. These observations suggest that intermediate filaments are somehow involved in anchoring the nucleus within the cytoplasm.
55. Structure of bacterial flagella
The main form of movement of bacteria is with the help of a flagellum. Bacterial flagella are fundamentally different from the flagella and cilia of eukaryotic cells. Based on the number of flagella, they are divided into: monotrichs - with one flagellum, polytrichs - with a bundle of flagella, peritrichs - with many flagella in different parts of the surface. Bacterial flagella have a very complex structure; they consist of three main parts: an outer long wavy filament (the flagellum itself), a hook, and a basal body. The flagellar filament is built from the flagellin protein. Its molecular weight varies depending on the type of bacteria (40 - 60 thousand). The globular subunits of flagellin are polymerized into helically twisted filaments so that a tubular structure is formed (not to be confused with eukaryotic microtubules!) with a diameter of 12–25 nm, hollow from the inside. Flagellins are not capable of movement. They can spontaneously polymerize into filaments with a constant wave pitch characteristic of each species. In living bacterial cells, the growth of flagella occurs at their distal end; It is likely that flagellin transport occurs through the hollow middle of the flagellum. Near the cell surface, the flagellum filament, the flagella, passes to a wider area, the so-called hook. It is about 45 nm long and consists of another protein. The bacterial basal body has nothing in common with the basal body of a eukaryotic cell. It consists of a rod connected to a hook and four rings. The two upper rings found in gram-negative bacteria are localized in the cell wall: one ring is immersed in the lipopolysaccharide membrane, the second in the murein (peptidoglycan) layer. The remaining two rings are localized in the plasma membrane of the cell. The basal bodies of Gram-positive bacteria have only two lower rings connected to the plasma membrane. By separating the flagellar filament mechanically and then causing lysis of bacterial cells, it was possible to isolate hooks and bacterial basal bodies. This is a protein structure that includes about 12 different proteins. By attaching bacterial flagella to a substrate using antibodies, the researchers observed the bacteria rotating. Consequently, the mechanism of flagellar movement is the rotation of the bacterial basal body around its axis. In this case, the flagellar filament describes a cone-shaped figure. It has been shown that numerous mutations in flagellins (changes in filament bending, “curling”, etc.) do not affect the ability of cells to move. Mutations in the basal component proteins often lead to loss of movement. The movement of bacterial flagella does not depend on ATP, but is carried out due to the potential difference on the surface of the plasma membrane. Another form of movement that occurs in cyanobacteria (blue-green algae) and some gram-positive bacteria is their sliding along the substrate. Its mechanism remains unclear; no special organelles of movement have yet been found in these bacteria.
To determine the localization of sites of biopolymer synthesis, to determine the pathways of substance transfer in a cell, to monitor the migration or conduct of individual cells, the method is widely used autoradiography– registration of substances labeled with isotopes. During an autoradiographic study, a monomer of one macromolecular compound (for example, an amino acid or nucleotide), one of the atoms of which is replaced by a radioactive isotope, is introduced into the cells into the medium. For example, instead of 12C, a 14C atom is introduced, instead of hydrogen, tritium 3H, etc. During the synthesis process, a labeled monomer molecule will also be included in the biopolymer. Its presence in a cell can be detected using photographic emulsion. If the cells in a layer or on a section are covered with a photoemulsion, then after some time, as a result of the decay of the isotope, B-particles scattering chaotically in different directions will enter the zone of the sensitive photolayer and activate the grains of silver bromide in it. The longer the exposure time, i.e., the contact of such a labeled cell with the photoemulsion, the more AgBr grains will be illuminated. After exposure, the drug must be developed; in this case, silver is restored only in illuminated granules; upon fixation, unexposed AgBr granules dissolve. As a result, from the mass of granules that covered the object, those that were activated by B-radiation will remain. Looking through a microscope at such preparations, on top of which a layer of photographic emulsion is applied, the researcher finds the localization of silver grains, which are located opposite the places where the labeled substance is contained.
This method has limitations: the accuracy will depend on the AgBr grain size and the particle energy. The greater the charge, the less accurately the location of the isotope can be determined. And the higher the energy of the particle and the longer its range, the farther from the site of decay the activation of AgBr grains will occur. Therefore, for the autoradiography method, special fine-grained photographic emulsions (0.2-0.3 microns) and isotopes with low energy B particles, mainly the hydrogen isotope, tritium 3H, are used. Any monomers of biological macromolecules can be labeled with tritium: nucleotides, amino acids, sugars, fatty acids. Labeled hormones, antibiotics, inhibitors, etc. are also used for autoradiographic studies. Water-soluble compounds cannot be studied autoradiographically, since during the treatment of cells with aqueous solutions (fixation, development, etc.) they may be lost. Another limitation of the method is the fairly high concentration of these substances, since at low concentrations the exposure time increases, and the danger of the appearance of a background of illuminated AgBr granules due to cosmic radiation increases.
The autoradiography method is one of the main methods that allows one to study the dynamics of synthetic processes and compare the intensity in different cells on the same preparation. Thus, for example, using this method, using labeled RNA monomers, it was shown that all RNA is synthesized only in the interphase nucleus, and the presence of cytoplasmic RNA is the result of the migration of synthesized molecules from the nucleus.
61. Vacuolar system
As is known, the cytoplasm itself, separated from the environment surrounding the cell by the plasma membrane, is heterogeneous in its structure. In addition to the seemingly structureless protoplasm, various membrane and non-membrane components are distinguished. Non-membrane components include microtubules and organelles built from them, and, in addition, various microfilaments and microfibrils. Membrane structures of the cytoplasm are separate or interconnected compartments, the contents of which are separated by membranes both from the hyaloplasm itself and from the plasma membrane. These cytoplasmic membrane structures have their own contents, different in composition, properties and functions from the hyaloplasm. Thus, the membrane elements of the cytoplasm are closed, enclosed volumetric zones (the term “compartment” is often used to describe them), distributed in a regular manner in the hyaloplasm. Membrane structures of the cytoplasm can be divided into two groups. One of them is the vacuolar system. It includes the endoplasmic reticulum, granular and smooth, and various vacuoles arising from this reticulum (plant cell vacuoles, microbodies, spherosomes, etc.). In addition, the vacuolar complex of the Golgi apparatus and lysosomes should be included in this system. All representatives of the vacuolar membrane system are characterized by the presence of a single limiting membrane. Another group of membrane components of the cytoplasm includes double-membrane organelles - mitochondria and plastids. In this case, they have closed and independent, external and internal membranes that do not transform into each other. This distinguishes them from the double-membrane nuclear envelope, where the outer membrane can be continuous with the endoplasmic reticulum membranes of the cytoplasm. Despite the fact that the vacuolar system includes components that are morphologically and functionally different, it represents a single whole. Its individual elements perform different functions, as if complementing and connecting each other.
50. Microtubules of interphase cells, structure and functions.
Microtubules are filamentous, non-branching structures consisting of tubulin proteins and proteins associated with them. Tubulins polymerize to form hollow tubes. The length of microtubules can reach several microns. The longest microtubules are found in the axoneme of sperm tails. Microtubules are found in the cytoplasm of interphase cells, where they are located singly or in small loose bundles or in the form of densely packed microtubules in centrioles, basal bodies, in cilia and flagella.
Microtubules are long, hollow cylinders whose walls are made of the polymerized protein tubulin. When polymerized, tubulin molecules form 13 longitudinal protofilaments, which curl into a hollow tube. The diameter of the tubulin monomer globule is 5 nm. Which corresponds to the thickness of a microtubule. The tubulin molecule consists of 2 subunits a - and b-tubulin.
The microtubule has a fast growing plus end and a slow growing minus end. When the protein concentration is sufficient, polymerization occurs spontaneously, without the consumption of ATP, but with the hydrolysis of one GTP molecule.
Microtubules are dynamic structures that can quickly polymerize and disassemble.
The isolated microtubules contain additional proteins associated with them, the so-called MAP proteins (for example, tau protein). These proteins stabilize microtubules and accelerate the process of tubulin polymerization. These proteins have binding sites for unpolymerized tubulin and binding sites for other cytoskeletal elements.
The average half-life of microtubules is only 5 minutes.
CILIA
The cilium consists of a basal body embedded in the cytoplasm and an axoneme, covered with a plasma membrane.
The axoneme consists of 9 doublets of microtubules located around the circumference, forming the outer wall of the axoneme cylinder, and two central microtubules. In microtubule doublets, there is an A-microtubule, consisting of 13 subunits, and a B-microtubule, consisting of 11 subunits. The A-microtubule carries two dynein arms facing the B-microtubule of the neighboring doublet. A spoke extends from the A-microtubule to the center of the axoneme, ending with a head on the central coupling surrounding the central vacuoles.
The basal body consists of 9 triplets, has spoke handles and a coupling.
CYTOPLASMA MICROTUBLES
1. skeletal
2. motor
Polymerization of microtubules (nucleation) occurs in the COMMT (usually the centrosome). Microtubules grow from the COMMT plus ends. Mature microtubules lose connection with the cell center. Microtubules create an elastic, stable cell skeleton. Microtubules participate in cell growth processes, while strengthening the cytoplasm, located in its peripheral layers. Microtubules play an important role in intracellular transport and, by their arrangement, set the directions for the movement of different structures. In this case, the proteins kinesin and dynein play an important role. Kinesin, when associated with a microtubule, acquires ATPase activity. When ATP is hydrolyzed, the conformation of the kinesin molecule changes and the movement of the particle in the direction towards the + end (dynein – minus end) is generated.
47. Microfilaments.
Actin microfilaments are found in all eukaryotic cells. They are especially abundant in highly specialized muscle fibers in cells that perform muscle contraction functions. Actin filaments are also part of special cellular components such as microvilli, ribbon junctions of epithelial cells, and cereocilia of sensitive cells. Actin microfilaments form bundles in the cytoplasm of moving living cells and a layer under the plasma membrane - the cortical layer. In many plant cells and cells of lower fungi, they are located in layers of moving cytoplasm.
The main protein of microfilaments is actin. This protein has a molecular weight of about 42 thousand and in monomer form has the form of a globule (G-actin). When polymerized, a thin fibril (F-actin) 6 nm thick is formed, which is a flat spiral ribbon. Actin microfilaments are polar in their properties. At sufficient concentration, G-actin begins to spontaneously polymerize. With such spontaneous polymerization of actin on the resulting microfilament filament, one of its ends quickly binds to G-actin (+ end of the microfilament) and therefore grows faster than the opposite (- end). If the concentration of G-actin is insufficient, then the resulting F-actin fibrils begin to disassemble. In solutions containing the so-called critical concentration of G-actin, a dynamic equilibrium is established between polymerization and depolymerization, as a result of which the length of the F-actin fibril will be constant. It follows that actin microfilaments are very dynamic structures that can arise and grow or, conversely, disassemble and disappear, depending on the presence of globular actin.
In living cells, such a seemingly unstable fibrillar system is stabilized by a mass of specific proteins associated with F-actin. So the tropomyosin protein, interacting with microfilaments, gives them the necessary rigidity. A number of proteins, such as filamin and α-actinin, form cross-links between F-actin filaments, resulting in the formation of a complex three-dimensional network that imparts a gel-like state to the cytoplasm. Other additional proteins can bind filaments into bundles (fimbrin), etc. In addition, there are proteins that interact with the ends of microfilaments and, preventing disassembly, stabilize them. The interaction of F-actin with this entire group of proteins regulates the aggregative state of microfilaments, their loose or, conversely, close arrangement, connection with other components. A special role in interaction with actin is played by myosin-type proteins, which together with actin form a complex capable of contraction when ATP is broken down.
Actin is a heterogeneous protein; different cells may have different variants, or isoforms, each encoded by its own genome. Thus, mammals have 6 different actins: one each in skeletal and cardiac muscles, two types in smooth muscles (one of them in blood vessels) and two non-muscle, cytoplasmic actins, which are a universal component of any mammalian cells. All actin isoforms are very similar in amino acid sequences; they have variant terminal regions that determine the rate of polymerization but do not affect contraction. This similarity of actins, despite some differences, determines their general properties.
58. Methods for electron microscopic study of cells
Contrasting corpuscular objects. Corpuscular objects include particles of viruses, phages, isolated cellular components (ribosomes, membranes, vacuoles, etc.), molecules. One of the widespread methods of contrasting biological objects is shading with metals. In this case, thermal evaporation of the metal is carried out in special vacuum installations. In this case, metal atoms fly away from the place of evaporation along straight trajectories. When they encounter an object, they are deposited on it in the form of a layer; its thickness will be greater in places perpendicular to the direction of flight of the metal particles. “Shadows” will appear in areas where the object shields the particle beam. Thus, the sputtered part of the object has a higher density than the sputtered substrate (background), and therefore the object will be visible. This method is widely used not only for contrasting viruses and ribosomes, but also for fairly thin molecules of nucleic acids. The disadvantage of this method is that it leads to an increase in the size of the object by the thickness of the sprayed layer, which in the best case reaches 10 - 15 A0. Another disadvantage is that it provides information only about the appearance and volume of particles. For contrasting shading, platinum, palladium, their alloys, and uranium are used. When negatively contrasting objects with solutions of heavy metal salts, ammonium molybdate, uranyl acetate, and phosphotungstic acid (PTA) are used. If aqueous solutions of such substances are mixed with biological objects, and then applied to substrate films and dried, then the objects (for example, viruses or protein complexes) will appear to be immersed in a thin layer of high-density amorphous substance. In an electron microscope, they appear as light objects on a dark background (like a photographic negative). The advantages of the method are that dissolved salts can penetrate deep into the object and further reveal its details. Negative contrast is widely used in the study of viruses and membrane enzyme complexes. Filamentous nucleic acid molecules are poorly detected by this method due to their small thickness. Heavy metal salts can be used in so-called positive contrast. In this case, the contrast agent binds to the structure and increases its electron density. Often, for positive contrasting of nucleic acids, solutions of uranyl acetate in alcohol or acetone are used. Uranyl acetate, contrasting nucleic acids, well stains the central cavities of spherical viruses, significantly increases the contrast of ribosomes and allows one to see thin threads of isolated nucleic acids.
Ultramicrotomy . When studying objects in an electron microscope, another complication arises - their thickness. The fact is that when a beam of electrons passes through an object, some of the electrons are absorbed, which leads to heating of the object and to its deformation. Therefore, it is necessary to have thin objects (no higher than 0.1 microns). The procedure for their manufacture is in principle similar to that used in light microscopy. For this purpose, cells and tissues are first fixed. Buffer solutions of glutaraldehyde or osmium tetroxide (OsO4) are used as fixatives. The most commonly used is double fixation: first glutaraldehyde, and then osmium, which is a heavy metal structure. Then, after dehydration, the fabrics are impregnated with epoxy resins or other plastics in liquid, monomeric form. When such plastics are polymerized, the object impregnated with them is enclosed in solid blocks that can already be cut into thin sections. Glass chips have a perfectly sharp and jagged cutting surface. But glass knives are very short-lived, they are used only once. Diamond knives are used: these are small diamonds sharpened in a special way; they serve for several years. The production of an ultra-thin section is carried out using thermal feeding of the object. A block with an object enclosed in plastic is mounted on a metal rod, which heats up and thereby moves the object forward by a certain amount in a known time. And if this thermal feed is coordinated with rhythmic cutting cycles, then a series of cuts of a given thickness can be obtained. This is achieved using special devices - ultramicrotomes. There are ultramicrotome designs where the object is fed mechanically. The area of the resulting ultrathin sections is usually very small (0.1 - 1 mm2), so all operations during ultramicrotomy are carried out under microscopic control. Sections mounted on grids with a backing must be additionally contrasted - “colored” using heavy metal salts. In this case, lead and uranium salts are also used, which, by binding to intracellular structures in the section, positively contrast them. In electron microscopic studies it turned out to be possible to use autoradiography methods. In this case, ultra-fine-grained emulsions are used (grain size about 0.02 - 0.06 microns). The disadvantage of this method is the very long exposure time, in some cases reaching several months. Techniques for preparing, ultrathin sectioning without fixation, and embedding cells in hard plastics are increasingly being used. These are methods of cryoultramicrotomy, i.e., obtaining sections from frozen tissues instantly cooled to the temperature of liquid nitrogen (–1960). In this case, almost instantaneous inhibition of all metabolic processes occurs, and water from the liquid phase passes into the solid, but not crystalline, its molecular structure is disordered (vitreous state). Such solid blocks can be cut into ultra-thin sections at liquid nitrogen temperature (the knife is also cooled). The resulting sections are used to identify enzyme activity in them, to carry out immunochemical reactions on them, for enzymatic digestion, etc. The study of sections obtained on cryoultratomes showed that the general structure and composition of cellular components in this case differs little from what visible using chemical fixation and conventional ultrathin sectioning techniques.
Other special methods of electron microscopy of biological objects
Freezing-etching method- consists in the fact that the object is first quickly frozen with liquid nitrogen, and then at the same temperature is transferred to a special vacuum installation. There, the frozen object is mechanically chopped off with a chilled knife. This exposes the internal zones of frozen cells. In a vacuum, part of the water that has passed into a glassy form is sublimated (“etching”), and the surface of the chip is successively covered with a thin layer of evaporated carbon and then metal. In this way, a replica is obtained from a chipped material that is frozen and retains its intravital structure. Then, at room temperature, the tissue or cells are dissolved in acids, but the replica film remains intact and is studied under an electron microscope. This method has two advantages: they study replicas from chips of native samples; They study the surface relief of cell membranes, which is unattainable by other methods. It turned out that in this case, the general organization of the cell and its components is similar to what we see during chemical fixation or cryotomy. This method made it possible to see that globules are located both on the surface and in the thickness of cell membranes, and that the membranes are not uniform in structure.
Recently, methods have begun to be used high voltage(or rather, ultra-high voltage) microscopy. Devices with accelerating voltages of 1–3 million V have been designed. These are very expensive devices, which limits their widespread use. The advantage of this class of electron microscopes is not that higher resolution can be obtained with them (at a shorter electron wavelength), but that with high energy electrons, which are less absorbed by the object, it is possible to view samples of large thickness (1 – 10 µm). The additional use of stereoscopic imaging makes it possible to obtain information about the three-dimensional organization of intracellular structures with high resolution (about 0.5 nm). This method is also promising in another respect: if at ultra-high energy of electrons their interaction with an object decreases, then in principle this can be used in studying the ultrastructure of living objects. Work is currently underway in this direction. Scanning (raster) electron microscopy method allows you to study a three-dimensional picture of the cell surface. In scanning electron microscopy, a thin beam of electrons (a probe) runs across the surface of an object, and the resulting information is transmitted to a cathode ray tube. The image can be obtained in reflected or secondary electrons. With this method, a fixed and specially dried object is coated with a thin layer of evaporated metal (most often gold), from which electrons are reflected and enter a receiving device that transmits a signal to a cathode ray tube. Thanks to the enormous depth of focus of a scanning microscope, which is much larger than that of a transmission microscope, an almost three-dimensional image of the surface under study is obtained. Using scanning electron microscopy, you can obtain information about the chemical composition in certain areas of cells. Thus, the method of X-ray spectral microanalysis is based on the identification and quantitative assessment of the content of chemical elements from the spectra of characteristic X-ray radiation arising from the interaction of primary electrons with atoms. To obtain such information, of course, objects should not be covered with a layer of metal, as with the conventional method of scanning electron microscopy. Moreover, the object must be prepared so that there is no loss or additional addition of elements. For this purpose, quickly frozen and vacuum-dried objects are used.
1) Cell wall- structural education. Function: gives strength and shape, protects the protoplast from external conditions, participates in the conduction and absorption of substances.
The basis of the cell membrane (composition) is high-polymer carbohydrates (cellulose, i.e. fiber - is not digested, indicates low productivity), cellulose molecules are collected in complex bundles (mycelium), mycelium is combined into fibrils, their spaces are filled with hemicellulose (semi-fiber - less stable compound) and pectin (useful, swell in water, are a source of energy).
There are primary and secondary cell membranes. Meristematic and young growing cells have primary cell shell, thin, rich in pectin and hemicellulose; Cellulose fibrils in the matrix of the primary cell wall are arranged in a disorderly manner.
Secondary cellular The shell is usually formed when the cell reaches its final size and is superimposed in layers on the primary one from the protoplast side. In the secondary cell membrane, cellulose predominates; its fibrils are arranged in an orderly, parallel manner, but their direction in each layer is different, which increases the strength of the cell membrane. In the secondary cell wall there are openings (pores), where the cells are separated only by the primary cell wall and plasmodesmata (cytoplasmic bridges connecting neighboring plant cells).
Cell wall modifications:
- Lignification of the cell membrane occurs as a result of the deposition of lignin (a non-carbohydrate component in the fibrils); the cells lose their elasticity, but can allow water to pass through. These cells are more often dead than alive. Some cell walls may include: wax, cutina, suberin. Functions: gives the cell shape; separates one cell from another, is the skeleton for each cell and gives strength to the entire plant, performs a protective function.
- Cork formation is caused by a special fat-like substance - suberin. Such shells become impermeable to water and gases; they also do not allow heat to pass through; the contents of cells with suberized shells die.
- Cutinization involves the release of the fat-like substance cutin. Usually the outer walls of the skin of leaves and herbaceous stems are cutinized. This makes them less permeable to water, reduces the evaporation of water in plants, and protects them from overheating and ultraviolet radiation. Cutin forms a film on the surface of the organ called cuticle.
- Mineralization of cell membranes is the deposition of: silica and calcium salts. The cell membranes of the skin of leaves and stems of cereals, sedges, and horsetails are most heavily encrusted. The leaves of cereals and sedges can injure your hands.
- Mucusing of shells is the transformation of cellulose and pectin substances into mucus and gum. Mucilage is clearly observed on flax seeds that were in water. The formation of mucilage promotes better absorption of water by the seeds and their attachment to the soil.
2) Reproduction: the ability of a single individual to give rise to a whole series of its own kind.
Divided into: sexual and asexual (proper asexual and vegetative)
Vegetative: new individuals develop from individual vegetative organs or their interactions. It is carried out thanks to regeneration (the ability to restore an organism from a part of the body). Bio significance: the new organism is similar to the maternal one.
Methods of vegetative propagation:
- propagation by cuttings (a part of the plant that is not infected is planted in a substrate, sporodina),
- propagation by grafting (by sprouting parts of several plants, used in gardening),
- propagation by tubers (fleshy tubers with pita are planted in the ground, viviparous buckwheat),
- propagation by offspring (form shoots on roots, aspen),
- propagation by bulbs (in autumn they are planted from the plant itself into the ground)
- propagation by means of tendrils (creeping shoots, rooting, drupes, strawberries)
- propagation by rhizomes (underground shoot, pita stock, lily of the valley, violet, wheatgrass)
The use of vegetative propagation by humans. The rest is 40 cm.
For a long time, people, cultivating plants, began to use vegetative propagation. For example, growing potatoes, strawberries, banana in all countries of the world it is carried out only by vegetative means - tubers, tendrils and rhizomes.
The use of vegetative reproduction of plants in agricultural practice is called artificial vegetative propagation.
The main methods of artificial vegetative propagation come down to repeating those that occur in plants under natural conditions.
People often use propagation by cuttings - parts of green or woody shoots (grapes, currants, gooseberries, roses, cloves, ficus), tubers (potatoes, dahlia, sweet potato, Jerusalem artichoke), leaves (saintpaulia, gloxinia, begonia), bulbs (onion, garlic, tulip, daffodil), dividing the bush (currants, pyrethrum) and layering (gooseberries, honeysuckle, clematis), mustache (strawberry), rhizomes (sugar cane, irises, phlox), root shoots (plum, raspberry, cherry, lilac).
3) Pumpkin. Shape: herbs. Tap root. Stem: climbing, creeping, climbing Leaf: simple, petiolate, without stipules.
Formula: dioecious
1) regular female Ca (5) Co (5) A 0 G (3) perianth under the ovary
2) correct male Ca (5) Co (5) A 2+2+1 G 0
The inflorescence is solitary. Fruit: pumpkin
Representatives: cucumber, melon, pumpkin, watermelon, zucchini
Meaning: food, fodder
The cell membrane is capable of thickening and modification. As a result of this, a secondary structure is formed. Thickening of the membrane occurs by applying new layers to the nerve membrane. Due to the fact that the layer extends through the hard shell, the cellulose fibrils in each layer lie parallel, and in adjacent layers - at an angle to each other. This achieves significant strength and hardness of the secondary shell. As the number of cellulose fibril layers increases and the wall thickness increases, it loses its elasticity and ability to grow. In the secondary cell wall, the cellulose content increases significantly, in some cases up to 60% or more. As cells continue to age, the shell matrix can be filled with various substances - lignin, suberin (lignification or suberization of the shell). Lignin is formed from hemicellulose and pectin substances.[...]
The cell wall of wood fiber has several layers: primary, which is called the outer shell of the fiber, and secondary (the wall, which in turn consists of three layers: outer, middle and inner). Between the primary cell walls there is a layer of intercellular substance, through which the fibers are connected to each other. The secondary wall is relatively thick and represents the bulk of the cell volume.[...]
In the secondary layers of the cell walls of pine wood, large quantities of mannan (22%) and uronic anhydride (25%) accumulated.[...]
[ ...]
Cell wall thickening phase. How does thickening occur? During the growth period, the protoplast is surrounded only by the primary wall. When the wood cell reaches its largest surface size, or shortly thereafter, the cell wall thickens. This is caused by the layering of a secondary wall on the primary one, and this new layer arises as a result of further activity of the protoplast inside the cell cavity. Naturally, cells in which the protoplast has disappeared cannot continue to thicken their walls. The formation of a secondary wall is a sign of an irreversible change in the cell, the further growth of which is already excluded, but further division is not necessarily excluded, provided that the daughter cells thus obtained occupy the same volume as the original cell. [...]
M.1ip - carpet, bedspread). It consists of tabular, thin-walled cells with dense cytoplasm. Usually it is single-rowed, but sometimes it is double-rowed or multi-rowed. Notoma cells are initially mononucleate, but later they often become binucleate or even multinucleate. The tapetum is a physiologically extremely active tissue: its cells contain enzymes, hormones, and nutritional material used in the process of microsporogesis. There are some reasons to consider the secretory type to be primary in evolutionary terms, and the amoeboid type to be secondary.[...]
It should be noted, however, that these data should be considered approximate, since the original preparations were not thoroughly purified.[...]
It is difficult to determine the cell wall location of polyuronide hemicelluloses because the reagents used to identify them also affect lignin. Some researchers suggest that hemicelluloses are the cementing substance between the fibrils and the various layers of the cell wall. Cohen even believes that secondary wall lignin is of the same nature as hemicelluloses. The basis for this assumption seems to be the fact that some carbohydrates, when treated with strong acids, can produce insoluble residues of a certain pattern. It should be emphasized, however, that areas, both carefully treated with reagents that dissolve hemicelluloses and not treated with them, give residues of a very similar structure when exposed to 72% sulfuric acid.[...]
To elucidate the composition of individual cell wall layers, an attempt was made to quantify xylouronides in different tracheid and libriform layers. Measurements were made on fibers from Japanese red pine, European fir, beech and birch. For this purpose, the fibers were carefully nitrated in a medium of acetic anhydride and carbon tetrachloride. Then the outer nitrated layer was removed by dissolving in acetone, after which the content of pentosans in the residue was controlled by furfural. It was found that pentosans in wood fibers are unevenly divided into layers. The largest amount of pentosans is found in the outer layers of the fibers and their concentration decreases from the periphery to the center. Thus, the outer layers of coniferous wood fibers contain 50-80% pentosans, while hardwoods contain almost 100%. In the secondary layers of the cell walls of conifers, the content of pentosans turned out to be no more than 2-4%, and in deciduous trees it was 8-10%. Thus, the chemical method confirmed the results obtained previously by the method of ultraviolet light sorption.[...]
A distinction is made between primary lignin, located in lignified cell walls (natural lignin) and secondary lignin - isolated lignin. The latter is largely a substance modified during the isolation process and contaminated with impurities of foreign substances. A change in lignin is expressed in the elimination of methoxyl groups, intramolecular condensation and other features.[...]
Many differences between tissue types are due to the structure of the cell wall, especially the secondary one. As we have already said, the formation of the primary cell wall occurs during the process of cell elongation, and, therefore, it must have the property of extensibility, while the secondary wall is formed after elongation has stopped.[...]
Preston
Simultaneously with these internal changes, the outer hard wall of the oospore splits at its apex into five teeth, giving rise to a seedling emerging from the central cell (Fig. 269, 3). The first division of the central cell occurs by a transverse septum perpendicular to its long axis and leads to the formation of two functionally different cells. From one, larger cell, a stem shoot is subsequently formed, which at the initial stage of development is called a pre-shoot, from another, smaller cell - the first rhizoid. Both of them grow by transverse cell divisions. The pre-adult grows upward and turns green quite quickly, filling with chloroplasts; the first rhizoid goes down and remains colorless (Fig. 269, 4). After a series of cell divisions, giving them the structure of single-row filaments, their differentiation into nodes and internodes occurs, and their further apical growth proceeds as described above for the stem. From the nodes of the pre-growth, secondary pre-shoots, whorls of leaves and lateral branches of the stem arise, from the nodes of the first rhizoid - secondary rhizoids and their whorled hairs. In this way, a thallus is formed, consisting of several stem shoots in the upper part and several complex rhizoids in the lower part (Fig. 2G9, 5).[...]
Supramolecular structure. Figure 6.10 shows a model of the structure of the cell wall. It includes 2 main layers: the primary wall P and the secondary wall. The latter is divided into 3 layers: 5], 5, Layer M, the middle plate, is an intercellular substance that connects cells to each other. [...]
Subsequent sections (Part II) will cover in detail the chemistry of cell walls, the relative amounts of lignin within them, and other related topics. However, concluding the consideration of the fourth and final phase of the ontogenesis of a woody cell, it is worth mentioning some phenomena that are in one way or another connected with lignification, as botanists call it. Like the formation and proliferation of cells, as well as the thickening of cell walls, lignification can only occur during the life of the cellular protoplast, since dead cells cannot lignify their walls. The lignification process can be completed in the layer of intercellular substance and in the primary wall, but can continue in the secondary wall, even if this last-named layer is still centripetally increasing in thickness. In tree wood, lignification often ends very quickly in the layer adjacent to the inner side of the cambium, usually almost simultaneously with the time when the new cells have reached their largest size and the secondary walls have reached their final thickness. This explains why sapwood, at the same moisture content, is as strong or almost as strong as heartwood.[...]
A detailed study of the distribution of lignin and polysaccharides in the lignified cell walls of spruce and birch wood by measuring the absorption intensity of a thin beam of ultraviolet rays passing through a transparent section confirmed the predominant location of lignin in the middle lamina and primary wall, as well as partially in the outer layers of the secondary wall. In the middle plate of spruce wood, the lignin content reaches 73%, and in the secondary wall - no more than 16%. It follows that polysaccharides are concentrated mainly in the secondary layer. An attempt was made to measure the relative positions of cellulose and hemicelluloses by this method. To do this, polysaccharides were first converted into colored compounds that absorb light.[...]
In most cells, alternating zones of greater or lesser lignin deposition are clearly visible, giving the appearance of concentric rings. In the opposite process, when the cell wall is treated with delignifying agents. reagents, the cellulose pattern remains the same. This indicates that there appear to be two interpenetrating systems, one consisting of cellulose and other polysaccharides, and the other of lignin. Bailey and Kerr showed that particle sizes reach 0.1¡x and less. The gaps or bands explain the relatively large "fibrils" seen by some researchers. In addition to the predominant concentric patterns, the grain of some types of wood exhibits an arrangement of radial lines or a combination of both types. Cells of compressed wood often have tough, almost solid bands of lignin near the cell cavity and radially arranged plates of it, separated by zones of polysaccharide substance, in the middle part of the cell wall.[...]
Lichens contain many elements and substances. All of them can be divided into two large groups - primary and secondary. Primary substances include those substances that directly participate in cellular metabolism; The body of lichens is built from them. Secondary products include the end products of metabolism, usually located on the walls of the hyphae. Many of these secondary lichen substances (in older literature they were called lichen acids) are specific to lichens and are not found in organisms from other systematic groups.[...]
Ritter, Lüdtke, et al. reported that when wood fibers are treated with various swelling agents, the secondary wall (and probably the primary wall as well) disintegrates into thread-like fragments or fibrils. Ritter divided these fibrils into spindle-shaped bodies, and these in turn into spherical units. The significance of such relatively large structural units (the length of the fusiform bodies is approximately 4[x) is unclear, due to the finely porous structure of the secondary wall described above. Neither the lignin residues after cellulose dissolution nor the cellulose residues after lignin dissolution show any noticeable gaps indicating the boundaries of these cell wall units. In addition, recent studies using an electron microscope have not established the presence of such relatively large units in the structure of cell walls.[...]
When assessing the effect of various wood-decaying fungi on plant tissue, it is necessary to take into account that their individual hyphae. move selectively through cell walls. Thus, white rot fungi prefer the middle plate and primary shell, where lignin is mainly concentrated. Red or brown rot fungi, on the contrary, prefer to pass through the secondary shell, which is richest in carbohydrates. Accordingly, the color of the wood damaged by them also differs. These issues will be discussed in more detail later.[...]
Studies of tracheids and libriforms using a polarizing and electron microscope, as well as radiography, have established the existence of five concentric layers in the cell walls: the outer, or primary, wall and the secondary wall. The secondary wall in turn is divided into three layers, usually designated 81, vg and B3. In addition, between the primary walls of neighboring cells there is a middle plate that glues them together (Fig. 35).[...]
The increase in yields when using water vapor is explained by the fact that the removal of valuable products from the reaction space is accelerated and the development of secondary decomposition reactions is delayed. In addition, when water vapor comes into contact with the capillary system of wood on its surface layers, steam condensation is possible, which creates conditions for thermal decomposition in an acidic aqueous environment. In this case, decomposition reactions occur primarily in the layers of the cell wall, which are located on the inner sides of the cell cavities and consist predominantly of non-heat-resistant hemicelluloses, which easily cleave off acetyl groups and part of the methoxyls associated with them, forming acetic acid and methyl alcohol, respectively.[... ]
It is hardly correct to call the segments that make up the filaments of the spherople cells cells, not only because they have many nuclei and chloroplasts (and, therefore, are clearly secondary formations), but also because the transverse partitions separating them are not similar to the cell walls of other multicellular organisms. green algae. They vary greatly in shape, as well as in the method and place of formation (Fig. 226, 4-6). Often transverse septa take the form of ring-shaped internal thickenings on the cell walls that do not close in the center, so that a hole remains through which the cytoplasmic strand passes (Fig. 226, 4). In other cases, instead of partitions, special plugs are formed. And finally, groups of radially converging cords can appear anywhere in the thread, resembling the skeletal cords of the caulerpa and playing a mechanical role.[...]
Outside the plasma membrane of their cells there is no additional dense cell wall or it consists of chitin, rarely cellulose. Storage carbohydrates are usually in the form of glycogen (animal starch).[...]
Marks-Figipi and Pepzel studied the change in the DP of cotton pulp at different stages of cotton ripening. They showed that the viscosity of cotton cellulose solutions decreased several hours after opening the box. Cellulose of the secondary cell wall in the fibers of unopened cotton bolls at low maturity (cellulose yield - 18%) has a single maximum on the distribution curve at DP 14,000. About 10% of the material has a lower molecular weight (DP 1500-2500), this cellulose is contained in primary cell wall.[...]
The position of the sites of microfibril formation in relation to the surface of the cytoplasmic membrane can be different. Thus, in bacteria this process occurs in an environment significantly removed from the cell surface and, therefore, from the membrane. Apparently, synthesis proceeds in a similar way in the thickened primary walls of the epidermal cells of oat coleoptiles, since cellulose synthesis in this case occurs evenly throughout the thickness of the cell wall. In the membranes of ascidians, cellulose deposition apparently also occurs in places remote from the surface of the secretory cells, although there is no sufficiently convincing evidence for this assumption. In contrast, microfibrils of secondary plant cell walls probably form on the inner surface of the wall, in close proximity to the cytoplasmic membrane. Since there is much more cellulose in the secondary walls than in the primary walls, it can be concluded that the majority of cellulose microfibrils are formed near the cytoplasmic membrane. However, this is not mandatory.[...]
One of the methods based on this principle is the method for determining the reactivity of cellulose from the pattern of swelling of xanthates in isopropyl alcohol. The swelling process during the interaction of a fiber with a solvent can be schematically represented as follows: the liquid penetrates into the fiber, as a result of which the volume of the fiber increases. Then the weak elastic outer layer of the secondary cell wall of the fiber ruptures and swellings (“beads”) form at the sites of rupture. The remains of this layer form constrictions and cuffs on the swollen fiber. Then the outer layer is separated and the fiber swells evenly, transverse stripes are formed on it and the fiber is divided into disk packs and individual disks, which subsequently dissolve.[...]
Dependence of wood strength on moisture content. Since the strength and stiffness of wood are determined in part by the cohesive forces that bind the molecules together, any agent that reduces these forces changes its overall strength. One such agent is water, so the strength of wood increases as the moisture content decreases, not only as a result of the increased density resulting from shrinkage, but also due to the presence of secondary valence cohesive forces1. Since the presence of water in an amount exceeding the saturation point of the fiber does not change the nature of the cell wall, the loss or acquisition of capillary (free) water has virtually no effect on the strength of wood. [...]
Structures containing a lot of lignin are dark brown to black, while weakly lignified areas are light yellow to amber. The results of this color reaction fully confirm previous work on cell wall chemistry. The secondary walls of the fibrous elements of hardwoods growing in temperate climates are lighter in color and therefore less lignified than the secondary walls of softwoods. The walls of vessels in hardwoods are darker in color than the surrounding fibrous elements, therefore they contain more lignin; the pore membranes are also highly lignified.[...]
This operation was carried out on lignified sections, previously freed from lignin using sodium chlorite in an acetic acid medium. The sections were then treated with p-phenylase; benzoyl chloride for the purpose of esterification of polysaccharides. The sections, brightly colored orange-red, were photometered after swelling in pyridine. By subjecting sections consisting of holocellulose to such treatment before and after removal of hemicelluloses, it was possible to establish that the bulk of hemicelluloses in spruce and birch wood is concentrated in the outer layers of the secondary wall. Thus, when extracting a cut of spruce holocellulose with 16% sodium hydroxide, it was found that up to 60-80% of the total amount of polysaccharides was extracted from the outer layers of the cell, about 50% from the middle of the cell wall, and only 16% from the layer B3 of alkali-soluble hemicelluloses. . A similar picture was observed for cross sections of libriform from birch wood.[...]
Experiments by Ritter, and later by Bailey et al. showed that, regardless of the possible presence of pectin polyuronides in the middle plate, it consists mainly of lignin, as chemists understand it (insoluble in cold 72% sulfuric acid, soluble after chlorination and treatment with weak bases or basic salts). In addition, Ritter proved that most of the lignin is located in this layer. This statement contradicted the prevailing view at the time that most of the lignin was present in other layers, especially in the secondary wall. It was later proven that in such cases the seemingly wide and voluminous secondary wall is actually like a spider's web, which, after drying, shrinks and turns into scattered pieces. If the primary walls are included in a complex middle plate, then it is very likely that most of the lignin is located here. [...]
Calcium channels are also found in plant cell membranes. The regulation of the entry of 45Ca2+ microsomes isolated from corn coleoptils and pumpkin hypocotyls by light, PAA, and the dependence of this reaction on calmodulin was shown. For the functioning of voltage-gated Ca2+ channels (charophytic algae Lieu11op,m), the presence of Mg2+ is necessary. The state of these voltage-gated channels is controlled by a system of enzymes that monitor the level of cAMP in the cell. Data were also obtained indicating a direct effect of exogenous cAMP on the uptake of 45Ca2+ in the cells of the cyclopsidae (mutant without a cell wall). The data shown in Fig. 4.1, indicate the regulatory effect of cAMP on the absorption of Ca2+ by cells. This indicates the possibility of mutual regulation of two systems of second messengers - cAMP and Ca2+. In experiments with animal cells, the increase in Ca2+ uptake under the influence of cAMP is explained by the phosphorylation of proteins of voltage-dependent Ca2+ channels and, as a result, an increase in their presence in the open state. [...]
Many studies have been devoted to studying the effect of ultrasound on cellulose fibers. Some researchers have compared or combined the effects of ultrasound with various mechanical influences. Thus, Yaime, Kronert and Neuhaus studied the effect of ultrasound on cellulose fibers in comparison with high-frequency mechanical vibrations and showed that ultrasound with a frequency of 20-3000 kHz loosens the fiber structure, increases the degree of its swelling and dehydration. The mechanical strength of paper made from such celluloses is increased, especially the tear strength. High-frequency mechanical vibrations act similarly. Iwasaki, Lindberg and Meyer believe that the general pattern of changes in fiber structure under the influence of ultrasound in an aqueous environment is similar to changes in fiber structure during mechanical grinding. In this case, profound changes in the morphological structure of the fibers occur, leading to shifts in the secondary cell wall, separation of large pieces from the primary wall, then to swelling of the secondary wall and its defibrillation. In the work of Safonova and Klenkova, when studying microphotographs of fibers subjected to ultrasound in water, it was shown that there are other, deeper disturbances in the structure of the fiber, which becomes penetrated by a whole network of numerous transverse channels. It is noted that early wood fibers and fibers that have not been dried are more susceptible to ultrasound.