Parts of a bacterial cell. Functions, structure and features of the bacterial membrane
The structure of bacteria has been well studied using electron microscopy of whole cells and their ultrathin sections, as well as other methods. The bacterial cell is surrounded by a membrane consisting of a cell wall and a cytoplasmic membrane. Under the envelope there is a protoplasm, consisting of cytoplasm with inclusions and a hereditary apparatus - an analogue of the nucleus, called a nucleoid (Fig. 2.2). There are additional structures: capsule, microcapsule, mucus, flagella, drank. Some bacteria can form spores under unfavorable conditions.
Rice. 2.2. The structure of the bacterial cell: 1 - capsule; 2 - cell wall; 3 - cytoplasmic membrane; 4 - mesosomes; 5 - nucleoid; 6 - plasmid; 7 - ribosomes; 8 - inclusions; 9 - flagellum; 10 - drank (villi)
Cell wall- a strong, elastic structure that gives the bacteria a certain shape and, together with the underlying cytoplasmic membrane, restrains high osmotic pressure in the bacterial cell. It participates in the process of cell division and transport of metabolites, has receptors for bacteriophages, bacteriocins and various substances. The thickest cell wall in gram-positive bacteria (Fig. 2.3). So, if the thickness of the cell wall of gram-negative bacteria is about 15-20 nm, then in gram-positive bacteria it can reach 50 nm or more.
The basis of the bacterial cell wall is peptidoglycan. Peptidoglycan is a polymer. It is represented by parallel polysaccharide glycan chains consisting of repeating residues of N-acetylglucosamine and N-acetylmuramic acid linked by a glycosidic bond. This bond is broken by lysozyme, which is acetylmuramidase.
A tetrapeptide is attached to N-acetylmuramic acid by covalent bonds. The tetrapeptide consists of L-alanine, which is bound to N-acetylmuramic acid; D-glutamine, which in gram-positive bacteria is combined with L-lysine, and in gram-
Rice. 2.3. Scheme of the architectonics of the bacterial cell wall
valuable bacteria - with diaminopimelic acid (DAP), which is a precursor of lysine in the process of bacterial biosynthesis of amino acids and is a unique compound found only in bacteria; The 4th amino acid is D-alanine (Figure 2.4).
The cell wall of gram-positive bacteria contains a small amount of polysaccharides, lipids and proteins. The main component of the cell wall of these bacteria is a multilayer peptidoglycan (murein, mucopeptide), which makes up 40-90% of the cell wall mass. Tetrapeptides of different layers of peptidoglycan in gram-positive bacteria are connected to each other by polypeptide chains of 5 glycine (pentaglycine) residues, which gives the peptidoglycan a rigid geometric structure (Fig. 2.4, b). With peptidoglycan to the tissue wall of gram-positive bacteria are covalently bound teichoic acids(from the Greek. tekhos- wall), the molecules of which are chains of 8-50 glycerol and ribitol residues connected by phosphate bridges. The shape and strength of bacteria is given by the rigid fibrous structure of a multilayer, cross-linked peptidoglycan.
Rice. 2.4. The structure of peptidoglycan: a - gram-negative bacteria; b - gram-positive bacteria
The ability of gram-positive bacteria during Gram staining to retain gentian violet in a complex with iodine (blue-violet color of bacteria) is associated with the property of multilayer peptidoglycan to interact with the dye. In addition, the subsequent treatment of the bacterial smear with alcohol causes the pores in the peptidoglycan to narrow and thereby retains the dye in the cell wall.
After exposure to alcohol, gram-negative bacteria lose their dye, which is due to the lower amount of peptidoglycan (5-10% of the mass of the cell wall); they are discolored with alcohol and turn red when treated with fuchsin or safranin. This is due to the structural features of the cell wall. Peptidoglycan in the cell wall of gram-negative bacteria is represented by 1-2 layers. The tetrapeptides of the layers are interconnected by a direct peptide bond between the amino group of DAP of one tetrapeptide and the carboxyl group of D-alanine of the tetrapeptide of the other layer (Fig. 2.4, a). Outside of peptidoglycan is a layer lipoprotein, linked to peptidoglycan via DAP. It is followed by outer membrane cell wall.
Outer membrane is a mosaic structure represented by lipopolysaccharides (LPS), phospholipids and proteins. Its inner layer is represented by phospholipids, and LPS is located in the outer layer (Fig. 2.5). Thus, the outdoor mem-
Rice. 2.5. Lipopolysaccharide structure
the brane is asymmetric. The LPS of the outer membrane consists of three fragments:
Lipid A is a conservative structure, almost the same in gram-negative bacteria. Lipid A is composed of phosphorylated glucoseamine disaccharide units to which long chains of fatty acids are attached (see Fig. 2.5);
The core, or core, of the crustal part (from lat. core- core), relatively conservative oligosaccharide structure;
A highly variable O-specific polysaccharide chain formed by repeating identical oligosaccharide sequences.
LPS is anchored in the outer membrane by lipid A, which causes the toxicity of LPS and is therefore identified with endotoxin. The destruction of bacteria by antibiotics releases large amounts of endotoxin, which can cause endotoxic shock in the patient. Lipid A leaves the nucleus, or the core part of the LPS. The most constant part of the LPS core is ketodeoxyoctonic acid. O-specific polysaccharide chain extending from the core part of the LPS molecule,
consisting of repeating oligosaccharide units, determines the serogroup, serovar (a type of bacteria detected using immune serum) of a particular strain of bacteria. Thus, the concept of LPS is associated with the concept of the O-antigen, by which bacteria can be differentiated. Genetic changes can lead to defects, shortening of bacterial LPS and the appearance of rough colonies of R-forms, which lose their O-antigenic specificity.
Not all gram-negative bacteria have a complete O-specific polysaccharide chain of repeating oligosaccharide units. In particular, bacteria of the genus Neisseria have a short glycolipid called lipooligosaccharide (LOS). It is comparable to the R-form, which has lost O-antigenic specificity, observed in mutant rough strains E. coli. The structure of the VOC resembles the structure of the glycosphingolipid of the human cytoplasmic membrane, therefore the VOC mimics the microbe, allowing it to escape the host's immune response.
Matrix proteins of the outer membrane permeate it in such a way that protein molecules called porins, border hydrophilic pores through which water and small hydrophilic molecules with a relative mass of up to 700 D.
Between the outer and cytoplasmic membrane there is periplasmic space, or periplasm containing enzymes (proteases, lipases, phosphatases, nucleases, β-lactamases), as well as components of transport systems.
When the synthesis of the bacterial cell wall is disturbed under the influence of lysozyme, penicillin, protective factors of the body and other compounds, cells with an altered (often spherical) shape are formed: protoplasts- bacteria completely devoid of the cell wall; spheroplasts- bacteria with a partially preserved cell wall. After removal of the cell wall inhibitor, such altered bacteria can reverse, i. E. acquire a full-fledged cell wall and restore its original shape.
Bacteria of the sphero or protoplast type that have lost the ability to synthesize peptidoglycan under the influence of antibiotics or other factors and are able to multiply are called L-shapes(from the name of the D. Lister Institute, where they first
some have been studied). L-forms can also arise as a result of mutations. They are osmotically sensitive, spherical, flask-shaped cells of various sizes, including those passing through bacterial filters. Some L-forms (unstable), upon removal of the factor that led to changes in bacteria, can reverse, returning to the original bacterial cell. L-forms can form many pathogens of infectious diseases.
Cytoplasmic membrane in electron microscopy of ultrathin sections, it is a three-layer membrane (2 dark layers 2.5 nm thick each are separated by a light - intermediate). In structure, it is similar to the plasmolemma of animal cells and consists of a double layer of lipids, mainly phospholipids, with embedded surface, as well as integral proteins, as if penetrating through the membrane structure. Some of them are permeases involved in the transport of substances. Unlike eukaryotic cells, sterols are absent in the cytoplasmic membrane of a bacterial cell (with the exception of mycoplasmas).
The cytoplasmic membrane is a dynamic structure with movable components, therefore it is presented as a mobile fluid structure. It surrounds the outer part of the cytoplasm of bacteria and is involved in the regulation of osmotic pressure, transport of substances and energy metabolism of the cell (due to enzymes of the electron transport chain, adenosine triphosphatase - ATPase, etc.). With excessive growth (in comparison with the growth of the cell wall), the cytoplasmic membrane forms invaginates - invaginations in the form of complexly twisted membrane structures, called mesosomes. Less intricately twisted structures are called intracytoplasmic membranes. The role of mesosomes and intracytoplasmic membranes is not fully understood. It is even assumed that they are an artifact arising after preparation (fixation) of a preparation for electron microscopy. Nevertheless, it is believed that derivatives of the cytoplasmic membrane participate in cell division, providing energy for the synthesis of the cell wall, take part in the secretion of substances, sporulation, i.e. in processes with high energy consumption. The cytoplasm occupies the bulk of the bacterial
It consists of soluble proteins, ribonucleic acids, inclusions, and numerous small granules - ribosomes responsible for the synthesis (translation) of proteins.
Ribosomes bacteria have a size of about 20 nm and a sedimentation coefficient of 70S, in contrast to 80S ribosomes characteristic of eukaryotic cells. Therefore, some antibiotics, by binding to bacterial ribosomes, suppress bacterial protein synthesis without affecting protein synthesis of eukaryotic cells. Bacterial ribosomes can dissociate into two subunits: 50S and 30S. rRNA - conservative elements of bacteria ("molecular clock" of evolution). 16S-rRNA is part of the small subunit of ribosomes, and 23S-rRNA is part of the large subunit of ribosomes. The study of 16S rRNA is the basis of genosystematics, making it possible to assess the degree of kinship of organisms.
The cytoplasm contains various inclusions in the form of granules of glycogen, polysaccharides, β-hydroxybutyric acid and polyphosphates (volutin). They accumulate when there is an excess of nutrients in the environment and serve as reserve substances for nutrition and energy needs.
Volutin has an affinity for basic dyes and is easily detected using special staining methods (for example, according to Neisser) in the form of metachromatic granules. With toluidine blue or methylene blue, volutin is colored red-violet, and the cytoplasm of the bacterium is colored blue. The characteristic arrangement of volutin granules is revealed in the diphtheria bacillus in the form of intensely staining cell poles. The metachromatic coloration of volutin is associated with a high content of polymerized inorganic polyphosphate. In electron microscopy, they look like electron-dense granules with a size of 0.1-1 microns.
Nucleoid- the equivalent of the nucleus in bacteria. It is located in the central zone of bacteria in the form of double-stranded DNA, tightly packed like a ball. The nucleoid of bacteria, unlike eukaryotes, does not have a nuclear envelope, nucleolus and basic proteins (histones). Most bacteria contain one chromosome, represented by a closed ring DNA molecule. But some bacteria have two ring-shaped chromosomes. (V. cholerae) and linear chromosomes (see section 5.1.1). Nucleoid is detected in a light microscope after staining with DNA specific
methods: according to Felgen or according to Romanovsky-Giemsa. On electron diffraction patterns of ultrathin sections of bacteria, the nucleoid has the form of light zones with fibrillar, filamentous DNA structures associated with certain areas with the cytoplasmic membrane or mesosome involved in chromosome replication.
In addition to the nucleoid, the bacterial cell contains extrachromosomal factors of heredity - plasmids (see section 5.1.2), which are covalently closed DNA rings.
Capsule, microcapsule, mucus.Capsule - a mucous structure with a thickness of more than 0.2 microns, firmly associated with the cell wall of bacteria and having clearly defined external boundaries. The capsule is distinguishable in smears-prints from the pathological material. In pure cultures of bacteria, the capsule is formed less frequently. It is revealed with special methods of staining the smear according to Burri-Hins, which create a negative contrasting of the substances of the capsule: ink creates a dark background around the capsule. The capsule consists of polysaccharides (exopolysaccharides), sometimes of polypeptides, for example, in the anthrax bacillus, it consists of polymers of D-glutamic acid. The capsule is hydrophilic and contains a large amount of water. It prevents bacterial phagocytosis. Antigenic capsule: antibodies to the capsule cause its enlargement (capsule swelling reaction).
Many bacteria form microcapsule- a mucous formation with a thickness of less than 0.2 microns, detected only by electron microscopy.
It should be distinguished from the capsule slime - mucoid exopolysaccharides without clear external boundaries. The mucus is water soluble.
Mucoid exopolysaccharides are characteristic of mucoid strains of Pseudomonas aeruginosa, which are often found in the sputum of patients with cystic fibrosis. Bacterial exopolysaccharides are involved in adhesion (adhesion to substrates); they are also called glycocalyx.
The capsule and mucus protect bacteria from damage, drying out, since, being hydrophilic, they bind water well, prevent the action of protective factors of the macroorganism and bacteriophages.
Flagella bacteria determine the mobility of the bacterial cell. Flagella are thin filaments that take on
stem from the cytoplasmic membrane, are longer than the cell itself. Flagella thickness 12-20 nm, length 3-15 microns. They consist of three parts: a spiral filament, a hook, and a basal corpuscle containing a rod with special discs (one pair of discs in gram-positive bacteria and two pairs in gram-negative bacteria). The flagella are attached to the cytoplasmic membrane and cell wall by discs. This creates the effect of an electric motor with a rod - a rotor that rotates the flagellum. The difference in proton potentials across the cytoplasmic membrane is used as an energy source. The rotation mechanism is provided by proton ATP synthetase. The rotation speed of the flagellum can reach 100 r / s. If a bacterium has several flagella, they begin to rotate synchronously, intertwining into a single bundle, forming a kind of propeller.
Flagella are composed of a protein called flagellin. (flagellum- flagellum), which is an antigen - the so-called H-antigen. Flagellin subunits are twisted in a spiral.
The number of flagella in bacteria of different species varies from one (monotrichous) in Vibrio cholerae to tens and hundreds, extending along the perimeter of the bacteria (peritrichus), in Escherichia coli, Proteus, etc. Lophotrichs have a bundle of flagella at one end of the cell. Amphitrichs have one flagellum or bundle of flagella at opposite ends of the cell.
Flagella are detected using electron microscopy of preparations sprayed with heavy metals, or in a light microscope after processing by special methods based on etching and adsorption of various substances, leading to an increase in the thickness of the flagella (for example, after silvering).
Villi, or drank (fimbriae)- filamentous formations, thinner and shorter (3-10 nm * 0.3-10 microns) than flagella. Peels extend from the surface of the cell and are composed of pilin protein. Several types of saws are known. Pili of the general type are responsible for attachment to the substrate, nutrition, and water-salt metabolism. They are numerous - several hundred per cell. Sex pills (1-3 per cell) create contact between cells, carrying out the transfer of genetic information between them by conjugation (see Chapter 5). Of particular interest are type IV saws, in which the ends are hydrophobic, as a result of which they curl, these saws are also called curls. Location
they move along the poles of the cell. These pili are found in pathogenic bacteria. They have antigenic properties, contact the bacteria with the host cell, and participate in biofilm formation (see Chapter 3). Many drank are receptors for bacteriophages.
Disputes - a peculiar form of dormant bacteria with a gram-positive type of cell wall structure. Spore-forming bacteria of the genus Bacillus, in which the size of the spore does not exceed the diameter of the cell are called bacilli. Spore-forming bacteria in which the size of the spore exceeds the diameter of the cell, which is why they take the shape of a spindle, are called clostridia, e.g. bacteria of the genus Clostridium(from lat. Clostridium- spindle). Spores are acid-resistant, therefore they are stained according to the Aujeszky method or the Ziehl-Nelsen method in red, and the vegetative cell in blue.
Spore formation, the shape and location of spores in the cell (vegetative) are a specific property of bacteria, which makes it possible to distinguish them from each other. The shape of the spores is oval and spherical, the location in the cell is terminal, i.e. at the end of the bacillus (in the causative agent of tetanus), subterminal - closer to the end of the bacillus (in causative agents of botulism, gas gangrene) and central (in the anthrax bacillus).
The process of sporulation (sporulation) goes through a number of stages, during which a part of the cytoplasm and the chromosome of the bacterial vegetative cell are separated, surrounded by an ingrowing cytoplasmic membrane - a prospore is formed.
The protoplast contains a nucleoid, a protein synthesizing system, and a glycolysis-based energy production system. Even aerobes lack cytochromes. Does not contain ATP, the energy for germination is stored in the form of 3-glycerol phosphate.
The prospore is surrounded by two cytoplasmic membranes. The layer surrounding the inner membrane of the spore is called wall of controversy, it consists of peptidoglycan and is the main source of the cell wall during spore germination.
A thick layer is formed between the outer membrane and the spore wall, consisting of peptidoglycan, which has many crosslinks - cortex.
Outside of the outer cytoplasmic membrane is located spore shell, consisting of keratin-like proteins, co-
containing multiple intramolecular disulfide bonds. This shell provides resistance to chemical agents. Spores of some bacteria have an additional cover - exosporium lipoprotein nature. Thus, a multilayer poorly permeable shell is formed.
Spore formation is accompanied by intensive consumption of spore, and then the forming shell of the spore of dipicolinic acid and calcium ions. The spore acquires thermal stability, which is associated with the presence of calcium dipicolinate in it.
The spore can persist for a long time due to the presence of a multilayer shell, calcium dipicolinate, low water content and sluggish metabolic processes. In soil, for example, the causative agents of anthrax and tetanus can persist for decades.
In favorable conditions, spores germinate through three successive stages: activation, initiation, growth. In this case, one bacterium is formed from one spore. Activation is the readiness to germinate. At a temperature of 60-80 ° C, the spore is activated for germination. Germination initiation takes several minutes. The sprouting stage is characterized by rapid growth, accompanied by the destruction of the shell and the emergence of the seedling.
Bacteria are microscopic, single-celled organisms. The structure of a bacterial cell has features that are the reason for the separation of bacteria into a separate kingdom of the living world.
Cell shell
Most bacteria have three shells:
- cell membrane;
- cell wall;
- mucous capsule.
Directly with the contents of the cell - the cytoplasm, the cell membrane comes into contact. She is thin and soft.
The cell wall is a dense, thicker membrane. Its function is to protect and support the cage. The cell wall and membrane have pores through which the substances it needs enter the cell.
Many bacteria have a mucous capsule that has a protective function and adheres to different surfaces.
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It is thanks to the mucous membrane that streptococci (one of the types of bacteria) adhere to the teeth and cause caries.
Cytoplasm
The cytoplasm is the inner content of the cell. It consists of 75% water. In the cytoplasm there are inclusions - drops of fat and glycogen. They are the storage nutrients of the cell.
Rice. 1. Diagram of the structure of a bacterial cell.
Nucleoid
Nucleoid means "like a nucleus". Bacteria do not have a real, or, as they say, a formed nucleus. This means that they do not have a nuclear envelope and nuclear space, like the cells of fungi, plants and animals. DNA is located right in the cytoplasm.
DNA functions:
- preserves hereditary information;
- implements this information by controlling the synthesis of protein molecules characteristic of a given type of bacteria.
The absence of a true nucleus is the most important feature of a bacterial cell.
Organelles
Unlike plant and animal cells, bacteria do not have membrane organelles.
But the cell membrane of bacteria in some places penetrates the cytoplasm, forming folds called the mesosome. The mesosome is involved in cell reproduction and energy exchange and, as it were, replaces membrane organelles.
The only organoid found in bacteria is the ribosome. These are small bodies that are located in the cytoplasm and synthesize proteins.
Many bacteria have a flagellum that helps them move around in a liquid environment.
Bacterial cell shapes
The shape of bacterial cells is different. Ball-shaped bacteria are called cocci. In the form of a comma - vibrios. Rod-shaped bacteria are bacilli. Spirillae have the appearance of a wavy line.
Rice. 2. Forms of bacterial cells.
Bacteria can only be seen under a microscope. The average cell size is 1-10 microns. There are bacteria up to 100 microns in length. (1 μm = 0.001 mm).
Spore formation
When unfavorable conditions occur, the bacterial cell goes into a dormant state, which is called a spore. The causes of sporulation can be:
- low and high temperatures;
- drought;
- lack of nutrition;
- life-threatening substances.
The transition occurs quickly, within 18-20 hours, and the cell can be in a state of spore for hundreds of years. When normal conditions are restored, the bacterium grows out of the spore in 4-5 hours and goes into normal life.
Rice. 3. Scheme of controversy formation.
Reproduction
Bacteria multiply by fission. The period from the birth of a cell to its division is 20-30 minutes. Therefore, bacteria are widespread on Earth.
What have we learned?
We learned that, in general terms, bacterial cells are similar to plant and animal cells, they have a membrane, cytoplasm, and DNA. The main difference between bacterial cells is the absence of a formed nucleus. Therefore, bacteria are called prenuclear organisms (prokaryotes).
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The structural components of a cell are the bacterial membrane, which consists of a cell wall, a cytoplasmic membrane, and sometimes a capsule; cytoplasm; ribosomes; various cytoplasmic inclusions; nucleoid (nucleus). In addition, some types of bacteria have spores, flagella, cilia (pili, fimbria) (Fig. 2).
Cell wall obligatory formation of bacteria of most species. Its structure depends on the type and belonging.
bacteria to groups differentiated by Gram staining. The mass of the cell wall is about 20% of the dry mass of the entire cell, the thickness is from 15 to 80 nm.
Rice. 3. Diagram of the structure of a bacterial cell
1 - capsule; 2 - cell wall; 3 - cytoplasmic membrane; 4 - cytoplasm; 5 - mesosomes; 6 - ribosomes; 7 - nucleoid; 8 - intracytoplasmic membrane formations; 9 - fatty drops; 10 - polysaccharide granules; 11 - polyphosphate granules; 12 - sulfur inclusions; 13 - flagella; 14 - basal body
The cell wall has pores up to 1 nm in diameter, so it is a semipermeable membrane through which nutrients penetrate and metabolic products are released.
These substances can penetrate into the microbial cell only after preliminary hydrolytic cleavage by specific enzymes secreted by bacteria into the external environment.
The chemical composition of the cell wall is not uniform, but it is constant for a certain type of bacteria, which is used for identification. The cell wall contains nitrogenous compounds, lipids, cellulose, polysaccharides, and pectin substances.
The most important chemical component of the cell wall is the complex polysaccharide peptide. It is also called peptidoglycan, glycopeptide, murein (from lat. murus - wall).
Murein is a structural polymer composed of glycan molecules formed by acetylglucosamine and acetylmuramic acid. Its synthesis is carried out in the cytoplasm at the level of the cytoplasmic membrane.
Peptidoglycan of the cell wall of various types has a specific amino acid composition and, depending on this, a certain chemotype, which is taken into account when identifying lactic acid and other bacteria.
In the cell wall of gram-negative bacteria, peptidoglycan is represented by one layer, while in the wall of gram-positive bacteria it forms several layers.
In 1884, Gram proposed a tissue staining method that was used to stain prokaryotic cells. If, during Gram staining, the fixed cells are treated with an alcohol solution of crystal violet paint, and then with a solution of iodine, then these substances form a stable colored complex with murein.
In homo-positive microorganisms, the colored violet complex does not dissolve under the influence of ethanol and, accordingly, does not fade; when stained with fuchsin (red paint), the cells remain stained dark violet.
In gram-negative microorganisms, gentian violet is dissolved in ethanol and washed out with water, and when stained with fuchsin, the cell turns red.
The ability of microorganisms to stain with analytical dyes and by the Gram method is called tinctorial properties . They must be studied in young (18-24 hour) cultures, since some gram-positive bacteria in old cultures lose their ability to stain positively according to the Gram method.
The significance of peptidoglycan lies in the fact that thanks to it, the cell wall has rigidity, i.e. elasticity, and is the protective framework of the bacterial cell.
When peptidoglycan is destroyed, for example, under the action of lysozyme, the cell wall loses its rigidity and collapses. The content of the cell (cytoplasm), together with the cytoplasmic membrane, takes on a spherical shape, that is, it becomes a protoplast (spheroplast).
Many synthesizing and destructive enzymes are associated with the cell wall. Cell wall components are synthesized in the cytoplasmic membrane and then transported to the cell wall.
Cytoplasmic membrane is located under the cell wall and fits snugly to its inner surface. It is a semi-permeable membrane that surrounds the cytoplasm and the inner contents of the protoplast cell. The cytoplasmic membrane is the thickened outer layer of the cytoplasm.
The cytoplasmic membrane is the main barrier between the cytoplasm and the environment, violation of its integrity leads to cell death. It contains proteins (50-75%), lipids (15-45%), in many species - carbohydrates (1-19%).
The main lipid component of the membrane is phospho- and glycolipids.
The cytoplasmic membrane, with the help of enzymes localized in it, performs various functions: synthesizes membrane lipids - components of the cell wall; membrane enzymes - selectively transfer various organic and inorganic molecules and ions through the membrane, the membrane participates in the transformation of cellular energy, as well as in the replication of chromosomes, in the transfer of electrochemical energy and electrons.
Thus, the cytoplasmic membrane provides selective entry into the cell and removal from it of various substances and ions.
Derivatives of the cytoplasmic membrane are mesosomes . These are spherical structures formed when the membrane is twisted into a curl. They are located on both sides - at the site of the formation of the cell septum or next to the zone of localization of nuclear DNA.
Mesosomes are functionally equivalent to the mitochondria of cells of higher organisms. They are involved in the redox reactions of bacteria, play an important role in the synthesis of organic substances, in the formation of the cell wall.
Capsule is a derivative of the outer layer of cell clumps and is a mucous membrane that surrounds one or more microbial cells. Its thickness can reach 10 microns, which is many times greater than the thickness of the bacteria itself.
The capsule has a protective function. The chemical composition of the capsule of bacteria is different. In most cases, it consists of complex polysaccharides, mucopolysaccharides, and sometimes polypeptides.
Capsule formation is usually a specific feature. However, the appearance of the microcapsule often depends on the culture conditions of the bacteria.
Cytoplasm- a complex colloidal system with a large amount of water (80-85%), in which proteins, carbohydrates, lipids, as well as mineral compounds and other substances are dispersed.
The cytoplasm is the content of a cell surrounded by a cytoplasmic membrane. It is divided into two functional parts.
One part of the cytoplasm is in the state of a sol (solution), has a homogeneous structure and contains a set of soluble ribonucleic acids, enzyme proteins and metabolic products.
The other part is represented by ribosomes, inclusions of various chemical nature, genetic apparatus, and other intracytoplasmic structures.
Ribosomes Are submicroscopic granules, which are spherical nucleoprotein particles with a diameter of 10 to 20 nm, a molecular weight of about 2-4 million.
Ribosomes of prokaryotes consist of 60% RNA (ribonucleic acid) located in the center, and 40 % the protein that covers the outside of the nucleic acid.
Cytoplasmic inclusions are metabolic products, as well as reserve products, due to which the cell lives in conditions of a lack of nutrients.
The genetic material of prokaryotes consists of a double strand of deoxyribonucleic acid (DNA) of a compact structure located in the central part of the cytoplasm and not separated from it by a membrane. The structure of the DNA of bacteria does not differ from the DNA of eukaryotes, but since it is not separated from the cytoplasm by a membrane, the genetic material is called nucleoid or genophore... Nuclear structures are spherical or horseshoe-shaped.
Controversy bacteria are dormant, not multiplying their form. They are formed inside the cell, they are round or oval-shaped formations. Spores form mainly gram-positive bacteria, rod-shaped with aerobic and anaerobic respiration in old cultures, as well as in unfavorable environmental conditions (lack of nutrients and moisture, accumulation of metabolic products in the environment, changes in pH and temperature of cultivation, presence or absence of atmospheric oxygen, etc. etc.) can switch to an alternative development program, resulting in disputes. In this case, one spore is formed in the cell. This indicates that sporulation in bacteria is an adaptation for the preservation of a species (individual) and is not a way of their reproduction. The process of sporulation occurs, as a rule, in the external environment for 18-24 hours.
A mature spore is approximately 0.1 of the maternal cell volume. Spores in different bacteria differ in shape, size, and location in the cell.
Microorganisms in which the diameter of the spore does not exceed the width of the vegetative cell are called bacilli, bacteria that have spores, the diameter of which is 1.5-2 times larger than the cell diameter, are called clostridia.
Inside the microbial cell, the spore can be located in the middle - the central position, at the end - the terminal position and between the center and the end of the cell - the subterminal position.
Flagella bacteria are locomotor organs (organs of movement), with the help of which bacteria can move at a speed of up to 50-60 microns / s. At the same time, in 1 s, bacteria overlap their body length by 50-100 times. The length of the flagella exceeds the length of bacteria by 5-6 times. The thickness of the flagella is on average 12-30 nm.
The number of flagella, their size and location are constant for certain types of prokaryotes and therefore are taken into account when identifying them.
Depending on the number and location of flagella, bacteria are divided into monotrichs (monopolar monotrichs) - cells with one flagellum at one end, lophotrichs (monopolar polytriches) - a bundle of flagella is located at one of the ends, amphitrichs (bipolar polytrichs) - flagella are located on each of poles, peritrichous - flagella are located over the entire surface of the cell (Fig. 4) and atrichs - bacteria devoid of flagella.
The nature of the movement of bacteria depends on the number of flagella, age, culture characteristics, temperature, the presence of various chemicals and other factors. Monotrichs have the greatest mobility.
Flagella are more often found in rod-shaped bacteria; they are not vital cell structures, since there are flagella-free variants of motile bacterial species.
Sizes - from 1 to 15 microns. Basic forms: 1) cocci (spherical), 2) bacilli (rod-shaped), 3) vibrios (bent in the form of a comma), 4) spirillae and spirochetes (spirally twisted).
Forms of bacteria:
1 - cocci; 2 - bacilli; 3 - vibrios; 4-7 - spirillae and spirochetes.
Bacterial cell structure:
1 - cytoplasmic membrane; 2 - cell wall; 3 - mucous capsule; 4 - cytoplasm; 5 - chromosomal DNA; 6 - ribosomes; 7 - mesosome; 8 - photosynthetic membranes; 9 - inclusions; 10 - flagella; 11 - drank.
The bacterial cell is limited by a membrane. The inner layer of the envelope is represented by the cytoplasmic membrane (1), above which is the cell wall (2); over the cell wall in many bacteria there is a mucous capsule (3). The structure and functions of the cytoplasmic membrane of eukaryotic and prokaryotic cells do not differ. The membrane can form folds called mesosomes(7). They can have different shapes (bag-shaped, tubular, lamellar, etc.).
Enzymes are located on the surface of mesosomes. The cell wall is thick, dense, rigid, consists of mureina(main component) and other organic substances. Murein is a regular network of parallel polysaccharide chains stitched together by short protein chains. Depending on the structural features of the cell wall, bacteria are divided into gram-positive(colored by Gram) and gram negative(not stained). In gram-negative bacteria, the wall is thinner, more complex, and there is a layer of lipids above the murein layer on the outside. The inner space is filled with cytoplasm (4).
The genetic material is represented by circular DNA molecules. These DNAs can be roughly divided into "chromosomal" and plasmid ones. "Chromosomal" DNA (5) - one, attached to the membrane, contains several thousand genes, unlike the chromosomal DNA of eukaryotes, it is not linear, not associated with proteins. The zone in which this DNA is located is called nucleoid. Plasmids- extrachromosomal genetic elements. They are small circular DNAs that are not bound to proteins, are not attached to the membrane, and contain a small number of genes. The number of plasmids can vary. The most studied plasmids carrying information about drug resistance (R-factor), participating in the sexual process (F-factor). A plasmid capable of uniting with a chromosome is called episome.
The bacterial cell lacks all membrane organelles characteristic of a eukaryotic cell (mitochondria, plastids, EPS, Golgi apparatus, lysosomes).
The cytoplasm of bacteria contains 70S-type ribosomes (6) and inclusions (9). Typically, ribosomes are assembled into polysomes. Each ribosome consists of small (30S) and large (50S) subunits. Function of ribosomes: assembly of the polypeptide chain. Inclusions can be represented by lumps of starch, glycogen, volutin, lipid drops.
Many bacteria have flagella(10) and drank (fimbria)(eleven). The flagella are not limited by a membrane, have a wavy shape, and are composed of spherical flagellin protein subunits. These subunits are arranged in a spiral and form a hollow cylinder 10–20 nm in diameter. The structure of the prokaryotic flagellum resembles one of the microtubules of the eukaryotic flagellum. The number and location of flagella may vary. Drank - straight threadlike structures on the surface of bacteria. They are thinner and shorter than flagella. They are short hollow cylinders made of pilin protein. Saws serve to attach bacteria to the substrate and to each other. During conjugation, special F-pili are formed, through which genetic material is transferred from one bacterial cell to another.
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Spore formation in bacteria, it is a way of experiencing unfavorable conditions. Spores are usually formed one at a time within the "mother cell" and are called endospores. Spores are highly resistant to radiation, extreme temperatures, drying and other factors that cause the death of vegetative cells.
Reproduction. Bacteria reproduce asexually - by dividing the "mother cell" in two. DNA replication takes place before division.
Rarely in bacteria there is a sexual process in which the recombination of genetic material occurs. It should be emphasized that gametes are never formed in bacteria, the cell contents do not fusion, but DNA is transferred from the donor cell to the recipient cell. There are three ways of transferring DNA: conjugation, transformation, transduction.
Conjugation- unidirectional transfer of the F-plasmid from the donor cell to the recipient cell in contact with each other. In this case, the bacteria are connected to each other by special F-pilas (F-fimbriae), through the channels of which DNA fragments are transferred. Conjugation can be divided into the following stages: 1) untwisting of the F-plasmid, 2) penetration of one of the F-plasmid strands into the recipient cell through the F-pill, 3) synthesis of the complementary strand on the single-stranded DNA template (occurs as in the donor cell (F +) and in the recipient cell (F -)).
Transformation- unidirectional transfer of DNA fragments from a donor cell to a recipient cell that are not in contact with each other. In this case, the donor cell either "secretes" a small fragment of DNA from itself, or the DNA enters the environment after the death of this cell. In any case, DNA is actively absorbed by the recipient cell and is incorporated into its own “chromosome”.
Transduction- transfer of a DNA fragment from a donor cell to a recipient cell using bacteriophages.
Viruses
Viruses are composed of nucleic acid (DNA or RNA) and proteins that form an envelope around this nucleic acid, i.e. represent a nucleoprotein complex. Some viruses contain lipids and carbohydrates. Viruses always contain one type of nucleic acid - either DNA or RNA. Moreover, each of the nucleic acids can be both single-stranded and double-stranded, both linear and circular.
The size of the viruses is 10–300 nm. The form of viruses: spherical, rod-shaped, filamentous, cylindrical, etc.
Capsid- the envelope of the virus, formed by protein subunits, folded in a certain way. The capsid protects the nucleic acid of the virus from various influences, ensures the deposition of the virus on the surface of the host cell. Supercapsid typical for complex viruses (HIV, influenza viruses, herpes). It occurs during the release of the virus from the host cell and is a modified section of the nuclear or outer cytoplasmic membrane of the host cell.
If the virus is inside the host cell, then it exists in the form of a nucleic acid. If the virus is outside the host cell, then it is a nucleoprotein complex, and this free form of existence is called virion... Viruses are highly specific, i.e. they can use a strictly defined circle of owners for their life.
The structure and chemical composition of the bacterial
cells
The general scheme of the structure of a bacterial cell is shown in Figure 2. The internal organization of a bacterial cell is complex. Each systematic group of microorganisms has its own specific structural features.
Cell wall. The bacterial cell is covered with a dense membrane. This surface layer, located outside the cytoplasmic membrane, is called the cell wall (Fig. 2, 14). The wall performs a protective and supporting function, and also gives the cell a constant, characteristic shape (for example, the shape of a rod or coccus) and represents the outer skeleton of the cell. This dense shell makes bacteria related to plant cells, which distinguishes them from animal cells that have soft shells.
Inside a bacterial cell, the osmotic pressure is several times, and sometimes tens of times, higher than in the external environment. Therefore, the cell would quickly rupture if it were not protected by such a dense, rigid structure as a cell wall.
The thickness of the cell wall is 0.01-0.04 microns. It accounts for 10 to 50% of the dry mass of bacteria. The amount of material that makes up the cell wall changes during bacterial growth and usually increases with age.
The main structural component of the walls, the basis of their rigid structure in almost all bacteria studied to date is murein (glycopeptide,
mucopeptide). This is an organic compound of a complex structure, which includes nitrogen-bearing sugars - amino sugar and 4-5 amino acids. Moreover, the amino acids of the cell walls have an unusual shape (D-stereoisomers), which is rarely found in nature.
The constituent parts of the cell wall, its components, form a complex strong structure (Fig. 3, 4 and 5).
With the help of the staining method first proposed in 1884 by Christian Gram, bacteria can be divided into two groups: gram-positive and
gram negative... Gram-positive organisms are able to bind some aniline dyes, such as crystal violet, and after treatment with iodine and then alcohol (or acetone) retain the iodine-dye complex. The same bacteria in which, under the influence of ethyl alcohol, this complex is destroyed (cells become discolored), are gram-negative.
The chemical composition of the cell walls of gram-positive and gram-negative bacteria is different.
In gram-positive bacteria, the cell walls include, in addition to mucopeptides, polysaccharides (complex, high molecular weight sugars), teichoic acids
(compounds complex in composition and structure, consisting of sugars, alcohols, amino acids and phosphoric acid). Polysaccharides and teichoic acids are associated with the wall framework - murein. What structure these constituent parts of the cell wall of gram-positive bacteria form, we do not yet know. With the help of electronic photographs, thin sections (layering) in the walls of gram-positive bacteria were not detected.
Probably, all these substances are very closely related to each other.
The walls of gram-negative bacteria are more complex in chemical composition, they contain a significant amount of lipids (fats) associated with proteins and sugars in complex complexes - lipoproteins and lipopolysaccharides. In general, there is less murein in the cell walls of gram-negative bacteria than in gram-positive bacteria.
The wall structure of gram-negative bacteria is also more complex. Using an electron microscope, it was found that the walls of these bacteria are multilayer (Fig.
6).
The inner layer consists of murein. Above it is a wider layer of loosely packed protein molecules. This layer is in turn covered with a lipopolysaccharide layer. The topmost layer consists of lipoproteins.
The cell wall is permeable: through it, nutrients freely pass into the cell, and metabolic products are released into the environment. Large, high molecular weight molecules do not pass through the shell.
Capsule. The cell wall of many bacteria from above is surrounded by a layer of mucous material - a capsule (Fig. 7). The thickness of the capsule can be many times the diameter of the cell itself, and sometimes it is so thin that it can only be seen through an electron microscope - a microcapsule.
The capsule is not an obligatory part of the cell, it is formed depending on the conditions in which the bacteria enter. It serves as a protective cover for the cell and participates in water exchange, protecting the cell from drying out.
In terms of chemical composition, capsules are most often polysaccharides.
Sometimes they consist of glycoproteins (complex complexes of sugars and proteins) and polypeptides (genus Bacillus), in rare cases - of fiber (genus Acetobacter).
The mucous substances released into the substrate by some bacteria cause, for example, the slimy-viscous consistency of spoiled milk and beer.
Cytoplasm. The entire contents of a cell, with the exception of the nucleus and the cell wall, are called cytoplasm. The liquid, structureless phase of the cytoplasm (matrix) contains ribosomes, membrane systems, mitochondria, plastids and other structures, as well as reserve nutrients. The cytoplasm has an extremely complex, fine structure (layered, granular). Many interesting details of the cell structure have been revealed with the help of an electron microscope.
The outer lipoprotein layer of the bacterial protoplast, which has special physical and chemical properties, is called the cytoplasmic membrane (Fig.
2, 15).
All vital structures and organelles are located inside the cytoplasm.
The cytoplasmic membrane plays a very important role - it regulates the entry of substances into the cell and the release of metabolic products outside.
Through the membrane, nutrients can enter the cell as a result of an active biochemical process with the participation of enzymes. In addition, the synthesis of some constituent parts of the cell occurs in the membrane, mainly the components of the cell wall and capsule.
Finally, the cytoplasmic membrane contains the most important enzymes (biological catalysts). The ordered arrangement of enzymes on membranes allows you to regulate their activity and prevent the destruction of some enzymes by others. Ribosomes are associated with the membrane - structural particles on which protein is synthesized.
The membrane consists of lipoproteins. It is strong enough and can provide the temporary existence of a cell without a shell. The cytoplasmic membrane accounts for up to 20% of the dry mass of the cell.
In electronic photographs of thin sections of bacteria, the cytoplasmic membrane appears as a continuous strand about 75A thick, consisting of a light layer
(lipids), enclosed between two darker (proteins). Each layer has a width
20-30A. Such a membrane is called elementary (Table 30, Fig. 8).
There is a connection between the plasma membrane and the cell wall in the form of desmoses.
- bridges. The cytoplasmic membrane often gives invagination - invagination into the cell. These invaginations form special membrane structures in the cytoplasm, called
mesosomes. Some types of mesosomes are bodies separated from the cytoplasm by their own membrane. Numerous vesicles and tubules are packed inside such membrane sacs (Fig. 2). These structures perform a wide variety of functions in bacteria. Some of these structures are mitochondrial analogs. Others perform the functions of the endoplasmic reticulum or the Golgi apparatus. The photosynthetic apparatus of bacteria is also formed by invagination of the cytoplasmic membrane.
After invagination of the cytoplasm, the membrane continues to grow and forms stacks (Table 30), which, by analogy with plant chloroplast granules, are called thylakoid stacks. In these membranes, which often fill most of the cytoplasm of a bacterial cell, pigments (bacteriochlorophyll, carotenoids) and enzymes are localized
(cytochromes) that carry out the process of photosynthesis.
,
The cytoplasm of bacteria contains ribosomes - protein-synthesizing particles with a diameter of 200A. There are more than a thousand of them in a cell. Ribosomes are made up of RNA and protein. In bacteria, many ribosomes are located freely in the cytoplasm, some of them may be associated with membranes.
Ribosomes are the centers of protein synthesis in the cell. Moreover, they often join together, forming aggregates called polyribosomes or polysomes.
The cytoplasm of bacterial cells often contains granules of various shapes and sizes.
However, their presence cannot be considered as some kind of permanent sign of a microorganism, usually it is largely associated with the physical and chemical conditions of the environment. Many cytoplasmic inclusions are composed of compounds that serve as a source of energy and carbon. These storage substances are formed when the body is supplied with sufficient nutrients, and, conversely, are used when the body is exposed to conditions that are less favorable in terms of nutrition.
In many bacteria, granules are composed of starch or other polysaccharides - glycogen and granulose. In some bacteria, when grown in a medium rich in sugars, droplets of fat are found inside the cell. Another widespread type of granular inclusions is volutin (metachromatin granules). These granules are composed of polymetaphosphate (a storage substance containing phosphoric acid residues).
Polymetaphosphate serves as a source of phosphate groups and energy for the body. Bacteria are more likely to accumulate volutin in unusual feeding conditions, such as in a sulfur-free environment. Sulfur droplets are found in the cytoplasm of some sulfur bacteria.
In addition to various structural components, the cytoplasm consists of a liquid part - a soluble fraction. It contains proteins, various enzymes, t-RNA, some pigments and low molecular weight compounds - sugars, amino acids.
As a result of the presence of low molecular weight compounds in the cytoplasm, a difference arises in the osmotic pressure of the cell contents and the external environment, and this pressure may be different for different microorganisms. The highest osmotic pressure was noted in gram-positive bacteria - 30 atm, in gram-negative bacteria it is much lower - 4-8 atm.
Nuclear apparatus. In the central part of the cell, a nuclear substance is localized - deoxyribonucleic acid a (DNA).
,
Bacteria do not have such a nucleus as higher organisms (eukaryotes), but have its analogue -
"Nuclear equivalent" - nucleoid(see Fig. 2, 8), which is an evolutionarily more primitive form of organization of nuclear matter. Microorganisms that do not have a real nucleus, but have its analogue, are classified as prokaryotes. All bacteria are prokaryotes. In the cells of most bacteria, most of the DNA is concentrated in one or more places. In eukaryotic cells, DNA is located in a specific structure - the nucleus. The kernel is surrounded by a shell- membrane.
In bacteria, DNA is less densely packed than in true nuclei; a nucleoid does not have a membrane, a nucleolus and a set of chromosomes. Bacterial DNA is not bound to the main proteins - histones - and is located in the nucleoid in the form of a bundle of fibrils.
Flagella. There are additional structures on the surface of some bacteria; the most widespread of them are flagella - the organs of movement of bacteria.
The flagellum is anchored under the cytoplasmic membrane using two pairs of discs.
Bacteria can have one, two, or many flagella. Their location is different: at one end of the cell, at two, over the entire surface, etc. (Fig. 9). Bacterial flagella have a diameter
0.01-0.03 microns, their length can be many times longer than the cell length. Bacterial flagella They consist of a protein - flagellin - and are twisted helical filaments.
There are thin villi on the surface of some bacterial cells -
fimbria.
Plant life: in 6 volumes. - M .: Education. Edited by A. L. Takhtadzhyan, Chief
editor Corresponding Member USSR Academy of Sciences, prof. A.A. Fedorov. 1974
- The structure and chemical composition of the bacterial cell
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CYTOPLASM (CPU)
Participate in sporulation.
Mesosomes
With excessive growth, in comparison with the growth of the CJ, the CPM forms invaginates (invaginations) - mesosomes. Mesosomes are the center of energy metabolism of the prokaryotic cell. Mesosomes are analogous to eukaryotic mitochondria, but they are simpler.
Well-developed and complexly organized mesosomes are characteristic of Gram + bacteria.
Bacterial cell wall
In Gram bacteria, mesosomes are less common and are simply organized (in the form of a loop). Mesosome polymorphism is observed even in the same species of bacteria. Rickettsiae have no mesosomes.
Mesosomes vary in size, shape, and location in the cell.
Mesosomes are distinguished by shape:
- - lamellar (lamellar),
- - vesicular (bubble-shaped),
- - tubular (tubular),
- - mixed.
By location in the cell, mesosomes are distinguished:
- - formed in the area of cell division and the formation of a transverse septum,
- - to which the nucleoid is attached;
- - formed as a result of invagination of the peripheral parts of the CPM.
Functions of mesosomes:
1. Strengthen the energy metabolism of cells, as they increase the total "working" surface of the membranes.
2. Participate in secretory processes(in some Gram + bacteria).
3. Participate in cell division. During reproduction, the nucleoid moves to the mesosome, receives energy, doubles and divides by amitosis.
Identification of mesosomes:
1. Electron microscopy.
Structure. Cytoplasm (protoplasm) is the content of the cell, surrounded by the CPM and occupying the bulk of the bacterial cell. CP is the internal environment of the cell and is a complex colloidal system consisting of water (about 75%) and various organic compounds (proteins, RNA and DNA, lipids, carbohydrates, minerals).
The layer of protoplasm located under the CPM is denser than the rest of the mass in the center of the cell. The fraction of the cytoplasm, which has a homogeneous consistency and contains a set of soluble RNA, enzyme proteins, products and substrates of metabolic reactions, is called cytosol. Another part of the cytoplasm is represented by various structural elements: nucleoid, plasmids, ribosomes and inclusions.
Cytoplasmic functions:
1. Contains cellular organelles.
Detection of cytoplasm:
1. Electron microscopy.
Structure. Nucleoid - the equivalent of the eukaryotic nucleus, although it differs from it in its structure and chemical composition. The nucleoid is not separated from the CP by the nuclear membrane, does not have nucleoli and histones, contains one chromosome, has a haploid (single) set of genes, and is not capable of mitotic division.
The nucleoid is located in the center of the bacterial cell and contains a double-stranded DNA molecule, a small amount of RNA and proteins. In most bacteria, a double-stranded DNA molecule with a diameter of about 2 nm, a length of about 1 m with a molecular weight of 1-3x109 Da is closed in a ring and tightly packed like a ball. Mycoplasmas have the lowest DNA molecular weight for cellular organisms (0.4–0.8 × 109 Da).
DNA in prokaryotes is structured in the same way as in eukaryotes (Fig. 25).
Rice. 25. DNA structure of prokaryotes:
BUT- a fragment of a DNA strand formed by alternating residues of deoxyribose and phosphoric acid. A nitrogenous base is attached to the first carbon atom of deoxyribose: 1 - cytosine; 2 - guanine.
B- DNA double helix: D- deoxyribose; F - phosphate; A - adenine; T is thymine; G - guanine; C - cytosine
The DNA molecule carries many negative charges, since each phosphate residue contains an ionized hydroxyl group. In eukaryotes, negative charges are neutralized by the formation of a complex of DNA with the main proteins - histones. There are no histones in prokaryotic cells, so the neutralization of charges is carried out by the interaction of DNA with polyamines and Mg2 + ions.
By analogy with eukaryotic chromosomes, bacterial DNA is often referred to as a chromosome. It is presented in the cell in the singular, since bacteria are haploid. However, before cell division, the number of nucleoids doubles, and during division it increases to 4 or more. Therefore, the terms "nucleoid" and "chromosome" do not always coincide. When certain factors (temperature, pH of the environment, ionizing radiation, salts of heavy metals, some antibiotics, etc.) act on cells, many copies of the chromosome are formed. When the influence of these factors is eliminated, as well as after the transition to the stationary phase, one copy of the chromosome is found in the cells.
For a long time, it was believed that there was no regularity in the distribution of DNA strands of the bacterial chromosome. Special studies have shown that the chromosomes of prokaryotes are a highly ordered structure. Part of the DNA in this structure is represented by a system of 20–100 independently supercoiled loops. Supercoiled loops correspond to DNA regions inactive at a given time and are located in the center of the nucleoid. Along the periphery of the nucleoid, there are despiralized areas on which messenger RNA (mRNA) is synthesized. Since the processes of transcription and translation in bacteria proceed simultaneously, the same mRNA molecule can be simultaneously associated with DNA and ribosomes.
In addition to the nucleoid, the cytoplasm of the bacterial cell can contain plasmids - factors of extrachromosomal heredity in the form of additional autonomous circular double-stranded DNA molecules with a lower molecular weight. In plasmids, hereditary information is also encoded, but it is not vital for a bacterial cell.
Nucleode functions:
1. Storage and transmission of hereditary information, including the synthesis of pathogenicity factors.
Nucleoid detection:
1. Electron microscopy: on electron diffraction patterns of ultrathin sections, the nucleoid looks like light zones of lower optical density with fibrillar, filamentous DNA structures (Fig. 26). Despite the absence of a nuclear membrane, the nucleoid is quite clearly delimited from the cytoplasm.
2. Phase contrast microscopy of native preparations.
3. Light microscopy after staining with DNA-specific methods according to Fehlgen, according to Pashkov or according to Romanovsky-Giemsa:
- the drug is fixed with methyl alcohol;
- the Romanovsky-Giemsa dye is poured onto the fixed preparation (a mixture of equal parts of three paints - azure, eosin and methylene blue, dissolved in methanol) for 24 hours;
- the paint is drained, the preparation is washed with distilled water, dried and microscoped: the nucleoid turns purple and is diffusely located in the cytoplasm, painted in a pale pink color.
Read also:
Features of the chemical composition of bacterial cells
The structure of a bacterial cell. The main differences between prokaryotes and eukaryotes. Functions of individual structural elements of a bacterial cell. Features of the chemical composition of the cell walls of gram-positive and gram-negative bacteria.
A bacterial cell consists of a cell wall, a cytoplasmic membrane, a cytoplasm with inclusions, and a nucleus called a nucleoid. There are additional structures: capsule, microcapsule, mucus, flagella, drank. Some bacteria can form spores under unfavorable conditions.
Differences in cell structure
1) Prokaryotes do not have a nucleus, but eukaryotes do.
2) Prokaryotes from organelles have only ribosomes (small, 70S), while eukaryotes, in addition to ribosomes (large, 80S), have many other organelles: mitochondria, EPS, cell center, etc.
3) A prokaryotic cell is much smaller than a eukaryotic cell: 10 times in diameter, 1000 times in volume.
1) In prokaryotes, DNA is circular, while in eukaryotes it is linear
2) In prokaryotes, DNA is naked, almost not connected to proteins, and in eukaryotes, DNA is connected to proteins in a 50/50 ratio, a chromosome is formed
3) In prokaryotes, DNA lies in a special area of the cytoplasm, which is called the nucleoid, and in eukaryotes, DNA lies in the nucleus.
Constant components of a bacterial cell.
Nucleoid - equivalent to the nucleus of prokaryotes
The cell wall is different in Gr + and Gr - bacteria. Determines and maintains a constant form, provides communication with the external environment, determines the antigenic specificity of bacteria, has important immunospecific properties; violation of cell wall synthesis leads to the formation of L-forms of bacteria.
Gr +: this color is associated with the content of teichoic and dipoteichoic acids in the COP, which penetrate it through and through and fix it in the cytoplasm. Peptidoglycan is thick, consists of a plasma membrane linked by beta-glycosidic bonds.
Gr -: a thin layer of peptidoglycans, the pectoral membrane is represented by lipopolysaccharide glycocoproteins, glycolipids.
CPM - consists of lipoproteins. It perceives all the chemical information entering the cell. Is the main barrier. Participates in the process of replication of nucleoid and plasmids; contains a large amount of enzymes; Participates in the synthesis of cell wall components.
Mesosomes - analogs of mitochondria in a bacterial cell
Ribosomes 70S are numerous small granules located in the cytoplasm.
UNCONTINUED:
Flagella: composed of the flagellin protein, originating from the CPM, the main function is motor.
They drank: due to them there is an attachment to the host cell
Plasmids. Capsule, Disputes, Inclusions.
Main article: Supramembrane complex
The supramembrane apparatus of bacteria is represented by the cell wall, the specificity of the organization of which serves as the basis for their subdivision into two non-taxonomic groups (gram-positive and gram-negative forms) and correlates with a very large number of morphofunctional, metabolic and genetic characteristics. The cell wall of prokaryotes is essentially a polyfunctional organoid, removed from the protoplast and carrying a significant proportion of the metabolic load of the cell.
Cell wall of gram-positive bacteria
Cell wall structure
In gram-positive bacteria (Fig. 12, A), the cell wall is generally simpler. The outer layers of the cell wall are formed by protein in a complex with lipids. In some bacterial species, a layer of surface protein globules was relatively recently discovered, the shape, size and nature of the arrangement of which are specific for the species. Inside the cell wall, as well as directly on its surface, enzymes are placed that break down substrates to low molecular weight components, which are subsequently transported through the cytoplasmic membrane into the cell. It also contains enzymes that synthesize extracellular polymers, such as capsular polysaccharides.
Polysaccharide capsule
The polysaccharide capsule, which envelops the cell wall of a number of bacteria from the outside, is mainly of private adaptive value, and its presence is not necessary to preserve the vital activity of the cell. Thus, it ensures the attachment of cells to the surface of dense substrates, accumulates some minerals and prevents their phagocytosis in pathogenic forms.
Murein
A hard murein layer is adjacent to the cytoplasmic membrane.
Murein, or peptidoglycan, is a cross-linked copolymer of acetylglucosamine and acetylmuramic acid. It is not excluded that the murein layer is one giant bag molecule that provides the rigidity of the cell wall and its individual shape.
Teichoic acids
The second polymer of the wall of gram-positive bacteria, teichoic acids, is in close contact with the murein layer. They are credited with the role of a cation accumulator and a regulator of ion exchange between the cell and the environment.
Cell wall of gram-negative bacteria
Cell wall structure
Compared to gram-positive forms, the cell wall of gram-negative bacteria is more complex and its physiological significance is incomparably wider. In addition to the murein layer, a second protein-lipid membrane is located closer to the surface (Fig. 12, B, C), which includes lipopolysaccharides. It is covalently linked to murein by cross-links from lipoprotein molecules. The main function of this membrane is the role of a molecular sieve, in addition, enzymes are located on its outer and inner surfaces.
3. The structure of the bacterial cell.
The space limited by the outer and cytoplasmic membranes is called periplasmic and is a unique feature of gram-negative bacteria. A whole set of enzymes is localized in its volume - phosphatases, hydrolases, nucleases, etc. They break down relatively high-molecular nutrient substrates, and also destroy their own cellular material released into the environment from the cytoplasm. To a certain extent, the periplasmic space can be likened to the lysosome of eukaryotes. In the periplasm zone, it is possible not only the most efficient course of enzymatic reactions, but also the isolation from the cytoplasm of compounds that pose a threat to its normal functioning. Material from the site http://wiki-med.com
Bacterial cell wall functions
In both gram-positive and gram-negative forms, the cell wall plays the role of a molecular sieve, selectively carrying out passive transport of ions, substrates and metabolites. In bacteria that have the ability to actively move due to flagella, the cell wall is a component of the locomotor mechanism. Finally, individual sections of the cell wall are closely associated with the cytoplasmic membrane in the nucleoid attachment zone and play an important role in its replication and segregation.
In one of the bacterial species, the process of destruction of the old cell membrane, which occurs during cell division, is provided by the work of at least four systems of hydrolytic enzymes present in the cell wall in a latent state. During cell division, a regular and strictly sequential in time activation of these systems is carried out, leading to the gradual destruction and sloughing of the old ("mother") membrane of the bacterial cell.
Material from the site http://Wiki-Med.com
On this page material on topics:
.the main component of the cell wall of gram-positive bacteria is
bacterial cell wall function
features of the structure of the bacterial cell wall
cell wall structure
characterization of the bacterial cell wall
The cell wall of gram-positive bacteria contains a small amount of polysaccharides, lipids, proteins. The main component of the cell wall of these bacteria is a multilayer peptidoglycan (murein, mucopeptide), which makes up 40-90% of the cell wall mass. Teichoic acids (from the Greek teichos - wall) are covalently bound to the peptidoglycan of the cell wall of gram-positive bacteria.
The cell wall of gram-negative bacteria includes an outer membrane bound by lipoprotein to the underlying layer of peptidoglycan. On ultrathin sections of bacteria, the outer membrane looks like a wavy three-layer structure, similar to the inner membrane, which is called the cytoplasmic membrane. The main component of these membranes is a bimolecular (double) lipid layer. The inner layer of the outer membrane is represented by phospholipids, and the outer layer is lipopolysaccharide (LPS). The lipopolysaccharide of the outer membrane consists of three fragments: lipid A - a conservative structure, practically the same in gram-negative bacteria; the core, or core, of the crustal part (Latin core - core), relatively conservative oligosaccharide structure (the most constant part of the LPS core is ketodeoxyoctonic acid); highly variable O-specific polysaccharide chain formed by repeating identical oligosaccharide sequences (O-antigen). The matrix proteins of the outer membrane permeate it in such a way that protein molecules called porins border the hydrophilic pores through which water and small hydrophilic molecules pass.
In case of violation of the synthesis of the cell wall of bacteria under the influence of lysozyme,
penicillin, the body's protective factors, cells with an altered (often spherical) shape are formed: protoplasts - bacteria completely devoid of a cell wall; spheroplasts are bacteria with a partially preserved cell wall. Bacteria of the sphero- or protoplast type that have lost the ability to synthesize peptidoglycan under the influence of antibiotics or other factors and are able to multiply are called L-forms.
They are osmotically sensitive, spherical, flask-shaped cells of various sizes, including those passing through bacterial filters. Some L-forms (unstable), upon removal of the factor that led to changes in bacteria, can reverse, "returning" to the original bacterial cell.
Between the outer and cytoplasmic membranes is the periplasmic space, or periplasm, containing enzymes (proteases, lipases, phosphatases, nucleases, beta-lactomases) and components of transport systems.
The cytoplasmic membrane in electron microscopy of ultrathin sections is a three-layer membrane (2 dark layers 2.5 nm thick are separated by a light - intermediate). In structure, it is similar to the plasmalemma of animal cells and consists of a double layer of phospholipids with embedded surface, as well as integral proteins, as if penetrating through the membrane structure. With excessive growth (in comparison with the growth of the cell wall), the cytoplasmic membrane forms invaginates - invaginations in the form of intricately twisted membrane structures called mesosomes. Less intricately twisted structures are called intracytoplasmic membranes.
Cytoplasm
The cytoplasm consists of soluble proteins, ribonucleic acids, inclusions and numerous small granules - ribosomes, responsible for the synthesis (translation) of proteins. Bacterial ribosomes have a size of about 20 nm and a sedimentation coefficient of 70S, in contrast to 80S ribosomes characteristic of eukaryotic cells. Ribosomal RNA (rRNA) are conservative elements of bacteria ("molecular clock" of evolution). 16S rRNA is part of the small subunit of ribosomes, and 23S rRNA is part of the large subunit of ribosomes. The study of 16S rRNA is the basis of genosystematics, making it possible to assess the degree of kinship of organisms.
The cytoplasm contains various inclusions in the form of granules of glycogen, polysaccharides, beta-hydroxybutyric acid and polyphosphates (volutin).
Cell wall
They are reserve substances for the nutrition and energy needs of bacteria. Volutin has an affinity for basic dyes and is easily detected using special staining methods (for example, according to Neisser) in the form of metachromatic granules. The characteristic arrangement of volutin granules is revealed in the diphtheria bacillus in the form of intensely staining cell poles.
Nucleoid
A nucleoid is the equivalent of a nucleus in bacteria. It is located in the central zone of bacteria in the form of double-stranded DNA, closed in a ring and tightly packed like a ball. The nucleus of bacteria, unlike eukaryotes, does not have a nuclear envelope, nucleolus and basic proteins (histones). Usually, a bacterial cell contains one chromosome, represented by a DNA molecule closed in a ring.
In addition to the nucleoid, represented by one chromosome, in the bacterial cell there are extrachromosomal factors of heredity - plasmids, which are covalently closed DNA rings.
Capsule, microcapsule, mucus
The capsule is a mucous structure with a thickness of more than 0.2 microns, firmly associated with the cell wall of bacteria and having clearly defined external boundaries. The capsule is distinguishable in smears-prints from the pathological material. In pure cultures of bacteria, the capsule is formed less frequently. It is detected with special methods of staining the smear (for example, according to Burri-Gins), which create a negative contrasting of the capsule substances: ink creates a dark background around the capsule. The capsule consists of polysaccharides (exopolysaccharides), sometimes of polypeptides, for example, in the anthrax bacillus, it consists of polymers of D-glutamic acid. The capsule is hydrophilic, prevents bacterial phagocytosis. Antigenic capsule: antibodies against the capsule cause the capsule to enlarge (capsule swelling reaction).
Many bacteria form a microcapsule - a mucous formation with a thickness of less than 0.2 microns, detected only by electron microscopy. From the capsule should be distinguished slie - mucoid exopolysaccharides that do not have clear boundaries. The mucus is water soluble.
Bacterial exopolysaccharides are involved in adhesion (adhesion to substrates), they are also called glycocalyx. Besides synthesis
exopolysaccharides by bacteria, there is another mechanism of their formation: by the action of extracellular enzymes of bacteria on disaccharides. As a result, dextrans and levans are formed.
Flagella
Bacterial flagella determine the motility of the bacterial cell. Flagella are thin filaments originating from the cytoplasmic membrane and are longer than the cell itself. Flagella thickness 12-20 nm, length 3-15 microns. They consist of 3 parts: a spiral filament, a hook and a basal corpuscle containing a rod with special discs (1 pair of discs for gram-positive bacteria and 2 pairs of discs for gram-negative bacteria). The flagella are attached to the cytoplasmic membrane and cell wall by discs. This creates the effect of an electric motor with a motor rod rotating the flagellum. Flagella are composed of a protein - flagellin (from flagellum - flagellum); is an H antigen. Flagellin subunits are twisted in a spiral.
The number of flagella in bacteria of various species varies from one (monotrichous) in Vibrio cholerae to tens and hundreds of flagella extending along the perimeter of the bacterium (peritrichus) in Escherichia coli, Proteus, etc. Lophotrichs have a bundle of flagella at one of the ends of the cell. Amphitrichs have one flagellum or bundle of flagella at opposite ends of the cell.
Drank
Drank (fimbriae, villi) - filamentous formations, thinner and shorter (3-10nm x 0.3-10μm) than flagella. Peels extend from the surface of the cell and are composed of the protein pilin, which has antigenic activity. There are pills responsible for adhesion, that is, for the attachment of bacteria to the affected cell, as well as pills responsible for nutrition, water-salt metabolism and sex (F-pills), or conjugation pills. They drank numerous - several hundred per cage. However, there are usually 1-3 sex pili per cell: they are formed by the so-called "male" donor cells containing transmissible plasmids (F-, R-, Col-plasmids). A distinctive feature of the genital pilus is the interaction with special "male" spherical bacteriophages, which are intensively adsorbed on the genital pilus.
Controversy
Spores are a peculiar form of dormant firmicut bacteria, i.e. bacteria
with a gram-positive type of cell wall structure. Spores are formed under unfavorable conditions for the existence of bacteria (drying, deficiency of nutrients, etc.) One spore (endospore) is formed inside the bacterial cell. Spore formation contributes to the preservation of the species and is not a way of reproduction, like in fungi. Spore-forming bacteria of the genus Bacillus have spores, not larger than the cell diameter Bacteria in which the spore size exceeds the cell diameter are called clostridia, for example, bacteria of the genus Clostridium (Latin Clostridium - spindle). Spores are acid-resistant, therefore they are stained according to the Aujeszky method or the Ziehl-Nielsen method in red, and the vegetative cell in blue.
The shape of the spores can be oval, spherical; the location in the cell is terminal, i.e. at the end of the stick (in the causative agent of tetanus), subterminal - closer to the end of the stick (in the causative agents of botuliema, gas gangrene) and central (in the anthrax bacillus). The spore persists for a long time due to the presence of a multilayer shell, calcium dipicolinate, low water content and sluggish metabolic processes. In favorable conditions, spores germinate through three successive stages: activation, initiation, germination.
Bacteria: habitats, structure, life processes, significance
2.b) The structure of the bacterial cell
The cell wall of bacteria determines their shape and ensures the preservation of the internal contents of the cell. According to the peculiarities of the chemical composition and structure of the cell wall, bacteria are differentiated by staining by gram ...
Bacterial cell wall biopolymers
Bacterial cell structure
The structure of bacteria is studied using electron microscopy of whole cells and their ultraviolet sections. The main structures of a bacterial cell are: the cell wall, the cytoplasmic membrane, the cytoplasm with inclusions and the nucleus ...
Humoral regulation of the body
3. Features of the structure, properties and functions of cell membranes
Variety of living cells
1.1 General plan of the structure of eukaryotic cells, also characterizing the structure of the animal cell
A cell is a structural and functional unit of living things. All eukaryotic cells are characterized by the presence of the following structures: 1) The cell membrane is an organoid that limits the contents of the cell from the environment ...
Variety of living cells
1.2 Features of the structure of a plant cell
In plant cells, organelles are found that are also characteristic of animals, for example, the nucleus, the endoplasmic reticulum, ribosomes, mitochondria, and the Golgi apparatus (see Fig. 2). They lack a cell center, and the function of lysosomes is performed by vacuoles ...
Variety of living cells
1.3 Features of the structure of the fungal cell
In most fungi, the cell in its structure and functions performed by it is generally similar to the plant cell. It consists of a hard shell and internal contents, which is the cytoplasmic system ...
Variety of living cells
1.4 General plan of the structure of prokaryotic cells, also characterizing the structure of the bacterial cell
The prokaryotic cell is structured as follows. The main feature of these cells is the absence of a morphologically pronounced nucleus, but there is a zone in which the DNA (nucleoid) is located.
Bacterial cell structure
Ribosomes are located in the cytoplasm ...
Fundamentals of Microbiology
1. Describe the structure of the bacterial cell. Sketch the organelles of the cell
Bacteria are microscopic plant organisms. Most of them are unicellular organisms that do not contain chlorophyll and reproduce by division. In shape, bacteria are spherical, rod-shaped and crimped ...
Features of the visual and auditory sensory systems
13. Simple, complex and supercomplex cells and their functions
"Simple" and "complex" cells. Neurons responding to simple linear stimuli (gaps, edges, or dark stripes) were called "simple", and those that respond to stimuli of complex configuration and to moving stimuli were called "complex" ...
Features of the structure of the cell
1. Cell as an elementary structural unit of an organism. The main components of the cell
The cell is the basic structural and functional unit of life, bounded by a semipermeable membrane and capable of self-reproduction. In a plant cell, first of all, it is necessary to distinguish between the cell membrane and the contents ...
Distribution and dynamics of the wild boar population in the Bryansk region
1.1 Structural features
The wild boar (Sus scrofa L.) is a massive animal with low, relatively thin legs. The body is relatively short, the anterior part is very massive, the posterior region of the shoulder blades is strongly elevated, the neck is thick, short, almost motionless ...
The structure, properties and functions of proteins
2. Functions of cell organelles
Cell organelles and their functions: 1. Cell membrane - consists of 3 layers: 1. rigid cell wall; 2. a thin layer of pectin substances; 3. thin cytoplasmic filament. The cell wall provides mechanical support and protection ...
4.1 Structural features
Thallus is a plasmodium capable of amoeba-like movements on the surface or inside the substrate. During sexual reproduction, plasmodia transform into fruiting bodies called sporocarps ...
Taxonomic group of slime mold
5.1 Structural features
The vegetative body in the form of a multinucleated protoplast, incapable of independent movement and located inside the cell of the host plant. Special sporulation is not formed. The wintering stage is represented by spores ...
The energy system of the cell. Classification of muscle tissue. Sperm structure
The energy system of the cell. General plan of the structure of mitochondria and plastids, their functions. Hypothesis about the symbiotic origin of mitochondria and chloroplasts
Eukaryotic cells have a unique organelle, mitochondria, in which ATP molecules are formed during oxidative phosphorylation. It is often said that mitochondria are the power plants of the cell (Figure 1) ...
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