Runny nose

Cellular structure of the body of prokaryotes or. Structure of a prokaryotic cell

All microorganisms included in the kingdom are characterized by a prokaryotic type of cell organization, which is determined by the characteristics of their ultrastructure, as well as the structure and functions of a number of macromolecules. Of all known cells, the prokaryotic is the simplest and probably the first cell, which arose about 3.6 billion years ago.

It is currently assumed that at some point in time, the evolution of cells went in two independent directions. Two groups of organisms appeared - prokaryotes, in which the nuclear material was not limited by an envelope, and eukaryotes, which had a formed nucleus with a nuclear envelope.

The main differences between prokaryotes and eukaryotes are as follows:

in prokaryotic cells there are no compartments or organelles limited from the cytoplasm by specialized intracellular lipoprotein membranes: endoplasmic reticulum (reticulum), mitochondria, Golgi apparatus, lysosomes, chloroplasts;

the nuclear structure of prokaryotes, called a nucleoid, does not have a nuclear envelope with a pore complex and is represented by a DNA macromolecule with proteins (without histones);

the genome of a prokaryotic cell is organized into one circular chromosome, which is a single replicon and is not divided by mitosis;

additional replicons can be represented by circular plasmid DNA molecules;

a prokaryotic cell contains only one type of ribosome with a sedimentation constant of 708, and some of the ribosomes are associated with the cytoplasmic membrane, which is never observed in eukaryotes;

The cell wall of prokaryotes contains a bioheteropolymer characteristic only of bacteria - peptidoglycan.

Some prokaryotes have structures not found in eukaryotes:

motile bacteria have special bacterial flagella made from flagellin proteins;

spore-forming forms of bacteria under unfavorable conditions transform into types of resting cells that are unique in their degree of stability - bacterial spores;

Prokaryotic cells are very small; the diameter of most bacterial cells does not exceed 1 μm, but the length can be significant, for example, in some spirochetes it is up to 500 μm. The small size of prokaryotes is believed to be associated with the absence of specialized membrane systems in their ultrastructure, which makes it difficult to coordinate intracellular processes in proportion to the increase in cell size.

The cellular structure clearly separates prokaryotes from viruses. Emphasizing the primitiveness of the organization of bacterial cells, it should be noted, however, that they have evolved in their direction over a much longer period of time than eukaryotic cells, and although the evolutionary capabilities of a prokaryotic cell are apparently limited, changes in their cellular organization occurred during the evolution process, which gradually led to its complication.

For a number of characteristics, bacteria have fundamental differences with eukaryotes, and knowledge of the features of their structure and functioning makes it possible to understand the possibility of selective antimicrobial action of chemotherapeutic drugs. The use of electron microscopy and fine cytochemical studies made it possible to study their ultrastructure (Fig. 1). The essential components of a bacterial cell are a cytoplasmic membrane surrounding the cytoplasm, which contains ribosomes and a nucleoid. The cells of all bacteria, with the exception of L-forms and mycoplasmas, have a cell wall. Other structures are additional and determine the morphological and functional characteristics of various species: capsules, flagella, pili, spores, inclusions.

Rice. 1. Scheme of the structure of a prokaryotic cell:

/ - capsule; 2 - cell wall; 3 - cytoplasmic membrane; 4 - mesosomes; 5 - cytoplasm; 6 - nucleoid; 7 - plasmid;

8 - ribosomes and polysomes; 9 - flagella; 10 - drank; 11 - glycogen granules; 12 - lipid droplets; 13 - volutin granules; 14 - sulfur inclusions

Surface structures. Capsule - This is the outer, uppermost mucous layer of a cell of varying thickness with a fibrillar or globular structure. It has a polysaccharide, mucopolysaccharide or polypeptide nature and contains up to 98% water. Depending on the thickness, a microcapsule (less than 0.2 microns thick) and a macrocapsule are distinguished. The capsule is not an obligatory structural element of the cell. The biological meaning of capsule formation is determined by a number of functions, including: protection from phagocytes and viruses, toxins and radiation; immunological mimicry in pathogenic bacteria; moisture retention in low humidity conditions; cell attachment to a dense surface.

Pili (fimbriae, villi, cilia) - These are straight cylindrical formations of a protein nature, 0.3-10 microns long, up to 10 nm in diameter, evenly covering the cell surface (up to several hundred per cell), and not performing a locomotor function.

There are general type pili, which promote the attachment of bacterial cells to the substrate, human cells (the phenomenon of adhesion of microorganisms) and sexual pili, which are involved in the transfer of genetic material from the donor cell to the recipient cell in the process of conjugation, as well as causing the adsorption of specific bacteriophages on cells.

Flagella - organs of movement of bacteria in the form of spirally curved cylindrical formations of a protein nature (flagellin proteins) on the cell surface 3-12 µm long and 10-30 nm thick, attached by a basal body (disk system) to the cytoplasmic membrane (see inclusion I). Number and location

The behavior of flagella can be different and is a species characteristic (Fig. 2). There are monotrichs (bacteria with one flagella at the end), amphitrichy (bacteria with flagella located at the poles), lophotrichs (cells with a bundle of flagella at one end) and peritrichs (with 2-30 flagella throughout the cell body).

Pili and flagella are not obligatory organelles of the bacterial cell.

Cell wall - one of the main structural elements of a bacterium, providing mechanical protection to the cell. Except for mycoplasmas and L-forms, the cells of all bacteria are covered with a cell wall, the thickness of which varies from 0.01 to 14 microns in different species. It is a dense elastic structure -

Rice. 2. The main forms of bacteria (according to A. A. Vorobyov et al., 1994):

/ - staphylococci; 2 - streptococci; 3 - sarcins; 4 - gonococci;

5- pneumococci; 6- pneumococcal capsules; 7- Corynebacterium diphtheria; 8 - clostridia; 9 - bacilli; 10 - vibrios; 11 - spirilla; 12 - treponsma; - borrelia; 14 - Leptospira; 15- actinomycetes; 16 - location of flagella: A - monotrichs; b - lophotrichs; c - amphitrichs; g - peritrichous

py, which surrounds the protoplast of the cell and gives it a permanent shape and rigidity. The cell wall prevents osmotic swelling and rupture of cells when they enter a hypotonic environment. Water, other small molecules, and various ions easily pass through tiny pores in the cell wall, but large molecules of proteins and nucleic acids do not pass through them.

The main chemical component of the cell wall is a specific heteropolymer - peptidoglycan (murein, mucopeptide, glucosaminopeptide, glycopeptide), consisting of chains in which N-acetylglucosamine and M-acetylmuramic acid residues alternate, interconnected by β-1,4-glycosidic bonds. This sharply distinguishes the envelope structures of bacteria from eukaryotic ones and creates the “Achilles heel” of bacteria used for antimicrobial chemotherapy.

Organization of the cytoplasm. Cytoplasmic membrane(CM) It is one of the obligatory cellular structures, has a thickness of 7-13 nm and is located directly under the cell wall, limiting the protoplast of the cell. The structure of the membranes of bacterial, animal and plant cells is very similar. Currently, most scientists have adopted the fluid-mosaic model of the CM structure. According to this model, the CM consists of a double layer (15-30% of phospholipid and triglyceride molecules with hydrophobic ends directed inward and hydrophilic “heads” directed outward. Protein molecules (50-70%) are mosaically immersed in it. The membrane also contains carbohydrates (2- 5%) and RNA. CM is a plastic “fluid” formation that plays a vital role in metabolism, is a semi-permeable structure, maintains osmotic pressure, controls both the entry of substances into the cell and the excretion of final metabolites through the system of substrate-specific permeases (enzymes). carriers localized on the membrane). Respiration processes that supply energy to the cell are associated with the CM, that is, those functions for which the membranes of mitochondria and chloroplasts are responsible in a eukaryotic cell.

There are so-called mesosomes - CM invaginations are mixed membrane systems formed by tubes, vesicles and lamellae. They are supposed to function as a center for the respiratory activity of bacteria, participate in cell division and the divergence of daughter chromosomes after replication.

Cytoplasm fills the volume of bacteria limited by the CM. This is a complex colloidal system that consists of proteins, nucleic acids, carbohydrates, lipids, minerals and 70-80% water. The cytoplasm is the location of intracellular organelles (nucleoid, ribosomes, various inclusions) and is involved in intracellular metabolism. Character-

The main features of the organization of the cytoplasm of prokaryotes in comparison with animal and plant cells are the absence of endoplasmic reticulum and high electron density.

Nucleoid - nuclear material of a bacterial cell. It is represented by a double strand of DNA macromolecule with a molecular weight of 2-3 10 in combination with proteins, among which there are no nuclear (histones and histone-like) proteins characteristic of eukaryotes. Unlike the real nucleus of eukaryotic cells, the nucleoid does not have a perforated nuclear membrane, is not divided by mitosis, and during the division period represents one circular chromosome that encodes all genetic information.

Plasmids - optional intracellular structures in the form of extrachromosomal circular DNA sections capable of self-replication. They cause the inheritance of additional traits: drug resistance, toxigenicity, bacteriocinogenicity, etc.

Ribosomes - organelles in which protein synthesis occurs. Each ribosome has dimensions of 20x30x30 nm and a sedimentation constant of 70S (since during ultracentrifugation, ribosomes sediment at a rate of about 70 Swedenberg units (S), in contrast to the larger cytoplasmic ribosomes of eukaryotes with a sedimentation constant of 808). In the free state, the bacterial ribosome is in the form of two subunits - 30S and 50S, both subunits contain approximately 40% ribosomal RNA and 60% protein. During protein synthesis, ribosomes, with the help of messenger RNA, form polysomes, usually associated with the CM. Bacteria can contain from 5,000 to 50,000 ribosomes, depending on the age of the cell and culture conditions.

Knowledge of the differences between the ribosomes of bacteria and eukaryotic cells is important for understanding the mechanisms of the antimicrobial action of those antibiotics that inhibit protein synthesis on bacterial ribosomes and do not affect the functions of 80S ribosomes.

Spores (endospores) of bacteria - resting forms of some types of gram-positive bacteria under unfavorable environmental conditions.

Sporulation occurs in several stages; when the spore is fully mature, the vegetative part of the cell lyses and dies (see incl. I, II).

In the process of sporulation (sporulation), several main stages can be distinguished. The cell that switches to sporulation stops growing; as a rule, it contains two or more nucleoids. At the first stage, part of the cellular DNA is localized in one of the poles of the cell. Then part of the cytoplasm containing

Another chromosome in it is separated by a cytoplasmic membrane, as if growing into the depths of the cell, and a prospore is formed, surrounded by a double membrane membrane.

Then, between the two membranes, the formation of a multilayer wall and cortex (cortex) of a peptidoglycan spore occurs. A polypeptide shell and exosporium are also formed outside the membranes, surrounding the spore in the form of a free sheath. A fully formed bacterial spore is a compacted area of the cell with a nucleoid and ribosomes, bounded by a dense multilayer membrane impregnated with calcium salts of dipicolinic acid.

Sporulation is characteristic of rod-shaped bacteria - bacilli and clostridia (see Fig. 2). There are central, terminal and subterminal locations of spores in the vegetative part of the cell, which is a differential diagnostic feature of the pathogen.

In one bacterium, one spore is formed, which is in the dormant stage, while all metabolic processes are practically reduced to zero, but the potential viability of the cell is preserved. Since an increase in the number of microorganisms does not occur in this process, sporulation in bacteria is not a method of reproduction, but only an adaptation for survival. Bacterial spores, unique in their degree of resistance to physical and chemical factors, can survive in the external environment without loss of viability for a long time (tens of years), making it difficult to combat spore-bearing pathogenic bacteria.

Intraplasmic inclusions. The term “inclusions” refers to such intracellular structures of bacteria that, obviously, are not absolutely necessary for their life. However, their nature and functions may be different. In some cases, the inclusions are metabolic products of the bacterial cell, in others they are a supply of nutrients.

Of the reserve polysaccharides, glucans are especially common - glycogen, starch, granulosa. They are detected in the cells of bacilli, clostridia, enterobacteria, etc.

Reserve lipids are represented by β-hydroxybutyric acid polyester and waxes. Waxes, esters of high molecular weight fatty acids and alcohols are characteristic of mycobacteria.

In corynebacteria, the phosphorus reserve is created in the form of polyphosphate grains (volutin), which have diagnostic value.

Prokaryotic cells- these are the most primitive, very simply structured organisms that retain the features of deep antiquity. TO prokaryotic(or prenuclear) organisms include bacteria and blue-green algae (cyanobacteria). Based on the similarity of structure and sharp differences from other cells, prokaryotes are classified into the independent kingdom of crushed cells.

Let's look at the structure prokaryotic cell using bacteria as an example. The genetic apparatus of a prokaryotic cell is represented by the DNA of a single circular chromosome, is located in the cytoplasm and is not delimited from it by a membrane. This analogue of the nucleus is called a nucleoid. DNA does not form complexes with proteins and therefore all genes that are part of the chromosome “work”, i.e. information is continuously read from them.

Prokaryotic cell surrounded by a membrane separating the cytoplasm from the cell wall, formed from a complex, highly polymeric substance. There are few organelles in the cytoplasm, but numerous small ribosomes are present (bacterial cells contain from 5,000 to 50,000 ribosomes).

Structure of a prokaryotic cell

The cytoplasm of a prokaryotic cell is penetrated by membranes that form the endoplasmic reticulum; it contains ribosomes that carry out protein synthesis.

The inner part of the cell wall of a prokaryotic cell is represented by a plasma membrane, the protrusions of which into the cytoplasm form mesosomes, which are involved in the construction of cell walls, reproduction, and are the site of DNA attachment. Respiration in bacteria occurs in mesosomes, and in blue-green algae in cytoplasmic membranes.

Many bacteria deposit reserve substances inside the cell: polysaccharides, fats, polyphosphates. Reserve substances, when included in metabolism, can prolong the life of a cell in the absence of external energy sources.

(1-cell wall, 2-outer cytoplasmic membrane, 3-chromosome (circular DNA molecule), 4-ribosome, 5-mesosome, 6-invagination of the outer cytoplasmic membrane, 7-vacuoles, 8-flagella, 9-stacks of membranes, in which photosynthesis occurs)

As a rule, bacteria reproduce by dividing in two. After cell elongation, a transverse partition is gradually formed, which is laid in the direction from the outside to the inside, then the daughter cells disperse or remain connected in characteristic groups - chains, packets, etc. The bacterium E. coli doubles its number every 20 minutes.

Bacteria are characterized by spore formation. It begins with the detachment of part of the cytoplasm from the mother cell. The detached part contains one genome and is surrounded by a cytoplasmic membrane. Then a cell wall, often multilayered, grows around the spore. In bacteria, the sexual process occurs in the form of an exchange of genetic information between two cells. The sexual process increases the hereditary variability of microorganisms.

Most living organisms are united in the superkingdom of eukaryotes, which includes the kingdom of plants, fungi and animals. Eukaryotic cells are larger prokaryotic cells, consist of a surface apparatus, a nucleus and a cytoplasm.

Eukaryotic cell

Eukaryotic(eukaryotic)cells contain a nucleus coordinating the vital activity of the cell, in which the hereditary apparatus of the body is located, and numerous organoids, performing various functions. Most eukaryotes are aerobes, that is, they use atmospheric oxygen for energy metabolism.

Let's look at the structure prokaryotic cell using bacteria as an example. The genetic apparatus of a prokaryotic cell is represented by the DNA of a single circular chromosome, located in the cytoplasm and not delimited from it by a membrane. This analogue of the nucleus is called a nucleoid. DNA does not form complexes with proteins and therefore all genes that are part of the chromosome “work”, i.e. information is continuously read from them.

The cytoplasm of a prokaryotic cell is penetrated by membranes that form the endoplasmic reticulum; it contains ribosomes that carry out protein synthesis.

CHARACTERISTICS OF MICROORGANISMS - OBJECTS OF BIOTECHNOLOGICAL PRODUCTION

Objects of industrial microbiology and biotechnology include bacteria, yeast, microscopic (mold) fungi, plant and animal cell cultures, as well as subcellular structures (viruses, plasmids, mitochondrial and chloroplast DNA, nuclear DNA).

Cellular forms, including prokaryotic and eukaryotic organisms, differ in many fundamental characteristics. However, the general, technologically important, properties of microorganisms are:

· high speed of metabolic processes. This is due to the large ratio of exchange surface to cell volume. For microorganisms, the entire cell surface is an exchange surface. Since bacterial cells are the smallest, they grow and develop the fastest of all microorganisms, followed by yeast and fungi. In turn, the rate of metabolic processes in microorganisms is tens and hundreds of thousands of times higher than in animals. For example, in the body of one bull weighing 500 kg, approximately 0.5 kg of protein is produced in 24 hours; during the same time, 500 kg of yeast can synthesize more than 50,000 kg of protein;

· plasticity of exchange – high ability to adapt (adapt to new conditions of existence). The incomparably greater flexibility of metabolic processes in microorganisms compared to plants and animals is explained by their ability to synthesize inducible enzymes, i.e. enzymes that are formed in the cell only in the presence of appropriate substances in the environment;

· high degree of variability. The higher degree of variability of microorganisms compared to macroorganisms is due to the fact that most microorganisms are single-celled organisms. It is easier to influence a single cell than an organism consisting of many cells. A high degree of variability, rapid growth and development, a high rate of metabolic processes, the formation of numerous offspring - all these properties of microorganisms make them extremely convenient objects for genetic analysis, since experiments can be carried out in a short time on a huge number of individuals.

The structure of a prokaryotic (bacterial) cell

A characteristic feature of prokaryotes is the absence of an intracellular membrane system.

Cell wall gives shape to the cell, protects the cell from external influences (is a mechanical barrier of the cell), protects the cell from the penetration of excess moisture into it.

Based on the chemical composition and structure of the cell wall, bacteria are divided into gram-positive (Gram+) and gram-negative (Gram-).

The Gram+ cell wall consists of peptidoglycan – mureina(up to 90–95%), teichoic acids, polysaccharides. It has a single-layer structure, tightly adjacent to the cytoplasmic membrane.

In Gram bacteria, the cell wall contains little murein (5–10%), teichoic acids are absent, and lipoproteins and lipopolysaccharides are contained in large quantities.

The cell wall of Gram-bacteria is much thinner than that of Gram+, but has a two-layer structure. The outer layer consists of lipoproteins and lipopolysaccharides, which prevent the penetration of toxic substances. Therefore, Gram-bacteria are more resistant to antibiotics and toxic chemicals, and the fight against these microorganisms in food production is less effective than against Gram+ bacteria.

Cytoplasmic membrane(CPM) plays an important role in cell nutrition and has selective permeability. It consists of a protein-lipid complex and has a three-layer structure. On the outer side of the membrane there are carrier proteins that transport nutrients into the cell, and on the inner side there are redox and hydrolytic enzymes. Between the two protein layers is a phospholipid layer.

Mesosomes – membranous formations, protrusions of the central nervous system. Thanks to them, the cell's exchange surface increases. They participate in energy processes, and also take part in the processes of cell division (reproduction).

Cytoplasm – intracellular contents, semi-liquid colloidal solution. It contains up to 70–80% of the cell’s mass of water, enzymes, nutritional substrates and cell metabolic products. All components of a prokaryotic cell are located in the cytoplasm.

Nucleoid – the carrier of hereditary information, the only chromosome of a prokaryotic cell, takes part in reproduction. This is a compact formation that occupies a central region in the cytoplasm and consists of a double-stranded spirally twisted DNA strand closed in a circle.

Many bacteria, along with chromosomal DNA, also contain extrachromosomal DNA, also represented by double helices closed in a ring. These autonomously replicating DNA elements are called plasmids.

Ribosomes – small granules containing RNA (60%) and protein (40%). Ribosomes carry out the synthesis of cellular proteins.

Spare substances. They consist of polysaccharide granules (granulosa glycogen), sulfur inclusions, fat droplets (contain poly-b-butyric acid), volutin (polyphosphate granules).

Motile forms of bacteria have flagella (8), long filaments consisting of a structural protein - flagellin. Flagella are attached to the CPM using two pairs of base discs – basal body (9).

Photosynthetic bacteria have cells in their cells thylakoids (10), through which photosynthesis occurs.

Mucous species of bacteria have capsule (11) or a mucous sheath, often consisting of polysaccharides, less often of polypeptides. This is an additional protective barrier of the cell, a source of reserve nutrients.

All living organisms can be classified into one of two groups (prokaryotes or eukaryotes) depending on the basic structure of their cells. Prokaryotes are living organisms consisting of cells that do not have a cell nucleus and membrane organelles. Eukaryotes are living organisms that contain a nucleus and membrane organelles.

The cell is a fundamental component of our modern definition of life and living things. Cells are seen as the basic building blocks of life and are used in defining what it means to be "alive".

Let's look at one definition of life: "Living things are chemical organizations composed of cells and capable of reproducing" (Keaton, 1986). This definition is based on two theories - the cell theory and the theory of biogenesis. was first proposed in the late 1830s by German scientists Matthias Jakob Schleiden and Theodor Schwann. They argued that all living things are made of cells. The theory of biogenesis, proposed by Rudolf Virchow in 1858, states that all living cells arise from existing (living) cells and cannot arise spontaneously from nonliving matter.

The components of cells are enclosed in a membrane, which serves as a barrier between the outside world and the internal components of the cell. The cell membrane is a selective barrier, meaning that it allows certain chemicals to pass through to maintain the balance necessary for cell function.

The cell membrane regulates the movement of chemicals from cell to cell in the following ways:

diffusion (the tendency of molecules of a substance to minimize concentration, that is, the movement of molecules from an area of higher concentration towards an area of lower until the concentration equalizes);
osmosis (the movement of solvent molecules through a partially permeable membrane in order to equalize the concentration of a solute that is unable to move through the membrane);
selective transport (using membrane channels and pumps).

Prokaryotes are organisms consisting of cells that do not have a cell nucleus or any membrane-bound organelles. This means that the genetic material DNA in prokaryotes is not bound in the nucleus. In addition, the DNA of prokaryotes is less structured than that of eukaryotes. In prokaryotes, DNA is single-circuit. Eukaryotic DNA is organized into chromosomes. Most prokaryotes consist of only one cell (unicellular), but there are a few that are multicellular. Scientists divide prokaryotes into two groups: and.

A typical prokaryotic cell includes:

plasma (cell) membrane;
cytoplasm;
ribosomes;
flagella and pili;
nucleoid;
plasmids;

Eukaryotes

Eukaryotes are living organisms whose cells contain a nucleus and membrane organelles. In eukaryotes, the genetic material is located in the nucleus, and the DNA is organized into chromosomes. Eukaryotic organisms can be unicellular or multicellular. are eukaryotes. Eukaryotes also include plants, fungi and protozoa.

A typical eukaryotic cell includes:

nucleolus;