Ultrastructure of a prokaryotic cell. Who are eukaryotes and prokaryotes: comparative characteristics of cells of different kingdoms Structure of prokaryotic cells drawing

The prokaryotic cell is much simpler than animal and plant cells. On the outside, it is covered with a cell wall that performs protective, formative and transport functions. The rigidity of the cell wall is provided by murein. Sometimes the bacterial cell is covered on top with a capsule or mucous layer.

The protoplasm of bacteria, like that of eukaryotes, is surrounded plasma membrane. Saccular, tubular or lamellar invaginations of the membrane contain mesosomes involved in the respiration process, bacteriochlorophyll and other pigments. The genetic material of prokaryotes does not form a nucleus, but is located directly in the cytoplasm. Bacterial DNA is a single circular molecule, each of which consists of thousands and millions of nucleotide pairs. The genome of a bacterial cell is much simpler than that of the cells of more developed creatures: on average, bacterial DNA contains several thousand genes.

Absent in prokaryotic cells endoplasmic reticulum, A ribosomes float freely in the cytoplasm. Prokaryotes do not have mitochondria; Their functions are partially performed by the cell membrane.

Prokaryotes

Bacteria are the smallest of organisms with a cellular structure; their sizes range from 0.1 to 10 microns. A typical printing point can accommodate hundreds of thousands of medium-sized bacteria. Bacteria can only be seen through a microscope, which is why they are called microorganisms or microbes; microorganisms are being studied microbiology . The branch of microbiology that studies bacteria is called bacteriology . This science began Anthony van Leeuwenhoek in the 17th century.

Bacteria - the oldest known organisms. Traces of the vital activity of bacteria and blue-green algae (stromatolites) belong to the Archean and date back to 3.5 billion years old.

Due to the possibility of gene exchange between representatives various kinds and even genera, it is quite difficult to systematize prokaryotes. A satisfactory taxonomy of prokaryotes has not yet been constructed; all existing systems are artificial and classify bacteria according to some group of characteristics, without taking into account their phylogenetic relationship. Previously, bacteria along with mushrooms And algae included in the subkingdom of lower plants. Currently, bacteria are classified as a separate superkingdom of prokaryotes. The most common classification system is Bergey system, which is based on the structure of the cell wall.

At the end of the 20th century, scientists discovered that cells of a relatively little-studied group of bacteria - archaebacteria – contain rRNA, different in structure from both the r-RNA of prokaryotes and the r-RNA of eukaryotes. The structure of the genetic apparatus of archaebacteria (presence introns and repeating sequences, processing, form ribosomes) brings them closer to eukaryotes; on the other hand, archaebacteria also have typical features of prokaryotes (absence of a nucleus in the cell, presence of flagella, plasmids and gas vacuoles, rRNA size, nitrogen fixation). Finally, archaebacteria differ from all other organisms in the structure of their cell wall, the type of photosynthesis, and some other characteristics. Archaebacteria are capable of existing in extreme conditions (for example, in hot springs at temperatures above 100 ° C, in the ocean depths at a pressure of 260 atm, in saturated salt solutions (30% NaCl)). Some archaebacteria produce methane, others use sulfur compounds to produce energy.

Apparently, archaebacteria are a very ancient group of organisms; "extreme" possibilities indicate the conditions characteristic of the Earth's surface in Archean era. It is believed that archaebacteria are closest to the hypothetical “pro-cells” that subsequently gave rise to all the diversity of life on Earth.

Recently it has become clear that there are three main types rRNA, presented, respectively, the first - in eukaryotic cells, the second - in the cells of real bacteria, as well as in mitochondria And chloroplasts eukaryotes, the third - in archaebacteria. Research in molecular genetics has forced us to take a fresh look at the theory of the origin of eukaryotes. It is now believed that three different branches of prokaryotes evolved simultaneously on ancient Earth - archaebacteria, eubacteria and urkaryotes , characterized by different structures and different methods of obtaining energy. Urkaryotes, which were essentially the nuclear-cytoplasmic component of eukaryotes, were subsequently included as symbionts representatives of various groups of eubacteria, which turned into mitochondria and chloroplasts of future eukaryotic cells.

Thus, the class rank previously allocated for archaebacteria is clearly insufficient. Currently, many researchers tend to divide prokaryotes into two kingdoms: archaebacteria and real bacteria (eubacteria ) or even separate archaebacteria into a separate superkingdom Archaea.

The classification of real bacteria is given in scheme.

IN bacterial cell There is no nucleus, chromosomes are freely located in the cytoplasm. In addition, the bacterial cell lacks membrane organelles: mitochondria, EPS, Golgi apparatus etc. The outside of the cell membrane is covered with a cell wall.

Most bacteria move passively, using water or air currents. Only some of them have movement organelles - flagella . Prokaryotic flagella are very simple in structure and consist of the flagellin protein, which forms a hollow cylinder with a diameter of 10–20 nm. They screw into the medium, propelling the cell forward. Apparently, this is the only structure known in nature that uses the wheel principle.

Based on their shape, bacteria are divided into several groups:

cocci (have a round shape);

bacilli (have a rod-shaped form);

spirilla (have the shape of a spiral);

vibrios (have the shape of a comma).

Based on the method of respiration, bacteria are divided into aerobes (most bacteria) and anaerobes (causative agents of tetanus, botulism, gas gangrene). The former need oxygen to breathe; for the latter, oxygen is useless or even poisonous.

Bacteria reproduce by dividing approximately every 20 minutes (under favorable conditions). DNA is replicated, with each daughter cell receiving its own copy of the parent DNA. Transfer of DNA between non-dividing cells is also possible (through the capture of “naked” DNA, using bacteriophages or by conjugation , when bacteria are connected to each other by copulatory fimbriae), however, an increase in the number of individuals does not occur. Reproduction is prevented by the sun's rays and the products of their own vital activity.

The behavior of bacteria is not particularly complex. Chemical receptors record changes in the acidity of the environment and the concentration of various substances: sugars, amino acids, oxygen. Many bacteria respond to changes in temperature or light, and some bacteria can sense the Earth's magnetic field.

Under unfavorable conditions, the bacterium becomes covered with a dense shell, the cytoplasm is dehydrated, and vital activity almost ceases. In this state, bacterial spores can remain in a deep vacuum for hours and tolerate temperatures from –240 °C to +100 °C.

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;

Bacteria cell

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

Let's look at the structure of a prokaryotic cell using bacteria as an example.

The genetic apparatus 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.

A prokaryotic cell is surrounded by a membrane that separates 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).

The cytoplasm is permeated with membranes that form the endoplasmic reticulum; it contains ribosomes that carry out protein synthesis.

The inner part of the cell wall is represented by the 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.

Structure of a prokaryotic cell

(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 diverge 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.

The shape of prokaryotic cells is not so diverse. The round cells are called cocci. Both archaea and eubacteria can have this form. Streptococci are cocci elongated in a chain. Staphylococci are “clusters” of cocci, diplococci are cocci united in two cells, tetrads are four, and sarcina are eight. Rod-shaped bacteria are called bacilli. Two rods - diplobacillus, elongated in a chain - streptobacilli. Other species include coryneform bacteria (with a club-like extension at the ends), spirilla (long curled cells), vibrios (short curved cells) and spirochetes (curl differently from spirilla).

The shape of a bacterial cell is one of the most important systematic characteristics.

There are 4 main cell forms:

1) Cocci are bacteria that have a spherical shape. Spherical bacteria after division can form:

a) diplococci - two cells in one capsule. Representatives: pneumococcus - the causative agent of pneumonia;

b) streptococci - formed by cocci in the form of a chain. Representatives: pathogens of sore throat and scarlet fever;

c) staphylococci - resemble a bunch of grapes. Representatives: different strains of staphylococci cause furunculosis, pneumonia, food poisoning and some other diseases.

2) Bacilli are straight, rod-shaped bacteria:

a) non-spore-forming rods are called bacteria. Representatives: common intestinal symbionts, pathogens of typhoid fever, nodule bacteria;

b) spore-forming rods are called bacilli. Representatives: a lot in the soil, for example, nitrogen-fixing bacteria, the causative agents of anthrax, the causative agent of tuberculosis - Koch's bacillus.

3) Spirilla, spirochetes- spiral shape.

A) spirilla are spiral rods with one flagellum. Representatives: ordinary inhabitants oral cavity.

b) spirochetes - the shape of the cells is very complex, but there are differences in the method of movement. Representatives: ordinary inhabitants of the oral cavity, the causative agent of syphilis.

4) Vibrios are short rods, always curved in the shape of a comma. Representatives: cholera causative agent.

Prokaryotes appeared on Earth about 3.5 billion years ago and were probably the first cellular life forms, giving rise to modern prokaryotes and eukaryotes.

In our article we will look at the structure of prokaryotes and eukaryotes. These organisms differ significantly in their level of organization. And the reason for this is the peculiarities of the structure of genetic information.

Features of the structure of prokaryotic cells

Prokaryotes are all living organisms whose cells do not contain a nucleus. Of the five modern representatives, only one belongs to them - Bacteria. The prokaryotes whose structure we are considering also include representatives of blue-green algae and archaea.

Despite the absence of a formed nucleus in their cells, they contain genetic material. This allows the storage and transmission of hereditary information, but limits the variety of methods of reproduction. All prokaryotes reproduce by dividing their cells in two. They are not capable of mitosis and meiosis.

The structure of prokaryotes and eukaryotes

The structural features of prokaryotes and eukaryotes that distinguish them are quite significant. In addition to the structure of the genetic material, this also applies to many organelles. Eukaryotes, which include plants, fungi and animals, contain mitochondria, the Golgi complex, the endoplasmic reticulum, and many plastids in their cytoplasm. Prokaryotes do not have them. The cell wall that both have is different chemical composition. In bacteria, it contains complex carbohydrates pectin or murein, while in plants it is based on cellulose, and in fungi it is chitin.

History of discovery

The structural features and functioning of prokaryotes became known to scientists only in the 17th century. And this despite the fact that these creatures have existed on the planet since its inception. In 1676, they were first examined through an optical microscope by its creator, Antonie van Leeuwenhoek. Like all microscopic organisms, the scientist called them “animalcules.” The term "bacteria" appeared only at the beginning of the 19th century. It was proposed by the famous German naturalist Christian Ehrenberg. The concept of “prokaryotes” arose later, during the era of the creation of the electron microscope. Moreover, at first, scientists established the fact of differences in the structure of the genetic apparatus of the cells of different creatures. E. Chatton in 1937 proposed to unite organisms on this basis into two groups: pro- and eukaryotes. This division still exists today. In the second half of the 20th century, a difference was discovered among the prokaryotes themselves: archaea and bacteria.

Features of the surface apparatus

The surface apparatus of prokaryotes consists of a membrane and a cell wall. Each of these parts has its own characteristics. Their membrane is formed by a double layer of lipids and proteins. Prokaryotes, whose structure is quite primitive, have two types of cell wall structure. Thus, in gram-positive bacteria it consists mainly of peptidoglycan, has a thickness of up to 80 nm and fits tightly to the membrane. A characteristic feature of this structure is the presence of pores in it, through which a number of molecules penetrate. The cell wall of gram-negative bacteria is very thin - up to a maximum of 3 nm. It does not fit tightly to the membrane. Some representatives of prokaryotes also have a mucous capsule on the outside. It protects organisms from drying out, mechanical damage, and creates an additional osmotic barrier.

Organelles of prokaryotes

The cell structure of prokaryotes and eukaryotes has its own significant differences, which primarily lie in the presence of certain organelles. These permanent structures determine the level of development of organisms as a whole. Prokaryotes lack most of them. Protein synthesis in these cells occurs in ribosomes. Aquatic prokaryotes contain aerosomes. These are gas cavities that provide buoyancy and regulate the degree of immersion of organisms. Only prokaryotic cells contain mesosomes. These folds of the cytoplasmic membrane only occur when chemical fixation methods are used during preparation for microscopy. The organelles of movement of bacteria and archaea are cilia or flagella. And attachment to the substrate is carried out by pili. These structures formed by protein cylinders are also called villi and fimbriae.

What is a nucleoid

But the most significant difference is in the structure of the gene of prokaryotes and eukaryotes. all these organisms have. In eukaryotes it is located inside the formed nucleus. This double-membrane organelle has its own matrix, called nucleoplasm, envelope and chromatin. Here, not only the storage of genetic information is carried out, but also the synthesis of RNA molecules. In the nucleoli, subunits of ribosomes are subsequently formed from them - organelles responsible for protein synthesis.

The structure of prokaryotic genes is simpler. Their hereditary material is represented by the nucleoid or nuclear region. DNA in prokaryotes is not packaged into chromosomes, but has a closed circular structure. The nucleoid also includes RNA and protein molecules. The latter functions resemble eukaryotic histones. They are involved in DNA duplication, RNA synthesis, restoration of chemical structure and nucleic acid breaks.

Features of life

Prokaryotes, whose structure is not very complex, carry out rather complex life processes. These are nutrition, breathing, reproduction of their own kind, movement, metabolism... And only one microscopic cell, the size of which ranges from up to 250 microns, is capable of all this! So we can only talk about primitiveness relatively.

The structural features of prokaryotes also determine the mechanisms of their physiology. For example, they are able to obtain energy in three ways. The first is fermentation. It is carried out by some bacteria. This process is based on redox reactions, during which ATP molecules are synthesized. This is a chemical compound that, when broken down, releases energy in several stages. Therefore, it is not for nothing that it is called a “cellular battery”. The next method is breathing. The essence of this process is the oxidation of organic substances. Some prokaryotes are capable of photosynthesis. Examples of these are blue-green algae and algae, which contain plastids in their cells. But archaea are capable of chlorophyll-free photosynthesis. During this process, carbon dioxide is not fixed, but ATP molecules are directly formed. So, in essence, this is true photophosphorylation.

Power type

Forms of reproduction

Prokaryotes, whose structure is represented by one cell, reproduce by dividing it into two parts or by budding. This feature is also due to their structure. The process of binary fission is preceded by doubling, or DNA replication. In this case, the nucleic acid molecule first unwinds, after which each strand is duplicated. The resulting chromosomes diverge to the poles. The cells increase in size, a constriction forms between them, and then their final separation occurs. Some bacteria are also capable of forming asexual reproduction cells - spores.

Bacteria and Archaea: Distinctive Features

For a long time, archaea, together with bacteria, were representatives of the Kingdom of Drobyanka. Indeed, they have many similar structural features. This is primarily the size and shape of their cells. However biochemical research showed that they have a number of similarities with eukaryotes. This is the nature of enzymes, under the influence of which the processes of synthesis of RNA and protein molecules occur.

Archaea have mastered almost all habitats. They are especially diverse in the composition of plankton. Initially, all archaea were classified as extremophiles, since they are able to live in hot springs, in reservoirs with high salinity, and at depths with significant pressure.

The importance of prokaryotes in nature and human life

The role of prokaryotes in nature is significant. First of all, they are the first living organisms to arise on the planet. Scientists have found that bacteria and archaea arose about 3.5 billion years ago. The theory of symbiogenesis suggests that some organelles of eukaryotic cells also originated from them. In particular, we are talking about plastids and mitochondria.

Many prokaryotes find their application in biotechnology to obtain medicines, antibiotics, enzymes, hormones, fertilizers, herbicides. Man has long been using beneficial features lactic acid bacteria for making cheese, kefir, yogurt, fermented products. With the help of these organisms, water bodies and soils are cleaned and ores of various metals are enriched. Bacteria form the intestinal microflora of humans and many animals. Along with archaea, they carry out the cycle of many substances: nitrogen, iron, sulfur, hydrogen.

On the other hand, many bacteria are the causative agent dangerous diseases, regulating the numbers of many species of plants and animals. These include plague, syphilis, cholera, anthrax, and diphtheria.

So, prokaryotes are organisms whose cells lack a formed nucleus. Their genetic material is represented by a nucleoid, consisting of a circular DNA molecule. Among modern organisms, prokaryotes include bacteria and archaea.

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 surface of the cell (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 the 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 flagellum at the end), amphitrichs (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 is different types ranges from 0.01-14 microns. 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 structure of the CM. 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 expected 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 ​​a 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 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.