Drawing of a prokaryotic. Who are eukaryotes and prokaryotes: comparative characteristics of cells from different kingdoms

prokaryotic cells- these are the most primitive, very simply arranged, preserving the features of ancient times. TO prokaryotic(or pre-nuclear) organisms include bacteria and blue-green algae (cyanobacteria). Based on the commonality of the structure and sharp differences from other cells, prokaryotic cells are isolated into an independent kingdom of shotguns.

Consider the structure prokaryotic cell on the example of bacteria. The genetic apparatus of a prokaryotic cell is represented by the DNA of a single ring chromosome, located in the cytoplasm and not separated from it by a membrane. Such an analogue of the nucleus is called a nucleoid. DNA does not form complexes with proteins, and therefore all the genes that make up the chromosome "work", i.e. information is continuously read from them.

prokaryotic cell surrounded by a membrane that separates the cytoplasm from the cell wall, formed from a complex, high-polymer 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 of a prokaryotic cell is permeated with membranes that form the endoplasmic reticulum, and 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 involved in the construction of cell partitions, reproduction, and are the site of DNA attachment. Respiration in bacteria is carried out in mesosomes, in blue-green algae in cytoplasmic membranes.

In many bacteria, reserve substances are deposited inside the cell: polysaccharides, fats, polyphosphates. Reserve substances, being included in the metabolism, can prolong the life of the 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 takes place)

Typically, bacteria reproduce by dividing in two. After cell elongation, a transverse septum is gradually formed, which is laid in the direction from the outside to the inside, then the daughter cells diverge or remain connected into characteristic groups - chains, packages, etc. The bacterium - Escherichia coli doubles its number every 20 minutes.

Bacteria are characterized by sporulation. It begins with the detachment of a 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 multilayer, grows around the spore. In bacteria, the sexual process is observed in the form of the exchange of genetic information between two cells. The sexual process increases the hereditary variability of microorganisms.

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

Lesson type: combined.

Methods: verbal, visual, practical, problem-search.

Lesson Objectives

Educational: to deepen students' knowledge of the structure of eukaryotic cells, to teach how to apply them in practical classes.

Developing: to improve the ability of students to work with didactic material; develop students' thinking by offering tasks for comparing prokaryotic and eukaryotic cells, plant cells and animal cells with the identification of similar and distinctive features.

Equipment: poster "The structure of the cytoplasmic membrane"; task cards; handout (the structure of a prokaryotic cell, a typical plant cell, the structure of an animal cell).

Intersubject communications: botany, zoology, human anatomy and physiology.

Lesson plan

I. Organizational moment

Check readiness for the lesson.
Checking the list of students.
Presentation of the topic and objectives of the lesson.

II. Learning new material

Division of organisms into pro- and eukaryotes

The shape of the cells is extremely diverse: some are rounded, others look like stars with many rays, others are elongated, etc. Cells are also different in size - from the smallest, hardly distinguishable in a light microscope, to those perfectly visible to the naked eye (for example, fish and frog eggs).

Any unfertilized egg, including giant fossilized dinosaur eggs that are kept in paleontological museums, were also once living cells. However, if we talk about the main elements of the internal structure, all cells are similar to each other.

prokaryotes (from lat. pro- before, before, instead of and Greek. karyon- nucleus) - these are organisms whose cells do not have a nucleus limited by a membrane, i.e. all bacteria, including archaebacteria and cyanobacteria. The total number of species of prokaryotes is about 6000. All the genetic information of a prokaryotic cell (genophore) is contained in a single circular DNA molecule. Mitochondria and chloroplasts are absent, and the functions of respiration or photosynthesis, which provide the cell with energy, are performed by the plasma membrane (Fig. 1). Prokaryotes reproduce without a pronounced sexual process by dividing in two. Prokaryotes are able to carry out a number of specific physiological processes: they fix molecular nitrogen, carry out lactic acid fermentation, decompose wood, and oxidize sulfur and iron.

After an introductory conversation, students consider the structure of a prokaryotic cell, comparing the main features of the structure with the types of eukaryotic cells (Fig. 1).

eukaryotes - These are higher organisms that have a clearly defined nucleus, which is separated from the cytoplasm by a membrane (karyomembrane). Eukaryotes include all higher animals and plants, as well as unicellular and multicellular algae, fungi and protozoa. Nuclear DNA in eukaryotes is enclosed in chromosomes. Eukaryotes have cellular organelles limited by membranes.

Differences between eukaryotes and prokaryotes

- Eukaryotes have a real nucleus: the genetic apparatus of a eukaryotic cell is protected by a shell similar to the shell of the cell itself.
– Organelles included in the cytoplasm are surrounded by a membrane.

The structure of plant and animal cells

The cell of any organism is a system. It consists of three interconnected parts: membrane, nucleus and cytoplasm.

In the study of botany, zoology and human anatomy, you have already become familiar with the structure of various types of cells. Let's briefly review this article.

Exercise 1. Determine from Figure 2 which organisms and tissue types correspond to the cells under the numbers 1-12. What is the reason for their shape?

The structure and functions of organelles of plant and animal cells

Using figures 3 and 4 and using the Biological Encyclopedic Dictionary and textbook, students complete the table comparing animal and plant cells.

Table. The structure and functions of organelles of plant and animal cells

cell organelles	The structure of organelles	Function	Presence of organelles in cells
			plants	animals
Chloroplast	It is a type of plastid	Colors plants green for photosynthesis
leukoplast	The shell consists of two elementary membranes; internal, growing into the stroma, forms a few thylakoids	Synthesizes and accumulates starch, oils, proteins
Chromoplast	Plastids with yellow, orange and red color, the color is due to pigments - carotenoids	Red, yellow color of autumn leaves, juicy fruits, etc.
	Occupies up to 90% of the volume of a mature cell, filled with cell sap	Maintenance of turgor, accumulation of reserve substances and metabolic products, regulation of osmotic pressure, etc.
microtubules	Composed of the protein tubulin, located near the plasma membrane	Participate in the deposition of cellulose on cell walls, the movement of various organelles in the cytoplasm. During cell division, microtubules form the basis of the division spindle structure.
Plasma membrane (CPM)	Consists of a lipid bilayer permeated with proteins immersed to various depths	Barrier, transport of substances, communication between cells
Smooth EPR	System of flat and branching tubules	Carries out the synthesis and release of lipids
Rough EPR	It got its name because of the many ribosomes on its surface.	Synthesis of proteins, their accumulation and transformation for release from the cell to the outside
	Surrounded by a double nuclear membrane with pores. The outer nuclear membrane forms a continuous structure with the ER membrane. Contains one or more nucleoli	Carrier of hereditary information, center of regulation of cell activity
cell wall	Composed of long cellulose molecules arranged in bundles called microfibrils	Outer frame, protective shell
Plasmodesmata	Tiny cytoplasmic channels that pierce cell walls	Unite protoplasts of neighboring cells
Mitochondria		ATP synthesis (energy storage)
golgi apparatus	Consists of a stack of flat sacs - cisterns, or dictyosomes	Synthesis of polysaccharides, formation of CPM and lysosomes
Lysosomes		intracellular digestion
Ribosomes	Composed of two unequal subunits large and small, into which they can dissociate	Site of protein biosynthesis
Cytoplasm	Consists of water with a large amount of dissolved substances containing glucose, proteins and ions	It contains other organelles of the cell and all processes of cellular metabolism are carried out.
Microfilaments	Actin fibers are usually arranged in bundles near the surface of cells	Involved in cell motility and reshaping
Centrioles	May be part of the mitotic apparatus of the cell. A diploid cell contains two pairs of centrioles	Participate in the process of cell division in animals; in zoospores of algae, mosses and in protozoa they form basal bodies of cilia
microvilli	protrusions of the plasma membrane	Increase the outer surface of the cell, microvilli together form the border of the cell

conclusions

1. The cell wall, plastids and the central vacuole are inherent only in plant cells.
2. Lysosomes, centrioles, microvilli are present mainly only in the cells of animal organisms.
3. All other organelles are characteristic of both plant and animal cells.

The structure of the cell membrane

The cell membrane is located outside the cell, delimiting the latter from the external or internal environment of the body. It is based on the plasmalemma (cell membrane) and the carbohydrate-protein component.

Cell wall functions:

- maintains the shape of the cell and gives mechanical strength to the cell and the organism as a whole;
- protects the cell from mechanical damage and the ingress of harmful compounds into it;
- performs recognition of molecular signals;
- regulates the exchange of substances between the cell and the environment;
- carries out intercellular interaction in a multicellular organism.

Cell wall function:

- represents an external frame - a protective shell;
- provides transport of substances (water, salts, molecules of many organic substances pass through the cell wall).

The outer layer of animal cells, unlike the cell walls of plants, is very thin and elastic. It is not visible under a light microscope and consists of a variety of polysaccharides and proteins. The surface layer of animal cells is called glycocalyx, performs the function of a direct connection of animal cells with the external environment, with all the substances surrounding it, does not play a supporting role.

Under the glycocalyx of the animal and cell wall of the plant cell, there is a plasma membrane that borders directly on the cytoplasm. The plasma membrane contains proteins and lipids. They are arranged in an orderly manner due to various chemical interactions with each other. Lipid molecules in the plasma membrane are arranged in two rows and form a continuous lipid bilayer. Protein molecules do not form a continuous layer, they are located in the lipid layer, plunging into it at different depths. Molecules of proteins and lipids are mobile.

Functions of the plasma membrane:

- forms a barrier that separates the internal contents of the cell from the external environment;
- provides transport of substances;
- provides communication between cells in the tissues of multicellular organisms.

Entry of substances into the cell

The surface of the cell is not continuous. In the cytoplasmic membrane there are numerous tiny holes - pores through which, with or without the help of special proteins, ions and small molecules can penetrate into the cell. In addition, some ions and small molecules can enter the cell directly through the membrane. The entry of the most important ions and molecules into the cell is not passive diffusion, but active transport, which requires energy. Transport of substances is selective. The selective permeability of the cell membrane is called semipermeability.

way phagocytosis inside the cell enter: large molecules of organic substances, such as proteins, polysaccharides, food particles, bacteria. Phagocytosis is carried out with the participation of the plasma membrane. In the place where the surface of the cell comes into contact with a particle of some dense substance, the membrane flexes, forms a depression and surrounds the particle, which in the "membrane capsule" is immersed inside the cell. A digestive vacuole is formed, and organic substances that have entered the cell are digested in it.

By phagocytosis, amoeba, ciliates, animal and human leukocytes feed. Leukocytes absorb bacteria, as well as a variety of solid particles that accidentally enter the body, thus protecting it from pathogenic bacteria. The cell wall of plants, bacteria and blue-green algae prevents phagocytosis, and therefore this pathway of substances entering the cell is not realized in them.

Liquid droplets containing various substances in a dissolved and suspended state also penetrate into the cell through the plasma membrane. This phenomenon was called pinocytosis. The process of fluid absorption is similar to phagocytosis. A drop of liquid is immersed in the cytoplasm in a "membrane package". Organic substances that enter the cell along with water begin to be digested under the influence of enzymes contained in the cytoplasm. Pinocytosis is widespread in nature and is carried out by the cells of all animals.

III. Consolidation of the studied material

What two large groups are all organisms divided into according to the structure of the nucleus?
What organelles are found only in plant cells?
What organelles are found only in animal cells?
What is the difference between the structure of the cell wall of plants and animals?
What are the two ways substances enter the cell?
What is the importance of phagocytosis for animals?

There are only two types of organisms on Earth: eukaryotes and prokaryotes. They differ greatly in their structure, origin and evolutionary development, which will be discussed in detail below.

In contact with

Signs of a prokaryotic cell

Prokaryotes are otherwise called pre-nuclear. A prokaryotic cell does not have other organelles that have a membrane sheath (, endoplasmic reticulum, Golgi complex).

They also have the following features:

1. without a shell and does not form bonds with proteins. Information is transmitted and read continuously.
2. All prokaryotes are haploid organisms.
3. Enzymes are located in a free state (diffusely).
4. They have the ability to sporulate under adverse conditions.
5. The presence of plasmids - small extrachromosomal DNA molecules. Their function is the transfer of genetic information, increasing resistance to many aggressive factors.
6. The presence of flagella and pili - external protein formations necessary for movement.
7. Gas vacuoles are cavities. Due to them, the body is able to move in the water column.
8. The cell wall in prokaryotes (specifically bacteria) consists of murein.
9. The main methods of obtaining energy in prokaryotes are chemo- and photosynthesis.

These include bacteria and archaea. Examples of prokaryotes: spirochetes, proteobacteria, cyanobacteria, krenarcheotes.

Attention! Despite the fact that prokaryotes lack a nucleus, they have its equivalent - a nucleoid (a circular DNA molecule devoid of shells), and free DNA in the form of plasmids.

The structure of a prokaryotic cell

bacteria

Representatives of this kingdom are among the most ancient inhabitants of the Earth and have a high survival rate in extreme conditions.

There are gram-positive and gram-negative bacteria. Their main difference lies in the structure of the cell membrane. Gram-positive have a thicker shell, up to 80% consists of a murein base, as well as polysaccharides and polypeptides. When stained by Gram they give a purple color. Most of these bacteria are pathogens. Gram-negative ones have a thinner wall, which is separated from the membrane by the periplasmic space. However, such a shell has increased strength and is much more resistant to the effects of antibodies.

Bacteria play a very important role in nature:

1. Cyanobacteria (blue-green algae) help maintain the right level of oxygen in the atmosphere. They form more than half of all O2 on Earth.
2. They contribute to the decomposition of organic remains, thereby taking part in the cycle of all substances, participate in the formation of soil.
3. Nitrogen fixers on the roots of legumes.
4. They purify water from waste, for example, the metallurgical industry.
5. They are part of the microflora of living organisms, helping to absorb nutrients as much as possible.
6. They are used in the food industry for fermentation. This is how cheeses, cottage cheese, alcohol, and dough are obtained.

Attention! In addition to the positive value, bacteria also play a negative role. Many of them cause deadly diseases such as cholera, typhoid fever, syphilis, and tuberculosis.

bacteria

Archaea

Previously, they were combined with bacteria into a single kingdom of Drobyanok. However, over time, it became clear that archaea have their own individual evolutionary path and are very different from other microorganisms in their biochemical composition and metabolism. Up to 5 types are distinguished, the most studied are Euryarchaeots and Crenarchaeotes. Archaeal features are:

• most of them are chemoautotrophs - they synthesize organic substances from carbon dioxide, sugar, ammonia, metal ions and hydrogen;
• play a key role in the nitrogen and carbon cycle;
• participate in digestion in humans and many ruminants;
• have a more stable and durable membrane shell due to the presence of ether bonds in glycerol-ether lipids. This allows archaea to live in highly alkaline or acidic environments, as well as under conditions of high temperatures;
• the cell wall, unlike bacteria, does not contain peptidoglycan and consists of pseudomurein.

The structure of eukaryotes

Eukaryotes are a kingdom of organisms whose cells contain a nucleus. In addition to archaea and bacteria, all living things on Earth are eukaryotes (for example, plants, protozoa, animals). Cells can vary greatly in their shape, structure, size, and function. Despite this, they are similar in the basics of life, metabolism, growth, development, ability to irritate and variability.

Eukaryotic cells can be hundreds or thousands of times larger than prokaryotic cells. They include the nucleus and cytoplasm with numerous membranous and non-membrane organelles. Membrane include: endoplasmic reticulum, lysosomes, Golgi complex, mitochondria,. Non-membrane: ribosomes, cell center, microtubules, microfilaments.

The structure of eukaryotes

Let us compare eukaryotic cells from different kingdoms.

The kingdoms of eukaryotes include:

• protozoa. Heterotrophs, some capable of photosynthesis (algae). They reproduce asexually, sexually and in a simple way into two parts. Most do not have a cell wall;
• plants. They are producers, the main way to obtain energy is photosynthesis. Most plants are immobile and reproduce asexually, sexually and vegetatively. The cell wall is made up of cellulose;
• mushrooms. Multicellular. Distinguish between lower and higher. They are heterotrophic organisms and cannot move independently. They reproduce asexually, sexually and vegetatively. They store glycogen and have a strong chitin cell wall;
• animals. There are 10 types: sponges, worms, arthropods, echinoderms, chordates and others. They are heterotrophic organisms. Capable of independent movement. The main storage substance is glycogen. The cell wall is made up of chitin, just like in fungi. The main mode of reproduction is sexual.

Table: Comparative characteristics of plant and animal cells

Structure	plant cell	animal cage
cell wall	Cellulose	Consists of glycocalyx - a thin layer of proteins, carbohydrates and lipids.
Core location	Located closer to the wall	Located in the central part
Cell Center	Exclusively in lower algae	Present
Vacuoles	Contains cell sap	Contractile and digestive.
Spare substance	Starch	Glycogen
plastids	Three types: chloroplasts, chromoplasts, leucoplasts	Missing
Nutrition	autotrophic	heterotrophic

Comparison of prokaryotes and eukaryotes

The structural features of prokaryotic and eukaryotic cells are significant, but one of the main differences concerns the storage of genetic material and the way energy is obtained.

Prokaryotes and eukaryotes photosynthesize differently. In prokaryotes, this process takes place on membrane outgrowths (chromatophores) stacked in separate piles. Bacteria do not have a fluorine photosystem, therefore they do not release oxygen, unlike blue-green algae, which form it during photolysis. The sources of hydrogen in prokaryotes are hydrogen sulfide, H2, various organic substances and water. The main pigments are bacteriochlorophyll (in bacteria), chlorophyll and phycobilins (in cyanobacteria).

Of all the eukaryotes, only plants are capable of photosynthesis. They have special formations - chloroplasts containing membranes laid in grana or lamellae. The presence of photosystem II allows oxygen to be released into the atmosphere during the process of water photolysis. The only source of hydrogen molecules is water. The main pigment is chlorophyll, and phycobilins are present only in red algae.

The main differences and characteristic features of prokaryotes and eukaryotes are presented in the table below.

Table: Similarities and differences between prokaryotes and eukaryotes

Comparison	prokaryotes	eukaryotes
Appearance time	Over 3.5 billion years	About 1.2 billion years
Cell sizes	Up to 10 µm	10 to 100 µm
Capsule	Eat. Performs a protective function. Associated with the cell wall	Absent
plasma membrane	Eat	Eat
cell wall	Composed of pectin or murein	There are other than animals
Chromosomes	Instead, circular DNA. Translation and transcription take place in the cytoplasm.	Linear DNA molecules. Translation takes place in the cytoplasm, while transcription takes place in the nucleus.
Ribosomes	Small 70S-type. Located in the cytoplasm.	Large 80S-type, can be attached to the endoplasmic reticulum, located in plastids and mitochondria.
membranous organelle	None. There are outgrowths of the membrane - mesosomes	There are: mitochondria, Golgi complex, cell center, EPS
Cytoplasm	Eat	Eat
	Missing	Eat
Vacuoles	Gas (aerosomes)	Eat
Chloroplasts	None. Photosynthesis takes place in bacteriochlorophylls	Present only in plants
Plasmids	Eat	Missing
Core	Absent	Eat
Microfilaments and microtubules.	Missing	Eat
Division methods	Constriction, budding, conjugation	Mitosis, meiosis
Interaction or contacts	Missing	Plasmodesmata, desmosomes or septa
Types of cell nutrition	Photoautotrophic, photoheterotrophic, chemoautotrophic, chemoheterotrophic	Phototrophic (in plants) endocytosis and phagocytosis (in others)

Differences between prokaryotes and eukaryotes

Similarities and differences between prokaryotic and eukaryotic cells

Conclusion

Comparison of a prokaryotic and eukaryotic organism is a rather laborious process that requires consideration of many nuances. They have much in common with each other in terms of structure, ongoing processes and properties of all living things. The differences lie in the functions performed, the methods of nutrition and internal organization. Those who are interested in this topic can use this information.

The prokaryotic cell is much simpler than the cells of animals and plants. Outside, it is covered with a cell wall that performs protective, shaping 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 by plasma membrane. Mesosomes involved in the process of respiration, bacteriochlorophyll and other pigments are found in sac-like, tubular or lamellar invaginations of the membrane. 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 base pairs. The genome of a bacterial cell is much simpler than that of cells of more advanced creatures: on average, bacterial DNA contains several thousand genes.

Not found in prokaryotic cells endoplasmic reticulum, A ribosomes float freely in the cytoplasm. Not in prokaryotes mitochondria; part of their function is performed by the cell membrane.

prokaryotes

Bacteria are the smallest of the organisms with a cellular structure; their sizes range from 0.1 to 10 µm. 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 part of microbiology that deals with bacteria is called bacteriology . The beginning of this science laid Anthony van Leeuwenhoek in the 17th century.

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

Due to the possibility of gene exchange between representatives of different species and even genera, it is rather difficult to systematize prokaryotes. Satisfactory taxonomy of prokaryotes has not yet been built; all existing systems are artificial and classify bacteria according to some group of characters, without taking into account their phylogenetic relationship. Previously, bacteria along with mushrooms And algae included in the sub-kingdom of lower plants. Bacteria are currently classified as a separate kingdom of prokaryotes. The most common classification system is Bergi system based on the structure of the cell wall.

At the end of the 20th century, scientists discovered that the cells of a relatively little-studied group of bacteria - archaebacteria - contain rRNA, which differ in their structure from both prokaryotic rRNA and eukaryotic rRNA. The structure of the genetic apparatus of archaebacteria (the presence introns and repeating sequences processing, form ribosome) brings them closer to eukaryotes; on the other hand, archaebacteria also have typical signs of prokaryotes (absence of a nucleus in the cell, the presence of flagella, plasmids and gas vacuoles, rRNA size, nitrogen fixation). Finally, archaebacteria differ from all other organisms in the structure of the cell wall, the type of photosynthesis, and some other features. Archaebacteria are able to exist 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, while others use sulfur compounds for energy.

Apparently, archaebacteria are a very ancient group of organisms; "extreme" possibilities testify to the conditions characteristic of the Earth's surface in archean era. It is believed that archaebacteria are closest to the hypothetical "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. Studies of molecular genetics have forced a new look at the theory of the origin of eukaryotes. It is currently believed that three different branches of prokaryotes evolved simultaneously on the ancient Earth - archaebacteria, eubacteria and urcaryotes , characterized by a different structure and different ways of obtaining energy. Urkaryotes, which were, in fact, 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 allocated earlier 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 kingdom of Archaea.

The classification of true bacteria is given in scheme.

IN bacterial cell there is no nucleus, the chromosomes are freely located in the cytoplasm. In addition, there are no membrane organelles in the bacterial cell: mitochondria, EPS, golgi apparatus etc. Outside, the cell membrane is covered with a cell wall.

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

Bacteria are divided into several groups according to their shape:

cocci (have a rounded shape);

bacilli (have a rod-shaped form);

spirilla (have the shape of a spiral);

vibrios (have the form of a comma).

According to the mode 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 replicates, each daughter cell receives its own copy of the parent DNA. It is also possible to transfer DNA between non-dividing cells (through the capture of "naked" DNA, with the help of bacteriophages or by conjugations , when bacteria are interconnected 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 register changes in the acidity of the environment and the concentration of various substances: sugars, amino acids, oxygen. Many bacteria react to changes in temperature or light, and some bacteria can sense the Earth's magnetic field.

Under unfavorable conditions, the bacterium is covered with a dense shell, the cytoplasm is dehydrated, and vital activity almost stops. In this state, spores of bacteria can stay for hours in a deep vacuum, endure temperatures from -240 ° C to +100 ° C.

Figure 1 - Image of a prokaryotic cell

Figure 4 - The structure of the flagellum of gram-negative bacteria.
1 - thread; 2 - hook; 3 - basal body; 4 - rod; 5 - L-ring; 6 - P-ring; 7 - S-ring; 8 - M-ring; 9 - CPM; 10 - periplasmic space; 11 - peptidoglycan layer; 12 - outer membrane

The structure of cells of lower prokaryotes is much simpler (Fig. 1). At the same time, the different structure of the nuclear apparatus is not the only feature that distinguishes a eukaryotic cell from a prokaryotic one.

One of the main structural components of the prokaryotic cell is cell wall (Fig. 2, 3). The composition of the cell membrane of bacteria includes complex molecular complexes consisting of proteins, polysaccharides and fat-like substances. Being rigid, it serves as a skeleton of the cell, giving it a certain shape. The cell membrane of prokaryotes forms a kind of barrier to the passage of dissolved substances from the environment into the cell. Cyanobacteria cells are covered with an elastic pectin membrane. In some types of bacteria, a layer of mucus forms on the surface of the cell, forming, as it were, a case - capsule .

Among the surface structures of the cells of many bacteria are flagella - organs of movement, which are long very thin threads, spiral, wavy or curved (Fig. 4).

Figure 3 - The cell wall of gram-negative bacteria (A) and the structure of the lipopolysaccharide molecule (B).
A. Cell wall of gram-negative bacteria 1 - cytoplasmic membrane; 2 - peptidoglycan layer; 3 - periplasmic space; 4 - protein molecules; 5 - phospholipid; 6 - lipopolysaccharide.
B. The structure of the lipopolysaccharide molecule 1 - lipid A; 2 - internal polysaccharide core; 3 - outer polysaccharide core; 4 - O-antigen

The length of the flagella can be many times the length of the body of the bacterium. The number and location of flagella are a characteristic species feature. Some bacterial species have one flagellum ( monotrichous ), in others, the flagella are arranged in bundles at one or both ends of the cell ( lophotrichous ), the third has one flagellum at both ends of the cell ( amphitrichous ), in the fourth, they cover the entire surface of the cell ( peritrichous ).

Closely adjacent to the shell is the cytoplasmic membrane. It has selective permeability - it allows certain substances to enter the cell and removes certain substances from it. Due to this ability, the membrane plays the role of an organelle that concentrates nutrients inside the cell and promotes the excretion of waste products. Inside the cell, there is always an increased osmotic pressure compared to the environment. The cytoplasmic membrane ensures its constancy. In addition, it is the site of localization of a number of enzyme systems, in particular redox enzymes associated with energy production (in eukaryotes they are located in mitochondria). Unlike eukaryotic cells, in a prokaryotic cell there is no division into compartments. Prokaryotic cells do not have either the Golgi complex or mitochondria, nor is there a directed movement of the cytoplasm in them. The phenomena of pinocytosis and phagocytosis are not characteristic of prokaryotes. Of the organelles, only ribosomes are similar to eukaryotic ribosomes.

In many bacterial cells, special membrane structures have been found - mesosomes formed as a result of the retraction of the cytoplasmic membrane into the cell. Their role has not yet been fully elucidated. There are assumptions about the participation of mesosomes in the most important intracellular processes of cell division, the synthesis of substances of the cell membrane, and in energy metabolism.