What structure of bacteria performs the function of movement. Bacterial cell. Features of the structure of a plant cell

According to scientists, bacteria are more than 3.5 billion years old. They existed on Earth long before the appearance of highly organized organisms. Being at the origins of life, bacterial organisms received an elementary structure of the prokaryotic type, characterized by the absence of a formed nucleus and nuclear membrane. One of the factors that influenced the formation of their biological properties is the bacterial membrane (cell wall).

The bacterial wall is designed to perform several fundamental functions:

be the skeleton of a bacterium;
give it a certain shape;
communicate with the external environment;
protect from the harmful effects of environmental factors;
participate in the division of a bacterial cell that does not have a nucleus and a nuclear membrane;
retain antigens and various types of receptors on its surface (typical of gram-negative bacteria).

Certain types of bacteria have an outer capsule, which is durable and serves to maintain the integrity of the microorganism long time. In this case, the bacterial membrane is an intermediate form between the cytoplasm and the capsule. Some bacteria (for example, leuconostoc) have the peculiarity of enclosing several cells in one capsule. This is called zoogel.

Chemical composition capsules are characterized by the presence of polysaccharides and a large amount of water. The capsule may also enable the bacterium to attach to a specific object.

The degree of its absorption by the bacterium depends on how easily the substance penetrates the shell. Molecules with long chain sections that are resistant to biodegradation are more likely to penetrate.

What is a shell?

The bacterial membrane consists of lipopolysaccharides, proteins, lipoproteins, and teichoic acids. The main component is murein (peptidoglycan).

The thickness of the cell wall can be different and reach 80 nm. The surface is not continuous, it has pores of various diameters, through which the microbe receives nutrients and releases the products of its vital activity.

The importance of the outer wall is evidenced by its significant weight - it can range from 10 to 50% of the dry mass of the entire bacterium. The cytoplasm can protrude, changing external relief bacteria.

The top of the shell can be covered with cilia or it can have flagella, which consist of flagellin, a specific protein substance. For attachment to the bacterial membrane, flagella have special structures - flat discs. Bacteria with one flagellum are called monotrichous, those with two are called amphitrichous, those with a tuft are called lophotrichous, and those with many tufts are called peritrichous. Microorganisms without flagella are called atrichia.

The cell membrane has an inner part that begins to form after the cell has completed growth. Unlike the outer one, it consists of a much smaller amount of water and has greater elasticity and strength.

The process of microbial wall synthesis begins inside the bacterium. To do this, it contains a network of polysaccharide complexes that alternate in a certain sequence (acetylglucosamine and acetylmuramic acid) and are linked to each other by strong peptide bonds. The assembly of the wall is carried out externally, on the plasma membrane, where the shell is located.

Since a bacterium does not have a nucleus, it does not have a nuclear membrane.

The shell is an unstained thin structure that cannot even be seen without special staining of the cells. For this purpose, plasmolysis and a darkened field of view are used.

Gram stain

To study the detailed structure of the cell in 1884, Christian Gram proposed special way her coloring, which was later named after him. Gram stain divides all microorganisms into gram-positive and gram-negative. Each species has its own biochemical and biological properties. The different colors are also due to the structure of the cell wall:

Gram-positive bacteria have a massive shell that includes polysaccharides, proteins and lipids. It is durable, the pores are minimal, the paint used for painting penetrates tightly and is practically not washed out. Such microorganisms acquire a blue-violet color.
Gram-negative bacterial cells have certain differences: their wall thickness is less, but the shell has two layers. The inner layer consists of peptidoglycan, which has a looser structure and wide pores. Gram staining paint is easily washed out with ethanol. The cell becomes discolored. In the future, the technique involves adding a contrasting red dye, which colors the bacteria red or pink.

The proportion of gram-positive microbes, harmless to humans, far exceeds gram-negative ones. To date, three groups of gram-negative microorganisms that cause diseases in humans have been classified:

cocci (streptococci and staphylococci);
non-spore-forming forms (corynebacteria and listeria);
spore-forming forms (bacillus, clostridia).

Characteristics of the periplasmic space

Between the bacterial wall and the cytoplasmic membrane there is a periplasmic space, which consists of enzymes. This component is an obligatory structure; it makes up 10-12% of the dry mass of the bacterium. If the membrane is destroyed for some reason, the cell dies. Genetic information is located directly in the cytoplasm and is not separated from it by the nuclear membrane.

Regardless of whether the microbe is gram-positive or gram-negative, it is the osmotic barrier of the microorganism, a transporter of organic and inorganic molecules deep into the cell. A certain role of the periplasm in the growth of the microorganism has also been proven.

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The bacterial cell as a whole is structured quite simply. It is separated from the external environment by a cytoplasmic membrane and filled with cytoplasm, in which the nucleoid zone is located, including a circular DNA molecule from which the transcribed mRNA can “hang”, to which, in turn, ribosomes are attached, synthesizing protein on its matrix simultaneously with the process of synthesis itself matrices. At the same time, DNA can be associated with proteins that carry out its replication and repair. Bacterial ribosomes are smaller than eukaryotic ones and have a sedimentation coefficient of 70S. They, like eukaryotic ones, are formed by two subunits - small (30S), which includes 16S rRNA, and large - 50S, which includes 23S and 5S rRNA molecules.

The photograph obtained using transmission microscopy (Fig. 1) clearly shows a light zone in which the genetic apparatus is located and the processes of transcription and translation occur. Ribosomes are visible as small granular inclusions.

Most often, in a bacterial cell, the genome is represented by only one DNA molecule, which is closed in a ring, but there are exceptions. Some bacteria may have several DNA molecules. For example, Deinococus radiodurans, a bacterium known for its phenomenal radiation resistance and the ability to comfortably withstand radiation doses 2,000 times the lethal dose for humans, has two copies of its genomic DNA. Bacteria are known to have three or four copies. In some species the DNA may not be closed in a circle, and some Agrobacterium contain one circular and one linear DNA.

In addition to the nucleoid, genetic material can be presented in the cell in the form of additional small circular DNA molecules - plasmids. Plasmids replicate independently of the nucleoid and often contain genes useful to the cell, giving the cell, for example, resistance to antibiotics, the ability to assimilate new substrates, the ability to conjugate, and much more. Plasmids can be transferred both from mother cell to daughter cell, and by horizontal transfer they can be transferred from one cell to another.

A bacterial cell is most often surrounded not only by a membrane, but also by a cell wall, and according to the type of cell wall structure, bacteria are divided into two groups - gram-positive and gram-negative.

The cell wall of bacteria is formed by peptidoglycan - murein. On molecular level The murein layer is a network formed by molecules of N-acetylglucosamine and N-acetylmuramic acid, linked together into long chains by β-1-4-glycosidic bonds, neighboring chains, in turn, are connected by cross peptide bridges (Fig. 2). This creates one large network surrounding the cell.

Gram-positive bacteria have a thick cell wall located on top of the membrane. Murein is cross-linked with another type of molecules - teichoic and lipoteichoic (if they are connected to membrane lipids) acids. It is believed that these molecules give the cell wall elasticity during lateral compression and stretching, acting like springs. Because the murein layer is thick, it stains easily with the Gram stain: the cells appear bright purple because the dye (Gentian or methyl violet) is stuck in the cell wall layer.

In gram-negative bacteria, the murein layer is very thin (the exception is cyanobacteria), therefore, when staining with Gram, the violet dye is washed out, and the cells are stained with the color of the second dye (Fig. 3).

The cell wall of gram-negative bacteria is covered on top by another, outer, membrane attached to the peptidoclycan by lipoproteins. The space between the cytoplasmic membrane and the outer membrane is called the periplasm. The outer membrane contains lipopolyproteins, lipopolysaccharides (LPS), as well as proteins that form hydrophilic pores. Components of the outer membrane are often responsible for the interaction of the cell with the external environment. It contains antigens, phage receptors, molecules involved in conjugation, etc.

Since the structure of the integument differs in gram-positive and gram-negative cells (Fig. 4, top), the apparatus that anchors the flagellum in the cell integument also differs (Fig. 4, bottom).

The flagellum of Gram-positive bacteria is anchored in the membrane by two protein rings (S-ring and M-ring) and is driven by a system of proteins that, consuming energy, cause the thread to spin. In gram-negative bacteria, in addition to this structure, there are two more rings that additionally fix the flagellum in the outer membrane and cell wall.

The flagellum itself in bacteria consists of the protein flagellin, the subunits of which are connected into a helix that has a cavity inside and forms a thread. The thread is flexibly attached to the anchoring and torsion apparatus using a hook.

In addition to flagella, there may be other outgrowths called pili on the surface of bacterial cells. These are protein pili that allow bacteria to attach to various surfaces (increasing the hydrophobicity of the cell) or take part in the transport of metabolites and the conjugation process (F-pili).

A bacterial cell usually does not contain any membrane structures inside, including vesicles, but there may be various types of inclusions (reserve lipids, sulfur) and gas bubbles surrounded by a protein membrane. Without a membrane, the cell can store polysaccharide molecules, cyanophycin (as a nitrogen depot), and can also contain carboxysomes - vesicles containing the RuBisCO enzyme, necessary for the fixation of carbon dioxide in the Calvin Cycle.

In microbiology, this term means a nutrient that can be absorbed by a microorganism

This name of the groups comes from the surname of the doctor G.K. Gram, who developed a method for staining bacterial cell walls, which makes it possible to distinguish cells with different types of cell wall structure.

Ribulose bisphosphate carboxylase/oxygenase

Bacteria (grass)

Shape and structure of bacterial cells

Bacteria are the most ancient group living organisms having sizes most often not exceeding 0.5 microns. Their structure can only be examined under an electron microscope (Fig. 2.1). Bacteria do not have mitochondria, lysosomes, or the Golgi complex. endoplasmic reticulum. They have no plastids, no formed nucleus, and the nuclear substance (DNA) is represented by one ring-shaped chromosome (nucleoid) located directly in the cytoplasm, but at one point attached to the cytoplasmic membrane. The cytoplasm contains many ribosomes, in which protein synthesis occurs intensively. Most bacteria are colorless, but some are green or purple. Bacteria are the most common organisms in nature; they are classified as prokaryotes, i.e. prenuclear organisms.

Rice. 2.1. Bacterium

The shape of bacteria is varied. Some of them look like single balls - cocci, which can form pairs - diplococci, four - tetracocci, and form chains - streptococci. Clusters of cocci look like packets - sarcina or grape bunches - staphylococci. Some bacteria are elongated in the form of rods - bacilli, others are curved in the shape of a comma - vibrios, or several times along the entire length - spirilla (Fig. 2.2).

Rice. 2.2. Bacterial cell shapes:

1 – cocci; 2, 3 – diplococci; 4 – streptococci; 5 – tetracocci; 6 – staphylococci; 7 – sarcins; 8, 9 – bacilli; 10 – chains of bacilli; 11 – vibrios; 12 – spirilla; 13 – flagellated, 14 – ciliated

Many bacteria have movement organoids- one or more flagella. Bacteria that do not have flagella, but are covered on the outside with mucus, are also capable of gliding movement. Some aquatic and soil bacteria, particularly cyanobacteria, can rise and fall by regulating the amount of gas in gas vacuoles present in the cytoplasm.

The bacterial cell is covered with a membrane consisting of cytoplasmic membrane And cell wall(Fig. 2.3). The membrane is composed of proteins and lipids. It is semi-permeable and ensures the selective entry of substances into the cell and the release of breakdown products into environment. On the surface of the invaginations of the cytoplasmic membrane into the bacterium, called mesosomes, there are oxidative enzymes involved in the respiration process. Such membrane invaginations play the role of mitochondria and some other organelles that are absent in the bacterial cell. In bacteria capable of photosynthesis (cyanobacteria, green bacteria, etc.), photosynthetic pigments are localized on mesosomes.

Rice. 2.3. Diagram of the structure of a bacterial cell:

1 – ribosomes; 2 – cell membrane; 3 – mucous capsule; 4 – nucleoid; 5 – cell wall; 6 – flagellum; 7 – mesosoma

The cell wall is also permeable to nutrients and waste products. It has a strong lattice of mureins (peptidoglycans), gives the bacterium a certain shape and protects it from environmental influences. In some bacteria, the cytoplasmic membrane and cell wall take part in the formation of another, outer layer of the membrane - capsule. A capsule is a semi-liquid mucous mass that covers the outside of the cell wall. It performs a protective function.

Structure of a bacterial cell

The cytoplasm of most bacteria is surrounded by membranes: a cell wall, a cytoplasmic membrane and a capsular (mucous) layer. These structures take part in metabolism; food products enter through the cell membranes and metabolic products are removed. They protect the cell from action harmful factors environment largely determine the surface properties of the cell (surface tension, electrical charge, osmotic state, etc.). These structures in a living bacterial cell are in constant functional interaction.

Cell wall. A bacterial cell is separated from the external environment by a cell wall. Its thickness is 10-20 nm, its mass reaches 20-50% of the cell mass. This is a complex multifunctional system that determines the constancy of the cell’s shape, its surface charge, anatomical integrity, the ability to adsorb phages, participation in immune reactions, contact with the external environment and protection from adverse external influences. The cell wall has elasticity and sufficient strength and can withstand intracellular pressure of 1-2 MPa.

The main components of the cell wall are peptidoglycans(glycopeptides, mucopeptides, mureins, glycosaminopeptides), which are found only in prokaryotes. A specific heteropolymer of peptidoglycan consists of alternating residues of N-acetylglucosamine and N-acetylmuramic acid, interconnected through β-1-4-glycosidic bonds, diaminopimelic acid (DAP), D-glutamic acid, L- and D-alanine in the ratio 1:1:1:1:2. The glycosidic and peptide bonds that hold peptidoglycan subunits together give them a molecular network or sac structure. The murein network of the prokaryotic cell wall also includes teichoic acids, polypeptides, lipopolysaccharides, lipoproteins, etc. The cell wall has rigidity; it is this property that determines the shape of the bacterial wall. The cell wall has tiny pores through which metabolic products are transported.

Gram stain. Most bacteria are divided into two groups depending on their chemical composition. This property was first noticed in 1884 by the Danish physicist H. Gram. The essence is that when staining bacteria with gentian violet (crystal violet, methyl violet, etc.), in some bacteria the paint with iodine forms a compound that is retained by the cells when they are treated with alcohol. Such bacteria are colored blue-violet and are called gram-positive (Gr +). Discolored bacteria are gram-negative (Gr -), they are stained with contrasting paint (magenta). Gram staining is diagnostic, but only for prokaryotes that have a cell wall.

In structure and chemical composition, gram-positive bacteria differ significantly from gram-negative ones. In gram-positive bacteria, the cell wall is thicker, homogeneous, amorphous, and contains a lot of murein, which is associated with teichoic acids. In gram-negative bacteria, the cell wall is thinner, layered, contains little murein (5-10%), and there are no teichoic acids.

Table 1.1 Chemical composition of Gr+ and Gr- bacteria

We cannot even imagine how many microorganisms constantly surround us. By holding the handrail on the bus, you have already planted about one hundred thousand bacteria on your hand; by going into a public toilet, you, again, have rewarded yourself with these microorganisms. Bacteria always and everywhere accompany humans. But there is no need to react negatively to this word, because bacteria are not only pathogenic, but also beneficial to the body.

Scientists were very surprised when they realized that some bacteria have retained their appearance for approximately a billion years. Such microorganisms were even compared to a Volkswagen car - the appearance of one of their models has not changed for 40 years, having an ideal shape.

Bacteria were among the first to appear on Earth, so they can deservedly be called long-livers. An interesting fact is that these cells do not have a formed nucleus, which is why to this day they attract a lot of attention to their structure.

What are bacteria?

Bacteria are microscopic organisms of plant origin. The structure of a bacterial cell (table, diagrams exist for a clear understanding of the types of these cells) depends on its purpose.

These cells are ubiquitous because they can multiply quickly. There is scientific evidence that in just six hours one cell can produce offspring of 250 thousand bacteria. These single-celled organisms come in many varieties that vary in shape.

Bacteria are very tenacious organisms; their spores can retain the ability to live for 30-40 years. These spores are transported by blowing wind, flowing water, and other means. Viability is maintained up to a temperature of 100 degrees and with slight frost. And yet, what structure does a bacterial cell have? The table describes the main components of bacteria; the functions of other organelles are outlined below.

Globular (cocci) bacteria

They are pathogenic in nature. Cocci are divided into groups depending on their location to each other:

Micrococci (small). Division occurs in one plane. Arrangement in a chaotic single order. Eat ready-made organic compounds, but do not depend on other organisms (saprophytes).
Diplococci (double). They divide in the same plane as micrococci, but form paired cells. Outwardly they resemble beans or lancelet.
Streptococci (in the form of a chain). The division is the same, but the cells are connected to each other and look like beads.
Staphylococcus (grape bunch). This species divides in several planes, producing a cluster of grape-like cells.
Tetracocci (four). Cells divide in two perpendicular planes, forming tetrads.
Sarcinas (ligament). Such cells divide in three planes, which are mutually perpendicular to each other. Moreover, in appearance they look like bags or bales, consisting of many individuals of an even number.

Cylindrical (rod) bacteria

Rods that form spores are divided into clostridia and bacilli. In size, these bacteria are short and very short. The end sections of the sticks are rounded, thickened or cut off. Depending on the location of the bacteria, several groups are distinguished: mono-, diplo- and streptobacteria.

Spiral-shaped (convoluted) bacteria

These microscopic cells come in two types:

Vibrios (with a single bend or generally straight).
Spirilla (large in size, but few curls).

Filamentous bacteria. There are two groups of such forms:

Temporary threads.
Permanent threads.

The structural features of a bacterial cell are that during its existence it is capable of changing shape, but polymorphism is not inherited. Various factors act on the cell during metabolism in the body, as a result of which quantitative changes in its appearance are observed. But as soon as the external action stops, the cell will take on its previous image. What are the structural features of a bacterial cell can be revealed by examining it using a microscope.

Structure of a bacterial cell, membrane

The shell gives and maintains the shape of the cell and protects the internal components from damage. Due to incomplete permeability, not all substances can enter the cell, which promotes the exchange of low- and high-molecular structures between the external environment and the cell itself. Also in the wall there are various chemical reactions. Using an electron microscope, it is not difficult to study the detailed structure of a bacterial cell.

The shell base contains the polymer murein. Gram-positive bacteria have a single-layer skeleton consisting of murein. Here there are polysaccharide and lipoprotein complexes, phosphates. In gram-negative cells, the murein skeleton has many layers. The outer layer adjacent to the cell wall is the cytoplasmic membrane. It also has certain layers containing proteins with lipids. The main function of the cytoplasmic membrane is to control the penetration of substances into the cell and their removal (osmotic barrier). This is a very important function for cells, as it helps protect cells.

Composition of the cytoplasm

The living semi-liquid substance that fills the cell cavity is called cytoplasm. A large amount of protein, a supply of nutrients (fats and fat-like substances) contains a bacterial cell. A photo taken during a microscope examination clearly shows the constituent parts inside the cytoplasm. The main composition includes ribosomes, arranged in a chaotic order and in large numbers. It also contains mesosomes containing redox enzymes. Due to them, the cell draws energy. The nucleus is presented in the form of a nuclear substance located in chromatin bodies.

Functions of ribosomes in cells

Ribosomes consist of subunits (2) and are nucleoproteins. By connecting with each other, these constituent elements form polysomes or polyribosomes. The main task These inclusions are protein synthesis, which occurs on the basis of genetic information. Sedimentation rate 70S.

Features of the bacterial nucleus

The genetic material (DNA) is located in the unformed nucleus (nucleoid). This nucleus is located in several places in the cytoplasm, being a loose shell. Bacteria that have such a nucleus are called prokaryotes. The nuclear apparatus lacks a membrane, a nucleolus, and a set of chromosomes. And deoxyribonucleic acid is located in it in fibril bundles. The diagram of the structure of a bacterial cell demonstrates in detail the structure of the nuclear apparatus.

Under certain conditions, bacteria may develop mucilaginous membranes. As a result, a capsule is formed. If mucus is very strong, then the bacteria turn into zooglea (general mucous mass).

Bacterial cell capsule

The structure of the bacterial cell has a peculiarity - the presence of a protective capsule consisting of polysaccharides or glycoproteins. Sometimes these capsules are composed of polypeptides or fiber. It is located on top of the cell membrane. The thickness of the capsule can be either thick or thin. Its formation occurs due to the conditions in which the cell finds itself. The main property of the capsule is to protect the bacteria from drying out.

In addition to the protective capsule, the structure of the bacterial cell provides for its motor ability.

Flagella on bacterial cells

Flagella are additional elements that carry out cell movement. They are presented in the form of threads of different lengths, which consist of flagellin. This is a protein that has the ability to contract.

The composition of the flagellum is three-component (filament, hook, basal body). Depending on their attachment and location, several groups of motile bacteria have been identified:

Monotrichs (these cells have 1 flagellum located polarly).
Lophotrichs (flagella in the form of a bundle at one end of the cell).
Amphitrichy (tufts at both ends).

There are many interesting facts about bacteria. So, it has long been proven that a mobile phone contains a huge number of these cells, even on a toilet seat there are fewer of them. Other bacteria allow us to live a quality life - eat, perform certain activities, and free our body from nutrient breakdown products without problems. Bacteria are truly diverse, their functions are multifaceted, but we should not forget about their pathological effect on the body, so it is important to monitor our own hygiene and the cleanliness around us.