Cytoplasmic membranes have a nature. biological membranes
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Depending on the type of bacteria, the cytoplasmic membrane makes up 8–15% of the dry mass of the cell. Its chemical composition is represented by a protein-lipid complex, in which the share of proteins is 50–75%, and the share of lipids is 15–50%. The main lipid component of the membrane is phospholipids. The protein fraction of the cytoplasmic membrane is represented by structural proteins with enzymatic activity. The protein composition of the cytoplasmic membrane is diverse. Thus, the cytoplasmic membrane of Escherichia coli bacteria contains about 120 different proteins. In addition, a small amount of carbohydrates was found in the membranes.
The cytoplasmic membrane of bacteria is generally similar in chemical composition to the membranes of eukaryotic cells, but bacterial membranes are richer in proteins, contain unusual fatty acids, and generally lack sterols.
The fluid-mosaic model developed for eukaryotic membranes is applicable to the structure of the cytoplasmic membrane of bacteria. According to this model, the membrane consists of a lipid bilayer. The hydrophobic “ends” of phospholipid and triglyceride molecules are directed inward, and
hydrophilic “heads” – outward. Protein molecules are embedded in the lipid bilayer. Based on the location and nature of interaction with the lipid bilayer, proteins of the cytoplasmic membrane are divided into peripheral and integral.
Peripheral proteins are associated with the membrane surface and are easily washed out of it when the ionic strength of the solvent changes. Peripheral proteins include NAD H2 dehydrogenases, as well as some proteins included in the ATPase complex, etc.
The ATPase complex is a group of protein subunits arranged in a certain way, in contact with the cytoplasm, periplasmic space and forming a channel through which protons move.
Integral proteins include proteins that are partially or completely immersed in the thickness of the membrane, and sometimes penetrate through it. The connection of integral proteins with lipids is determined mainly by hydrophobic interactions.
Integral membrane proteins of E. coli bacteria include, for example, cytochrome b and iron-sulfur proteins.
The cytoplasmic membrane performs a number of functions essential for the cell:
Maintaining the internal constancy of the cell cytoplasm. This is achieved due to the unique property of the cytoplasmic membrane - its semi-permeability. It is permeable to water and low molecular weight substances, but not permeable to ionized compounds.
Transport of such substances into the cell and exit outside is carried out due to specialized transport systems that are localized in the membrane. Such transport systems function through active transport mechanisms and a system of specific permease enzymes;
Transport of substances into the cell and their removal out;
The electron transport chain and oxidative phosphorylation enzymes are localized in the cytoplasmic membrane;
The cytoplasmic membrane is associated with the synthesis of the cell wall and capsule due to the presence in it of specific carriers for the molecules that form them;
Flagella are attached to the cytoplasmic membrane. The energy supply for flagella is associated with the cytoplasmic membrane.
Mesosomes are invaginations of the cytoplasmic membrane into the cytoplasm. (lamellar (lamellar), vesicular (bubble-shaped) and tubular (tubular))
In the cells of some bacteria, mesosomes of a mixed type are also found: consisting of lamellae, tubes and vesicles. Complexly organized and well-developed mesosomes are characteristic of gram-positive bacteria. In gram-negative bacteria they are much less common and are relatively simply organized. Based on their location in the cell, mesosomes are distinguished, formed in the zone of cell division and the formation of the transverse septum; mesosomes to which the nucleoid is attached; mesosomes formed as a result of invagination of peripheral sections of the cytoplasmic membrane.
Beneath the bacterial cell wall is the cytoplasmic membrane (CPM). It separates the cell contents from the cell wall and is an essential structure of any cell.
The thickness of the bacterial CPM is usually about 6-8 nm. It accounts for up to 15% of the dry mass of the cell. It consists of lipids (15-45%), proteins (45-60%) and a small amount of carbohydrates (about 10%). Lipids are represented by phospholipids - up to 30% of the dry weight of the membrane. Among them, phosphatidyl glycerol and diphosphatidyl glyceride (cardiolipin) predominate - an essential component of the mitochondrial membranes of eukaryotes. Smaller quantities contain phosphatidylinositol and phosphatide yl-
ethanolamine. In addition to phospholipids, various glycolipids, small amounts of carotenoids and quinones were found in the membrane. In the composition of lipids derived from glycerol, fatty acids atypical for membranes were identified - saturated or monounsaturated with 16-18 carbon atoms, as well as acids not found in eukaryotic membranes - cyclopropane and branched fatty acids with 15-17 carbon atoms. The set of fatty acids, as well as the membrane lipids consisting of them, is species specific for prokaryotes.
Membrane lipids are small polar molecules bearing hydrophilic (heads) and hydrophobic (tails) groups. In an aqueous environment, they spontaneously form a closed bimolecular layer - a bilayer. This layer serves as a significant barrier to ions and polar compounds. Organized into a bimolecular layer, lipids form the structural basis of the membrane, maintain mechanical stability and impart hydrophobicity to it.
Proteins make up more than half of the dry mass of the membrane. There are more than 20 different types. Based on differences in the strength of bonds with lipids and location in the membrane, proteins are divided into integral and peripheral. Integral proteins are immersed in the hydrophobic region of the membrane, where they form numerous bonds with the hydrocarbon chains of lipids,
creating lipoprotein complexes. Peripheral proteins are localized on the surface of the hydrophilic layer and are often attached to integral proteins (Fig. 3.14).
Fig.3.14. Structure of the cytoplasmic membrane: 1 - lipids; 2 - glycoproteins; 3 - peripheral proteins; 4 - integral proteins
Based on their functions within membranes, membrane proteins can be divided into two groups: structural and dynamic.
The functions of structural proteins are limited to maintaining the structural integrity of the membrane. They are located on the surface of the hydrophilic lipid layer, acting as a molecular bandage.
Dynamic proteins include proteins that are directly involved in all processes occurring on the membrane. They are divided into three classes: transport, involved in the transport of compounds in and out of the cell; catalytic, performing the functions of enzymes in reactions occurring on the membrane; receptor proteins that specifically bind certain compounds (toxins, hormones) on the outside of the membrane.
Carbohydrates in the membrane are not in a free state, but are interconnected with proteins and lipids into glycoproteins. They're like
As a rule, they are localized only on the outer surface of the membrane and function as recognition receptors for environmental factors.
The cytoplasmic membrane of bacteria, like all other biological membranes, is an asymmetric liquid crystalline structure. The asymmetry is due to differences in the chemical structure of protein molecules and their location in the lipid bilayer of the membrane. Some proteins are located on the surface of the bilayer, others are immersed in its thickness, and others pass through from the inner to the outer surface of the bilayer. The strictly defined orientation of membrane proteins, in turn, is due to the fact that they are synthesized and incorporated into the membrane asymmetrically. The outer and inner surfaces of the membrane also differ in enzymatic activity. Depending on conditions (for example, temperature), the CPM can be in different phase states: liquefied or crystalline. During the transition from one liquid crystalline phase to another, the mobility of the membrane components and its packing density change, which, in turn, leads to a disruption of its functional activity.
Structural organization and functions of the cytoplasmic membrane. To explain the nature and mechanism of the numerous functions of the CPM, the most suitable is the fluid-mosaic model of the organization of biological membranes, proposed by R. Singer and A. Nicholson in 1972. According to this model, membranes are two-dimensional solutions of globular proteins and lipids oriented in a certain way. Lipids form a bilayer in which the hydrophilic “heads” of the molecules face outward, and the hydrophobic “tails” are immersed in the thickness of the membrane, while possessing sufficient flexibility. Membrane lipids and many proteins move freely in the bilayer, but only in the lateral direction (lateral diffusion). In the transverse direction, i.e., from one surface of the membrane to the opposite, proteins cannot move, and lipids move extremely slowly (once every few hours). The reason for the absence or low activity of transverse diffusion appears to be the asymmetric distribution of lipids:
Some lipids are more abundant in the outer part of the bilayer, while others are more abundant in the inner part. The consequence of this is the unequal electron density (conductivity) of the bilayer in the transverse direction.
The CPM is in a liquid crystalline or liquefied state only under certain, so-called
biological temperatures. When the temperature decreases (below the melting point, Tm), the lipids transform into a crystalline state, the degree of viscosity increases until the membrane hardens. The temperature that causes the membrane to harden is determined by the content of unsaturated and
branched fatty acids. The more of them there are in the membrane, the lower the temperature of the transition of lipids from the liquid crystalline state to the crystalline state.
Prokaryotes have the ability to regulate membrane fluidity by changing the number of double bonds and the chain length of fatty acid molecules. Thus, in E. coli, when the environmental temperature decreases from 42° C to 27° C, the ratio of saturated and unsaturated fatty acids in the membrane decreases from 1.6 to 1.0, i.e., the content of unsaturated fatty acids reaches the level of saturated ones. This prevents an increase in viscosity and ensures that cells maintain physiological activity at low temperatures.
The CPM performs numerous vital functions in prokaryotes. They are mainly determined by proteins localized in it, which act as channels, receptors, energy regenerators, enzymes, transport functions and others. The CPM is the main osmotic barrier, which, due to the presence of membrane transport mechanisms, selectively transports substances into the cell and removes metabolic products from it. The selective permeability of the CPM is due to substrate-specific permeases localized in it, which actively transport various organic and mineral substances through the membrane. The CPM contains enzymes for the biosynthesis of membrane lipids and macromolecules that make up the cell wall, outer membrane, and capsule. The CPM is the site of localization of redox enzymes that carry out
electron transport, oxidative and photosynthetic phosphorylation, electrochemical energy generation
transmembrane potential (A// +) and chemical (ATP). CPM
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performs important functions in the biosynthesis and translocation of secreted proteins by gram-negative bacteria. The biosynthesis of these proteins is carried out on ribosomes attached to the CPM. Gram-negative bacteria have special receptor proteins on the CPM that “recognize” signals from the large ribosomal subunit about ribosome attachment and the beginning of protein synthesis. Membrane receptor proteins interact with the large subunit of the ribosome, forming a ribosome-membrane complex on which the synthesis of secreted proteins is carried out. In this way, for example, E. coli synthesizes alkaline phosphatase, Bac. subtilis - a-amylase. The CPM also ensures the transfer of these proteins into the periplasmic space. The CPM plays a great role in the regulation of cell division, replication of chromosomes and plasmids, and the subsequent segregation of these genetic elements between newly formed daughter cells.
All prokaryotes, along with the cytoplasmic membrane, contain its derivatives - intracellular membranes that perform specialized functions. The cytoplasmic membrane is capable of forming all kinds of invaginations (invaginations). These invaginations make up intracellular membranes, which have different lengths, packaging and localization in the cytoplasm. They can be collected into complex balls - lamellar, honeycomb or tubular formations. Less complex membranes take the form of simple loops or tubules of varying lengths. Regardless of the complexity of the organization of intracellular membranes, they are all derivatives of the cytoplasmic membrane. The size of their active surface exceeds that of the cytoplasmic membrane. This gives grounds to judge the great functional activity of these structures in cells.
A particularly rich intracellular membrane apparatus is found in nitrogen-fixing and photosynthetic bacteria, Brucella, and nitrifying bacteria. Photosynthetic bacteria (Rhodospirillum rubrum) have membranes that look like closed bubbles - vesicles. Their formation begins with invagination of the cytoplasmic membrane, which then forms a tube. Constrictions appear on the tube, dividing it into a series of bubbles. These vesicles are called chromatophores. They contain light-absorbing pigments - bacteriochlorophylls and carotenoids, electron transport enzymes - ubiquinones and cytochromes, components of the phosphorylation system. In some photosynthetic prokaryotes, in particular in purple sulfur bacteria and cyanobacteria, the photosynthetic apparatus is represented by stacks of membranes that have a flattened shape and, by analogy with the grana of chloroplasts of green plants, are called thylakoids (Fig. 3.15). 
They concentrate photosynthetic pigments, enzymes of the electron transport chain and phosphorylation systems. A feature of cyanobacterial thylakoids is the lack of connection with the cytoplasmic membrane. This is the only group of prokaryotes that has a differentiated membrane system.
In nitrifying bacteria, the intracellular membrane apparatus has the form of plates, or lamellae, consisting of flat vesicles (Fig. 3.16). 
Of the intracellular membranes, mesosomes have the most complex structure. They are spirally twisted, flat or spherical tubular bodies. Mesosomes are formed during cell division in the zone of formation of the transverse septum. They take part in chromosome replication and distribution of genomes between daughter cells, and in the synthesis of cell wall substances. To participate
mesosomes in cell division are indicated by its connection with the DNA of the nucleoid. Well-developed mesosomes are found only in gram-positive bacteria.
The information accumulated to date suggests that the membrane structures of bacteria are sufficiently differentiated and ensure the course of various metabolic processes in the cell.
- Cytoplasm and cytoplasmic inclusions
In the cytoplasm, two fractions are distinguished. One of them presents the structural elements: ribosomes, aerosomes,
carboxysomes, storage inclusions, genetic apparatus, Another fraction contains a complex mixture of soluble RNA, enzyme proteins, pigments, minerals, products and substrates of metabolic reactions. This fraction is called cytosol.
Due to the presence of various organic compounds, the cytoplasm of bacterial cells is characterized by increased viscosity. It is 800-8000 times greater than the viscosity of water (approaching the viscosity of glycerin). Young cells in lag phase or early stages of logarithmic phase have lower cytoplasmic viscosity; in aging people, the viscosity increases, resembling a gel in consistency. The degree of cytoplasmic viscosity characterizes not only the age of the cell, but also its physiological activity. An increase in cytoplasmic viscosity in old cultures is one of the factors responsible for a decrease in the physiological activity of cells. Cytoplasm is the medium that connects all intracellular structures into a single system.
Ribosomes. The cytoplasm of a bacterial cell constantly contains spherical structures, 15-20 nm in size, with a molecular weight of 3106.
Ribosomes consist of 60-65% ribosomal RNA and 35-40% protein. The latter are rich in basic amino acids. During ultracentrifugation, bacterial ribosomes sediment at a rate of about 70 Svedberg units (S)7, which is why they are called 708-ribosomes. Cytoplasmic ribosomes of eukaryotes are larger and are called 80S ribosomes (their sedimentation constant is 80S).
Each ribosome consists of two subunits: 30S and 50S, which differ in the size of the RNA molecules and the amount of protein they contain. The large subunit (50S) contains two rRNA molecules - 5S and 23S and 35 molecules of various proteins. The small subunit (30S) includes one molecule of 16 rRNA and 21 molecules of different types of proteins. The number of ribosomes in a cell is not constant - from 5,000 to 90,000. It is determined by the age of the cell and the conditions of bacterial cultivation. The minimum amount is contained at the beginning of the lag phase, and the maximum - in the exponential phase of culture growth. In E. coli, during the period of active growth on a complete nutrient medium, 5-6 ribosomes are synthesized in 1 second. Most of them in the cytoplasm of bacteria are in a free state, and the rest is
S = 1 swedberg unit = 10"13 cm (s) field unit.
united by strands of messenger RNA into polysomes. The number of ribosomes in polysomes can reach several dozen. This indicates a high protein-synthesizing activity of the cell, since ribosomes are the site of protein synthesis. They are figuratively called protein “factories”.
Gas vacuoles (aerosomes). These structures are characteristic only of some water and soil bacteria. They are found in phototrophic sulfur bacteria, colorless filamentous bacteria, as well as in bacteria of the genus Renobacter. There are up to 40-60 of them in a cell (Fig. 3.17). Gas vacuoles are surrounded by thin 
Rice. 3.17. Renobacter vocuolatum cell with aerosomes (magnification x 70,000)
protein membrane. They contain gas bubbles, the number of which is not constant. The composition and pressure of gas in bubbles and aerosomes are generally determined by the amount of gases dissolved in the environment. Aerosomes are either in a compressed state or filled with a gas medium. Their condition is regulated by hydrostatic pressure of the environment. A sharp increase in pressure causes compression of aerosomes and the cells lose their buoyancy.
Aerosomes regulate the buoyancy of the cell, providing the ability to move it to favorable conditions of aeration, lighting, and nutrient content. A special feature is their one-time operation when filled with gas. After compression under the influence of hydrostatic pressure, they are not refilled with gas and
are gradually destroyed. The cell can reproduce them only by forming them anew.
When the aerosomes are filled with gas, the bacteria are held on the surface of the water; when they are compressed, they sink into its thickness or settle to the bottom of the reservoir. This unique method of movement was developed in the process of evolution mainly in bacteria that lack flagella and, consequently, the ability to actively move.
Phycobilisomes. These intracellular structures are characteristic of cyanobacteria. They have the form of granules with a diameter of 28-55 nm and are the site of localization of water-soluble pigments - phycobiliproteins, which determine the color of cyanobacteria and are involved in photosynthesis.
Chlorosomes, or chlorobium vesicles, are structures in which the photosynthetic apparatus of green bacteria of the genus Chlorobium is localized. They have an elongated shape, 100-150 nm long, 50-70 nm wide, surrounded by a single-layer protein membrane. Chlorosomes are located in a dense layer under the cytoplasmic membrane, but are physically separated from it. The chlorosomes of green bacteria contain photosynthesis pigments - bacteriochlorophylls, which absorb light quanta and transfer energy to the reaction centers of photosynthesis.
Carboxysomes. The cells of certain types of phototrophic (cyanobacteria, some purple bacteria) and chemolithotrophic (nitrifying bacteria) prokaryotes contain polyhedron-shaped structures with a size of 90-500 nm. In accordance with the function they perform, they are called carboxysomes. They contain the enzyme ribulose diphosphate carboxylase, which catalyzes the reaction of carbon dioxide with ribulose diphosphate in the Calvin cycle. In autotrophic bacteria they are the site of carbon dioxide fixation. Carboxysomes are surrounded by a single-layer protein membrane, which protects the enzyme from the effects of intracellular proteases.
Spare nutrients* In addition to the described structural elements, the cytoplasm of bacteria contains granules of various shapes and sizes in the form of inclusions. Their presence in
cell is not constant and is associated with the composition of the nutrient medium and the physiological state of the culture. Many cytoplasmic inclusions consist of compounds that serve as a source of energy and a source of nutrients. They are usually formed in cultures on fresh, nutrient-rich media, when cell growth is inhibited for some reason, or after the end of a period of active growth. The chemical composition of inclusions is different and not the same in different types of bacteria. They can be polysaccharides, lipids, crystals and granules of inorganic substances.
Of the polysaccharides, we should first of all mention starch, glycogen and a starch-like substance - granulosa. The most common is glycogen. It is found in bacilli, salmonella, Escherichia coli, sardines, etc. In spore-bearing anaerobes of the genus Clostridium, the cells contain small granulosa granules. These inclusions are used by the cell as sources of energy and carbon.
Lipids accumulate in the cytoplasm of bacteria in the form of small drops and grains. In many bacteria, lipid inclusions are represented by poly-p-hydroxybutyric acid, which often accounts for up to 50% of the dry biomass of bacteria. Bacteria of the genus Bacillus and phototrophic bacteria are especially rich in this compound. Poly-p-hydroxybutyric acid is synthesized in large quantities during the growth of microorganisms on media rich in carbohydrates. In each polylactide chain, p-hydroxybutyric acid residues account for up to 60%, and therefore this compound is an ideal “storehouse” of energy for bacteria. Some microorganisms accumulate waxes and neutral fats (triglycerides). Thus, in mycobacteria and actinomycetes, waxes sometimes make up up to 40% of the dry mass; yeast cells of the genus Candida and Rhodotorula are rich in neutral fats; their number reaches almost 60%.
All lipid inclusions in microorganisms serve as a source of energy and carbon.
In the cells of many bacteria, special inclusions called volutin grains are often found. By chemical nature, volutin is a polyphosphate. Volutin name
comes from the species name of the sulfur bacteria Spirillum volutans, in which these inclusions were first described. Volutin has the property of metachromasia, i.e. causes color changes in some dyes. If bacteria are stained with methylene blue or toluidine blue, the volutin grains become purple or red-violet. In this regard, researchers V. Babes and E. Ernst, who first described these inclusions, called them metachromatic grains. Volutin grains have a spherical shape, up to 0.5 microns in size. They are formed under conditions of good nutrition of microorganisms, especially in media rich in carbohydrates, as well as in the presence of glycerol in the environment. Volutin is found in the cells of both pathogenic and saprophytic bacteria, for example, spirilla, azotobacter, and the causative agent of diphtheria.
Volutin is used by the cell mainly as a source of phosphate groups and partly energy.
In colorless and purple sulfur bacteria, during the oxidation of sulfides, mineral sulfur is deposited inside the cell in the form of drops. Sulfur accumulation occurs in environments rich in hydrogen sulfide H2S. When sulfides are depleted from the environment, bacteria use intracellular sulfur. For colorless sulfur bacteria it serves as a source of energy, for photosynthetic purple sulfur bacteria it serves as an electron donor.
In cyanobacteria, the reserve substance is cyanophycin. It is a polypeptide consisting of arginine and aspartic acid. It serves as a source of nitrogen when there is a lack of it in the environment. The accumulation of cyanophycin granules occurs in the stationary phase of culture growth and can amount to up to 8% of the dry weight of the cell.
Any living cell is separated from the environment by a thin membrane of a special structure - the cytoplasmic membrane (CPM). Eukaryotes have numerous intracellular membranes that separate the organelle space from the cytoplasm, while for most prokaryotes the CPM is the only cell membrane. In some bacteria and archaea, it can penetrate into the cytoplasm, forming outgrowths and folds of various shapes.
The CPM of any cells are built according to a single plan and consist of phospholipids (Fig. 3.5, A). In bacteria, they contain two fatty acids, usually with 16-18 carbon atoms in the chain and with saturated or one unsaturated bonds, connected by an ester bond to two hydroxyl groups of glycerol. The fatty acid composition of bacteria can vary in response to environmental changes, particularly temperature. When the temperature decreases, the amount of unsaturated fatty acids in the composition of phospholipids increases, which significantly affects the fluidity of the membrane. Some fatty acids may be branched or contain a cyclopropane ring. The third OH group of glycerol is connected to the phosphoric acid residue and through it to the head group. The head groups of phospholipids may have different chemical natures in different prokaryotes (phosphatidylethanolamine, phosphatidylglycerol, cardiolipin, phosphatidylserine, lecithin, etc.), but they are simpler in structure than in eukaryotes. For example, at E. coli, they are represented by 75% phosphatidylethanolamine, 20% phosphatidylglycerol, the rest consist of cardiolipin (diphosphatidylglycerol), phosphatidylserine and trace amounts of other compounds. Other bacteria have more complex types of membrane lipids. Some cells form glycolipids such as monogalactosyl diglyceride. Archaeal membrane lipids differ from eukaryotic and bacterial ones. Instead of fatty acids, they contain higher isoprenoid alcohols attached to glycerol by a simple rather than an ester bond.
Rice. 3.5.
A- phospholipid; b- bilayer membrane
O O o O o o
Such molecules make up a membrane bilayer, where the hydrophobic parts face inward, and the hydrophilic parts face outward, into the environment and into the cytoplasm (Fig. 3.5, b). Numerous proteins are embedded in or intersect the bilayer and can diffuse within the membrane, sometimes forming complex complexes. Membrane proteins have a number of important functions, including the conversion and storage of metabolic energy, regulation of the absorption and release of all nutrients and metabolic products. In addition, they recognize and transmit many signals reflecting changes in the environment and trigger the corresponding cascade of reactions leading to a cellular response. This organization of membranes is well explained by the liquid crystal model with a mosaic interspersed with membrane proteins (Fig. 3.6).
Rice. 3.6.
Most biological membranes have a thickness of 4 to 7 nm. Cell membranes are clearly visible in a transmission electron microscope when contrasted with heavy metals. In electron micrographs they look like three-layer formations: two outer dark layers show the position of the polar groups of lipids, and the light middle layer shows the hydrophobic inner space (Fig. 3.7).
Another technique for studying membranes is to obtain cleaved cells frozen at liquid nitrogen temperature and contrast the resulting surfaces by sputtering heavy metals
(platinum, gold, silver). The resulting preparations are viewed under a scanning electron microscope. In this case, one can see the surface of the membrane and the mosaic membrane proteins included in it, which do not extend through the membrane, but are connected by special hydrophobic anchor regions to the hydrophobic region of the bilayer.
Rice. 3.7.
The CPM has the property of selective permeability, preventing the free movement of most substances in and out of the cell, and also plays a significant role in cell growth and division, movement, and the export of surface and extracellular proteins and carbohydrates (exopolysaccharides). If a cell is placed in an environment with a higher or lower osmotic pressure than inside the cytoplasm, then water will leave the cell or water will enter it. This reflects the property of water to equalize solution gradients. In this case, the cytoplasm contracts or expands (the phenomenon of plasmolysis/deplasmolysis). Most bacteria, however, do not change their shape in such experiments due to the presence of a rigid cell wall.
The CPM regulates the flow of nutrients and metabolites. The presence of a hydrophobic layer formed by membrane lipids prevents the passage of any polar molecules and macromolecules through it. This property allows cells, which generally exist in dilute solutions, to retain useful macromolecules and metabolic precursors. The cell membrane is also designed to carry out a transport function. Typically, prokaryotes have a large number of very specific transport systems. Transport is an integral part of the overall bioenergetics of the cell, which creates and uses various ionic gradients through the CPM to transport substances and form other gradients necessary for the cell. The CPM plays a significant role in cell movement, growth and division. Many metabolic processes are concentrated in the membrane of prokaryotes. Membrane proteins perform important functions: they participate in the transformation and storage of energy, regulate the absorption and release of all nutrients and metabolic products, recognize and transmit signals about changes in the environment.
The cytoplasmic cell membrane consists of three layers:
External - protein;
Middle - bimolecular layer of lipids;
Internal - protein.
The membrane thickness is 7.5-10 nm. The bimolecular layer of lipids is the matrix of the membrane. The lipid molecules of both layers interact with the protein molecules immersed in them. From 60 to 75% of membrane lipids are phospholipids, 15-30% are cholesterol. Proteins are represented mainly by glycoproteins. Distinguish integral proteins, permeating the entire membrane, and peripheral located on the outer or inner surface.
integral proteins form ion channels that ensure the exchange of certain ions between extra- and intracellular fluid. They are also enzymes that carry out counter-gradient transport of ions across the membrane.
Peripheral proteins are chemoreceptors on the outer surface of the membrane that can interact with various physiologically active substances.
Membrane functions:
1. Ensures the integrity of the cell as a structural unit of tissue.
Carries out the exchange of ions between the cytoplasm and extracellular fluid.
Provides active transport of ions and other substances into and out of the cell.
Performs the perception and processing of information coming to the cell in the form of chemical and electrical signals.
Mechanisms of cell excitability. History of research into bioelectric phenomena.
Most information transmitted in the body takes the form of electrical signals (for example, nerve impulses). The presence of animal electricity was first established by the natural scientist (physiologist) L. Galvani in 1786. In order to study atmospheric electricity, he suspended neuromuscular preparations of frog legs on a copper hook. When these paws touched the iron railings of the balcony, muscle contraction occurred. This indicated the action of some kind of electricity on the nerve of the neuromuscular drug. Galvani believed that this was due to the presence of electricity in the living tissues themselves. However, A. Volta established that the source of electricity is the place of contact of two dissimilar metals - copper and iron. In physiology Galvani's first classical experiment is considered to be touching the nerve of the neuromuscular preparation with bimetallic tweezers made of copper and iron. To prove he was right, Galvani produced second experience. He threw the end of the nerve innervating the neuromuscular preparation onto the cut of its muscle. As a result, it was reduced. However, this experience did not convince Galvani’s contemporaries. Therefore, another Italian, Matteuci, performed the following experiment. He superimposed the nerve of one frog neuromuscular preparation onto the muscle of the second, which contracted under the influence of an irritating current. As a result, the first drug also began to shrink. This indicated the transfer of electricity (action potential) from one muscle to another. The presence of a potential difference between the damaged and undamaged areas of the muscle was first accurately established in the 19th century using a string galvanometer (ammeter) by Matteuci. Moreover, the cut had a negative charge, and the surface of the muscle had a positive charge.
Cytoplasm- an obligatory part of the cell, enclosed between the plasma membrane and the nucleus; is divided into hyaloplasm (the main substance of the cytoplasm), organelles (permanent components of the cytoplasm) and inclusions (temporary components of the cytoplasm). Chemical composition of the cytoplasm: the basis is water (60-90% of the total mass of the cytoplasm), various organic and inorganic compounds. The cytoplasm has an alkaline reaction. A characteristic feature of the cytoplasm of a eukaryotic cell is constant movement ( cyclosis). It is detected primarily by the movement of cell organelles, such as chloroplasts. If the movement of the cytoplasm stops, the cell dies, since only by being in constant motion can it perform its functions.
Hyaloplasma ( cytosol) is a colorless, slimy, thick and transparent colloidal solution. It is in it that all metabolic processes take place, it ensures the interconnection of the nucleus and all organelles. Depending on the predominance of the liquid part or large molecules in the hyaloplasm, two forms of hyaloplasm are distinguished: sol- more liquid hyaloplasm and gel- thicker hyaloplasm. Mutual transitions are possible between them: the gel turns into a sol and vice versa.
Functions of the cytoplasm:
- combining all cell components into a single system,
- environment for the passage of many biochemical and physiological processes,
- environment for the existence and functioning of organelles.
Cell walls
Cell membranes limit eukaryotic cells. In each cell membrane, at least two layers can be distinguished. The inner layer is adjacent to the cytoplasm and is represented by plasma membrane(synonyms - plasmalemma, cell membrane, cytoplasmic membrane), over which the outer layer is formed. In an animal cell it is thin and is called glycocalyx(formed by glycoproteins, glycolipids, lipoproteins), in a plant cell - thick, called cell wall(formed by cellulose).
All biological membranes have common structural features and properties. It is currently generally accepted fluid mosaic model of membrane structure. The basis of the membrane is a lipid bilayer formed mainly by phospholipids. Phospholipids are triglycerides in which one fatty acid residue is replaced by a phosphoric acid residue; The section of the molecule containing the phosphoric acid residue is called the hydrophilic head, the sections containing the fatty acid residues are called the hydrophobic tails. In the membrane, phospholipids are arranged in a strictly ordered manner: the hydrophobic tails of the molecules face each other, and the hydrophilic heads face outward, towards the water.
In addition to lipids, the membrane contains proteins (on average ≈ 60%). They determine most of the specific functions of the membrane (transport of certain molecules, catalysis of reactions, receiving and converting signals from the environment, etc.). There are: 1) peripheral proteins(located on the outer or inner surface of the lipid bilayer), 2) semi-integral proteins(immersed in the lipid bilayer to varying depths), 3) integral, or transmembrane, proteins(pierce the membrane through, contacting both the external and internal environment of the cell). Integral proteins are in some cases called channel-forming or channel proteins, since they can be considered as hydrophilic channels through which polar molecules pass into the cell (the lipid component of the membrane would not let them through).
A - hydrophilic phospholipid head; B - hydrophobic phospholipid tails; 1 - hydrophobic regions of proteins E and F; 2 — hydrophilic regions of protein F; 3 - branched oligosaccharide chain attached to a lipid in a glycolipid molecule (glycolipids are less common than glycoproteins); 4 - branched oligosaccharide chain attached to a protein in a glycoprotein molecule; 5 - hydrophilic channel (functions as a pore through which ions and some polar molecules can pass).
The membrane may contain carbohydrates (up to 10%). The carbohydrate component of membranes is represented by oligosaccharide or polysaccharide chains associated with protein molecules (glycoproteins) or lipids (glycolipids). Carbohydrates are mainly located on the outer surface of the membrane. Carbohydrates provide receptor functions of the membrane. In animal cells, glycoproteins form a supra-membrane complex, the glycocalyx, which is several tens of nanometers thick. It contains many cell receptors, and with its help cell adhesion occurs.
Molecules of proteins, carbohydrates and lipids are mobile, capable of moving in the plane of the membrane. The thickness of the plasma membrane is approximately 7.5 nm.
Functions of membranes
Membranes perform the following functions:
- separation of cellular contents from the external environment,
- regulation of metabolism between the cell and the environment,
- dividing the cell into compartments (“compartments”),
- place of localization of “enzymatic conveyors”,
- ensuring communication between cells in the tissues of multicellular organisms (adhesion),
- signal recognition.
The most important membrane property— selective permeability, i.e. membranes are highly permeable to some substances or molecules and poorly permeable (or completely impermeable) to others. This property underlies the regulatory function of membranes, ensuring the exchange of substances between the cell and the external environment. The process of substances passing through the cell membrane is called transport of substances. There are: 1) passive transport- the process of passing substances without energy consumption; 2) active transport- the process of passage of substances that occurs with the expenditure of energy.
At passive transport substances move from an area of higher concentration to an area of lower, i.e. along the concentration gradient. In any solution there are solvent and solute molecules. The process of moving solute molecules is called diffusion, and the movement of solvent molecules is called osmosis. If the molecule is charged, then its transport is also affected by the electrical gradient. Therefore, people often talk about an electrochemical gradient, combining both gradients together. The speed of transport depends on the magnitude of the gradient.
The following types of passive transport can be distinguished: 1) simple diffusion— transport of substances directly through the lipid bilayer (oxygen, carbon dioxide); 2) diffusion through membrane channels- transport through channel-forming proteins (Na +, K +, Ca 2+, Cl -); 3) facilitated diffusion- transport of substances using special transport proteins, each of which is responsible for the movement of certain molecules or groups of related molecules (glucose, amino acids, nucleotides); 4) osmosis— transport of water molecules (in all biological systems the solvent is water).
Necessity active transport occurs when it is necessary to ensure the transport of molecules across a membrane against an electrochemical gradient. This transport is carried out by special carrier proteins, the activity of which requires energy expenditure. The energy source is ATP molecules. Active transport includes: 1) Na + /K + pump (sodium-potassium pump), 2) endocytosis, 3) exocytosis.
Work Na + /K + -pump. For normal functioning, the cell must maintain a certain ratio of K + and Na + ions in the cytoplasm and in the external environment. The concentration of K + inside the cell should be significantly higher than outside it, and Na + - vice versa. It should be noted that Na + and K + can diffuse freely through the membrane pores. The Na + /K + pump counteracts the equalization of the concentrations of these ions and actively pumps Na + out of the cell and K + into the cell. The Na + /K + pump is a transmembrane protein capable of conformational changes, as a result of which it can attach both K + and Na +. The Na + /K + pump cycle can be divided into the following phases: 1) addition of Na + from the inside of the membrane, 2) phosphorylation of the pump protein, 3) release of Na + in the extracellular space, 4) addition of K + from the outside of the membrane , 5) dephosphorylation of the pump protein, 6) release of K + in the intracellular space. Almost a third of all energy required for cell functioning is spent on the operation of the sodium-potassium pump. During one cycle of operation, the pump pumps out 3Na + from the cell and pumps in 2K +.
Endocytosis- the process of absorption by the cell of large particles and macromolecules. There are two types of endocytosis: 1) phagocytosis- capture and absorption of large particles (cells, cell parts, macromolecules) and 2) pinocytosis— capture and absorption of liquid material (solution, colloidal solution, suspension). The phenomenon of phagocytosis was discovered by I.I. Mechnikov in 1882. During endocytosis, the plasma membrane forms an invagination, its edges merge, and structures delimited from the cytoplasm by a single membrane are laced into the cytoplasm. Many protozoa and some leukocytes are capable of phagocytosis. Pinocytosis is observed in intestinal epithelial cells and in the endothelium of blood capillaries.
Exocytosis- a process reverse to endocytosis: the removal of various substances from the cell. During exocytosis, the vesicle membrane merges with the outer cytoplasmic membrane, the contents of the vesicle are removed outside the cell, and its membrane is included in the outer cytoplasmic membrane. In this way, hormones are removed from the cells of the endocrine glands; in protozoa, undigested food remains are removed.
Go to lectures No. 5"Cell Theory. Types of cellular organization"
Go to lectures No. 7“Eukaryotic cell: structure and functions of organelles”
