It developed in such a way that the function of each of its systems was the result of the function of the sum of the cells that make up the organs and tissues of this system. Each cell of the body has a set of structures and mechanisms that allow it to carry out its own metabolism and perform its own function.

The cell contains cytoplasmic or surface membrane; cytoplasm, which has a number of organelles, inclusions, elements of the cytoskeleton; nucleus containing the nuclear genome. Cell organelles and the nucleus are delimited in the cytoplasm by internal membranes. Each structure of the cell performs its function in it, and all of them taken together ensure the viability of the cell and the performance of its specific functions.

Key role in cellular functions and their regulation belongs to the cytoplasmic membrane of the cell.

General principles of the structure of the cytoplasmic membrane

All cell membranes share the same structural principle.(Fig. 1), which is based on physiochemical properties complex lipids and proteins that make up their composition. Cell membranes are located in the aquatic environment and to understand the physicochemical phenomena that affect them structural organization, it is useful to describe the interaction of lipid and protein molecules with water molecules and with each other. A number of properties of cell membranes also follow from consideration of this interaction.

It is known that the plasma membrane of a cell is represented by a double layer of complex lipids covering the surface of the cell throughout its entire length. To create a lipid bilayer, only those lipid molecules that possess amphiphilic (amphipathic) properties could be selected by nature and included in its structure. Molecules of phospholipids and cholesterol meet these conditions. Their properties are such that one part of the molecule (glycerol for phospholipids and cyclopentane for cholesterol) has polar (hydrophilic) properties, and the other (fatty acid radicals) has nonpolar (hydrophobic) properties.

Rice. 1. The structure of the cytoplasmic membrane of the cell.

If a certain number of phospholipids and cholesterol molecules are placed in an aqueous medium, they will spontaneously begin to assemble into ordered structures and form closed bubbles ( liposomes), in which part of the aquatic environment is enclosed, and the surface becomes covered with a continuous double layer ( bilayer) phospholipid molecules and cholesterol. When considering the nature of the spatial arrangement of phospholipids and cholesterol molecules in this bilayer, it is clear that the molecules of these substances are located with their hydrophilic parts towards the outer and inner water spaces, and hydrophobic - in opposite directions - inside the bilayer.

What causes the molecules of these lipids to spontaneously form bilayer structures in an aqueous medium, similar to the structure of a cell membrane bilayer? The spatial arrangement of amphiphilic lipid molecules in an aqueous medium is dictated by one of the requirements of thermodynamics. The most probable spatial structure that lipid molecules will form in an aqueous medium will be structure with minimum free energy.

Such a minimum of free energy in the spatial structure of lipids in water will be achieved when both the hydrophilic and hydrophobic properties of the molecules are realized in the form of the corresponding intermolecular bonds.

When considering the behavior of complex amphiphilic lipid molecules in water, some properties of cell membranes. It is known that if the plasma membrane is mechanically damaged(for example, pierce it with an electrode or remove the nucleus through a puncture and place another nucleus in the cell), then in a moment due to the forces of intermolecular interaction of lipids and water the membrane will spontaneously restore integrity. Under the influence of the same forces, one can observe fusion of bilayers of two membranes when they come into contact(eg, vesicles and presynaptic membranes in synapses). The ability of membranes to merge upon their direct contact is part of the mechanisms of membrane structure renewal, transport of membrane components from one subcellular space to another, and also part of the mechanisms of endo- and exocytosis.

Energy of intermolecular bonds in the lipid bilayer very low, therefore, conditions are created for the rapid movement of lipid and protein molecules in the membrane and for changing the structure of the membrane when exposed to mechanical forces, pressures, temperatures and other factors. The presence of a double lipid layer in the membrane forms a closed space, isolates the cytoplasm from the surrounding aquatic environment and creates an obstacle for the free passage of water and substances soluble in it through the cell membrane. The thickness of the lipid bilayer is about 5 nm.

Cell membranes also contain proteins. Their molecules are 40-50 times larger in volume and mass than the molecules of membrane lipids. Due to proteins, the membrane thickness reaches 7-10 nm. Despite the fact that the total masses of proteins and lipids in most membranes are almost equal, the number of protein molecules in the membrane is ten times less than that of lipid molecules.

What happens if a protein molecule is placed in a phospholipid bilayer of liposomes, the outer and inner surfaces of which are polar, and the intralipid is non-polar? Under the influence of the forces of intermolecular interactions of lipids, protein and water, the formation of such a spatial structure will occur in which the non-polar regions of the peptide chain will tend to settle down in the depth of the lipid bilayer, while the polar ones will occupy a position on one of the surfaces of the bilayer and may also be immersed. into the external or internal aqueous environment of the liposome. A very similar nature of the arrangement of protein molecules also takes place in the lipid bilayer of cell membranes (Fig. 1).

Typically, protein molecules are localized in the membrane separately from one another. The very weak forces of hydrophobic interactions between hydrocarbon radicals of lipid molecules and nonpolar regions of the protein molecule (lipid-lipid, lipid-protein interactions) arising in the non-polar part of the lipid bilayer do not prevent the processes of thermal diffusion of these molecules in the bilayer structure.

When the structure of cell membranes was studied using subtle research methods, it turned out that it is very similar to that which is spontaneously formed by phospholipids, cholesterol and proteins in the aquatic environment. In 1972, Singer and Nichols proposed a fluid-mosaic model of the structure of the cell membrane and formulated its basic principles.

According to this model, the structural basis of all cell membranes is a liquid-like continuous double layer of amphipathic molecules of phospholipids, cholesterol, glycolipids, spontaneously forming it in the aquatic environment. In the lipid bilayer, protein molecules are asymmetrically located that perform specific receptor, enzymatic, and transport functions. Protein and lipid molecules are mobile and can rotational movements, diffuse in the plane of the bilayer. protein molecules are capable of changing their spatial structure (conformation), shifting and changing their position in the lipid bilayer of the membrane, plunging to different depths or floating up to its surface. The structure of the lipid bilayer of the membrane is heterogeneous. It has areas (domains) called "rafts", which are enriched in sphingolipids and cholesterol. "Rafts" differ in phase state from the state of the rest of the membrane in which they are located. The structural features of membranes depend on the function they perform and the functional state.

The study of the composition of cell membranes confirmed that their main components are lipids, which make up about 50% of the mass of the plasma membrane. About 40-48% of the membrane mass is accounted for by proteins and 2-10% by carbohydrates. Residues of carbohydrates are either incorporated into proteins, forming glycoproteins, or lipids, forming glycolipids. Phospholipids are the main structural lipids of plasma membranes and make up 30-50% of their mass.

Carbohydrate residues of glycolipid molecules are usually located on the outer surface of the membrane and are immersed in an aqueous medium. They play an important role in intercellular, cell-matrix interactions and recognition of antigens by cells of the immune system. Cholesterol molecules embedded in the phospholipid bilayer contribute to maintaining the ordered arrangement of fatty acid chains of phospholipids and their liquid crystal state. Due to the presence of high conformational mobility of acyl radicals of fatty acids of phospholipids, they form a rather loose packing of the lipid bilayer and structural defects can form in it.

Protein molecules are capable of penetrating the entire membrane so that their end sections protrude beyond its transverse limits. Such proteins are called transmembrane, or integral. The membranes also contain proteins that are only partially immersed in the membrane or located on its surface.

Many specific functions of membranes are determined by protein molecules, for which the lipid matrix is ​​a direct microenvironment and the implementation of functions by protein molecules depends on its properties. Among the most important functions membrane proteins can be distinguished: receptor - binding to such signal molecules as neurotransmitters, hormones, ingerleukins, growth factors, and signal transmission to post-receptor structures of the cell; enzymatic - catalysis of intracellular reactions; structural - participation in the formation of the structure of the membrane itself; transport - the transfer of substances through membranes; channel-forming - the formation of ionic and water channels. Proteins, together with carbohydrates, are involved in the implementation of adhesion-sticking, gluing cells during immune reactions, combining cells into layers and tissues, and ensure the interaction of cells with the extracellular matrix.

The functional activity of membrane proteins (receptors, enzymes, carriers) is determined by their ability to easily change their spatial structure (conformation) when interacting with signaling molecules, the action of physical factors, or changing the properties of the microenvironment. The energy required to implement these conformational changes in the structure of proteins depends both on the intramolecular forces of interaction between individual sections of the peptide chain and on the degree of fluidity (microviscosity) of membrane lipids immediately surrounding the protein.

Carbohydrates in the form of glycolipids and glycoproteins make up only 2-10% of the membrane mass; their number in different cells is variable. Thanks to them, some types of intercellular interactions are carried out, they take part in the recognition of foreign antigens by the cell and, together with proteins, create a kind of antigenic structure of the surface membrane of their own cell. By such antigens, cells recognize each other, unite into tissue and stick together for a short time to transmit signal molecules to each other.

Due to the low interaction energy of the substances included in the membrane and the relative orderliness of their arrangement, the cell membrane acquires a number of properties and functions that cannot be reduced to a simple sum of the properties of the substances that form it. Insignificant effects on the membrane, comparable to the energy of intermolecular bonds of proteins and lipids, can lead to a change in the conformation of protein molecules, the permeability of ion channels, changes in the properties of membrane receptors, and other numerous functions of the membrane and the cell itself. The high sensitivity of the structural components of the plasma membrane is of decisive importance in the perception of information signals by the cell and their transformation into cell responses.

Functions of the cytoplasmic membrane of the cell

The cytoplasmic membrane performs many functions that provide the vital needs of the cell. and, in particular, a number of functions necessary for the perception and transmission of information signals by the cell.

Among the most important functions of the plasma membrane are:

• delimitation of the cell from the environment while maintaining the shape, volume and significant differences between the cellular content and the extracellular space;
• the transfer of substances into and out of the cell based on the properties of selective permeability, active and other modes of transport;
• maintenance of the transmembrane electrical potential difference (membrane polarization) at rest, its change under various influences on the cell, generation and conduction of excitation;
• participation in the detection (reception) of signals of a physical nature, signal molecules due to the formation of sensory or molecular receptors and the transmission of signals into the cell;
• the formation of intercellular contacts (tight, gap and desmosomal contact) in the composition of the formed tissues or during adhesion of cells of various tissues;
• creation of a hydrophobic microenvironment for the manifestation of the activity of enzymes associated with the membrane;
• ensuring the immune specificity of the cell due to the presence in the structure of the membrane of antigens of a protein or glycoprotein nature. Immune specificity is important when cells combine into tissue and interact with immune surveillance cells in the body.

The above list of functions of cell membranes indicates that they are involved in the implementation of not only cellular functions, but also the basic processes of vital activity of organs, tissues and the whole organism. Without knowledge of a number of phenomena and processes provided by membrane structures, it is impossible to understand and consciously perform certain diagnostic procedures and therapeutic measures. For example, for the correct use of many medicinal substances, it is necessary to know to what extent each of them penetrates through cell membranes from the blood into the tissue fluid and into the cells.

cytoplasmic membrane

Image of a cell membrane. Small blue and white balls correspond to the hydrophilic "heads" of lipids, and the lines attached to them correspond to the hydrophobic "tails". The figure shows only integral membrane proteins (red globules and yellow helices). Yellow oval dots inside the membrane - cholesterol molecules Yellow-green chains of beads on the outside of the membrane - oligosaccharide chains that form the glycocalyx

The biological membrane also includes various proteins: integral (penetrating the membrane through), semi-integral (immersed at one end into the outer or inner lipid layer), surface (located on the outer or adjacent to the inner sides of the membrane). Some proteins are the points of contact of the cell membrane with the cytoskeleton inside the cell, and the cell wall (if any) outside. Some of the integral proteins function as ion channels, various transporters, and receptors.

Functions of biomembranes

• barrier - provides a regulated, selective, passive and active metabolism with the environment. For example, the peroxisome membrane protects the cytoplasm from peroxides dangerous to the cell. Selective permeability means that the permeability of a membrane to various atoms or molecules depends on their size, electric charge and chemical properties. Selective permeability ensures separation of the cell and cellular compartments from environment and supply them with the necessary substances.

• transport - through the membrane there is a transport of substances into the cell and out of the cell. Transport through membranes provides: the delivery of nutrients, the removal of end products of metabolism, the secretion of various substances, the creation of ionic gradients, the maintenance of the appropriate pH and ionic concentration in the cell, which are necessary for the operation of cellular enzymes.

Particles that for some reason are not able to cross the phospholipid bilayer (for example, due to hydrophilic properties, since the membrane inside is hydrophobic and does not allow hydrophilic substances to pass through, or because of their large size), but necessary for the cell, can penetrate the membrane through special carrier proteins (transporters) and channel proteins or by endocytosis.

In passive transport, substances cross the lipid bilayer without energy expenditure, by diffusion. A variant of this mechanism is facilitated diffusion, in which a specific molecule helps a substance to pass through the membrane. This molecule may have a channel that allows only one type of substance to pass through.

Active transport requires energy, as it occurs against a concentration gradient. There are special pump proteins on the membrane, including ATPase, which actively pumps potassium ions (K +) into the cell and pumps sodium ions (Na +) out of it.

• matrix - provides a certain relative position and orientation of membrane proteins, their optimal interaction;

• mechanical - ensures the autonomy of the cell, its intracellular structures, as well as connection with other cells (in tissues). Cell walls play an important role in providing mechanical function, and in animals - intercellular substance.

• energy - during photosynthesis in chloroplasts and cellular respiration in mitochondria, energy transfer systems operate in their membranes, in which proteins also participate;

• receptor - some proteins sitting in the membrane are receptors (molecules with which the cell perceives certain signals).

For example, hormones circulating in the blood only act on target cells that have receptors corresponding to those hormones. Neurotransmitters (chemicals that conduct nerve impulses) also bind to specific receptor proteins on target cells.

• enzymatic - membrane proteins are often enzymes. For example, the plasma membranes of intestinal epithelial cells contain digestive enzymes.

• implementation of generation and conduction of biopotentials.

With the help of the membrane, a constant concentration of ions is maintained in the cell: the concentration of the K + ion inside the cell is much higher than outside, and the concentration of Na + is much lower, which is very important, since this maintains the potential difference across the membrane and generates a nerve impulse.

• cell marking - there are antigens on the membrane that act as markers - "labels" that allow the cell to be identified. These are glycoproteins (that is, proteins with branched oligosaccharide side chains attached to them) that play the role of "antennas". Due to the myriad of side chain configurations, it is possible to make a specific marker for each cell type. With the help of markers, cells can recognize other cells and act in concert with them, for example, when forming organs and tissues. It also allows the immune system to recognize foreign antigens.

Structure and composition of biomembranes

Membranes are composed of three classes of lipids: phospholipids, glycolipids, and cholesterol. Phospholipids and glycolipids (lipids with carbohydrates attached to them) consist of two long hydrophobic hydrocarbon "tails" that are associated with a charged hydrophilic "head". Cholesterol stiffens the membrane by occupying the free space between the hydrophobic lipid tails and preventing them from bending. Therefore, membranes with a low cholesterol content are more flexible, while those with a high cholesterol content are more rigid and brittle. Cholesterol also serves as a “stopper” that prevents the movement of polar molecules from and into the cell. An important part of the membrane is made up of proteins penetrating it and responsible for various properties of membranes. Their composition and orientation in different membranes differ.

Cell membranes are often asymmetric, that is, the layers differ in lipid composition, the transition of an individual molecule from one layer to another (the so-called flip flop) is difficult.

Membrane organelles

These are closed single or interconnected sections of the cytoplasm, separated from the hyaloplasm by membranes. Single-membrane organelles include endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles, peroxisomes; to two-membrane - nucleus, mitochondria, plastids. Outside, the cell is limited by the so-called plasma membrane. The structure of the membranes of various organelles differs in the composition of lipids and membrane proteins.

Selective permeability

Cell membranes have selective permeability: glucose, amino acids, fatty acids, glycerol and ions slowly diffuse through them, and the membranes themselves actively regulate this process to a certain extent - some substances pass through, while others do not. There are four main mechanisms for the entry of substances into the cell or out of the cell: diffusion, osmosis, active transport and exo- or endocytosis. The first two processes are passive, i.e. do not require energy costs; the last two are active processes associated with energy consumption.

The selective permeability of the membrane during passive transport is due to special channels - integral proteins. They penetrate the membrane through and through, forming a kind of passage. The elements K, Na and Cl have their own channels. With respect to the concentration gradient, the molecules of these elements move in and out of the cell. When irritated, the sodium ion channels open, and there is a sharp influx of sodium ions into the cell. This results in an imbalance in the membrane potential. Then membrane potential is being restored. Potassium channels are always open, through them ions slowly enter the cell

Any living cell is separated from the environment by a thin shell of a special structure - the cytoplasmic membrane (CPM). Eukaryotes have numerous intracellular membranes separating the space of organelles from the cytoplasm, while for most prokaryotes the CMP 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 changes in the environment, especially temperature. With a decrease in temperature, the amount of unsaturated fatty acids in the composition of phospholipids increases, which largely affects the fluidity of the membrane. Some fatty acids may be branched or contain a cyclopropane ring. The third OH group of glycerol is linked to the phosphoric acid residue and through it to the head group. The head groups of phospholipids can have different chemical nature in different prokaryotes (phosphatidylethanolamine, phosphatidylglycerol, cardiolipin, phosphatidylserine, lecithin, etc.), but they are simpler than in eukaryotes. For example, at E. coli they are represented by 75% phosphatidylethanolamine, 20% by phosphatidylglycerol, the rest consist of cardiolipin (diphosphatidylglycerol), phosphatidylserine and trace amounts of other compounds. Other bacteria have more complex types membrane lipids. Some cells form glycolipids such as monogalactosyl diglyceride. Archaeal membrane lipids differ from eukaryotic and bacterial ones. Instead of fatty acids, they have higher isoprenoid alcohols attached to glycerol by a simple rather than an ester bond.

Rice. 3.5.

a- phospholipid; b- bilayer membrane

Oh oh oh oh oh

Such molecules make up a membrane bilayer, where the hydrophobic parts are turned inward, and the hydrophilic parts are turned outward, into the environment and into the cytoplasm (Fig. 3.5, b). Numerous proteins are immersed or crossed into the bilayer, which can diffuse inside the membrane, sometimes forming complex complexes. Membrane proteins have a number of important functions, including the conversion and storage of metabolic energy, the regulation of the absorption and release of all nutrients and metabolic products. In addition, they recognize and transmit many signals that reflect 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 are 4 to 7 nm thick. Cell membranes are clearly visible in a transmission electron microscope when contrasted with heavy metals. On electron micrographs, they look like three-layer formations: two outer dark layers show the position of the polar groups of lipids, and a light middle layer shows the hydrophobic interior (Fig. 3.7).

Another technique for studying membranes is to obtain chips of cells frozen at the temperature of liquid nitrogen and contrast the resulting surfaces using heavy metal deposition.

(platinum, gold, silver). The resulting preparations are viewed in 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.

CPM has the property of selective permeability, preventing the free movement of most substances into 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 exit the cell or water will enter it. This reflects the property of water to equalize solution gradients. At the same time, the cytoplasm shrinks 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 exist in most cases 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 general bioenergetics of the cell, which creates and uses various ionic gradients through the CPM for the transfer of substances and the formation of other gradients necessary for the cell. The CMP 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 conversion 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 its both layers interact with the protein molecules immersed in them. From 60 to 75% of membrane lipids are phospholipids, 15-30% cholesterol. Proteins are represented mainly by glycoproteins. Distinguish integral proteins spanning the entire membrane, and peripheral located on the outer or inner surface.

integral proteins form ion channels that provide the exchange of certain ions between the extra- and intracellular fluid. They are also enzymes that carry out antigradient transport of ions across the membrane.

Peripheral proteins are chemoreceptors on the outer surface of the membrane, which can interact with various physiologically active substances.

Membrane functions:

1. Ensures the integrity of the cell as a structural unit of the 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.

Produces the perception and processing of information coming to the cell in the form of chemical and electrical signals.

Mechanisms of cell excitability. History of the study of bioelectric phenomena.

Basically, the information transmitted in the body is in the form of electrical signals (for example, nerve impulses). The presence of animal electricity was first established by the naturalist (physiologist) L. Galvani in 1786. In order to study atmospheric electricity, he hung neuromuscular preparations of frog legs on a copper hook. When these paws touched the iron railing of the balcony, the muscles contracted. This indicated the action of some kind of electricity on the nerve of the neuromuscular preparation. Galvani considered that this was due to the presence of electricity in the living tissues themselves. However, A. Volta found that the source of electricity is the place of contact of two dissimilar metals - copper and iron. In physiology Galvani's first classical experience it is considered to touch the nerve of the neuromuscular preparation with bimetallic tweezers made of copper and iron. To prove his case, Galvani produced second experience. He threw the end of the nerve innervating the neuromuscular preparation over the cut of his muscle. The result was a contraction. However, this experience did not convince Galvani's contemporaries. Therefore, another Italian Matteuchi made the following experiment. He superimposed the nerve of one neuromuscular frog preparation on the muscle of the second, which contracted under the influence of an irritating current. As a result, the first drug also began to decline. This indicated the transfer of electricity (action potential) from one muscle to another. The presence of a potential difference between the damaged and undamaged parts of the muscle was first accurately established in the 19th century using a string galvanometer (ammeter) by Matteuchi. Moreover, the cut had a negative charge, and the surface of the muscle was positive.

cell membrane also called plasma (or cytoplasmic) membrane and plasmalemma. This structure not only separates the internal contents of the cell from the external environment, but also enters into the composition of most cell organelles and the nucleus, in turn separating them from the hyaloplasm (cytosol) - the viscous-liquid part of the cytoplasm. Let's agree to call cytoplasmic membrane one that separates the contents of the cell from the external environment. The remaining terms refer to all membranes.

The structure of the cell membrane

The basis of the structure of the cell (biological) membrane is a double layer of lipids (fats). The formation of such a layer is associated with the features of their molecules. Lipids do not dissolve in water, but condense in it in their own way. One part of a single lipid molecule is a polar head (it is attracted by water, i.e., hydrophilic), and the other is a pair of long non-polar tails (this part of the molecule is repelled by water, i.e., hydrophobic). This structure of the molecules makes them "hide" their tails from the water and turn their polar heads towards the water.

As a result, a lipid bilayer is formed, in which the non-polar tails are inside (facing each other), and the polar heads are facing out (to the external environment and cytoplasm). The surface of such a membrane is hydrophilic, but inside it is hydrophobic.

In cell membranes, phospholipids predominate among lipids (they belong to complex lipids). Their heads contain a residue of phosphoric acid. In addition to phospholipids, there are glycolipids (lipids + carbohydrates) and cholesterol (belongs to sterols). The latter gives the membrane rigidity, being located in its thickness between the tails of the remaining lipids (cholesterol is completely hydrophobic).

Due to electrostatic interaction, certain protein molecules are attached to the charged heads of lipids, which become surface membrane proteins. Other proteins interact with non-polar tails, partially sink into the bilayer, or penetrate it through and through.

Thus, the cell membrane consists of a bilayer of lipids, surface (peripheral), immersed (semi-integral), and penetrating (integral) proteins. In addition, some proteins and lipids on the outside of the membrane are associated with carbohydrate chains.

it fluid mosaic model of the membrane structure was put forward in the 70s of the XX century. Prior to this, a sandwich model of the structure was assumed, according to which the lipid bilayer is located inside, and on the inside and outside the membrane is covered with continuous layers of surface proteins. However, the accumulation of experimental data disproved this hypothesis.

The thickness of membranes in different cells is about 8 nm. Membranes (even different sides of one) differ from each other in the percentage of different types of lipids, proteins, enzymatic activity, etc. Some membranes are more liquid and more permeable, others are more dense.

Breaks in the cell membrane easily merge due to the physicochemical characteristics of the lipid bilayer. In the plane of the membrane, lipids and proteins (unless they are fixed by the cytoskeleton) move.

Functions of the cell membrane

Most of the proteins immersed in the cell membrane perform an enzymatic function (they are enzymes). Often (especially in the membranes of cell organelles) enzymes are arranged in a certain sequence so that the reaction products catalyzed by one enzyme pass to the second, then the third, etc. A conveyor is formed that stabilizes surface proteins, because they do not allow enzymes to swim along the lipid bilayer.

The cell membrane performs a delimiting (barrier) function from the environment and at the same time a transport function. It can be said that this is its most important purpose. The cytoplasmic membrane, having strength and selective permeability, maintains the constancy of the internal composition of the cell (its homeostasis and integrity).

In this case, the transport of substances occurs in various ways. Transport along a concentration gradient involves the movement of substances from an area with a higher concentration to an area with a lower one (diffusion). So, for example, gases diffuse (CO 2, O 2).

There is also transport against the concentration gradient, but with the expenditure of energy.

Transport is passive and lightweight (when some carrier helps him). Passive diffusion across the cell membrane is possible for fat-soluble substances.

There are special proteins that make membranes permeable to sugars and other water-soluble substances. These carriers bind to transported molecules and drag them across the membrane. This is how glucose is transported into the red blood cells.

Spanning proteins, when combined, can form a pore for the movement of certain substances through the membrane. Such carriers do not move, but form a channel in the membrane and work similarly to enzymes, binding a specific substance. The transfer is carried out due to a change in the conformation of the protein, due to which channels are formed in the membrane. An example is the sodium-potassium pump.

The transport function of the eukaryotic cell membrane is also realized through endocytosis (and exocytosis). Thanks to these mechanisms, large molecules of biopolymers, even whole cells, enter the cell (and out of it). Endo- and exocytosis are not characteristic of all eukaryotic cells (prokaryotes do not have it at all). So endocytosis is observed in protozoa and lower invertebrates; in mammals, leukocytes and macrophages absorb harmful substances and bacteria, i.e., endocytosis performs a protective function for the body.

Endocytosis is divided into phagocytosis(cytoplasm envelops large particles) and pinocytosis(capture of liquid droplets with substances dissolved in it). The mechanism of these processes is approximately the same. Absorbed substances on the cell surface are surrounded by a membrane. A vesicle (phagocytic or pinocytic) is formed, which then moves into the cell.

Exocytosis is the removal of substances from the cell by the cytoplasmic membrane (hormones, polysaccharides, proteins, fats, etc.). These substances are enclosed in membrane vesicles that fit the cell membrane. Both membranes merge and the contents are outside the cell.

The cytoplasmic membrane performs a receptor function. To do this, on its outer side there are structures that can recognize a chemical or physical stimulus. Some of the proteins penetrating the plasmalemma are externally connected to polysaccharide chains (forming glycoproteins). These are peculiar molecular receptors that capture hormones. When a particular hormone binds to its receptor, it changes its structure. This, in turn, triggers the cellular response mechanism. At the same time, channels can open, and certain substances can begin to enter the cell or be removed from it.

The receptor function of cell membranes has been well studied based on the action of the hormone insulin. When insulin binds to its glycoprotein receptor, the catalytic intracellular part of this protein (the enzyme adenylate cyclase) is activated. The enzyme synthesizes cyclic AMP from ATP. Already it activates or inhibits various enzymes of cellular metabolism.

The receptor function of the cytoplasmic membrane also includes the recognition of neighboring cells of the same type. Such cells are attached to each other by various intercellular contacts.

In tissues, with the help of intercellular contacts, cells can exchange information with each other using specially synthesized low molecular weight substances. One example of such an interaction is contact inhibition, when cells stop growing after receiving information that the free space is occupied.

Intercellular contacts are simple (membranes of different cells are adjacent to each other), locking (invagination of the membrane of one cell into another), desmosomes (when the membranes are connected by bundles of transverse fibers penetrating into the cytoplasm). In addition, there is a variant of intercellular contacts due to mediators (intermediaries) - synapses. In them, the signal is transmitted not only chemically, but also electrically. Synapses transmit signals between nerve cells, as well as from nerve to muscle.