Cell membrane structure and functions table briefly. Cell membrane: structure and functions

Plasma membrane , or plasmalemma,- the most permanent, basic, universal membrane for all cells. It is a thin (about 10 nm) film covering the entire cell. The plasmalemma consists of protein molecules and phospholipids (Fig. 1.6).

Phospholipid molecules are arranged in two rows - with hydrophobic ends inward, hydrophilic heads towards the internal and external aqueous environment. In some places, the bilayer (double layer) of phospholipids is penetrated through and through by protein molecules (integral proteins). Inside such protein molecules there are channels - pores through which water-soluble substances pass. Other protein molecules penetrate the lipid bilayer halfway on one side or the other (semi-integral proteins). There are peripheral proteins on the surface of the membranes of eukaryotic cells. Lipid and protein molecules are held together due to hydrophilic-hydrophobic interactions.

Properties and functions of membranes. All cell membranes are mobile fluid structures, since lipid and protein molecules are not interconnected covalent bonds and are capable of moving quite quickly in the plane of the membrane. Thanks to this, membranes can change their configuration, i.e., they have fluidity.

Membranes are very dynamic structures. They quickly recover from damage and also stretch and contract with cellular movements.

Membranes of different types of cells differ significantly both in chemical composition and in the relative content of proteins, glycoproteins, lipids in them, and, consequently, in the nature of the receptors they contain. Each cell type is therefore characterized by an individuality, which is determined mainly glycoproteins. Branched chain glycoproteins protruding from the cell membrane are involved in factor recognition external environment, as well as in mutual recognition of related cells. For example, an egg and a sperm recognize each other by cell surface glycoproteins, which fit together as separate elements of a whole structure. Such mutual recognition is a necessary stage preceding fertilization.

A similar phenomenon is observed in the process of tissue differentiation. In this case, cells similar in structure, with the help of recognition areas of the plasmalemma, are correctly oriented relative to each other, thereby ensuring their adhesion and tissue formation. Associated with recognition transport regulation molecules and ions through the membrane, as well as an immunological response in which glycoproteins play the role of antigens. Sugars can thus function as information molecules (like proteins and nucleic acids). The membranes also contain specific receptors, electron carriers, energy converters, and enzyme proteins. Proteins are involved in ensuring the transport of certain molecules into or out of the cell, provide a structural connection between the cytoskeleton and cell membranes, or serve as receptors for receiving and converting chemical signals from environment.

The most important property of the membrane is also selective permeability. This means that molecules and ions pass through it at different speeds, and the larger the size of the molecules, the slower the speed at which they pass through the membrane. This property defines the plasma membrane as osmotic barrier. Water and gases dissolved in it have the maximum penetrating ability; Ions pass through the membrane much more slowly. The diffusion of water through a membrane is called by osmosis.

There are several mechanisms for transporting substances across the membrane.

Diffusion-penetration of substances through a membrane along a concentration gradient (from an area where their concentration is higher to an area where their concentration is lower). Diffuse transport of substances (water, ions) is carried out with the participation of membrane proteins, which have molecular pores, or with the participation of the lipid phase (for fat-soluble substances).

With facilitated diffusion special membrane transport proteins selectively bind to one or another ion or molecule and transport them across the membrane along a concentration gradient.

Active transport involves energy costs and serves to transport substances against their concentration gradient. He carried out by special carrier proteins that form the so-called ion pumps. The most studied is the Na - / K - pump in animal cells, which actively pumps Na + ions out while absorbing K - ions. Due to this, a higher concentration of K - and a lower concentration of Na + is maintained in the cell compared to the environment. This process requires ATP energy.

As a result of active transport using a membrane pump in the cell, the concentration of Mg 2- and Ca 2+ is also regulated.

During the process of active transport of ions into the cell, various sugars, nucleotides, and amino acids penetrate through the cytoplasmic membrane.

Macromolecules of proteins, nucleic acids, polysaccharides, lipoprotein complexes, etc. do not pass through cell membranes, unlike ions and monomers. Transport of macromolecules, their complexes and particles into the cell occurs in a completely different way - through endocytosis. At endocytosis (endo...- inward) a certain area of the plasmalemma captures and, as it were, envelops extracellular material, enclosing it in a membrane vacuole that arises as a result of invagination of the membrane. Subsequently, such a vacuole connects with a lysosome, the enzymes of which break down macromolecules into monomers.

The reverse process of endocytosis is exocytosis (exo...- out). Thanks to it, the cell removes intracellular products or undigested residues enclosed in vacuoles or pu-

zyryki. The vesicle approaches the cytoplasmic membrane, merges with it, and its contents are released into the environment. This is how digestive enzymes, hormones, hemicellulose, etc. are removed.

Thus, biological membranes, as the main structural elements of a cell, serve not just as physical boundaries, but are dynamic functional surfaces. Numerous biochemical processes take place on the membranes of organelles, such as active absorption of substances, energy conversion, ATP synthesis, etc.

Functions of biological membranes the following:

They delimit the contents of the cell from the external environment and the contents of organelles from the cytoplasm.

They ensure the transport of substances into and out of the cell, from the cytoplasm to organelles and vice versa.

They act as receptors (receiving and converting chemicals from the environment, recognizing cell substances, etc.).

They are catalysts (providing near-membrane chemical processes).

Participate in energy conversion.

Biological membranes- common name functionally active surface structures bounding cells (cellular, or plasma membranes) and intracellular organelles (membranes of mitochondria, nuclei, lysosomes, endoplasmic reticulum, etc.). They contain lipids, proteins, heterogeneous molecules (glycoproteins, glycolipids) and, depending on the function performed, numerous minor components: coenzymes, nucleic acids, antioxidants, carotenoids, inorganic ions, etc.

The coordinated functioning of membrane systems - receptors, enzymes, transport mechanisms - helps maintain cell homeostasis and at the same time quickly respond to changes in the external environment.

TO basic functions of biological membranes can be attributed:

· separation of the cell from the environment and the formation of intracellular compartments (compartments);

· control and regulation of the transport of a huge variety of substances through membranes;

· participation in ensuring intercellular interactions, transmitting signals into the cell;

food energy conversion organic matter into energy chemical bonds ATP molecules.

The molecular organization of the plasma (cellular) membrane is approximately the same in all cells: it consists of two layers of lipid molecules with many specific proteins included in it. Some membrane proteins have enzymatic activity, while others bind nutrients from the environment and transport them into the cell across membranes. Membrane proteins are distinguished by the nature of their connection with membrane structures. Some proteins called external or peripheral , are loosely bound to the surface of the membrane, others, called internal or integral , immersed inside the membrane. Peripheral proteins are easily extracted, while integral proteins can only be isolated using detergents or organic solvents. In Fig. Figure 4 shows the structure of the plasma membrane.

The outer, or plasma, membranes of many cells, as well as the membranes of intracellular organelles, for example, mitochondria, chloroplasts, were isolated in free form and their molecular composition was studied. All membranes contain polar lipids in quantities ranging from 20 to 80% of their mass, depending on the type of membrane; the rest is mainly proteins. Thus, in the plasma membranes of animal cells, the amount of proteins and lipids, as a rule, is approximately the same; the inner mitochondrial membrane contains about 80% proteins and only 20% lipids, and the myelin membranes of brain cells, on the contrary, contain about 80% lipids and only 20% proteins.

Rice. 4. Structure of the plasma membrane

The lipid part of the membrane is a mixture various kinds polar lipids. Polar lipids, which include phosphoglycerolipids, sphingolipids, and glycolipids, are not stored in fat cells, but are integrated into cell membranes, and in strictly defined proportions.

All polar lipids in membranes are constantly renewed during metabolism, with normal conditions a dynamic stationary state is established in the cell, in which the rate of lipid synthesis is equal to the rate of their decay.

The membranes of animal cells contain mainly phosphoglycerolipids and, to a lesser extent, sphingolipids; triacylglycerols are found only in trace amounts. Some membranes of animal cells, especially the outer plasma membrane, contain significant amounts of cholesterol and its esters (Fig. 5).

Fig.5. Membrane lipids

Currently, the generally accepted model of membrane structure is the fluid mosaic model, proposed in 1972 by S. Singer and J. Nicholson.

According to it, proteins can be likened to icebergs floating in a lipid sea. As mentioned above, there are 2 types of membrane proteins: integral and peripheral. Integral proteins penetrate through the membrane; they are amphipathic molecules. Peripheral proteins do not penetrate the membrane and are less tightly bound to it. The main continuous part of the membrane, that is, its matrix, is the polar lipid bilayer. At normal cell temperatures, the matrix is in a liquid state, which is ensured by a certain ratio between saturated and unsaturated fatty acids in the hydrophobic tails of polar lipids.

The liquid-mosaic model also assumes that on the surface of integral proteins located in the membrane there are R-groups of amino acid residues (mainly hydrophobic groups, due to which the proteins seem to “dissolve” in the central hydrophobic part of the bilayer). At the same time, on the surface of peripheral, or external proteins, there are mainly hydrophilic R-groups, which are attracted to the hydrophilic charged polar heads of lipids due to electrostatic forces. Integral proteins, which include enzymes and transport proteins, are active only if they are located inside the hydrophobic part of the bilayer, where they acquire the spatial configuration necessary for the manifestation of activity (Fig. 6). It should be emphasized once again that covalent bonds are not formed either between the molecules in the bilayer or between the proteins and lipids of the bilayer.

Fig.6. Membrane proteins

Membrane proteins can move freely in the lateral plane. Peripheral proteins literally float on the surface of the bilayer “sea,” while integral proteins, like icebergs, are almost completely immersed in the hydrocarbon layer.

For the most part, membranes are asymmetrical, that is, they have unequal sides. This asymmetry is manifested in the following:

· firstly, that the inner and outer sides of the plasma membranes of bacterial and animal cells differ in the composition of polar lipids. For example, the inner lipid layer of human erythrocyte membranes contains mainly phosphatidylethanolamine and phosphatidylserine, and the outer layer contains phosphatidylcholine and sphingomyelin.

Secondly, some transport systems in membranes act only in one direction. For example, in the membranes of erythrocytes there is a transport system (“pump”) that pumps Na + ions from the cell into the environment, and K + ions into the cell due to the energy released during the hydrolysis of ATP.

Thirdly, the outer surface of plasma membranes contains very large number oligosaccharide groups, which are glycolipid heads and oligosaccharide side chains of glycoproteins, while there are practically no oligosaccharide groups on the inner surface of the plasma membrane.

The asymmetry of biological membranes is maintained due to the fact that the transfer of individual phospholipid molecules from one side of the lipid bilayer to the other is very difficult for energy reasons. A polar lipid molecule is able to move freely on its side of the bilayer, but is limited in its ability to jump to the other side.

Lipid mobility depends on the relative content and type of unsaturated fatty acids present. The hydrocarbon nature of the fatty acid chains imparts to the membrane properties of fluidity and mobility. In the presence of cis-unsaturated fatty acids, the cohesion forces between the chains are weaker than in the case of saturated fatty acids alone, and lipids remain highly mobile even at low temperatures.

On the outside of the membranes there are specific recognition regions, the function of which is to recognize certain molecular signals. For example, it is through the membrane that some bacteria perceive slight changes in the concentration of a nutrient, which stimulates their movement towards the food source; this phenomenon is called chemotaxis.

The membranes of various cells and intracellular organelles have a certain specificity due to their structure, chemical composition and functions. The following main groups of membranes in eukaryotic organisms are distinguished:

plasma membrane (outer cell membrane, plasmalemma),

· nuclear membrane,

endoplasmic reticulum,

membranes of the Golgi apparatus, mitochondria, chloroplasts, myelin sheaths,

excitable membranes.

In prokaryotic organisms, in addition to the plasma membrane, there are intracytoplasmic membrane formations; in heterotrophic prokaryotes they are called mesosomes. The latter are formed by invagination of the outer cell membrane and in some cases retain contact with it.

Red blood cell membrane consists of proteins (50%), lipids (40%) and carbohydrates (10%). The bulk of carbohydrates (93%) are associated with proteins, the rest with lipids. In the membrane, lipids are arranged asymmetrically, in contrast to the symmetrical arrangement in micelles. For example, cephalin is found predominantly in the inner lipid layer. This asymmetry is apparently maintained due to the transverse movement of phospholipids in the membrane, carried out with the help of membrane proteins and due to metabolic energy. The inner layer of the erythrocyte membrane contains mainly sphingomyelin, phosphatidylethanolamine, phosphatidylserine, and the outer layer contains phosphatidylcholine. The red blood cell membrane contains an integral glycoprotein glycophorin, consisting of 131 amino acid residues and penetrating the membrane, and the so-called band 3 protein, consisting of 900 amino acid residues. The carbohydrate components of glycophorin perform a receptor function for influenza viruses, phytohemagglutinins, and a number of hormones. Another integral protein was also found in the erythrocyte membrane, containing few carbohydrates and penetrating the membrane. They call him tunnel protein(component a), since it is assumed to form a channel for anions. A peripheral protein associated with the inner side of the erythrocyte membrane is spectrin.

Myelin membranes , surrounding the axons of neurons, are multilayered, they contain large number lipids (about 80%, half of them are phospholipids). The proteins of these membranes are important for fixing membrane salts lying on top of each other.

Chloroplast membranes. Chloroplasts are covered with a two-layer membrane. The outer membrane has some similarities with that of mitochondria. In addition to this surface membrane, chloroplasts have an internal membrane system - lamellae. The lamellae form either flattened vesicles - thylakoids, which, located one above the other, are collected in packs (granas) or form a stromal membrane system (stromal lamellae). The lamellae of the grana and stroma on the outer side of the thylakoid membrane are concentrated hydrophilic groups, galacto- and sulfolipids. The phytol part of the chlorophyll molecule is immersed in the globule and is in contact with the hydrophobic groups of proteins and lipids. The porphyrin nuclei of chlorophyll are mainly localized between the contacting membranes of the grana thylakoids.

Inner (cytoplasmic) membrane of bacteria its structure is similar to the internal membranes of chloroplasts and mitochondria. Enzymes of the respiratory chain and active transport are localized in it; enzymes involved in the formation of membrane components. The predominant component of bacterial membranes are proteins: the protein/lipid ratio (by weight) is 3:1. The outer membrane of gram-negative bacteria, compared to the cytoplasmic membrane, contains a smaller amount of various phospholipids and proteins. Both membranes differ in lipid composition. The outer membrane contains proteins that form pores for the penetration of many low-molecular substances. A characteristic component of the outer membrane is also a specific lipopolysaccharide. A number of outer membrane proteins serve as receptors for phages.

Virus membrane. Among viruses, membrane structures are characteristic of those containing a nucleocapsid, which consists of protein and nucleic acid. This “core” of viruses is surrounded by a membrane (envelope). It also consists of a lipid bilayer with embedded glycoproteins located mainly on the surface of the membrane. In a number of viruses (microviruses), 70-80% of all proteins are contained in the membranes; the remaining proteins are contained in the nucleocapsid.

Thus, cell membranes are very complex structures; their constituent molecular complexes form an ordered two-dimensional mosaic, which imparts biological specificity to the membrane surface.

Cell membrane

Image of a cell membrane. The small blue and white balls correspond to the hydrophobic heads of the phospholipids, and the lines attached to them correspond to the hydrophilic 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 - chains of oligosaccharides forming the glycocalyx

A biological membrane also includes various proteins: integral (penetrating the membrane through), semi-integral (immersed at one end in 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 between the cell membrane and the cytoskeleton inside the cell, and the cell wall (if there is one) outside. Some of the integral proteins function as ion channels, various transporters and receptors.

Functions

barrier - ensures regulated, selective, passive and active metabolism with the environment. For example, the peroxisome membrane protects the cytoplasm from peroxides that are dangerous to the cell. Selective permeability means that the permeability of the membrane to different atoms or molecules depends on their size, electrical charge and chemical properties. Selective permeability ensures that the cell and cellular compartments are separated from the environment and supplied with the necessary substances.
transport - transport of substances into and out of the cell occurs through the membrane. Transport through membranes ensures: delivery of nutrients, removal of metabolic end products, secretion of various substances, creation of ion gradients, maintenance of optimal ion concentrations in the cell that are necessary for the functioning of cellular enzymes.
Particles that for any reason are unable 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 due to 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 expending energy along a concentration gradient by diffusion. A variant of this mechanism is facilitated diffusion, in which a specific molecule helps a substance 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 - ensures 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 a major role in ensuring mechanical function, and in animals, the 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 located in the membrane are receptors (molecules with the help of which the cell perceives certain signals).
For example, hormones circulating in the blood act only on target cells that have receptors corresponding to these hormones. Neurotransmitters ( chemicals, ensuring the conduction of nerve impulses) also bind to special receptor proteins of 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 ensures the maintenance of the potential difference on the membrane and the generation of 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”. Because of the myriad configurations of side chains, 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, in the formation of organs and tissues. This 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) consist of two long hydrophobic hydrocarbon tails that are connected to a charged hydrophilic head. Cholesterol gives the membrane rigidity by occupying the free space between the hydrophobic tails of lipids and preventing them from bending. Therefore, membranes with a low cholesterol content are more flexible, and those with a high cholesterol content are more rigid and fragile. Cholesterol also serves as a “stopper” that prevents the movement of polar molecules from the cell and into the cell. An important part of the membrane consists of proteins that penetrate it and are responsible for the various properties of membranes. Their composition and orientation differ in different membranes.

Cell membranes are often asymmetrical, 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 the endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles, peroxisomes; to double membranes - nucleus, mitochondria, plastids. 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, to a certain extent, actively regulate this process - some substances pass through, but others do not. There are four main mechanisms for the entry of substances into the cell or their removal from the cell to the outside: diffusion, osmosis, active transport and exo- or endocytosis. The first two processes are passive in nature, that is, they do not require energy; 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 right through, forming a kind of passage. The elements K, Na and Cl have their own channels. Relative to the concentration gradient, the molecules of these elements move in and out of the cell. When irritated, the sodium ion channels open and a sudden influx of sodium ions into the cell occurs. In this case, an imbalance of membrane potential occurs. After which the membrane potential is restored. Potassium channels are always open, allowing potassium ions to slowly enter the cell.

Literature

Antonov V.F., Smirnova E.N., Shevchenko E.V. Lipid membranes during phase transitions. - M.: Science, 1994.
Gennis R. Biomembranes. Molecular structure and functions: translation from English. = Biomembranes. Molecular structure and function (by Robert B. Gennis). - 1st edition. - M.: Mir, 1997. - ISBN 5-03-002419-0
Ivanov V. G., Berestovsky T. N. Lipid bilayer of biological membranes. - M.: Nauka, 1982.
Rubin A. B. Biophysics, textbook in 2 vols. - 3rd edition, corrected and expanded. - M.: Moscow University Publishing House, 2004. -

The cell membrane (plasma membrane) is a thin, semi-permeable membrane that surrounds cells.

Function and role of the cell membrane

Its function is to protect the integrity of the interior by allowing some necessary substances into the cage, and not allowing others to enter.

It also serves as the basis for attachment to some organisms and to others. Thus, the plasma membrane also provides the shape of the cell. Another function of the membrane is to regulate cell growth through balance and.

In endocytosis, lipids and proteins are removed from the cell membrane as substances are absorbed. During exocytosis, vesicles containing lipids and proteins fuse with the cell membrane, increasing cell size. , and fungal cells have plasma membranes. Internal ones, for example, are also enclosed in protective membranes.

Cell membrane structure

The plasma membrane is mainly composed of a mixture of proteins and lipids. Depending on the location and role of the membrane in the body, lipids can make up 20 to 80 percent of the membrane, with the remainder being proteins. While lipids help give the membrane flexibility, proteins control and maintain chemical composition cells and also help in the transport of molecules across the membrane.

Membrane lipids

Phospholipids are the main component of plasma membranes. They form a lipid bilayer in which the hydrophilic (water-attracted) head regions spontaneously organize to face the aqueous cytosol and extracellular fluid, while the hydrophobic (water-repelled) tail regions face away from the cytosol and extracellular fluid. The lipid bilayer is semi-permeable, allowing only some molecules to diffuse across the membrane.

Cholesterol is another lipid component of animal cell membranes. Cholesterol molecules are selectively dispersed between membrane phospholipids. This helps maintain the rigidity of cell membranes by preventing phospholipids from becoming too dense. Cholesterol is absent in plant cell membranes.

Glycolipids are located on the outer surface of cell membranes and are connected to them by a carbohydrate chain. They help the cell recognize other cells in the body.

Membrane proteins

The cell membrane contains two types of associated proteins. Proteins of the peripheral membrane are external and are associated with it by interacting with other proteins. Integral membrane proteins are introduced into the membrane and most pass through. Parts of these transmembrane proteins are located on both sides of it.

Plasma membrane proteins have a number of different functions. Structural proteins provide support and shape to cells. Membrane receptor proteins help cells communicate with their external environment via hormones, neurotransmitters and other signaling molecules. Transport proteins, such as globular proteins, transport molecules across cell membranes by facilitated diffusion. Glycoproteins have a carbohydrate chain attached to them. They are embedded in the cell membrane, helping in the exchange and transport of molecules.

Organelle membranes

Some cellular organelles are also surrounded by protective membranes. core,

The cell membrane is an ultrathin film on the surface of a cell or cellular organelle, consisting of a bimolecular layer of lipids with embedded proteins and polysaccharides.

Membrane functions:

· Barrier - provides regulated, selective, passive and active metabolism with the environment. For example, the peroxisome membrane protects the cytoplasm from peroxides that are dangerous to the cell. Selective permeability means that the permeability of the membrane to different atoms or molecules depends on their size, electric charge and chemical properties. Selective permeability ensures that the cell and cellular compartments are separated from the environment and supplied with the necessary substances.
· Transport - transport of substances into and out of the cell occurs through the membrane. Transport through membranes ensures: delivery of nutrients, removal of metabolic end products, secretion of various substances, creation of ion gradients, maintenance of optimal pH and ion concentrations in the cell, which are necessary for the functioning of cellular enzymes. Particles that for any reason are unable 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 due to 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 expending energy along a concentration gradient by diffusion. A variant of this mechanism is facilitated diffusion, in which a specific molecule helps a substance 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 - ensures 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 a major role in ensuring mechanical function, and in animals, the 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 located in the membrane are receptors (molecules with the help of which the cell perceives certain signals). For example, hormones circulating in the blood act only on target cells that have receptors corresponding to these hormones. Neurotransmitters (chemicals that ensure the conduction of nerve impulses) also bind to special receptor proteins in 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 ensures the maintenance of the potential difference on the membrane and the generation of 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”. Because of the myriad configurations of side chains, 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, in the formation of organs and tissues. This also allows the immune system to recognize foreign antigens.

Some protein molecules diffuse freely in the plane of the lipid layer; in the normal state, parts of protein molecules that exit along different sides cell membrane do not change their position.

The special morphology of cell membranes determines their electrical characteristics, among which the most important are capacitance and conductivity.

Capacitive properties are mainly determined by the phospholipid bilayer, which is impermeable to hydrated ions and at the same time thin enough (about 5 nm) to allow efficient charge separation and storage, and electrostatic interaction of cations and anions. In addition, the capacitive properties of cell membranes are one of the reasons that determine the time characteristics of electrical processes occurring on cell membranes.

Conductivity (g) -- reciprocal electrical resistance and equal to the ratio of the total transmembrane current for a given ion to the value that determined its transmembrane potential difference.

Can diffuse through the phospholipid bilayer various substances, and the degree of permeability (P), i.e. the ability of the cell membrane to pass these substances, depends on the difference in the concentrations of the diffusing substance on both sides of the membrane, its solubility in lipids and the properties of the cell membrane. The rate of diffusion for charged ions under constant field conditions in a membrane is determined by the mobility of ions, the thickness of the membrane, and the distribution of ions in the membrane. For nonelectrolytes, the permeability of the membrane does not affect its conductivity, since nonelectrolytes do not carry charges, i.e., they cannot carry electric current.

The conductivity of a membrane is a measure of its ionic permeability. An increase in conductivity indicates an increase in the number of ions passing through the membrane.

An important property of biological membranes is fluidity. All cell membranes are mobile fluid structures: most of their constituent lipid and protein molecules are capable of moving quite quickly in the plane of the membrane