Cell membrane. Cytoplasmic membrane: functions, structure

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 are 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.

This 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 connected from the outside 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.

Cells are characterized by a membrane principle of structure.

biological membrane - a thin film, protein-lipid structure, 7 - 10 nm thick, located on the surface of cells (cell membrane), forming the walls of most organelles and the shell of the nucleus.

In 1972, S. Singer and G. Nichols proposed fluid mosaic model structure of the cell membrane. Later it was practically confirmed. When viewed under an electron microscope, three layers can be seen. Medium, light, forms the basis of the membrane - the bilipid layer formed by liquid phospholipids ("lipid sea"). Molecules of membrane lipids (phospholipids, glycolipids, cholesterol, etc.) have hydrophilic heads and hydrophobic tails, therefore they are orderly oriented in the bilayer. The two dark layers are proteins that are located differently relative to the lipid bilayer: peripheral (adjacent) - most proteins are located on both surfaces of the lipid layer; semi-integral (semi-submerged) - penetrate only one layer of lipids; integral (submerged) pass through both layers. Proteins have hydrophobic regions that interact with lipids, and hydrophilic regions on the surface of the membrane in contact with the aqueous contents of the cell, or tissue fluid.

Functions of biological membranes:

1) delimits the contents of the cell from the external environment and the contents of organelles, the nucleus from the cytoplasm;

2) provide transport of substances into and out of the cell, into the cytoplasm from organelles and vice versa;

3) participate in receiving and converting signals from the environment, recognizing cell substances, etc.;

4) provide near-membrane processes;

5) participate in the transformation of energy.

Cytoplasmic membrane (plasmalemma, cell membrane, plasma membrane) - the biological membrane surrounding the cell; the main component of the surface apparatus, universal for all cells. Its thickness is about 10 nm. It has a structure characteristic of biological membranes. In the cytoplasmic membrane, the hydrophilic heads of lipids face the outer and inner sides of the membrane, while the hydrophobic tails face the inside of the membrane. Peripheral proteins associated with the polar heads of lipid molecules by hydrostatic interactions. They do not form a continuous layer. Peripheral proteins bind the plasmalemma to supra- or submembrane structures of the surface apparatus. Some lipid and protein molecules in the plasmalemma of animal cells have covalent bonds with oligo-iposaccharide molecules located on the outer surface of the membrane. Highly branched molecules form glycolipids and glycoproteins with lipids and proteins, respectively. saccharide layer glycocalyx (lat. glycis- sweet and callum- thick skin) covers the entire surface of the cell and is an epimembrane complex of the animal cell. Oligosaccharide and polysaccharide chains (antennas) perform a number of functions: recognition of external signals; adhesion of cells, their correct orientation during tissue formation; immune response, where glycoproteins play the role of an immune response.

Rice. The structure of the plasmalemma

The chemical composition of the plasma membrane: 55% - proteins, 35-40% - lipids, 2-10% - carbohydrates.

The outer cell membrane forms a mobile surface of the cell, which can have outgrowths and protrusions, performs undulating oscillatory movements, macromolecules are constantly moving in it. The cell surface is heterogeneous: its structure is not the same in different areas, and the physiological properties of these areas are also not the same. Some enzymes (about 200) are localized in the plasmalemma, so the effect of environmental factors on the cell is mediated by its cytoplasmic membrane. The surface of the cell has high strength and elasticity, it is easily and quickly restored after minor damage.

The structure of the plasma membrane determines its properties:

Plasticity (fluidity), allows the membrane to change its shape and size;

The ability to self-closing, enables the membrane to restore integrity when ruptured;

Selective permeability provides the passage of various substances through the membrane at different speeds.

The main functions of the cytoplasmic membrane:

determines and maintains the shape of the cell ( shaping);

delimits the internal contents of the cell ( barrier), playing the role of a mechanical barrier; the actual barrier function is provided by the bilipid layer, preventing the contents from spreading and preventing the penetration of foreign substances into the cell;

Protects the cell from mechanical influences ( protective);

Regulates the metabolism between the cell and the environment, ensuring the constancy of the intracellular composition ( regulatory);

recognizes external signals, “recognizes” certain substances (for example, hormones) ( receptor); some plasma membrane proteins (hormone receptors; B-lymphocyte receptors; integral proteins that perform specific enzymatic functions that carry out processes of parietal digestion) are able to recognize certain substances and bind to them, thus receptor backs are involved in the selection of molecules entering the cell;

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 the physicochemical properties of complex lipids and proteins that make up them. Cell membranes are located in an aqueous medium, and to understand the physicochemical phenomena that affect their 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 hydrophilic and hydrophobic properties of molecules are realized in the form of 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 mechanical forces, pressures, temperatures, and other factors act on it. 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 take 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 with the help of 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 perform rotational movements, diffuse in the plane of the bilayer. Protein molecules are able to change their spatial structure (conformation), shift and change 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 high conformational mobility of acyl radicals of fatty acids in 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 of membrane proteins, the following can be singled out: 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.

Cytoplasm- an obligatory part of the cell, enclosed between the plasma membrane and the nucleus; It is subdivided into hyaloplasm (the main substance of the cytoplasm), organelles (permanent components of the cytoplasm) and inclusions (temporary components of the cytoplasm). The 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 is alkaline. 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 being in constant motion can it perform its functions.

Hyaloplasm ( cytosol) is a colorless, slimy, thick and transparent colloidal solution. It is in it that all metabolic processes take place, it provides 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- denser hyaloplasm. Mutual transitions are possible between them: the gel turns into a sol and vice versa.

Functions of the cytoplasm:

1. integration of all components of the cell into a single system,
2. environment for the passage of many biochemical and physiological processes,
3. environment for the existence and functioning of organelles.

Cell walls

Cell walls limit eukaryotic cells. At least two layers can be distinguished in each cell membrane. 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. Currently generally accepted fluid mosaic model of the 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 in which the residue of phosphoric acid is located is called the hydrophilic head, the sections in which fatty acid residues are located are called 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 outwards, 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.). Distinguish: 1) peripheral proteins(located on the outer or inner surface of the lipid bilayer), 2) semi-integral proteins(immersed in the lipid bilayer to different depths), 3) integral or transmembrane proteins(permeate the membrane through and through, while in contact with both the external and internal environment of the cell). Integral proteins in some cases are called channel-forming, or channel, 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 head of the phospholipid; C, hydrophobic tails of the phospholipid; 1 - hydrophobic regions of proteins E and F; 2, hydrophilic regions of protein F; 3 - a 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). Basically, carbohydrates are located on the outer surface of the membrane. Carbohydrates provide receptor functions of the membrane. In animal cells, glycoproteins form an epimembrane complex, the glycocalyx, several tens of nanometers thick. Many cell receptors are located in it, with its help cell adhesion occurs.

Molecules of proteins, carbohydrates and lipids are mobile, able to move in the plane of the membrane. The thickness of the plasma membrane is approximately 7.5 nm.

Membrane functions

The membranes perform the following functions:

1. separation of cellular contents from the external environment,
2. regulation of metabolism between the cell and the environment,
3. division of the cell into compartments ("compartments"),
4. location of "enzymatic conveyors",
5. providing communication between cells in the tissues of multicellular organisms (adhesion),
6. 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, which ensures the exchange of substances between the cell and the external environment. The process by which substances pass through the cell membrane is called transport of substances. Distinguish: 1) passive transport- the process of passing substances, going without energy; 2) active transport- the process of passing substances, going with the cost of energy.

At passive transport substances move from an area with a higher concentration to an area with a lower one, i.e. along the concentration gradient. In any solution there are molecules of the solvent and the solute. The process of movement of solute molecules is called diffusion, the movement of solvent molecules is called osmosis. If the molecule is charged, then its transport is affected by the electrical gradient. Therefore, one often speaks of 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, water is the solvent).

Necessity active transport occurs when it is necessary to ensure the transfer of molecules through the membrane against the 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 freely diffuse through the membrane pores. The Na+/K+ pump counteracts the equalization of these ion concentrations 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, so that it can attach both K + and Na + . The cycle of operation of Na + /K + -pump can be divided into the following phases: 1) attachment of Na + from the inside of the membrane, 2) phosphorylation of the pump protein, 3) release of Na + in the extracellular space, 4) attachment of K + from the outside of the membrane , 5) dephosphorylation of the pump protein, 6) release of K + in the intracellular space. The sodium-potassium pump consumes almost a third of all the energy necessary for the life of the cell. 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 the 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 the epithelial cells of the intestine, in the endothelium of blood capillaries.

Exocytosis- the reverse process of endocytosis: the removal of various substances from the cell. During exocytosis, the vesicle membrane fuses 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 excreted from the cells of the endocrine glands, and in protozoa, undigested food remains.

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