Membrane - what is it? Biological membrane: functions and structure. The structure and properties of biological cell membranes

cell membrane (also cytolemma, plasmalemma, or plasma membrane) - an elastic molecular structure consisting of proteins and lipids. Separates the contents of any cell from external environment, ensuring its integrity; regulates the exchange between the cell and the environment; intracellular membranes divide the cell into specialized closed compartments - compartments or organelles, in which certain environmental conditions are maintained.

If there is one in the cell (usually in plant cells), it covers the cell membrane.

The cell membrane is a double layer (bilayer) of lipid class molecules, most of which are so-called complex lipids - phospholipids. Lipid molecules have a hydrophilic (“head”) and a hydrophobic (“tail”) part. During the formation of membranes, the hydrophobic portions of the molecules turn inward, while the hydrophilic portions turn outward. 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),
• superficial (located on the outer or adjacent to the inner sides of the membrane).

Some proteins are the contact points of the cell membrane with the cytoskeleton inside the cell and the cell wall outside.

Membrane functions:

• Barrier - provides a regulated, selective, passive and active metabolism with the environment.
• Transport - through the membrane, substances are transported into the cell and out of the cell. Transport across membranes provides: delivery nutrients, removal of end products of metabolism, secretion of various substances, creation of ionic gradients, maintenance of optimal pH and concentration of ions in the cell, which are necessary for the functioning of cellular enzymes.
• 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.
• Energy - during photosynthesis in chloroplasts and cellular respiration in mitochondria, energy transfer systems operate in their membranes, in which proteins also participate.

Membranes are composed of three classes of lipids:

• phospholipids,
• glycolipids,
• cholesterol.

Phospholipids and glycolipids(lipids with carbohydrates attached to them) consist of two long hydrophobic hydrocarbon "tails" that are connected to a charged hydrophilic "head".

cholesterol stiffens the membrane free space between hydrophobic tails of lipids and not allowing them to bend. 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 proteins, penetrating it and responsible for various properties of membranes. Their composition and orientation in different membranes differ. Next to the proteins are annular lipids - they are more ordered, less mobile, contain more saturated fatty acids and are released from the membrane along with the protein. Without annular lipids, membrane proteins do not work.

Cell membranes are often asymmetrical that is, the layers differ in lipid composition, the outer layer contains mainly phosphatidylinositol, phosphatidylcholine, sphingomyelins and glycolipids, while the inner layer contains phosphatidylserine, phosphatidylethanolamine and phosphatidylinositol. The transition of an individual molecule from one layer to another (the so-called flip flop) is difficult, but can occur spontaneously, about once every 6 months, or with the help of flippase proteins and scramblase of the plasma membrane. If phosphatidylserine appears in the outer layer, this is a signal for macrophages to destroy the cell.

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

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 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 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. After that, the membrane potential is restored. Potassium channels are always open, through which potassium ions slowly enter the cell.

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 has alkaline reaction. Feature eukaryotic cell cytoplasm - 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 in 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 environment and 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); four) osmosis- transport of water molecules (in all biological systems, water is the solvent).

Need 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 operation cycle of Na + /K + -pump can be divided into the following phases: 1) addition of Na + with inside membranes, 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 structures separated 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.

Go to lectures number 5"Cell Theory. Types of cellular organization»

Go to lectures number 7"Eukaryotic cell: structure and functions of organelles"

The structure of the biomembrane. The cell-bounding membranes and membrane organelles of eukaryotic cells share a common chemical composition and structure. They include lipids, proteins and carbohydrates. Membrane lipids are mainly represented by phospholipids and cholesterol. Most membrane proteins are complex proteins such as glycoproteins. Carbohydrates do not occur on their own in the membrane, they are associated with proteins and lipids. The thickness of the membranes is 7-10 nm.

According to the currently accepted fluid mosaic model of membrane structure, lipids form a double layer, or lipid bilayer, in which the hydrophilic "heads" of lipid molecules are turned outward, and the hydrophobic "tails" are hidden inside the membrane (Fig. 2.24). These “tails”, due to their hydrophobicity, ensure the separation of the aqueous phases of the internal environment of the cell and its environment. With lipids various types interactions bound proteins. Some of the proteins are located on the surface of the membrane. Such proteins are called peripheral, or superficial. Other proteins are partially or completely immersed in the membrane - these are integral, or immersed proteins. Membrane proteins perform structural, transport, catalytic, receptor, and other functions.

Membranes are not like crystals, their components are constantly in motion, as a result of which gaps appear between lipid molecules - pores through which various substances can enter or leave the cell.

Biological membranes differ in their location in the cell, their chemical composition, and their functions. The main types of membranes are plasma and internal.

plasma membrane(Fig. 2.24) contains about 45% lipids (including glycolipids), 50% proteins and 5% carbohydrates. Chains of carbohydrates that make up complex proteins-glycoproteins and complex lipids-glycolipids protrude above the surface of the membrane. Plasmalemmal glycoproteins are extremely specific. So, for example, through them there is a mutual recognition of cells, including sperm and eggs.

On the surface of animal cells, carbohydrate chains form a thin surface layer - glycocalyx. It has been found in almost all animal cells, but its severity is not the same (10-50 microns). The glycocalyx provides a direct connection of the cell with the external environment; extracellular digestion occurs in it; receptors are located in the glycocalyx. The cells of bacteria, plants and fungi, in addition to the plasmalemma, are also surrounded by cell membranes.

Internal membranes eukaryotic cells delimit various parts of the cell, forming a kind of "compartments" - compartments, which contributes to the separation of various processes of metabolism and energy. They may differ in chemical composition and performed functions, but overall plan their buildings are preserved.

Membrane functions:

1. Limiting. It consists in the fact that they separate the internal space of the cell from the external environment. The membrane is semi-permeable, that is, only those substances that are necessary for the cell can freely overcome it, while there are mechanisms for transporting the necessary substances.

2. Receptor. It is associated primarily with the perception of environmental signals and the transfer of this information into the cell. Special receptor proteins are responsible for this function. Membrane proteins are also responsible for cellular recognition according to the “friend or foe” principle, as well as for the formation of intercellular connections, the most studied of which are the synapses of nerve cells.

3. catalytic. Numerous enzyme complexes are located on the membranes, as a result of which intensive synthetic processes take place on them.

4. Energy transforming. Associated with the formation of energy, its storage in the form of ATP and expenditure.

5. Compartmentalization. The membranes also delimit the space inside the cell, thereby separating the initial substances of the reaction and the enzymes that can carry out the corresponding reactions.

6. Formation of intercellular contacts. Despite the fact that the thickness of the membrane is so small that it cannot be distinguished with the naked eye, on the one hand, it serves as a fairly reliable barrier for ions and molecules, especially water-soluble ones, and on the other hand, it ensures their transfer into the cell and out.

membrane transport. Due to the fact that cells as elementary biological systems are open systems, to ensure metabolism and energy, maintain homeostasis, growth, irritability and other processes, the transfer of substances through the membrane is required - membrane transport (Fig. 2.25). Currently, the transport of substances across the cell membrane is divided into active, passive, endo- and exocytosis.

Passive transport- this is a type of transport that occurs without the expenditure of energy from a higher concentration to a lower one. Small non-polar molecules (0 2 , CO 2 ) soluble in lipids easily penetrate the cell by simple diffusion. Insoluble in lipids, including charged small particles, are picked up by carrier proteins or pass through special channels (glucose, amino acids, K +, PO 4 3-). This type of passive transport is called facilitated diffusion. Water enters the cell through pores in the lipid phase, as well as through special channels lined with proteins. The transport of water across a membrane is called osmosis(Fig. 2.26).

Osmosis is extremely important in the life of a cell, because if it is placed in a solution with a higher concentration of salts than in a cell solution, then water will begin to leave the cell, and the volume of living contents will begin to decrease. In animal cells, the cell as a whole shrinks, and in plant cells, the cytoplasm lags behind the cell wall, which is called plasmolysis(Fig. 2.27).

When a cell is placed in a solution less concentrated than the cytoplasm, water transport occurs in reverse direction- in a cell. However, there are limits to the extensibility of the cytoplasmic membrane, and the animal cell eventually ruptures, while in the plant cell this is not allowed by a strong cell wall. The phenomenon of filling everything with cellular content inner space cells called deplasmolysis. Intracellular salt concentration should be taken into account when preparing medicines, especially for intravenous administration, as this can lead to damage to blood cells (for this, saline with a concentration of 0.9% sodium chloride is used). This is no less important in the cultivation of cells and tissues, as well as organs of animals and plants.

active transport proceeds with the expenditure of ATP energy from a lower concentration of a substance to a higher one. It is carried out with the help of special proteins-pumps. Proteins pump ions K +, Na +, Ca 2+ and others through the membrane, which contributes to the transport of the most important organic matter, as well as the emergence of nerve impulses, etc.

Endocytosis- this is an active process of absorption of substances by the cell, in which the membrane forms invaginations, and then forms membrane vesicles - phagosomes in which the absorbed objects are enclosed. The primary lysosome then fuses with the phagosome to form secondary lysosome, or phagolysosome, or digestive vacuole. The contents of the vesicle are cleaved by lysosome enzymes, and the cleavage products are absorbed and assimilated by the cell. Undigested residues are removed from the cell by exocytosis. There are two main types of endocytosis: phagocytosis and pinocytosis.

Phagocytosis- this is the process of capture by the cell surface and absorption of solid particles by the cell, and pinocytosis- liquids. Phagocytosis occurs mainly in animal cells (single-celled animals, human leukocytes), it provides their nutrition, and often the protection of the body (Fig. 2.28).

Through pinocytosis, the absorption of proteins, antigen-antibody complexes in the process of immune reactions, etc. occurs. However, many viruses also enter the cell through pinocytosis or phagocytosis. In the cells of plants and fungi, phagocytosis is practically impossible, as they are surrounded by strong cell membranes.

Exocytosis is the reverse process of endocytosis. Thus, undigested food residues are released from the digestive vacuoles, the substances necessary for the life of the cell and the organism as a whole are removed. For example, the transmission of nerve impulses occurs due to the release of chemical mediators by the neuron that sends the impulse - mediators, and in plant cells, auxiliary carbohydrates of the cell membrane are released in this way.

Cell walls of plant cells, fungi and bacteria. Outside of the membrane, the cell can secrete a strong framework - cell membrane, or cell wall.

In plants, the cell wall is made up of cellulose, packed in bundles of 50-100 molecules. The gaps between them are filled with water and other carbohydrates. The shell of a plant cell is permeated with channels - plasmodesmata(Fig. 2.29), through which the membranes of the endoplasmic reticulum pass.

The plasmodesmata transport substances between cells. However, the transport of substances, such as water, can also occur along the cell walls themselves. Over time, various substances, including tannins or fat-like substances, accumulate in the cell membrane of plants, which leads to lignification or corking of the cell wall itself, the displacement of water and the death of cellular contents. Between the cell walls of neighboring plant cells there are jelly-like pads - middle plates that fasten them together and cement the body of the plant as a whole. They are destroyed only in the process of fruit ripening and when the leaves fall.

The cell walls of fungal cells are formed chitin- carbohydrate containing nitrogen. They are strong enough and are the outer skeleton of the cell, but still, like in plants, they prevent phagocytosis.

In bacteria, the cell wall contains carbohydrate with fragments of peptides - murein, however, its content differs significantly in different groups bacteria. Outside of the cell wall, other polysaccharides can also be released, forming a mucous capsule that protects bacteria from external influences.

The shell determines the shape of the cell, serves as a mechanical support, performs a protective function, provides the osmotic properties of the cell, limiting the stretching of the living contents and preventing the rupture of the cell, which increases due to the influx of water. In addition, water and substances dissolved in it overcome the cell wall before entering the cytoplasm or, conversely, when leaving it, while water is transported along the cell walls faster than through the cytoplasm.

cell membrane - molecular structure that is made up of lipids and proteins. Its main properties and functions:

• separation of the contents of any cell from the external environment, ensuring its integrity;
• management and adjustment of the exchange between the environment and the cell;
• intracellular membranes divide the cell into special compartments: organelles or compartments.

The word "membrane" in Latin means "film". If we talk about the cell membrane, then this is a combination of two films that have different properties.

The biological membrane includes three types of proteins:

1. Peripheral - located on the surface of the film;
2. Integral - completely penetrate the membrane;
3. Semi-integral - at one end penetrate into the bilipid layer.

What are the functions of the cell membrane

1. Cell wall - a strong shell of the cell, which is located outside the cytoplasmic membrane. It performs protective, transport and structural functions. Present in many plants, bacteria, fungi and archaea.

2. Provides a barrier function, that is, selective, regulated, active and passive metabolism with the external environment.

3. Able to transmit and store information, and also takes part in the process of reproduction.

4. Performs transport function which can transport substances into and out of the cell across the membrane.

5. The cell membrane has one-way conductivity. Due to this, water molecules can pass through the cell membrane without delay, and molecules of other substances penetrate selectively.

6. With the help of the cell membrane, water, oxygen and nutrients are obtained, and the products of cellular metabolism are removed through it.

7. Performs cell exchange across membranes, and can perform them through 3 main types of reactions: pinocytosis, phagocytosis, exocytosis.

8. The membrane provides the specificity of intercellular contacts.

9. There are numerous receptors in the membrane that are able to perceive chemical signals - mediators, hormones and many other biologically active substances. So she is able to change the metabolic activity of the cell.

10. Basic properties and functions of the cell membrane:

• matrix
• Barrier
• Transport
• Energy
• Mechanical
• Enzymatic
• Receptor
• Protective
• Marking
• Biopotential

What is the function of the plasma membrane in the cell?

1. Delimits the contents of the cell;
2. Carries out the flow of substances into the cell;
3. Provides removal of a number of substances from the cell.

cell membrane structure

Cell membranes include lipids of 3 classes:

• Glycolipids;
• Phospholipids;
• Cholesterol.

Basically, the cell membrane consists of proteins and lipids, and has a thickness of no more than 11 nm. From 40 to 90% of all lipids are phospholipids. It is also important to note glycolipids, which are one of the main components of the membrane.

The structure of the cell membrane is three-layered. A homogeneous liquid bilipid layer is located in the center, and proteins cover it from both sides (like a mosaic), partly penetrating into the thickness. Proteins are also necessary for the membrane to pass inside the cells and transport out of them special substances that cannot penetrate the fat layer. For example, sodium and potassium ions.

• It is interesting -

Cell structure - video

Table number 2

Question 1 (8)

cell membrane(or cytolemma, or plasmalemma, or plasma membrane) separates the contents of any cell from the external environment, ensuring its integrity; regulates the exchange between the cell and the environment; intracellular membranes divide the cell into specialized closed compartments - compartments or organelles, in which certain environmental conditions are maintained.

Functions of the cell or plasma membrane

The membrane provides:

1) Selective penetration into and out of the cell of molecules and ions necessary to perform specific cell functions;
2) Selective transport of ions across the membrane, maintaining a transmembrane electric potential difference;
3) The specifics of intercellular contacts.

Due to the presence in the membrane of numerous receptors that perceive chemical signals - hormones, mediators and other biologically active substances, it is able to change the metabolic activity of the cell. Membranes provide the specificity of immune manifestations due to the presence of antigens on them - structures that cause the formation of antibodies that can specifically bind to these antigens.
The nucleus and organelles of the cell are also separated from the cytoplasm by membranes that prevent the free movement of water and substances dissolved in it from the cytoplasm to them and vice versa. This creates conditions for the separation of biochemical processes occurring in different compartments (compartments) inside the cell.

cell membrane structure

cell membrane- elastic structure, thickness from 7 to 11 nm (Fig. 1.1). It consists mainly of lipids and proteins. From 40 to 90% of all lipids are phospholipids - phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, sphingomyelin and phosphatidylinositol. An important component membranes are glycolipids represented by cerebrisides, sulfatides, gangliosides and cholesterol.

The main structure of the cell membrane is a double layer of phospholipid molecules. Due to hydrophobic interactions, the carbohydrate chains of lipid molecules are held near each other in an extended state. Groups of phospholipid molecules of both layers interact with protein molecules immersed in the lipid membrane. Due to the fact that most of the lipid components of the bilayer are in a liquid state, the membrane has mobility and undulates. Its sections, as well as proteins immersed in the lipid bilayer, will mix from one part to another. Mobility (fluidity) of cell membranes facilitates the transport of substances through the membrane.

cell membrane proteins represented mainly by glycoproteins.

Distinguish

integral proteins penetrating through the entire thickness of the membrane and

peripheral proteins attached only to the surface of the membrane, mainly to its inner part.

Peripheral proteins almost all function as enzymes (acetylcholinesterase, acid and alkaline phosphatases, etc.). But some enzymes are also represented by integral proteins - ATPase.

integral proteins provide a selective exchange of ions through the membrane channels between the extracellular and intracellular fluid, and also act as proteins - carriers of large molecules.

Membrane receptors and antigens can be represented by both integral and peripheral proteins.

Proteins adjacent to the membrane from the cytoplasmic side belong to cell cytoskeleton. They can attach to membrane proteins.

So, protein strip 3(band number during protein electrophoresis) of erythrocyte membranes is combined into an ensemble with other cytoskeleton molecules - spectrin through the low molecular weight protein ankyrin

Spectrin is the main protein of the cytoskeleton, constituting a two-dimensional network to which actin is attached.

Actin forms microfilaments, which are the contractile apparatus of the cytoskeleton.

cytoskeleton allows the cell to exhibit flexibly elastic properties, provides additional strength to the membrane.

Most integral proteins are glycoproteins. Their carbohydrate part protrudes from the cell membrane to the outside. Many glycoproteins have a large negative charge due to the significant content of sialic acid (for example, the glycophorin molecule). This provides the surface of most cells with a negative charge, helping to repel other negatively charged objects. Carbohydrate protrusions of glycoproteins carry blood group antigens, other antigenic determinants of the cell, and act as hormone-binding receptors. Glycoproteins form adhesive molecules that cause cells to attach to each other, i.e. close intercellular contacts.