Cell membrane and its functions. What is the function of the outer cell membrane? The structure of the outer cell membrane

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. Proteins are associated with lipids through various types of interactions. 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. Plasmalemma 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 different 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 functions, but they retain the general plan of the structure.

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 membrane thickness 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 the cell, because if it is placed in a solution with a higher concentration of salts than in the 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 is transported in the opposite direction - into the 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 the entire internal space of the cell with cellular contents is called deplasmolysis. Intracellular salt concentration should be taken into account in the preparation of drugs, especially for intravenous administration, as this can lead to damage to blood cells (for this, physiological 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 substances, 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).

By way of pinocytosis, the absorption of proteins, antigen-antibody complexes in the process of immune reactions, etc. occurs. However, many viruses also enter the cell by way of 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 plant body 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 varies significantly in different groups of 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.

The membrane is a hyperfine structure that forms the surface of organelles and the cell as a whole. All membranes have a similar structure and are connected in one system.

Chemical composition

Cell membranes are chemically homogeneous and consist of proteins and lipids of various groups:

phospholipids;
galactolipids;
sulfolipids.

They also contain nucleic acids, polysaccharides and other substances.

Physical Properties

At normal temperature, the membranes are in a liquid-crystalline state and constantly fluctuate. Their viscosity is close to that of vegetable oil.

The membrane is recoverable, strong, elastic and has pores. The thickness of the membranes is 7 - 14 nm.

Model

The structure of membranes is usually described using a fluid mosaic model. The membrane has a frame - two rows of lipid molecules, tightly, like bricks, adjacent to each other.

Rice. 1. Sandwich-type biological membrane.

On both sides, the surface of lipids is covered with proteins. The mosaic pattern is formed by protein molecules unevenly distributed on the surface of the membrane.

According to the degree of immersion in the bilipid layer, protein molecules are divided into three groups:

transmembrane;
submerged;
superficial.

Proteins provide the main property of the membrane - its selective permeability for various substances.

Membrane types

All cell membranes according to localization can be divided into the following types:

outdoor;
nuclear;
organelle membranes.

The outer cytoplasmic membrane, or plasmolemma, is the boundary of the cell. Connecting with elements of the cytoskeleton, it maintains its shape and size.

Rice. 2. Cytoskeleton.

The nuclear membrane, or karyolemma, is the boundary of the nuclear content. It is built from two membranes, very similar to the outer one. The outer membrane of the nucleus is connected to the membranes of the endoplasmic reticulum (ER) and, through pores, to the inner membrane.

EPS membranes penetrate the entire cytoplasm, forming surfaces on which various substances are synthesized, including membrane proteins.

Organoid membranes

Most organelles have a membrane structure.

Walls are built from one membrane:

Golgi complex;
vacuoles;
lysosomes.

Plastids and mitochondria are built from two layers of membranes. Their outer membrane is smooth, and the inner one forms many folds.

Features of the photosynthetic membranes of chloroplasts are embedded chlorophyll molecules.

Animal cells have a carbohydrate layer called the glycocalyx on the surface of the outer membrane.

Rice. 3. Glycocalyx.

The glycocalyx is most developed in the cells of the intestinal epithelium, where it creates conditions for digestion and protects the plasmolemma.

Table "Structure of the cell membrane"

What have we learned?

We examined the structure and functions of the cell membrane. The membrane is a selective (selective) barrier of the cell, nucleus and organelles. The structure of the cell membrane is described by a fluid-mosaic model. According to this model, protein molecules are embedded in a double layer of viscous lipids.

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Cell structure

Cell theory.

Plan

Cell is the basic structural unit of a living organism.

1.Cell theory.

2. The structure of the cell.

3. Evolution of the cell.

In 1665 R. Hooke first discovered plant cells. In 1674 A. Leeuwenhoek discovered the animal cell. In 1839 T. Schwann and M. Schleiden formulated the cell theory. The main position of the cell theory was that the cell is the structural and functional basis of living systems. But they mistakenly believed that cells are formed from a structureless substance. In 1859 R. Virchow proved that new cells are formed only by dividing the previous ones.

Basic provisions of cell theory :

1) The cell is the structural and functional unit of all living things. All living organisms are made up of cells.

2) All cells are basically similar in chemical composition and metabolic processes.

3) New cells are formed by dividing existing ones.

4) All cells store and implement hereditary information in the same way.

5) The vital activity of a multicellular organism as a whole is due to the interaction of its constituent cells.

According to the structure, 2 types of cells are distinguished:

prokaryotes

eukaryotes

Prokaryotes include bacteria and blue-green algae. Prokaryotes differ from eukaryotes in the following: they do not have membrane organelles present in a eukaryotic cell (mitochondria, endoplasmic reticulum, lysosomes, Golgi complex, chloroplasts).

The most important difference is that they do not have a nucleus surrounded by a membrane. Prokaryotic DNA is represented by one folded circular molecule. Prokaryotes also lack cell center centrioles, so they never divide by mitosis. They are characterized by amitosis - direct rapid division.

Eukaryotic cells are cells of unicellular and multicellular organisms. They consist of three main components:

The cell membrane that surrounds the cell and separates it from the external environment;

Cytoplasm containing water, mineral salts, organic compounds, organelles and inclusions;

The nucleus that contains the genetic material of the cell.

1 - polar head of the phospholipid molecule

2 - fatty acid tail of the phospholipid molecule

3 - integral protein

4 - peripheral protein

5 - semi-integral protein

6 - glycoprotein

7 - glycolipid

The outer cell membrane is inherent in all cells (animals and plants), has a thickness of about 7.5 (up to 10) nm and consists of lipid and protein molecules.

At present, the fluid-mosaic model of the construction of the cell membrane is widespread. According to this model, lipid molecules are arranged in two layers, with their water-repellent ends (hydrophobic - fat-soluble) facing each other, and water-soluble (hydrophilic) - to the periphery. Protein molecules are embedded in the lipid layer. Some of them are located on the outer or inner surface of the lipid part, others are partially immersed or penetrate the membrane through and through.

Membrane functions :

Protective, border, barrier;

Transport;

Receptor - is carried out at the expense of proteins - receptors, which have a selective ability for certain substances (hormones, antigens, etc.), enter into chemical interactions with them, conduct signals inside the cell;

Participate in the formation of intercellular contacts;

They provide the movement of some cells (amoeboid movement).

Animal cells have a thin layer of glycocalyx on top of the outer cell membrane. It is a complex of carbohydrates with lipids and carbohydrates with proteins. The glycocalyx is involved in intercellular interactions. The cytoplasmic membranes of most cell organelles have exactly the same structure.

In plant cells outside of the cytoplasmic membrane. the cell wall is made up of cellulose.

Transport of substances across the cytoplasmic membrane .

There are two main mechanisms for substances to enter or leave the cell:

1. Passive transport.

2. Active transport.

Passive transport of substances occurs without the expenditure of energy. An example of such transport is diffusion and osmosis, in which the movement of molecules or ions is carried out from a region of high concentration to a region of lower concentration, for example, water molecules.

Active transport - in this type of transport, molecules or ions penetrate the membrane against a concentration gradient, which requires energy. An example of active transport is the sodium-potassium pump, which actively pumps sodium out of the cell and absorbs potassium ions from the external environment, transferring them into the cell. The pump is a special membrane protein that sets it in motion with ATP.

Active transport maintains a constant cell volume and membrane potential.

Substances can be transported by endocytosis and exocytosis.

Endocytosis - the penetration of substances into the cell, exocytosis - out of the cell.

During endocytosis, the plasma membrane forms an invagination or outgrowths, which then envelop the substance and, lacing off, turn into vesicles.

There are two types of endocytosis:

1) phagocytosis - the absorption of solid particles (phagocyte cells),

2) pinocytosis - the absorption of liquid material. Pinocytosis is characteristic of amoeboid protozoa.

By exocytosis, various substances are removed from the cells: undigested food residues are removed from the digestive vacuoles, their liquid secret is removed from the secretory cells.

Cytoplasm -(cytoplasm + nucleus form protoplasm). The cytoplasm consists of a watery ground substance (cytoplasmic matrix, hyaloplasm, cytosol) and various organelles and inclusions in it.

Inclusions– cell waste products. There are 3 groups of inclusions - trophic, secretory (gland cells) and special (pigment) values.

Organelles - These are permanent structures of the cytoplasm that perform certain functions in the cell.

Allocate organelles of general importance and special. Special ones are found in most cells, but are present in significant numbers only in cells that perform a specific function. These include microvilli of intestinal epithelial cells, cilia of the epithelium of the trachea and bronchi, flagella, myofibrils (providing muscle contraction, etc.).

Organelles of general importance include EPS, the Golgi complex, mitochondria, ribosomes, lysosomes, centrioles of the cell center, peroxisomes, microtubules, microfilaments. Plant cells contain plastids and vacuoles. Organelles of general importance can be divided into organelles having a membrane and non-membrane structure.

Organelles having a membrane structure are two-membrane and one-membrane. Two-membrane cells include mitochondria and plastids. To single-membrane - endoplasmic reticulum, Golgi complex, lysosomes, peroxisomes, vacuoles.

Membraneless organelles: ribosomes, cell center, microtubules, microfilaments.

Mitochondria – These are round or oval organelles. They consist of two membranes: internal and external. The inner membrane has outgrowths - cristae, which divide the mitochondria into compartments. The compartments are filled with a substance - a matrix. The matrix contains DNA, mRNA, tRNA, ribosomes, calcium and magnesium salts. This is where protein biosynthesis takes place. The main function of mitochondria is the synthesis of energy and its accumulation in ATP molecules. New mitochondria are formed in the cell as a result of the division of old ones.

plastids – organelles found predominantly in plant cells. They are of three types: chloroplasts containing a green pigment; chromoplasts (pigments of red, yellow, orange color); leucoplasts (colorless).

Chloroplasts, thanks to the green pigment chlorophyll, are able to synthesize organic substances from inorganic ones using the energy of the sun.

Chromoplasts give bright colors to flowers and fruits.

Leucoplasts are able to accumulate reserve nutrients: starch, lipids, proteins, etc.

Endoplasmic reticulum ( EPS ) is a complex system of vacuoles and channels that are limited by membranes. There are smooth (agranular) and rough (granular) EPS. Smooth has no ribosomes on its membrane. It contains the synthesis of lipids, lipoproteins, the accumulation and removal of toxic substances from the cell. Granular EPS has ribosomes on membranes in which proteins are synthesized. Then the proteins enter the Golgi complex, and from there out.

Golgi complex (Golgi apparatus) is a stack of flattened membrane sacs - cisterns and a system of bubbles associated with them. The stack of cisterns is called a dictyosome.

Functions of the Golgi complex : protein modification, polysaccharide synthesis, substance transport, cell membrane formation, lysosome formation.

Lysosomes – are membrane-bound vesicles containing enzymes. They carry out intracellular cleavage of substances and are divided into primary and secondary. Primary lysosomes contain enzymes in an inactive form. After entering the organelles of various substances, enzymes are activated and the process of digestion begins - these are secondary lysosomes.

Peroxisomes have the appearance of bubbles bounded by a single membrane. They contain enzymes that break down hydrogen peroxide, which is toxic to cells.

Vacuoles – These are plant cell organelles that contain cell sap. Cell sap may contain spare nutrients, pigments, and waste products. Vacuoles are involved in the creation of turgor pressure, in the regulation of water-salt metabolism.

Ribosomes – organelles consisting of large and small subunits. They can be located either on the ER or located freely in the cell, forming polysomes. They are composed of rRNA and protein and are produced in the nucleolus. Protein synthesis takes place in ribosomes.

Cell Center – found in the cells of animals, fungi, lower plants and absent in higher plants. It consists of two centrioles and a radiant sphere. The centriole has the form of a hollow cylinder, the wall of which consists of 9 triplets of microtubules. When dividing, cells form threads of the mitotic spindle, which ensure the divergence of chromatids in the anaphase of mitosis and homologous chromosomes during meiosis.

microtubules – tubular formations of various lengths. They are part of the centrioles, mitotic spindle, flagella, cilia, perform a supporting function, promote the movement of intracellular structures.

Microfilaments – filamentous thin formations located throughout the cytoplasm, but there are especially many of them under the cell membrane. Together with microtubules, they form the cytoskeleton of the cell, determine the flow of the cytoplasm, intracellular movements of vesicles, chloroplasts, and other organelles.

Cell- this is not only a liquid, enzymes and other substances, but also highly organized structures called intracellular organelles. Organelles for a cell are no less important than its chemical components. So, in the absence of organelles such as mitochondria, the supply of energy extracted from nutrients will immediately decrease by 95%.

Most organelles in a cell are covered membranes composed primarily of lipids and proteins. There are membranes of cells, endoplasmic reticulum, mitochondria, lysosomes, Golgi apparatus.

Lipids are insoluble in water, so they create a barrier in the cell that prevents the movement of water and water-soluble substances from one compartment to another. Protein molecules, however, make the membrane permeable to various substances through specialized structures called pores. Many other membrane proteins are enzymes that catalyze numerous chemical reactions, which will be discussed in the following chapters.

Cell (or plasma) membrane is a thin, flexible and elastic structure with a thickness of only 7.5-10 nm. It consists mainly of proteins and lipids. The approximate ratio of its components is as follows: proteins - 55%, phospholipids - 25%, cholesterol - 13%, other lipids - 4%, carbohydrates - 3%.

lipid layer of the cell membrane prevents water penetration. The basis of the membrane is a lipid bilayer - a thin lipid film consisting of two monolayers and completely covering the cell. Throughout the membrane are proteins in the form of large globules.

Schematic representation of the cell membrane, reflecting its main elements
- phospholipid bilayer and a large number of protein molecules protruding above the membrane surface.
Carbohydrate chains are attached to proteins on the outer surface
and to additional protein molecules inside the cell (this is not shown in the figure).

lipid bilayer consists mainly of phospholipid molecules. One end of such a molecule is hydrophilic, i.e. soluble in water (a phosphate group is located on it), the other is hydrophobic, i.e. soluble only in fats (it contains a fatty acid).

Due to the fact that the hydrophobic part of the molecule phospholipid repels water but is attracted to similar parts of the same molecules, phospholipids have a natural property to attach to each other in the thickness of the membrane, as shown in Fig. 2-3. The hydrophilic part with a phosphate group forms two membrane surfaces: the outer one, which is in contact with the extracellular fluid, and the inner one, which is in contact with the intracellular fluid.

Middle lipid layer impermeable to ions and aqueous solutions of glucose and urea. Fat-soluble substances, including oxygen, carbon dioxide, alcohol, on the contrary, easily penetrate this area of the membrane.

molecules cholesterol, which is part of the membrane, are also naturally lipids, since their steroid group has a high solubility in fats. These molecules seem to be dissolved in the lipid bilayer. Their main purpose is the regulation of the permeability (or impermeability) of membranes for water-soluble components of body fluids. In addition, cholesterol is the main regulator of membrane viscosity.

Cell membrane proteins. In the figure, globular particles are visible in the lipid bilayer - these are membrane proteins, most of which are glycoproteins. There are two types of membrane proteins: (1) integral, which penetrate the membrane through; (2) peripheral, which protrude only above one surface without reaching the other.

Many integral proteins form channels (or pores) through which water and water-soluble substances, especially ions, can diffuse into the intra- and extracellular fluid. Due to the selectivity of the channels, some substances diffuse better than others.

Other integral proteins function as carrier proteins, carrying out the transport of substances for which the lipid bilayer is impermeable. Sometimes carrier proteins act in the direction opposite to diffusion, such transport is called active. Some integral proteins are enzymes.

Integral membrane proteins can also serve as receptors for water-soluble substances, including peptide hormones, since the membrane is impermeable to them. The interaction of a receptor protein with a certain ligand leads to conformational changes in the protein molecule, which, in turn, stimulates the enzymatic activity of the intracellular segment of the protein molecule or signal transmission from the receptor into the cell using a second messenger. Thus, integral proteins built into the cell membrane involve it in the process of transferring information about the external environment into the cell.

Molecules of peripheral membrane proteins often associated with integral proteins. Most peripheral proteins are enzymes or play the role of a dispatcher for the transport of substances through membrane pores.

cell membrane

Image of a cell membrane. 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 - oligosaccharide chains that form the glycocalyx

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

Functions

barrier - provides a regulated, selective, passive and active metabolism with the environment. For example, the peroxisome membrane protects the cytoplasm from peroxides dangerous to the cell. Selective permeability means that the permeability of a membrane to various atoms or molecules depends on their size, electrical charge, and chemical properties. Selective permeability ensures the separation of the cell and cellular compartments from the environment and supply them with the necessary substances.
transport - through the membrane there is a transport of substances into the cell and out of the cell. Transport through the membranes provides: the delivery of nutrients, the removal of end products of metabolism, the secretion of various substances, the creation of ionic gradients, the maintenance of the optimal concentration of ions in the cell, which are necessary for the functioning of cellular enzymes.
Particles that for some 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 because of their large size), but necessary for the cell, can penetrate the membrane through special carrier proteins (transporters) and channel proteins or by endocytosis.
In passive transport, substances cross the lipid bilayer without energy expenditure along the concentration gradient by diffusion. A variant of this mechanism is facilitated diffusion, in which a specific molecule helps a substance to pass through the membrane. This molecule may have a channel that allows only one type of substance to pass through.
Active transport requires energy, as it occurs against a concentration gradient. There are special pump proteins on the membrane, including ATPase, which actively pumps potassium ions (K +) into the cell and pumps sodium ions (Na +) out of it.
matrix - provides a certain relative position and orientation of membrane proteins, their optimal interaction.
mechanical - ensures the autonomy of the cell, its intracellular structures, as well as connection with other cells (in tissues). Cell walls play an important role in providing mechanical function, and in animals - intercellular substance.
energy - during photosynthesis in chloroplasts and cellular respiration in mitochondria, energy transfer systems operate in their membranes, in which proteins also participate;
receptor - some proteins located in the membrane are receptors (molecules with which the cell perceives certain signals).
For example, hormones circulating in the blood only act on target cells that have receptors corresponding to these hormones. Neurotransmitters (chemicals that conduct nerve impulses) also bind to specific receptor proteins on target cells.
enzymatic - membrane proteins are often enzymes. For example, the plasma membranes of intestinal epithelial cells contain digestive enzymes.
implementation of generation and conduction of biopotentials.
With the help of the membrane, a constant concentration of ions is maintained in the cell: the concentration of the K + ion inside the cell is much higher than outside, and the concentration of Na + is much lower, which is very important, since this maintains the potential difference across the membrane and generates a nerve impulse.
cell marking - there are antigens on the membrane that act as markers - "labels" that allow the cell to be identified. These are glycoproteins (that is, proteins with branched oligosaccharide side chains attached to them) that play the role of "antennas". Due to the myriad of side chain configurations, it is possible to make a specific marker for each cell type. With the help of markers, cells can recognize other cells and act in concert with them, for example, when forming organs and tissues. It also allows the immune system to recognize foreign antigens.

Structure and composition of biomembranes

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

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

Membrane organelles

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

Selective permeability

Cell membranes have selective permeability: glucose, amino acids, fatty acids, glycerol and ions slowly diffuse through them, and the membranes themselves actively regulate this process to a certain extent - some substances pass through, while others do not. There are four main mechanisms for the entry of substances into the cell or 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 them potassium ions slowly enter the cell.

Literature

Antonov V. F., Smirnova E. N., Shevchenko E. V. Lipid membranes during phase transitions. - M .: Nauka, 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, revised and expanded. - M .: Moscow University Press, 2004. - ISBN 5-211-06109-8
Bruce Alberts, et al.