EntranceBrief characteristics of the cell membrane. Cell membrane structure and functions
Table No. 2
Question 1 (8)
Cell membrane(or cytolemma, or plasmalemma, or plasma membrane) 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.
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 electrical potential difference;
3) Specificity of intercellular contacts.
Due to the presence in the membrane of numerous receptors that perceive chemical signals - hormones, mediators and other biological active substances, it is capable of changing 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, which prevent the free movement of water and substances dissolved in it from the cytoplasm into them and vice versa. This creates conditions for the separation of biochemical processes occurring in different 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 of the membrane are glycolipids, represented by cerebrisides, sulfatides, gangliosides and cholesterol.
Basic 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 elongated 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 makes wave-like movements. Its sections, as well as proteins immersed in the lipid bilayer, are mixed from one part to another. The mobility (fluidity) of cell membranes facilitates the processes of transport of substances across the membrane.
Cell membrane proteins are 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 selective exchange of ions through membrane channels between extracellular and intracellular fluid, and also act as proteins that transport large molecules.
Membrane receptors and antigens can be represented by both integral and peripheral proteins.
Proteins adjacent to the membrane from the cytoplasmic side are classified as cell cytoskeleton. They can attach to membrane proteins.
So, protein band 3(band number during protein electrophoresis) of erythrocyte membranes is combined into an ensemble with other cytoskeletal molecules - spectrin through the low molecular weight protein ankyrin
Spectrin is a major cytoskeletal protein 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 flexible-elastic properties and 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 their significant sialic acid content (for example, the glycophorin molecule). This provides the surfaces of most cells with a negative charge, helping to repel other negatively charged objects. Carbohydrate protrusions of glycoproteins are carriers of blood group antigens, other antigenic determinants of the cell, and they act as receptors that bind hormones. Glycoproteins form adhesive molecules that cause cells to attach to one another, i.e. close intercellular contacts.
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 is also part of most cellular 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 the one that separates the contents of the cell from the external environment. The remaining terms denote all membranes.
Structure of the cell membrane
The structure of the cellular (biological) membrane is based on a double layer of lipids (fats). The formation of such a layer is associated with the characteristics 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 to 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 molecules causes them to “hide” their tails from the water and turn their polar heads towards the water.
The result is a lipid bilayer in which the nonpolar tails are inward (facing each other) and the polar heads are outward (toward the external environment and cytoplasm). The surface of such a membrane is hydrophilic, but inside it is hydrophobic.
In cell membranes, phospholipids predominate among the lipids (they belong to complex lipids). Their heads contain a phosphoric acid residue. In addition to phospholipids, there are glycolipids (lipids + carbohydrates) and cholesterol (related to sterols). The latter imparts rigidity to the membrane, being located in its thickness between the tails of the remaining lipids (cholesterol is completely hydrophobic).
Due to electrostatic interaction, some protein molecules are attached to the charged lipid heads, which become surface membrane proteins. Other proteins interact with nonpolar tails, are partially buried in the bilayer, or penetrate through it.
Thus, cell membrane consists of a bilayer of lipids, surface (peripheral), embedded (semi-integral) and permeating (integral) proteins. In addition, some proteins and lipids on the outside of the membrane are associated with carbohydrate chains.
This fluid mosaic model of membrane structure was put forward in the 70s of the XX century. Previously, a sandwich model of 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 refuted this hypothesis.
The thickness of membranes in different cells is about 8 nm. Membranes (even different sides of the same) differ in percentage various types lipids, proteins, enzymatic activity, etc. Some membranes are more liquid and more permeable, others are more dense.
Cell membrane breaks easily merge due to physical and chemical characteristics lipid bilayer. In the plane of the membrane, lipids and proteins (unless they are anchored by the cytoskeleton) move.
Functions of the cell membrane
Most proteins immersed in the cell membrane perform an enzymatic function (they are enzymes). Often (especially in the membranes of cell organelles) enzymes are located in a certain sequence so that the reaction products catalyzed by one enzyme move to the second, then the third, etc. A conveyor is formed that stabilizes surface proteins, because they do not allow the enzymes to float along the lipid bilayer.
The cell membrane serves as a delimiter (barrier) from environment and at the same time transport function. We can say 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). For example, gases (CO 2 , O 2 ) diffuse.
There is also transport against a concentration gradient, but with energy consumption.
Transport can be passive and facilitated (when it is assisted by some carrier). 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. Such carriers bind to transported molecules and pull them through the membrane. This is how glucose is transported inside red blood cells.
Threading proteins combine to form a pore for the movement of certain substances across the membrane. Such carriers do not move, but form a channel in the membrane and work similarly to enzymes, binding a specific substance. Transfer occurs due to a change in protein conformation, resulting in the formation of channels 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). Thus, 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(capturing droplets of liquid with substances dissolved in it). The mechanism of these processes is approximately the same. Absorbed substances on the surface of cells 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 (hormones, polysaccharides, proteins, fats, etc.) by the cytoplasmic membrane. These substances are contained in membrane vesicles that fit the cell membrane. Both membranes merge and the contents appear outside the cell.
The cytoplasmic membrane performs a receptor function. To do this, structures are located on its outer side that can recognize a chemical or physical stimulus. Some of the proteins that penetrate 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. In this case, channels can open, and certain substances can begin to enter or exit the cell.
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 (adenylate cyclase enzyme) is activated. The enzyme synthesizes cyclic AMP from ATP. Already it activates or suppresses various enzymes of cellular metabolism.
The receptor function of the cytoplasmic membrane also includes 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 substances. One example of such an interaction is contact inhibition, when cells stop growing after receiving information that free space is occupied.
Intercellular contacts can be simple (the membranes of different cells are adjacent to each other), locking (invaginations of the membrane of one cell into another), desmosomes (when the membranes are connected by bundles of transverse fibers that penetrate 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.
The basic structural unit of a living organism is the cell, which is a differentiated section of the cytoplasm surrounded by a cell membrane. Due to the fact that the cell performs many important functions, such as reproduction, nutrition, movement, the membrane must be plastic and dense.
History of the discovery and research of the cell membrane
In 1925, Grendel and Gorder conducted a successful experiment to identify the “shadows” of red blood cells, or empty membranes. Despite several serious mistakes, scientists discovered the lipid bilayer. Their work was continued by Danielli, Dawson in 1935, and Robertson in 1960. As a result of many years of work and accumulation of arguments, in 1972 Singer and Nicholson created a fluid-mosaic model of the membrane structure. Further experiments and studies confirmed the works of scientists.
Meaning
What is a cell membrane? This word began to be used more than a hundred years ago; translated from Latin it means “film”, “skin”. This is how the cell boundary is designated, which is a natural barrier between the internal contents and the external environment. The structure of the cell membrane implies semi-permeability, due to which moisture and nutrients and breakdown products can freely pass through it. This shell can be called the main structural component of the cell organization.
Let's consider the main functions of the cell membrane
1. Separates the internal contents of the cell and components of the external environment.
2. Helps maintain a constant chemical composition of the cell.
3. Regulates proper metabolism.
4. Provides communication between cells.
5. Recognizes signals.
6. Protection function.
"Plasma Shell"
The outer cell membrane, also called the plasma membrane, is an ultramicroscopic film whose thickness ranges from five to seven nanomillimeters. It consists mainly of protein compounds, phospholides, and water. The film is elastic, easily absorbs water, and quickly restores its integrity after damage.
It has a universal structure. This membrane occupies a border position, participates in the process of selective permeability, removal of decay products, and synthesizes them. The relationship with its “neighbors” and reliable protection of the internal contents from damage makes it an important component in such matters as the structure of the cell. The cell membrane of animal organisms is sometimes covered with a thin layer - the glycocalyx, which includes proteins and polysaccharides. Plant cells outside the membrane are protected by a cell wall, which serves as support and maintains shape. The main component of its composition is fiber (cellulose) - a polysaccharide that is insoluble in water.
Thus, the outer cell membrane has the function of repair, protection and interaction with other cells.
Structure of the cell membrane
The thickness of this movable shell varies from six to ten nanomillimeters. The cell membrane of a cell has a special composition, the basis of which is a lipid bilayer. Hydrophobic tails, inert to water, are located on the inside, while hydrophilic heads, interacting with water, face outward. Each lipid is a phospholipid, which is the result of the interaction of substances such as glycerol and sphingosine. The lipid framework is closely surrounded by proteins, which are arranged in a non-continuous layer. Some of them are immersed in the lipid layer, the rest pass through it. As a result, areas permeable to water are formed. The functions performed by these proteins are different. Some of them are enzymes, the rest are transport proteins that transfer various substances from the external environment to the cytoplasm and back.
The cell membrane is permeated through and closely connected by integral proteins, and the connection with peripheral ones is less strong. These proteins perform an important function, which is to maintain the structure of the membrane, receive and convert signals from the environment, transport substances, and catalyze reactions that occur on membranes.
Compound
The basis of the cell membrane is a bimolecular layer. Thanks to its continuity, the cell has barrier and mechanical properties. At different stages of life, this bilayer can be disrupted. As a result, structural defects of through hydrophilic pores are formed. In this case, absolutely all functions of such a component as the cell membrane can change. The core may suffer from external influences.
Properties
The cell membrane of a cell has interesting features. Due to its fluidity, this membrane is not a rigid structure, and the bulk of the proteins and lipids that make up it move freely on the plane of the membrane.
In general, the cell membrane is asymmetrical, so the composition of the protein and lipid layers differs. Plasma membranes in animal cells, on their outer side, have a glycoprotein layer that performs receptor and signaling functions, and also plays big role during the process of combining cells into tissue. The cell membrane is polar, that is, the charge on the outside is positive and the charge on the inside is negative. In addition to all of the above, the cell membrane has selective insight.
This means that, in addition to water, only a certain group of molecules and ions of dissolved substances are allowed into the cell. The concentration of a substance such as sodium in most cells is much lower than in the external environment. Potassium ions have a different ratio: their amount in the cell is much higher than in the environment. In this regard, sodium ions tend to penetrate the cell membrane, and potassium ions tend to be released outside. Under these circumstances, the membrane activates a special system that plays a “pumping” role, leveling the concentration of substances: sodium ions are pumped to the surface of the cell, and potassium ions are pumped inside. This feature is one of the most important functions of the cell membrane.
This tendency of sodium and potassium ions to move inward from the surface plays a big role in the transport of sugar and amino acids into the cell. In the process of actively removing sodium ions from the cell, the membrane creates conditions for new intakes of glucose and amino acids inside. On the contrary, in the process of transferring potassium ions into the cell, the number of “transporters” of decay products from inside the cell to the external environment is replenished.
How does cell nutrition occur through the cell membrane?
Many cells take up substances through processes such as phagocytosis and pinocytosis. In the first option, a flexible outer membrane creates a small depression in which the captured particle ends up. The diameter of the recess then becomes larger until the enclosed particle enters the cell cytoplasm. Through phagocytosis, some protozoa, such as amoebas, are fed, as well as blood cells - leukocytes and phagocytes. Similarly, cells absorb fluid, which contains the necessary nutrients. This phenomenon is called pinocytosis.
The outer membrane is closely connected to the endoplasmic reticulum of the cell.
Many types of main tissue components have protrusions, folds, and microvilli on the surface of the membrane. Plant cells on the outside of this shell are covered with another, thick and clearly visible under a microscope. The fiber they are made of helps form support for plant tissues, such as wood. Animal cells also have a number of external structures that sit on top of the cell membrane. They are exclusively protective in nature, an example of this is chitin contained in the integumentary cells of insects.
In addition to the cellular membrane, there is an intracellular membrane. Its function is to divide the cell into several specialized closed compartments - compartments or organelles, where a certain environment must be maintained.
Thus, it is impossible to overestimate the role of such a component of the basic unit of a living organism as the cell membrane. The structure and functions suggest a significant expansion of the total surface area of the cell and an improvement in metabolic processes. This molecular structure consists of proteins and lipids. Separating the cell from the external environment, the membrane ensures its integrity. With its help, intercellular connections are maintained at a fairly strong level, forming tissues. In this regard, we can conclude that the cell membrane plays one of the most important roles in the cell. The structure and functions performed by it differ radically in different cells, depending on their purpose. Through these features, a variety of physiological activities of cell membranes and their roles in the existence of cells and tissues is achieved.
The structure of the biomembrane. The cell-bounding membranes and membrane organelles of eukaryotic cells have 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 independently in the membrane; they are associated with proteins and lipids. The thickness of the membranes is 7-10 nm.
According to the currently generally accepted fluid mosaic model of membrane structure, lipids form a double layer, or lipid bilayer, in which the hydrophilic “heads” of lipid molecules face outward, and the hydrophobic “tails” are hidden inside the membrane (Fig. 2.24). These “tails”, due to their hydrophobicity, ensure the separation of aqueous phases internal environment cell and its environment. Proteins are associated with lipids through various types of interactions. Some 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 submerged 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, chemical composition and 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, which are part of complex proteins-glycoproteins and complex lipids-glycolipids, protrude above the surface of the membrane. Plasmalemma glycoproteins are extremely specific. For example, they are used for mutual recognition of cells, including sperm and egg.
On the surface of animal cells, carbohydrate chains form thin surface layer -glycocalyx. It is detected in almost all animal cells, but the degree of its expression varies (10-50 µm). The glycocalyx provides direct communication between the cell and the external environment, where extracellular digestion occurs; Receptors are located in the glycocalyx. In addition to the plasmalemma, the cells of bacteria, plants and fungi are also surrounded by cell membranes.
Internal membranes eukaryotic cells delimit different parts of the cell, forming peculiar “compartments” - compartments, which promotes the separation of various metabolic and energy processes. They may vary according to chemical composition and the functions performed, but their general structural plan remains the same.
Membrane functions:
1. Limiting. The idea is that they separate the internal space of the cell from the external environment. The membrane is semi-permeable, that is, only those substances that the cell needs can freely pass through it, and there are mechanisms for transporting the necessary substances.
2. Receptor. It is primarily associated 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 occur on them.
4. Energy transforming. Associated with the formation of energy, its storage in the form of ATP and consumption.
5. Compartmentalization. Membranes also delimit the space inside the cell, thereby separating the starting materials 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, it, on the one hand, serves as a fairly reliable barrier for ions and molecules, especially water-soluble ones, and on the other, ensures their transport into and out of the cell.
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 - membrane transport is required (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 energy consumption from a higher concentration to a lower one. Lipid-soluble small non-polar molecules (0 2, C0 2) easily penetrate the cell by simple diffusion. Those 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. Transport of water through a membrane is called by 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 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 transport occurs in the opposite direction - into the cell. However, there are limits to the extensibility of the cytoplasmic membrane, and an animal cell eventually ruptures, while a plant cell does not allow this to happen due to its strong cell wall. The phenomenon of filling the entire internal space of a cell with cellular contents is called deplasmolysis. The intracellular concentration of salts should be taken into account when preparing medications, especially for intravenous administration, as this can lead to damage to blood cells (for this, saline solution with a concentration of 0.9% sodium chloride is used). This is no less important when cultivating cells and tissues, as well as animal and plant organs.
Active transport proceeds with the expenditure of ATP energy from a lower concentration of a substance to a higher one. It is carried out using special pump proteins. Proteins pump K+, Na+, Ca2+ and other ions across the membrane, which facilitates the transport of essential organic matter, as well as the emergence 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 contained. Then the primary lysosome fuses with the phagosome and forms secondary lysosome, or phagolysosome, or digestive vacuole. The contents of the vesicle are digested by lysosome enzymes, and the breakdown 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 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 (unicellular animals, human leukocytes), it provides their nutrition, and often protection of the body (Fig. 2.28).
By pinocytosis, proteins, antigen-antibody complexes are absorbed during immune reactions, etc. However, many viruses also enter the cell by pinocytosis or phagocytosis. In plant and fungal cells, phagocytosis is practically impossible, as they are surrounded by durable cell membranes.
Exocytosis- a process reverse to endocytosis. In this way, undigested food remains are released from the digestive vacuoles, and substances necessary for the life of the cell and the body as a whole are removed. For example, the transmission of nerve impulses occurs due to the release of chemical messengers by the neuron sending the impulse - mediators, and in plant cells this is how auxiliary carbohydrates of the cell membrane are secreted.
Cell walls of plant cells, fungi and bacteria. Outside the membrane, the cell can secrete a strong framework - cell membrane, or cell wall.
In plants, the basis of the cell wall is cellulose, packed in bundles of 50-100 molecules. The spaces between them are filled with water and other carbohydrates. Shell plant cell permeated with channels - plasmodesmata(Fig. 2.29), through which the membranes of the endoplasmic reticulum pass.
Plasmodesmata transport substances between cells. However, transport of substances, such as water, can also occur along the cell walls themselves. Over time, various substances accumulate in the cell wall of plants, including tannins or fat-like substances, which leads to lignification or suberization of the cell wall itself, displacement of water and death of cellular contents. Between the cell walls of neighboring plant cells there are jelly-like spacers - middle plates that hold them together and cement the plant body as a whole. They are destroyed only during the process of fruit ripening and when the leaves fall.
The cell walls of fungal cells are formed chitin- a carbohydrate containing nitrogen. They are quite strong and are the external skeleton of the cell, but still, like in plants, they prevent phagocytosis.
In bacteria, the cell wall contains carbohydrates with peptide fragments - murein, however, its content varies significantly among different groups of bacteria. Other polysaccharides can also be released outside the cell wall, forming a mucous capsule that protects bacteria from external influences.
The membrane determines the shape of the cell, serves as a mechanical support, performs a protective function, and provides osmotic properties cells, limiting the stretching of the living contents and preventing 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 through the cell walls faster than through the cytoplasm.
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Cells are separated from the internal environment of the body by a 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 electrical potential difference;
3) Specificity 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 capable of changing 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, which prevent the free movement of water and substances dissolved in it from the cytoplasm into them and vice versa. This creates conditions for the separation of biochemical processes occurring in different compartments inside the cell.
Cell membrane structure
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The cell membrane is an elastic structure, with a thickness of 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 of the membrane are glycolipids, represented by cerebrosides, sulfatides, gangliosides and cholesterol.
Rice. 1.1 Organization of the membrane.
Basic 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 elongated 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 makes wave-like movements. Its sections, as well as proteins immersed in the lipid bilayer, are mixed from one part to another. The mobility (fluidity) of cell membranes facilitates the processes of transport of substances across the membrane.
Cell membrane proteins are represented mainly by glycoproteins. There are:
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 silk phosphatases, etc.). But some enzymes are also represented by integral proteins - ATPase.
Integral proteins provide selective exchange of ions through membrane channels between extracellular and intracellular fluid, and also act as proteins that transport large molecules.
Membrane receptors and antigens can be represented by both integral and peripheral proteins.
Proteins adjacent to the membrane from the cytoplasmic side are classified as cell cytoskeleton . They can attach to membrane proteins.
So, protein band 3 (band number during protein electrophoresis) of erythrocyte membranes is combined into an ensemble with other cytoskeletal molecules - spectrin through the low molecular weight protein ankyrin (Fig. 1.2).
Rice. 1.2 Scheme of the arrangement of proteins in the near-membrane cytoskeleton of erythrocytes. 1 - spectrin; 2 - ankyrin; 3 - protein of band 3; 4 - protein band 4.1; 5 - band protein 4.9; 6 - actin oligomer; 7 - protein 6; 8 - gpicophorin A; 9 - membrane.
Spectrin is a major cytoskeletal protein 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 flexible-elastic properties and 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 their significant sialic acid content (for example, the glycophorin molecule). This provides the surfaces of most cells with a negative charge, helping to repel other negatively charged objects. Carbohydrate protrusions of glycoproteins are carriers of blood group antigens, other antigenic determinants of the cell, and they act as receptors that bind hormones. Glycoproteins form adhesive molecules that cause cells to attach to one another, i.e. close intercellular contacts.
Features of metabolism in the membrane
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Membrane components are subject to many metabolic transformations under the influence of enzymes located on or within their membrane. These include oxidative enzymes, which play an important role in the modification of hydrophobic elements of membranes - cholesterol, etc. In membranes, when enzymes - phospholipases are activated - formation occurs from arachidonic acid biologically active compounds - prostaglandins and their derivatives. As a result of activation of phospholipid metabolism, thromboxanes and leukotrienes are formed in the membrane, which have a powerful effect on platelet adhesion, the process of inflammation, etc.
The processes of renewal of its components continuously occur in the membrane . Thus, the lifetime of membrane proteins ranges from 2 to 5 days. However, there are mechanisms in the cell that ensure the delivery of newly synthesized protein molecules to membrane receptors, which facilitate the incorporation of the protein into the membrane. “Recognition” of this receptor by the newly synthesized protein is facilitated by the formation of a signal peptide, which helps to find the receptor on the membrane.
Membrane lipids are also characterized by a significant rate of exchange, which requires membranes for the synthesis of these components large quantity fatty acids.
The specificity of the lipid composition of cell membranes is influenced by changes in the human environment and the nature of his diet.
For example, an increase in dietary fatty acids with unsaturated bonds increases liquid state lipids in cell membranes of various tissues, leads to a change in the ratio of phospholipids to sphingomyelins and lipids to proteins that is favorable for the function of the cell membrane.
Excess cholesterol in membranes, on the contrary, increases the microviscosity of their bilayer of phospholipid molecules, reducing the rate of diffusion of certain substances through cell membranes.
Food enriched with vitamins A, E, C, P improves lipid metabolism in erythrocyte membranes and reduces membrane microviscosity. This increases the deformability of red blood cells, making it easier for them to perform transport function(Chapter 6).
Deficiency of fatty acids and cholesterol in food disrupts the lipid composition and functions of cell membranes.
For example, fat deficiency disrupts the functions of the neutrophil membrane, which inhibits their ability to move and phagocytosis (active capture and absorption of microscopic foreign living objects and solid particles single-celled organisms or some cells).
In the regulation of the lipid composition of membranes and their permeability, regulation of cell proliferation an important role is played by reactive oxygen species formed in the cell in conjunction with normally occurring metabolic reactions (microsomal oxidation, etc.).
Generated reactive oxygen species- superoxide radical (O 2), hydrogen peroxide (H 2 O 2), etc. are extremely reactive substances. Their main substrate in free radical oxidation reactions are unsaturated fatty acids that are part of the phospholipids of cell membranes (the so-called lipid peroxidation reactions). The intensification of these reactions can cause damage to the cell membrane, its barrier, receptor and metabolic functions, modification of molecules nucleic acids and proteins, which leads to mutations and inactivation of enzymes.
IN physiological conditions the intensification of lipid peroxidation is regulated by the antioxidase system of cells, represented by enzymes that inactivate reactive oxygen species - superoxide dismutase, catalase, peroxidase and substances with antioxidant activity - tocopherol (vitamin E), ubiquinone, etc. A pronounced protective effect on cell membranes (cytoprotective effect) when Prostaglandins E and J2 have various damaging effects on the body, “quenching” the activation of free radical oxidation. Prostaglandins protect the gastric mucosa and hepatocytes from chemical damage, neurons, neuroglial cells, cardiomyocytes - from hypoxic damage, skeletal muscles - during heavy physical activity. Prostaglandins, by binding to specific receptors on cell membranes, stabilize the bilayer of the latter and reduce the loss of phospholipids by the membranes.
Functions of membrane receptors
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A chemical or mechanical signal is first perceived by cell membrane receptors. The consequence of this is a chemical modification of membrane proteins, leading to the activation of “second messengers” that ensure rapid propagation of the signal in the cell to its genome, enzymes, contractile elements, etc.
Transmembrane signal transmission in a cell can be schematically represented as follows:
1) The receptor, excited by the received signal, activates the γ-proteins of the cell membrane. This occurs when they bind guanosine triphosphate (GTP).
2) The interaction of the GTP-γ-protein complex, in turn, activates the enzyme - the precursor of secondary messengers, located on the inner side of the membrane.
The precursor of one secondary messenger, cAMP, formed from ATP, is the enzyme adenylate cyclase;
The precursor of other secondary messengers - inositol triphosphate and diacylglycerol, formed from membrane phosphatidylinositol-4,5-diphosphate, is the enzyme phospholipase C. In addition, inositol triphosphate mobilizes another secondary messenger in the cell - calcium ions, which are involved in almost all regulatory processes in the cell. For example, the resulting inositol triphosphate causes the release of calcium from the endoplasmic reticulum and an increase in its concentration in the cytoplasm, thereby including various shapes cellular response. With the help of inositol triphosphate and diacylglycerol, the function of smooth muscles and B cells of the pancreas is regulated by acetylcholine, the anterior lobe of the pituitary gland by thyrogropin-releasing factor, the response of lymphocytes to antigen, etc.
In some cells, the role of a second messenger is played by cGMP, formed from GTP with the help of the enzyme guanylate cyclase. It serves, for example, as a second messenger for natriuretic hormone in the smooth muscle of the walls of blood vessels. cAMP serves as a secondary messenger for many hormones - adrenaline, erythropoietin, etc. (Chapter 3).