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

Molecules of phospholipids are arranged in two rows - hydrophobic ends inward, hydrophilic heads to the internal and external aquatic environment. AT separate places the bilayer (double layer) of phospholipids is permeated through with protein molecules (integral proteins). Inside such protein molecules there are channels - pores through which water-soluble substances pass. Other protein molecules permeate the lipid bilayer half from one side or the other (semi-integral proteins). On the surface of the membranes of eukaryotic cells there are peripheral proteins. Lipid and protein molecules are held together by hydrophilic-hydrophobic interactions.

Properties and functions of membranes. All cell membranes are mobile fluid structures, since the molecules of lipids and proteins are not linked by covalent bonds and are able to move quite quickly in the plane of the membrane. Due to this, the membranes can change their configuration, i.e. they have fluidity.

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

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

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

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

There are several mechanisms for the transport of substances across the membrane.

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

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

active transport is associated with energy costs and serves to transport substances against their concentration gradient. He carried out by special carrier proteins, which form the so-called ion pumps. The most studied is the Na - / K - pump in animal cells, actively pumping out Na + ions, while absorbing K - ions. Due to this, a large concentration of K - and a lower Na + in comparison with the environment are maintained in the cell. This process consumes the energy of ATP.

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

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

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

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

bubbles. The vesicle approaches the cytoplasmic membrane, merges with it, and its contents are released into the environment. How digestive enzymes, hormones, hemicellulose, etc. are excreted.

Thus, biological membranes, as the main structural elements of the cell, serve not just as physical boundaries, but as dynamic functional surfaces. On the membranes of organelles, numerous biochemical processes are carried out, such as active absorption of substances, energy conversion, ATP synthesis, etc.

Functions of biological membranes the following:

Separate the contents of the cell from external environment and contents of organelles from the cytoplasm.

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

They play the role of receptors (receiving and converting signals from the environment, recognition of cell substances, etc.).

They are catalysts (providing membrane chemical processes).

Participate in the transformation of energy.

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 of the membrane 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.

Image of a cell membrane. Small blue and white balls correspond to the hydrophilic "heads" of lipids, and the lines attached to them correspond to the hydrophobic "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 inner sides membranes). 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 of biomembranes

• 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 across membranes provides: delivery nutrients, removal of metabolic end products, secretion of various substances, creation of ionic gradients, maintenance of the appropriate pH and ionic concentration in the cell, which are necessary for the operation of cellular enzymes.

Particles that for some reason are not able 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, 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 those 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. Outside, the cell is limited by the so-called plasma membrane. 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. Then membrane potential is being restored. Potassium channels are always open, through them potassium ions slowly enter the cell.

Links

• Bruce Alberts, et al. Molecular Biology Of The Cell. - 5th ed. - New York: Garland Science, 2007. - ISBN 0-8153-3218-1 - textbook on molecular biology in English. language

• Rubin A.B. Biophysics, textbook in 2 vols. . - 3rd edition, revised and expanded. - Moscow: Moscow University Press, 2004. - ISBN 5-211-06109-8

• Gennis R. Biomembranes. Molecular structure and functions: translation from English. = Biomembranes. Molecular structure and function (by Robert B. Gennis). - 1st edition. - Moscow: Mir, 1997. - ISBN 5-03-002419-0

• Ivanov V.G., Berestovsky T.N. lipid bilayer of biological membranes. - Moscow: Nauka, 1982.

• Antonov V.F., Smirnova E.N., Shevchenko E.V. Lipid membranes during phase transitions. - Moscow: Nauka, 1994.

see also

• Vladimirov Yu. A., Damage to the components of biological membranes in pathological processes

Wikimedia Foundation. 2010 .

In 1972, the theory was put forward that a partially permeable membrane surrounds the cell and performs a number of vital tasks, and the structure and function of cell membranes are significant issues regarding the proper functioning of all cells in the body. became widespread in the 17th century, along with the invention of the microscope. It became known that plant and animal tissues are composed of cells, but due to the low resolution of the device, it was impossible to see any barriers around the animal cell. In the 20th century, the chemical nature of the membrane was studied in more detail, it was found that lipids are its basis.

The structure and functions of cell membranes

The cell membrane surrounds the cytoplasm of living cells, physically separating intracellular components from the external environment. Fungi, bacteria and plants also have cell walls that provide protection and prevent the passage of large molecules. Cell membranes also play a role in the development of the cytoskeleton and the attachment of other vital particles to the extracellular matrix. This is necessary in order to hold them together, forming the tissues and organs of the body. Structural features of the cell membrane include permeability. The main function is protection. The membrane consists of a phospholipid layer with embedded proteins. This part is involved in processes such as cell adhesion, ion conduction, and signaling systems and serves as an attachment surface for several extracellular structures, including the wall, glycocalyx, and internal cytoskeleton. The membrane also maintains the potential of the cell by acting as a selective filter. It is selectively permeable to ions and organic molecules and controls the movement of particles.

Biological mechanisms involving the cell membrane

1. Passive diffusion: some substances (small molecules, ions), such as carbon dioxide (CO2) and oxygen (O2), can penetrate the plasma membrane by diffusion. The shell acts as a barrier to certain molecules and ions that can be concentrated on either side.

2. Transmembrane protein channels and transporters: Nutrients such as glucose or amino acids must enter the cell, and some metabolic products must leave it.

3. Endocytosis is the process by which molecules are taken up. AT plasma membrane a slight deformity (invagination) is created in which the substance to be transported is swallowed. It requires energy and is thus a form of active transport.

4. Exocytosis: occurs in various cells to remove undigested residues of substances brought by endocytosis, to secrete substances such as hormones and enzymes, and transport the substance completely through the cell barrier.

molecular structure

The cell membrane is a biological membrane, consisting mainly of phospholipids and separating the contents of the entire cell from the external environment. The formation process occurs spontaneously under normal conditions. In order to understand this process and correctly describe the structure and functions of cell membranes, as well as properties, it is necessary to assess the nature of phospholipid structures, which are characterized by structural polarization. When phospholipids in the aqueous environment of the cytoplasm reach a critical concentration, they combine into micelles, which are more stable in the aqueous environment.

Membrane properties

• Stability. This means that after the formation of the membrane is unlikely to disintegrate.
• Strength. The lipid membrane is sufficiently reliable to prevent the passage of a polar substance; both dissolved substances (ions, glucose, amino acids) and much larger molecules (proteins) cannot pass through the formed boundary.
• dynamic character. This is perhaps the most important property when considering the structure of the cell. The cell membrane can be subjected to various deformations, it can fold and bend without collapsing. Under special circumstances, such as the fusion of vesicles or budding, it can be broken, but only temporarily. At room temperature its lipid components are in constant, chaotic motion, forming a stable fluid boundary.

Liquid mosaic model

Speaking about the structure and functions of cell membranes, it is important to note that in the modern view, the membrane as a liquid mosaic model was considered in 1972 by scientists Singer and Nicholson. Their theory reflects three main features of the membrane structure. The integrals provide a mosaic template for the membrane, and they are capable of lateral in-plane movement due to the variable nature of lipid organization. Transmembrane proteins are also potentially mobile. An important feature structure of the membrane is its asymmetry. What is the structure of a cell? Cell membrane, nucleus, proteins and so on. The cell is the basic unit of life, and all organisms are made up of one or more cells, each with a natural barrier separating it from its environment. This outer border of the cell is also called the plasma membrane. It consists of four various types molecules: phospholipids, cholesterol, proteins and carbohydrates. The liquid mosaic model describes the structure of the cell membrane as follows: flexible and elastic, similar in consistency to vegetable oil, so that all individual molecules simply float in the liquid medium, and they are all able to move sideways within this shell. A mosaic is something that contains many different details. In the plasma membrane, it is represented by phospholipids, cholesterol molecules, proteins and carbohydrates.

Phospholipids

Phospholipids make up the basic structure of the cell membrane. These molecules have two distinct ends: a head and a tail. The head end contains a phosphate group and is hydrophilic. This means that it is attracted to water molecules. The tail is made up of hydrogen and carbon atoms called fatty acid chains. These chains are hydrophobic, they do not like to mix with water molecules. This process is similar to what happens when you pour vegetable oil into water, that is, it does not dissolve in it. The structural features of the cell membrane are associated with the so-called lipid bilayer, which consists of phospholipids. Hydrophilic phosphate heads are always located where there is water in the form of intracellular and extracellular fluid. The hydrophobic tails of phospholipids in the membrane are organized in such a way that they keep them away from water.

Cholesterol, proteins and carbohydrates

When people hear the word "cholesterol", people usually think it's bad. However, cholesterol is actually a very important component of cell membranes. Its molecules consist of four rings of hydrogen and carbon atoms. They are hydrophobic and occur among the hydrophobic tails in the lipid bilayer. Their importance lies in maintaining consistency, they strengthen the membranes, preventing crossover. Cholesterol molecules also keep the phospholipid tails from coming into contact and hardening. This guarantees fluidity and flexibility. Membrane proteins act as enzymes to accelerate chemical reactions, act as receptors for specific molecules or transport substances across the cell membrane.

Carbohydrates, or saccharides, are found only on the extracellular side of the cell membrane. Together they form the glycocalyx. It provides cushioning and protection to the plasma membrane. Based on the structure and type of carbohydrates in the glycocalyx, the body can recognize the cells and determine if they should be there or not.

Membrane proteins

The structure of the cell membrane cannot be imagined without such a significant component as protein. Despite this, they can be significantly inferior in size to another important component - lipids. There are three main types of membrane proteins.

• Integral. They completely cover the bi-layer, cytoplasm and extracellular environment. They perform a transport and signaling function.
• Peripheral. Proteins are attached to the membrane by electrostatic or hydrogen bonds at their cytoplasmic or extracellular surfaces. They are involved mainly as a means of attachment for integral proteins.
• Transmembrane. They perform enzymatic and signaling functions, and also modulate the basic structure of the lipid bilayer of the membrane.

Functions of biological membranes

The hydrophobic effect, which regulates the behavior of hydrocarbons in water, controls structures formed by membrane lipids and membrane proteins. Many properties of membranes are conferred by carriers of lipid bilayers, which form the basic structure for all biological membranes. Integral membrane proteins are partially hidden in the lipid bilayer. Transmembrane proteins have a specialized organization of amino acids in their primary sequence.

Peripheral membrane proteins are very similar to soluble proteins, but they are also membrane bound. Specialized cell membranes have specialized cell functions. How do the structure and functions of cell membranes affect the body? The functionality of the whole organism depends on how biological membranes are arranged. From intracellular organelles, extracellular and intercellular interactions of membranes, the structures necessary for organizing and performing biological functions. Many structural and functional features are common to bacteria and enveloped viruses. All biological membranes are built on a lipid bilayer, which determines the presence of a number of common characteristics. Membrane proteins have many specific functions.

• Controlling. Plasma membranes of cells determine the boundaries of the interaction of the cell with the environment.
• Transport. The intracellular membranes of cells are divided into several functional blocks with different internal composition, each of which is supported by the necessary transport function in combination with control permeability.
• signal transduction. Membrane fusion provides a mechanism for intracellular vesicular alert and obstruction different kind viruses can freely enter the cell.

Significance and conclusions

The structure of the outer cell membrane affects the entire body. It plays an important role in protecting integrity by allowing only selected substances to penetrate. It is also a good base for anchoring the cytoskeleton and cell wall, which helps in maintaining the shape of the cell. Lipids make up about 50% of the membrane mass of most cells, although this varies depending on the type of membrane. The structure of the outer cell membrane of mammals is more complex, it contains four main phospholipids. An important property of lipid bilayers is that they behave like a two-dimensional fluid in which individual molecules can freely rotate and move laterally. Such fluidity is an important property of membranes, which is determined depending on temperature and lipid composition. Due to the hydrocarbon ring structure, cholesterol plays a role in determining the fluidity of membranes. biological membranes for small molecules allows the cell to control and maintain its internal structure.

Considering the structure of the cell (cell membrane, nucleus, and so on), we can conclude that the body is a self-regulating system that cannot harm itself without outside help and will always look for ways to restore, protect and properly function each cell.

Cell— self-regulating structural and functional unit of tissues and organs. The cellular theory of the structure of organs and tissues was developed by Schleiden and Schwann in 1839. Subsequently, using electron microscopy and ultracentrifugation, it was possible to elucidate the structure of all the main organelles of animal and plant cells (Fig. 1).

Rice. 1. Scheme of the structure of the cell of animal organisms

The main parts of the cell are the cytoplasm and the nucleus. Each cell is surrounded by a very thin membrane that limits its contents.

The cell membrane is called plasma membrane and is characterized by selective permeability. This property allows the necessary nutrients and chemical elements to penetrate into the cell, and excess products to leave it. The plasma membrane consists of two layers of lipid molecules with the inclusion of specific proteins in it. The main membrane lipids are phospholipids. They contain phosphorus, a polar head, and two non-polar long-chain fatty acid tails. Membrane lipids include cholesterol and cholesterol esters. In accordance with the fluid mosaic model of the structure, membranes contain inclusions of protein and lipid molecules that can mix relative to the bilayer. Each type of membrane of any animal cell is characterized by its relatively constant lipid composition.

Membrane proteins are divided into two types according to their structure: integral and peripheral. Peripheral proteins can be removed from the membrane without destroying it. There are four types of membrane proteins: transport proteins, enzymes, receptors, and structural proteins. Some membrane proteins have enzymatic activity, others bind certain substances and promote their transfer into the cell. Proteins provide several pathways for the movement of substances across membranes: they form large pores consisting of several protein subunits that allow water molecules and ions to move between cells; form ion channels specialized for the movement of certain types of ions across the membrane under certain conditions. Structural proteins are associated with the inner lipid layer and provide the cytoskeleton of the cell. Cytoskeleton attaches mechanical strength cell membrane. In various membranes, proteins account for 20 to 80% of the mass. Membrane proteins can move freely in the lateral plane.

Carbohydrates are also present in the membrane, which can covalently bind to lipids or proteins. There are three types of membrane carbohydrates: glycolipids (gangliosides), glycoproteins and proteoglycans. Most membrane lipids are in a liquid state and have a certain fluidity, i.e. the ability to move from one area to another. On the outside membranes have receptor sites that bind various hormones. Other specific sections of the membrane can> t recognize and bind some proteins alien to these cells and various biologically active compounds.

The inner space of the cell is filled with cytoplasm, in which most enzyme-catalyzed reactions of cellular metabolism take place. The cytoplasm consists of two layers: the inner, called the endoplasm, and the peripheral, the ectoplasm, which has a high viscosity and is devoid of granules. The cytoplasm contains all the components of a cell or organelle. The most important of the cell organelles are the endoplasmic reticulum, ribosomes, mitochondria, the Golgi apparatus, lysosomes, microfilaments and microtubules, peroxisomes.

Endoplasmic reticulum is a system of interconnected channels and cavities penetrating the entire cytoplasm. It provides transport of substances from the environment and inside cells. The endoplasmic reticulum also serves as a depot for intracellular Ca 2+ ions and serves as the main site for lipid synthesis in the cell.

Ribosomes - microscopic spherical particles with a diameter of 10-25 nm. Ribosomes are freely located in the cytoplasm or attached to the outer surface of the membranes of the endoplasmic reticulum and the nuclear membrane. They interact with informational and transport RNA, and protein synthesis is carried out in them. They synthesize proteins that enter the cisterns or the Golgi apparatus and are then released outside. Ribosomes that are free in the cytoplasm synthesize protein for use by the cell itself, and ribosomes associated with the endoplasmic reticulum produce protein that is excreted from the cell. Various functional proteins are synthesized in ribosomes: carrier proteins, enzymes, receptors, cytoskeletal proteins.

golgi apparatus formed by a system of tubules, cisterns and vesicles. It is associated with the endoplasmic reticulum, and the biologically active substances that have entered here are stored in a compacted form in secretory vesicles. The latter are constantly separated from the Golgi apparatus, transported to the cell membrane and merge with it, and the substances contained in the vesicles are removed from the cell in the process of exocytosis.

Lysosomes - particles surrounded by a membrane with a size of 0.25-0.8 microns. They contain numerous enzymes involved in the breakdown of proteins, polysaccharides, fats, nucleic acids, bacteria and cells.

Peroxisomes formed from a smooth endoplasmic reticulum, resemble lysosomes and contain enzymes that catalyze the decomposition of hydrogen peroxide, which is cleaved under the influence of peroxidases and catalase.

Mitochondria contain outer and inner membranes and are the "energy station" of the cell. Mitochondria are round or elongated structures with a double membrane. The inner membrane forms folds protruding into the mitochondria - cristae. ATP is synthesized in them, the substrates of the Krebs cycle are oxidized, and many biochemical reactions are carried out. ATP molecules formed in mitochondria diffuse into all parts of the cell. Mitochondria contain a small amount of DNA, RNA, ribosomes, and with their participation, renewal and synthesis of new mitochondria takes place.

Microfilaments are thin protein filaments, consisting of myosin and actin, and form the contractile apparatus of the cell. Microfilaments are involved in the formation of folds or protrusions of the cell membrane, as well as in the movement of various structures inside cells.

microtubules form the basis of the cytoskeleton and provide its strength. The cytoskeleton gives cells their characteristic appearance and shape, serves as an attachment site for intracellular organelles and various bodies. AT nerve cells bundles of microtubules are involved in the transport of substances from the cell body to the ends of axons. With their participation, the functioning of the mitotic spindle during cell division is carried out. They play the role of motor elements in the villi and flagella in eukaryotes.

Nucleus is the main structure of the cell, is involved in the transmission of hereditary traits and in the synthesis of proteins. The nucleus is surrounded by a nuclear membrane containing many nuclear pores through which various substances are exchanged between the nucleus and the cytoplasm. Inside it is the nucleolus. The important role of the nucleolus in the synthesis of ribosomal RNA and histone proteins has been established. The rest of the nucleus contains chromatin, consisting of DNA, RNA, and a number of specific proteins.

Functions of the cell membrane

Cell membranes play an important role in the regulation of intracellular and intercellular metabolism. They are selective. Their specific structure makes it possible to provide barrier, transport and regulatory functions.

barrier function It manifests itself in limiting the penetration of compounds dissolved in water through the membrane. The membrane is impermeable to large protein molecules and organic anions.

Regulatory function membrane is the regulation of intracellular metabolism in response to chemical, biological and mechanical influences. Various influences are perceived by special membrane receptors with a subsequent change in the activity of enzymes.

transport function through biological membranes can be carried out passively (diffusion, filtration, osmosis) or with the help of active transport.

Diffusion - the movement of a gas or solute along a concentration and electrochemical gradient. The diffusion rate depends on the permeability of the cell membrane, as well as the concentration gradient for uncharged particles, electric and concentration gradients for charged particles. simple diffusion occurs through the lipid bilayer or through channels. Charged particles move along the electrochemical gradient, while uncharged particles follow the chemical gradient. For example, oxygen, steroid hormones, urea, alcohol, etc. penetrate through the lipid layer of the membrane by simple diffusion. Various ions and particles move through the channels. Ion channels are formed by proteins and are divided into gated and uncontrolled channels. Depending on the selectivity, there are ion-selective ropes that allow only one ion to pass through, and channels that do not have selectivity. Channels have a mouth and a selective filter, and controlled channels have a gate mechanism.

Facilitated diffusion - a process in which substances are transported across a membrane by special membrane carrier proteins. In this way, amino acids and monosugars enter the cell. This mode of transport is very fast.

Osmosis - movement of water across a membrane from a solution with a lower osmotic pressure to a solution with a higher osmotic pressure.

Active transport - transfer of substances against a concentration gradient using transport ATPases (ion pumps). This transfer occurs with the expenditure of energy.

Na + /K + -, Ca 2+ - and H + pumps have been studied to a greater extent. Pumps are located on cell membranes.

A type of active transport is endocytosis and exocytosis. With the help of these mechanisms, larger substances (proteins, polysaccharides, nucleic acids) that cannot be transported through the channels are transported. This transport is more common in the epithelial cells of the intestine, renal tubules, and vascular endothelium.

At In endocytosis, cell membranes form invaginations into the cell, which, when laced, turn into vesicles. During exocytosis, vesicles with contents are transferred to the cell membrane and merge with it, and the contents of the vesicles are released into the extracellular environment.

The structure and functions of the cell membrane

To understand the processes that ensure the existence electrical potentials in living cells, first of all, you need to understand the structure of the cell membrane and its properties.

At present, the fluid-mosaic model of the membrane, proposed by S. Singer and G. Nicholson in 1972, enjoys the greatest recognition. The basis of the membrane is a double layer of phospholipids (bilayer), the hydrophobic fragments of the molecule of which are immersed in the thickness of the membrane, and the polar hydrophilic groups are oriented outward, those. into the surrounding aquatic environment (Fig. 2).

Membrane proteins are localized on the membrane surface or can be inserted on different depth into the hydrophobic zone. Some proteins penetrate the membrane through and through, and different hydrophilic groups of the same protein are found on both sides of the cell membrane. Proteins found in the plasma membrane play a very important role: they are involved in the formation of ion channels, play a role diaphragm pumps and carriers of various substances, and can also perform a receptor function.

The main functions of the cell membrane: barrier, transport, regulatory, catalytic.

The barrier function is to limit the diffusion of water-soluble compounds through the membrane, which is necessary to protect cells from foreign, toxic substances and to maintain a relatively constant content of various substances inside the cells. So, the cell membrane can slow down the diffusion of various substances by 100,000-10,000,000 times.

Rice. 2. Three-dimensional scheme of the fluid-mosaic model of the Singer-Nicolson membrane

Globular integral proteins embedded in a lipid bilayer are shown. Some proteins are ion channels, others (glycoproteins) contain oligosaccharide side chains involved in the recognition of each other by cells and in the intercellular tissue. Cholesterol molecules are closely adjacent to the phospholipid heads and fix the adjacent areas of the "tails". Internal plots tails of phospholipid molecules are not limited in their movement and are responsible for membrane fluidity (Bretscher, 1985)

There are channels in the membrane through which ions penetrate. Channels are potential dependent and potential independent. Potential-gated channels open when the potential difference changes, and potential-independent(hormone-regulated) open when the receptors interact with substances. Channels can be opened or closed thanks to gates. Two types of gates are built into the membrane: activation(in the depth of the channel) and inactivation(on the surface of the channel). The gate can be in one of three states:

• open state (both types of gate are open);
• closed state (activation gate closed);
• inactivation state (inactivation gates are closed).

Another characteristic feature of membranes is the ability to selectively transport inorganic ions, nutrients, and various products exchange. There are systems of passive and active transfer (transport) of substances. Passive transport is carried out through ion channels with or without the help of carrier proteins, and its driving force is the difference in the electrochemical potentials of ions between the intra- and extracellular space. The selectivity of ion channels is determined by its geometric parameters and the chemical nature of the groups lining the channel walls and mouth.

At present, channels with selective permeability for Na + , K + , Ca 2+ ions and also for water (the so-called aquaporins) are the most well studied. The diameter of ion channels, according to various studies, is 0.5-0.7 nm. Bandwidth channels can vary, through one ion channel can pass 10 7 - 10 8 ions per second.

Active transport occurs with the expenditure of energy and is carried out by the so-called ion pumps. Ion pumps are molecular protein structures embedded in the membrane and carrying out the transfer of ions towards a higher electrochemical potential.

The operation of the pumps is carried out due to the energy of ATP hydrolysis. Na + / K + - ATPase, Ca 2+ - ATPase, H + - ATPase, H + / K + - ATPase, Mg 2+ - ATPase, which ensure the movement of Na +, K +, Ca 2+ ions, respectively, are well studied. , H+, Mg 2+ isolated or conjugated (Na+ and K+; H+ and K+). The molecular mechanism of active transport has not been fully elucidated.