The basic structural unit of a living organism is a cell, which is a differentiated section of the cytoplasm surrounded by a cell membrane. In view of the fact that the cell performs many important functions, such as reproduction, nutrition, movement, the shell must be plastic and dense.

History of the discovery and research of the cell membrane

In 1925, Grendel and Gorder made a successful experiment to identify the "shadows" of erythrocytes, or empty shells. Despite several gross mistakes made, scientists discovered the lipid bilayer. Their work was continued by Danielli, Dawson in 1935, Robertson in 1960. As a result of many years of work and the accumulation of arguments in 1972, Singer and Nicholson created a fluid mosaic model of the structure of the membrane. Further experiments and studies confirmed the works of scientists.

Meaning

What does it represent cell membrane? This word began to be used more than a hundred years ago, translated from Latin it means "film", "skin". This is the designation of the border of the cell, which is a natural barrier between the internal contents and external environment. The structure of the cell membrane suggests semi-permeability, due to which moisture and nutrients and decay products can freely pass through it. This shell can be called the main structural component of the organization of the cell.

Consider the main functions of the cell membrane

1. Separates the internal contents of the cell and the components of the external environment.

2. Helps maintain a constant chemical composition of the cell.

3. Regulates the correct metabolism.

4. Provides interconnection between cells.

5. Recognizes signals.

6. Protection function.

"Plasma Shell"

The outer cell membrane, also called the plasma membrane, is an ultramicroscopic film that is five to seven nanometers thick. It consists mainly of protein compounds, phospholide, water. The film is elastic, easily absorbs water, and also quickly restores its integrity after damage.

Differs in a universal structure. This membrane occupies a boundary position, participates in the process of selective permeability, excretion of decay products, synthesizes them. relationship with neighbors and reliable protection internal contents from damage makes it an important component in such a matter as the structure of the cell. The cell membrane of animal organisms is sometimes covered thinnest layer- glycocalyx, which includes proteins and polysaccharides. Plant cells outside the membrane are protected by a cell wall that acts as a 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 performs the function of repair, protection and interaction with other cells.

The structure of the cell membrane

The thickness of this movable shell varies from six to ten nanometers. The cell membrane of a cell has a special composition, the basis of which is the lipid bilayer. Hydrophobic tails, inert to water, placed with inside, while hydrophilic heads interacting with water face outwards. Each lipid is a phospholipid, which is the result of the interaction of substances such as glycerol and sphingosine. The lipid scaffold is closely surrounded by proteins, which are located in a non-continuous layer. Some of them are immersed in the lipid layer, the rest pass through it. As a result, water-permeable areas are formed. The functions performed by these proteins are different. Some of them are enzymes, the rest are transport proteins that carry various substances from the external environment to the cytoplasm and vice versa.

The cell membrane is permeated through and closely connected with integral proteins, while 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. Due to its continuity, the cell has barrier and mechanical properties. On the different stages this bilayer may be disrupted in its vital functions. As a result, structural defects of through hydrophilic pores are formed. In this case, absolutely all functions of such a component as a cell membrane can change. In this case, the nucleus may suffer from external influences.

Properties

The cell membrane of a cell has interesting features. Due to its fluidity, this shell is not a rigid structure, and the bulk of the proteins and lipids that make up its composition move freely on the plane of the membrane.

In general, the cell membrane is asymmetric, so the composition of the protein and lipid layers is different. Plasma mambrana in animal cells with their own outer side have a glycoprotein layer that performs receptor and signal functions, and also plays an important role in the process of combining cells into tissue. The cell membrane is polar, that is, the charge on the outside is positive, and on the inside it 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. For potassium ions, a different ratio is characteristic: their number in the cell is much higher than in 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 performs a “pumping” role, leveling the concentration of substances: sodium ions are pumped out to the cell surface, and potassium ions are pumped inward. This feature part of the most important functions of the cell membrane.

This tendency of sodium and potassium ions to move inward from the surface plays a large 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 inflows 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 is the cell nourished through the cell membrane?

Many cells take in substances through processes such as phagocytosis and pinocytosis. In the first variant, a small recess is created by a flexible outer membrane, in which the captured particle is located. Then the diameter of the recess becomes larger until the surrounded particle enters the cell cytoplasm. Through phagocytosis, some protozoa, such as amoeba, as well as blood cells - leukocytes and phagocytes, are fed. Similarly, cells absorb fluid that contains the necessary nutrients. This phenomenon is called pinocytosis.

The outer membrane is closely connected to the endoplasmic reticulum of the cell.

In many types of basic tissue components, protrusions, folds, and microvilli are located on the surface of the membrane. Plant cells on the outside of this shell are covered with another one, thick and clearly visible under a microscope. The fiber they are made of helps form the 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 the chitin contained in the integumentary cells of insects.

In addition to the cell 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 a cell membrane. Structure and function suggest significant expansion total area cell surface, improving 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 bonds are maintained at a sufficiently strong level, forming tissues. In this regard, we can conclude that one of the most important roles in the cell is played by the cell membrane. The structure and functions performed by it are radically different in different cells, depending on their purpose. Through these features, a variety of physiological activity is achieved. cell membranes and their roles in the existence of cells and tissues.

The study of the structure of organisms, as well as plants, animals and humans, is the branch of biology called cytology. Scientists have found that the contents of the cell, which is inside it, is quite complex. It is surrounded by the so-called surface apparatus, which includes the outer cell membrane, supra-membrane structures: glycocalyx and microfilaments, pelicule and microtubules that form its submembrane complex.

In this article, we will study the structure and functions of the outer cell membrane, which is part of the surface apparatus various kinds cells.

What are the functions of the outer cell membrane?

As described earlier, outer membrane is part of the surface apparatus of each cell, which successfully separates its internal contents and protects cell organelles from adverse environmental conditions. Another function is to ensure the exchange of substances between the cell contents and the tissue fluid, therefore, the outer cell membrane transports molecules and ions entering the cytoplasm, and also helps to remove toxins and excess toxic substances from the cell.

The structure of the cell membrane

membranes or plasma membranes various types cells are very different. Mainly, the chemical structure, as well as the relative content of lipids, glycoproteins, proteins in them and, accordingly, the nature of the receptors in them. External which is determined primarily by the individual composition of glycoproteins, takes part in the recognition of environmental stimuli and in the reactions of the cell itself to their actions. Some types of viruses can interact with proteins and glycolipids of cell membranes, as a result of which they penetrate into the cell. Herpes and influenza viruses can use to build their protective shell.

And viruses and bacteria, the so-called bacteriophages, attach to the cell membrane and dissolve it at the point of contact with the help of a special enzyme. Then a molecule of viral DNA passes into the hole formed.

Features of the structure of the plasma membrane of eukaryotes

Recall that the outer cell membrane performs the function of transport, that is, the transfer of substances into and out of it into the external environment. To carry out such a process, a special structure is required. Indeed, the plasmalemma is a constant, universal system of the surface apparatus for all. This is a thin (2-10 Nm), but fairly dense multilayer film that covers the entire cell. Its structure was studied in 1972 by such scientists as D. Singer and G. Nicholson, they also created a fluid-mosaic model of the cell membrane.

The main chemical compounds that form it are ordered molecules of proteins and certain phospholipids, which are interspersed in a liquid lipid environment and resemble a mosaic. Thus, the cell membrane consists of two layers of lipids, the non-polar hydrophobic "tails" of which are located inside the membrane, and the polar hydrophilic heads face the cytoplasm of the cell and the intercellular fluid.

The lipid layer is penetrated by large protein molecules that form hydrophilic pores. It is through them that aqueous solutions of glucose and mineral salts are transported. Some protein molecules are located both on the outer and inner surfaces of the plasmalemma. Thus, on the outer cell membrane in the cells of all organisms with nuclei, there are carbohydrate molecules bound by covalent bonds with glycolipids and glycoproteins. The content of carbohydrates in cell membranes ranges from 2 to 10%.

The structure of the plasmalemma of prokaryotic organisms

The outer cell membrane in prokaryotes performs similar functions to cell plasma membranes. nuclear organisms, namely: the perception and transmission of information coming from the external environment, the transport of ions and solutions into and out of the cell, the protection of the cytoplasm from foreign reagents from the outside. It can form mesosomes - structures that arise when the plasmalemma protrudes into the cell. They may contain enzymes involved in the metabolic reactions of prokaryotes, for example, in DNA replication, protein synthesis.

Mesosomes also contain redox enzymes, while photosynthetics contain bacteriochlorophyll (in bacteria) and phycobilin (in cyanobacteria).

The role of outer membranes in intercellular contacts

Continuing to answer the question of what functions the outer cell membrane performs, let us dwell on its role in plant cells. In plant cells, pores are formed in the walls of the outer cell membrane, passing into the cellulose layer. Through them, the exit of the cytoplasm of the cell to the outside is possible; such thin channels are called plasmodesmata.

Thanks to them, the connection between neighboring plant cells is very strong. In human and animal cells, the sites of contact between adjacent cell membranes are called desmosomes. They are characteristic of endothelial and epithelial cells, and are also found in cardiomyocytes.

Auxiliary formations of the plasmalemma

To understand how plant cells differ from animals, it helps to study the structural features of their plasma membranes, which depend on what functions the outer cell membrane performs. Above it in animal cells is a layer of glycocalyx. It is formed by polysaccharide molecules associated with proteins and lipids of the outer cell membrane. Thanks to the glycocalyx, adhesion (sticking) occurs between cells, leading to the formation of tissues, therefore it takes part in the signaling function of the plasmalemma - the recognition of environmental stimuli.

How is the passive transport of certain substances across cell membranes

As mentioned earlier, the outer cell membrane is involved in the process of transporting substances between the cell and the external environment. There are two types of transport through the plasmalemma: passive (diffusion) and active transport. The first includes diffusion, facilitated diffusion and osmosis. The movement of substances along the concentration gradient depends primarily on the mass and size of the molecules passing through the cell membrane. For example, small non-polar molecules easily dissolve in the middle lipid layer of the plasmalemma, move through it and end up in the cytoplasm.

Large molecules of organic substances penetrate into the cytoplasm with the help of special carrier proteins. They are species-specific and, when combined with a particle or ion, passively transport them across the membrane along a concentration gradient (passive transport) without expending energy. This process underlies such property of the plasmalemma as selective permeability. In the process, the energy of ATP molecules is not used, and the cell saves it for other metabolic reactions.

Active transport of chemical compounds across the plasmalemma

Since the outer cell membrane ensures the transfer of molecules and ions from the external environment into the cell and back, it becomes possible to remove the products of dissimilation, which are toxins, to the outside, that is, to the intercellular fluid. occurs against a concentration gradient and requires the use of energy in the form of ATP molecules. It also involves carrier proteins called ATPases, which are also enzymes.

An example of such transport is the sodium-potassium pump (sodium ions pass from the cytoplasm to the external environment, and potassium ions are pumped into the cytoplasm). The epithelial cells of the intestine and kidneys are capable of it. Varieties of this method of transfer are the processes of pinocytosis and phagocytosis. Thus, having studied what functions the outer cell membrane performs, it can be established that heterotrophic protists, as well as cells of higher animal organisms, for example, leukocytes, are capable of pino- and phagocytosis.

Bioelectric processes in cell membranes

It has been established that there is a potential difference between the outer surface of the plasmalemma (it is positively charged) and the parietal layer of the cytoplasm, which is negatively charged. It was called the resting potential, and it is inherent in all living cells. BUT nervous tissue has not only a resting potential, but is also capable of conducting weak biocurrents, which is called the process of excitation. The outer membranes of nerve cells-neurons, receiving irritation from receptors, begin to change charges: sodium ions massively enter the cell and the surface of the plasmalemma becomes electronegative. And the parietal layer of the cytoplasm, due to an excess of cations, receives a positive charge. This explains why the outer cell membrane of the neuron is recharged, which causes conduction. nerve impulses underlying the excitation process.

Outer cell membrane (plasmalemma, cytolemma, plasma membrane) of animal cells covered on the outside (i.e., on the side not in contact with the cytoplasm) with a layer of oligosaccharide chains covalently attached to membrane proteins (glycoproteins) and, to a lesser extent, to lipids (glycolipids). This carbohydrate coating of the membrane is called glycocalyx. The purpose of the glycocalyx is not yet very clear; there is an assumption that this structure takes part in the processes of intercellular recognition.

In plant cells on top of the outer cell membrane is a dense cellulose layer with pores through which communication is carried out between neighboring cells through cytoplasmic bridges.

Cells mushrooms on top of the plasmalemma - a dense layer chitin.

At bacteria – mureina.

Properties of biological membranes

1. Ability to self-assemble after destructive impacts. This property is determined by the physicochemical characteristics of phospholipid molecules, which in an aqueous solution come together so that the hydrophilic ends of the molecules turn outward, and the hydrophobic ends inward. Proteins can be incorporated into ready-made phospholipid layers. The ability to self-assemble is essential at the cellular level.

2. Semi-permeability(selectivity in the transmission of ions and molecules). Ensures the maintenance of the constancy of the ionic and molecular composition in the cell.

3. Membrane fluidity. Membranes are not rigid structures; they constantly fluctuate due to the rotational and oscillatory movements of lipid and protein molecules. This provides a high rate of enzymatic and other chemical processes in the membranes.

4. Fragments of membranes do not have free ends, as they are closed in bubbles.

Functions of the outer cell membrane (plasmalemma)

The main functions of the plasmalemma are as follows: 1) barrier, 2) receptor, 3) exchange, 4) transport.

1. barrier function. It is expressed in the fact that the plasmalemma limits the contents of the cell, separating it from the external environment, and intracellular membranes divide the cytoplasm into separate reactionary compartments.

2. receptor function. One of the most important functions of the plasmalemma is to ensure communication (connection) of the cell with the external environment through the receptor apparatus present in the membranes, which has a protein or glycoprotein nature. The main function of the receptor formations of the plasmalemma is the recognition of external signals, due to which the cells are correctly oriented and form tissues in the process of differentiation. The activity of various regulatory systems, as well as the formation of an immune response, is associated with the receptor function.

exchange function is determined by the content of enzyme proteins in biological membranes, which are biological catalysts. Their activity varies depending on the pH of the medium, temperature, pressure, the concentration of both the substrate and the enzyme itself. Enzymes determine the intensity of key reactions metabolism, as well as orientation.

Transport function of membranes. The membrane provides selective penetration into the cell and from the cell into the environment of various chemicals. The transport of substances is necessary to maintain the appropriate pH in the cell, the proper ionic concentration, which ensures the efficiency of cellular enzymes. Transport supplies nutrients that serve as a source of energy, as well as material for the formation of various cellular components. It determines the removal of toxic waste from the cell, the secretion of various useful substances and the creation of ionic gradients necessary for nervous and muscle activity. Changes in the rate of transfer of substances can lead to disturbances in bioenergetic processes, water-salt metabolism, excitability and other processes. Correction of these changes underlies the action of many drugs.

There are two main ways in which substances enter the cell and out of the cell into the external environment;

passive transport,

active transport.

Passive transport goes along the gradient of chemical or electrochemical concentration without the expenditure of ATP energy. If the molecule of the transported substance has no charge, then the direction of passive transport is determined only by the difference in the concentration of this substance on both sides of the membrane (chemical concentration gradient). If the molecule is charged, then its transport is affected by both the chemical concentration gradient and the electrical gradient (membrane potential).

Both gradients together constitute an electrochemical gradient. Passive transport of substances can be carried out in two ways: simple diffusion and facilitated diffusion.

With simple diffusion salt ions and water can penetrate through the selective channels. These channels are formed by some transmembrane proteins that form end-to-end transport pathways that are open permanently or only for a short time. Through the selective channels, various molecules penetrate, having the size and charge corresponding to the channels.

There is another way of simple diffusion - this is the diffusion of substances through the lipid bilayer, through which fat-soluble substances and water easily pass. The lipid bilayer is impermeable to charged molecules (ions), and at the same time, uncharged small molecules can freely diffuse, and the smaller the molecule, the faster it is transported. The rather high rate of water diffusion through the lipid bilayer is precisely due to the small size of its molecules and the absence of a charge.

With facilitated diffusion proteins are involved in the transport of substances - carriers that work on the principle of "ping-pong". In this case, the protein exists in two conformational states: in the “pong” state, the binding sites of the transported substance are open on the outside of the bilayer, and in the “ping” state, the same sites open on the other side. This process is reversible. From which side the binding site of a substance will be open at a given time depends on the concentration gradient of this substance.

In this way, sugars and amino acids pass through the membrane.

With facilitated diffusion, the rate of transport of substances increases significantly in comparison with simple diffusion.

In addition to carrier proteins, some antibiotics, such as gramicidin and valinomycin, are involved in facilitated diffusion.

Because they provide ion transport, they are called ionophores.

Active transport of substances in the cell. This type of transport always comes with the cost of energy. The source of energy needed for active transport is ATP. A characteristic feature of this type of transport is that it is carried out in two ways:

with the help of enzymes called ATPases;

transport in membrane packaging (endocytosis).

AT the outer cell membrane contains enzyme proteins such as ATPases, whose function is to provide active transport ions against a concentration gradient. Since they provide the transport of ions, this process is called an ion pump.

There are four main ion transport systems in the animal cell. Three of them provide transfer through biological membranes. Na + and K +, Ca +, H +, and the fourth - the transfer of protons during the operation of the mitochondrial respiratory chain.

An example of an active ion transport mechanism is sodium-potassium pump in animal cells. It maintains a constant concentration of sodium and potassium ions in the cell, which differs from the concentration of these substances in the environment: normally, there are less sodium ions in the cell than in the environment, and more potassium.

As a result, according to the laws of simple diffusion, potassium tends to leave the cell, and sodium diffuses into the cell. In contrast to simple diffusion, the sodium-potassium pump constantly pumps out sodium from the cell and injects potassium: for three molecules of sodium thrown out, there are two molecules of potassium introduced into the cell.

This transport of sodium-potassium ions is ensured by the ATP-dependent enzyme, which is localized in the membrane in such a way that it penetrates its entire thickness. Sodium and ATP enter this enzyme from the inside of the membrane, and potassium from the outside.

The transfer of sodium and potassium across the membrane occurs as a result of conformational changes that the sodium-potassium-dependent ATPase undergoes, which is activated when the concentration of sodium inside the cell or potassium in the environment increases.

ATP hydrolysis is required to power this pump. This process is provided by the same enzyme sodium-potassium-dependent ATP-ase. At the same time, more than one third of the ATP consumed by the animal cell at rest is spent on the work of the sodium - potassium pump.

Violation correct operation sodium - potassium pump leads to various serious diseases.

The efficiency of this pump exceeds 50%, which is not achieved by the most advanced machines created by man.

Many active transport systems are driven by energy stored in ionic gradients rather than by direct hydrolysis of ATP. All of them work as cotransport systems (facilitating the transport of low molecular weight compounds). For example, the active transport of certain sugars and amino acids into animal cells is determined by the sodium ion gradient, and the higher the sodium ion gradient, the greater the rate of glucose absorption. Conversely, if the concentration of sodium in the intercellular space decreases markedly, glucose transport stops. In this case, sodium must join the sodium - dependent glucose carrier protein, which has two binding sites: one for glucose, the other for sodium. Sodium ions penetrating into the cell contribute to the introduction of the carrier protein into the cell along with glucose. Sodium ions that have entered the cell along with glucose are pumped back out by the sodium-potassium-dependent ATPase, which, by maintaining the sodium concentration gradient, indirectly controls glucose transport.

Transport of substances in membrane packaging. Large molecules of biopolymers practically cannot penetrate through the plasmalemma by any of the above-described mechanisms of transport of substances into the cell. They are captured by the cell and absorbed in the membrane package, which is called endocytosis. The latter is formally divided into phagocytosis and pinocytosis. The capture of solid particles by the cell is phagocytosis, and liquid - pinocytosis. During endocytosis, the following stages are observed:

reception of the absorbed substance due to receptors in the cell membrane;

invagination of the membrane with the formation of a bubble (vesicles);

separation of the endocytic vesicle from the membrane with the expenditure of energy - phagosome formation and restoration of membrane integrity;

Fusion of phagosome with lysosome and formation phagolysosomes (digestive vacuole) in which the digestion of absorbed particles occurs;

removal of undigested material in the phagolysosome from the cell ( exocytosis).

In the animal kingdom endocytosis is a characteristic way of feeding many unicellular organisms (for example, in amoebas), and among multicellular organisms this type of digestion of food particles is found in endodermal cells in coelenterates. As for mammals and humans, they have a reticulo-histio-endothelial system of cells with the ability to endocytosis. Examples are blood leukocytes and liver Kupffer cells. The latter line the so-called sinusoidal capillaries of the liver and capture various foreign particles suspended in the blood. Exocytosis- this is also a way of removing from the cell of a multicellular organism the substrate secreted by it, which is necessary for the function of other cells, tissues and organs.

All living organisms on Earth are made up of cells, and each cell is surrounded by a protective shell - a membrane. However, the functions of the membrane are not limited to protecting organelles and separating one cell from another. The cell membrane is a complex mechanism that is directly involved in reproduction, regeneration, nutrition, respiration, and many other important cell functions.

The term "cell membrane" has been used for about a hundred years. The word "membrane" in translation from Latin means "film". But in the case of a cell membrane, it would be more correct to speak of a combination of two films interconnected in a certain way, moreover, different sides of these films have different properties.

The cell membrane (cytolemma, plasmalemma) is a three-layer lipoprotein (fat-protein) shell that separates each cell from neighboring cells and the environment, and carries out a controlled exchange between cells and the environment.

Of decisive importance in this definition is not that the cell membrane separates one cell from another, but that it ensures its interaction with other cells and the environment. The membrane is a very active, constantly working structure of the cell, on which many functions are assigned by nature. From our article, you will learn everything about the composition, structure, properties and functions of the cell membrane, as well as the danger posed to human health by disturbances in the functioning of cell membranes.

History of cell membrane research

In 1925, two German scientists, Gorter and Grendel, were able to conduct a complex experiment on human red blood cells, erythrocytes. Using osmotic shock, the researchers obtained the so-called "shadows" - empty shells of red blood cells, then put them in one pile and measured the surface area. The next step was to calculate the amount of lipids in the cell membrane. With the help of acetone, the scientists isolated lipids from the "shadows" and determined that they were just enough for a double continuous layer.

However, during the experiment, two gross errors were made:

The use of acetone does not allow all lipids to be isolated from the membranes;

The surface area of ​​the "shadows" was calculated by dry weight, which is also incorrect.

Since the first error gave a minus in the calculations, and the second - a plus, the overall result turned out to be surprisingly accurate, and the German scientists brought in scientific world The most important discovery is the lipid bilayer of the cell membrane.

In 1935, another pair of researchers, Danielly and Dawson, after long experiments on bilipid films, came to the conclusion that proteins are present in cell membranes. There was no other way to explain why these films have such a high surface tension. Scientists have presented to the public a schematic model of a cell membrane, similar to a sandwich, where the role of slices of bread is played by homogeneous lipid-protein layers, and between them instead of oil is emptiness.

In 1950, with the help of the first electron microscope, the Danielly-Dawson theory was partially confirmed - microphotographs of the cell membrane clearly showed two layers consisting of lipid and protein heads, and between them a transparent space filled only with tails of lipids and proteins.

In 1960, guided by these data, the American microbiologist J. Robertson developed a theory about the three-layer structure of cell membranes, which for a long time considered to be the only correct one. However, as science developed, more and more doubts were born about the homogeneity of these layers. From the point of view of thermodynamics, such a structure is extremely unfavorable - it would be very difficult for cells to transport substances in and out through the entire “sandwich”. In addition, it has been proven that cell membranes of different tissues have different thickness and the method of attachment, which is due to the different functions of the organs.

In 1972, microbiologists S.D. Singer and G.L. Nicholson was able to explain all the inconsistencies of Robertson's theory with the help of a new, fluid-mosaic model of the cell membrane. Scientists have found that the membrane is heterogeneous, asymmetric, filled with fluid, and its cells are in constant motion. And the proteins that make up it have a different structure and purpose, in addition, they are located differently relative to the bilipid layer of the membrane.

Cell membranes contain three types of proteins:

Peripheral - attached to the surface of the film;

semi-integral- partially penetrate the bilipid layer;

Integral - completely penetrate the membrane.

Peripheral proteins are associated with the heads of membrane lipids through electrostatic interaction, and they never form a continuous layer, as was previously believed. And semi-integral and integral proteins serve to transport oxygen into the cell and nutrients, as well as for deriving decay products from it, and for several other important functions, which you will learn about later.

The cell membrane performs the following functions:

Barrier - the permeability of the membrane for different types molecules are not the same. To bypass the cell membrane, the molecule must have a certain size, Chemical properties and electric charge. Harmful or inappropriate molecules, due to the barrier function of the cell membrane, simply cannot enter the cell. For example, with the help of the peroxide reaction, the membrane protects the cytoplasm from peroxides that are dangerous for it;

Transport - a passive, active, regulated and selective exchange passes through the membrane. Passive metabolism is suitable for fat-soluble substances and gases consisting of very small molecules. Such substances penetrate into and out of the cell without energy expenditure, freely, by diffusion. The active transport function of the cell membrane is activated when necessary, but difficult to transport substances need to be carried into or out of the cell. For example, those with big size molecules, or unable to cross the lipid layer due to hydrophobicity. Then protein pumps begin to work, including ATPase, which is responsible for the absorption of potassium ions into the cell and the ejection of sodium ions from it. Regulated transport is essential for secretion and fermentation functions, such as when cells produce and secrete hormones or gastric juice. All these substances leave the cells through special channels and in a given volume. And the selective transport function is associated with the very integral proteins that penetrate the membrane and serve as a channel for the entry and exit of strictly defined types of molecules;

Matrix - the cell membrane determines and fixes the location of organelles relative to each other (nucleus, mitochondria, chloroplasts) and regulates the interaction between them;

Mechanical - provides a restriction of one cell from another, and, at the same time time is correct the connection of cells into a homogeneous tissue and the resistance of organs to deformation;

Protective - both in plants and in animals, the cell membrane serves as the basis for building a protective frame. An example is hard wood, dense peel, prickly thorns. There are many examples in the animal kingdom protective function cell membranes - turtle shell, chitinous shell, hooves and horns;

Energy - the processes of photosynthesis and cellular respiration would be impossible without the participation of cell membrane proteins, because it is with the help of protein channels that cells exchange energy;

Receptor - proteins embedded in the cell membrane may have another important function. They serve as receptors through which the cell receives a signal from hormones and neurotransmitters. And this, in turn, is necessary for the conduction of nerve impulses and the normal course of hormonal processes;

Enzymatic - another important function inherent in some proteins of cell membranes. For example, in the intestinal epithelium, digestive enzymes are synthesized with the help of such proteins;

Biopotential- the concentration of potassium ions inside the cell is much higher than outside, and the concentration of sodium ions, on the contrary, is greater outside than inside. This explains the potential difference: inside the cell the charge is negative, outside it is positive, which contributes to the movement of substances into the cell and out in any of the three types of metabolism - phagocytosis, pinocytosis and exocytosis;

Marking - on the surface of cell membranes there are so-called "labels" - antigens consisting of glycoproteins (proteins with branched oligosaccharide side chains attached to them). Because side chains can have great multitude configurations, each type of cell receives its own unique label, which allows other cells of the body to recognize them “by sight” and respond to them correctly. That is why, for example, human immune cells, macrophages, easily recognize a foreigner that has entered the body (infection, virus) and try to destroy it. The same thing happens with diseased, mutated and old cells - the label on their cell membrane changes and the body gets rid of them.

Cellular exchange occurs across membranes, and can be carried out through three main types of reactions:

Phagocytosis is a cellular process in which phagocytic cells embedded in the membrane capture and digest solid particles of nutrients. In the human body, phagocytosis is carried out by membranes of two types of cells: granulocytes (granular leukocytes) and macrophages (immune killer cells);

Pinocytosis is the process of capturing liquid molecules that come into contact with it by the surface of the cell membrane. For nutrition by the type of pinocytosis, the cell grows thin fluffy outgrowths in the form of antennae on its membrane, which, as it were, surround a drop of liquid, and a bubble is obtained. First, this bubble protrudes above the surface of the membrane, and then it is “swallowed” - it hides inside the cell, and its walls merge with inner surface cell membrane. Pinocytosis occurs in almost all living cells;

Exocytosis is a reverse process in which vesicles with a secretory functional fluid (enzyme, hormone) are formed inside the cell, and it must somehow be removed from the cell into the environment. To do this, the bubble first merges with the inner surface of the cell membrane, then protrudes outward, bursts, expels the contents and again merges with the surface of the membrane, this time with outside. Exocytosis takes place, for example, in the cells of the intestinal epithelium and the adrenal cortex.

Cell membranes contain three classes of lipids:

Phospholipids;

Glycolipids;

Cholesterol.

Phospholipids (a combination of fats and phosphorus) and glycolipids (a combination of fats and carbohydrates), in turn, consist of a hydrophilic head, from which two long hydrophobic tails extend. But cholesterol sometimes occupies the space between these two tails and does not allow them to bend, which makes the membranes of some cells rigid. In addition, cholesterol molecules streamline the structure of cell membranes and prevent the transition of polar molecules from one cell to another.

But the most important component, as can be seen from previous section about the functions of cell membranes are proteins. Their composition, purpose and location are very diverse, but there is something in common that unites them all: annular lipids are always located around the proteins of cell membranes. These are special fats that are clearly structured, stable, have more saturated fatty acids in their composition, and are released from membranes along with "sponsored" proteins. This is a kind of personal protective shell for proteins, without which they simply would not work.

The structure of the cell membrane is three-layered. A relatively homogeneous liquid bilipid layer lies in the middle, and proteins cover it on both sides with a kind of mosaic, partially penetrating into the thickness. That is, it would be wrong to think that the outer protein layers of cell membranes are continuous. Proteins, in addition to their complex functions, are needed in the membrane in order to pass inside the cells and transport out of them those substances that are not able to penetrate the fat layer. For example, potassium and sodium ions. For them, special protein structures are provided - ion channels, which we will discuss in more detail below.

If you look at the cell membrane through a microscope, you can see a layer of lipids formed by the smallest spherical molecules, along which, like the sea, large protein cells float. different shapes. Exactly the same membranes divide inner space each cell into compartments in which the nucleus, chloroplasts and mitochondria are comfortably located. If there were no separate “rooms” inside the cell, the organelles would stick together and would not be able to perform their functions correctly.

A cell is a set of organelles structured and delimited by membranes, which is involved in a complex of energy, metabolic, informational and reproductive processes that ensure the vital activity of the organism.

As can be seen from this definition, the membrane is the most important functional component of any cell. Its significance is as great as that of the nucleus, mitochondria and other cell organelles. BUT unique properties membranes are determined by its structure: it consists of two films stuck together in a special way. Molecules of phospholipids in the membrane are located with hydrophilic heads outward, and hydrophobic tails inward. Therefore, one side of the film is wetted by water, while the other is not. So, these films are connected to each other with non-wettable sides inward, forming a bilipid layer surrounded by protein molecules. This is the very “sandwich” structure of the cell membrane.

Ion channels of cell membranes

Let us consider in more detail the principle of operation of ion channels. What are they needed for? The fact is that only fat-soluble substances can freely penetrate through the lipid membrane - these are gases, alcohols and fats themselves. So, for example, in red blood cells there is a constant exchange of oxygen and carbon dioxide, and for this our body does not have to resort to any additional tricks. But what about when it becomes necessary to transport aqueous solutions, such as sodium and potassium salts, through the cell membrane?

It would be impossible to pave the way for such substances in the bilipid layer, since the holes would immediately tighten and stick together back, such is the structure of any adipose tissue. But nature, as always, found a way out of the situation and created special protein transport structures.

There are two types of conductive proteins:

Transporters are semi-integral protein pumps;

Channeloformers are integral proteins.

Proteins of the first type are partially immersed in the bilipid layer of the cell membrane, and look out with their heads, and in the presence of the desired substance, they begin to behave like a pump: they attract the molecule and suck it into the cell. And proteins of the second type, integral, have an elongated shape and are located perpendicular to the bilipid layer of the cell membrane, penetrating it through and through. Through them, as through tunnels, substances that are unable to pass through fat move into and out of the cell. It is through ion channels that potassium ions penetrate into the cell and accumulate in it, while sodium ions, on the contrary, are brought out. There is a difference electrical potentials necessary for the proper functioning of all cells in our body.

The most important conclusions about the structure and functions of cell membranes

Theory always looks interesting and promising if it can be usefully applied in practice. Discovery of the structure and functions of cell membranes human body allowed scientists to make a real breakthrough in science in general, and in medicine in particular. It is no coincidence that we have dwelled on ion channels in such detail, because it is here that lies the answer to one of the most important questions of our time: why do people increasingly get sick with oncology?

Cancer claims about 17 million lives worldwide every year and is the fourth leading cause of all deaths. According to WHO, the incidence of cancer is steadily increasing, and by the end of 2020 it could reach 25 million per year.

What explains the real epidemic of cancer, and what does the function of cell membranes have to do with it? You will say: the reason is in a bad environmental situation, malnutrition, bad habits and heavy heredity. And, of course, you will be right, but if we talk about the problem in more detail, then the reason is the acidification of the human body. The negative factors listed above lead to disruption of the cell membranes, inhibit breathing and nutrition.

Where there should be a plus, a minus is formed, and the cell cannot function normally. But cancer cells do not need either oxygen or an alkaline environment - they are able to use an anaerobic type of nutrition. Therefore, in conditions of oxygen starvation and off-scale pH levels, healthy cells mutate, wanting to adapt to the environment, and become cancerous cells. This is how a person gets cancer. To avoid this, you just need to consume enough clean water daily, and discard carcinogens in food. But, as a rule, people are well aware of harmful products and the need for quality water, and do nothing - they hope that the trouble will bypass them.

Knowing the features of the structure and functions of the cell membranes of different cells, doctors can use this information to provide targeted, targeted therapeutic effects on the body. Many modern medications, getting into our body, they are looking for the desired "target", which can be ion channels, enzymes, receptors and biomarkers of cell membranes. This method of treatment allows you to achieve better results with minimal side effects.

Antibiotics of the latest generation, when released into the blood, do not kill all the cells in a row, but look for exactly the cells of the pathogen, focusing on markers in its cell membranes. The latest drugs against migraine, triptans, narrow only the inflamed vessels of the brain, while almost no effect on the heart and peripheral circulatory system. And they recognize the necessary vessels precisely by the proteins of their cell membranes. There are many such examples, so we can say with confidence that knowledge about the structure and functions of cell membranes underlies the development of modern medical science, and saves millions of lives every year.

Education: Moscow Medical Institute. I. M. Sechenov, specialty - "Medicine" in 1991, in 1993 "Occupational diseases", in 1996 "Therapy".

Membranes are extremely viscous and at the same time plastic structures that surround all living cells. Functions cell membranes:

1. The plasma membrane is a barrier that maintains a different composition of the extra- and intracellular environment.

2. Membranes form specialized compartments inside the cell, i.e. numerous organelles - mitochondria, lysosomes, Golgi complex, endoplasmic reticulum, nuclear membranes.

3. Enzymes involved in energy conversion in processes such as oxidative phosphorylation and photosynthesis are localized in membranes.

Structure and composition of membranes

The basis of the membrane is a lipid bilayer, in the formation of which phospholipids and glycolipids participate. The lipid bilayer is formed by two rows of lipids, the hydrophobic radicals of which are hidden inside, and the hydrophilic groups are turned outward and are in contact with the aqueous medium. Protein molecules seem to be “dissolved” in the lipid bilayer.

Structure of membrane lipids

Membrane lipids are amphiphilic molecules, because the molecule has both a hydrophilic region (polar heads) and a hydrophobic region, represented by hydrocarbon radicals of fatty acids, spontaneously forming a bilayer. There are three main types of lipids in membranes: phospholipids, glycolipids, and cholesterol.

The lipid composition is different. The content of one or another lipid, apparently, is determined by the variety of functions performed by these lipids in membranes.

Phospholipids. All phospholipids can be divided into two groups - glycerophospholipids and sphingophospholipids. Glycerophospholipids are classified as derivatives of phosphatidic acid. The most common glycerophospholipids are phosphatidylcholines and phosphatidylethanolamines. Sphingophospholipids are based on the amino alcohol sphingosine.

Glycolipids. In glycolipids, the hydrophobic part is represented by alcohol ceramide, and the hydrophilic part is represented by a carbohydrate residue. Depending on the length and structure of the carbohydrate part, cerebrosides and gangliosides are distinguished. Polar "heads" of glycolipids are located on the outer surface of plasma membranes.

Cholesterol (CS). CS is present in all membranes of animal cells. Its molecule consists of a rigid hydrophobic core and a flexible hydrocarbon chain. The only hydroxyl group at the 3-position is the "polar head". For an animal cell, the average molar ratio of cholesterol / phospholipids is 0.3-0.4, but in the plasma membrane this ratio is much higher (0.8-0.9). The presence of cholesterol in membranes reduces the mobility of fatty acids, reduces the lateral diffusion of lipids, and therefore can affect the functions of membrane proteins.

Membrane Properties:

1. Selective permeability. The closed bilayer provides one of the main properties of the membrane: it is impermeable to most water-soluble molecules, since they do not dissolve in its hydrophobic core. Gases such as oxygen, CO 2 and nitrogen have the ability to easily penetrate into the cell due to the small size of the molecules and weak interaction with solvents. Also, molecules of a lipid nature, for example, steroid hormones, easily penetrate through the bilayer.

2. Liquidity. The membranes are characterized by fluidity (fluidity), the ability of lipids and proteins to move. Two types of phospholipid movements are possible: somersault (called “flip-flop” in the scientific literature) and lateral diffusion. In the first case, phospholipid molecules opposing each other in the bimolecular layer turn over (or somersault) towards each other and change places in the membrane, i.e. the outside becomes the inside and vice versa. Such jumps are associated with the expenditure of energy. More often, rotations around the axis (rotation) and lateral diffusion are observed - movement within the layer parallel to the membrane surface. The speed of movement of molecules depends on the microviscosity of membranes, which, in turn, is determined by the relative content of saturated and unsaturated fatty acids in the composition of lipids. Microviscosity is lower if unsaturated fatty acids predominate in the composition of lipids, and higher if the content of saturated fatty acids is high.

3. Asymmetry of membranes. The surfaces of the same membrane differ in the composition of lipids, proteins and carbohydrates (transverse asymmetry). For example, phosphatidylcholines predominate in the outer layer, while phosphatidylethanolamines and phosphatidylserines predominate in the inner layer. The carbohydrate components of glycoproteins and glycolipids come to the outer surface, forming a continuous pouch called the glycocalyx. There are no carbohydrates on the inner surface. Proteins - hormone receptors are located on the outer surface of the plasma membrane, and the enzymes regulated by them - adenylate cyclase, phospholipase C - on the inside, etc.

Membrane proteins

Membrane phospholipids act as a solvent for membrane proteins, creating a microenvironment in which the latter can function. Proteins account for 30 to 70% of the mass of membranes. The number of different proteins in the membrane varies from 6-8 in the sarcoplasmic reticulum to more than 100 in the plasma membrane. These are enzymes, transport proteins, structural proteins, antigens, including antigens of the main histocompatibility system, receptors for various molecules.

By localization in the membrane, proteins are divided into integral (partially or completely immersed in the membrane) and peripheral (located on its surface). Some integral proteins cross the membrane once (glycophorin), while others cross the membrane many times. For example, the retinal photoreceptor and β 2 -adrenergic receptor crosses the bilayer 7 times.

Peripheral proteins and domains of integral proteins located on the outer surface of all membranes are almost always glycosylated. Oligosaccharide residues protect the protein from proteolysis and are also involved in ligand recognition or adhesion.