The cell membrane is the planar structure from which the cell is built. It is present in all organisms. Its unique properties ensure the vital activity of cells.

Types of membranes

There are three types of cell membranes:

• outdoor;
• nuclear;
• organelle membranes.

The outer cytoplasmic membrane creates the boundaries of the cell. It should not be confused with the cell wall or membrane found in plants, fungi and bacteria.

The difference between the cell wall and the cell membrane is in a much greater thickness and the predominance of the protective function over the exchange. The membrane is located under the cell wall.

The nuclear membrane separates the contents of the nucleus from the cytoplasm.

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Among the cell organelles there are those whose shape is formed by one or two membranes:

• mitochondria;
• plastids;
• vacuoles;
• Golgi complex;
• lysosomes;
• endoplasmic reticulum (ER).

Membrane structure

According to modern concepts, the structure of the cell membrane is described using a fluid mosaic model. The basis of the membrane is the lipid layer - two levels of lipid molecules forming a plane. Protein molecules are located on both sides of the bilipid layer. Some proteins are immersed in the bilipid layer, some pass through it.

Rice. 1. Cell membrane.

Animal cells have a complex of carbohydrates on the membrane surface. When studying the cell under a microscope, it was noted that the membrane is in constant motion and is heterogeneous in structure.

The membrane is a mosaic both in the morphological and functional sense, since its different parts contain different substances and have different physiological properties.

Properties and functions

Any border structure performs protective and exchange functions. This applies to all types of membranes.

The implementation of these functions is facilitated by such properties as:

• plastic;
• high ability to recover;
• semipermeability.

The property of semi-permeability lies in the fact that some substances are not passed through the membrane, while others are passed freely. This is how the controlling function of the membrane is carried out.

Also, the outer membrane provides communication between cells due to numerous outgrowths and the release of an adhesive that fills the intercellular space.

Transport of substances across the membrane

Substances pass through the outer membrane in the following ways:

• through the pores with the help of enzymes;
• through the membrane directly;
• pinocytosis;
• phagocytosis.

The first two ways transport ions and small molecules. Large molecules enter the cell by pinocytosis (in liquid state) and phagocytosis (in solid form).

Rice. 2. Scheme of pino- and phagocytosis.

The membrane wraps around the food particle and closes it into the digestive vacuole.

Water and ions pass into the cell without energy expenditure, by passive transport. Large molecules move by active transport, with the expenditure of energy resources.

intracellular transport

From 30% to 50% of the cell volume is occupied by the endoplasmic reticulum. This is a kind of system of cavities and channels that connects all parts of the cell and provides an ordered intracellular transport of substances.

Rice. 3. EPS drawing.

Thus, a significant mass of cell membranes is concentrated in the EPS.

What have we learned?

We found out what a cell membrane is in biology. It is the structure upon which all living cells are built. Its significance in the cell lies in: delimiting the space of organelles, the nucleus and the cell as a whole, ensuring the selective entry of substances into the cell and nucleus. The membrane contains lipid and protein molecules.

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Universal biological membrane formed by a double layer of phospholipid molecules with a total thickness of 6 microns. In this case, the hydrophobic tails of the phospholipid molecules are turned inward, towards each other, and the polar hydrophilic heads are turned outward of the membrane, towards the water. Lipids provide the main physicochemical properties of membranes, in particular, their fluidity at body temperature. Proteins are embedded in this lipid double layer.

They are subdivided into integral(permeate the entire lipid bilayer), semi-integral(penetrate up to half of the lipid bilayer), or surface (located on the inner or outer surface of the lipid bilayer).

At the same time, protein molecules are located in the lipid bilayer mosaically and can "swim" in the "lipid sea" like icebergs, due to the fluidity of the membranes. According to their function, these proteins can be structural(maintain a certain structure of the membrane), receptor(to form receptors for biologically active substances), transport(carry out the transport of substances through the membrane) and enzymatic(catalyze certain chemical reactions). This is currently the most recognized fluid mosaic model The biological membrane was proposed in 1972 by Singer and Nikolson.

Membranes perform a delimiting function in the cell. They divide the cell into compartments, compartments in which processes and chemical reactions can proceed independently of each other. For example, the aggressive hydrolytic enzymes of lysosomes, which are able to break down most organic molecules, are separated from the rest of the cytoplasm by a membrane. In the event of its destruction, self-digestion and cell death occur.

Having a common structural plan, different biological cell membranes differ in their chemical composition, organization and properties, depending on the functions of the structures they form.

Plasma membrane, structure, functions.

The cytolemma is the biological membrane that surrounds the outside of the cell. This is the thickest (10 nm) and complexly organized cell membrane. It is based on a universal biological membrane, covered on the outside glycocalyx, and from the inside, from the side of the cytoplasm, submembrane layer(Fig.2-1B). Glycocalyx(3-4 nm thick) is represented by the outer, carbohydrate sections of complex proteins - glycoproteins and glycolipids that make up the membrane. These carbohydrate chains play the role of receptors that ensure that the cell recognizes neighboring cells and intercellular substance and interacts with them. This layer also includes surface and semi-integral proteins, the functional sites of which are located in the supramembrane zone (for example, immunoglobulins). The glycocalyx contains histocompatibility receptors, receptors for many hormones and neurotransmitters.

Submembrane, cortical layer formed by microtubules, microfibrils and contractile microfilaments, which are part of the cytoskeleton of the cell. The submembrane layer maintains the shape of the cell, creates its elasticity, and provides changes in the cell surface. Due to this, the cell participates in endo- and exocytosis, secretion, and movement.

Cytolemma fulfills a bunch of functions:

1) delimiting (the cytolemma separates, delimits the cell from the environment and ensures its connection with the external environment);

2) recognition by this cell of other cells and attachment to them;

3) recognition by the cell of the intercellular substance and attachment to its elements (fibers, basement membrane);

4) transport of substances and particles into and out of the cytoplasm;

5) interaction with signal molecules (hormones, mediators, cytokines) due to the presence of specific receptors for them on its surface;

1. provides cell movement (formation of pseudopodia) due to the connection of the cytolemma with the contractile elements of the cytoskeleton.

The cytolemma contains numerous receptors, through which biologically active substances ( ligands, signal molecules, first messengers: hormones, mediators, growth factors) act on the cell. Receptors are genetically determined macromolecular sensors (proteins, glyco- and lipoproteins) built into the cytolemma or located inside the cell and specialized in the perception of specific signals of a chemical or physical nature. Biologically active substances, when interacting with the receptor, cause a cascade of biochemical changes in the cell, while transforming into a specific physiological response (change in cell function).

All receptors have a common structural plan and consist of three parts: 1) supramembrane, which interacts with a substance (ligand); 2) intramembrane, carrying out signal transfer; and 3) intracellular, immersed in the cytoplasm.

Types of intercellular contacts.

The cytolemma is also involved in the formation of special structures - intercellular connections, contacts, which provide close interaction between adjacent cells. Distinguish simple and complex intercellular connections. AT simple At intercellular junctions, the cytolemmas of cells approach each other at a distance of 15-20 nm and the molecules of their glycocalyx interact with each other (Fig. 2-3). Sometimes the protrusion of the cytolemma of one cell enters the depression of the neighboring cell, forming serrated and finger-like connections (connections "like a lock").

Complex intercellular connections are of several types: locking, fastening and communication(Fig. 2-3). To locking compounds include tight contact or blocking zone. At the same time, the integral proteins of the glycocalyx of neighboring cells form a kind of mesh network along the perimeter of neighboring epithelial cells in their apical parts. Due to this, intercellular gaps are locked, delimited from the external environment (Fig. 2-3).

Rice. 2-3. Various types of intercellular connections.

1. Simple connection.
2. Tight connection.
3. Adhesive band.
4. Desmosome.
5. Hemidesmosome.
6. Slotted (communication) connection.
7. Microvilli.

(According to Yu. I. Afanasiev, N. A. Yurina).

To linking, anchoring compounds include adhesive belt and desmosomes. Adhesive band located around the apical parts of the cells of a single-layer epithelium. In this zone, the integral glycocalyx glycoproteins of neighboring cells interact with each other, and submembrane proteins, including bundles of actin microfilaments, approach them from the cytoplasm. Desmosomes (adhesion patches)– paired structures about 0.5 µm in size. In them, the glycoproteins of the cytolemma of neighboring cells closely interact, and from the side of the cells in these areas, bundles of intermediate filaments of the cell cytoskeleton are woven into the cytolemma (Fig. 2-3).

To communication connections refer gap junctions (nexuses) and synapses. Nexuses have a size of 0.5-3 microns. In them, the cytolemmas of neighboring cells converge up to 2-3 nm and have numerous ion channels. Through them, ions can pass from one cell to another, transmitting excitation, for example, between myocardial cells. synapses characteristic of the nervous tissue and are found between nerve cells, as well as between nerve and effector cells (muscle, glandular). They have a synaptic cleft, where, when a nerve impulse passes from the presynaptic part of the synapse, a neurotransmitter is released that transmits a nerve impulse to another cell (for more details, see the chapter "Nervous tissue").

cell membrane

Image of a cell membrane. Small blue and white balls correspond to the hydrophobic "heads" of the phospholipids, and the lines attached to them correspond to the hydrophilic "tails". The figure shows only integral membrane proteins (red globules and yellow helices). Yellow oval dots inside the membrane - cholesterol molecules Yellow-green chains of beads on the outside of the membrane - oligosaccharide chains that form the glycocalyx

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

Functions

• barrier - provides a regulated, selective, passive and active metabolism with the environment. For example, the peroxisome membrane protects the cytoplasm from peroxides dangerous to the cell. Selective permeability means that the permeability of a membrane to various atoms or molecules depends on their size, electrical charge, and chemical properties. Selective permeability ensures the separation of the cell and cellular compartments from the environment and supply them with the necessary substances.
• transport - through the membrane there is a transport of substances into the cell and out of the cell. Transport through the membranes provides: the delivery of nutrients, the removal of end products of metabolism, the secretion of various substances, the creation of ionic gradients, the maintenance of the optimum and concentration of ions in the cell, which are necessary for the functioning of cellular enzymes.
  Particles that for some reason are unable to cross the phospholipid bilayer (for example, due to hydrophilic properties, since the membrane is hydrophobic inside and does not allow hydrophilic substances to pass through, or because of their large size), but necessary for the cell, can penetrate the membrane through special carrier proteins (transporters) and channel proteins or by endocytosis.
  In passive transport, substances cross the lipid bilayer without energy expenditure along the concentration gradient by diffusion. A variant of this mechanism is facilitated diffusion, in which a specific molecule helps a substance to pass through the membrane. This molecule may have a channel that allows only one type of substance to pass through.
  Active transport requires energy, as it occurs against a concentration gradient. There are special pump proteins on the membrane, including ATPase, which actively pumps potassium ions (K +) into the cell and pumps sodium ions (Na +) out of it.
• matrix - provides a certain relative position and orientation of membrane proteins, their optimal interaction.
• mechanical - ensures the autonomy of the cell, its intracellular structures, as well as connection with other cells (in tissues). Cell walls play an important role in providing mechanical function, and in animals - intercellular substance.
• energy - during photosynthesis in chloroplasts and cellular respiration in mitochondria, energy transfer systems operate in their membranes, in which proteins also participate;
• receptor - some proteins located in the membrane are receptors (molecules with which the cell perceives certain signals).
  For example, hormones circulating in the blood only act on target cells that have receptors corresponding to these hormones. Neurotransmitters (chemicals that conduct nerve impulses) also bind to specific receptor proteins on target cells.
• enzymatic - membrane proteins are often enzymes. For example, the plasma membranes of intestinal epithelial cells contain digestive enzymes.
• implementation of generation and conduction of biopotentials.
  With the help of the membrane, a constant concentration of ions is maintained in the cell: the concentration of the K + ion inside the cell is much higher than outside, and the concentration of Na + is much lower, which is very important, since this maintains the potential difference across the membrane and generates a nerve impulse.
• cell marking - there are antigens on the membrane that act as markers - "labels" that allow the cell to be identified. These are glycoproteins (that is, proteins with branched oligosaccharide side chains attached to them) that play the role of "antennas". Due to the myriad of side chain configurations, it is possible to make a specific marker for each cell type. With the help of markers, cells can recognize other cells and act in concert with them, for example, when forming organs and tissues. It also allows the immune system to recognize foreign antigens.

Structure and composition of biomembranes

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

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

Membrane organelles

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

Selective permeability

Cell membranes have selective permeability: glucose, amino acids, fatty acids, glycerol and ions slowly diffuse through them, and the membranes themselves actively regulate this process to a certain extent - some substances pass through, while others do not. There are four main mechanisms for the entry of substances into the cell or their removal from the cell to the outside: diffusion, osmosis, active transport and exo- or endocytosis. The first two processes are passive in nature, that is, they do not require energy; the last two are active processes associated with energy consumption.

The selective permeability of the membrane during passive transport is due to special channels - integral proteins. They penetrate the membrane through and through, forming a kind of passage. The elements K, Na and Cl have their own channels. With respect to the concentration gradient, the molecules of these elements move in and out of the cell. When irritated, the sodium ion channels open, and there is a sharp influx of sodium ions into the cell. This results in an imbalance in the membrane potential. After that, the membrane potential is restored. Potassium channels are always open, through which potassium ions slowly enter the cell.

see also

Literature

• Antonov V. F., Smirnova E. N., Shevchenko E. V. Lipid membranes during phase transitions. - M .: Nauka, 1994.
• Gennis R. Biomembranes. Molecular structure and functions: translation from English. = Biomembranes. Molecular structure and function (by Robert B. Gennis). - 1st edition. - M .: Mir, 1997. - ISBN 5-03-002419-0
• Ivanov V. G., Berestovsky T. N. lipid bilayer of biological membranes. - M .: Nauka, 1982.
• Rubin A. B. Biophysics, textbook in 2 vols. - 3rd edition, revised and expanded. - M .: Moscow University Press, 2004. - ISBN 5-211-06109-8
• Bruce Alberts, et al.

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 stacked 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 one gave a plus, the overall result turned out to be surprisingly accurate, and German scientists brought the most important discovery to the scientific world - 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 attention of 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 - two layers consisting of lipid and protein heads were clearly visible on micrographs of the cell membrane, and between them there was 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 was considered the only true 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 the cell membranes of different tissues have different thickness and method of attachment, which is due to different functions of 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 and nutrients into the cell, as well as to remove decay products from it and more for several important features, which you will learn about later.

The cell membrane performs the following functions:

Barrier - the permeability of the membrane for different types of molecules is 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 a large molecular size, or unable to cross the bilipid 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 - ensures the restriction of one cell from another, and, at the same time, the correct 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. In the animal world, there are also many examples of the protective function of 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: the charge is negative inside the cell, positive outside, 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). Since side chains can have a huge variety of configurations, each type of cell receives its own unique label that allows other cells in 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 vesicle protrudes above the surface of the membrane, and then it is “swallowed” - it hides inside the cell, and its walls merge with the inner surface of the 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 bulges outward, bursts, expels the contents and again merges with the surface of the membrane, this time from the 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 the previous section on 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 of various shapes float. Exactly the same membranes divide the internal space of 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. And the unique properties of the membrane are due to 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 a 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 in electrical potentials, so necessary for the proper functioning of all the cells of 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. The discovery of the structure and functions of the cell membranes of the 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 poor environmental conditions, malnutrition, bad habits and severe 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 drink enough clean water daily, and give up carcinogens in food. But, as a rule, people are well aware of harmful products and the need for high-quality water, and do nothing - they hope that 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 drugs, getting into our body, are looking for the right "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 newest anti-migraine drugs, triptans, only constrict the inflamed vessels in the brain, with 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".

9.5.1. One of the main functions of membranes is participation in the transport of substances. This process is provided by three main mechanisms: simple diffusion, facilitated diffusion and active transport (Figure 9.10). Remember the most important features of these mechanisms and examples of the transported substances in each case.

Figure 9.10. Mechanisms of transport of molecules across the membrane

simple diffusion- transfer of substances through the membrane without the participation of special mechanisms. Transport occurs along a concentration gradient without energy consumption. Small biomolecules - H2O, CO2, O2, urea, hydrophobic low molecular weight substances are transported by simple diffusion. The rate of simple diffusion is proportional to the concentration gradient.

Facilitated diffusion- the transfer of substances across the membrane using protein channels or special carrier proteins. It is carried out along the concentration gradient without energy consumption. Monosaccharides, amino acids, nucleotides, glycerol, some ions are transported. Saturation kinetics is characteristic - at a certain (saturating) concentration of the transferred substance, all carrier molecules take part in the transfer and the transport speed reaches the limit value.

active transport- also requires the participation of special carrier proteins, but the transfer occurs against a concentration gradient and therefore requires energy. With the help of this mechanism, Na+, K+, Ca2+, Mg2+ ions are transported through the cell membrane, and protons through the mitochondrial membrane. The active transport of substances is characterized by saturation kinetics.

9.5.2. An example of a transport system that performs active ion transport is Na+,K+ -adenosine triphosphatase (Na+,K+ -ATPase or Na+,K+ -pump). This protein is located in the thickness of the plasma membrane and is able to catalyze the reaction of ATP hydrolysis. The energy released during the hydrolysis of 1 ATP molecule is used to transfer 3 Na + ions from the cell to the extracellular space and 2 K + ions in the opposite direction (Figure 9.11). As a result of the action of Na + , K + -ATPase, a concentration difference is created between the cytosol of the cell and the extracellular fluid. Since the transport of ions is non-equivalent, a difference in electrical potentials arises. Thus, an electrochemical potential arises, which is the sum of the energy of the difference in electric potentials Δφ and the energy of the difference in the concentrations of substances ΔС on both sides of the membrane.

Figure 9.11. Scheme of Na+, K+ -pump.

9.5.3. Transfer through membranes of particles and macromolecular compounds

Along with the transport of organic substances and ions carried out by carriers, there is a very special mechanism in the cell designed to absorb and remove macromolecular compounds from the cell by changing the shape of the biomembrane. Such a mechanism is called vesicular transport.

Figure 9.12. Types of vesicular transport: 1 - endocytosis; 2 - exocytosis.

During the transfer of macromolecules, sequential formation and fusion of vesicles (vesicles) surrounded by a membrane occur. According to the direction of transport and the nature of the transferred substances, the following types of vesicular transport are distinguished:

Endocytosis(Figure 9.12, 1) - the transfer of substances into the cell. Depending on the size of the resulting vesicles, there are:

a) pinocytosis - absorption of liquid and dissolved macromolecules (proteins, polysaccharides, nucleic acids) using small bubbles (150 nm in diameter);

b) phagocytosis — absorption of large particles, such as microorganisms or cell debris. In this case, large vesicles are formed, called phagosomes with a diameter of more than 250 nm.

Pinocytosis is characteristic of most eukaryotic cells, while large particles are absorbed by specialized cells - leukocytes and macrophages. At the first stage of endocytosis, substances or particles are adsorbed on the membrane surface; this process occurs without energy consumption. At the next stage, the membrane with the adsorbed substance deepens into the cytoplasm; the resulting local invaginations of the plasma membrane are laced from the cell surface, forming vesicles, which then migrate into the cell. This process is connected by a system of microfilaments and is energy dependent. The vesicles and phagosomes that enter the cell can merge with lysosomes. Enzymes contained in lysosomes break down substances contained in vesicles and phagosomes to low molecular weight products (amino acids, monosaccharides, nucleotides), which are transported to the cytosol, where they can be used by the cell.

Exocytosis(Figure 9.12, 2) - the transfer of particles and large compounds from the cell. This process, like endocytosis, proceeds with the absorption of energy. The main types of exocytosis are:

a) secretion - removal from the cell of water-soluble compounds that are used or affect other cells of the body. It can be carried out both by non-specialized cells and cells of the endocrine glands, the mucosa of the gastrointestinal tract, adapted for the secretion of the substances they produce (hormones, neurotransmitters, proenzymes), depending on the specific needs of the body.

Secreted proteins are synthesized on ribosomes associated with the membranes of the rough endoplasmic reticulum. These proteins are then transported to the Golgi apparatus, where they are modified, concentrated, sorted, and then packaged into vesicles, which are cleaved into the cytosol and subsequently fuse with the plasma membrane so that the contents of the vesicles are outside the cell.

Unlike macromolecules, small secreted particles, such as protons, are transported out of the cell using facilitated diffusion and active transport mechanisms.

b) excretion - removal from the cell of substances that cannot be used (for example, the removal of a reticular substance from reticulocytes during erythropoiesis, which is an aggregated remnant of organelles). The mechanism of excretion, apparently, consists in the fact that at first the excreted particles are in the cytoplasmic vesicle, which then merges with the plasma membrane.