The plasma membrane of an animal cell. The structure of the plasma membrane in detail

It has a thickness of 8-12 nm, so it is impossible to examine it with a light microscope. The structure of the membrane is studied using an electron microscope.

The plasma membrane is formed by two layers of lipids - the lipid layer, or bilayer. Each molecule consists of a hydrophilic head and a hydrophobic tail, and in biological membranes, lipids are located with heads outward, tails inward.

Numerous protein molecules are immersed in the bilipid layer. Some of them are on the surface of the membrane (external or internal), others penetrate the membrane.

Functions of the plasma membrane

The membrane protects the contents of the cell from damage, maintains the shape of the cell, selectively passes the necessary substances into the cell and removes metabolic products, and also provides communication between cells.

The barrier, delimiting function of the membrane provides a double layer of lipids. It does not allow the contents of the cell to spread, mix with the environment or intercellular fluid, and prevents the penetration of dangerous substances into the cell.

A number of the most important functions of the cytoplasmic membrane are carried out due to the proteins immersed in it. With the help of receptor proteins, it can perceive various irritations on its surface. Transport proteins form the thinnest channels through which potassium, calcium, and other ions of small diameter pass into and out of the cell. Proteins - provide vital processes in itself.

Large food particles that are unable to pass through thin membrane channels enter the cell by phagocytosis or pinocytosis. The common name for these processes is endocytosis.

How does endocytosis occur - the penetration of large food particles into the cell

The food particle comes into contact with the outer membrane of the cell, and an invagination forms in this place. Then the particle, surrounded by a membrane, enters the cell, a digestive one is formed, and digestive enzymes penetrate into the formed vesicle.

The white blood cells that can capture and digest foreign bacteria are called phagocytes.

In the case of pinocytosis, the invagination of the membrane does not capture solid particles, but droplets of liquid with substances dissolved in it. This mechanism is one of the main pathways for the penetration of substances into the cell.

Plant cells covered over the membrane with a solid layer of the cell wall are not capable of phagocytosis.

The reverse process of endocytosis is exocytosis. Synthesized substances (for example, hormones) are packed into membrane vesicles, approach, are embedded in it, and the contents of the vesicle are ejected from the cell. Thus, the cell can also get rid of unnecessary metabolic products.

The plasma membrane performs a number of important functions:

1) Barrier. The barrier function of the plasma membrane is to limit the free diffusion of substances from cell to cell, to prevent the leakage of water-soluble contents of the cell. But since the cell must receive the necessary nutrients, release the end products of metabolism, and regulate the intracellular concentrations of ions, special mechanisms for the transfer of substances through the cell membrane have been formed in it.

2) Transport. The transport function is Ensuring the entry and exit of various substances into and out of the cell. An important property of the membrane is selective permeability, or semipermeability. It easily passes water and water-soluble gases and repels polar molecules such as glucose or amino acids.

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

passive transport;

active transport;

transport in membrane packaging.

Passive transport.Diffusion - This is the movement of particles of the medium, leading to the transfer of a substance from an area where its concentration is high to an area with a low concentration. During diffusion transport, the membrane functions as an osmotic barrier. The rate of diffusion depends on the size of the molecules and their relative solubility in fats. The smaller the molecules and the more fat-soluble (lipophilic) they are, the faster they will move through the lipid bilayer. Diffusion can be neutral(transfer of uncharged molecules) and lightweight(with the help of special carrier proteins). Facilitated diffusion is faster than neutral diffusion. Water has the maximum penetrating power, since its molecules are small and uncharged. Diffusion of water across a cell membrane is called osmosis. It is assumed that special "pores" exist in the cell membrane for the penetration of water and some ions. Their number is small, and the diameter is about 0.3-0.8 nm. Easily soluble molecules in the lipid bilayer, such as O, and uncharged polar molecules of small diameter (CO, urea) diffuse most rapidly through the membrane.

The transfer of polar molecules (sugars, amino acids) carried out with the help of special membrane transport proteins is called facilitated diffusion. Such proteins are found in all types of biological membranes, and each specific protein is designed to carry molecules of a certain class. Transport proteins are transmembrane; their polypeptide chain crosses the lipid bilayer several times, forming through passages in it. This ensures the transfer of specific substances through the membrane without direct contact with it. There are two main classes of transport proteins: carrier proteins (transporters) And channel-forming proteins (protein channels). Carrier proteins carry molecules across the membrane by first changing their configuration. Channel-forming proteins form water-filled pores in the membrane. When the pores are open, molecules of specific substances (usually inorganic ions of the right size and charge) pass through them. If the molecule of the transported substance has no charge, then the direction of transport is determined by the concentration gradient. If the molecule is charged, then its transport, in addition to the concentration gradient, is also affected by the electric charge of the membrane (membrane potential). The inner side of the plasmalemma is usually negatively charged relative to the outer side. The membrane potential facilitates the penetration of positively charged ions into the cell and prevents the passage of negatively charged ions.

active transport. Active transport is the movement of substances against an electrochemical gradient. It is always carried out by transporter proteins and is closely associated with an energy source. Carrier proteins have binding sites with the transported substance. The more such sites associated with the substance, the higher the rate of transport. The selective transfer of one substance is called uniport. The transfer of several substances is carried out cotransport systems. If the transfer goes in one direction, it is symport, if in opposite antiport. For example, glucose is transported from the extracellular fluid into the cell in a uniportal manner. The transfer of glucose and Na 4 from the intestinal cavity or tubules of the kidneys, respectively, to the cells of the intestine or blood is carried out symportally, and the transfer of C1 ~ and HCO "is antiport. .

An example of a carrier protein that uses the energy released during ATP hydrolysis to transport substances is Na + -TO + pump, found in the plasma membrane of all cells. The Na + -K pump works on the antiport principle, pumping Na "out of the cell and K t into the cell against their electrochemical gradients. The Na + gradient creates osmotic pressure, maintains cell volume and ensures the transport of sugars and amino acids. A third of all energy is spent on this pump necessary for the vital activity of cells.When studying the mechanism of action of the Na + -K + pump, it was found that it is an ATPase enzyme and a transmembrane integral protein.In the presence of Na + and ATP, under the action of ATPase, terminal phosphate is separated from ATP and attached to the residue of aspartic acid on the ATPase molecule. The ATPase molecule is phosphorylated, changes its configuration and Na + is excreted from the cell. Following the excretion of Na from the cell, transport of K "into the cell always occurs. For this, the previously attached phosphate is cleaved from ATPase in the presence of K. The enzyme is dephosphorylated, restores its configuration, and K 1 is "pumped" into the cell.

ATPase is formed by two subunits, large and small. The large subunit consists of thousands of amino acid residues that cross the bilayer several times. It has catalytic activity and can be reversibly phosphorylated and dephosphorylated. The large subunit on the cytoplasmic side has sites for binding Na + and ATP, and on the outside - sites for binding K + and ouabain. The small subunit is a glycoprotein and its function is not yet known.

Na + -K pump has an electrogenic effect. It removes three positively charged Na f ions from the cell and introduces two K ions into it. As a result, a current flows through the membrane, forming an electrical potential with a negative value in the inner part of the cell relative to its outer surface. Na "-K + pump regulates cell volume, controls the concentration of substances inside the cell, maintains osmotic pressure, and participates in the creation of membrane potential.

Transport in membrane packaging. The transfer of macromolecules (proteins, nucleic acids, polysaccharides, lipoproteins) and other particles through the membrane is carried out through the sequential formation and fusion of vesicles (vesicles) surrounded by the membrane. The process of vesicular transport occurs in two stages. Initially, the vesicle membrane and plasmalemma stick together and then merge. For the course of stage 2, it is necessary that water molecules be displaced by interacting lipid bilayers, which approach each other up to a distance of 1-5 nm. It is believed that this process is activated by special fusion proteins(they have been isolated so far only in viruses). Vesicular transport has important feature- absorbed or secreted macromolecules in the vesicles usually do not mix with other macromolecules or organelles of the cell. Bubbles can fuse with specific membranes, which ensures the exchange of macromolecules between the extracellular space and the contents of the cell. Similarly, macromolecules are transferred from one cell compartment to another.

The transport of macromolecules and particles into a cell is called endocytosis. In this case, the transported substances are enveloped by a part of the plasma membrane, a bubble (vacuole) is formed, which moves inside the cell. Depending on the size of the formed vesicles, two types of endocytosis are distinguished - pinocytosis and phagocytosis.

pinocytosis provides absorption of liquid and dissolved substances in the form of small bubbles (d=150 nm). Phagocytosis - this is the absorption of large particles, microorganisms or fragments of organelles, cells. In this case, large vesicles, phagosomes or vacuoles (d-250 nm or more) are formed. In protozoa, the phagocytic function is a form of nutrition. In mammals, the phagocytic function is carried out by macrophages and neutrophils, which protect the body from infection by engulfing invading microbes. Macrophages are also involved in the disposal of old or damaged cells and their fragments (in the human body, macrophages absorb more than 100 old red blood cells daily). Phagocytosis begins only when the absorbed particle binds to the surface of the phagocyte and activates specialized receptor cells. The binding of particles to specific membrane receptors causes the formation of pseudopodia, which envelop the particle and, merging at the edges, form a bubble - phagosome. The formation of a phagosome and proper phagocytosis occurs only if, during the enveloping process, the particle is in constant contact with the plasmalemma receptors, as if "zipping up".

A significant part of the material absorbed by the cell by endocytosis ends up in lysosomes. Large particles are included in phagosomes which then fuse with lysosomes to form phagolysosomes. Fluid and macromolecules taken up during pinocytosis are initially transferred to endosomes, which also fuse with lysosomes to form endolysosomes. Various hydrolytic enzymes present in lysosomes quickly destroy macromolecules. Hydrolysis products (amino acids, sugars, nucleotides) are transported from lysosomes to the cytosol, where they are used by the cell. Most of the membrane components of endocytic vesicles from phagosomes and endosomes are returned by exocytosis to the plasma membrane and reutilized there. The main biological significance of endocytosis is the acquisition of building blocks through intracellular digestion of macromolecules in lysosomes.

The absorption of substances in eukaryotic cells begins in specialized areas of the plasma membrane, the so-called bordered pits. On electron micrographs, the pits look like invaginations of the plasma membrane, the cytoplasmic side of which is covered with a fibrous layer. The layer, as it were, borders small pits of the plasmalemma. The pits occupy about 2% of the total surface of the eukaryotic cell membrane. Within a minute, the pits grow, invaginate deeper and deeper, are drawn into the cell and then, narrowing at the base, split off, forming bordered vesicles. It has been established that approximately a quarter of the membrane in the form of bordered vesicles is split off from the plasma membrane of fibroblasts within one minute. The vesicles quickly lose their border and acquire the ability to merge with the lysosome.

Endocytosis may be non-specific(constitutive) and specific(receptor). At nonspecific endocytosis the cell captures and absorbs substances that are completely alien to it, for example, soot particles, dyes. Initially, particles are deposited on the glycocalyx of the plasmalemma. Positively charged protein groups are especially well precipitated (adsorbed), since the glycocalyx carries a negative charge. Then the morphology of the cell membrane changes. It can either sink, forming invaginations (invaginations), or, conversely, form outgrowths that seem to fold, separating small volumes of the liquid medium. The formation of invaginations is more typical for cells of the intestinal epithelium, amoebae, and outgrowths - for phagocytes and fibroblasts. These processes can be blocked by respiratory inhibitors. The resulting vesicles - primary endosomes - can merge with each other, increasing in size. Subsequently, they combine with lysosomes, turning into an endolysosome - a digestive vacuole. The intensity of liquid-phase nonspecific pinocytosis is quite high. Macrophages form up to 125, and epithelial cells of the small intestine up to a thousand pinosomes per minute. The abundance of pinosomes leads to the fact that the plasmalemma is quickly spent on the formation of many small vacuoles. Restoration of the membrane proceeds quite rapidly during recyclization during exocytosis due to the return of vacuoles and their incorporation into the plasmalemma. In macrophages, the entire plasma membrane is replaced in 30 minutes, and in fibroblasts in 2 hours.

A more efficient way to absorb specific macromolecules from the extracellular fluid is specific endocytosis(mediated by receptors). In this case, macromolecules bind to complementary receptors on the cell surface, accumulate in the bordered fossa, and then, forming an endosome, are immersed in the cytosol. Receptor endocytosis ensures the accumulation of specific macromolecules at its receptor. Molecules that bind to a receptor on the surface of the plasmalemma are called ligands. With the help of receptor endocytosis in many animal cells, cholesterol is absorbed from the extracellular environment.

The plasma membrane takes part in the removal of substances from the cell (exocytosis). In this case, the vacuoles approach the plasmalemma. At the points of contact, the plasmolemma and the vacuole membrane merge and the contents of the vacuole enter the environment. In some protozoa, sites on the cell membrane for exocytosis are predetermined. So, in the plasma membrane of some ciliated ciliates there are certain areas with the correct arrangement of large globules of integral proteins. Mucocysts and trichocysts of ciliates that are completely ready for secretion have a halo of integral protein globules on the upper part of the plasmalemma. These sections of the membrane of the mucocysts and trichocysts are in contact with the surface of the cell. A peculiar exocytosis is observed in neutrophils. They are able, under certain conditions, to release their lysosomes into the environment. In some cases, small outgrowths of the plasmalemma containing lysosomes are formed, which then break off and pass into the environment. In other cases, there is invagination of the plasmalemma deep into the cell and its capture of lysosomes located far from the cell surface.

The processes of endocytosis and exocytosis are carried out with the participation of the system of fibrillar components of the cytoplasm associated with the plasmolemma.

Receptor function of the plasmalemma. This is one of the main, universal for all cells, is the receptor function of the plasmalemma. It determines the interaction of cells with each other and with the external environment.

The whole variety of informational intercellular interactions can be schematically represented as a chain of successive reactions signal-receptor-secondary messenger-response (signal-response concept). The transfer of information from cell to cell is carried out by signaling molecules that are produced in some cells and specifically affect others that are sensitive to the signal (target cells). Signal molecule - primary intermediary binds to receptors located on target cells that respond only to certain signals. Signal molecules - ligands - approach their receptor like a key to a lock. Ligands for membrane receptors (plasmalemma receptors) are hydrophilic molecules, peptide hormones, neurotransmitters, cytokines, antibodies, and for nuclear receptors - fat-soluble molecules, steroid and thyroid hormones, vitamin D. Membrane proteins or glycocalyx elements can act as receptors on the cell surface - polysaccharides and glycoproteins. It is believed that areas sensitive to individual substances are scattered over the surface of the cell or collected in small zones. So, on the surface of prokaryotic cells and animal cells there are a limited number of places with which viral particles can bind. Membrane proteins (carriers and channels) recognize, interact and carry only certain substances. Cell receptors are involved in the transmission of signals from the surface of the cell into it. The diversity and specificity of the sets of receptors on the cell surface leads to the creation of a very complex system of markers that make it possible to distinguish one's own cells from those of others. Similar cells interact with each other, their surfaces can stick together (conjugation in protozoa, tissue formation in multicellular). Cells that do not perceive markers, as well as those that differ in the set of determinant markers, are destroyed or rejected. When the receptor-ligand complex is formed, transmembrane proteins are activated: converter protein, amplifier protein. As a result, the receptor changes its conformation and interacts with the precursor of the second messenger located in the cell - messenger. Messengers can be ionized calcium, phospholipase C, adenylate cyclase, guanylate cyclase. Under the influence of the messenger, the activation of enzymes involved in the synthesis cyclic monophosphates - AMP or HMF. The latter alter the activity of two types of protein kinase enzymes in the cell cytoplasm, leading to the phosphorylation of numerous intracellular proteins.

The most common formation of cAMP, under the influence of which the secretion of a number of hormones - thyroxine, cortisone, progesterone, increases, the breakdown of glycogen in the liver and muscles, the frequency and strength of heart contractions, osteodestruction, and reverse absorption of water in the nephron tubules increase.

The activity of the adenylate cyclase system is very high - the synthesis of cAMP leads to a ten thousandth increase in the signal.

Under the action of cGMP, the secretion of insulin by the pancreas, histamine by mast cells, serotonin by platelets increases, and smooth muscle tissue is reduced.

In many cases, the formation of a receptor-ligand complex results in a change in the membrane potential, which in turn leads to a change in the permeability of the plasmalemma and metabolic processes in the cell.

On the plasma membrane there are specific receptors that respond to physical factors. So, in photosynthetic bacteria, chlorophylls are located on the cell surface that react to light. In light-sensitive animals, the plasma membrane contains a whole system of phogoreceptor proteins-rhodopsins, with the help of which the light stimulus is transformed into a chemical signal, and then an electrical impulse.

or plasmalemma, occupies a special place among various cell membranes. This is a superficial peripheral structure that limits the cell from the outside, which determines its direct connection with the extracellular environment, and, consequently, with all substances and stimuli that act on the cell. Therefore, the plasma membrane plays the role of a barrier, a barrier between the complexly organized intracellular contents and the external environment. In this case, the plasmalemma performs not only the role of a mechanical barrier, but, most importantly, it limits the free flow of low- and high-molecular substances in both directions through the membrane. Moreover, the plasmalemma acts as a structure that “recognizes”, receptors, various chemicals and selectively regulates the transport of these substances into and out of the cell. In other words, the plasma membrane performs functions associated with regulated selective transmembrane transport of substances and plays the role of a primary cell analyzer. In this regard, the plasmalemma can be considered a cellular organelle that is part of the vacuolar system of the cell. Like other membranes of this system (the membranes of lysosomes, endosomes, the Golgi apparatus, etc.), it arises and is updated due to the synthetic activity of the endoplasmic reticulum and has a similar composition. Oddly enough, the plasma membrane can be likened to the membrane of an intracellular vacuole, but turned inside out: it is not surrounded by hyaloplasm, but surrounds it.

Barrier-transport role of the plasmalemma

Surrounding the cell from all sides, the plasma membrane acts as a mechanical barrier. In order to pierce it with microneedles or micropipettes, quite a lot of effort is required. With the pressure of a microneedle on it, it first strongly bends, and only then breaks through. Artificial lipid membranes are less stable. This mechanical stability of the plasma membrane may be determined by additional components such as the glycocalyx and the cortical layer of the cytoplasm (Fig. 127).

Glycocalyx is a layer external to the lipoprotein membrane containing polysaccharide chains of membrane integral proteins - glycoproteins. These chains contain such carbohydrates as mannose, glucose, N-acetylglucosamine, sialic acid, etc. Such carbohydrate heteropolymers form branching chains, between which glycolipids and proteoglycans isolated from the cell can be located. The layer of glycocalyx is heavily watered, has a jelly-like consistency, which significantly reduces the rate of diffusion of various substances in this zone. Hydrolytic enzymes secreted by the cell, which are involved in the extracellular cleavage of polymers (extracellular digestion) to monomeric molecules, which are then transported to the cytoplasm through the plasma membrane, can also “get stuck” here.

As shown by electron microscopic studies, especially with the use of special methods of contrasting polysaccharides, the glycocalyx has the form of a loose fibrous layer 3-4 nm thick, covering the entire surface of the cell. The glycocalyx is especially well expressed in the brush border of the cells of the absorbing intestinal epithelium (enterocytes), however, it is found in almost all animal cells, but the degree of its severity is different (Fig. 128).

The mechanical stability of the plasma membrane, in addition, is provided by the structure of the cortical layer adjacent to it from the side of the cytoplasm and intracellular fibrillar structures.

cortical(from the word cortex- bark, peel) layer cytoplasm, in close contact with the lipoprotein outer membrane, has a number of features. Here, in a thickness of 0.1-0.5 microns, there are no ribosomes and membrane vesicles, but fibrillar elements of the cytoplasm - microfilaments and often microtubules - are found in large numbers. The main fibrillar component of the cortical layer is a network of actin microfibrils. A number of auxiliary proteins are also located here, which are necessary for the movement of sections of the cytoplasm (for more details on the skeletal-motor system of cells, see). The role of these actin-associated proteins is very important, as it explains their participation in the connection, in the "anchoring" of the integral proteins of the plasma membrane.

In many protozoa, especially ciliates, the plasma membrane takes part in the formation pellicles- a rigid layer that often determines the shape of the cell. Membrane sacs can adjoin the plasma membrane here from the inside; in this case, there are three membrane layers near the surface of the cells: the plasma membrane itself and two membranes of the pellicular alveoli. In the ciliates of the shoe, the pellicle forms thickenings, located in the form of hexagons, in the center of which there are cilia (Fig. 129). The rigidity of pellicular formations can also be associated with elements of the cytoplasm underlying the plasma membrane, with the cortical layer. Thus, in the crests of the euglena pellicle near the membrane, in addition to membrane vacuoles, parallel bundles of microtubules and microfilaments are found. This fibrillar peripheral reinforcement, together with the folded multilayer membrane periphery, creates a rigid pellicle structure.

The barrier role of the plasmalemma also consists in limiting the free diffusion of substances. Model experiments on artificial lipid membranes showed that they are permeable to water, gases, small non-polar molecules of fat-soluble substances, but completely impermeable to charged molecules (ions) and large uncharged ones (sugars) (Fig. 130).

Natural membranes also limit the rate of penetration of low molecular weight compounds into the cell.

Transmembrane transport of ions and low molecular weight compounds

The plasma membrane, like other lipoprotein cell membranes, is semipermeable. This means that different molecules pass through it at different speeds and the larger the size of the molecules, the lower the speed of their passage through the membrane. This property defines the plasma membrane as an osmotic barrier. Water and gases dissolved in it have the maximum penetrating ability, ions penetrate the membrane much more slowly (about 10 4 times slower). Therefore, if a cell, for example, an erythrocyte, is placed in an environment where the salt concentration is lower than in the cell (hypotension), then water from the outside will rush into the cell, which will lead to an increase in the volume of the cell and to rupture of the plasma membrane ("hypotonic shock"). On the contrary, when an erythrocyte is placed in salt solutions of a higher concentration than in the cell, water will escape from the cell into the external environment. At the same time, the cell will wrinkle, decrease in volume.

Such passive transport of water out of the cell and into the cell still proceeds at a low rate. The rate of water penetration through the membrane is about 10 -4 cm/s, which is 100,000 times less than the rate of diffusion of water molecules through an aqueous layer 7.5 nm thick. In this regard, it was concluded that in the cell membrane, in its lipoprotein layer, there are special "pores" for the penetration of water and ions. Their number is not so great: the total area with the size of a single "pore" of about 0.3-0.8 nm should be only 0.06% of the entire cell surface.

Unlike artificial bilayer lipid membranes, natural membranes, primarily the plasma membrane, are capable of transporting ions and many monomers, such as sugars, amino acids, etc. The permeability for ions is low, and the rate of passage of different ions is not the same. Higher passage rate for cations (K + , Na +) and much lower for anions (Сl -).

The transport of ions through the plasmalemma is carried out due to the participation in this process of membrane transport proteins - permease. These proteins can carry one substance in one direction (uniport) or several substances simultaneously (symport), or, together with the import of one substance, remove another from the cell (antiport). So, glucose can enter the cells symportally together with the Na + ion.

Ion transport can take place along the concentration gradient,passively, without additional energy consumption. Thus, the Na + ion penetrates into the cell from the external environment, where its concentration is higher than in the cytoplasm. In the case of passive transport, some membrane transport proteins form molecular complexes - channels, through which solute molecules pass through the membrane by simple diffusion along a concentration gradient. Some of these channels are permanently open, while the other part can close or open in response to either binding to signaling molecules or changes in the intracellular ion concentration. In other cases, special membrane carrier proteins selectively bind to one or another ion and carry it through the membrane (facilitated diffusion) (Fig. 131).

The presence of such protein transport channels and carriers, it would seem, should lead to an equilibrium in the concentrations of ions and low molecular weight substances on both sides of the membrane. In fact, this is not so: the concentration of ions in the cytoplasm of cells differs sharply not only from that in the external environment, but even from the blood plasma that bathes the cells in the animal body (Table 14).

As can be seen in this case, the total concentration of monovalent cations both inside and outside the cells is practically the same (150 mM), i.e. isotonic. But it turns out that in the cytoplasm the concentration of K + is almost 50 times higher, and Na + is lower than in blood plasma. Moreover, this difference is maintained only in a living cell: if the cell is killed or the metabolic processes in it are suppressed, then after a while the ionic differences on both sides of the plasma membrane will disappear. You can simply cool the cells to +2 °C, and after a while the concentration of K + and Na + on both sides of the membrane will become the same. When the cells are heated, this difference is restored. This phenomenon is due to the fact that there are membrane protein carriers in cells that work against the concentration gradient, while expending energy due to ATP hydrolysis. This type of work is called activetransport, and it is done with protein ion pumpsowls. The plasma membrane contains a two-subunit molecule (K + /Na +)-nacoca, which is also an ATPase. During operation, this pump pumps out three Na + ions in one cycle and pumps two K + ions into the cell against the concentration gradient. In this case, one ATP molecule is spent, which goes to ATPase phosphorylation, as a result of which Na + is transferred through the membrane from the cell, and K + gets the opportunity to bind to the protein molecule and then is transferred into the cell (Fig. 132). As a result of active transport with the help of membrane pumps, the concentration in the cell of the divalent cations Mg 2+ and Ca 2+ is also regulated, also with the consumption of ATP.

Rice. 132. (K + /Na +)-nacoc 1 - Na + binding site; 2 - binding site K + ; 3 - membrane

Such a constant work of permeases and pumps creates a constant concentration of ions and low molecular weight substances in the cell, i.e. creates the so-called homeostasis - the constancy of the concentrations of osmotically active substances. It should be noted that approximately 80% of the total ATP of the cell is spent on maintaining homeostasis.

In combination with active transport of ions across the plasma membrane, various sugars, nucleotides and amino acids are transported. Thus, the active transport of glucose, which symportically (simultaneously) enters the cell together with the flow of the passively transported Na + ion, will depend on the activity of the (K + /Na +) pump. If this pump is blocked, then soon the difference in the concentration of Na + on both sides of the membrane will disappear, while the diffusion of Na + into the cell will decrease, and at the same time the flow of glucose into the cell will stop. As soon as the work of (K + /Na +)-ATPase is restored and a difference in the concentration of ions occurs, the diffuse flow of Na + and, at the same time, glucose transport will immediately increase. Similarly, through the membrane and the flow of amino acids, which are transported by special carrier proteins that work as symport systems, simultaneously transporting ions.

The active transport of sugars and amino acids in bacterial cells is due to a gradient of hydrogen ions.

In itself, the participation of special membrane proteins in the passive or active transport of low molecular weight compounds indicates the high specificity of this process. Even in the case of passive ion transport, proteins “recognize” a given ion, interact with it, bind specifically, change their conformation and function. Consequently, already on the example of the transport of simple substances, membranes act as analyzers, as receptors. This receptor role is especially manifested when biopolymers are absorbed by the cell.

Vesicular transport: endocytosis and exocytosis

macromolecules such as proteins, nucleic acids, polysaccharides, lipoprotein complexes and others, do not pass through cell membranes, as opposed to how ions and monomers are transported. The transport of micromolecules, their complexes, particles into and out of the cell is carried out in a completely different way - through vesicular transfer. This term means that various macromolecules, biopolymers or their complexes cannot enter the cell through the plasma membrane. And not only through it: any cell membranes are not capable of transmembrane transfer of biopolymers, with the exception of membranes that have special protein complex carriers - porins (membranes of mitochondria, plastids, peroxisomes). Macromolecules enter the cell or from one membrane compartment to another enclosed within vacuoles or vesicles. Such vesicular transfer can be divided into two types: exocytosis- removal of macromolecular products from the cell, and endocytosis- absorption of macromolecules by the cell (Fig. 133).

Rice. 133. Comparison of endocytosis ( A) and exocytosis ( b)

During endocytosis, a certain section of the plasmalemma captures, as it were, enveloping the extracellular material, enclosing it in a membrane vacuole that has arisen due to the invagination of the plasma membrane. In such a primary vacuole, or endosome any biopolymers, macromolecular complexes, parts of cells or even whole cells can enter, where they then disintegrate, depolymerize to monomers, which, by means of transmembrane transfer, enter the hyaloplasm. The main biological significance of endocytosis is the acquisition of building blocks through intracellular digestvaniya, which is carried out at the second stage of endocytosis, after the fusion of the primary endosome with the lysosome - a vacuole containing a set of hydrolytic enzymes.

Endocytosis is formally divided into pinocytosis And phagocytosis(Fig. 134). Phagocytosis- capture and absorption by the cell of large particles (sometimes even cells or their parts) - was first described by I.I. Mechnikov. Phagocytosis occurs both in unicellular (for example, in amoeba, some predatory ciliates) and in multicellular animals. In the latter case, it is carried out with the help of specialized cells. Such cells, phagocytes, are characteristic of both invertebrates (amoebocytes of blood or cavity fluid) and vertebrates (neutrophils and macrophages). pinocytosis was originally defined as the absorption of water or aqueous solutions of various substances by the cell. It is now known that both phagocytosis and pinocytosis proceed very similarly, and therefore the use of these terms can only reflect differences in the volumes and mass of absorbed substances. What these processes have in common is that the absorbed substances on the surface of the plasma membrane are surrounded by a membrane in the form of a vacuole - an endosome, which moves inside the cell.

Rice. 134. Scheme of phagocytosis ( A) and pinocytosis ( b)

Endocytosis, including pinocytosis and phagocytosis, can be non-specific, or constitutive, constant and specific, mediated by receptors (receptor). Nonspecific endocytosis(pinocytosis and phagocytosis) is so called because it proceeds as if automatically and can often lead to the capture and absorption of substances that are completely alien or indifferent to the cell, for example, particles of soot or dyes.

Nonspecific endocytosis is often accompanied by initial sorption of the entrapping material by the plasma membrane glycocalyx. The glycocalyx, due to the acidic groups of its polysaccharides, has a negative charge and binds well to various positively charged groups of proteins. With such adsorption nonspecific endocytosis, macromolecules and small particles (acidic proteins, ferritin, antibodies, virions, colloidal particles) are absorbed. Liquid-phase pinocytosis leads to the absorption together with the liquid medium of soluble molecules that do not bind to the plasmalemma.

At the next stage, a change in the morphology of the cell surface occurs: either small invaginations of the plasma membrane occur, i.e. invagination, or outgrowths appear on the surface of the cell in the form of folds, or "frills" (from the English ruffle), which, as it were, overlap, fold, separating small volumes of the liquid medium (Fig. 135 and 136). The first type of occurrence of a pinocytic vesicle - pinosomes, is characteristic of cells of the intestinal epithelium, endothelium, and amoebas; the second - for phagocytes and fibroblasts. These processes depend on the supply of energy: respiration inhibitors block these processes.

This restructuring of the surface is followed by the process of adhesion and fusion of contacting membranes, which leads to the formation of a pinocytic vesicle (pinosome), which detaches from the cell surface and goes deep into the cytoplasm. Both nonspecific and receptor endocytosis, leading to the cleavage of membrane vesicles, occurs in specialized regions of the plasma membrane. These are the so-called lined pits. They are called so because from the side of the cytoplasm, the plasma membrane is covered (clothed) with a thin (about 20 nm) fibrous layer, which on ultrathin sections, as it were, borders, covers small protrusions - pits (Fig. 137). Almost all animal cells have these pits; they occupy about 2% of the cell surface. The border layer consists mainly of the clathrin protein associated with a number of additional proteins. Three molecules of clathrin, together with three molecules of a low molecular weight protein, form the structure of a triskelion, resembling a three-beam swastika (Fig. 138). Clathrin triskelions on the inner surface of the pits of the plasma membrane form a loose network consisting of pentagons and hexagons, generally resembling a basket. The clathrin layer covers the entire perimeter of the separating primary endocytic vacuoles - bordered vesicles.

Clathrin belongs to one of the types of so-called dressing proteins (COP - coated proteins). These proteins bind to integral receptor proteins from the side of the cytoplasm and form a dressing layer along the perimeter of the emerging pinosome, the primary endosomal vesicle, i.e. "bordered" bubble. In the separation of the primary endosome, proteins are also involved - dynamins, which polymerize around the neck of the separating vesicle (Fig. 139).

After the bordered vesicle separates from the plasmalemma and begins to be transferred deep into the cytoplasm, the clathrin layer disintegrates, dissociates, and the endosome membrane (pinosomes) acquires its usual form. After the loss of the clathrin layer, the endosomes begin to fuse with each other.

The membranes of the bordered pits contain relatively little cholesterol, which can determine the decrease in membrane stiffness and contribute to the formation of bubbles. The biological meaning of the appearance of a clathrin “coat” along the periphery of the vesicles may be that it provides adhesion of the bordered vesicles to the elements of the cytoskeleton and their subsequent transport in the cell, and also prevents them from merging with each other.

The intensity of liquid-phase nonspecific pinocytosis can be very high. So, the epithelial cell of the small intestine forms up to 1000 pinosomes per second, and macrophages - about 125 pinosomes per minute. The size of pinosomes is small, their lower limit is 60-130 nm, but their abundance leads to the fact that during endocytosis the plasmalemma is quickly replaced, as if “spent” on the formation of many small vacuoles. For example, in macrophages, the entire plasma membrane is replaced in 30 minutes, in fibroblasts - in 2 hours.

The further fate of endosomes can be different, some of them can return to the cell surface and merge with it, but most of them enter the process of intracellular digestion. Primary endosomes contain mostly foreign molecules trapped in the liquid medium and do not contain hydrolytic enzymes. Endosomes can fuse with each other, while increasing in size. They then fuse with primary lysosomes, which introduce enzymes into the endosome cavity that hydrolyze various biopolymers. The action of these lysosomal hydrolases causes intracellular digestion - the breakdown of polymers to monomers.

As already mentioned, during phagocytosis and pinocytosis, cells lose a large area of ​​the plasma membrane (see macrophages), which, however, is quickly restored during membrane recycling due to the return of vacuoles and their incorporation into the plasma membrane. This is due to the fact that small vesicles can separate from endosomes or vacuoles, as well as from lysosomes, which again merge with the plasmalemma. With such recycling, a kind of “shuttle” transfer of membranes occurs: plasmalemma-pinosome-vacuole-plasmalemma. This leads to the restoration of the original area of ​​the plasma membrane. With such a return - membrane recycling, all absorbed material is retained in the remaining endosome.

Specific, or receptor-mediated endocytosis has a number of differences from nonspecific. The main thing is that molecules are absorbed for which there are specific receptors on the plasma membrane that are associated only with this type of molecules. Often such molecules that bind to receptor proteins on the surface of cells are called ligands.

Receptor-mediated endocytosis was first described in the accumulation of proteins in avian oocytes. Proteins of yolk granules - vitellogenins, are synthesized in various tissues, but then they enter the ovaries with the blood flow, where they bind to special membrane receptors of oocytes and then enter the cell with the help of endocytosis, where yolk granules are deposited.

Another example of selective endocytosis is the transport of cholesterol into the cell. This lipid is synthesized in the liver and, in combination with other phospholipids and a protein molecule, forms the so-called low-density lipoprotein (LDL), which is secreted by liver cells and spreads throughout the body with blood (Fig. 140). Special receptors of the plasma membrane, diffusely located on the surface of various cells, recognize the protein component of LDL and form a specific receptor-ligand complex. Following this, such a complex moves to the zone of bordered pits and internalizes - it is surrounded by a membrane and plunges into the depths of the cytoplasm. It has been shown that mutant receptors can bind LDL, but do not accumulate in the area of ​​bordered pits. In addition to LDL receptors, more than two dozen other substances involved in receptor endocytosis of various substances have been found. They all use the same internalization pathway through the bordered pits. Probably, their role is in the accumulation of receptors: one and the same bordered pit can collect about 1000 receptors of different classes. However, in fibroblasts, LDL receptor clusters are located in the zone of bordered pits even in the absence of a ligand in the medium.

The further fate of the absorbed LDL particle is that it undergoes decay in the composition secondary lysosome. After immersion in the cytoplasm of a bordered vesicle loaded with LDL, there is a rapid loss of the clathrin layer, membrane vesicles begin to merge with each other, forming an endosome - a vacuole containing absorbed LDL particles still associated with receptors on the membrane surface. Then the dissociation of the ligand-receptor complex occurs; small vacuoles are split off from the endosome, the membranes of which contain free receptors. These vesicles are recycled, incorporated into the plasma membrane, and thereby the receptors return to the cell surface. The fate of LDL is that after fusion with lysosomes, they are hydrolyzed to free cholesterol, which can be incorporated into cell membranes.

Endosomes are characterized by a lower pH value (4-5), a more acidic environment than other cell vacuoles. This is due to the presence in their membranes of proton pump proteins that pump in hydrogen ions with the simultaneous consumption of ATP (H + -dependent ATPase). The acidic environment within endosomes plays a critical role in the dissociation of receptors and ligands. In addition, an acidic environment is optimal for the activation of hydrolytic enzymes in lysosomes, which are activated when lysosomes fuse with endosomes, which leads to the formation endolysosomes, where the breakdown of absorbed biopolymers occurs.

In some cases, the fate of dissociated ligands is not related to lysosomal hydrolysis. Thus, in some cells, after binding of plasma membrane receptors to certain proteins, clathrin-coated vacuoles sink into the cytoplasm and are transferred to another area of ​​the cell, where they fuse again with the plasma membrane, and the bound proteins dissociate from the receptors. This is how the transfer is carried out - transcytosis, of some proteins through the wall of the endothelial cell from the blood plasma into the intercellular environment (Fig. 141). Another example of transcytosis is the transfer of antibodies. Thus, in mammals, maternal antibodies can be transmitted to the baby through milk. In this case, the receptor-antibody complex remains unchanged in the endosome.

As already mentioned, phagocytosis is a variant of endocytosis and is associated with the absorption by the cell of large aggregates of macromolecules, up to living or dead cells. As well as pinocytosis, phagocytosis can be non-specific (for example, the absorption of particles of colloidal gold or dextran polymer by fibroblasts or macrophages) and specific, mediated by receptors on the surface of the plasma membrane of phagocytic cells. During phagocytosis, large endocytic vacuoles are formed - Fgosom, which then fuse with lysosomes to form phagolysosomes.

On the surface of cells capable of phagocytosis (in mammals, these are neutrophils and macrophages), there is a set of receptors that interact with ligand proteins. Thus, in bacterial infections, antibodies to bacterial proteins bind to the surface of bacterial cells, forming a layer in which the F c -regions of antibodies look outward. This layer is recognized by specific receptors on the surface of macrophages and neutrophils, and at the sites of their binding, absorption of the bacterium begins by enveloping it with the plasma membrane of the cell (Fig. 142).

The plasma membrane is involved in the removal of substances from the cell with the help of exocytosis- the reverse process of endocytosis (see Fig. 133). In the case of exocytosis, intracellular products enclosed in vacuoles or vesicles and separated from the hyaloplasm by a membrane approach the plasma membrane. At their points of contact, the plasma membrane and the vacuole membrane merge, and the bubble is emptied into the environment. With the help of exocytosis, the process of recycling of membranes involved in endocytosis occurs.

Exocytosis is associated with the release of various substances synthesized in the cell. Secreting, i.e. releasing substances into the environment, cells can produce and release low molecular weight compounds (acetylcholine, biogenic amines, etc.), as well as, in most cases, macromolecules (peptides, proteins, lipoproteins, peptidoglycans, etc.). Exocytosis, or secretion, in most cases is carried out in response to an external signal (nerve impulse, exposure to a hormone, mediator, etc.), although in some cases exocytosis occurs constantly (secretion of fibronectin and collagen by fibroblasts). Similarly, some polysaccharides (hemicelluloses) involved in the formation of cell walls are removed from the cytoplasm of plant cells.

Most secreted substances are used by other cells of multicellular organisms (secretion of milk, digestive juices, hormones, etc.). But often cells secrete substances for their own needs. For example, the growth of the plasma membrane is carried out due to the incorporation of sections of the membrane as part of exocytic vacuoles, some of the elements of the glycocalyx are secreted by the cell in the form of glycoprotein molecules, etc.

Hydrolytic enzymes isolated from cells by exocytosis can be sorbed in the glycocalyx layer and provide membrane-bound extracellular cleavage of various biopolymers and organic molecules. Membrane non-cellular digestion is of great importance for animals. It was found that in the intestinal epithelium of mammals in the area of ​​the so-called brush border of the absorbing epithelium, which is especially rich in glycocalyx, a huge amount of various enzymes is found. Some of these enzymes are of pancreatic origin (amylase, lipases, various proteinases, etc.), and some are secreted by the epithelial cells themselves (exohydrolases, which break down mainly oligomers and dimers with the formation of transported products).

The receptor role of the plasmalemma

We have already met with this feature of the plasma membrane when getting acquainted with its transport functions. Carrier proteins and pumps are also receptors that recognize and interact with certain ions. Receptor proteins bind to ligands and participate in the selection of molecules entering cells.

Membrane proteins or glycocalyx elements - glycoproteins can act as such receptors on the cell surface. Such sensitive to individual substances areas can be scattered over the surface of the cell or collected in small areas.

Different cells of animal organisms may have different sets of receptors or different sensitivity of the same receptor.

The role of many cell receptors is not only in the binding of specific substances or the ability to respond to physical factors, but also in the transmission of intercellular signals from the surface into the cell. At present, the system of signal transmission to cells with the help of certain hormones, which include peptide chains, has been well studied. These hormones bind to specific receptors on the surface of the cell's plasma membrane. Receptors, after binding to the hormone, activate another protein, which is already in the cytoplasmic part of the plasma membrane, adenylate cyclase. This enzyme synthesizes the cyclic AMP molecule from ATP. The role of cyclic AMP (cAMP) is that it is a secondary messenger - an activator of kinase enzymes that cause modifications of other enzyme proteins. So, when the pancreatic hormone glucagon, produced by A-cells of the islets of Langerhans, acts on the liver cell, it binds to a specific receptor, which stimulates the activation of adenylate cyclase. Synthesized cAMP activates protein kinase A, which in turn activates a cascade of enzymes that ultimately break down glycogen (animal storage polysaccharide) to glucose. The action of insulin is the opposite: it stimulates the entry of glucose into the liver cells and its deposition in the form of glycogen.

In general, the chain of events unfolds as follows: the hormone interacts specifically with the receptor part of this system and, without penetrating into the cell, activates adenylate cyclase, which synthesizes cAMP. The latter activates or inhibits an intracellular enzyme or group of enzymes. Thus, the command (signal from the plasma membrane) is transmitted inside the cell. The efficiency of this adenylate cyclase system is very high. Thus, the interaction of one or several hormone molecules can lead, due to the synthesis of many cAMP molecules, to a signal amplification thousands of times. In this case, the adenylate cyclase system serves as a converter of external signals.

There is another way in which other second messengers are used - this is the so-called phosphatidylinositol way. Under the action of the appropriate signal (some nerve mediators and proteins), the enzyme phospholipase C is activated, which cleaves the phosphatidylinositol diphosphate phospholipid, which is part of the plasma membrane. The hydrolysis products of this lipid, on the one hand, activate protein kinase C, which activates the kinase cascade, which leads to certain cellular reactions, and on the other hand, leads to the release of calcium ions, which regulates a number of cellular processes.

Another example of receptor activity is the receptors for acetylcholine, an important neurotransmitter. Acetylcholine, being released from the nerve ending, binds to the receptor on the muscle fiber, which causes an impulsive flow of Na + into the cell (membrane depolarization), immediately opening about 2000 ion channels in the area of ​​the neuromuscular ending.

The diversity and specificity of the sets of receptors on the surface of cells lead to the creation of a very complex system of markers that make it possible to distinguish one's own cells (of the same individual or of the same species) from those of others. Similar cells enter into interactions with each other, leading to adhesion of surfaces (conjugation in protozoa and bacteria, the formation of tissue cell complexes). In this case, cells that differ in the set of determinant markers or do not perceive them are either excluded from such interaction, or (in higher animals) are destroyed as a result of immunological reactions.

The plasma membrane is associated with the localization of specific receptors that respond to physical factors. So, in the plasma membrane or in its derivatives in photosynthetic bacteria and blue-green algae, receptor proteins (chlorophylls) interacting with light quanta are localized. In the plasma membrane of light-sensitive animal cells, there is a special system of photoreceptor proteins (rhodopsin), with the help of which the light signal is converted into a chemical one, which in turn leads to the generation of an electrical impulse.

Intercellular recognition

In multicellular organisms, due to intercellular interactions, complex cellular ensembles are formed, the maintenance of which can be carried out in different ways. In germinal, embryonic tissues, especially in the early stages of development, cells remain connected to each other due to the ability of their surfaces to stick together. This property adhesion(connection, adhesion) of cells can be determined by the properties of their surface, which specifically interact with each other. The mechanism of these connections is well studied, it is provided by the interaction between glycoproteins of plasma membranes. With such intercellular interaction of cells between plasma membranes, there always remains a gap about 20 nm wide, filled with glycocalyx. Treatment of tissue with enzymes that violate the integrity of the glycocalyx (mucases that act hydrolytically on mucins, mucopolysaccharides) or damage the plasma membrane (proteases) leads to the isolation of cells from each other, to their dissociation. However, if the dissociation factor is removed, the cells can reassemble and reaggregate. So it is possible to dissociate cells of sponges of different colors, orange and yellow. It turned out that two types of aggregates are formed in a mixture of these cells: some consist only of yellow, others only of orange cells. In this case, mixed cell suspensions self-organize, restoring the original multicellular structure. Similar results were obtained with separated cell suspensions of amphibian embryos; in this case, there is a selective spatial separation of ectoderm cells from the endoderm and from the mesenchyme. Moreover, if tissues of late stages of embryonic development are used for reaggregation, then various cell ensembles with tissue and organ specificity independently assemble in a test tube, epithelial aggregates similar to renal tubules are formed, etc.

Transmembrane glycoproteins are responsible for the aggregation of homogeneous cells. Molecules of the so-called CAM-proteins (cell adhesion molecules) are directly responsible for the connection - adhesion, of cells. Some of them connect cells with each other due to intermolecular interactions, others form special intercellular connections, or contacts.

Interactions between adhesive proteins can be homophilly, when neighboring cells communicate with each other using homogeneous molecules, and heterophile when various kinds of CAMs on neighboring cells are involved in adhesion. Intercellular binding occurs through additional linker molecules.

There are several classes of CAM proteins: cadherins, immunoglobulin-like N-CAMs (nerve cell adhesion molecules), selectins, integrins.

Cadherins are integral fibrillar membrane proteins that form parallel homodimers. Separate domains of these proteins are associated with Ca 2+ ions, which gives them a certain rigidity. There are more than 40 types of cadherins. Thus, E-cadherin is characteristic of cells of preimplanted embryos and epithelial cells of adult organisms. P-cadherin is characteristic of trophoblast, placenta, and epidermis cells; N-cadherin is located on the surface of nerve cells, lens cells, and on cardiac and skeletal muscles.

Nerve cell adhesion molecules(N-CAM) belong to the immunoglobulin superfamily, they form connections between nerve cells. Some of the N-CAMs are involved in the connection of synapses, as well as in the adhesion of cells of the immune system.

selectins- integral proteins of the plasma membrane, are involved in the adhesion of endothelial cells, in the binding of platelets, leukocytes.

Integrins are heterodimers, with α and β chains. Integrins primarily connect cells with extracellular substrates, but they can also participate in cell adhesion to each other.

As already mentioned, a complex complex reaction, an immune reaction, develops against foreign macromolecules (antigens) that enter the body. Its essence lies in the fact that some of the lymphocytes produce special proteins-antibodies that specifically bind to antigens. Thus, macrophages recognize antigen-antibody complexes with their surface receptors and absorb them (for example, the absorption of bacteria during phagocytosis).

In the body of all vertebrates, in addition, there is a system of reception of foreign cells or their own, but with altered plasma membrane proteins, for example, during viral infections or mutations, often associated with tumor degeneration of cells.

On the surface of all vertebrate cells are proteins of the so-called major histocompatibility complex(MHC - major histocompatibility complex). These are integral proteins, glycoproteins, heterodimers. It is very important to remember that each individual has a different set of these MHC proteins. This is due to the fact that they are very polymorphic, since each individual has a large number of alternative forms of the same gene (more than 100); in addition, there are 7-8 loci encoding MHC molecules. This leads to the fact that each cell of a given organism, having a set of MHC proteins, will differ from the cells of an individual of the same species. A special form of lymphocytes - T-lymphocytes, recognize the MHC of their body, but the slightest changes in the structure of the MHC (for example, association with a virus or the result of a mutation in individual cells) lead to the fact that T-lymphocytes recognize such changed cells and destroy them, but not by phagocytosis. They secrete specific perforin proteins from secretory vacuoles, which are embedded in the cytoplasmic membrane of the altered cell, form transmembrane channels in it, making the plasma membrane permeable, which leads to the death of the altered cell (Fig. 143 and 144).

Special intercellular connections (contacts)

In addition to such relatively simple adhesive (but specific) bonds (Fig. 145), there are a number of special intercellular structures - contacts, or compounds that perform certain functions. These are locking, anchoring and communication connections (Fig. 146).

Locking, or tight, connection characteristic of single-layered epithelium. This is the zone where the outer layers of the two plasma membranes are as close as possible. The three-layer membrane is often seen in this contact: the two outer osmophilic layers of both membranes seem to merge into one common layer 2–3 nm thick. The fusion of membranes does not occur over the entire area of ​​​​tight contact, but is a series of point convergence of membranes (Fig. 147, A and 148).

On planar preparations of plasma membrane fractures in the zone of tight contact, using the freezing and chipping method, it was found that the points of contact of the membranes are rows of globules. These are the proteins occludin and claudin - special integral proteins of the plasma membrane, built in rows. Such rows of globules, or stripes, can intersect in such a way that they form, as it were, a lattice, or network, on the cleavage surface. This structure is very typical for epithelia, especially glandular and intestinal. In the latter case, tight contact forms a continuous zone of fusion of plasma membranes, encircling the cell in its apical (upper, looking into the intestinal lumen) part (see Fig. 148). Thus, each cell of the layer is, as it were, surrounded by a tape of this contact. Such structures can also be seen with special stains in a light microscope. They received from morphologists the name of the closing plates. It turned out that in this case the role of the closing tight contact is not only in the mechanical connection of cells with each other. This contact area is poorly permeable to macromolecules and ions, and thus it locks, blocks the intercellular cavities, isolating them (and with them the internal environment of the body) from the external environment (in this case, the intestinal lumen).

This can be demonstrated using electron-dense contrasters such as lanthanum hydroxide solution. If the lumen of the intestine or duct of some gland is filled with a solution of lanthanum hydroxide, then on sections under an electron microscope, the zones where this substance is located have a high electron density and will be dark. It turned out that neither the zone of tight contact nor the intercellular spaces below it darken. If the tight contacts are damaged (by light enzymatic treatment or removal of Ca 2+ ions), then lanthanum also penetrates into the intercellular regions. Similarly, tight junctions have been shown to be impermeable to hemoglobin and ferritin in the tubules of the kidneys. Thus, tight junctions are not only barriers for macromolecules, they are impermeable to liquids and ions.

Closing, or tight, contact occurs between all types of single-layer epithelium (endothelium, mesothelium, ependyma).

anchoring, or coupling, connections, or contacts, so called because they connect not only the plasma membranes of neighboring cells, but also bind to the fibrillar elements of the cytoskeleton (Fig. 149). This kind of compounds is characterized by the presence of two types of proteins. The first type is represented by transmembrane linker (binding) proteins that are involved either in the actual intercellular connection or in the connection of the plasmalemma with the components of the extracellular matrix (basement membrane of epithelia, extracellular structural proteins of connective tissue).

The second type includes intracellular proteins that connect or anchor the membrane elements of such contact with the cytoplasmic fibrils of the cytoskeleton.

Anchoring junctions include intercellular anchoring point junctions, anchoring bands, focal junctions, or anchorage plaques; all these contacts bind within cells to actin microfilaments. Another group of anchoring intercellular connections are desmosomes And hemidesmosomes; they bind to other elements of the cytoskeleton - with intermediate filaments.

Intercellular pinpoint junctions have been found in many non-epithelial tissues, but the structure has been more clearly described. adhesive (adhesiveny) tapes in single-layer epithelium (Fig. 150). This structure encircles the entire perimeter of the epithelial cell, similar to what happens in the case of a tight junction. Most often, such a belt, or tape, lies below the tight connection (see Fig. 146). In this place, the plasma membranes are not brought together, but even somewhat moved apart at a distance of 25–30 nm, and a zone of increased density is visible between them. This is nothing more than the sites of interaction of transmembrane glycoproteins, which specifically adhere to each other and provide a mechanical connection between the membranes of two neighboring cells. These linker proteins belong to E-cadherins, proteins that provide specific recognition of homogeneous membranes by cells. The destruction of this layer of glycoproteins leads to the isolation of individual cells and to the destruction of the epithelial layer. On the cytoplasmic side near the membrane, an accumulation of some dense substance is seen, to which is adjoined a layer of thin (6-7 nm) filaments lying along the plasma membrane in the form of a bundle that runs along the entire perimeter of the cell. Thin filaments are actin fibrils, they bind to the plasma membrane through the proteins catenin, vinculin and α-actinin, which form a dense peri-membrane layer.

The functional significance of such a ribbon connection lies not only in the mechanical adhesion of cells to each other: when the actin filaments in the ribbon are reduced, the shape of the cell can change. It is believed that the cooperative contraction of actin fibrils in all cells of the epithelial sheet can cause a change in its geometry, for example, folding into a tube, similar to what occurs during the formation of the neural tube in vertebrate embryos.

focal contacts, or clutch plaques, occur in many cells and are especially well studied in fibroblasts. They are built according to the general plan with adhesive tapes, but are expressed in the form of small areas - plaques - on the plasmalemma. In this case, transmembrane linker integrin proteins bind specifically to extracellular matrix proteins (for example, fibronectin) (Fig. 151). From the side of the cytoplasm, these same glycoproteins are associated with membrane proteins, which also includes vinculin, which in turn is associated with a bundle of actin filaments. The functional significance of focal contacts lies both in anchoring the cell to extracellular structures and in creating a mechanism that allows cells to move.

Desmosomes- structures in the form of plaques or buttons, also connect cells to each other (Fig. 152 and 153, A). In the intercellular space, a dense layer is also visible here, represented by interacting integral membrane cadherins - desmogleins, which link cells to each other. On the cytoplasmic side, a layer of desmoplakin protein is adjacent to the plasmalemma, with which the intermediate filaments of the cytoskeleton are associated. Desmosomes are found most often in epithelia, in which case the intermediate filaments contain keratins. Heart muscle cells - cardiomyocytes, contain desmin fibrils as part of desmosomes. In the vascular endothelium, desmosomes contain vimentin intermediate filaments.

Hemidesmosomes in principle, they are similar in structure to the desmosome, but they are a connection of cells with intercellular structures. So, in epithelium, linker glycoproteins (integrins) of desmosomes interact with proteins of the so-called basement membrane, which includes collagen, laminin, proteoglycans, etc.

The functional role of desmosomes and hemidesmosomes is purely mechanical - they firmly adhere cells to each other and to the underlying extracellular matrix, which allows epithelial layers to withstand heavy mechanical loads. Similarly, desmosomes tightly bind heart muscle cells to each other, which allows them to perform a huge mechanical load while remaining bound into a single contractile structure.

Unlike tight contact, all types of bonding contacts are permeable to aqueous solutions and play no role in limiting diffusion.

Gap contacts are considered communication connections of cells. These structures are involved in the direct transfer of chemicals from cell to cell, which can not only play a major physiological role in the functioning of specialized cells, but also provide intercellular interactions during the development of the organism, during the differentiation of its cells. A characteristic of this type of contacts is the convergence of the plasma membranes of two neighboring cells at a distance of 2-3 nm (see Fig. 147, b and 153, b). It is this circumstance that for a long time did not allow us to distinguish this type of contact from a dense separating (closing) contact on ultrathin sections. When using lanthanum hydroxide, it has been observed that some of the tight contacts leak the contraster. In this case, lanthanum filled a thin gap about 3 nm wide between the adjacent plasma membranes of neighboring cells. This gave rise to the term gap contact. Further progress in deciphering its structure was achieved using the freeze-chipping method. It turned out that gap junction zones (from 0.5 to 5 µm in size) on cleavages of membranes are dotted with hexagonally arranged (with a period of 8–10 nm) particles 7–8 nm in diameter, having a channel about 2 nm wide in the center. These particles are called connexons(Fig. 154). There can be from 10-20 to several thousand connexons in the gap contact zones, depending on the functional characteristics of the cells. Connexons have been isolated preparatively and consist of six subunits connectin- a protein with a molecular weight of about 30 thousand. Combining with each other, connectins form a cylindrical aggregate - a connexon, in the center of which there is a channel. Individual connexons are embedded in the plasma membrane in such a way that they pierce through it. One connexon on the cell's plasma membrane is precisely opposed by a connexon on the plasma membrane of the neighboring cell, so that the channels of the two connexons form a single unit. Connexons play the role of direct intercellular channels through which ions and low molecular weight substances can diffuse from cell to cell. Connexons can close, changing the diameter of the internal channel, and thereby participate in the regulation of the transport of molecules between cells.

When studying the giant cells of the salivary glands of Diptera, it became clear what functional significance the gap junctions have. Due to their size, microelectrodes can be easily introduced into such cells in order to study the electrical conductivity of their membranes. It turned out that if electrodes are introduced into two adjacent cells, their plasma membranes exhibit low electrical resistance, i.e. current flows between cells. Moreover, it was found that when a fluorescent dye is injected into one cell, the label is quickly detected in neighboring cells. Using different fluorochromes on mammalian tissue culture cells, it was found that substances with a molecular weight of no more than 1-1.5 thousand and a size of no more than 1.5 nm can be transported through gap junctions (in insects, substances with a molecular weight of up to 2 thousand). Among these substances were various ions, amino acids, nucleotides, sugars, vitamins, steroids, hormones, cAMP. Neither proteins nor nucleic acids can pass through gap junctions.

This ability of gap junctions to serve as a place for the transport of low molecular weight compounds is used in those cellular systems where a fast transmission of an electrical impulse (excitation wave) from cell to cell is needed without the participation of a nerve mediator. So, all muscle cells of the myocardium of the heart are connected using gap junctions (in addition, the cells there are also connected by adhesive contacts) (see Fig. 147, b). This creates a condition for the synchronous reduction of a huge number of cells. With the growth of the culture of embryonic cardiac muscle cells (myocardiocytes), some cells in the layer begin to spontaneously contract independently of each other with different frequencies, and only after the formation of gap junctions between them do they begin to beat synchronously, as a single contracting layer of cells. In the same way, a joint contraction of smooth muscle cells in the uterine wall is ensured.

Gap junctions can serve the purpose of metabolic cooperation between cells by exchanging various molecules, hormones, cAMP or metabolites. An example is the co-cultivation of thymidine kinase mutant cells with normal cells: in the event of gap junctions between these cell types, mutant cells received thymidine triphosphate from normal cells through gap junctions and could participate in DNA synthesis.

In early vertebrate embryos, starting from the eight-cell stage, most cells are connected to each other by gap junctions. As the embryo differentiates, gap junctions between all cells disappear and only remain between groups of specialized cells. For example, during the formation of the neural tube, the connection of the cells of this structure with the rest of the epidermis is interrupted and they are separated.

The integrity and functioning of gap junctions are highly dependent on the level of Ca 2+ ions inside the cell. Normally, the concentration of calcium in the cytoplasm is very low. If Ca 2+ is injected into one of the cells of the tissue culture layer, then there is no increase in the level of Ca 2+ in the cytoplasm in neighboring cells; the cells, as it were, are disconnected from their neighbors, they cease to conduct electricity and dyes. After some time, after the introduced calcium is accumulated by mitochondria, the structure and functions of gap junctions are restored. This property is very important for maintaining the integrity and operation of the entire layer of cells, since damage to one of them is not transmitted to the neighboring one through gap junctions, which stop working as intercellular diffusion channels.

Synaptic contact (synapses). This type of contacts is characteristic of nervous tissue and occurs both between two neurons and between a neuron and some other element - a receptor or effector (for example, a neuromuscular ending). Synapses are areas of contact between two cells specialized for one-way transmission of excitation or inhibition from one element to another (Fig. 155). In principle, this kind of functional load, the transmission of an impulse, can also be carried out by other types of contacts (for example, a gap contact in the heart muscle), however, in a synaptic connection, high efficiency in the implementation of a nerve impulse is achieved. Synapses are formed on the processes of nerve cells - these are the terminal sections of dendrites and axons. Interneuronal synapses usually look like pear-shaped extensions - plaques at the end of the process of a nerve cell. Such a terminal extension of the process of one of the nerve cells can contact and form a synaptic connection both with the body of another nerve cell and with its processes. Peripheral processes of nerve cells (axons) form specific contacts with effector or receptor cells. Therefore, a synapse is a structure that forms between regions of two cells (as well as a desmosome). The membranes of these cells are separated by an intercellular space - a synaptic cleft about 20-30 nm wide. Often in the lumen of this slit, a fine-fibred material perpendicular to the membranes is visible. The membrane in the area of ​​synaptic contact of one cell is called presynaptic, the membrane of another cell that receives the impulse is called postsynaptic. In an electron microscope, both membranes look dense and thick. Near the presynaptic membrane, a huge number of small vacuoles are revealed - synaptic vesicles filled with neurotransmitters. Synaptic vesicles at the time of passage of the nerve impulse eject their contents into the synaptic cleft. The postsynaptic membrane often looks thicker than ordinary membranes due to the accumulation of many thin fibrils around it from the side of the cytoplasm.

Plasmodesma. This type of intercellular communication is found in plants. Plasmodesmata are thin tubular cytoplasmic channels connecting two adjacent cells. The diameter of these channels is usually 20-40 nm. The membrane that limits these channels directly passes into the plasma membranes of neighboring cells. Plasmodesmata pass through the cell wall that separates the cells (Figures 156 and 157). Thus, in some plant cells, plasmodesmata connect the hyaloplasm of neighboring cells, so formally there is no complete distinction, separation of the body of one cell from another, it is rather a syncytium: the union of many cell territories with the help of cytoplasmic bridges. Membrane tubular elements can penetrate inside the plasmodesmata, connecting the cisterns of the endoplasmic reticulum of neighboring cells. Plasmodesmata are formed during cell division, when the primary cell wall is being built. In newly divided cells, the number of plasmodesmata can be very high (up to 1000 per cell); with cell aging, their number decreases due to ruptures with an increase in the thickness of the cell wall.

The functional role of plasmodesmata is very great: with their help, intercellular circulation of solutions containing nutrients, ions and other compounds is ensured. Lipid droplets can move along the plasmodesmata. Plasmodesmata infect cells with plant viruses. However, experiments show that free transport through plasmodesmata is limited to particles with a mass of no more than 800 Da.

Cell wall (shell) of plants

If you isolate any cell from an animal's body and place it in water, then after a short time the cell will burst after swelling, i.e. she is lysing. This is due to the fact that water enters the cytoplasm through the plasma membrane, into a zone with a higher concentration of salts and organic molecules. This increases the internal volume of the cell until the plasma membrane ruptures. This does not happen in the organism of animals, because the cells of lower and higher animals exist surrounded by liquids of the internal environment, the concentration of salts and substances in which is close to that in the cytoplasm. Free-living in fresh water, unicellular protozoa do not lyse (in the absence of a cell wall) due to the fact that they constantly have a cellular pump pumping water out of the cytoplasm - the contractile vacuole.

If we place bacterial or plant cells in water, they will not lyse until their cell wall is intact. By exposure to a set of different enzymes, these walls can be dissolved. In this case, swelling and rupture (lysis) of cells occur immediately. Therefore, under natural conditions, the cell wall prevents this process, which is fatal for the cell. Moreover, the presence of cell walls is one of the main factors regulating the flow of water into the cell. Cells of bacteria and plants live most of all in a hypotonic aquatic environment, they do not have contractile (excretory) vacuoles to pump out water, but a strong cell wall protects them from extreme swelling. As water enters the cell, internal pressure arises - turgor, which prevents the further flow of water.

Interestingly, in many lower plants, such as green algae, the cells have a well-formed cell membrane, but during sexual reproduction, when mobile zoospores are formed, the latter lose their cell membrane and pulsating vacuoles appear in them.

The cell wall of plants is formed with the participation of the plasma membrane and is an extracellular (extracellular) multilayer formation that protects the surface of the cell and serves as the outer skeleton of the plant cell (Fig. 158). The cell wall of plants consists of two components: an amorphous plastic gel-like matrix (base) with a high water content and a supporting fibrillar system. Additional polymeric substances and salts, often included in the composition of the shells, give them rigidity and make them non-wettable.

Chemically, the main components of plant membranes are structural polysaccharides. The composition of the matrix of plant membranes includes heterogeneous groups of polysaccharides that dissolve in concentrated alkalis, hemicelluloses and pectin substances. Hemicelluloses are branching polymer chains consisting of various hexoses (glucose, mannose, galactose, etc.), pentoses (xylose, arabinose) and uronic acids (glucuronic and galacturonic). These components of hemicelluloses are combined with each other in different quantitative ratios and form various combinations. Chains of hemicellulose molecules do not crystallize and do not form elementary fibrils. Due to the presence of polar groups of uronic acids, they are highly hydrated.

Pectic substances are a heterogeneous group that includes branched, highly hydrated polymers that carry negative charges due to the many residues of galacturonic acid. Due to the properties of its components, the matrix is ​​a soft plastic mass reinforced with fibrils.

The fibrous components of plant cell membranes usually consist of cellulose, a linear, non-branching polymer of glucose. The molecular weight of cellulose varies from 5·10 4 to 5·10 5 , which corresponds to 300-3000 glucose residues. Such linear cellulose molecules can be combined into bundles or fibers. In the cell wall, cellulose forms fibrils, which consist of submicroscopic microfibrils up to 25 nm thick, which in turn consist of many parallel chains of cellulose molecules.

Quantitative ratios of cellulose to matrix substances (hemicellulose) can be very different for different objects. Over 60% of the dry mass of the primary membranes is their matrix and about 30% is the skeletal substance - cellulose. In raw cell membranes, almost all water is associated with hemicelluloses; therefore, the mass of the main substance in the swollen state reaches 80% of the wet mass of the entire membrane, while the content of fibrous substances is reduced to only 12%. In cotton hairs, the cellulose component is 90%; in wood, cellulose accounts for 50% of the cell wall components.

In addition to cellulose, hemicellulose and pectins, cell membranes contain additional components that give them special properties. So, inlay (inclusion inside) of the shells with lignin (a polymer of coniferyl alcohol) leads to lignification of the cell walls, increasing their strength (Fig. 159). Lignin mixes the plastic substances of the matrix in such shells and plays the role of the main substance with high strength. The matrix is ​​often reinforced with minerals (SiO 2 , CaCO 3 , etc.).

On the surface of the cell membrane, various adcrusting substances, such as cutin and suberin, can accumulate, leading to cell suberization. In the cells of the epidermis, wax is deposited on the surface of the cell membranes, which forms a waterproof layer that prevents the cell from losing water.

Due to its porous, loose structure, the plant cell wall is largely permeable to low molecular weight compounds such as water, sugars and ions. But macromolecules do not penetrate well through cellulose shells: the size of the pores in the shells, which allows free diffusion of substances, is only 3-5 nm.

Experiments with labeled compounds have shown that during the growth of the cell membrane, the release of substances from which it is built occurs over the entire surface of the cell. Amorphous substances of the matrix, hemicelluloses and pectins are synthesized in the vacuoles of the Golgi apparatus and are released through the plasmalemma by exocytosis. Cellulose fibrils are synthesized by special enzymes built into the plasmalemma.

The membranes of differentiated, mature cells are usually multi-layered, the cellulose fibrils in the layers are oriented differently, and their number can also vary significantly. Usually describe the primary, secondary and tertiary cell membranes (see Fig. 158). In order to understand the structure and appearance of these membranes, it is necessary to get acquainted with how they are formed after cell division.

During division of plant cells, after the divergence of chromosomes in the equatorial plane of the cells, an accumulation of small membrane vesicles appears, which in the central part of the cells begin to merge with each other (Fig. 160). This process of fusion of small vacuoles occurs from the center of the cell to the periphery and continues until the membrane vesicles merge with each other and with the plasma membrane of the lateral surface of the cell. This is how it is formed cellnaya plate, or phragmoplast. In its central part there is an amorphous substance of the matrix, which filled the merging bubbles. It has been proven that these primary vacuoles originate from the membranes of the Golgi apparatus. The composition of the primary cell wall also includes a small amount of a protein (about 10%) rich in hydroxyproline and having many short oligosaccharide chains, which determines this protein as a glycoprotein. Along the periphery of the cell plate, when observed in polarized light, a noticeable birefringence is detected, caused by the fact that oriented cellulose fibrils are located in this place. Thus, the growing primary cell wall already consists of three layers: the central one - the middle plate, consisting only of an amorphous matrix, and two peripheral ones - the primary membrane containing hemicellulose and cellulose fibrils. If the middle plate is a product of the activity of the original cell, then the primary membrane is formed due to the release of hemicellulose and cellulose fibrils by two new cell bodies. And all further increase in the thickness of the cell (or rather, intercellular) wall will occur due to the activity of two daughter cells, which secrete substances of the cell membrane from opposite sides, thickening by layering more and more new layers. As from the very beginning, the release of the substances of the matrix is ​​carried out due to the approach of the vesicles of the Golgi apparatus to the plasma membrane, their fusion with the membrane and the release of their contents outside the cytoplasm. Here, outside the cell, on its plasma membrane, the synthesis and polymerization of cellulose fibrils takes place. This is how the secondary cell membrane is gradually formed. It is difficult to determine and be able to distinguish the primary shell from the secondary one with sufficient accuracy, since they are interconnected by several intermediate layers.

The main mass of the cell wall that has completed its formation is the secondary membrane. It gives the cell its final shape. After the cell is divided into two daughter cells, new cells grow, their volume increases and their shape changes; cells are often elongated. At the same time, there is an increase in the thickness of the cell membrane and a restructuring of its internal structure.

During the formation of the primary cell wall, there are still few cellulose fibrils in its composition, and they are located more or less perpendicular to the future longitudinal axis of the cell. Later, during the period of elongation (elongation of the cell due to the growth of vacuoles in the cytoplasm), the orientation of these transversely directed fibrils undergoes passive changes: the fibrils begin to be located at right angles to each other and eventually become elongated more or less parallel to the longitudinal axis of the cell. The process is constantly going on: in the old layers (closer to the center of the shell), the fibrils undergo passive shifts, and the deposition of new fibrils in the inner layers (closest to the cell membrane) continues in accordance with the original shell construction plan. This process creates the possibility of fibrils sliding relative to each other, and the rearrangement of the cell membrane reinforcement is possible due to the gelatinous state of the components of its matrix. Subsequently, when hemicellulose is replaced by lignin in the matrix, the mobility of fibrils sharply decreases, the shell becomes dense, and lignification occurs.

Often, under the secondary membrane, a tertiary membrane is found, which can be considered as a dried-up remnant of the degenerated layer of the cytoplasm itself.

It should be noted that in plant cell division, the formation of the primary membrane is not in all cases preceded by the formation of a cell plate. Thus, in the green algae Spirogyra, new transverse septa arise by the formation of protrusions on the side walls of the original cell, which, gradually growing towards the center of the cell, close and divide the cell in two.

As already mentioned, if a cell is deprived of its membrane in an aqueous hypotonic medium, then lysis, cell rupture, will occur. It turned out that by selecting the appropriate concentrations of salts and sugars, it is possible to equalize the osmotic pressure outside and inside the cells, devoid of their membranes. At the same time, such protoplasts acquire a spherical shape (spheroplasts). If there is a sufficient amount of nutrients and salts in the environment where the protoplasts are located (among them, Ca 2+ is needed), then the cells are restored again, regenerate their cell membrane. Moreover, in the presence of hormones (auxins) they are capable of dividing and creating cell colonies, which can give rise to the growth of the whole plant from which the cell was taken.

The main fibrous component of the cell wall of large groups of fungi (basibiomycetes, ascomycetes, zygomycetes) is chitin; it is a polysaccharide in which the main saccharide is N-acetylglucosamine. The composition of the fungal cell wall, in addition to chitin, may include matrix substances, glycoproteins, and various proteins synthesized in the cytoplasm and released by the cell to the outside.

Cell walls of bacteria

The supporting frame of the cell wall of bacteria and blue-green algae is also largely a homogeneous polymer - peptidoglycan, or murein. The rigid frame surrounding the bacterial cell is one giant bag-shaped molecule of a complex polysaccharide - a peptide. This frame is called a murein bag. The basis of the structure of the murein sac is a network of parallel polysaccharide chains built from alternating disaccharides (acetylglucosamine combined with acetylmuramic acid) linked by numerous peptide cross-links (Fig. 161). The length of the chains can be huge - up to several hundred disaccharide blocks. The basis of the peptide part of murein is made up of tetrapeptides formed by various amino acids.

The bacterial wall can be up to 20-30% of the dry mass of the bacterium. This is due to the fact that, in addition to the multilayer murein framework, its composition includes a large number of additional components, as in the matrix of the plant wall. In gram-positive bacteria (when stained according to Gram - stained with crystal violet, treated with iodine, washed with alcohol - the bacteria perceive the dye differently: gram-positive ones remain stained after treatment with alcohol, gram-negative ones become discolored), the accompanying components are polymeric substances woven in a complex way into the murein network. These include teichoic acids, polysaccharides, polypeptides and proteins. The cell wall of gram-positive bacteria is very rigid, its murein network is multilayered.

The walls of gram-negative bacteria contain a single-layer murein network, which makes up 12% of the dry mass of the wall. The accompanying components make up to 80% of the dry mass. These are lipoproteins, complex lipopolysaccharides. They form a complex outer lipoprotein membrane. Consequently, the periphery of gram-negative bacteria contains an outer membrane, then a single-layer murein network, below it is a plasma membrane (Fig. 162). The outer membrane provides the structural integrity of the cell, serves as a barrier that limits the free access of various substances to the plasma membrane. It may also contain receptors for bacteriophages. It contains pori squirrelsus, which are involved in the transfer of many low molecular weight substances. Porin molecules form trimers that pass through the thickness of the membrane. One of the functions of these proteins is the formation of hydrophilic pores in the membrane, through which diffusion of molecules weighing no more than 900 Da occurs. Sugars, amino acids, small oligosaccharides and peptides pass freely through the pores. The pores are formed by different porins, have different permeability.

Between the outer lipoprotein membrane of the bacterial wall and the plasma membrane lies periplasmic spacestvo, or periplasm. Its thickness is usually about 10 nm, it contains a thin (1-3 nm) murein layer and a solution containing two types of specific proteins: hydrolytic enzymes and transport proteins. Due to the presence of hydrolases, the periplasm is sometimes considered as an analogue of the eukaryotic lysosomal compartment. Periplasmic transport proteins bind and transport sugars, amino acids, etc. from the outer membrane to the plasmalemma.

Bacterial wall precursors are synthesized inside the cell, and the walls are assembled outside the plasma membrane.

Under the action of the enzyme lysozyme, it is possible to break the murein framework and dissolve the bacterial wall. Under hypotonic conditions, the cells are destroyed in this case, as the naked cells of animals and plants are destroyed; under isotonic conditions, spherical protoplasts are formed, which are able to produce their cell wall again.

cell membrane also called plasma (or cytoplasmic) membrane and plasmalemma. This structure not only separates the internal contents of the cell from the external environment, but also enters into the composition of most cell organelles and the nucleus, in turn separating them from the hyaloplasm (cytosol) - the viscous-liquid part of the cytoplasm. Let's agree to call cytoplasmic membrane one that separates the contents of the cell from the external environment. The remaining terms refer to all membranes.

The basis of the structure of the cell (biological) membrane is a double layer of lipids (fats). The formation of such a layer is associated with the features of their molecules. Lipids do not dissolve in water, but condense in it in their own way. One part of a single lipid molecule is a polar head (it is attracted by water, i.e., hydrophilic), and the other is a pair of long non-polar tails (this part of the molecule is repelled by water, i.e., hydrophobic). This structure of the molecules makes them "hide" their tails from the water and turn their polar heads towards the water.

As a result, a lipid bilayer is formed, in which the non-polar tails are inside (facing each other), and the polar heads are facing out (to the external environment and cytoplasm). The surface of such a membrane is hydrophilic, but inside it is hydrophobic.

In cell membranes, phospholipids predominate among lipids (they are complex lipids). Their heads contain a residue of phosphoric acid. In addition to phospholipids, there are glycolipids (lipids + carbohydrates) and cholesterol (belongs to sterols). The latter gives the membrane rigidity, being located in its thickness between the tails of the remaining lipids (cholesterol is completely hydrophobic).

Due to electrostatic interaction, certain protein molecules are attached to the charged heads of lipids, which become surface membrane proteins. Other proteins interact with non-polar tails, partially sink into the bilayer, or penetrate it through and through.

Thus, the cell membrane consists of a bilayer of lipids, surface (peripheral), immersed (semi-integral), and penetrating (integral) proteins. In addition, some proteins and lipids on the outside of the membrane are associated with carbohydrate chains.

This fluid mosaic model of the membrane structure was put forward in the 70s of the XX century. Prior to this, a sandwich model of the structure was assumed, according to which the lipid bilayer is located inside, and on the inside and outside the membrane is covered with continuous layers of surface proteins. However, the accumulation of experimental data disproved this hypothesis.

The thickness of membranes in different cells is about 8 nm. Membranes (even different sides of one) differ from each other in the percentage of different types of lipids, proteins, enzymatic activity, etc. Some membranes are more liquid and more permeable, others are more dense.

Breaks in the cell membrane easily merge due to the physicochemical characteristics of the lipid bilayer. In the plane of the membrane, lipids and proteins (unless they are fixed by the cytoskeleton) move.

Functions of the cell membrane

Most of the proteins immersed in the cell membrane perform an enzymatic function (they are enzymes). Often (especially in the membranes of cell organelles) enzymes are arranged in a certain sequence so that the reaction products catalyzed by one enzyme pass to the second, then the third, etc. A conveyor is formed that stabilizes surface proteins, because they do not allow enzymes to swim along the lipid bilayer.

The cell membrane performs a delimiting (barrier) function from the environment and at the same time a transport function. It can be said that this is its most important purpose. The cytoplasmic membrane, having strength and selective permeability, maintains the constancy of the internal composition of the cell (its homeostasis and integrity).

In this case, the transport of substances occurs in various ways. Transport along a concentration gradient involves the movement of substances from an area with a higher concentration to an area with a lower one (diffusion). So, for example, gases diffuse (CO 2, O 2).

There is also transport against the concentration gradient, but with the expenditure of energy.

Transport is passive and lightweight (when some carrier helps him). Passive diffusion across the cell membrane is possible for fat-soluble substances.

There are special proteins that make membranes permeable to sugars and other water-soluble substances. These carriers bind to transported molecules and drag them across the membrane. This is how glucose is transported into the red blood cells.

Spanning proteins, when combined, can form a pore for the movement of certain substances through the membrane. Such carriers do not move, but form a channel in the membrane and work similarly to enzymes, binding a specific substance. The transfer is carried out due to a change in the conformation of the protein, due to which channels are formed in the membrane. An example is the sodium-potassium pump.

The transport function of the eukaryotic cell membrane is also realized through endocytosis (and exocytosis). Thanks to these mechanisms, large molecules of biopolymers, even whole cells, enter the cell (and out of it). Endo- and exocytosis are not characteristic of all eukaryotic cells (prokaryotes do not have it at all). So endocytosis is observed in protozoa and lower invertebrates; in mammals, leukocytes and macrophages absorb harmful substances and bacteria, i.e. endocytosis performs protective function for the body.

Endocytosis is divided into phagocytosis(cytoplasm envelops large particles) and pinocytosis(capture of liquid droplets with substances dissolved in it). The mechanism of these processes is approximately the same. Absorbed substances on the cell surface are surrounded by a membrane. A vesicle (phagocytic or pinocytic) is formed, which then moves into the cell.

Exocytosis is the removal of substances from the cell by the cytoplasmic membrane (hormones, polysaccharides, proteins, fats, etc.). These substances are enclosed in membrane vesicles that fit the cell membrane. Both membranes merge and the contents are outside the cell.

The cytoplasmic membrane performs a receptor function. To do this, on its outer side there are structures that can recognize a chemical or physical stimulus. Some of the proteins penetrating the plasmalemma are externally connected to polysaccharide chains (forming glycoproteins). These are peculiar molecular receptors that capture hormones. When a particular hormone binds to its receptor, it changes its structure. This, in turn, triggers the cellular response mechanism. At the same time, channels can open, and certain substances can begin to enter the cell or be removed from it.

The receptor function of cell membranes has been well studied based on the action of the hormone insulin. When insulin binds to its glycoprotein receptor, the catalytic intracellular part of this protein (the enzyme adenylate cyclase) is activated. The enzyme synthesizes cyclic AMP from ATP. Already it activates or inhibits various enzymes of cellular metabolism.

The receptor function of the cytoplasmic membrane also includes the recognition of neighboring cells of the same type. Such cells are attached to each other by various intercellular contacts.

In tissues, with the help of intercellular contacts, cells can exchange information with each other using specially synthesized low molecular weight substances. One example of such an interaction is contact inhibition, when cells stop growing after receiving information that the free space is occupied.

Intercellular contacts are simple (membranes of different cells are adjacent to each other), locking (invagination of the membrane of one cell into another), desmosomes (when the membranes are connected by bundles of transverse fibers penetrating into the cytoplasm). In addition, there is a variant of intercellular contacts due to mediators (intermediaries) - synapses. In them, the signal is transmitted not only chemically, but also electrically. Synapses transmit signals between nerve cells, as well as from nerve to muscle.

Biological membranes form the basis of the structural organization of the cell. The plasma membrane (plasmalemma) is the membrane that surrounds the cytoplasm of a living cell. Membranes are made up of lipids and proteins. Lipids (mainly phospholipids) form a double layer in which the hydrophobic "tails" of the molecules face inside the membrane, and the hydrophilic tails - to its surfaces. Protein molecules can be located on the outer and inner surface of the membrane, they can be partially immersed in the lipid layer or penetrate it through. Most of the immersed membrane proteins are enzymes. This is a fluid-mosaic model of the structure of the plasma membrane. Protein and lipid molecules are mobile, which ensures the dynamism of the membrane. The membranes also contain carbohydrates in the form of glycolipids and glycoproteins (glycocalix) located on the outer surface of the membrane. The set of proteins and carbohydrates on the membrane surface of each cell is specific and is a kind of cell type indicator.

Membrane functions:

1. Dividing. It consists in the formation of a barrier between the internal contents of the cell and the external environment.
2. Ensuring the exchange of substances between the cytoplasm and the external environment. Water, ions, inorganic and organic molecules(transport function). Products formed in the cell (secretory function) are excreted into the external environment.
3. Transport. Transport across the membrane can take place in different ways. Passive transport is carried out without energy expenditure, by simple diffusion, osmosis or facilitated diffusion with the help of carrier proteins. Active transport is by carrier proteins and requires energy input (eg sodium-potassium pump). material from the site

Large molecules of biopolymers enter the cell as a result of endocytosis. It is divided into phagocytosis and pinocytosis. Phagocytosis is the capture and absorption of large particles by the cell. The phenomenon was first described by I.I. Mechnikov. First, substances adhere to the plasma membrane, to specific receptor proteins, then the membrane sags, forming a depression.

A digestive vacuole is formed. It digests the substances that have entered the cell. In humans and animals, leukocytes are capable of phagocytosis. Leukocytes engulf bacteria and other solid particles.

Pinocytosis is the process of capturing and absorbing liquid droplets with substances dissolved in it. Substances adhere to membrane proteins (receptors), and a drop of solution is surrounded by a membrane, forming a vacuole. Pinocytosis and phagocytosis occur with the expenditure of ATP energy.

1. Secretory. Secretion - the release by the cell of substances synthesized in the cell into the external environment. Hormones, polysaccharides, proteins, fat droplets are enclosed in membrane-bound vesicles and approach the plasmalemma. The membranes merge, and the contents of the vesicle are released into the environment surrounding the cell.
2. Connection of cells in tissue (due to folded outgrowths).
3. Receptor. There are a large number of receptors in membranes - special proteins, the role of which is to transmit signals from the outside to the inside of the cell.

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On this page, material on the topics:

• the structure of a biological membrane briefly
• structure and function of the plasma membrane
• plasma membrane structure and function
• plasma membrane briefly
• plasma membrane structure and functions briefly