An element taking part in active transport. Transport of substances across cell membranes

ENT electroradiography began to be used after the invention of special plates (selenium and others), which replace X-ray films. Electroradiography is also based on the use of x-ray, however, it is much cheaper, since it eliminates the need to use expensive silver (on X-ray films). The method is also attractive because the image of the organ under study can be obtained in 2-3 minutes and an unlimited number of times to apply it on ordinary paper.

Often electroradiography used to diagnose pathological conditions of the paranasal sinuses or fractures of the nasal bones.

Image on paper it turns out clear, contrasting. The details of the structure of bone structures are well transferred. The picture clearly shows pathological changes in the mucous membrane, bony walls of the sinuses. Electroradiography uses the same patient patterns as for standard X-ray examinations.

ENT ultrasound

Significant progress in medicine achieved with the introduction of the achievements of radio electronics and physics into its practice. So, widespread use in the diagnosis of diseases internal organs received ultrasonic biolocation. The method also proved to be effective in the diagnosis of some diseases of the ENT organs. The expediency of the use of ultrasound in the diagnosis of pathology of the paranasal sinuses has been proved. Currently, ENT clinics are equipped with different kinds such devices.

Method ultrasonic dowsing based on the registration of reflected signals at the boundaries of media and tissues, different in density. In particular, the density of the head tissues is different, therefore, their acoustic resistance is not the same, and therefore the speed of propagation of ultrasonic waves in them. Air is an absolute obstacle for the ultrasound wave, therefore it goes around the airways and is not reflected on the echogram. If there is pus or other fluid, polyps or tumor in the cavity, they are visualized on the echogram.

To receive ultrasound imaging a thin layer of petroleum jelly is applied to the skin of the cheek or forehead and the sensor is pressed firmly. Depending on the condition, for example, of the maxillary sinus, an image of a curve is obtained on the device screen, the shape and location of which depend on the nature of the pathological changes.

The method is simple, informative and is harmless to the patient. It allows the otorhinolaryngologist to carry out quick and high-quality diagnostics, which is very important for timely and effective treatment. Ultrasonic biolocation is especially indicated for preventive medical examinations.

Repeated echolocation in the course of treatment allows one to judge its effectiveness, the need to change therapy and the timing of its termination. The method can be used for dynamic observation of patients who are registered with dispensaries.

ENT (otolaryngologist) is a doctor who specializes in the diagnosis, treatment and prevention of diseases of the ear, nose, paranasal sinuses, pharynx and larynx in adults and children.

Diseases of the ear, nose and throat are among the most common human diseases. Since the ENT organs are located close to each other and functionally interconnected, in many cases complex treatment is required. Inattention to the first symptoms (sore throat or in the ears, enlargement of the submandibular lymph nodes, difficulty in nasal breathing) can lead to the spread of the inflammatory process and the development of complications.

A timely appointment with an otolaryngologist and the treatment prescribed by him will allow you to quickly restore health and avoid problems in the future. If you need an appointment with an otolaryngologist in Moscow, please contact the Family Doctor JSC. You can make an appointment with a highly qualified specialist at any convenient time. Surgical treatment of ENT diseases is carried out in our Hospital Center and Surgical Hospital.

What diseases should you go to see an ENT doctor with?

The competence of the ENT doctor includes diseases requiring both therapeutic and surgical treatment, including:

infectious diseases accompanied by damage to the ENT organs;

injuries to the nose, ear and throat;

deformation of the nasal septum;

violation of the sense of smell;

diseases and injuries of the paranasal sinuses;

pharyngitis - inflammation of the pharynx;

tonsillitis - inflammation of the tonsils;

laryngitis - inflammation of the larynx;

tracheitis - inflammation of the trachea;

otitis media - inflammation of the outer, middle and inner ear;

hearing impairment;

balance disorders associated with ear diseases;

You may need to see an otolaryngologist if symptoms such as

runny nose, sneezing, nasal discharge, itchy nose;

violation of the sense of smell;

frequent nosebleeds;

pain in the nose, throat, or ear;

labored breathing;

swallowing disorder;

extraneous sounds in the ear;

dizziness, prolonged headaches of unknown origin;

When the first symptoms appear, make an appointment with an ENT doctor. Modern methods diagnostics allow detecting ENT diseases at the initial stages and shorten the duration of treatment.

Methods for the diagnosis of ENT diseases

Outpatient reception of an ENT doctor in the clinics of the Family Doctor JSC network is carried out using the most diagnostic (including endoscopic) equipment. Along with taking anamnesis and instrumental diagnostics of ENT organs, the doctor may prescribe (or conduct) additional studies, including:

For visualization of specific anatomical structures.

- X-ray examination of the pharynx with contrasting with a special substance, which makes it possible to identify foreign bodies, tumors and deformities.

Allows you to obtain three-dimensional images of the area of ​​interest.

And to determine the condition of the hearing organs.

It is carried out for those patients who, for any reason, cannot give feedback during the research process.

Laboratory diagnostics (the doctor will take biological material from the problem area right at the reception and transfer it to the laboratory for research).

Treatment of ENT diseases

Treatment will be assigned to you immediately - based on the data of the initial examination. At the next appointment, the ENT doctor, having received the results of instrumental and laboratory diagnostics, will, if necessary, make changes in order to achieve the maximum result in the shortest possible time. Prescribed treatment may include methods such as:

drug therapy for ENT diseases.

Hardware for rhinitis and sinusitis.

Allows you to quickly stop exacerbation of chronic tonsillitis.

With eustachitis and hearing loss.

According to Politzer for diagnostic and therapeutic purposes.

Membrane transport proteins take part in the transport of ions through the plasmalemma. These proteins can conduct one substance in one direction (uniport) or several substances at the same time (symport), and also, together with the import of one substance, remove another substance from the cell (antiport). Glucose, for example, can enter cells sympathetically along with the Na + ion. Ion transport can occur along the concentration gradient, i.e., passively, without additional energy consumption. In the case of passive transport, some membrane transport proteins form molecular complexes, channels through which dissolved molecules pass through the membrane by simple diffusion along the concentration gradient. Some of these channels are constantly open, others can be closed or opened in response either to binding to signaling molecules or to a change in the intracellular concentration of ions. In other cases, special membrane carrier proteins selectively bind to one or another ion and transport it across the membrane (facilitated diffusion). The concentration of ions in the cytoplasm of cells differs sharply not only from the concentration in the external environment, but even from the blood plasma that bathes the cells in the body of higher animals. The total concentration of monovalent cations both inside the cells and outside is practically the same (150 mM), isotonic. But in the cytoplasm, the concentration of K + is almost 50 times higher, and Na + is lower than in blood plasma, and this difference is maintained only in a living cell: if a cell is killed or metabolic processes are suppressed in it, then after a while ionic differences on both sides the plasma membrane will disappear. You can simply cool the cells to +2 o С, and after a while the concentrations 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 spending energy due to the hydrolysis of ATP. This type of transfer of substances is called active transport, and it is carried out using protein ion pumps. The plasma membrane contains a two-subunit molecule (K + + Na +) - a pump, which is also an ATPase. This pump pumps out 3 Na + ions in one cycle and pumps 2 K + ions into the cell against the concentration gradient. In this case, one ATP molecule, going to the phosphorylation of ATPase, as a result of which Na + is transported through the membrane from the cell, and K + is able to bind to a protein molecule and then be transferred into the cell. As a result of active transport with the help of membrane pumps, the concentration of bivalent cations Mg 2+ and Ca + is also regulated in the cell, also with the consumption of ATP. In combination with active transport of ions, various sugars, nucleotides and amino acids penetrate through the plasma membrane. Thus, the active transport of glucose, which simultaneously (simultaneously) penetrates into 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 ion concentration is created, then the diffuse flux of Na + and simultaneously the transport of glucose immediately increase. Like this

the transport of amino acids is carried out, which are carried across the membrane by special carrier proteins that work as symport systems, simultaneously transferring ions. Active transport of sugars and amino acids into bacterial cells due to the gradient of hydrogen ions. The very participation of special membrane proteins in the passive or active transport of low molecular weight compounds shows the high specificity of this process. Even in the case of passive ionic transport, proteins “recognize” this ion, interact with it, bind specifically, change their conformation and function. Consequently, even by the example of the transport of simple substances, membranes act as analyzers, as receptors. The receptor function of the membrane is especially manifested when biopolymers are absorbed by the cell.

Intercellular contacts.

In multicellular organisms, due to intercellular interactions, complex cell assemblies are formed, the maintenance of which is carried out in different ways. In embryonic, embryonic tissues, especially on early stages development, cells remain in communication with 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. Sometimes, especially in monolayer epitheliums, the plasma membranes of neighboring cells form multiple invaginations, resembling a carpentry suture. This creates additional strength for the intercellular junction. In addition to such a simple adhesive (but specific) connection, there are a number of special intercellular structures, contacts or connections that perform specific functions. These are locking, anchorage and communication connections. Locking, or tight, the connection is characteristic of unilamellar epithelium. This is the zone where the outer layers of the two plasma membranes are as close as possible. A three-layer membrane is often seen in this contact: the two outer osmiophilic layers of both membranes seem to merge into one common layer 2 - 3 nm thick. On planar preparations of plasma membrane fractures in the zone of close contact using the freezing and cleavage method, it was found that the contact points of the membranes are globules (most likely, special integral proteins of the plasma membrane) arranged in rows. Such rows of globules, or stripes, can intersect so that they form a lattice or network on the cleavage surface. This structure is very characteristic of the epithelium, especially the glandular and intestinal ones. V the latter case tight contact forms a continuous zone of fusion of plasma membranes, encircling the cell in the apical (upper, looking into the intestinal lumen) part of it. Thus, each cell of the layer is, as it were, surrounded by a tape of this contact. Such structures with special colors can also be seen under a light microscope. They received the name of the end plates from morphologists. In this case, the role of the closing tight contact is not only in the mechanical connection of cells with each other. This area of ​​contact is poorly permeable to macromolecules and ions, and thereby it locks, blocks the intercellular cavities, isolating them (and together with them the internal environment of the body) from the external environment (in this case, the intestinal lumen). Although all tight junctions are barriers to macromolecules, their permeability to small molecules is different in different epithelia. Anchoring (adhesion) connections, or contacts, are so called because they not only connect the plasma membranes of neighboring cells, but also bind to the fibrillar elements of the cytoskeleton. This type of compound is characterized by the presence of two types of proteins. One of them is represented by transmembrane linker (binding) proteins, which are involved either in the intercellular junction itself or in the junction 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 connecting, or anchoring, membrane elements of such contact with cytoplasmic fibrils of the cytoskeleton. Intercellular point adhesion junctions are found in many non-epithelial tissues, but the structure of adhesion (adhesive) is more clearly described tapes, or belts, in monolayer epithelium. This structure encircles the entire perimeter of the epithelial cell, just as it does in the case of a tight junction. Most often, such a belt, or tape, lies below the tight connection. In this place, the plasma membranes are brought together, and even the distance of 25 - 30 nm is somewhat moved apart, and a zone of increased density is visible between them. These are nothing more than places of interaction of transmembrane glycoproteins, which, with the participation of Ca ++ ions, specifically adhere to each other and provide a mechanical connection of the membranes of two neighboring cells. Linker proteins belong to cadherins - receptor proteins that provide specific recognition of homogeneous membranes by cells. The destruction of the glycoprotein layer leads to the isolation of individual cells and the destruction of the epithelial layer. From the cytoplasmic side near the membrane, an accumulation of dense matter is visible, to which a layer of thin (6 - 7 nm) filaments is adjacent, lying along the plasma membrane in the form of a bundle extending along the entire perimeter of the cell. Thin filaments are actin fibrils; they bind to the plasma membrane through the protein vinculin, which forms a dense peri-membrane layer. The functional significance of the tape connection lies not only in the mechanical adhesion of cells to each other: when the actin filaments in the tape contract, the shape of the cell can change. Focal contacts, or clutch plaques, are found in many cells and are especially well studied in fibroblasts. They are built according to a general plan with adhesive tapes, but are expressed in the form of small areas - plaques on the plasmalemma. In this case, transmembrane linker proteins specifically bind to extracellular matrix proteins, such as fibronectin. From the side of the cytoplasm, these same glycoproteins are associated with membrane proteins, which includes vinculin, which in turn is associated with a bundle of actin filaments. The functional significance of focal contacts lies both in the anchoring of the cell to extracellular structures and in the creation of a mechanism that allows cells to move. Desmosomes that look like plaques or buttons also connect cells to each other. In the intercellular space, a dense layer is also visible here, represented by interacting integral membrane glycoproteins - desmogleins, which also, depending on Ca ++ ions, link cells to each other. On the cytoplasmic side, a layer of protein desmoplakin is adjacent to the plasmolemma, to which intermediate filaments of the cytoskeleton are connected. Desmosomes are found most often in the epithelium, in which case the intermediate filaments contain keratins. The cells of the heart muscle, cardiomyocytes, contain desmin fibrils as part of the desmosomes. In the entothelium of the vessels, the desmosomes include vimentin intermediate filaments. Semi-desmosomes are similar in structure to the desmosome, but represent a connection of cells with intercellular structures. The functional role of desmosomes and semi-desmosomes is purely mechanical: they link cells to each other and to the underlying extracellular matrix. Unlike tight contact, all types coupling contacts permeable to aqueous solutions and play no role in limiting diffusion. Slotted contacts are considered to be communication links of cells. These structures are involved in live transmission chemical substances from cell to cell. This type of contact is characterized by the convergence of the plasma membranes of two neighboring cells at a distance of 2 - 3 nm. Using the freezing-chipping method. It turned out that on cleavage membranes, the gap contact zones (0.5 to 5 μm in size) are dotted with particles 7 - 8 nm in diameter, located hexagonally with a period of 8 - 10 nm and having about 2 wells in the center of the channel. These particles are called connexons. In the gap contact zones, there can be from 10 - 20 to several thousand connexons, depending on functional features cells. Connexons were isolated preparatively. They are made up of six connectin-protein subunits. Uniting with each other, connectins form a cylindrical aggregate - a connexon, in the center of which is a canal. Individual connexons are built into the plasma membrane so that they pierce it through and through. One connexon on the plasma membrane of a cell is exactly opposed by a connexon on the plasma membrane of a neighboring cell, so that the channels of the two connexons form a single whole. 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 thus participate in the regulation of the transport of molecules between cells. Neither proteins nor nucleic acids can pass through gap junctions. The ability of gap junctions to pass low molecular weight compounds underlies fast transfer electrical impulse (excitation wave) from cell to cell without the participation of a nerve mediator. Synaptic contact (synapses)... Synapses are areas of contact between two cells, specialized for one-way transmission of excitation or inhibition from one element to another. This type of contact is characteristic of nerve tissue and occurs both between two neurons and between neurons and some other element — a receptor or effector. The neuromuscular terminal is also an example of synaptic contact. Interneuronal synapses usually look like pear-shaped extensions (plaques). Synaptic plaques can contact both the body of another neuron and its processes. Peripheral processes of nerve cells (axons) form specific contacts with effector cells (muscle or glandular) or receptor cells. Consequently, a synapse is a specialized structure that forms between the regions of two cells (just like the desmosome). At the sites of synaptic contacts, cell membranes are separated by an intercellular space - a synaptic cleft about 20-30 nm wide. Often, in the lumen of the slit, a thin-fibrous material is seen perpendicular to the membranes. The membrane of one cell transmitting excitation in the area of ​​synaptic contact is called presynaptic, the membrane of another cell that receives the impulse is called postsynaptic. A huge number of small vacuoles - synaptic vesicles filled with neurotransmitters - are found near the presynaptic membrane. The contents of the synaptic vesicles at the time of the passage of the nerve impulse are released by exocytosis into the synaptic cleft. Postsynaptic membrane often looks thicker than ordinary membranes due to the accumulation of many thin fibrils near it from the side of the cytoplasm. Plasmodesmata. This type of intercellular communication is found in plants. Plasmodesmata are thin tubular cytoplasmic canals that connect two adjacent cells. The diameter of these channels is usually 20 - 40 nm. The membrane limiting these channels directly passes into the plasma membranes of neighboring cells. Plasmodesmata pass through the cell wall that separates the cells. Membrane tubular elements connecting the cisterns of the endoplasmic reticulum of neighboring cells can penetrate inside the plasmodesmata. Plasmodesmata are formed during division, when the primary cell membrane is built. In newly divided cells, the number of plasmodesmata can be very large (up to 1000 per cell). With aging of cells, their number decreases due to ruptures with an increase in the thickness of the cell wall. Lipid droplets can move along the plasmodesmata. Cells are infected with plant viruses through the plasmodesmata.

The exchange of a cell with the environment with various substances and energy is vital necessary condition her existence.

To maintain consistency chemical composition and the properties of the cytoplasm in conditions when there are significant differences in the chemical composition and properties of the environment and the cytoplasm of the cell, there must be special transport mechanisms selectively moving substances through.

In particular, cells must have mechanisms for delivering oxygen and nutrients from the environment and removing metabolites into it. Concentration gradients of various substances exist not only between the cell and the external environment, but also between the cell organelles and the cytoplasm, and transport flows of substances are observed between different compartments of the cell.

Of particular importance for the perception and transmission of information signals is the maintenance of the transmembrane difference in the concentrations of mineral ions Na +, K +, Ca 2+... The cell spends a significant part of its metabolic energy to maintain the concentration gradients of these ions. The energy of electrochemical potentials stored in ionic gradients ensures the constant readiness of the plasma membrane of the cell to respond to stimuli. The entry of calcium into the cytoplasm from the intercellular environment or from cell organelles ensures the response of many cells to hormonal signals, controls the release of neurotransmitters, and triggers.

Rice. Classification of transport types

To understand the mechanisms of the transition of substances through cell membranes it is necessary to take into account both the properties of these substances and the properties of the membranes. The substances transported differ molecular weight, carried charge, solubility in water, lipids and a number of other properties. Plasma and other membranes are represented by large areas of lipids, through which fat-soluble non-polar substances easily diffuse and water and water-soluble substances of a polar nature do not pass. For the transmembrane movement of these substances, the presence of special channels in the cell membranes is necessary. The transport of molecules of polar substances becomes difficult with an increase in their size and charge (in this case, additional transfer mechanisms are required). The transfer of substances against concentration and other gradients also requires the participation of special carriers and energy expenditures (Fig. 1).

Rice. 1. Simple, facilitated diffusion and active transport of substances through cell membranes

For transmembrane movement high molecular weight compounds, supramolecular particles and components of cells that are unable to penetrate through membrane channels, special mechanisms are used - phagocytosis, pinocytosis, exocytosis, transfer through intercellular spaces. Thus, the transmembrane movement of various substances can be carried out using different methods, which are usually subdivided according to the signs of the participation of special carriers and energy consumption. There are passive and active transport across the cell membranes.

Passive transport- transfer of substances through a biomembrane along a gradient (concentration, osmotic, hydrodynamic, etc.) and without energy consumption.

Active transport- transport of substances through a biomembrane against a gradient and with energy consumption. In humans, 30-40% of all energy generated in the course of metabolic reactions is spent on this type of transport. In the kidneys, 70-80% of the oxygen consumed is used for active transport.

Passive transport of substances

Under passive transport they understand the transfer of a substance through membranes along various gradients (electrochemical potential, concentration of a substance, electric field, osmotic pressure, etc.), which does not require direct expenditure of energy for its implementation. Passive transport of substances can occur through simple and facilitated diffusion. It is known that under diffusion understand the chaotic movement of particles of matter in various media, due to the energy of its thermal vibrations.

If the molecule of a substance is electrically neutral, then the direction of diffusion of this substance will be determined only by the difference (gradient) of concentrations of the substance in media separated by a membrane, for example, outside and inside the cell or between its compartments. If a molecule, ions of a substance carry an electric charge, then diffusion will be influenced both by the difference in concentration, the magnitude of the charge of this substance, and the presence and sign of charges on both sides of the membrane. The algebraic sum of the forces of concentration and electrical gradients on the membrane determines the magnitude of the electrochemical gradient.

Simple diffusion is carried out due to the presence of concentration gradients of a certain substance, electric charge or osmotic pressure between the sides of the cell membrane. For example, the average content of Na + ions in blood plasma is 140 mM / L, and in erythrocytes - approximately 12 times less. This concentration difference (gradient) creates driving force, which ensures the transition of sodium from plasma to erythrocytes. However, the rate of such a transition is low, since the membrane has a very low permeability to Na + ions. The permeability of this membrane to potassium is much greater. The processes of simple diffusion do not consume the energy of cellular metabolism.

The rate of simple diffusion is described by Fick's equation:

dm / dt = -kSΔC / x,

where dm/ dt- the amount of a substance that diffuses per unit of time; To - diffusion coefficient characterizing the membrane permeability for a diffusing substance; S- diffusion surface area; ΔC- the difference in the concentration of the substance on both sides of the membrane; NS Is the distance between the points of diffusion.

From the analysis of the diffusion equation, it is clear that the rate of simple diffusion is directly proportional to the concentration gradient of the substance between the sides of the membrane, the membrane permeability for a given substance, and the diffusion surface area.

Obviously, the most easily to move through the membrane by diffusion will be those substances, the diffusion of which is carried out both along the concentration gradient and along the electric field gradient. However, an important condition for the diffusion of substances through membranes is physical properties membrane and, in particular, its permeability to the substance. For example, Na + ions, the concentration of which is higher outside the cell than inside it, and the inner surface of the plasma membrane is negatively charged, should easily diffuse into the cell. However, the diffusion rate of Na + ions through the plasma membrane of the cell at rest is lower than that of K + ions, which diffuses along the concentration gradient from the cell, since the membrane permeability at rest for K + ions is higher than for Na + ions.

Since the hydrocarbon radicals of phospholipids that form the membrane bilayer have hydrophobic properties, hydrophobic substances can easily diffuse through the membrane, in particular, readily soluble in lipids (steroid, thyroid hormones, some narcotic substances, etc.). Low-molecular substances of a hydrophilic nature, mineral ions diffuse through passive ion channels of membranes formed by channel-forming protein molecules, and, possibly, through stacking faults in the membrane of phospholium molecules that arise and disappear in the membrane as a result of thermal fluctuations.

Diffusion of substances in tissues can be carried out not only through cell membranes, but also through other morphological structures, for example, from saliva into the dentinal tissue of a tooth through its enamel. In this case, the conditions for diffusion remain the same as through cell membranes. For example, for the diffusion of oxygen, glucose, mineral ions from saliva into the tooth tissue, their concentration in saliva must exceed the concentration in the tooth tissues.

Under normal conditions, non-polar and small electrically neutral polar molecules can pass in significant quantities through the phospholipid bilayer by simple diffusion. Transport of significant amounts of other polar molecules is carried out by carrier proteins. If the participation of a carrier is necessary for the transmembrane transition of a substance, then instead of the term "diffusion" the term is often used transport of matter through the membrane.

Facilitated diffusion, as well as simple "diffusion" of a substance, is carried out along the gradient of its concentration, but in contrast to simple diffusion, a specific protein molecule, a carrier, is involved in the transfer of a substance through the membrane (Fig. 2).

Facilitated diffusion Is a type of passive transfer of ions across biological membranes, which is carried out along a concentration gradient using a carrier.

The transfer of a substance using a carrier protein (transporter) is based on the ability of this protein molecule to integrate into the membrane, permeating it and forming channels filled with water. The carrier can reversibly bind to the transported substance and, at the same time, reversibly change its conformation.

It is assumed that the carrier protein is capable of being in two conformational states. For example, in a state a this protein has an affinity for the carried substance, its binding sites are turned inward and it forms a pore open to one side of the membrane.

Rice. 2. Facilitated diffusion. Description in text

Having bound with a substance, the carrier protein changes its conformation and goes into the state 6 ... During this conformational transformation, the carrier loses its affinity with the transported substance, it is released from the bond with the carrier and is transferred to a pore on the other side of the membrane. After that, the protein again makes a return to state a. This transfer of a substance by a transporter protein across the membrane is called uniform.

Through facilitated diffusion, such low-molecular substances as glucose can be transported from the interstitial spaces to the cells, from the blood to the brain, some amino acids and glucose from the primary urine to the blood in the renal tubules can be reabsorbed, amino acids and monosaccharides can be absorbed from the intestine. The speed of transport of substances by facilitated diffusion can reach up to 10 8 particles per second through the channel.

In contrast to the rate of transfer of a substance by simple diffusion, which is directly proportional to the difference in its concentrations on both sides of the membrane, the transfer rate of a substance with facilitated diffusion increases in proportion to an increase in the difference in concentration of a substance to a certain maximum value, above which it does not increase, despite an increase in the difference in concentration of the substance in both sides of the membrane. Achievement of the maximum transfer rate (saturation) in the process of facilitated diffusion is explained by the fact that at the maximum transfer rate all molecules of the carrier proteins are involved.

Exchange diffusion- with this type of transport of substances, the exchange of molecules of the same substance, located on different sides of the membrane, can occur. The concentration of the substance on each side of the membrane remains unchanged.

A kind of exchange diffusion is the exchange of a molecule of one substance for one or more molecules of another substance. For example, in smooth muscle cells of blood vessels and bronchi, in contractile myocytes of the heart, one of the ways to remove Ca 2+ ions from cells is to exchange them for extracellular Na + ions. For three ions of incoming Na +, one Ca 2+ ion is removed from the cell. An interdependent (conjugate) movement of Na + and Ca 2+ through the membrane in opposite directions is created (this type of transport is called antiport). Thus, the cell is freed from the excess amount of Ca 2+ ions, which is a prerequisite for the relaxation of smooth myocytes or cardiomyocytes.

Active transport of substances

Active transport substances through - this is the transfer of substances against their gradients, carried out with the expenditure of metabolic energy. This type of transport differs from the passive one in that the transfer is carried out not along the gradient, but against the gradients of the concentration of the substance and it uses the energy of ATP or other types of energy, for the creation of which ATP was spent earlier. If the direct source of this energy is ATP, then this transfer is called primary active. If energy is used for transfer (concentration, chemical, electrochemical gradients), previously stored due to the operation of ion pumps that consumed ATP, then such transport is called secondary-active, as well as conjugate. An example of conjugated, secondary-active transport is the absorption of glucose in the intestine and its reabsorption in the kidneys with the participation of Na ions and GLUT1 transporters.

Thanks to active transport, the forces of not only concentration, but also electrical, electrochemical and other gradients of matter can be overcome. As an example of the work of primary-active transport, we can consider the work of a Na + -, K + -pump.

Active transfer of Na + and K + ions is provided by a protein-enzyme - Na + -, K + -ATP-ase, capable of cleaving ATP.

The protein Na K -ATP-ase is contained in the cytoplasmic membrane of almost all cells of the body, accounting for 10% or more of the total protein content in the cell. More than 30% of the total metabolic energy of the cell is spent on the operation of this pump. Na + -, K + -ATP-ase can be in two conformational states - S1 and S2. In the S1 state, the protein has an affinity for the Na ion and 3 Na ions bind to three high-affinity binding sites turned into the cell. The addition of the Na "ion stimulates ATP-ase activity, and as a result of ATP hydrolysis, Na + -, K + -ATP-ase is phosphorylated due to the transfer of a phosphate group to it and carries out a conformational transition from the S1 state to the S2 state (Fig. 3).

As a result of a change in the spatial structure of the protein, the binding sites of Na ions turn to the outer surface of the membrane. The affinity of the binding sites for Na + ions sharply decreases, and, being released from the bond with the protein, it is transferred to the extracellular space. In the conformational state S2, the affinity of Na + -, K-ATPase centers for K ions increases, and they attach two K ions from the extracellular environment. The addition of K ions causes dephosphorylation of the protein and its reverse conformational transition from the S2 state to the S1 state. Together with the rotation of the binding sites on the inner surface of the membrane, two K ions are released from the bond with the carrier and are transferred inside. Such transfer cycles are repeated at a rate sufficient to maintain a non-uniform distribution of Na + and K + ions in the cell and the intercellular medium in a resting cell and, as a consequence, maintain a relatively constant potential difference across the membrane of excitable cells.

Rice. 3. Schematic representation of the operation of the Na + -, K + -pump

The substance strophanthin (ouabain), secreted from the foxglove plant, has a specific ability to block the work of the Na + -, K + - pump. After its introduction into the body, as a result of the blockade of pumping out the Na + ion from the cell, a decrease in the efficiency of the Na + -, Ca 2 -exchange mechanism and the accumulation of Ca 2+ ions in the contractile cardiomyocytes are observed. This leads to increased myocardial contraction. The drug is used to treat insufficiency of the pumping function of the heart.

In addition to Na "-, K + -ATP-ase, there are several types of transport ATP-ase, or ion pumps. Among them, a pump that transports hydrogen runs (cell mitochondria, renal tubular epithelium, gastric parietal cells); calcium pumps (pacemaker and contractile cells of the heart, muscle cells of striated and smooth muscles). For example, in the cells of skeletal muscles and myocardium, the protein Ca 2+ -ATP-ase is built into the membranes of the sarcoplasmic reticulum and thanks to its work, a high concentration of Ca 2+ ions in its intracellular storage facilities (cisterns, longitudinal tubes of the sarcoplasmic reticulum).

In some cells, the forces of the transmembrane electrical potential difference and the sodium concentration gradient arising from the operation of the Na +, Ca 2+ pump are used to carry out secondary-active types of transfer of substances through the cell membrane.

Secondary active transport characterized by the fact that the transfer of a substance through the membrane is carried out due to the concentration gradient of another substance, which was created by the mechanism of active transport with the expenditure of ATP energy. There are two types of secondary active transport: symport and antiport.

Symptom is called the transfer of a substance, which is associated with the simultaneous transfer of another substance in the same direction. The symptomatic mechanism is used to transfer iodine from the extracellular space to thyroid gland thyrocytes, glucose and amino acids during their absorption from the small intestine into enterocytes.

Antiport is called the transfer of a substance, which is associated with the simultaneous transfer of another substance, but in the opposite direction. An example of an antiport transfer mechanism is the work of the previously mentioned Na + -, Ca 2+ - exchanger in cardiomyocytes, K + -, H + -exchange mechanism in the epithelium of the renal tubules.

It can be seen from the examples given that the secondary-active transport is carried out by using the forces of the gradient of Na + ions or K + ions. The Na + ion or K ion moves through the membrane towards its lower concentration and pulls another substance along with it. In this case, a specific carrier protein built into the membrane is usually used. For example, the transport of amino acids and glucose during their absorption from the small intestine into the blood occurs due to the fact that the carrier protein of the membrane of the epithelium of the intestinal wall binds to the amino acid (glucose) and the Na + ion and only then changes its position in the membrane in such a way that it transfers the amino acid ( glucose) and Na + ion into the cytoplasm. To carry out such transport, it is necessary that the concentration of the Na + ion outside the cell is much higher than inside, which is ensured by the constant work of Na +, K + - ATP-ase and the expenditure of metabolic energy.