Introduction

Membrane transport is the transport of substances through the cell membrane into the cell or from the cell, carried out using various mechanisms - simple diffusion, facilitated diffusion and active transport.

The most important property of a biological membrane is its ability to pass various substances into and out of the cell. This is of great importance for self-regulation and maintaining a constant cell composition. This function of the cell membrane is performed due to selective permeability, that is, the ability to pass some substances and not pass others.

Passive transport

Distinguish between passive and active transport. Passive transport occurs without energy consumption along an electrochemical gradient. Passive includes diffusion (simple and lightweight), osmosis, filtration. Active transport requires energy and occurs in spite of a concentration or electrical gradient.

Types of passive transport

Types of passive transport of substances:

• Simple diffusion
• Osmosis
• Diffusion of ions
• Facilitated diffusion

Simple diffusion

Diffusion is the process by which gas or solutes are diffused and filled to fill the available volume.

Molecules and ions dissolved in a liquid are in chaotic motion, colliding with each other, solvent molecules and the cell membrane. A collision of a molecule or ion with a membrane can have a twofold outcome: the molecule will either "bounce" off the membrane or pass through it. When the probability of the latter event is high, the membrane is said to be permeable to the substance.

If the concentration of the substance on both sides of the membrane is different, a flow of particles occurs, directed from a more concentrated solution to a dilute one. Diffusion occurs until the concentration of the substance on both sides of the membrane is equalized. Both highly water-soluble (hydrophilic) substances and hydrophobic, poorly or completely insoluble in water, pass through the cell membrane.

Hydrophobic, highly fat-soluble substances diffuse due to dissolution in membrane lipids. Water and substances well soluble in it penetrate through temporary defects in the hydrocarbon region of the membrane, the so-called kinks, as well as through the pores, permanently existing hydrophilic sections of the membrane.

In the case when the cell membrane is impermeable or poorly permeable to a solute, but permeable to water, it is subjected to the action of osmotic forces. At a lower concentration of a substance in the cell than in the environment, the cell shrinks; if the concentration of solute in the cell is higher, water rushes into the cell.

Osmosis is the movement of water molecules (solvent) through a membrane from a smaller area to a higher concentration of solute. Osmotic pressure is the lowest pressure that must be applied to a solution in order to prevent the solvent from flowing through the membrane into a solution with a higher concentration of the substance.

Solvent molecules, like the molecules of any other substance, are set in motion by the force arising from the difference in chemical potentials. When any substance is dissolved, the chemical potential of the solvent decreases. Therefore, in the area where the concentration of the solute is higher, the chemical potential of the solvent is lower. Thus, the solvent molecules, moving from a solution with a lower concentration to a solution with a higher concentration, move in the thermodynamic sense "downward", "along the gradient".

Cell volume is largely governed by the amount of water they contain. The cell is never in a state of complete equilibrium with the environment. The continuous movement of molecules and ions through the plasma membrane changes the concentration of substances in the cell and, accordingly, the osmotic pressure of its contents. If a cell secretes any substance, then in order to maintain a constant value of osmotic pressure, it must either release an appropriate amount of water or absorb an equivalent amount of another substance. Since the environment surrounding most cells is hypotonic, it is important for cells to prevent large amounts of water from entering them. Maintaining a constant volume even in an isotonic environment requires energy expenditure, therefore the concentration of substances incapable of diffusion (proteins, nucleic acids, etc.) in the cell is higher than in the pericellular environment. In addition, metabolites are constantly accumulating in the cell, which disrupts the osmotic balance. The need for energy expenditure to maintain a constant volume is easily demonstrated in experiments with refrigeration or metabolic inhibitors. Under such conditions, cells swell quickly.

To solve the "osmotic problem", cells use two methods: they pump out the components of their contents into the interstitium or the water entering them. In most cases, cells use the first option - pumping out substances, more often than ions, using a sodium pump for this (see below).

In general, the volume of cells without rigid walls is determined by three factors:

• a) the amount of substances contained in them and incapable of penetration through the membrane;
• b) concentration in the interstitium of compounds capable of passing through the membrane;
• c) the ratio of the rates of penetration and pumping of substances from the cell.

An important role in the regulation of the water balance between the cell and the environment is played by the elasticity of the plasma membrane, which creates a hydrostatic pressure that prevents water from entering the cell. If there is a difference in hydrostatic pressures in two regions of the medium, water can be filtered through the pores of the barrier separating these regions.

Filtration phenomena underlie many physiological processes, such as, for example, the formation of primary urine in the nephron, the exchange of water between blood and tissue fluid in capillaries.

Diffusion of ions

Diffusion of ions occurs mainly through specialized protein structures of the membrane - ion channels, when they are in an open state. Cells can have a different set of ion channels depending on the type of tissue. Distinguish between sodium, potassium, calcium, sodium-calcium and chlorine channels. The transport of ions through channels has a number of features that distinguish it from simple diffusion. This is especially true for calcium channels.

Ionic channels can be in open, closed and inactive states. The transition of the channel from one state to another is controlled either by a change in the electrical potential difference across the membrane, or by the interaction of physiologically active substances with receptors. Accordingly, ion channels are divided into voltage-gated and receptor-gated. The selective permeability of the ion channel for a particular ion is determined by the presence of special selective filters at its mouth.

Facilitated diffusion

In addition to water and ions, many substances (from ethanol to complex drugs) penetrate through biological membranes by simple diffusion. At the same time, even comparatively small polar molecules, for example, glycols, monosaccharides and amino acids, practically do not penetrate the membrane of most cells due to simple diffusion. Their transfer is carried out by facilitated diffusion. Facilitated is the diffusion of a substance along the gradient of its concentration, which is carried out with the participation of special protein carrier molecules.

The transport of Na +, K +, Cl-, Li +, Ca2 +, HCO3- and H + can also be carried out by specific carriers. The characteristic features of this type of membrane transport are a high, in comparison with simple diffusion, the rate of transfer of a substance, dependence on the structure of its molecules, saturation, competition and sensitivity to specific inhibitors - compounds that inhibit facilitated diffusion.

All of the listed features of facilitated diffusion are the result of the specificity of carrier proteins and their limited number in the membrane. When a certain concentration of the transferred substance is reached, when all carriers are occupied by the transported molecules or ions, its further increase will not lead to an increase in the number of transferred particles - the phenomenon of saturation. Substances similar in molecular structure and transported by the same carrier will compete for the carrier - a phenomenon of competition.

There are several types of transport of substances through facilitated diffusion.

Uniport, when molecules or ions are transferred through the mebrana, regardless of the presence or transfer of other compounds (transport of glucose, amino acids through the basement membrane of epithelial cells);

Symptom, in which their transfer is carried out simultaneously and unidirectionally with other compounds (sodium-dependent transport of sugars and amino acids Na + K +, 2Cl- and cotran-sport);

Antiport - (the transport of a substance is due to the simultaneous and oppositely directed transport of another compound or ion (Na + / Ca2 +, Na + / H + Cl- / HCO3- - exchanges).

Symport and antiport are types of cotransport in which the speed of transfer is controlled by all participants in the transport process.

The nature of the carrier proteins is unknown. According to the principle of operation, they are divided into two types. The carriers of the first type make shuttle movements through the membrane, and of the second, they are embedded in the membrane, forming a channel. Their action can be simulated using antibiotics-ionophores, a carrier of alkali metals. So, one of them - (valinomycin) - acts as a true carrier, transporting potassium across the membrane. Molecules of gramicidin A, another ionophore, stick into the membrane one after another, forming a "channel" for sodium ions.

Most cells have a facilitated diffusion system. However, the list of metabolites carried by this mechanism is rather limited. These are mainly sugars, amino acids and some ions. Compounds that are intermediate metabolic products (phosphorylated sugars, amino acid metabolism products, macroergs) are not transported using this system. Thus, facilitated diffusion serves to transfer those molecules that the cell receives from the environment. An exception is the transport of organic molecules through the epithelium, which will be considered separately.

Passive transport - transport of substances along the concentration gradient, which does not require energy consumption. Transport of hydrophobic substances through the lipid bilayer occurs passively. All channel proteins and some carriers passively pass through themselves. Passive transport involving membrane proteins is called facilitated diffusion.

Other carrier proteins (sometimes called pump proteins) transport substances across the membrane with the expenditure of energy, which is usually supplied during the hydrolysis of ATP. This type of transport is carried out against the concentration gradient of the carried substance and is called active transport.

Symport, antiport and uniport

Membrane transport of substances also differs in the direction of their movement and the amount of substances carried by this carrier:

1) Uniport - transport of one substance in one direction depending on the gradient

2) Symport - transport of two substances in one direction through one carrier.

3) Antiport - movement of two substances in different directions through one carrier.

Uniport implements, for example, a voltage-dependent sodium channel through which sodium ions move into the cell during the generation of the action potential.

Symport carries out a glucose transporter located on the outer (facing into the intestinal lumen) side of the intestinal epithelium cells. This protein simultaneously captures a glucose molecule and a sodium ion and, changing conformation, transfers both substances into the cell. In this case, the energy of an electrochemical gradient is used, which, in turn, is created due to hydrolysis of ATP with sodium-potassium ATPase.

Antiport carries out, for example, sodium-potassium ATPase (or sodium-dependent ATPase). It transfers potassium ions into the cell. and from the cell - sodium ions.

The work of sodium-potassium atPhase as an example of antiport and active transport

Initially, this carrier attaches three ions to the inner side of the membrane Na +. These ions change the conformation of the active site of the ATPase. After such activation, ATPase is able to hydrolyze one ATP molecule, and the phosphate ion is fixed on the surface of the carrier from the inner side of the membrane.

The released energy is spent on changing the ATPase conformation, after which three ions Na + and ion (phosphate) are on the outside of the membrane. Here the ions Na + are split off and replaced by two ions K +. Then the conformation of the carrier changes to the original one, and the ions K + end up on the inside of the membrane. Here the ions K + are split off, and the carrier is ready for work again.

More succinctly, the action of ATPase can be described as follows:

1) It "takes" three ions from inside the cell Na +, then breaks down the ATP molecule and attaches phosphate to itself

2) "Ejects" ions Na + and attaches two ions K + from the external environment.

3) Detaches phosphate, two ions K + throws it out inside the cell

As a result, a high concentration of ions is created in the extracellular environment. Na +, and inside the cell there is a high concentration K +. Job Na + , K + - ATPase creates not only a concentration difference, but also a charge difference (it works as an electrogenic pump). A positive charge is generated on the outside of the membrane, and a negative charge on the inside.

With passive transfer, water, ions, and some low-molecular compounds move freely due to the difference in concentration and equalize the concentration of the substance inside and outside the cell. In passive transport, physical processes such as diffusion, osmosis and filtration play a major role (Fig. 24-26).

If a substance moves through the membrane from an area of \u200b\u200bhigh concentration to a low concentration without energy consumption by the cell, then such transport is called passive, or diffusion ). There are two types of diffusion: simple and lightweight ... The cell membrane is permeable to some substances and impermeable to others. If the cell membrane is permeable to solute molecules, it does not interfere with diffusion.

Simple diffusion characteristic of small neutral molecules (H 2 O, CO 2, O 2), as well as hydrophobic low molecular weight organic substances. These molecules can pass without any interaction with membrane proteins through the pores or channels of the membrane as long as the concentration gradient is maintained.

Facilitated diffusion... It is characteristic of hydrophilic molecules that are also transported across the membrane along the concentration gradient, but with the help of special membrane carrier proteins. For facilitated diffusion, in contrast to simple diffusion, high selectivity is characteristic, since the carrier protein has a binding center complementary to the transported substance, and the transfer is accompanied by conformational changes in the protein.

One of the possible mechanisms of facilitated diffusion can be as follows: the transport protein (translocase) binds the substance, then approaches the opposite side of the membrane, releases this substance, assumes the initial conformation, and is ready to perform the transport function again. Little is known about how the protein itself moves. Another possible transfer mechanism involves the participation of several carrier proteins. In this case, the initially bound compound itself passes from one protein to another, sequentially binding with one or another protein until it is on the opposite side of the membrane.

As for the transport of ions, it is carried out, as a rule, using diffusion through special ion channels (Fig. 27).

Fig. 27. The main mechanisms of transmembrane transmission of signaling information: I - the passage of a fat-soluble signaling molecule through the cell membrane; II - binding of a signaling molecule to a receptor and activation of its intracellular fragment; III - regulation of the activity of the ion channel; IV - transmission of signaling information using secondary transmitters. 1 - medicine; 2 - intracellular receptor; 3 - cellular (transmembrane) receptor; 4 - intracellular transformation (biochemical reaction); 5 - ion channel; 6 - ion flow; 7 - secondary intermediary; 8 - enzyme or ion channel; 9 - secondary intermediary.

Thus, there are several mechanisms for the transport of substances.

The first mechanism is that a lipid-soluble signaling molecule passes through the cell membrane and activates an intracellular receptor (for example, an enzyme). This is how nitric oxide, a number of fat-soluble hormones (glucocorticoids, mineralocorticoids, sex hormones and thyroid hormones) and vitamin D act. They stimulate gene transcription in the cell nucleus and, thus, the synthesis of new proteins. The mechanism of action of hormones is to stimulate the synthesis of new proteins in the cell nucleus, which remain active for a long time in the cell.

The second mechanism of signal transmission across the cell membrane is binding to cellular receptors that have extracellular and intracellular fragments (that is, transmembrane receptors). These receptors mediate the first stage of the action of insulin and a number of other hormones. The extracellular and intracellular portions of these receptors are linked by a polypeptide bridge across the cell membrane. The intracellular fragment has enzymatic activity, which increases when a signaling molecule binds to a receptor. Accordingly, the rate of intracellular reactions in which this fragment is involved increases.

The third mechanism of information transfer is the action on the receptors that regulate the opening or closing of ion channels. Natural signaling molecules that interact with such receptors include, in particular, acetylcholine, gamma-aminobutyric acid (GABA), glycine, aspartate, glutamate, and others, which are mediators of various physiological processes. When they interact with the receptor, the transmembrane conductance for individual ions increases, which causes a change in the electrical potential of the cell membrane. For example, acetylcholine, interacting with H-cholinergic receptors, increases the entry of sodium ions into the cell and causes depolarization and muscle contraction. The interaction of gamma-aminobutyric acid with its receptor leads to an increase in the flow of chlorine ions into cells, an increase in polarization and the development of inhibition (suppression) of the central nervous system. This signaling mechanism is distinguished by the speed of development of the effect (milliseconds).

The fourth mechanism of transmembrane transmission of a chemical signal is realized through receptors that activate an intracellular secondary transmitter. When interacting with such receptors, the process proceeds in four stages. The signaling molecule is recognized by a receptor on the surface of the cell membrane, as a result of their interaction, the receptor activates the G-protein on the inner surface of the membrane. The activated G protein alters the activity of either the enzyme or the ion channel. This leads to a change in the intracellular concentration of the secondary mediator, through which the effects are already directly realized (metabolic and energy processes change). This mechanism for transmitting signaling information allows you to amplify the transmitted signal. So if the interaction of a signaling molecule (for example, norepinephrine) with a receptor lasts for several milliseconds, then the activity of the secondary transmitter, to which the receptor transmits a signal over the relay race, persists for tens of seconds.

Secondary messengers are substances that are formed inside the cell and are important components of numerous intracellular biochemical reactions. The intensity and results of cell activity, and the functioning of all tissue, largely depend on their concentration. The most famous secondary mediators are cyclic adenosine monophosphate (cAMP), cyclic guanosine monophosphate (cGMP), calcium and potassium ions, etc.

Osmosis - a special type of water diffusion through a semi-permeable membrane into the area of \u200b\u200bhigher concentration of a solute. As a result of this movement, a significant pressure is created inside the cell, which is called osmotic. This pressure can even destroy the cell.

For example, if erythrocytes are placed in clean water, then under the action of osmosis, water will penetrate into them faster than leave. This environment is called hypotonic. As water penetrates, the erythrocyte will swell and “burst”. Another situation is an isotonic environment. If erythrocytes are placed in water containing 0.87% sodium chloride, then no osmotic pressure is created. This is due to the fact that with an equal concentration of solution inside and outside the cell, the water moves in the same way in both directions. An environment is considered hypertensive when the concentration of substances dissolved in it is higher than in the cell. A cell (erythrocyte) in such an environment begins to lose water, shrinks and dies.

All these features of osmosis are taken into account when introducing medicinal substances. As a rule, drugs intended for injection are prepared in isotonic solution. This prevents blood cells from swelling or shrinking when the drug is injected. Nasal drops are also prepared in isotonic solution to avoid swelling or dehydration of the cells in the nasal mucosa.

Osmosis also explains some of the effects of drugs, such as the laxative effect of Epsom salts (magnesium sulfate) and other saline laxatives. In the intestinal lumen, they form a hypertensive environment. Under the influence of osmosis, water leaves the cells of the intestinal epithelium, intercellular space and blood into the intestinal lumen, stretches the intestinal walls, liquefies its contents and accelerates emptying.

Filtration - movement of water molecules and substances dissolved in it through the cell membrane in the direction opposite to the action of osmotic pressure.

This process becomes possible if the solution in the cell is under pressure that is higher than the osmotic pressure. For example, the heart pumps blood into the vessels under a certain pressure. In the thinnest capillaries, this pressure increases and becomes sufficient to force water and substances dissolved in the blood to leave the capillaries into the intercellular space. The so-called interstitial fluid is formed, it plays an important role in the delivery of nutrients to the cells and the removal of the final metabolic products from them. After fulfilling its functions, tissue fluid in the form of lymph returns to the bloodstream through the lymphatic vessels.

Filtration also plays an important role in kidney function. In the capillaries of the kidneys, the blood is under great pressure, which causes filtration of water and substances dissolved in it from the blood vessels into the thinnest renal tubules. Then part of the water and substances necessary for the body are absorbed again and enter the general bloodstream, and the rest forms urine and is excreted from the body.

Membrane transport is the transition of ions and molecules of a substance through the membrane from the medium to the cell and in the opposite direction.

Depending on the nature of the bond between the transport of an ion or molecule with the transfer of other ions and molecules, the following are distinguished:

1) uniport - transport, regardless of the transport of other connections;

2) cotransport - coordinated (interdependent) transport across the membrane; it includes symport (simultaneous and unidirectional transport of two different substances) and antiport (simultaneous transport across the membrane in opposite directions).

Depending on the change in the free energy of the system, two types of transport are distinguished:

Passive transport (simple diffusion).

Active transport - transfer of non-electrolytes and ions against the chemical gradient. or electrochemical. potential associated with energy costs (transfer through the membrane of amino acids and monosaccharides).

31. Passive transport. Fick, Nernst-Planck equation, Teorella.
Passive transport - transfer of non-electrolytes and ions through the membrane along the chemical gradient. or electrochemical. potential, accompanied by a decrease in free energy (simple diffusion).

The driving force behind simple diffusion is the chemical difference. the potentials of a given substance in two regions between which diffusion occurs. Chem. potential - a value numerically equal to the free energy per 1 mole of substance; defined as a partial derivative of free energy.

The basic thermodynamic principle governing the stationary distribution of diffusing molecules in a system with a membrane is that the chemical potentials of a given substance on both sides of the membrane must be equal.

If dn moles of matter are transferred through the membrane separating compartments I and II, then this process is accompanied by a change in the free energy of the system by the value:

dG \u003d (II - I) dn.

Diffusion stops and the system goes into a state of thermodynamic equilibrium when II \u003d I.

Fick's I law has the form:

The flow of a substance can be represented taking into account the permeability coefficient (P) of the membrane for a given substance:

,

where c I and c II are the concentrations of the diffusing substance in the aqueous solution. [P] \u003d cm / s.

The permeability coefficient depends on the properties of the membrane and the substances carried:

where D is the diffusion coefficient, is the distribution coefficient of the substance between the aqueous solution and the membrane, which characterizes the solubility of the substance in the lipid phase of the membrane, and d is the membrane thickness.

The driving force of the passive flow of ions through the membrane is the gradient of the electrochemical potential. Electrochemical potential ion for conditions under which the activity of the ion corresponds to its concentration (c) is equal to:

where is the electric potential, z is the valence of the ion, F is the Faraday number, 0 is the standard chemical potential.

Electrochemical potential is a measure of the work required to transfer 1 mole from a solution at a given concentration and given electrical potential to an infinitely distant point in a vacuum. This work consists of the costs of overcoming the forces of chemical interaction (0 + RTlnc) and the transfer of charges in an electric field (zF).

The diffusion of ions in solution and in a homogeneous uncharged membrane is described by the equation of electrodiffusion Nernst-Planck:

,

where u is the ion mobility, D \u003d uRT. The first term on the right-hand side of the equation describes free diffusion, while the second describes the migration of ions in an electric field.

Theorell's equation:Flux density during passive transport: J \u003d - cU (dm / dx), where m is the electrochemical potential, U is the mobility of particles, c is the concentration.

32. Types of passive transport across the membrane. Simple and easy diffusion.

Passive transport Is the transfer of non-electrolytes and ions through the membrane along the gradient of the chemical or electrochemical potential, accompanied by a decrease in free energy. Passive transport includes simple diffusion through the lipid bilayer and facilitated diffusion through channels in the membrane and with the help of carriers. Simple and easy diffusion processes are aimed at leveling gradients and establishing equilibrium in the system.
Diffusion- spontaneous movement of a substance from places with a higher concentration to places with a lower concentration of the substance due to the chaotic thermal movement of molecules.
Differences between light diffusion and simple:
1) the transfer of the substance with the participation of the carrier is faster;
2) facilitated diffusion has the property of saturation: with an increase in concentration on one side of the membrane, the density of the substance flux increases only up to a certain limit, when all the carrier molecules are already occupied;

3) with facilitated diffusion, there is competition between the transferred substances in cases where different substances are transferred by the carrier; Moreover, some substances are better tolerated than others, and the addition of some substances complicates the transport of others; So, glucose is better tolerated from sugars than fructose, fructose is better than xylose;

33. Ionic channels: mechanism of work, selectivity.
Ionic channels are integral glycoproteins that, as a result of external influences (change in the potential on the membrane), change the membrane permeability for various ions. Ionic channels provide the implementation of the most important physiological processes: transmission of electrical and chemical / signals, contraction, secretion.

Ion channels of biomembranes are characterized by selective permeability for ions (selectivity) and the ability to open and close under various influences on the membrane. - The "gate" mechanism of the channels is controlled by an external stimulus sensor (primary mediator receptor).

Ionic channels work by a facilitated diffusion mechanism. When the channels are activated, the movement of ions along them follows the concentration gradient. The speed of movement through the membrane is 10 ions per second. Channel selectivity is determined by the presence of a selective filter. Its role is played by the initial section of the channel, which has a certain charge, configuration, and size (diameter), which allows only a certain type of ions to enter the channel. Some of the ion channels are non-selective, such as "leakage" channels. These are membrane channels through which K + ions leave the cell at rest along the concentration gradient, however, a small amount of Na + ions also enter the cell at rest along the concentration gradient through these channels.

34. Main families of ion channels.

The ion channel is an integral protein that forms a pore in the membrane for the exchange of K +, Na +, H +, Ca 2+, Cl - ions with the environment, as well as water, and is able to change its permeability.

Sodium channels have a simple structure: a protein of three different subunits that form a pore-like structure - that is, a tube with an internal lumen. The channel can be in three states: closed, open, and inactivated (closed and non-excitable). This is provided by the localization of negative and positive charges in the protein itself; these charges are attracted to the opposite ones existing on the membrane, and thus the channel opens and closes when the state of the membrane changes. When it is open, sodium ions can freely penetrate through it into the cell along a concentration gradient.

Potassium channels are simpler: these are separate subunits with a trapezoidal cross-section; they are located almost close to each other, but there is always a gap between them. Potassium channels do not close completely, at rest potassium freely leaves the cytoplasm (along the concentration gradient).

Calcium channels are transmembrane proteins of complex structure, consisting of several subunits. Sodium, barium and hydrogen ions also enter through these channels. Distinguish between voltage-gated and receptor-gated calcium channels. Through voltage-dependent channels, Ca 2+ ions pass through the membrane as soon as its potential drops below a certain critical level. In the second case, the flow of Ca 2+ through the membranes is regulated by specific agonists (acetylcholine, catecholamines, serotonin, histamine, etc.) during their interaction with cell receptors. Currently, there are several types of calcium channels with different properties (conductivity, opening duration) and having different tissue localization.

Passive transportincludes simple and easy diffusion - processes that do not require energy input. Diffusion - transport of molecules and ions across the membrane from an area with a high to an area with their low concentration, those. substances come along a concentration gradient. Diffusion of water through semipermeable membranes is called osmosis. Water is also able to pass through membrane pores formed by proteins and carry molecules and ions of substances dissolved in it. The mechanism of simple diffusion is the transfer of small molecules (for example, О2, Н2О, СО2); this process is not very specific and proceeds at a rate proportional to the concentration gradient of transported molecules on both sides of the membrane.

Facilitated diffusionis carried out through channels and (or) carrier proteins that have specificity for the transported molecules. The ion channels are transmembrane proteins, which form small water pores through which small water-soluble molecules and ions are transported along an electrochemical gradient. Carrier proteins are also transmembrane proteins that undergo reversible conformational changes that provide for the transport of specific molecules through the plasmolemma. They function in both passive and active transport mechanisms.

Active transport is an energy-consuming process by which the transfer of molecules is carried out using carrier proteins against an electrochemical gradient. An example of a mechanism providing the oppositely directed active transport of ions is the sodium-potassium pump (represented by the carrier protein Na + -K + -ATPase), due to which Na + ions are removed from the cytoplasm, and K + ions are simultaneously transferred into it. The concentration of K + inside the cell is 10-20 times higher than outside, and the concentration of Na is vice versa. Such a difference in ion concentration is provided by the work (Na * -K *\u003e pump. To maintain this concentration, three Na ions are transferred from the cell for every two K * ions into the cell. In this process, a protein in the membrane takes part, which acts as an enzyme that breaks down ATP, releasing the energy needed to operate the pump.
The participation of specific membrane proteins in passive and active transport indicates the high specificity of this process. This mechanism ensures the maintenance of a constant cell volume (by regulating osmotic pressure), as well as membrane potential. The active transport of glucose into the cell is carried out by a carrier protein and is combined with the unidirectional transfer of the Na + ion.

Lightweight transport ions is mediated by special transmembrane proteins - ion channels that provide the selective transfer of certain ions. These channels consist of the transport system itself and a gate mechanism that opens the channel for some time in response to a change in the membrane potential, (b) mechanical action (for example, in the hair cells of the inner ear), binding of a ligand (signaling molecule or ion).

Membrane transport of substances also differs in the direction of their movement and the amount of substances carried by this carrier:

• Uniport - transport of one substance in one direction depending on the gradient
• Symport is the transport of two substances in one direction through one carrier.
• Antiport - movement of two substances in different directions through a single carrier.

Uniport implements, for example, a voltage-dependent sodium channel through which sodium ions move into the cell during the generation of the action potential.

Symport carries out a glucose transporter located on the outer (facing into the intestinal lumen) side of the intestinal epithelium cells. This protein simultaneously captures a glucose molecule and a sodium ion and, changing conformation, transfers both substances into the cell. In this case, the energy of an electrochemical gradient is used, which, in turn, is created due to the hydrolysis of ATP with sodium-potassium ATPase.

Antiport carries out, for example, sodium-potassium ATPase (or sodium-dependent ATPase). It transfers potassium ions into the cell. and from the cell - sodium ions. Initially, this carrier attaches three ions to the inner side of the membrane Na +. These ions change the conformation of the active site of the ATPase. After such activation, ATPase is able to hydrolyze one ATP molecule, and the phosphate ion is fixed on the surface of the carrier from the inner side of the membrane.

The released energy is spent on changing the ATPase conformation, after which three ions Na + and ion (phosphate) are on the outside of the membrane. Here the ions Na + are split off and replaced by two ions K +. Then the conformation of the carrier changes to the original one, and the ions K + end up on the inside of the membrane. Here the ions K + split off, and the carrier is again ready for work