There are active and passive transfer (transport) of neutral molecules and ions through biomembranes. Active transport - occurs when energy is consumed due to ATP hydrolysis or proton transfer along the respiratory chain of mitochondria. Passive transport is not associated with the expenditure of chemical energy by the cell: it is carried out as a result of the diffusion of substances towards a lower electrochemical potential.

An example of active transport is the transfer of potassium and sodium ions through the cytoplasmic membranes K - into the cell, and Na - out of it, the transfer of calcium through the sarcoplasmic reticulum of skeletal and cardiac muscles into the reticulum vesicles, the transfer of hydrogen ions through the membranes of mitochondria from the matrix - out: all these processes occur due to the energy of ATP hydrolysis and are carried out by special enzymes - transport ATP phases. The best-known example of passive transport is the movement of ions and potassium across the cytoplasmic membrane of nerve fibers during the propagation of an action potential.

Passive transfer of substances through biomembranes.Diffusion of uncharged molecules.

It is customary to distinguish the following types of passive transfer of substances (including ions) through membranes:

2. Transfer through pores (channels)

3. Transport by carriers due to:

a) diffusion of the carrier together with the substance in the membrane (mobile carrier);

b) relay-race transfer of a substance from one carrier molecule to another, the carrier molecules form a temporary chain across the membrane.

Transport by mechanism 2 and 3 is sometimes called facilitated diffusion.

Transport of non-electrolytes by simple andfacilitated diffusion

Various substances are transported through membranes by two main mechanisms: by diffusion (passive transport) and by active transport. The permeability of membranes for various solutes depends on the size and charge of these molecules. Because the interior of membranes is made up of hydrocarbon chains, many small, neutral and non-polar molecules can pass through a bimolecular membrane by normal diffusion. In other words, these molecules can be said to be soluble in the membrane.

The most important of these substances is glucose, which is transported across membranes only in combination with a carrier molecule. This role is usually played by protein. The glucose-carrier complex is readily soluble in the membrane and can therefore diffuse across the membrane. Such a process is called facilitated diffusion . The total rate of glucose transport increases dramatically in the presence of the hormone insulin. It is not yet entirely clear whether the action of insulin is to increase the concentration of the transporter or whether this hormone stimulates the formation of a complex between glucose and the transporter.

The main mechanism of passive transport of substances, due to the presence of a concentration gradient, is diffusion.

Diffusion - this is a spontaneous process of penetration of a substance from an area of ​​​​higher concentration into an area of ​​\u200b\u200blower concentration as a result of thermal chaotic movement of molecules.

Mathematical description of the diffusion process Dar Rick. According to Rick's law, the diffusion rate is directly proportional to the concentration and area gradient S, through which diffusion occurs:

The minus sign on the right side of the equation shows that diffusion occurs from an area of ​​higher concentration to an area of ​​lower concentration of a substance.

"D" called diffusion coefficient . The diffusion coefficient is numerically equal to the amount of a substance diffusing per unit time through a unit area at a concentration gradient equal to one. "D" depends on the nature of the substance and on the temperature. It characterizes the ability of a substance to diffuse.

Since it is difficult to determine the concentration gradient of a cell membrane, a simpler equation proposed by Kolleider and Berlund is used to describe the diffusion of substances through cell membranes:

where From 1 and From 2- the concentration of the substance on opposite sides of the membrane, R- permeability coefficient, similar to the diffusion coefficient. Unlike the diffusion coefficient, which depends only on the nature of the substance and temperature, "R" also depends on the properties of the membrane and on its functional state.

The penetration of dissolved particles with an electric charge through the cell membrane depends not only on the concentration gradient of the membrane. In this regard, ion transport can occur in the direction opposite to the concentration gradient, in the presence of an oppositely directed electrical gradient. The combination of concentration and electrical gradients is called the electrochemical gradient. Passive transport of ions across membranes always follows an electrochemical gradient.

The main gradients inherent in living organisms are concentration, osmotic, electrical and fluid hydrostatic pressure gradients.

In accordance with this gradient, there are the following types of passive transport of substances in cells and tissues: diffusion, osmosis, electroosmosis and abnormal osmosis, filtration.

Of great importance for the life of cells is the phenomenon of conjugated transport of substances and ions, which consists in the fact that the transfer of one substance (ion) against the electrochemical potential (“uphill”) is due to the simultaneous transfer of another ion through the membrane in the direction of decreasing electrochemical potential (“downhill”). "). This is shown schematically in the figure. The work of transport ATPases and the transfer of protons during the operation of the respiratory chain of mitochondria is often called primary active transport, and the transport of substances associated with it is called secondary active transport.

transfer phenomenon. General transport equation.

A group of phenomena caused by the chaotic motion of molecules and leading to the transfer of mass, kinetic energy and momentum is called transfer phenomenon .

These include diffusion - the transfer of matter, heat conduction - the transfer of kinetic energy and internal friction - the transfer of momentum.

The general transport equation describing these phenomena can be obtained on the basis of molecular kinetic theory.

Let a certain physical quantity be transferred through the area "S" (figure) as a result of the chaotic movement of molecules.

At distances equal to the mean free path, to the right and to the left of the site, we construct rectangular parallelepipeds of small thickness " l» ( l<< ). Объем каждого параллелепипеда равен

V = Sl.

If the concentration of molecules is " P”, then inside the selected parallelepiped there is “ S l p» molecules.

All molecules due to their chaotic motion can be conditionally represented by six groups, each of which moves along or against the direction of one of the coordinate axes. That is, in the direction perpendicular to the site " S, moves the molecules. Since volume "1" is located at a distance from the site " S”, then these molecules will reach it without collision. The same number of molecules will reach the area " S» on the left.

Each molecule is able to transfer a certain value of "Z" (mass, momentum, kinetic energy), and all molecules - or , where H = nZ- physical quantity carried by molecules enclosed in a unit volume. As a result, through the platform S» from volumes 1 and 2 for the time interval «Dt» the value is transferred

To determine the time "Dt", we assume that all molecules from the allocated volumes move with the same average speeds. Then the molecules in volume 1 or 2 that have reached the area " S, cross it during the time interval

Dividing (1) by (2), we get that the value transferred over the time interval "Dt" is equal to

The change in the value of "H" per unit length "dx" is called the gradient of the value "H". Since (H 1 - H 2) is a change in "H" at a distance equal to 2, then

After substituting (4) into (3) and multiplying the resulting equation by the time, we find the flow of the unbearable physical quantity "H" for the time interval "Dt" through the area "S":

This is the general transport equation used in the study of diffusion, thermal conductivity, viscosity.

Diffusion. Passive transport of non-electrolytes through biomembranes,Rick's equation. Transport of non-electrolytes across membranes bysimple and facilitated (in combination with a carrier) diffusion.

Diffusion is a process that leads to a spontaneous decrease in concentration gradients in a solution until a uniform distribution of particles is established. The diffusion process plays an important role in many chemical and biological systems. It is diffusion, for example, that determines mainly the access of carbon dioxide to active photosynthetic structures in chloroplasts. To understand the features of the transport of dissolved molecules across cell membranes, detailed knowledge of diffusion is required. Let us consider some basic principles of diffusion in solutions.

Imagine a vessel, on the left side of which there is a pure solvent, and on the right side - a solution prepared with the same solvent. Let first these two parts of the vessel be separated by a flat vertical wall. If we now remove the wall, then due to the random movement of molecules in all directions, the boundary between the solution and the solvent will shift to the left until the entire system becomes homogeneous. In 1855, Rick, studying diffusion processes, discovered that the diffusion rate, that is, the number of solute molecules "n" crossing the vertical plane per unit time, is directly proportional to the cross-sectional area "S" and the concentration gradient. In this way,

where D- diffusion coefficient (measured in m 2 / s in "SI"). The minus sign indicates that diffusion is from an area of ​​high concentration to an area of ​​low concentration. This means that the concentration gradient in the direction of diffusion is negative. Equation (1) is known as Rick's first law of diffusion. Physical laws are intuitive conclusions that cannot be deduced from simpler statements and the consequences of which do not contradict experiment. These conclusions include the laws of mechanics and thermodynamics; so is Rick's law.

Let us now consider the diffusion process in more detail. Let us single out in space the volume element " S x dx", as it shown on the picture

The rate at which solute molecules enter the volume element through the section "x" is equal to The rate of change in the concentration gradient as "x" changes is equal to

Therefore, the rate at which the solute molecules leave the volume element through a section remote from the first by "dx" is equal to

The rate of accumulation of solute molecules in the volume element is the difference between these two quantities:

However, the same particle accumulation rate is equal to , so one can write

Equation (6) is called the diffusion equation or Rick's second law of diffusion, from which it follows that the change in concentration over time at a certain distance "x" from the initial plane is proportional to the rate of change in the concentration gradient in the direction "x" at the moment "t".

To solve equation (6), it is required to use special methods (developed by Rurier), the description of which is omitted, the result obtained has a simple form:

where C 0 is the initial concentration of the substance at the reference point at the zero moment of time.

According to equation (7), it is possible to plot the dependence of the concentration gradient on the “x” coordinate at various times “t”. Optical methods (for example, by measuring the refractive index) can determine the concentration gradients at various distances from the boundary along which diffusion began.

Molecular mechanism of active ion transport

There are four main systems of active ion transport in a living cell, three of which ensure the transfer of sodium, potassium, calcium and proton ions through biological membranes due to the energy of ATP hydrolysis as a result of the work of special carrier enzymes called transport ATPases. The fourth mechanism - the transfer of protons during the operation of the respiratory chain of mitochondria - has not yet been studied enough. Of the transport ATP-ases, the H + - ATP-ase, consisting of several subunits, is the most complex, the simplest is the Ca 2+ ATP-ase, consisting of one polypeptide chain (subunit) with a molecular weight of about 100,000. Let us consider the mechanism of transfer of calcium ions of this ATPase.

The first step in the work of Ca 2+ ATP-zy is the binding of substrates: Ca 2+ and ATP in complex with Mg 2+ (Mg ATP). These two ligands attach to different sites on the surface of the enzyme molecule facing outward of the sarcoplasmic reticulum (SR) vesicle.

Ligand - a small molecule (ion, hormone, drug, etc.).

The second stage of the enzyme's work is the hydrolysis of ATP. In this case, the formation of an enzyme-phosphate complex (E-P) occurs.

The third stage of the enzyme's work is the transition of the Ca 2+ binding center to the other side of the membrane - translocation.

The release of high-energy bond energy occurs at the fourth stage of the work of Ca 2+ ATP-ase during the hydrolysis of E-P. This energy is by no means wasted (i.e., does not turn into heat), but is used to change the binding constant of calcium ions with the enzyme. The transfer of calcium from one side of the membrane to the other is thus associated with energy consumption, which can be 37.4 - 17.8 = 19.6 kJ / mol. It is clear that the energy of ATP hydrolysis is sufficient for the transfer of two calcium ions.

The transfer of calcium from the area of ​​​​lower (1-4 x 10 -3 M) to the area of ​​\u200b\u200bhigh concentrations (1-10 x 10 -3 M) is the work that Ca, the transport ATPase, does in muscle cells.

To repeat the cycle, the return of calcium-binding centers from the inside to the outside is required, that is, one more conformational change in the enzyme molecule.

The molecular mechanism of operation of these two "pumps" is close in many respects.

The main steps in the work of Na + K + ATPases are as follows:

1. Accession from the outside of two K + ions and one Mg 2+ ATP molecule:

2 K + + Mg ATP + E ® (2 K +) (Mg ATP) E

2. Hydrolysis of ATP and the formation of enzyme phosphate:

(2 K +) (Mg ATP) E ® Mg ATP + (2 K +) E - P

3. Transfer of binding centers K + inside (translocation 1):

(2K +)E - P ® E - P(2K +)

4. Detachment of both potassium ions and replacement of these ions with three Na ions located inside the cell:

E - P(2 K +) + 3 Na i + ® E - P(3 Na +) + 2 K + i

5. Hydrolysis E - P:

E - P(3 Na +) ® E(3 Na +) + P (phosphate)

6. Transfer of binding centers together with Na + ions outwards (translocation 2):

E(3Na+) ® (3Na+)E

7. Removal of 3 Na + and addition of 2 K + outside:

2 K 0 + + 3 Na + (E) ® 3 Na + + (2 K +)E

The transfer of 2 K + inside the cell and the release of 3 Na + outside ultimately leads to the transfer of one positive ion from the cytoplasm to the environment, and this contributes to the appearance of a membrane potential (with a minus sign inside the cell).

Thus the Na + K + pump is electrogenic.

Permeability

Permeability is the ability of cells and tissues to absorb, release and transport chemicals, passing them through cell membranes, vascular walls and epithelial cells. Living cells and tissues are in a state of continuous exchange of chemicals with the environment, receiving food from it and removing metabolic products into it. The main diffusion barrier to the movement of substances is the cell membrane. In 1899, Overton discovered that the ease of passage of substances through the cell membrane depended on the ability of these substances to dissolve in fats. At the same time, a number of polar substances penetrated the cells regardless of their solubility in fats, which could be explained by the existence of water pores in the membranes.

Currently, there are passive permeability, active transport of substances and special cases of permeability associated with phagocytosis and pinocytosis.

The main types of diffusion are the diffusion of substances by dissolving in membrane lipids, the diffusion of substances through polar pores, the diffusion of ions through uncharged pores. Special types of diffusion are facilitated and exchange. It is provided by special fat-soluble carrier substances that are able to bind the transported substance on one side of the membrane, diffuse with it through the membrane and release it on the other side of the membrane. The role of specific ion carriers is performed by some antibiotics, called ionophores (valinomine, nigericin, monensin, poenoic antibiotics nystatin, aifotericin B, and a number of others).

Ionophores can in turn be divided into three classes depending on the charge of the carrier and the structure of the ring: a neutral carrier with a closed covalent bond ring (valinomycin, nactins, polyesters), a charged carrier with a ring closed by a hydrogen bond (nigericin, monensin). Charged carriers hardly penetrate in the charged form through model and biological membranes, while in the neutral form they diffuse freely in the membrane. The neutral form is formed by complexing the anionic form of the carrier with the cation. Thus, charged carriers are able to exchange cations located predominantly on one side of the membrane for cations of the solution washing the opposite side of the membrane.

The most common type of passive diffusion of cell membranes is porous. Data on the osmotic properties of cells testify in favor of the actually existing porous mechanism of permeability.

Classic osmotic pressure equation:

where p is the osmotic pressure, c is the concentration of the solute, R is the gas constant, T is the absolute temperature, includes an additional term s that varies from zero to 1. This constant, called the reflection coefficient, corresponds to the ease of passage of the solute through the membrane in comparison with the passage of a water molecule.

The type of permeability, characteristic only of living cells and tissues, is called active transport. Active transport is the transfer of a substance through the cell membrane from the surrounding solution (homocellular active transport) or through cellular active transport, which flows against the gradient of the electrochemical activity of the substance with the expenditure of free energy of the body. It has now been proven that the molecular system responsible for the active transport of substances is located in the cell membrane.

It has now been proven that the main element of the ion pump is Na + K + ATPase. The study of the properties of this membrane enzyme showed that the enzyme is active only in the presence of potassium and sodium ions, with sodium ions activating the enzyme from the side of the cytoplasm, and ions from the surrounding solution. A specific inhibitor of the enzyme is the acid glycoside suabain. In the membranes of mitochondria, another molecular system is known, which ensures the pumping of hydrogen ions by the enzyme H + - ATPase.

P. Mitchell, the author of the chemiosmotic theory of oxidative phosphorylation in mitochondria, introduced the concept of secondary active transport of substances. There are three methods of transmembrane ion transport in conjugating membranes. Unidirectional transfer of ions in the direction of the electrochemical gradient by free diffusion or with the help of a specific carrier - uniport. In the latter case, the uniport is identical to facilitated diffusion. A more complicated situation arises when two substances interact with the same carrier. This case of symport implies the obligatory conjugation of the flows of two substances in the process of their transfer through the membrane in one direction. The symport of two ions is electrically neutral, but the osmotic balance is disturbed in this case.

It should be emphasized that during symport, the electrochemical gradient that determines the movement of one of the ions (for example, a sodium ion or a hydrogen ion) can cause the movement of another substance (for example, carar molecules or amino acids), which is carried by a common carrier. The third type of ionic conjugation - actiport - characterizes the situation in which two ions of the same sign are balanced across the membrane in such a way that the transfer of one of them requires the transfer of the other in the opposite direction. The transfer is generally electrically neutral and osmotically balanced. This type of transfer is identical to exchange diffusion.

Less studied are two special types of permeability - phagocytosis - the process of capturing and absorbing large solid particles, and pinocytosis - the process of capturing and absorbing part of the cell surface of the surrounding fluid with substances dissolved in it.

All types of permeability are to some extent characteristic of multicellular tissues of the membranes of the walls of blood vessels, epithelium of the kidneys, intestinal mucosa and stomach.

Various kinetic methods are used to study passive and active permeability. The labeled atom method is the most widely used.

Vital dyes are widely used in the study of permeability. The essence of the method is to observe, using a microscope, the rate of penetration of dye molecules into the cell. Currently, fluorescent labels are widely used, among them sodium fluorescein, chlortetracycline, etc. Much merit in the development of the method of vital dyes belongs to D.N. Nasonov, V.Ya. Aleksandrov and A.S. Troshin.

The osmotic properties of cells and subcellular particles make it possible to use this quality to study the permeability of water and substances soluble in it. The essence of the osmotic method lies in the fact that using a microscope or measuring the light scattering of a suspension of particles, a change in the volume of particles is observed depending on the tonicity of the surrounding solution.

Increasingly, potentiometric methods are used to study cell membranes. A wide range of ion-specific electrodes allows you to study the transport kinetics of many ions - K + , Na + , Ca 2+ , H + , CI - and others, as well as organic ions - acetate, salicylates, etc.

simple diffusion

By the path of simple diffusion, particles of a substance move through the lipid bilayer. The direction of simple diffusion is determined only by the difference in the concentrations of the substance on both sides of the membrane. By simple diffusion, hydrophobic substances (O 2 , N 2 , benzene) and polar small molecules (CO 2 , H 2 O, urea) penetrate the cell. Polar relatively large molecules (amino acids, monosaccharides), charged particles (ions) and macromolecules (DNA, proteins) do not penetrate.

Facilitated diffusion

Most substances are transported through the membrane with the help of transport proteins (carrier proteins) immersed in it. All transport proteins form a continuous protein passage across the membrane. With the help of carrier proteins, both passive and active transport of substances is carried out. Polar substances (amino acids, monosaccharides), charged particles (ions) pass through membranes with the help of facilitated diffusion, with the participation of channel proteins or carrier proteins. The participation of carrier proteins provides a higher rate of facilitated diffusion compared to simple passive diffusion. The rate of facilitated diffusion depends on a number of reasons: on the transmembrane concentration gradient of the transported substance, on the amount of the carrier that binds to the transported substance, on the rate of binding of the substance by the carrier on one surface of the membrane (for example, on the outer), on the rate of conformational changes in the carrier molecule, in as a result of which the substance is transported through the membrane and released on the other side of the membrane. Facilitated diffusion does not require special energy costs due to ATP hydrolysis. This feature distinguishes facilitated diffusion from active transmembrane transport.

Carrier proteins

Carrier proteins are transmembrane proteins that specifically bind the molecule of the transported substance and, by changing the conformation, carry out the transfer of the molecule through the lipid layer of the membrane. Carrier proteins of all types have specific binding sites for the transported molecule. They can provide both passive and active membrane transport.

see also

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See what "Passive transport" is in other dictionaries:

passive transport- - transfer of substances along a concentration gradient, without energy consumption (for example, diffusion, osmosis). General chemistry: textbook / A. V. Zholnin ... Chemical terms

- (from Latin transporto I transfer, move, transfer) in living organisms, includes the delivery of the necessary compounds to certain organs and tissues (using the circulatory system in animals and the conducting system in plants), their absorption by cells and ... ... Biological encyclopedic dictionary

Membrane transport The transport of substances across the cell membrane into or out of the cell by various mechanisms of simple diffusion, facilitated diffusion, and active transport. The most important property of biological ... ... Wikipedia

The material exchange between the nucleus and the cytoplasm of the cell is carried out through the nuclear pores of the transport channels penetrating the two-layer nuclear envelope. The transition of macromolecules from the nucleus to the cytoplasm and vice versa is called nuclear ... ... Wikipedia

The transfer of a substance through a cell or intracellular membrane (transmembrane A.t.) or through a layer of cells (transcellular A.t.), flowing against a concentration gradient from an area of ​​​​low concentration to an area of ​​\u200b\u200bhigh concentration, i.e. at a cost ... ... Wikipedia

The transport function of proteins is the participation of proteins in the transfer of substances into and out of cells, in their movements within cells, as well as in their transport by blood and other fluids throughout the body. There are different types of transport that are carried out with ... ... Wikipedia

The exchange of substances between the nucleus and the cytoplasm of the cell is carried out through the nuclear pores of the transport channels penetrating the two-layer nuclear envelope. The transition of molecules from the nucleus to the cytoplasm and vice versa is called nuclear ... ... Wikipedia

The material exchange between the nucleus and the cytoplasm of the cell is carried out through the nuclear pores of the transport channels penetrating the two-layer nuclear envelope. The transition of macromolecules from the nucleus to the cytoplasm and vice versa is called nuclear ... ... Wikipedia

The material exchange between the nucleus and the cytoplasm of the cell is carried out through the nuclear pores of the transport channels penetrating the two-layer nuclear envelope. The transition of macromolecules from the nucleus to the cytoplasm and vice versa is called nuclear ... ... Wikipedia

Books

• Physiology and molecular biology of cell membranes, A. G. Kamkin, I. S. Kiseleva. The textbook presents modern concepts of electrophysiology and molecular biology of cell membranes. The issues of molecular organization of biological membranes, passive…

The exchange of a cell with the external environment by various substances and energy is a vital condition for its existence.

To maintain the constancy of the chemical composition and properties of the cytoplasm in conditions where there are significant differences in the chemical composition and properties of the external 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 a transmembrane difference in the concentrations of mineral ions. Na + , K + , Ca 2+. The cell spends a significant part of its metabolic energy on maintaining the concentration gradients of these ions. The energy of electrochemical potentials stored in ionic gradients ensures the constant readiness of the cell's plasma membrane 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 in, and launches.

Rice. Classification of transport types

To understand the mechanisms of the passage of substances through cell membranes, it is necessary to take into account both the properties of these substances and the properties of membranes. Transported substances differ in molecular weight, transferred charge, solubility in water, lipids, and a number of other properties. Plasma and other membranes are represented by vast 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 cell membranes is necessary. The transport of molecules of polar substances becomes more 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 consumption (Fig. 1).

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

For the transmembrane movement of macromolecular compounds, supramolecular particles and cell components that are unable to penetrate through membrane channels, special mechanisms are used - phagocytosis, pinocytosis, exocytosis, and 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 in them and energy consumption. There are passive and active transport across cell membranes.

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

active transport- the transfer of substances through a biomembrane against a gradient and with energy consumption. In humans, 30-40% of all energy generated during 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 understand the transfer of a substance through membranes along various kinds of gradients (electrochemical potential, substance concentration, electric field, osmotic pressure, etc.), which does not require a 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 the concentrations of the substance in the media separated by the 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 by both the difference in concentrations, 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 the concentration and electrical gradients on the membrane determines the magnitude of the electrochemical gradient.

simple diffusion 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 it is approximately 12 times less. This concentration difference (gradient) creates a driving force that ensures the transition of sodium from plasma to red blood cells. However, the rate of such a transition is low, since the membrane has a very low permeability for Na + ions. The permeability of this membrane for potassium is much greater. The energy of cellular metabolism is not spent on the processes of simple diffusion.

The rate of simple diffusion is described by the Fick equation:

dm/dt = -kSΔC/x,

where dm/ dt- the amount of substance diffusing per unit of time; to - diffusion coefficient characterizing the permeability of the membrane for a diffusing substance; S- diffusion surface area; ∆C is the difference in the concentrations of the substance on both sides of the membrane; X is the distance between diffusion points.

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 permeability of the membrane for a given substance, and the diffusion surface area.

It is obvious that the most easy 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 the physical properties of the membrane and, in particular, its permeability to the substance. For example, Na+ ions, whose concentration 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 rate of diffusion 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 permeability of the membrane at rest for K+ ions is higher than for Na+ ions.

Since the hydrocarbon radicals of the phospholipids that form the bilayer of the membrane have hydrophobic properties, substances of a hydrophobic nature, in particular, readily soluble in lipids (steroid, thyroid hormones, some narcotic substances, etc.), can easily diffuse through the membrane. 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 packing defects in the membrane of phospholioid 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 the tooth through its enamel. In this case, the conditions for the implementation of diffusion remain the same as through cell membranes. For example, for the diffusion of oxygen, glucose, mineral ions from saliva into the tissues of the tooth, their concentration in saliva must exceed the concentration in the tissues of the tooth.

Under normal conditions, non-polar and small electrically neutral polar molecules can pass in significant quantities through the phospholipid bilayer by simple diffusion. The 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 the term "diffusion" is often used instead of the term transport of a substance across a membrane.

Lightweight diffusion, as well as simple “diffusion” of a substance, is carried out along its concentration gradient, but unlike simple diffusion, a specific protein molecule, a carrier, is involved in the transfer of a substance through the membrane (Fig. 2).

Facilitated diffusion- This is a type of passive transfer of ions through biological membranes, which is carried out along a concentration gradient with the help of a carrier.

The transfer of a substance with the help of a carrier protein (transporter) is based on the ability of this protein molecule to integrate into the membrane, penetrating it and forming channels filled with water. The carrier can reversibly bind to the transferred 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 transported substance, its binding sites are turned inward and it forms a pore that is open to one side of the membrane.

Rice. 2. Facilitated diffusion. Description in text

Having contacted the substance, the carrier protein changes its conformation and passes into the state 6 . During this conformational transformation, the carrier loses its affinity for the transferred substance, it is released from its bond with the carrier and is transferred to a pore on the other side of the membrane. After that, the protein again returns to state a. This transport of a substance by a transporter protein across a membrane is called uniport.

Through facilitated diffusion, low molecular weight substances such as glucose can be transported from interstitial spaces to cells, from blood to the brain, some amino acids and glucose from primary urine can be reabsorbed into the blood in the renal tubules, amino acids and monosaccharides can be absorbed from the intestine. The rate 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 rate of transfer of a substance during facilitated diffusion increases in proportion to the increase in the difference in the concentrations of a substance up to a certain maximum value, above which it does not increase, despite the increase in the difference in concentrations of the substance along both sides of the membrane. The achievement of the maximum rate (saturation) of the transfer in the process of facilitated diffusion is explained by the fact that at the maximum rate, all carrier protein molecules are involved in the transfer.

exchange diffusion- with this type of transport of substances, an 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 variation 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 Ca2+ ions from cells is to exchange them for extracellular Na+ ions. For every three ions of incoming Na+, one Ca2+ ion is removed from the cell. An interdependent (coupled) 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 an excess amount of Ca 2+ ions, which is a necessary condition 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 concentration gradients of the substance, and it uses the energy of ATP or other types of energy, the creation of which ATP was spent earlier. If the direct source of this energy is ATP, then such a transfer is called primary active. If the transfer uses energy (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 conjugated. An example of coupled, 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 substance gradients can be overcome. As an example of the operation of primary active transport, we can consider the operation of the Na + -, K + - pump.

The active transfer of Na + and K + ions is provided by a protein-enzyme - Na + -, K + -ATP-ase, capable of splitting ATP.

Protein Na K -ATPase 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 + -ATPase 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 attach to its three high-affinity binding sites that are turned inside the cell. The addition of the Na ion stimulates ATPase activity, and as a result of ATP hydrolysis, Na+ -, K+ -ATPase 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, having been released from the bond with the protein, it is transferred to the extracellular space. In the S2 conformational state, 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 centers to 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 an uneven distribution of Na+ and K+ ions in the cell and intercellular medium in the resting cell and, as a consequence, to maintain a relatively constant potential difference across the membrane of excitable cells.

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

The substance strophanthin (ouabain), isolated 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 contractile cardiomyocytes are observed. This leads to an increase in myocardial contraction. The drug is used to treat insufficiency of the pumping function of the heart.

In addition to Na "-, K + -ATPase, there are several more types of transport ATPases, or ion pumps. Among them are a pump that transports hydrogen runs (mitochondria of cells, epithelium of the renal tubules, parietal cells of the stomach); calcium pumps (pacemaker and contractile cells of the heart, muscle cells of striated and smooth muscles). storage facilities (cistern, longitudinal tubules of the sarcoplasmic reticulum).

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

secondary active transport is 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.

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

Antiport 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 mechanism of transfer is the work of the previously mentioned Na + -, Ca 2+ - exchanger in cardiomyocytes, K + -, H + - exchange mechanism in the epithelium of the renal tubules.

From the above examples it can be seen that secondary active transport is carried out by using the gradient forces of Na+ ions or K+ ions. The Na + ion or K ion moves through the membrane towards its lower concentration and pulls another substance 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 membrane carrier protein 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. For the implementation of such transport, it is necessary that the concentration of the Na + ion outside the cell be much higher than inside, which is ensured by the constant work of Na +, K + - ATP-ase and the expenditure of metabolic energy.

Passive transport includes simple and facilitated diffusion - processes that do not require energy expenditure. Diffusion is the transport of molecules and ions across a membrane from an area of ​​high concentration to an area of ​​low concentration. Substances move along a concentration gradient. The diffusion of water across semipermeable membranes is called osmosis. Water is also able to pass through membrane pores formed by proteins and transport molecules and ions of substances dissolved in it. The mechanism of simple diffusion carries out the transfer of small molecules (for example, O2, H2O, CO2); this process is of little specificity and proceeds at a rate proportional to the concentration gradient of transported molecules on both sides of the membrane. Facilitated diffusion occurs through channels and/or carrier proteins that are specific for the molecules being transported. The ion channels are transmembrane proteins that form small water pores through which small water-soluble molecules and ions are transported along the electrochemical gradient. Carrier proteins are also transmembrane proteins that undergo reversible conformational changes that ensure the transport of specific molecules across the plasmalemma. They function in the mechanisms of both passive and active transport.

active transport is an energy-intensive process due to which the transfer of molecules is carried out with the help of carrier proteins against an electrochemical gradient. An example of a mechanism that provides 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. This difference in ion concentrations is ensured by the operation of the (Na * -K *> pump. To maintain this concentration, three Na ions are transferred from the cell for every two K * ions into the cell. This process involves a protein in the membrane that acts as an enzyme that breaks down ATP, releasing the energy needed to run the pump.
The participation of specific membrane proteins in passive and active transport indicates the high specificity of this process. This mechanism maintains the constancy of the cell volume (by regulating osmotic pressure), as well as the membrane potential. 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 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) a change in membrane potential, (b) mechanical action (for example, in the hair cells of the inner ear), (c) binding of a ligand (signaling molecule or ion).

Transport across the membrane of small molecules.

Membrane transport may involve the unidirectional transport of molecules of a substance or the joint transport of two different molecules in the same or opposite directions.

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 the gases dissolved in it have the maximum penetrating power. One of the most important properties of the plasma membrane is associated with the ability to pass various substances into or out of the cell. This is necessary to maintain the constancy of its composition (i.e. homeostasis).

Ion transport.

Unlike artificial bilayer lipid membranes, natural membranes, and primarily the plasma membrane, are still capable of transporting ions. The permeability for ions is small, and the speed of passage of different ions is not the same. Higher transmission speed for cations (K+, Na+) and much lower for anions (Cl-). The transport of ions through the plasmalemma occurs due to the participation in this process of membrane transport proteins - permeases. These proteins can transport 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). For example, glucose can enter cells symportally together with the Na+ ion. Ion transport can take place along the concentration gradient- passively without additional energy consumption. For example, the Na+ ion enters the cell from the external environment, where its concentration is higher than in the cytoplasm.

It would seem that the presence of protein transport channels and carriers 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.

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 +20C, 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 active transport, and it is carried out with the help of protein ion pumps. The plasma membrane contains a two-subunit molecule (K + + Na +)-pump, which is also an ATPase. During operation, 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 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. As a result of active transport with the help of membrane pumps, the concentration of divalent cations Mg2+ and Ca2+ is also regulated in the cell, also with the consumption of ATP.

Thus, the active transport of glucose, which symportically (simultaneously) enters the cell along with the flow of the passively transported Na+ ion, will depend on the activity of the (K+ + Na+) pump. If this (K + -Na +) - 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 is created, the diffuse flow of Na + immediately increases and at the same time the transport of glucose. 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 involved 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.

And active transport. Passive transport occurs without energy consumption along an electrochemical gradient. Passive ones include diffusion (simple and facilitated), osmosis, filtration. Active transport requires energy and occurs in spite of a concentration or electrical gradient.
active transport
This is the transport of substances in spite of the concentration or electrical gradient, which occurs with energy costs. There are primary active transport, which requires the energy of ATP, and secondary (the creation of ion concentration gradients on both sides of the membrane due to ATP, and the energy of these gradients is already used for transport).
Primary active transport is widely used in the body. It is involved in creating a difference in electrical potentials between the inner and outer sides of the cell membrane. With the help of active transport, various concentrations of Na +, K +, H +, SI "" and other ions are created in the middle of the cell and in the extracellular fluid.
The transport of Na+ and K+ - Na+,-K+-Hacoc has been studied better. This transport occurs with the participation of a globular protein with a molecular weight of about 100,000. The protein has three Na + binding sites on the inner surface and two K + binding sites on the outer surface. There is a high activity of ATPase on the inner surface of the protein. The energy generated during ATP hydrolysis leads to conformational changes in the protein and, at the same time, three Na + ions are removed from the cell and two K + ions are introduced into it. With the help of such a pump, a high concentration of Na + in the extracellular fluid and a high concentration of K + - in the cell.
Recently, Ca2+ pumps have been intensively studied, due to which the Ca2+ concentration in the cell is tens of thousands of times lower than outside it. There are Ca2 + pumps in the cell membrane and in cell organelles (sarcoplasmic reticulum, mitochondria). Ca2+ pumps also function at the expense of the carrier protein in the membranes. This protein has a high ATPase activity.
secondary active transport. Due to the primary active transport, a high concentration of Na + is created outside the cell, conditions arise for the diffusion of Na + into the cell, but together with Na +, other substances can enter it. This transport "is directed in one direction, is called symporta. Otherwise, the entry of Na + stimulates the exit of another substance from the cell, these are two flows directed in different directions - an antiport.
An example of a symport would be the transport of glucose or amino acids along with Na+. The carrier protein has two sites for Na + binding and for glucose or amino acid binding. Five separate proteins have been identified to bind five types of amino acids. Other types of symport are also known - transport of N + together with into the cell, K + and Cl- from the cell, etc.
In almost all cells, there is an antiport mechanism - Na + enters the cell, and Ca2 + leaves it, or Na + - into the cell, and H + - out of it.
Mg2 +, Fe2 +, HCO3- and many other substances are actively transported through the membrane.
Pinocytosis is one of the types of active transport. It lies in the fact that some macromolecules (mainly proteins, the macromolecules of which have a diameter of 100-200 nm) are attached to the membrane receptors. These receptors are specific for different proteins. Their attachment is accompanied by the activation of the contractile proteins of the cell - actin and myosin, which form and close the cavity with this extracellular protein and a small amount of extracellular fluid. This creates a pinocytic vesicle. It secretes enzymes that hydrolyze this protein. Hydrolysis products are absorbed by cells. Pinocytosis requires the energy of ATP and the presence of Ca2+ in the extracellular environment.
Thus, there are many modes of transport of substances across cell membranes. Different types of transport can occur on different sides of the cell (in the apical, basal, and lateral membranes). An example of this would be the processes that take place in