Passive transport is the transport of substances along a concentration gradient that does not require energy. Hydrophobic substances are passively transported through the lipid bilayer. All protein-channels and some carriers pass substances passively through themselves. Passive transport involving membrane proteins is called facilitated diffusion.

Other carrier proteins (sometimes called pump proteins) transport substances across the membrane at the expense of energy, which is usually supplied by ATP hydrolysis. This type of transport occurs against the concentration gradient of the transported 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 - the movement of two substances in different directions through one carrier.

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

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

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

Work of sodium-potassium atPase as an example of antiport and active transport

Initially, this carrier attaches three ions to the inside of the membrane Na+ . These ions change the conformation of the ATPase active site. 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 inside 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+ split off, and is replaced by two ions K+ . Then the conformation of the carrier changes to the original one, and the ions K+ appear on the inner side of the membrane. Here the ions K+ are split off, and the carrier is ready for work again.

More briefly, the actions of ATPase can be described as follows:

1) It “takes” three ions from inside the cell Na+ , then splits the ATP molecule and attaches phosphate to itself

2) "Throws out" ions Na+ and adds two ions K+ from the external environment.

3) Removes phosphate, two ions K+ throws into the cell

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

Introduction

Membrane transport - the transport of substances through the cell membrane into or out of 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 maintenance of a constant composition of the cell. This function of the cell membrane is carried out due to selective permeability, that is, the ability to let some substances through and not others.

Passive transport

Distinguish between passive 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.

Types of passive transport

Types of passive transport of substances:

• simple diffusion
• Osmosis
• ion diffusion
• Facilitated diffusion

simple diffusion

Diffusion is the process by which a gas or solute spreads and fills the entire available volume.

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

If the concentration of a 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 substances pass through the cell membrane.

Hydrophobic, highly lipid-soluble substances diffuse due to dissolution in membrane lipids. Water and substances soluble in it penetrate through temporary defects in the hydrocarbon region of the membrane, the so-called. kinks, as well as through 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 osmotic forces. At a lower concentration of a substance in the cell than in the environment, the cell shrinks; if the concentration of the solute in the cell is higher, water rushes into the cell.

Osmosis is the movement of water molecules (solvent) through a membrane from an area of ​​​​lower to an area of ​​\u200b\u200bhigher concentration of a solute. Osmotic pressure is the smallest 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 a substance.

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

The volume of cells is largely regulated 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 across the plasma membrane changes the concentration of substances in the cell and, accordingly, the osmotic pressure of its contents. If a cell secretes a 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 the cells to prevent large amounts of water from entering them. Maintaining a constant volume even in an isotonic environment requires energy consumption, 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 constantly accumulate in the cell, which disrupts the osmotic balance. The need to expend energy to maintain a constant volume is easily demonstrated in experiments with cooling or metabolic inhibitors. Under such conditions, the cells swell rapidly.

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

In general, the volume of cells that do not have rigid walls is determined by three factors:

• a) the amount of substances contained in them and incapable of penetrating through the membrane;
• b) the 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 hydrostatic pressure that prevents water from entering the cell. If there is a difference in hydrostatic pressures in two areas of the medium, water can be filtered through the pores of the barrier separating these areas.

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

Diffusion of ions

Diffusion of ions occurs mainly through specialized protein structures of the membrane - ion channels, when they are in the open state. Depending on the type of tissue, cells can have a different set of ion channels. There are sodium, potassium, calcium, sodium-calcium and chloride 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.

Ion channels can be in open, closed and inactivated states. The transition of a 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-dependent and receptor-gated. The selective permeability of an ion channel for a particular ion is determined by the presence of special selective filters at its mouth.

Facilitated diffusion

Through biological membranes, in addition to water and ions, many substances (from ethanol to complex drugs) penetrate by simple diffusion. At the same time, even relatively small polar molecules, such as 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 its concentration gradient, 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 rate of substance transfer compared to simple diffusion, dependence on the structure of its molecules, saturation, competition, and sensitivity to specific inhibitors - compounds that inhibit facilitated diffusion.

All of the above 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 the carriers are occupied by the transported molecules or ions, its further increase will not lead to an increase in the number of transported particles - the phenomenon of saturation. Substances that are similar in molecular structure and transported by the same carrier will compete for the carrier - the phenomenon of competition.

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

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

Symport, 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 co-transport);

Antiport - (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 transfer rate is controlled by all participants in the transport process.

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

Most cells have a facilitated diffusion system. However, the list of metabolites transported by this mechanism is rather limited. Basically, these are sugars, amino acids and some ions. Compounds that are intermediate products of metabolism (phosphorylated sugars, products of amino acid metabolism, macroergs) are not transported using this system. Thus, facilitated diffusion serves to transport 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.

Membrane transport - a special case of the phenomenon of the transfer of substances through a biological membrane.

Transfer events include:

ü mass transfer of matter (diffusion);

ü momentum transfer (viscosity);

ü energy transfer (thermal conductivity);

charge transfer (conductivity).

Types of membrane transport:

Passive - the transfer of molecules and ions along a chemical (or electrochemical) potential gradient or the transfer of molecules from places with a higher concentration of a substance to places with a lower concentration of a substance. This is a spontaneous process (ΔG<0 - энергия Гиббса уменьшается).

The flux density of a substance through a membrane is determined Theorell equation:

ü J - mol / (m 2 s)

ü - chemical or electrochemical potential gradient (means a change in the chemical or electrochemical potential during the transfer of a substance through a membrane with a thickness x)

ü U - coefficient of mobility of molecules.

ü C is the concentration of the substance.

Passive transport of non-electrolytes (for example, glucose) during normal diffusion is determined by Fick equation, which is derived on the basis of substitution and differentiation of the expression for the chemical potential of substances - into the Theorell equation

ü - concentration gradient of a substance (is the driving force for the transfer of a substance)

ü RTU \u003d D - diffusion coefficient - m 2 / s.

ü R - Universal gas constant.

the “-” sign indicates that the total flux density of the substance is directed in the direction of decreasing the concentration of the substance.

Passive transport of electrolytes (ions K +, Na +, Ca 2+, Mg 2+, etc.) during normal diffusion is determined the Nernst-Planck equation, which is derived on the basis of substitution and differentiation of the expression for the electrochemical potential of substances - into the Theorell equation:

ü Z - ion charge;

ü F =96500 C/mol - Faraday number.

ü φ - electric potential - V (volt);

ü - electric potential gradient;

and - are the driving forces for the transport of electrolytes in passive transport.

Types of diffusion:

ü ordinary (transfer of gas molecules O 2, CO 2, H 2 O molecules, etc.)

ü facilitated - carried out along the gradient of the chemical (electrochemical) potential with the participation of a carrier protein.

Facilitated diffusion properties:

ü The presence of the effect of saturation (the number of carrier proteins in the membrane is fixed);

ü Selectivity (for each substance its own carrier protein);

ü Sensitivity to inhibitors;

The presence of carriers changes the kinetics (speed) of transport, and it becomes similar to the equations of enzymatic catalysis, only the carrier acts as an enzyme, and the transferred substance (S) acts as a substrate:

- facilitated diffusion equation

Kt - the transport constant corresponds to the Michaelis constant and is equal to the concentration S at Js=Jmax/2.

Active transport - the transfer of substances against the gradient of the chemical ((electrochemical potential or the transfer of molecules from places with a lower concentration of a substance to places with a higher concentration of a substance. This is not a spontaneous process (ΔG> 0 - the Gibbs energy increases), it is conjugate.

Primary active transport - transport of substances associated with the reaction of ATP hydrolysis, during which energy is released, which is used to transport substances through the membrane against the chemical potential gradient.

PAT examples:

ü transport of K + and Na + in the outer cytoplasmic membranes;

ü H+ transport in mitochondria;

ü Ca 2+ transport in outer cytoplasmic membranes.

Secondary active transport - transport of substances associated with a spontaneous process of transfer of Na + ions through the membrane along the gradient of the electrochemical potential of substances.

Examples of BAT:

ü transport of sugars (amino acids) due to the energy of the gradient of the electrochemical potential of Na + ions (symport);

ü Na + - Ca 2+ - exchange is the transport of Ca 2+ ions due to the energy of the gradient of the electrochemical potential of Na + ions (antiport).

Transport ATPases of prokaryotic and eukarytic cells are divided into 3 types: P-type, V-type, F-type.

ATP-zams of the cytoplasmic membrane of this type include:

ü Na, + K + - ATPase

ü Ca 2+ - ATPase eukaryotic plasma membrane

ü H+–ATPase

Intracellular ATP-ases P-type:

Ca 2+ is the ATPase of the endo-(sarco) plasma reticulum of eukaryotes.

K+ is the ATPase of the outer membranes of prokaryotes. They are quite simple, they act like a pump.

V-type ATPases are found in membranes in yeast vacuoles, in lysosomes, endosomes, secretory granules of animal cells (H+-ATP-ases).

F-type ATPases found in bacterial membranes, chloroplasts, mitochondria.

Ion channels (uniport) are classified:

A) by type of ions: sodium, potassium, calcium and chloride channels;

B) according to the method of regulation:

1) potential-sensitive

2) chemosensitive (receptor-controlled)

3) intracellular substances (ions).

In the process of cation transfer, two main conditions (factors) must be met:

1. Steric– coincidence of the sizes of the cation and the hydration shell with the sizes of the channel.

2. Energy– interaction of the cation with carboxyl (negatively charged groups) of the channel itself.

The lipid bilayers are largely impermeable to the vast majority of substances, and therefore transport through the lipid phase requires significant energy costs.

Distinguish active transport And passive transport(diffusion).

Passive transport

Passive transport is the transfer of molecules along a concentration or electrochemical gradient, that is, it is determined only by the difference in the concentration of the transferred substance on opposite sides of the membrane or by the direction of the electric field and is carried out without the expenditure of ATP energy. Two types of diffusion are possible: simple and facilitated.

simple diffusion occurs without the participation of a membrane protein. The rate of simple diffusion is well described by the usual diffusion laws for substances soluble in the lipid bilayer; it is directly proportional to the degree of hydrophobicity of the molecule, i.e., its fat solubility, as well as to the concentration gradient. The mechanism of diffusion of water-soluble substances is less studied. The transfer of substances across the lipid bilayer, such as compounds such as ethanol, is possible through temporary pores in the membrane formed by breaks in the lipid layer during the movement of membrane lipids. By the mechanism of simple diffusion, transmembrane transport of gases is carried out (for example,

Rice. 22.5.

0 2 and C0 2), water, some simple organic ions and a number of low molecular weight fat-soluble compounds. It should be remembered that simple diffusion is non-selective and has a low speed.

Facilitated diffusion, unlike simple diffusion, it is facilitated by the participation of specific membrane proteins in this process. Consequently, facilitated diffusion is a diffusion process associated with a chemical reaction of the interaction of the transported substance with the carrier. This process is specific and proceeds at a higher rate than simple diffusion.

Two types of membrane transport proteins are known: carrier proteins, called translocases or permeases, And channel-forming proteins. Transport proteins bind specific substances and carry them through the bilayer along their concentration gradient or electrochemical potential, and, therefore, this process, as in simple diffusion, does not require ATP energy.

The specific mechanism of functioning of translocases during facilitated diffusion is not well understood. It is believed that after the binding of the transferred substance to the carrier protein, a series of conformational changes of the latter occur, allowing the bound substance to be transported from one side of the membrane to the other according to the scheme (Fig. 22.5).

Another possible variant of the transfer mechanism is according to the so-called relay type, when the transport protein is not able to cross the bilayer at all. In this case, the transported substance may itself pass from one protein to another until it is on the opposite side of the membrane.

Channel proteins (or channel proteins) form transmembrane hydrophilic channels through which solute molecules of appropriate size and charge can pass by facilitated diffusion. In contrast to the transport carried out by translocases, transport through channels does not have high specificity, but can be carried out at a much higher rate, reaching saturation over a wide range of concentrations of the transported substance (Fig. 22.6). Some canapes are permanently open, while others open only in response to the binding of the transported substance. This leads to a change in the conformation of the transport protein, as a result of which a hydrophilic channel opens in the membrane and the substance is released from the other side of the membrane (see Fig. 22.6).

Rice. 22.6.

Until now, the structure and mechanism of functioning of transport proteins have not been studied enough, which is largely due to the difficulty of isolating them in a solubilized form. Apparently, the most common way of transmembrane transport of substances by the mechanism of facilitated diffusion is transport with the help of channel-forming substances.

Rice. 22.7.

Proteins - carriers of all types, resemble enzymes associated with membranes, and the process of facilitated diffusion is an enzymatic reaction in a number of properties: 1) transport proteins are highly specific and have binding sites (sites) for the transported molecule (by analogy, a substrate); 2) when all binding sites are occupied (i.e., the protein is saturated), the transport rate reaches its maximum value, denoted U tlh(Fig. 22.7); 3) the carrier protein has a characteristic binding constant K m equal to the concentration of the transported substance, at which the transport speed is half of its maximum value (similarly K m for the enzyme-substrate system), transport proteins are sensitive to changes in the pH value of the medium; 4) they are inhibited by competitive or noncompetitive inhibitors. However, unlike an enzymatic reaction, the molecule of the transported substance does not undergo a covalent transformation when interacting with the transport protein (Fig. 22.7).

Facilitated diffusion is usually characteristic of water-soluble substances: carbohydrates, amino acids, metabolically important organic acids, and some ions. Facilitated diffusion also transports steroid hormones, a number of fat-soluble vitamins, and other molecules of this class. Practically directed flows of substances in the cell by simple and facilitated diffusion never stop, since the substances that enter the cell are involved in metabolic transformations, and their loss is constantly replenished by transmembrane transfer along the concentration gradient.

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 through 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 coupled 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 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 = n Z- 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. Thus,

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 provide 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, which varies from zero to 1. This constant, called the reflection coefficient, corresponds to the ease of passage of a 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 the rate of penetration of dye molecules into the cell using a microscope. Currently, fluorescent labels are widely used, among them sodium fluorescein, chlortetracycline, etc. 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.