In the previous paragraphs, the characteristic features of molecular thermal motion, thermal equilibrium and the processes that occur when thermal equilibrium is disturbed (radiation, thermal conductivity and convection) were considered. All this still does not, however, give a complete picture of molecular thermal motion. We must turn to the phenomenon of diffusion - to a phenomenon that obliges us to move from the concept of thermal equilibrium to the concept of thermodynamic equilibrium.

Diffusion is the process of gradual mutual penetration of two substances that border on each other due to the chaotic movement of molecules. One of the first experiments on the study of diffusion was made by the German physicist Loschmidt. He took two glass tubes, closed at one end, about half a meter long and 2.5 cm in diameter; he filled one tube with carbon dioxide, and the other with hydrogen, and placed them in a vertical position so that the open ends of the tubes touched; In this case, the tube with carbon dioxide was at the bottom (the latter was necessary so that the mixing of both gases occurred only due to molecular movements, and not due to the different gravity of these gases). The contents of the tubes were examined after half an hour; it turned out that 37% of carbon dioxide penetrated into the upper tube from the lower one.

If the gas molecules did not collide at all, then, due to their high velocities, they would already run considerable distances in a straight line in a small part of a second. Therefore, the process of mixing two gases in contact with each other would proceed extremely quickly. Loschmidt's experiment shows that in reality the diffusion of a gas does not occur particularly rapidly. This can already be seen from everyday phenomena: for example, if in one corner of the room

a bottle of perfume is opened, and if the air in the room is at macroscopic rest, then it will take a long time before we feel the appearance of the smell of perfume in the opposite corner of the room.

The comparative slowness of the diffusion process is the result of molecular collisions, as a result of which the molecule can be thrown back in the direction from which it came. We know that as a result of collisions, the molecule describes an extremely intricate zigzag trajectory; for 1 sec. it will follow this trajectory for several hundred meters, and yet it may be very close to the starting position. Therefore, the process of diffusion of gases proceeds the slower, the greater the number of collisions experienced by the molecule per second, or, in other words, the smaller the mean free path of the molecule.

Two adjoining gases always diffuse into each other (except if they instantly combine chemically). This cannot be said without some reservations about liquids. Two liquids diffuse into each other indefinitely only if they are able to mix with each other. Therefore, it is possible, for example, to observe the mutual diffusion of water and alcohol, water and ether, kerosene and vegetable oil. But there are liquids that do not completely mix with each other. When such liquids merge, diffusion is first observed, but when a certain amount of the first liquid dissolves in the second and a certain amount of the second liquid dissolves in the first, then diffusion stops and no matter how long these solutions are in contact, their chemical composition no longer changes (thermodynamic equilibrium sets in , § 98). Some liquids are so slightly soluble in each other that diffusion of one liquid into another is practically not observed (for example, water and mercury).

Diffusion of liquids is observed especially easily if one of the liquids is colorless and the other is colored. You can use, for example, water and a solution of copper sulfate in water. The glass cylinder is filled halfway with water, and then, using a funnel with a long tube, a heavier solution of copper sulphate is poured onto the bottom of the cylinder. The boundary between the two fluids, sharp at first, will gradually blur, but it will take several months for the two fluids to mix completely. This shows that the number of collisions experienced by a molecule in a liquid medium is many times greater than in the case of a gaseous medium. The reason for this, of course, is that a unit volume of liquids contains a much larger number of molecules than a unit volume of gas.

The law of diffusion in a liquid medium (also applicable to a gaseous medium) was found by the German physicist Fick. This law

is expressed by the formula

where is the amount of a diffusing substance (for example, copper sulphate) passing in time through an area located perpendicular to the direction in which the substance moves; c, and the concentration of the diffusing substance in two layers separated from each other at a distance finally, the diffusion coefficient. This coefficient depends on the nature of the medium, on the nature of the diffusing substance and on the conditions under which the medium and the diffusing substance are located (for liquids - on temperature, for gases - on temperature and density).

It is assumed that the concentration in a liquid or gaseous column changes uniformly along the length of the column, i.e., and that the column is in a steady state, i.e., in each section, its concentration does not change over time.

More generally, Fick's law can be expressed by the following formula:

those. the amount of a substance diffusing over a period of time through the area normal to the line I along which diffusion occurs, is proportional to the time of the area and the concentration gradient

From the above formulas, it is easy to see that the diffusion coefficient is numerically equal to the amount of diffusing substance penetrating per unit time through a unit surface, provided that the difference in concentrations on two surfaces spaced from each other by a unit length is equal to one.

It is easy to see that the dimension of the diffusion coefficient . In the absolute system of units, the diffusion coefficient is measured in For different gases under normal conditions, it has values ​​from about 0.1 to for liquids (i.e., times less than for gases).

Comparing the formula expressing Fick's law with the formulas expressing Fourier's law for heat conduction and Ohm's law for electric current, it is easy to see that all three laws are formally similar. In the case of diffusion, the concentration difference plays the same role that the temperature difference plays in the phenomenon of heat conduction and the potential difference in the phenomenon of electric current.

A rigorous experimental verification of Fick's law was carried out by N. A. Umov in 1888-1891. Umov showed that Fick's law is accurate only for cases of complete isothermality of the medium and low concentrations of solutions.

In any homogeneous substance, gaseous or liquid, the molecules of one part of the substance are incessantly diffusing into another part of the substance; this is the so-called self-diffusion. Recently self-diffusion has been investigated experimentally; for this purpose, a small amount of a radioactive variety of molecules of the same substance is introduced into a certain area of ​​\u200b\u200bthe substance and the spread of radioactive properties throughout the mass of the substance is monitored.

The self-diffusion coefficient of a gas, as was theoretically established by Maxwell, is equal to the product of one third of the average velocity of molecules and their mean free path:

This formula could be derived by the same simple reasoning that follows (in §§ 93 and 94) to derive similar formulas for the thermal conductivity and viscosity of gases. But usually in applications of physics one has to deal not with self-diffusion, but with mutual diffusion of substances. In this case, the theoretical calculation is more complicated. However, in the end it turns out that the coefficient of mutual diffusion of gases can be calculated "according to the Rule of Mixing" from the self-diffusion coefficients of both gases, namely: if the self-diffusion coefficient of the first gas, the self-diffusion coefficient of the second gas, and and - the number of molecules of each of these gases per unit volume mixture of gases in the place where we are interested in the course of mutual diffusion, then

This equation is valid only when gases diffuse into each other, being under the same pressure, in this case the diffusion flow is stationary and the total concentration of both gases in different parts of the mixture is the same and unchanged in time, i.e. Under the indicated condition, the diffusion coefficient of the first gas in the second is equal to the diffusion coefficient of the second gas in the first:

The coefficients of self-diffusion and interdiffusion depend on the density of the gas to the same extent as the free path; the free path is inversely proportional to the density of the gas (§ 89), and therefore the diffusion coefficient is inversely proportional to the density of the gas. If a

The diffusion coefficient at pressure and absolute temperature then at pressure and temperature the diffusion coefficient of the gas will be:

As for the dependence of the diffusion coefficient on the percentage composition of the gas mixture (on the ratio, then experience, in accordance with the refined theory, shows that the diffusion coefficient changes little with a change in the percentage composition of the mixture.

The coefficients of self-diffusion and interdiffusion of some gases at normal temperature and density (at are given in the tables below.

Gas self-diffusion coefficients

(see scan)

Interdiffusion coefficients of gases

(see scan)

For liquids, the diffusion coefficient has a value, as mentioned above, hundreds of thousands of times smaller than for gases. For example, the diffusion coefficient of table salt in water at 10 ° C is:

The diffusion coefficient of sugar in water is almost three times less than the indicated diffusion coefficient of table salt. The diffusion coefficient of hydrogen in water has the highest value - about

Comparing diffusion in liquids and gases, it should be noted that very large concentration gradients are often realized in liquid solutions. Therefore, the intensity of the diffusion flow in liquids often turns out to be not at all as small as might be expected, judging by the small value of the diffusion coefficient.

The phenomenon of diffusion plays an important role in nature and technology. The roots of plants capture the substances necessary for the plant from soil water due to the diffusion flow into the roots. The intensity of this diffusion flow is maintained by the fact that inside the roots the substances necessary for the plant are quickly "assimilated", i.e., chemically converted, so that the concentration of these substances at the surface of the roots is constantly reduced, which causes the diffusion of the necessary substances from the surrounding soil to the roots. . As for the substances that are useless and harmful to the plant, they are not processed by the plant into other substances, and therefore their concentration inside and near the surface of the roots quickly compares with the concentration of these substances in the surrounding soil; this stops the diffusive influx. Thus, diffusion helps the plant to “select” and extract from the soil those substances that the plant needs to build its cells.

Similarly, diffusion is used by the tissues of the digestive system of animals and humans to "select" and extract substances from food that the body needs. Food turns into a soluble state in the stomach and intestines, and the substances needed by the body diffuse through the walls of the digestive tract.

In technology, diffusion is constantly used to extract (extract) various substances, such as sugar from raw beets, tannins, dyes, and various substances in chemical industries (Chile saltpeter, caustic soda, etc.).

A. Einstein (in 1905) developed the theory of diffusion of liquids, using his equations for Brownian motion and applying the Stokes law (§ 53) to the motion of the molecules of a dissolved substance. This led Einstein to formula

where is the diffusion coefficient of the solute, is the viscosity of the solution, is the Boltzmann constant, is the absolute temperature, and is some effective radius of the diffusing substance molecule.

Einstein's formula satisfactorily determines the value for solutions of certain substances, the molecules of which are large in comparison with the molecules of the solvent.

Another formula for the diffusion coefficient of liquids will be explained in § 117.

The phenomenon of diffusion is also observed in solids. For example, when iron is heated with charcoal, the charcoal diffuses into the iron. The phenomenon of diffusion of carbon into iron is used in carburizing

(with surface carburization of iron products) in order to obtain products after hardening with a hard outer layer, but a viscous core (carburation is carried out by heating an iron or steel product in soot, charcoal or coke, or by placing the product at a temperature of 600-1000 ° in gaseous carbon monoxide).

The diffusion coefficient in solid metals is by the order of 1,000,000 times less than in liquids; therefore, diffusion in solids is called a “secular” process (nevertheless, diffusion in solid metals, consisting of individual grains of different chemical composition, significantly affects metal properties).

The first Fick equation allows you to determine the total flow j atoms through a unit surface per unit time between two adjacent planes of crystals of a lattice located at a distance Δ (fig.8.1).

Fig.8.1. The total flux j of atoms through a unit surface

per unit time between two adjacent planes 1 and 2 of the lattice crystals,

located at a distance ∆

The number of jumps of atoms in two opposite directions is equally probable, we substitute ½ into the equations of oncoming flows of atoms:

,

where is the concentration of atoms in planes 1 and 2 of the crystal lattice, respectively, at / m 3, is the average time between jumps of atoms C.

Then the total flow of atoms:

(8.1)

According to Lagrange's mean value theorem

(8.2)

Substituting equation (6.2) into (6.1), we obtain:

(8.3)

where

Proportionality factor D is called the diffusion coefficient.

The sign (-) in the equation means that in the case under consideration the total flow j and the concentration gradient of the substance are directed oppositely, i.e. diffusion goes in the direction of lower concentrations.

Sometimes the concept of the frequency of atomic jumps is introduced:

Since during the time the number of jumps , then for two directions of the x-axis

Let be the frequency of jumps of the atom to one of the nearest nodes of the crystal lattice of this type. Then the total frequency of atomic jumps ,

where To is the coordination number or the number of nearest equidistant atoms, and the diffusion coefficient

Near the melting point, the atom performs diffuse jumps on average 10 million times per second ( = 10 7 s -1).

According to A. Einstein, the diffusion path of an atom

,

and the total distance it travels in time

Taking for and near t° pl. Δ ≈ 0.3 nm, G\u003d 10 7 s -1 we get that for 100 hours (360000 s) diffusion , and

In this case, the atom is displaced from its initial position by 0.57 nm.

Diffusion coefficient depends on temperature:

where is the pre-exponential factor, which changes from 10 -6 to 10 -4 m 2 s during self-diffusion in metals.

Q is the diffusion activation energy.

where is the universal gas constant, equal to 8.31441 J / (mol K), R=KN A

N A- Avogadro's number \u003d 6.022045 * 10 23 mol -1.

Activation energy Q different metals varies from 100 to 600 kJ/mol.

8.2. Diffusion mechanisms in metals and polymers

The question of determining the diffusion mechanism is complex. Defects in the crystal lattice, especially vacancies, play a huge influence.

Possible diffusion mechanisms (Fig. 8.2):

Simple exchange (1)

Cyclic exchange (2)

Vacancy (3)

Simple interstitial (4)

Interstitial displacement mechanism

Crowdion.

The coefficient of boundary diffusion ( D) is 3-5 orders of magnitude greater than the volume diffusion coefficient.

So, any theory of diffusion (dyes in fibrous materials, components in plastics, ion exchange in ion-exchange materials, as well as particles in crystalline substances, including metals, semiconductors, oxides, ceramics, glasses, etc.) is based on the laws Fika. There are two Fick's laws - the first and the second.

Fick's first law describes quasi-stationary processes when a membrane (plate) permeable to exchanging particles separates two media (which can be liquid or gaseous) with substantially constant conditions at the interfaces. This membrane can be inert with respect to diffusible substances (for example, porous glass that separates aqueous salt solutions of various concentrations or salt composition) or active with respect to one or more diffusible components (for example, a palladium membrane that passes hydrogen through itself at high temperature from due to specific sorption processes at its boundary and practically impervious to other gases).

The equation describing Fick's first law is:

where j is the flow of matter through a unit surface, D is the diffusion coefficient (in the general case, the interdiffusion coefficient), C- concentration across the thickness of the membrane, equal to the difference in the concentrations of the transferred substance on both sides of the membrane, x- membrane thickness.

Obviously, this equation is not applicable to the zinc coating formation processes we are discussing, since the processes we are studying are nonstationary.

Fick's second law describes non-stationary processes, and it is it that must be used to describe the regularities that both metallurgists and workers of other specialties deal with when dealing with problems of mass transfer in solids.

Let's consider its action on the following example. Let us take two identical samples having a flat surface and consisting of a metal that, under the influence of neutron irradiation, is capable of creating radioactive atoms of the same nature. We irradiate one of the two samples with a neutron flux in order to create radioactivity in it, connect the irradiated and non-irradiated samples closely on the surfaces, and to speed up the process, we will maintain this composition at an elevated temperature. Due to thermal motion, radioactive atoms from one part of the sample will diffuse into its second part, and this process will be promoted the more, the higher the temperature and the longer the time of the experiment. Then we separate the samples, and in each sample we measure the radioactivity layer by layer (the technology of this type of experiment is very well developed). As a result of the experiment, the curves shown in Fig. rice. 7.38, which are appropriately processed to calculate the effective diffusion coefficients. The concentration of radioactive ions at the interface will be equal to half that in the original left sample, and the diffusion process itself will be described by the equation:

The method of processing such curves, as follows from the literature, was proposed by a physicist named Matano, and, as a rule, is called the Matano method and sometimes the Matano-Boltzmann method (probably due to the fact that the method arose as a result of the analysis of solutions of diffusion equations obtained Boltzmann, one of the great physicists of the century before last).

If the surface of the sample is in contact with some medium in liquid form, then the concentration of this medium at the interface, as a rule, remains constant, but this feature of the experiment, provided that the effective diffusion coefficient is constant, has little effect on the shape of the front in the iron sample ( fig.7.39).

For the galvanizing process, it is necessary to simulate just such a picture. In this case, the concentration of the diffusing substance at the boundary of two media is practically constant, and the diffusion of the substance into another medium will continue until it reaches a steady state.

Rice. 7.38. The shape of the diffusion front at the contact of two solid samples, in one of which (in this case on the left) radioactive atoms were created by neutron irradiation, for two values ​​of the experiment time.

Rice. 7.39

The equation of non-stationary diffusion is described, as already mentioned, by Fick's second law, which for diffusion with a constant concentration at the boundary of two phases has the following form:

where n= 2, 1, or 0 for a ball, an infinite cylinder, and an infinite plate.

For an infinite plate, the equation is:

Below are the corresponding solutions for the degree of completion of the exchange as a function of time at constant diffusion coefficients:

for the ball:

for plate:

and for an infinite cylinder:

μ are the roots of the zero-order Bessel function, Bt = π 2 F 0

N- the degree of completion of the exchange process

F 0 \u003d D * t / l 2- dimensionless parameter, where (D - diffusion coefficient, t - time, l - linear parameter)

These equations show what fraction of atoms (from the maximum possible) is accumulated in the absorbing part of the sample.

The analysis shows that the resulting curves shown in rice. 7.39, in no way resemble the typical front of zinc sorption by the iron surface, the picture of which can be seen on rice. 7.40. According to the curve obtained for rice. 7.39, the greatest thickness should have ζ - and G1-phases, and δ -phase should have an intermediate thickness (about η -phase we will talk a little later). Similar results (that is, not matching the front shown in rice. 7.39) have been found in a significant amount of research, and this is where the mind game begins.

Some begin to look for the reason that, since the body under study has a crystalline structure, the diffusion coefficients in different directions are different. Indeed, this has been proven in a number of cases on single crystals. But here's the problem: steel is a polycrystalline body, and for the galvanizing process, this can hardly explain the experimental regularities mentioned above.

Others are looking for the reason for the deviation from the theoretical dependence in the Matano method in that it is necessary to use not the concentration gradient, but the chemical potential gradient in the equation of Fick's second law. In this case, the equation becomes much more complicated, and it is not known what results - reflecting or not reflecting reality - will be obtained.

Finally, the third took a logically more correct path. In fact, during diffusion in a metal with an impurity (alloy), not one type of particles diffuses, but at least two. These two types of particles diffuse towards each other, moreover, they have different mobility. If we count the speed of their movement from some imaginary plane ( rice 7.41 ), then it will be found that after some time of the experiment this plane will move towards that part of the sample that contains faster particles ( Kirkendal effect).

Rice. 7.40.

Rice. 7.41. The essence of the Kirkendal effect. The brass plate is surrounded by a layer of electrolytically deposited copper, and molybdenum wire marks are preliminarily fixed on the border of the brass sample. As a result of keeping the sample for several hundred hours at an elevated temperature, the marks moved inside the sample.

When analyzing data on the kinetics of the formation of a zinc-iron coating on a sample, any facts are scrutinized, including the type and structure of the iron-zinc alloys formed, but no article to date has analyzed the shape of the zinc front in the coating. Meanwhile, it is the shape of the front that says a lot, and it is precisely the elucidation of the reasons for its formation that can become the key to a quantitative description of the rate of formation of iron-zinc layers. Let us pay attention to the following. In almost all studies in the low-temperature region (we did not find reliable information about the shape of the front in the high-temperature region), a breaking front shape is formed that is close to that shown in Fig. rice. 7.40. This shape does not strongly depend on the process temperature, the thickness of the resulting coating, and the presence or absence of silicon (phosphorus) in the sample. Meanwhile, there are very few processes that are characterized by such a form of the front. One of these processes is the combustion process with the rapid removal of the resulting combustion products from the surface. For a burning ball, for example, the combustion process is described by the equation:

where R- the radius of the ball before the start of combustion, r- radius of the combustion coordinate, D- diffusion coefficient.

Obviously, if we make a flat sample with protection of the side surfaces, then the combustion process will occur only on one of the surfaces without changing its real area, that is, the rate of sample thickness decrease will be proportional to time. An example of such a process is the "smoking of a cigarette" by an automatic smoker at a constant rate of air being sucked through the sample.

Meanwhile, in the vast majority of studies, an inverse square dependence of the layer formation rate (the rate of iron leaching into the melt) on time is observed, that is, the dependence is fulfilled:

However, it is necessary to carefully check the last statement before accepting it as an axiom.

On the rice. 7.42 and 7.43 data are given on the dependence of the rate of accumulation of iron in the melt on time at various temperatures. The book states that plotting these data in coordinates produces straight lines for all temperatures, except for the data at 510°C, where there is a straight line relationship. Let's check this statement.

free diffusion. Fick's equation.

Diffusion is the process of transfer of substances from an area with a high concentration to an area with a lower concentration due to the thermal movement of molecules.

The diffusion of uncharged particles decreases towards this gradient until it reaches an equilibrium state, passive transport, since it does not require the expenditure of external energy. Diffusion characteristic - the flow of matter (φ) mass transfer through the surface S perpendicular to the flow of matter per unit time φ=φ/t

The ratio of the flow of matter to the area of ​​the flux density j=φ/s

Fick's diffusion equation

j=-Ddc/dx=-DSgradC

"=" - shows the flow direction in the direction of decreasing concentration (i.e. against gradC) D-diffusion coefficient D=RT/(6πηrN_A)

For biomembranes, the distribution coefficient of the substance between the lipid layers and water is essential. Therefore j=D_k/l(C_2-C_1)

Through simple diffusion, low molecular weight hydrophobic organic substances (fatty acids) penetrate the phospholipid bilayer

Ticket number 18

Peculiarities of passive transport of ions. membrane permeability. The role of carriers and channels in the passive transport of hydrophilic substances across biological membranes. Structure and basic properties of membrane channels. Facilitated diffusion.

Electrodiffusion-diffusion of electrically charged particles (ions) under the influence of conceptual and electrical gradients. The lipid bilayer is impermeable to ions, they can only penetrate through special structures - ion channels, which are formed by integral proteins. The driving force of diffusion is not only the difference in conc. Ions inside and outside the cell, but also the difference in the EC (electro-chemical) potentials created by these ions on both sides of the membrane => the diffuse flow of ions is determined by the gradient of the EC potential. The EC potential determines the free energy of the ion and takes into account all the forces that can induce the ion to move. For a solute: μ = μ0 + R*T*lnC + z*F*φ

where μ0 is the standard chemical potential, depending on the nature of the solvent.

С - substance concentration R - gas constant T - temperature z - ion valency F - Faraday number φ - electric potential

Zav-Th ion flux density from EH potential. The U-mobility of ions, dµ/dx-EC gradient, is determined by the Theorell equation. Substituting the expression for the EC potential in the Theorell equation, one can obtain the urNerist-Planck taking into account 2x grad C, which determine the diffusion of ions. ϳ=D dc/dx-uƶFCdȹ/dx

An ion channel is an integral protein or protein complex embedded in the cell membrane. When passing through the channel, the ion experiences the action of electric fields created by charges located on the inner side of the channel.

Membrane ion channels are integral membrane proteins that form a hole in the membrane filled with water. A number of ion channels were found in the plasmalemma, which are of high specificity, allowing the movement of the duct of one type of ion. There are Na, Cl channels, each of them has selective filters that can pass only certain ions. The permeability of ion channels can change due to the presence of a gate certain groups of atoms in the composition of proteins, forms a channel. Conformational changes in the gate due to a change in the EH potential or the action of specific chemical substances that perform a signal function.

Facilitated diffusion of hydrophobic molecules. Large hydrophilic molecules (sugars, amino acids) move through the membrane. with the help of carriers. This type of transport is diffusion, since the transport in the island moves along gradC without additional energy. Another feature of facilitated diffusion is the Saturation phenomenon. The flow of the substance transported by the diffusion path grows depending on gradC only up to a certain value. Then the increase in the flow stops, since the transport system is completely occupied. The kinetics of the diffusion region is displayed by Michaelis Menten's control. ϳ=ϳ_max C_e/(C_e+K_m) KM-Michaelis constant is equal to the concentration of the island outside the flux density is equal to half the maximum.

Features of facilitated diffusion include the following:

1) the transfer of a substance with the participation of a carrier occurs much faster;

2) facilitated diffusion has the property of saturation: with an increase in concentration on one side of the membrane, the flux density of a substance increases only to a certain limit, when all carrier molecules are already occupied;

3) with facilitated diffusion, competition of transferred substances is observed in cases where different substances are transferred by the carrier; while some substances are better tolerated than others, and the addition of some substances makes it difficult to transport others; thus, among sugars, glucose is better tolerated than fructose, fructose is better than xylose, and xylose is better than arabinose, etc.;

4) there are substances that block facilitated diffusion - they form a strong complex with carrier molecules, for example, phloridzin inhibits the transport of sugars through a biological membrane.

Fick's equation

In most practical cases, the concentration C is used instead of the chemical potential. The direct replacement of µ by C becomes incorrect in the case of high concentrations, since the chemical potential is related to the concentration according to a logarithmic law. If we do not consider such cases, then the above formula can be replaced by the following:

which shows that the flux density of the substance J is proportional to the diffusion coefficient D and the concentration gradient. This equation expresses Fick's first law (Adolf Fick is a German physiologist who established the laws of diffusion in 1855). Fick's second law relates spatial and temporal changes in concentration (diffusion equation):

The diffusion coefficient D depends on the temperature. In a number of cases, in a wide temperature range, this dependence is the Arrhenius equation.

Diffusion processes are of great importance in nature:

Nutrition, respiration of animals and plants;

The penetration of oxygen from the blood into human tissues.

Passive transport

Passive transport is the transfer of substances from places with a large value of the electrochemical potential to places with its lower value.

In experiments with artificial lipid bilayers, it was found that the smaller the molecule and the less it forms hydrogen bonds, the faster it diffuses through the membrane. So, the smaller the molecule and the more fat-soluble (hydrophobic or non-polar) it is, the faster it will permeate the membrane. Diffusion of substances across the lipid bilayer is caused by a concentration gradient in the membrane. Molecules of lipid-insoluble substances and water-soluble hydrated ions (surrounded by water molecules) penetrate the membrane through lipid and protein pores. Small non-polar molecules are easily soluble and diffuse rapidly. Uncharged polar molecules at small sizes are also soluble and diffuse.

Importantly, water permeates the lipid bilayer very quickly despite being relatively insoluble in fats. This is due to the fact that its molecule is small and electrically neutral.

Osmosis is the preferential movement of water molecules across semi-permeable membranes (permeable to solute and permeable to water) from places with a lower concentration of a solute to places with a higher concentration. Osmosis is, in essence, the simple diffusion of water from places of higher concentration to places of lower water concentration. Osmosis plays an important role in many biological phenomena. The phenomenon of osmosis causes hemolysis of erythrocytes in hypotonic solutions.

So, membranes can pass water and non-polar molecules through simple diffusion.

Differences between facilitated diffusion and simple

• 1) the transfer of a substance with the participation of a carrier occurs much faster;
• 2) facilitated diffusion has the property of saturation: with an increase in concentration on one side of the membrane, the flux density of a substance increases only to a certain limit, when all carrier molecules are already occupied;
• 3) with facilitated diffusion, competition of transferred substances is observed in cases where different substances are transferred by the carrier; while some substances are better tolerated than others, and the addition of some substances makes it difficult to transport others; So among sugars, glucose is better tolerated than fructose, fructose is better than xylose, and xylose is better than arabinose, etc. etc.;
• 4) there are substances that block facilitated diffusion - they form a strong complex with carrier molecules, for example, phloridzin inhibits the transport of sugars through a biological membrane.