Mechanisms of transport of substances through biological membranes. biological membranes. Pump for sodium and potassium

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In animals with a closed vascular system, the extracellular fluid is conventionally divided into two components:

1) interstitial fluid
2) circulating blood plasma.

Interstitial fluid is the part of the extracellular fluid that is located outside the vascular system and bathes the cells.

About 1/3 of the total body water is extracellular fluid, the remaining 2/3 is intracellular fluid.

The concentrations of electrolytes and colloids differ significantly in plasma, interstitial and intracellular fluids. The most pronounced differences are the relatively low content of anionic proteins in the interstitial fluid, compared with the intracellular fluid and blood plasma, and higher concentrations of sodium and chlorine in the interstitial fluid, and potassium in the intracellular fluid.

The unequal composition of various liquid media of the body is largely due to the nature of the barriers separating them. Cell membranes separate the intracellular fluid from the extracellular fluid, while capillary walls separate the interstitial fluid from the plasma. Transport of substances across these barriers can occur passively through diffusion, filtration and osmosis, as well as through active transport.

Passive transport

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Rice. 1.12 Types of passive and active transport of substances across the membrane.

Schematically, the main types of transport of substances through the cell membrane are shown in Fig. 1.12

Fig.1.12 Types of passive and active transport of substances through the membrane.

3 - facilitated diffusion,

Passive transfer of substances through cell membranes does not require the expenditure of metabolic energy.

Types of passive transport

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Types of passive transport of substances:

• simple diffusion
• Osmosis
• Diffusion of ions
• Facilitated diffusion

simple diffusion

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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 last event is high, the membrane is said to permeable to thissubstances.

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. They pass through the cell membrane as highly soluble in water. (hydrophilic) substances, and hydrophobic, poorly or completely insoluble in it.

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. kinky, and also through pores, permanently existing hydrophilic regions 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

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

Osmotic pressure called the smallest pressure that must be applied to the solution in order to prevent the solvent from flowing through the membrane into a solution with a higher concentration of the substance.

Solvent molecules, like the molecules of any other substance, are set in motion by 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 refrigeration 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 for this sodium pump(see below).

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

1) the amount of substances contained in them and incapable of penetrating through the membrane;
2) the concentration in the interstitium of compounds that can pass through the membrane;
3) 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.

Filtration phenomena 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

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Diffusion of ions occurs mainly through specialized protein structures of the membrane - ion kacash, when they are open. Depending on the type of tissue, cells can have a different set of ion channels.

Distinguish between 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 may be in open, closed and inactivated states. The transition of the channel from one state to another is controlled either by a change in the electrical potential difference across the membrane, or by the interaction of physiologically active substances with receptors.

Accordingly, ion channels are divided into potential dependent and receptor-driven. 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

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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 through the membrane of most cells due to simple diffusion. They are transferred through facilitated diffusion.

Diffusion is called light substances along its concentration gradient, which is carried out with the participation of special protein carrier molecules.

Transport Na + , K + , Cl - , Li + , Ca 2+ , HCO 3 - and H + can also carry out 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 carriers are occupied by the transported molecules or ions, its further increase will not lead to an increase in the number of transported particles - saturation phenomenon. Substances that are similar in molecular structure and transported by the same carrier will compete for the carrier - competition phenomenon.

There are several types of transport of substances through facilitated diffusion (Fig. 1.13):

Rice. 1.13 Classification of methods of transport through the membrane.

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 + / Ca 2+, Na + / H + Cl - / HCO 3 - - exchanges).

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

The nature of the carrier proteins is unknown. According to the principle of 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.

active transport

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active transport carried out by transport adenosine triphosphatases (ATPases) and occurs due to the energy of ATP hydrolysis.

Figure 1.12 shows the types of passive and active transport of substances through the membrane.

1,2 - simple diffusion through the bilayer and ion channel,
3 - facilitated diffusion,
4 - primary active transport,
5 - secondary active transport.

Types of active transport

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Types of active transport of substances:

primary active transport,

secondary active transport.

primary active transport

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The transport of substances from a medium with a low concentration to a medium with a higher concentration cannot be explained by movement along a gradient, i.e. diffusion. This process is carried out due to the energy of ATP hydrolysis or energy due to the concentration gradient of any ions, most often sodium. If the source of energy for the active transport of substances is the hydrolysis of ATP, and not the movement of some other molecules or ions through the membrane, transport calledprimary active.

The primary active transfer is carried out by transport ATPases, which are called ion pumps. In animal cells, the most common Na +, K + - ATPase (sodium pump), which is an integral protein of the plasma membrane and Ca 2+ - ATPase, contained in the plasma membrane of the sarco-(endo)-plasmic reticulum. All three proteins have a common property - the ability to be phosphorylated and form an intermediate phosphorylated form of the enzyme. In the phosphorylated state, the enzyme can be in two conformations, which are commonly referred to as E 1 and E 2 .

Enzyme conformation - this is a way of spatial orientation (laying) of the polypeptide chain of its molecule. These two conformations of the enzyme are characterized by different affinities for transported ions, i.e. different ability to bind transported ions.

Na + /K + - ATPase provides conjugated active transport of Na + from the cell and K + into the cytoplasm. In the Na + /K + - ATPase molecule, there is a special area (site) in which the binding of Na and K ions occurs. With the conformation of the enzyme E 1, this area is turned inside the plasma reticulum. For the implementation of this stage of the conversion of Ca 2+ -ATPase, the presence of magnesium ions in the sarcoplasmic reticulum is necessary. Subsequently, the cycle of the enzyme is repeated.

secondary active transport

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secondary active transport is the transfer of a substance across the membrane against its concentration gradient due to the energy of the concentration gradient of another substance created in the process of active transport. In animal cells, the main source of energy for secondary active transport is the energy of the sodium ion concentration gradient, which is created due to the work of Na + /K + - ATPase. For example, the cell membrane of the mucous membrane of the small intestine contains a protein that carries out the transfer (symport) of glucose and Na + to epitheliocytes. Glucose transport is carried out only if Na +, simultaneously binding with glucose to the specified protein, is transferred along the electrochemical gradient. The electrochemical gradient for Na+ is maintained by the active transport of these cations out of the cell.

In the brain, the work of Na + -pump is associated with reverse absorption (reabsorption) of mediators - physiologically active substances that are released from nerve endings under the action of excitatory factors.

In cardiomyocytes and smooth muscle cells, the functioning of Na + , K + -ATPase is associated with the transport of Ca 2+ through the plasma membrane, due to the presence in the cell membrane of a protein that carries out countertransport (antiport) of Na + and Ca 2+ . Calcium ions are transported across the cell membrane in exchange for sodium ions and due to the energy of the concentration gradient of sodium ions.

A protein was found in cells that exchanges extracellular sodium ions for intracellular protons - Na + /H + - exchanger. This carrier plays an important role in maintaining a constant intracellular pH. The rate at which Na + /Ca 2+ and Na + /H + - exchange is carried out is proportional to the electrochemical Na + gradient across the membrane. With a decrease in the extracellular concentration of Na + inhibition of Na + , K + -ATPase by cardiac glycosides or in a potassium-free environment, the intracellular concentration of calcium and protons is increased. This increase in intracellular concentration of Ca 2+ with inhibition of Na + , K + -ATPase underlies the use of cardiac glycosides in clinical practice to enhance heart contractions.

The exchange of substances between the cell and its environment occurs constantly. The mechanisms of transport of substances into and out of the cell depend on the size of the transported particles. Small molecules and ions are transported by the cell directly across the membrane in the form of passive and active transport.

Passive transport carried out without energy expenditure, along the concentration gradient by simple diffusion, filtration, osmosis or facilitated diffusion.

Diffusion – penetration of substances through the membrane along the concentration gradient; diffuse transport of substances (water, ions) is carried out with the participation of integral membrane proteins, which have molecular pores, or with the participation of the lipid phase (for fat-soluble substances).

Facilitated diffusion - transfer with the help of special carrier proteins (permeases), which selectively bind to one or another ion or molecule and carry them across the membrane. In this case, the particles move faster than with conventional diffusion.

Osmosis - the entry of water into the cells from a hypotonic solution.

Active transport consists in the movement of substances against a concentration gradient with the help of transport proteins (porins, ATP-ases, etc.), which form diaphragm pumps, with the expenditure of ATP energy (potassium-sodium pump, regulation of the concentration of calcium and magnesium ions in cells, the intake of monosaccharides, nucleotides, amino acids).

The transfer of macromolecules and larger particles occurs by pinocytosis and phagocytosis due to the ability of the cell membrane to form protrusions. The edges of these protrusions close, capturing the liquid surrounding the cell (pinocytosis) or solid particles (phagocytosis) and formed bubbles surrounded by a membrane.

pinocytosis - one of the main ways of penetration into the cell of macromolecular compounds. The resulting pinocytic vacuoles range in size from 0.01 to 1-2 microns. Then the vacuole plunges into the cytoplasm and laces off. At the same time, the wall of the pinocytic vacuole completely retains the structure of the plasma membrane that gave rise to it. Pinocytosis and phagocytosis are fundamentally similar processes in which four phases can be distinguished: the intake of substances by pino- or phagocytosis, their cleavage under the action of enzymes secreted by lysosomes, the transfer of cleavage products into the cytoplasm (due to changes in the permeability of vacuole membranes) and the release of metabolic products.

Depending on the type and direction of transport, there are endocytosis (transfer into the cell by direct pino or phagocytosis) and exocytosis (transfer from the cell by reverse pino - or phagocytosis).

6. CYTOPLASMA, ITS STRUCTURE, CHEMICAL COMPOSITION.

Cytoplasm - an essential component of the cell. Complex and diverse processes of synthesis, respiration, growth take place in it; the phenomena of irritability and heredity are inherent in it, i.e. all those properties that characterize life.

The cytoplasm is a viscous transparent colorless mass with a specific gravity of 1.04 - 1.06. Light refracts a little more than water. The cytoplasm is elastic, elastic, does not mix with water. In many cells, its movement can be observed: in cells with one large central vacuole - circular (cyclosis), in cells with many vacuoles and strands of cytoplasm between them - striated. The current of the cytoplasm involves the movement of cellular organelles.

The cytoplasm is differentiated into a structureless mass - hyaloplasm and formed formations - cellular organelles. Hyaloplasm (cytoplasmic matrix) - a complex colloidal system formed by proteins, nucleic acids, carbohydrates, water and other substances. Depending on the physiological state and the impact of the external environment, hyaloplasm can be in the form of a sol (liquid) or gel (more elastic dense substance). Hyaloplasm is the internal environment of the cell, where reactions of intracellular metabolism take place.

In the hyaloplasm of cells, between the nuclear membrane and the cytoplasmic membrane, there is a cytoskeleton. It is formed by a developed network of filaments (protein tubes): microfilaments (6–8 nm) formed by the actin protein; intermediate fibers (10 nm) consisting of various fibrillar proteins (cytokeratins, etc.); microtubules (about 25 nm) built from tubulin and able to contract. The cytoskeleton determines the shape of the cell, participates in various movements of the cell itself (during division) and in the intracellular movement of organelles and individual compounds.

Hyaloplasm functions:

1) is the internal environment of the cell, in which many chemical processes take place;

2) unites all cellular structures and provides chemical interaction between them;

3) determines the location of organelles in the cell;

4) provides intracellular transport of substances (amino acids, sugars, etc.) and the movement of organelles (the movement of chloroplasts in plant cells);

5) is a zone of movement of ATP molecules;

6) determines the shape of the cell.

Cytoplasm is a complex chemical multicomponent system containing 75-86% water, 10-20% proteins, 2-3% lipids, 1-2% carbohydrates, 1% mineral salts. This is the total and approximate composition of the cytoplasm, which does not reflect the complexity of its chemical structure.

The cytoplasm in the dissolved state contains a large amount of free amino acids and nucleotides, many intermediate products that arise during the synthesis and breakdown of molecules. A large number of ions Na + , K + , Mg 2+ , Cl - , HCO 3 2- , HPO 4 2- and others are also found.

Similar information.

Passive transport includes simple and facilitated diffusion - processes that do not require energy expenditure. Diffusion- transport of molecules and ions through the membrane from an area with a high to an area with a low concentration, i.e. Substances move along a concentration gradient. Diffusion of water across semipermeable membranes is called osmosis. Water is also able to pass through membrane pores formed by proteins and carry molecules and ions of substances dissolved in it. The mechanism of simple diffusion is the transfer of small molecules (for example, 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 is carried out through channels and (or) carrier proteins that have specificity in relation to the transported molecules. 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 the 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 actual transport system and a gate mechanism that opens the channel for some time in response to a change in the membrane potential, (b) mechanical action (for example, in the hair cells of the inner ear), binding of a ligand (signal molecule or ion).

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

• Uniport - transport of one substance in one direction depending on the gradient
• Symport is the transport of two substances in the same direction through one carrier.
• Antiport is 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. 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+ split off, and the carrier is ready for work again

Transport of substances:

Transfer of substances through biol. The membrane is associated with such important biological phenomena as intracellular homeostasis of ions, bioelectric potentials, excitation and conduction of a nerve impulse, storage and transformation of energy.

There are several types of transport:

1 . Uniport- this is the transport of a substance through the membrane, regardless of the presence and transfer of other compounds.

2. Contransport- this is the transfer of one substance associated with the transport of another: symport and antiport

a) where a unidirectional transfer is called symport - absorption of amino acids through the membrane of the small intestine,

b) oppositely directed - antiport(sodium-potassium pump).

The transport of substances can be - passive and active transport (transfer)

Passive transport is not associated with energy costs, it is carried out by diffusion (directed movement) along concentration (from mac towards min), electric or hydrostatic gradients. Water moves along the water potential gradient. Osmosis is the movement of water across a semi-permeable membrane.

active transport carried out against gradients (from min to mac), is associated with energy consumption (mainly the energy of ATP hydrolysis) and is associated with the work of specialized membrane carrier proteins (ATP synthetase).

Passive transfer can be carried out:

a. By simple diffusion through the lipid bilayers of the membrane, as well as through specialized formations - channels. By diffusion through the membrane penetrate into the cell:

uncharged molecules, highly soluble in lipids, incl. many poisons and medicines,

gases- oxygen and carbon dioxide.

ions- they enter through the penetrating channels of the membrane, which are lipoprotein structures. They serve to transport certain ions (for example, cations - Na, K, Ca, anions Cl, P,) and can be in an open or closed state. The conductance of the channel depends on the membrane potential, which plays an important role in the mechanism of generation and conduction of a nerve impulse.

b. Facilitated diffusion . In some cases, the transfer of matter coincides with the direction of the gradient, but significantly exceeds the speed of simple diffusion. This process is called facilitated diffusion; it occurs with the participation of carrier proteins. The process of facilitated diffusion does not require energy. In this way, sugars, amino acids, nitrogenous bases are transported. Such a process occurs, for example, when sugars are absorbed from the intestinal lumen by epithelial cells.

in. Osmosis – movement of the solvent through the membrane

active transport

The transfer of molecules and ions against the electrochemical gradient (active transport) is associated with significant energy costs. Often the gradients reach large values, for example, the concentration gradient of hydrogen ions on the plasma membrane of the cells of the gastric mucosa is 106, the concentration gradient of calcium ions on the membrane of the sarcoplasmic reticulum is 104, while the ion fluxes against the gradient are significant. As a result, energy costs for transport processes reach, for example, in humans, more than 1/3 of the total energy of metabolism.

Active ion transport systems have been found in the plasma membranes of cells of various organs, for example:

sodium and potassium - sodium pump. This system pumps sodium out of the cell and potassium into the cell (antiport) against their electrochemical gradients. The transfer of ions is carried out by the main component of the sodium pump - Na +, K + -dependent ATP-ase due to ATP hydrolysis. For every hydrolyzed ATP molecule, three sodium ions and two potassium ions are transported. .

There are two types of Ca 2 + -ATP-az. One of them ensures the release of calcium ions from the cell into the intercellular environment, the other - the accumulation of calcium from the cellular contents into the intracellular depot. Both systems are able to create a significant calcium ion gradient.

K+, H+-ATPase was found in the mucous membrane of the stomach and intestines. It is able to transport H+ across the membrane of mucosal vesicles during ATP hydrolysis.

Anion-sensitive ATP-ase was found in microsomes of the frog stomach mucosa, capable of antiporting bicarbonate and chloride upon ATP hydrolysis.

Proton pump in mitochondria and plastids

secretion of HCI in the stomach,

uptake of ions by plant root cells

Violation of membrane transport functions, in particular, an increase in membrane permeability, is a well-known universal sign of cell damage. More than 20 so-calledtransport diseases, among which:

renal glycosuria,

cystinuria,

malabsorption of glucose, galactose and vitamin B12,

hereditary spherocytosis (hemolytic anemia, erythrocytes are spherical, while the membrane surface decreases, lipid content decreases, membrane permeability to sodium increases. Spherocytes are removed from the bloodstream faster than normal erythrocytes).

In a special group of active transport, the transfer of substances (large particles) is distinguished by - andendo- andexocytosis.

Endocytosis(from the Greek. endo - inside) the entry of substances into the cell, includes phagocytosis and pinocytosis.

Phagocytosis (from the Greek Phagos - devouring) is the process of capturing solid particles, foreign living objects (bacteria, cell fragments) by unicellular organisms or multicellular cells, the latter are called phagocytes or devouring cells. Phagocytosis was discovered by I. I. Mechnikov. Usually, during phagocytosis, the cell forms protrusions, cytoplasm- pseudopodia that flow around the captured particles.

But the formation of pseudopodia is not necessary.

Phagocytosis plays an important role in the nutrition of unicellular and lower multicellular animals, which are characterized by intracellular digestion, and is also characteristic of cells that play an important role in the phenomena of immunity and metamorphosis. This form of absorption is characteristic of connective tissue cells - phagocytes, which perform a protective function, actively phagocytize placental cells, cells lining the body cavity, and the pigment epithelium of the eyes.

In the process of phagocytosis, four successive phases can be distinguished. In the first (optional) phase, the phagocyte approaches the object of absorption. Here, the positive reaction of the phagocyte to the chemical stimulation of chemotaxis is essential. In the second phase, adsorption of the absorbed particle on the surface of the phagocyte is observed. In the third phase, the plasma membrane in the form of a sac envelops the particle, the edges of the sac close and detach from the rest of the membrane, and the resulting vacuole is inside the cell. In the fourth phase, the swallowed objects are destroyed and digested inside the phagocyte. Of course, these stages are not delimited, but imperceptibly pass one into another.

Cells can also absorb liquids and macromolecular compounds in a similar way. This phenomenon was called p and not ts and toz and (Greek rupo - drink and sutoz - cell). Pinocytosis is accompanied by vigorous movement of the cytoplasm in the surface layer, leading to the formation of an invagination of the cell membrane, which extends from the surface in the form of a tubule into the cell. At the end of the tubule, vacuoles form, which break off and pass into the cytoplasm. Pinocytosis is most active in cells with intensive metabolism, in particular in the cells of the lymphatic system, malignant tumors.

Through pinocytosis, macromolecular compounds penetrate into cells: nutrients from the bloodstream, hormones, enzymes and other substances, including medicinal ones. Electron microscopic studies have shown that fat is absorbed by intestinal epithelial cells through pinocytosis, phagocytic cells of the renal tubules and growing oocytes.

Foreign bodies that have entered the cell by phagocytosis or pinocytosis are exposed to lytic enzymes inside the digestive vacuoles or directly in the cytoplasm. The intracellular reservoirs of these enzymes are lysosomes.

Functions of endocytosis

carried out, nutrition(eggs absorb yolk proteins in this way: phagosomes are the digestive vacuoles of protozoa)

Protective and immune responses (leukocytes engulf foreign particles and immunoglobulins)

Transport(renal tubules absorb proteins from primary urine).

Selective endocytosis certain substances (yolk proteins, immunoglobulins, etc.) occurs upon contact of these substances with substrate-specific receptor sites on the plasma membrane.

Materials that enter the cell by endocytosis are broken down ("digested"), accumulated (e.g., yolk proteins), or expelled again from the opposite side of the cell by exocytosis ("cytopempsis").

Exocytosis(from Greek exo - outside, outside) - a process opposite to endocytosis: for example, from the endoplasmic reticulum, the Golgi apparatus, various endocytic vesicles, lysosomes merge with the plasma membrane, releasing their contents to the outside.

INTRODUCTION

Since the time of R. Virchow, it has been known that a living cell is an elementary cell of a biological organization that provides all the functions of an organism. Among the diverse phenomena occurring in the cell, an important place is occupied by active and passive transport of substances, osmosis, filtration and bioelectrogenesis. It has now become obvious that these phenomena are determined in one way or another by the barrier properties of cell membranes. A cell is an open system that continuously exchanges matter and energy with the environment. In many cases of biological transport, the basis for the transfer of substances is their diffusion through a cellular or multicellular membrane. The methods of diffusion transfer are diverse (Fig. 1): diffusion of fat-soluble substances through the lipid part of the membrane, transfer of hydrophilic substances through the pores formed by membrane lipids and proteins, facilitated diffusion with the participation of special carrier molecules, and selective transport of ions through ion channels. However, in the process of evolution, a living cell has created a special method of transfer, called active transport. In this case, the transfer of the substance goes against the concentration drop and, therefore, is associated with the use of energy, the universal source of which in the cell is the adenosine triphosphoric acid molecule.

TRANSPORT OF SUBSTANCES THROUGH BIOLOGICAL MEMBRANES

Living systems at all levels of organization are open systems. The elementary cell of life - the cell and cellular organelles are also open systems. Therefore, the transport of substances through biological membranes is a necessary condition for life. The transfer of substances through membranes is associated with the processes of cell metabolism, bioenergetic processes, the formation of biopotentials, the generation of a nerve impulse, etc. Violation of the transport of substances through biomembranes leads to various pathologies. Treatment is often associated with the penetration of drugs through cell membranes.

Passive and active transport of substances

The transport of substances across biological membranes can be divided into two main types: passive and active. The definitions of passive and active transport are related to the concept of electrochemical potential. It is known that the driving force of any transfer is the energy drop. Free energy (Gibbs energy) is determined at constant pressure, temperature and the number of particles carried. The latter circumstance is convenient for describing the transfer of substance particles through a membrane from one surface to another.

Electrochemical potential - a value numerically equal to the Gibbs energy per mole of a given substance placed in an electric field. For diluted solutions

where R \u003d 8.31 J / (K "mol) is the universal gas constant, F \u003d 96,500 C / mol (Faraday number), Z is the charge of the electrolyte ion (in elementary charge units), j is the potential of the electric field.

Passive transport goes in the direction of the difference in the electrochemical potential of the substance, occurs spontaneously and does not require the free energy of ATP.

Active transport is a process in which the transfer occurs from a place with a lower value of the electrochemical potential to a place with a higher value. This process, accompanied by an increase in energy, cannot proceed spontaneously, but only in conjunction with the process of ATP hydrolysis, that is, due to the expenditure of Gibbs energy stored in ATP macroergic bonds.

The density of the flow of matter jm - the amount of matter per unit time through a unit area - with passive transport obeys the Teorell equation

where U is the mobility of particles, C is the concentration. The minus sign indicates that the transfer occurs in the direction of decreasing.

Substituting in (2) the expression for the electrochemical potential (1), we obtain the Nernst-Planck equation for dilute solutions:

So, there can be two reasons for the transfer of matter during passive transport: the concentration gradient dC / dx and the electrostatic potential gradient dj / dx. In some cases, due to the conjugation of these two reasons, a passive transfer of matter from places with a lower concentration to places with a higher concentration can occur due to the energy of the electric field.

In the case of non-electrolytes (Z = 0) or a constant electric field (dj / dx = 0), the Theorell equation goes into the equation

According to Einstein's relation, URT = D, where D is the diffusion coefficient, and, substituting, we obtain Fick's law

Types of passive transport

On fig. 1 shows the main types of diffusion of substances through the membrane. Diffusion is the spontaneous movement of matter from places with a higher concentration of matter to places with a lower concentration of matter due to the chaotic thermal motion of particles. Diffusion of a substance across the lipid bilayer is caused by a concentration gradient across the membrane. Flux density of matter according to Fick's law

where is the concentration of a substance in the membrane near one of its surfaces and - near the other, l is the thickness of the membrane.

Since it is difficult to measure concentrations, in practice they use a formula that relates the flux density of a substance through a membrane with the concentrations of this substance not inside the membrane, but outside in solutions near the membrane surfaces - C1 and C2:

jm = P (C1 - C2),

where P is the permeability coefficient.

K - distribution coefficient - shows what part of the concentration at the surface outside the membrane is the concentration at the surface of the membrane, but inside it.

Equations (6) and (8) show that the permeability coefficient

This coefficient is convenient because it has the dimension of linear velocity (in m/s) and can be determined from the results of measuring membrane potentials.

The permeability coefficient, as can be seen from the formula, the greater, the greater the diffusion coefficient D, the thinner the membrane and the better the substance dissolves in the lipid phase of the membrane (the greater the K). Non-polar substances are readily soluble in the lipid phase of the membrane, for example: organic and fatty acids, esters. Naturally, these substances pass through cell membranes relatively easily, having an increased affinity for the lipid phase of the membranes. At the same time, polar substances pass poorly through the lipid bilayer of the membrane: water, inorganic salts, sugars, amino acids. Thus, the P values ​​for water and urea are 10 µm/s and 1 pm/s, respectively. At first glance, it seems difficult to explain the relatively large value of P for water, a polar substance insoluble in lipids. Obviously, in this case we can talk about the transfer of water through water-filled protein and lipid pores. However, recently, in addition to hydrophilic pores, the penetration of small polar molecules through the membrane is associated with the formation between the fatty acid tails of phospholipid molecules during their thermal movement of small free cavities - kinks (from the English kink - loop). Due to the thermal motion of the tails of phospholipid molecules, kinks can move across the membrane and carry small molecules that have entered them, primarily water molecules.

Molecules of lipid-insoluble substances and water-soluble hydrated ions surrounded by water molecules penetrate the membrane through hydrophilic lipid and protein pores. For fat-insoluble substances and ions, the membrane acts as a molecular sieve: the larger the particle size, the lower the permeability of the membrane for this substance. The selectivity of the transfer is ensured by a set of pores of a certain radius in the membrane, corresponding to the size of the penetrating particle. This distribution depends on the membrane potential. Thus, the pores selective for potassium ions in the erythrocyte membrane have a relatively low permeability coefficient equal to 4 pm/s at a membrane potential of 80 mV, which decreases fourfold as the potential decreases to 40 mV. The permeability of the squid axon membrane for potassium ions at the excitation potential level is determined by potassium channels, the radius of which is numerically estimated as the sum of the crystal radius of the potassium ion and the thickness of one hydration shell (0.133 nm + 0.272 nm = 0.405 nm). It should be emphasized that the selectivity of ion channels is not absolute, channels are available for other ions, but with lower P values.

The maximum value of P corresponds to potassium ions. Ions with large crystalline radii (rubidium, cesium) have smaller P, apparently because their dimensions with one hydration shell exceed the channel size. Less obvious is the reason for the relatively low P for lithium and sodium ions, which have a smaller radius compared to potassium. Based on the concept of a membrane as a molecular sieve, one could think that they should freely pass through potassium channels. One of the possible solutions to this contradiction was proposed by L. Mullins. He assumes that in solution outside the pore, each ion has a hydration shell consisting of three spherical layers of water molecules. When entering a pore, a hydrated ion "undresses", losing water layer by layer. A pore will be permeable to an ion if its diameter exactly matches the diameter of any of these spherical shells. As a rule, an ion remains in a pore with one hydration shell. The above calculation shows that the radius of the potassium pore will be 0.405 nm in this case. Hydrated sodium and lithium ions that are not a multiple of the pore size will have difficulty passing through it. A peculiar "quantization" of hydrated ions in terms of their size during passage through the pores was noted.

Facilitated diffusion occurs with the participation of carrier molecules. It is known, for example, that the antibiotic valinomycin is a carrier of potassium ions. Valinomycin is a peptide with a molecular weight of 1111. In the lipid phase, the valinomycin molecule has the shape of a cuff lined inside with polar groups and outside with non-polar hydrophobic residues of valine molecules.

Features of the chemical structure of valinomycin make it possible to form a complex with potassium ions that enter the cuff molecule, and at the same time, valinomycin is soluble in the lipid phase of the membrane, since its outside molecule is nonpolar. Potassium ions are held within the molecule due to the forces of the ion-dipole interaction. Molecules of valinomycin at the surface of the membrane can capture potassium ions from the surrounding solution. By diffusing in the membrane, the molecules carry potassium across the membrane and donate ions to the solution on the other side of the membrane. Thus, the shuttle transfer of potassium ions through the membrane occurs.

Differences between facilitated diffusion and simple:

1) the transfer of ions with the participation of the carrier is much faster compared to free diffusion;

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 transported substances is observed in cases where different substances are transported by one carrier; while some substances are better tolerated than others, and the addition of some substances makes it difficult to transport others;

4) there are substances that block facilitated diffusion; they form a strong complex with carrier molecules, preventing further transfer.

A kind of facilitated diffusion is transport by immobile carrier molecules fixed in a certain way across the membrane. In this case, the molecule of the transferred substance is transferred from one carrier molecule to another like a relay race.

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 essentially the diffusion of water from places of higher concentration to places of lower concentration. Osmosis plays an important role in many biological phenomena. The phenomenon of osmosis causes hemolysis of erythrocytes in hypotonic solutions and turgor in plants.

Active transport of substances across biological membranes. Using experience

Active transport of substances across biological membranes is of great importance. Due to active transport in the body, differences in concentrations, differences in electrical potentials, pressures are created that support life processes, that is, from the point of view of thermodynamics, active transfer keeps the body in a non-equilibrium state, supports life, since balance is the death of the body. The existence of active transport of substances through biological membranes was first proved in the experiments of Using (1949) using the example of the transfer of sodium ions through the skin of a frog. The experience is very instructive and deserves detailed consideration.

The experimental Using chamber, filled with normal Ringer's solution, was divided into two parts with freshly isolated frog skin. In the experiment, unidirectional flows of sodium ions through the skin of a frog in the forward and reverse directions were studied.

From the equation describing passive transport (2), the Using-Theorell equation follows for the ratio of these flows in the case of passive transport

On the isolated skin of a frog that separates Ringer's solution, a potential difference arises jin - jex (the inner side of the skin is positive relative to the outer). The installation had a special device: an electric battery with a potentiometer - a voltage divider, with the help of which the potential difference on the frog's skin was compensated: Dj = jin - jout = 0, which was controlled by a voltmeter. In addition, the concentration of sodium ions from the outer and inner sides was maintained the same. Under these conditions, as can be seen from the Using-Theorell equation,

jm, ext = jm, ext.

The total flow of ions through the membrane should be absent. Its presence would indicate the transfer of ions against the concentration drop, that is, active transfer. To prove this, 22Na radioactive isotopes were added to the left side of the experimental chamber, and 24Na to the right. 22Na decays with the emission of hard g-quanta, the emission of 24Na was detected by soft b-rays. It was shown that the 22Na flux is greater than the 24Na flux. The milliammeter readings also testified to the presence of current in the circuit.

These experimental data provided irrefutable evidence that the transfer of sodium ions through the skin of a frog does not obey the passive transport equation. Moreover, it turned out that the total flow of sodium ions is extremely sensitive to factors that affect the energy metabolism in skin cells: the presence of oxygen, the action of uncouplers of oxidative phosphorylation, and the action of low temperatures. Therefore, we should be talking about a special way of ion transfer, later called active. Later, it was found that the active transport of sodium ions in the frog skin is provided by ion pumps localized in the cells of the basal epithelium. The pump was blocked by a specific inhibitor, ouabain.

Further studies have shown that in biological membranes there are several types of ion pumps operating due to the free energy of ATP hydrolysis - special systems of integral proteins (transport ATPases). Three types of electrogenic ion pumps are currently known. The transfer of ions by transport ATPases occurs due to the conjugation of transfer processes with chemical reactions due to the energy of cell metabolism.

During the work of K + -Na + -ATPase, due to the energy of high-energy bonds released during the hydrolysis of each ATP molecule, two potassium ions are transferred into the cell and three sodium ions are simultaneously pumped out of the cell. Thus, an increased concentration of potassium ions in the cell compared to the intercellular medium and a reduced concentration of sodium ions are created, which is of great physiological significance. Ca-ATPase provides an active transfer of two calcium ions, a proton pump - two protons per ATP molecule.

The molecular mechanism of the work of ionic ATPases is not fully understood. Nevertheless, the main stages of this complex enzymatic process can be traced. In the case of K-Na-ATPase (we will designate it for brevity as E), there are seven stages of ion transfer associated with ATP hydrolysis. Designations E1 and E2 correspond to the location of the active site of the enzyme on the inner and outer surfaces of the membrane, respectively (adenosine diphosphate - ADP, inorganic phosphate - P, the active complex is marked with an asterisk):

1) E + ATP E*ATP,

2) E*ATP + 3Na *Na3,

3) *Na3 [E1 ~ P]*Na3 + ADP,

4) [E1 ~ P] * Na3 [E2 ~ P] * Na3,

5) [E2 ~ P] * Na3 + 2K [E2 - P] * K2 + 3Na,

6) [E2 - P] * K2 [E1 - P] * K2,

7) [E1 - P] * E + P + 2K.

The scheme shows that the key stages of the enzyme's work are: 1) the formation of an enzyme complex with ATP on the inner surface of the membrane (this reaction is activated by magnesium ions); 2) binding by the complex of three sodium ions; 3) phosphorylation of the enzyme with the formation of adenosine diphosphate; 4) coup (flip-flop) of the enzyme inside the membrane; 5) the reaction of ion exchange of sodium for potassium occurring on the outer surface of the membrane; 6) reverse turnover of the enzyme complex with the transfer of potassium ions into the cell; and 7) return of the enzyme to its original state with the release of potassium ions and inorganic phosphate (P). Thus, for a complete cycle, three sodium ions are released from the cell, the cytoplasm is enriched with two potassium ions, and one ATP molecule is hydrolyzed.

Secondary active ion transport

In addition to the ion pumps discussed above, similar systems are known in which the accumulation of substances is associated not with ATP hydrolysis, but with the work of redox enzymes or photosynthesis. The transport of substances in this case is secondary, mediated by the membrane potential and/or ion concentration gradient in the presence of specific carriers in the membrane. This transport mechanism is called secondary active transport. This mechanism is considered in most detail by Peter Mitchell (1966) in the chemiosmotic theory of oxidative phosphorylation. In the plasmatic and subcellular membranes of living cells, the simultaneous functioning of primary and secondary active transport is possible. An example is the inner membrane of mitochondria. Inhibition of ATPase in it does not deprive the particle of the ability to accumulate substances due to secondary active transport. This method of accumulation is especially important for those metabolites for which there are no pumps (sugars, amino acids).

Currently, three schemes of secondary active transport have been studied in depth. For simplicity, the transport of monovalent ions with the participation of carrier molecules is considered. This implies that the carrier in a loaded or unloaded state equally well crosses the membrane. The energy source is the membrane potential and/or the concentration gradient of one of the ions. The schemes are shown in fig. 5. Unidirectional ion transfer in combination with a specific carrier is called a uniport. In this case, a charge is transferred through the membrane either by a complex, if the carrier molecule is electrically neutral, or by an empty carrier, if the transfer is provided by a charged carrier. The result of the transfer will be the accumulation of ions due to a decrease in the membrane potential. This effect is observed during the accumulation of potassium ions in the presence of valinomycin in energized mitochondria.

The counter transfer of ions with the participation of a single carrier molecule is called antiport. It is assumed here that the carrier molecule forms a strong complex with each of the transferred ions. The transfer is carried out in two stages: first, one ion crosses the membrane from left to right, then the second ion crosses the membrane in the opposite direction. The membrane potential does not change in this case. What is the driving force of this process? Obviously, the difference in concentrations of one of the transferred ions. If initially there was no difference in the concentration of the second ion, then the transfer will result in the accumulation of the second ion due to a decrease in the difference in concentrations of the first. A classic example of an antiport is the transfer of potassium and hydrogen ions through the cell membrane with the participation of the antibiotic nigericin molecule.

The joint unidirectional transfer of ions with the participation of a two-site carrier is called symport. It is assumed that the membrane can contain two electrically neutral particles: a carrier in a complex with a cation and anion, and an empty carrier. Since the membrane potential does not change in such a transfer scheme, the cause of the transfer may be the difference in the concentrations of one of the ions. It is believed that the accumulation of amino acids by cells is carried out according to the symport scheme. The potassium-sodium pump creates an initial concentration gradient of sodium ions, which then, according to the symport scheme, contribute to the accumulation of amino acids. It follows from the symport scheme that this process must be accompanied by a significant shift in osmotic equilibrium, since two particles are transported through the membrane in one cycle in one direction.

In the process of life, the boundaries of the cell are crossed by various substances, the flows of which are effectively regulated. The cell membrane copes with this task with transport systems built into it, including ion pumps, a system of carrier molecules, and highly selective ion channels.

At first glance, such an abundance of transfer systems seems redundant, because the operation of only ion pumps makes it possible to provide the characteristic features of biological transport: high selectivity, transfer of substances against diffusion forces and an electric field. The paradox, however, is that the number of flows to be regulated is infinitely large, while there are only three pumps. In this case, the mechanisms of ion conjugation, known as secondary active transport, in which diffusion processes play an important role, acquire particular importance. Thus, the combination of active transport of substances with the phenomena of diffusion transfer in the cell membrane is the basis that ensures the vital activity of the cell.

AND PERMEABILITY OF MEMBRANES

In terms of permeability, lipid pores fundamentally differ from protein channels in their origin and exceptional dynamism. While protein channels have strictly defined dimensions that persist throughout the life of the cell, the dimensions of lipid pores during the process of wicking vary widely. However, this variability has a limit. If the pore radius is less than the critical one, then the pore must pass through all intermediate radii and reach the minimum size in the process of leakage. The question of the possibility of complete wicking of lipid pores remains open. It is assumed that the complete tightening of the pore is prevented by powerful hydration forces, which manifest themselves when the walls of hydrophilic pores approach each other.

Lipid pores, in contrast to protein ion channels, do not have pronounced selectivity, which correlates with their relatively large initial sizes. It is clear, however, that in the process of wicking, lipid pores can reach arbitrarily small sizes, including those comparable to the sizes of protein ion channels, which can lead to a redistribution of ion currents in the membrane, for example, upon excitation. It is further known that after the stress is turned off, the lipid bilayer membrane can return to a state of low conductivity, which implies that the pores have reached a size insufficient for the passage of hydrated ions. Thus, hydrophilic lipid pores are universal in that they can be used by the cell for the transport of macromolecular substances, ions, and water molecules.

Studies of the permeability of lipid pores are currently developing in two directions: in the first, the largest possible pores are studied, in the second, on the contrary, lipid pores of the minimum radius are studied. In the first case, we are talking about electrotransfection - a method of introducing DNA molecules into living cells or liposomes with the aim of transferring and intracellular introduction of foreign genetic material. It turned out that a high-strength external electric field facilitates the penetration of a giant DNA molecule into the membrane particle. As can be seen from, the maximum size of the critical pore corresponds to the liquid-crystalline state of the lipid bilayer in the absence of an external electric field and is equal to 9 nm. The application of an external electric field with a strength of 100 kV/m reduces the critical pore radius to 1 nm in a time of 0.2 s. Since the membranes are preserved in this case, the size of the lipid pores in them obviously does not exceed this lower limit. The paradox is that the effective diameter of the statistical coil of DNA, which must get inside the particle, reaches 2000 nm. Truly a problem about a camel penetrating through the eye of a needle. Therefore, it is obvious that the DNA molecule must penetrate the membrane in the form of an unraveled single strand. It is known that the end of the thread has a diameter of 2 nm and thus can only just enter the pore. However, free diffusion of the DNA strand in the pore is hardly possible in this case. Unfortunately, the mechanism of this phenomenon is still unclear. It is assumed, in particular, that the DNA molecule is able to expand the pore and thus slip through the membrane. DNA penetration can be facilitated by additional forces of electrophoresis and electroosmosis, taking into account the total negative charge of the DNA molecule. It is possible that the pores with the ends of the DNA molecule fixed in them play the role of an anchor that holds the molecule in a certain place near the surface of the vesicle membrane, and the transfer process itself is a type of pinocytosis. The study of this interesting from the point of view