Basic properties of the cell membrane.

♦ Has selective permeability, which changes with various states cells. It depends:

· on the mechanical factor (correspondence between the size of the channels and the diameter of the ions);

· electrostatic factor (channel charge);

· competition between ions (Na + and Ca 2+).

♦ Has channels through which ions penetrate:

· potential-dependent channels – open when the potential difference changes;

· potential-independent (ligand-dependent, hormone-regulated) – open when receptors interact with substances.

♦ Channels can be opened or closed thanks to gates. There are two types of gates:

· activation (deep in the channel);

· inactivation (on the surface of the channel).

♦ The gate can be in one of 3 states:

· open (both types of gates are open);

· closed (activation gate is closed);

· inactivation (inactivation gates are closed).

Membrane transport mechanisms.

Transport of substances across the cell membrane is carried out by diffusion through the lipid bilayer or through two classes of membrane proteins - carriers or channels.

Plasma membrane permeability.

♦ If proteins were not present in the plasma membrane, the ease with which molecules pass through the phospholipid bilayer along a concentration gradient would depend on the size of the molecule, its fat solubility, and electric charge.

♦ Fat-soluble (non-polar) molecules diffuse quickly. Examples: O 2, CO 2, N 2.

♦ Fat-insoluble (polar) molecules diffuse quickly provided they are small in size and electrically neutral. Examples: H 2 O and urea. Only large uncharged polar molecules such as glucose and sucrose diffuse.

♦ Charged molecules (ions), even if small size(Na + , K + , Cl –) practically do not penetrate the lipid bilayer in the absence of special transport mechanisms.

♦ Transport of ions and large polar molecules is ensured by special transmembrane proteins.

♦ Carrier proteins transfer substances by the physical movement of one part of the protein molecule relative to another. Transport by vectors can be passive or active (requires an energy source).

♦ Channels transport substances along their electrochemical gradient. Such transport does not require direct expenditure of metabolic energy and is therefore called passive transport.

♦ Transport of macromolecules, such as protein, is accompanied by cotransport of the substance within the membrane environment.

♦ If a substance is transported out of the cell, the process is called exocytosis.

♦ Transport into the cell is called endocytosis.

A. Terminology. Currently, various authors interpret the terms “permeability” and “conductivity” differently. By the permeability of a cell membrane we mean its ability to pass water and particles - charged (ions) and uncharged according to the laws of diffusion and filtration. The permeability of the cell membrane is determined by the following factors: 1) the presence of various ion channels in the membrane - controlled (with a gate mechanism) and uncontrolled (leak channels); 2) channel sizes and particle sizes; 3) solubility of particles in the membrane (the cell membrane is permeable to lipids soluble in it and impermeable to peptides).

The term conductivity should only be used in relation to charged particles. Therefore, by conductivity we mean the ability of charged particles (ions) to pass through a cell membrane according to an electrochemical gradient (a combination of electrical and concentration gradients).

As is known, ions, like uncharged particles, pass through the membrane from an area of ​​high concentration to an area of ​​low concentration. With a large concentration gradient and good permeability of the membrane separating the corresponding solutions, the conductivity of ions can be high, and a one-way flow of ions is observed. When the concentration of ions on both sides of the membrane is equalized, the conductivity of the ions will decrease, the one-way flow of ions will stop, although the permeability will remain the same - high. In addition, the conductivity of the ion, with constant membrane permeability, also depends on the charge of the ion; like charges repel, unlike charges attract, i.e. An important role in the conductivity of an ion is played by its electric charge. A situation is possible when, with good permeability of the membrane, the conductivity of ions through the membrane turns out to be low or zero - in the absence of a driving force (concentration and / or electrical gradients).

Thus, the conductivity of an ion depends on its electrochemical gradient and on the permeability of the membrane; the larger they are, the better the conductivity of the ion through the membrane. Movement of ions into and out of the cell according to concentration and electrical gradients cell at rest carried out mainly through uncontrollable(without gate mechanism) channels (leakage channels). Uncontrolled channels are always open; they practically do not change their throughput when electrically applied to the cell membrane and excited. Unmanaged channels are divided into ion-selective channels (eg potassium slow ungated channels) and ion-nonselective channels. The latter allow various ions to pass through; K+, Ka+, C1".

B. The role of cell membrane permeability and various ions in the formation of PP(Fig. H.2.).

cannot leave the cell, so inside the cell at rest there are more negative ions than positive ones. For this reason, the inside of the cell has a negative charge. Interestingly, at all points of the cell the negative charge is almost the same. This is evidenced by the same magnitude of PP when the microelectrode is introduced to different depths inside the cell, as was the case in the experiments of Hodgkin, Huxley and Katz. Charge

inside the cell is negative both absolutely (the hyaloplasm of the cell contains more anions than cations) and relative to the outer surface of the cell membrane. This is evidenced by the results of an experiment with perfusion of the internal contents of the squid giant axon with saline solutions. With a decrease in the concentration of K + ions in the perfusion, PP decreases, and with an increase in their concentration, PP increases. When the cell is at rest, a dynamic balance is established between the number of K+ ions leaving the cell and entering the cell. Electrical and concentration gradients oppose each other: according to the concentration gradient, K+ tends to leave the cell, the negative charge inside the cell and the positive charge on the outer surface of the cell membrane prevent this. When the concentration and electrical gradients are balanced, the number of K+ ions leaving the cell is compared with the number of K+ ions entering the cell. In this case, the so-called equilibrium potential.

Equilibrium potential for the ion can be calculated using the Nernst formula. The concentration of a positively charged ion located outside the cell is written in the numerator in the Nernst formula, and the concentration of the ion located inside the cell is written in the denominator. For negatively charged ions the arrangement is opposite.

Contribution of Na + and Cl - to the creation of PP. The permeability of the cell membrane at rest for the N3+ ion is very low, much lower than for the K+ ion, nevertheless it is present, therefore the Ka* ions, according to concentration and electrical gradients, tend and pass into the cell in small quantities. This leads to a decrease in PP, since on the outer surface of the cell membrane the total number of positively charged ions decreases, although slightly, and some of the negative ions inside the cell are neutralized by positively charged Na + ions entering the cell. Ion input Na+ inside cells reduces PP. The effect of SG on the PP value is opposite and depends on the permeability of the cell membrane for SG ions. The fact is that the SG ion, according to the concentration gradient, tends and passes into the cell. An electrical gradient prevents the entry of the SG ion into the cell, since the charge inside the cell is negative, as is the charge of the SG. An equilibrium occurs between the forces of the concentration gradient, which promotes the entry of the SG ion into the cell, and the electrical gradient, which prevents the entry of the SG ion into the cell. Therefore, the intracellular concentration of SG ions is significantly less than the extracellular one. When the SG ion enters the cell, the number of negative charges outside the cell decreases slightly, and inside the cell increases: the SG ion is added to the large, proteinaceous anions located inside the cell. Due to their large size, these anions cannot pass through the channels of the cell membrane to the outside of the cell - into the interstitium. Thus, CI - ion, penetrating into the cell, increases PP. Partially, as outside the cell, ions + and C1" inside the cells they neutralize each other. As a result, the combined entry of Na + and C1~ ions into the cell does not significantly affect the PP value.

B. A certain role in the formation of PP is played by the surface charges of the cell membrane itself and Ca 2+ ions. External and internal the surfaces of the cell membrane carry their own electrical charges, mostly with a negative sign. These are polar molecules of the cell membrane: glycolipids, phospholipids, glycoproteins. Fixed external negative charges, neutralizing the positive charges of the outer surface of the membrane, reduce PP. Fixed internal negative charges of the cell membrane, on the contrary, when added to anions inside the cell, increase PP.

The role of Ca 2+ ions in the formation of PP is that they interact with external negative fixed charges of the cell membrane and neutralize them, which leads to an increase and stabilization of PP.

Thus, PP- it is the algebraic sum of not only all the charges of ions outside and inside the cell, but also algebraic the sum of the negative external and internal surface charges of the membrane itself.

When taking measurements, the potential surrounding the cell environment is taken equal to zero. Relative to zero potential external environment the potential of the internal environment of a neuron, as noted, is on the order of -60-80 mV. Cell damage leads to an increase in the permeability of cell membranes, as a result of which the difference in permeability for K + and N3+ ions decreases. At the same time, the PP decreases. Similar changes occur during tissue ischemia. In severely damaged cells, the PP may decrease to the level of Donann equilibrium, when the concentration inside and outside the cell will be determined only by the selective permeability of the cell membrane in the resting state of the cell, which can lead to disruption of the electrical activity of neurons. However, normally, ions move according to the electrochemical gradient, but the PP is not disturbed.

The plant continuously absorbs from the outside various substances necessary for its nutrition, and also constantly releases substances into the external environment. Wednesday. This indicates that the plant membranes are permeable, that is, they are capable of allowing absorbed and excreted substances to pass through them. But cell membranes do not allow all substances to pass through, but only some. Thus, they have selective permeability. Facts confirming that the villages in the class. act not only passively, but also actively:

1. Substances penetrate into the cell not only along a concentration gradient (from high to lower) - passively, but also against the gradient, when more substances accumulate in the cell than in the environment. For example, the concentration of iodine and bromine in the thalli of algae growing in seawater.

2. Not only low molecular weight compounds, but also substances with large molecules can enter and be released from the cell.

3. When any salt is in a solution, the ions that make up its molecule will not penetrate into the cell evenly, but in different ratios (different numbers of cations or anions).

Firstly, the permeability of cytoplasmic membranes varies greatly during the life of the plant. It depends on temperature, light, moisture content and action poisonous substances. Secondly, permeability is associated with respiration, which can be observed under the action of substances that stimulate and weaken this process. Thus, under the action of macroenergetic ATP, the supply of substances to the roots increases, and cellular poisons (cyanide, fluoride, etc.) suppress this process. There are several theories that try to explain permeability by active processes. Main - Carrier theory. Essence: it is assumed that the substance itself (A) cannot penetrate the membrane. Then it connects with a special carrier (X) that transports it; then the transferred substance is released and remains inside the cell, and the carrier with another substance (B) comes out and again becomes capable of transfer. The transfer of a substance into a cell can be schematically depicted in the following way: A + X =AX ->> AX =A + X.

Reverse transfer: B + X =BX ->> BX =B + X.

It is assumed that the carriers are org. substances of protein nature; one for cation transfer, etc. for anion transport. Their action is based on the process of exchange adsorption, when absorbed and transferred ions are exchanged for others, which are in excess in the cell. Cations, usually, exchanged for H + and Na +, Anions - for OH and HCO 3. In bacteria they are represented by Ant (such as gramicidin and valine). In higher plants, their role is played by membrane ATPases and ion pumps.

Ion pumps- special formations built into cell membranes. These are globules consisting of 3 subunits. Two of them are a protein channel through which ions move, the third is the enzyme ATPase, with the participation of which ATP breaks down with the release of a phosphoric acid residue and a certain amount of energy. K+, Na+-pump, which removes sodium from the cell and pumps in potassium; protic or hydrogen, carrying out H+ and pumping in other substances. Action of ion pumps: there is always a lot of sodium ions in the external environment (for example, in soil). Due to its high concentration, it moves along a gradient and passively penetrates into cells. But the plant does not need such a large amount of sodium, so an excess of it is quickly created in the cells, which is released from them due to the released energy of ATP. Potassium, on the contrary, is needed by the plant in larger quantities than sodium, but there is little of it in the soil, and it cannot be supplied along the gradient itself. Then, simultaneously with the release of sodium, potassium is forced to be pumped in. The transport of many other substances - mineral and organic - in plants is carried out by the H-pump, which serves as the main transporter. The proton pump is involved in loading the phloem with sugars produced during photosynthesis.

Permeability is the ability of tissues, cells and sub cellular structures(cell nuclei, etc.) allow gases, water and various substances. The penetration of substances through biological membranes occurs passively or through active transfer with the participation of special mechanisms. The permeability of membranes to various agents depends both on the physicochemical properties of the latter and on the characteristics of the membranes themselves.

Permeability disorders can occur as a result of various damaging factors: high and low temperatures, irradiation, certain substances (for example), lack of oxygen, vitamins, hormones, etc. Permeability disorders play an important role in the pathogenesis of many disease processes: (see. ), (see), shock (see), infectious diseases, disorders of excretory processes, etc. Changes in permeability can be both a manifestation of a protective reaction and the cause of many serious disorders.

Permeability is the ability of cells and tissues to pass and absorb solutions and gases from environment and highlight them out. Permeability is a general biological problem associated with the relationship of the organism with the environment, with metabolism and is important for physiology and pathology.

There are the following theories of selective permeability of cells and tissues, which interpret the substrate and conditions of this process differently. According to the membrane theory of cellular permeability, the distribution of substances between the cell and the environment is explained by the presence of a submicroscopic membrane that is selectively permeable to molecules and ions. The protoplasm of cells is considered a colloid, in which almost all water is in a free state and has the properties of a solvent. The sorption theory of permeability is based on the idea of ​​protoplasm as a phase immiscible with water, in which water and ions are in a bound state. The entry of substances into the cell is regulated by the entire protoplasm and is determined by sorption factors (solubility, chemical binding, adsorption, etc.). According to modern concepts, cell membranes (see Cell) have a total thickness of 70-80 A and consist of two parallel layers of lipid molecules, oriented with polar groups to the surface of the membrane, with layers of protein adsorbed on them. In addition, in the cytoplasm there is a system of membrane formations associated with endoplasmic reticulum and mitochondria.

Low molecular weight substances, water, gases can penetrate into the cell under the influence of osmotic forces (see Osmotic pressure), by diffusion (see) and ultrafiltration (see), without energy costs(passive transfer). For ions, permeability depends on the electrical charge, the potential gradient between the outer and inner surfaces of the membranes.

Active transfer refers to processes that occur with the expenditure of energy generated in the cell during metabolism (phosphorylation, dephosphorylation, formation of complex substance complexes, the presence of carrier molecules, participation of enzymes, etc.). In this case, substances can move against the concentration gradient. Thus, the content of K ions in erythrocytes is 20 times higher than Na ions, however, K ions accumulate in them, and Na ions enter the plasma against a 50-fold concentration gradient. One of the ways substances penetrate the cell is pinocytosis (see). This process consists of the adsorption of substances by the cell membrane, reducing its surface tension and invagination into the cytoplasm with the formation of pinocytic vacuoles; subsequently, their shell is destroyed, and the substances are included in cellular metabolism.

The selective permeability of substances depends on both the structure and chemical structure cell membranes, and on size, electrical charge, hydration, and solubility of substances in lipoids. Unlike strong acids and bases, which do not penetrate the cell, weak acids and bases, which contain predominantly undissociated molecules, have great penetrating ability. When the active reaction shifts to the acidic or alkaline side, accompanied by a change in the degree of dissociation of molecules, the penetration of substances into the cell increases or decreases. Thus, it has been established that tertiary ammonium compounds, which do not carry a charge, penetrate the brain, in contrast to ionized quaternary amines and their salts.

In the body, many tissues are membranes that have selective permeability (endothelium of capillaries and serous cavities, intestinal wall, skin epithelium, etc.). The permeability of such membranes depends not only on their constituent cellular structures, but also on the permeability of the intercellular substance. The permeability of histo-hematological barriers, which regulate the relative constancy of the internal environment of organs and tissues, is important (see Barrier functions).

Violations of permeability are an essential link in the pathogenesis of many pathological processes (allergy, inflammation, edema, shock), in the mechanism of changes in absorption (see), secretion, excretion, metabolism. Of particular importance in clinical pathology are disturbances in capillary permeability, observed in many infectious, toxic, allergic and other diseases (dysentery, brucellosis, scarlet fever, influenza, rheumatism, typhoid and typhus, tonsillitis, nephritis, etc.). Violations of vascular permeability have been noted in diseases of the cardiovascular system (rheumatic pancarditis, myocarditis, septic endocarditis, hypertension, atherosclerosis), respiratory organs (pulmonary emphysema, pneumonia, pneumosclerosis), kidneys, liver, skin, nervous system. Changes in vascular permeability are characteristic of different stages of radiation sickness.

Impairments in the permeability of histo-hematological barriers are also important in the pathogenesis of a number of diseases. In particular, the permeability of the blood-brain barrier increases with traumatic brain injury, inflammation of the meninges, some forms of epilepsy, cerebrovascular accidents, shock, radiation sickness and other pathological processes. There is evidence of the effects of various medicinal substances on the permeability of capillaries, blood-brain and other histo-hematological barriers, which makes it possible to regulate permeability disorders under pathological conditions.

Membrane transport

Transport of substances in and out of the cell, as well as between the cytoplasm and various subcellular organelles (mitochondria, nucleus, etc.) is ensured by membranes. If membranes were a blind barrier, then the intracellular space would be inaccessible to nutrients, and waste products could not be removed from the cell. At the same time, with complete permeability, the accumulation of certain substances in the cell would be impossible. The transport properties of the membrane are characterized by semi-permeability: some compounds can penetrate through it, while others cannot:

Membrane permeability for various substances

One of the main functions of membranes is the regulation of substance transfer. There are two ways to transfer substances across a membrane: passive and active transport:

Passive transport. If a substance moves through a membrane from an area of ​​high concentration to a low concentration (i.e., along the concentration gradient of this substance) without the cell expending energy, then such transport is called passive, or diffusion. There are two types of diffusion: simple and facilitated.

Simple diffusion is characteristic of small neutral molecules (H2O, CO2, O2), as well as hydrophobic low molecular weight organic substances. These molecules can pass without any interaction with membrane proteins through membrane pores or channels as long as the concentration gradient is maintained.

Facilitated diffusion. Characteristic of hydrophilic molecules that are transported through the membrane also along a concentration gradient, but with the help of special membrane proteins - carriers. Facilitated diffusion, in contrast to simple diffusion, is characterized by high selectivity, since the transporter protein has a binding center complementary to the transported substance, and the transfer is accompanied by conformational changes in the protein. One of the possible mechanisms of facilitated diffusion may be as follows: a transport protein (translocase) binds a substance, then approaches the opposite side of the membrane, releases this substance, takes on its original conformation and is again ready to perform the transport function. Little is known about how the protein itself moves. Another possible mechanism transfer involves the participation of several carrier proteins. In this case, the initially bound compound itself moves from one protein to another, sequentially binding with one or the other protein until it ends up on the opposite side of the membrane.