MEMBRANES BIOLOGICAL (Latin membrana a thin skin, a cover) — functionally active superficial structures of cells thickness in several molecular layers limiting cytoplasm and most intracellular structures, and also forming uniform intracellular system of tubules, folds and the closed cavities.
The membrane limiting cytoplasm of a cell outside call plasmatic or a cytoplasmic membrane, a cell membrane or a plasmolemma. The name of intracellular (subcellular) membranes usually comes from the name of the subcellular structures limited or formed by them. E.g., distinguish mitochondrial, nuclear and lysosomic membranes, membranes of a complex of Golgi, an endoplasmic reticulum, a sarcoplasmic reticulum etc. (see. Cell ).
M.'s thickness. apprx. 10 nanometers. however owing to rather close packing of the main molecular components (proteins and lipids) in M., and also the big total area of cellular membranes they make usually more than a half of all mass of a cell (in terms of dry weight). The functions which are carried out by M., are extremely important and various: formation of cellular structures, maintenance of an intracellular homeostasis, participation in process of excitement and carrying out nervous impulse, photo, mekhano-and chemoreception, absorption, secretion and gas exchange, tissue respiration, storage and transformation of energy etc. Is defined by all this obshchebiol. value M. as the universal and dominating form of the structural and functional organization of living matter.
Huge value M. is defined by importance of the listed above functions in processes of normal life activity, and also variety of diseases and patol, the states arising at disturbances of functions M. and shown at various levels of the organization — from a cell and its subcellular systems to fabrics, bodies and an organism in general. The overwhelming number of the known diseases of the person and animals are either a direct consequence of disturbances of membranes, or the processes and states to a greater or lesser extent interfaced to them.
the Term «membrane» for designation of superficial formations of a cell was entered it. researchers to Moths (H. Mohl, 1851) at the description of a plasmolysis of cells of plants and Neghelli (Page W. Nageli, 1855) during the studying of the mechanism of the osmotic phenomena and penetrations into cells of dyes. In 1877 W. F. Ph. Pfeffer proved existence of a cellular membrane, having shown a community of osmotic properties of the cells and osmometers formed by artificial semipermeable membranes. In the 80th 19 century of X. de Fris found out that cytoplasm of plant cells is concluded between two membranes — a plasmolemma and a tonoplast.
The first instructions on the fact that lipids are a part of a cell membrane were received by Overton (E. Overton, 1895 — 1902), to-ry found a feedforward between solubility of many substances in lipids and their speed of penetration into a cell. In 1925 E. Gorter and F. Grendell experimentally showed that in a membrane of erythrocytes the quantity of lipids are enough for creation of a double continuous layer. It allowed them to suggest that the superficial membrane of an erythrocyte contains a dimolecular lipidic layer. Approximately in the same time of H. Fricke measured the electric capacity of membranes of erythrocytes and received size — 0,81 mkf/cm 2 , what corresponds to a dielectric layer (lipid) 3,3 nanometers thick and thickness of the dimolecular layer formed fat to-tami with 16 — 17 carbon atoms.
In the 30th 20 century of Harvey and Danielle (E. N. Harvey, J, F, Danielli) showed that the size of surface intention on border of fatty drops and cytoplasms of cells is lower (~ than 0,1 dynes/cm), than for pure limit of the section a lipid — water (~ 10 dynes/cm). It indicates a possibility of spontaneous formation of associates of soluble proteins with the oriented layers of lipids. Developing these representations, Danielle and H. Davson in 1935 would make the first hypothesis of M.'s structure., according to a cut the membrane consists of the double lipidic layer covered from two parties with layers of globular proteins.
A direct opportunity to observe biol, membranes appeared only in the 50th 20 century owing to development of a method of a submicroscopy and techniques of preparation of ultrathin sections. The received pictures of membranes allowed to present to M. as three-layered structures about 10 nanometers thick for plasmatic and a little smaller — for subcellular membranes. J. D. Robertson made a hypothesis of uniformity of a structure of all biol, membranes and offered the unitary scheme of a structure of a membrane. According to Robertson, proteins M-. can be developed on a surface of a double lipidic layer under the influence of forces of electrostatic interaction with the loaded heads of molecules of phospholipids; on an outer surface of a membrane also molecules of glycoproteins are located. This scheme reflected the important principle of a structure of membranes — its asymmetry.
Gradually under the influence of the new facts, and first of all granularity of structure of membranes, edge it was looked through in the pictures received at big increase, initial ideas of a trekhsloynost of membranes were reconsidered. In the beginning Luxi (J. Lucy) suggests about the micellar organization of a lipidic layer in a membrane. F. S. Sjostrand made a hypothesis of the globular organization of a cytoplasmic membrane in general. Later Green (D. E. Green) offered the scheme of the organization of membranes from subunits and formulated the principle of the repeating units in relation to an inner membrane of mitochondrions. In spite of the fact that this model was carefully developed, she did not offer a satisfactory explanation for the well-known fact of low-permeability of membranes for ions. At the same time the teterogenny particles forming membranes did not manage to be received at fragmentation of membranes. Also the fact that more than modest part, and also almost total absence of the account a lipid - proteinaceous interaction was assigned to lipids in this model raised doubts. In other models e.g., in Benson's model) the lipid - proteinaceous is assigned to interaction, on the contrary, central' a part in formation of membranes.
Further it turned out that the failure from ideas of existence of a continuous dimolecular layer was premature. On new views of a squirrel do not cover a surface of a lipidic layer, and as if float on a surface in the form of the separate globular molecules or particles to a greater or lesser extent shipped in a membrane. This zhidkomozaichny model offered by Lenard and Singer (J. Lenard, S. Singer), allows to explain well a number of the facts, in particular dependence of many fiziol, functions of membranes and activity of separate membrane enzymes from a phase condition of lipids in a membrane and degree of its flowability (viscosity). However protein-protein interaction in this model is considered insufficiently, and she does not allow to explain well experimentally established fact of retention of structure and key parameters of a membrane at extraction from it a significant amount of lipids. These facts were considered further in the belkovokristallichesky model offered G. Vanderkooi and Green different actually only by existence of the extended proteinaceous structures which are formed as a result of implementation of dalnodeystvuyushchy squirrels - proteinaceous bonds.
The most popular models M., became zhidkomozaichny. At the same time becomes more and more clear that membranes differ from each other: they are various on structure and are specific in the functional relation. The thin organization of membranes is closely interconnected with their functional state; it is both characterized by extreme sensitivity to action of external factors. At the same time nek-ry membranes show the separate lines inherent to various models, and sometimes (e.g., at nek-ry bacteria) membranes represent as if a set of the fragments corresponding some to one of the developed models.
The structure and structure of biological membranes
In the membranes received from different sources, the maintenance of a lipid hesitates from 25 to 70% (on weight), and lipidic structure mnogokomponenten and is exclusively changeable. The only general characteristic of lipids of various membranes is obligatory existence in their structure of the so-called amfipatichny lipids showing at the same time hydrophilic and hydrophobic properties. The proteinaceous structure of membranes is also exclusively various. Membranes contain a large number of various proteins with relative a pier. it is powerful (weighing) from 25 000 to 230 000. The exception is made only by the membranes of sticks of a retina containing nearly one protein — rhodopsin, and the myelin containing three types of proteins. Depending on degree of water repellency (i.e. numbers and localizations of the hydrophobic amino-acid remains in a polypeptide chain) proteins or partially, or are entirely submersed in a lipidic layer of a membrane and penetrate it through. In the functional relation membrane proteins subdivide into enzymes, receptors, proteins of transport systems and structural proteins. Also carbohydrates (up to 10% of the general dry mass of a membrane) in the form of glycoproteins and glycolipids are a part of the majority of membranes.
Main components M. are, as a rule, synthesized out of membrane system of mechanisms. Their inclusion in membranes is still insufficiently studied. Exchange proteinaceous and lipidic complexes probably participate in the simplest cases (e.g., phospholipids). Possibly and interface of processes of biosynthesis to bystry inclusion of components in a membrane (e.g., Na+, K+ - ATP-ases). Apparently, separate components can be built in membranes independently from each other since in membranes, as a rule, there are no specific centers of growth. At the same time does not raise doubts that nek-ry a lipid - dependent enzymes in certain situations are built in a membrane along with a lipidic environment.
Membrane structures are created at the expense of rather weak forces of hydrophobic and electrostatic (van-der-vaalsovykh) interactions (see. Molecule , Structure of substance ). Covalent bonds in formation of membrane structures play a supporting role. In this regard membranes possess a row special physical. - chemical properties. So, molecular components keep quite high mobility in membranes. Distinguish the intramolecular mobility connected with rotary mobility around single bonds, rotary mobility of molecules in general, motion of the molecules in the planes of a membrane (lateral mobility) and vertical mobility of molecular components of membranes (or partial, or with transition of a molecule from one half of a membrane in another).
Usually membranes function at temperatures when the zhirnokislotny remains of phospholipids are in liquid (more precisely liquid crystal) a state (see. Liquid crystals ). In this state diffusion rate of phospholipids provides movement of a lipidic molecule in time about one second. Transitions of lipids from one half of a dimolecular layer of a membrane to another (so-called transitions flip-flop) are made rather seldom. This process, obviously, shall be slowed down in membranes with the expressed asymmetry of lipidic structure, napr, in membranes of erythrocytes, in to-rykh sphingomyelin and phosphatidylsincaline there are mainly in an outside half of a membrane, and a phosphatidylethanolamine and phosphatidylserine — on its interior.
Proteinaceous molecules also show quite big freedom of the movement in a membrane. So, the speed of rotary motion and lateral mobility of nek-ry glycoproteins (antigenic and receptor proteins) corresponds to the speed of their free diffusion in the environment of the corresponding viscosity. Also vertical mobility of membrane proteins which is usually interfaced to their functional state is found. E.g., depth of immersion of rhodopsin in a membrane changes depending on a functional condition of protein. The free movement, however, is inherent not in all membrane proteins. Often they form steady plotnoupakovanny educations (area of intercellular contacts, plaques of purple membranes of Halobacterium, etc.) or strictly oriented systems (components an electron - transport chains of mitochondrions, chlorolayers, an endoplasmic reticulum, etc.). Obviously, mobility of proteins is limited when they are in a lipidic microenvironment, excellent on structure from the ground mass of lipids of membranes.
According to various types of mobility of membrane components considerable heterogeneity in viscosity of various sites M. would be observed. Low values of viscosity are observed in a hydrocarbon layer of lipids, and viscosity decreases to the middle of a layer according to a gradient of rotary mobility of hydrocarbon chains of lipids. The minimum values of the viscosity in the middle of a lipidic bi-layer measured at temperatures are higher than a point of phase change, make the 100-th shares puaz. The viscosity of a polar layer of a membrane measured on rotary and lateral mobility of molecules corresponds to units puaz. At the same time the general viscosity of membranes as textural features of a cell, measured in size of mechanical deformation of a membrane in general, reaches 10 7 — 10 8 puaz. Nek-ry other physical. - chemical properties of membranes are given in the table.
Table. Comparison of some physical and chemical properties of biological and artificial phospholipidic membranes
Important universal property M. various taxonomical, fabric and organ and tsitol, origins their ability to the restructurings caused by various exogenous and internal causes is. As a rule, restructurings happen without gaps or formation of new strong covalent bonds and come down to changes intermolecular (and intramolecular) interactions that leads to transitions from one minimum of the general free energy of intermolecular interactions in a membrane to another. In various conditions these transitions can be local or extend to considerable sites of a membrane (and also to take all membrane). Restructurings are accompanied by huge number of the major functions of membranes, and also by numerous patol, states, however in most cases mechanisms and a role of these transitions are studied insufficiently.
The most important functions of biological membranes
For a cell and subcellular particles of a membrane carry out a role of the mechanical barrier limiting them from external space. At the same time, obviously, membranes are the not rigidly fixed structures, and the flexible, labile, constantly renewed educations remaining strong and elastic at deformation.
Real operating conditions of a cell are often accompanied by existence of considerable mechanical gradients on its surface, is preferential owing to osmotic and hydrostatic pressure in a cell.
The main mechanical loading is born in this case by a cell wall (cover) constructed at the higher plants generally of cellulose, pectin and an ekstepsin, and at bacteria — complex polysaccharide of murein. In zooblasts need for a rigid cover is absent as osmotic pressure is usually balanced by activity of systems of active transport (see. Transport of ions ). In some cases nek-ry rigidity to cells is given by the nadmembranny layer 5 — 10 nanometers thick formed by glycoproteins, glycolipids and acid mucopolysaccharides and also the proteinaceous structures of cytoplasm adjoining an inner surface of a plasma membrane.
In the cells deprived of a rigid cell wall depending on conditions of a microenvironment the plasma membrane can form various type outgrowths and protrusions.
The structures created with the participation of intracellular membranes differ in high functional specificity and considerable lability. Intracellular membranes often form complex membrane systems, such as internal and outside membranes mitochondrions (see), piles of disks of receptor cells of a retina, association of membranes of an endoplasmic and sarcoplasmic reticulum, etc. The total surface of membranes is very big that, obviously, allows them to carry out big functional loads. The only cell having only one plasma membrane is the erythrocyte of mammals.
Transfer of substances through biological membranes and regulation of this process — one of the central functions of cellular membranes. These processes are interfaced with such the major biol, the phenomena as maintenance of an intracellular homeostasis, excitement and carrying out nervous impulse, storage and transformation of energy, processes of metabolism etc.
Transfer of substances through cellular membranes consists of motion of the molecules of solutes and of the movement of the water. Water gets through membranes by nearly 50 times quicker, than it could be expected from calculations on the basis of quantity of the hydrogen bindings formed by it. This fact always served as a weighty argument in favor of existence in a membrane of a time. However a number of the experimental facts would contradict existence in M. any fixed time, and hypotheses of the mechanisms explaining bystry water intrusion through membranes need specification.
The movement of ions through membranes happens or is passive (by diffusion on concentration or electric gradients), or by the active transport going against chemical or electrochemical potential with energy consumption (preferential energy of hydrolysis of ATP) and interfaced to work of specialized membrane systems — so-called membrane pumps.
The passive movement of ions (in particular cations) through purely lipidic membrane is strongly complicated mainly because of big energy of dehydration of ions — the process necessary for penetration of ions into a lipidic phase of a membrane.
In the presence of various ionophores (see), the cations facilitating dehydration (valinomitsin, nonaktin, nigeritsin) or creating in a membrane the channel of an ionic conduction (gramicidin), passive permeability of purely lipidic membranes for cations increases on several orders and becomes frequent M., comparable with permeability.
In M. functions of a passive ionic conduction are performed by the specific lipoprotein structures penetrating a membrane — so-called channels. They can be in the «open» or «closed» state, and their selectivity (i.e. ability to pass only certain ions) is defined by the geometry of the channel, electric charges of structures surrounding the channel or the proteinaceous subunits controlling its work. High performance of action of ion channels provides performance of these or those specific functions of membranes at their rather small amount (e.g., the number of natrium channels in membranes of various nervous cells fluctuates from tens to several hundred on 1 micron 2 , and their total area would make only shares of percent from all M. Square. cells).
Important feature of channels of passive permeability for ions of sodium and potassium (e.g., membranes of erythrocytes) is their high sensitivity to the content of intracellular calcium. At increase of ion concentration of calcium in a cell the passive movement of ions of sodium and potassium on a gradient of their concentration can exceed the flows created by a sodium pompe that leads a cell to a number of adverse effects up to death.
Passive flows of ions through M. are directed to alignment of gradients and reduction of ionic system in balance. Active transport — the opposite process which is carrying out transfer of ions against a gradient of their concentration and supporting stationary conditions of existence of a cell (see. Transport of ions ).
In M. various origin systems of active transport of ions of sodium, potassium, calcium and hydrogen are found. The sodium pompe (the system which is pumping over ions of sodium from a cell and potassium ions in a cell against their electrochemical gradients) is most studied. The sodium pompe works, as a rule, in the electrogene mode, i.e. the relation of number of the postponed ions of sodium to potassium (Na + / K + ) it is more than unit — usually it is equal to 3/2. Processes of transfer of ions of sodium and potassium in the pump are interfaced (release of sodium from a cell is impossible on beskaliyevy Wednesday). However it is not clear whether these processes are always inseparable.
The hydrolysis of ATP providing with energy transfer of ions is carried out by the main component of a sodium pompe — Na + = K + - dependent ATP-ase; on each hydrolyzed molecule ATP about three ions of sodium and two — potassium are transported. The exact architecture of a sodium pompe and mechanisms of its work are not found out yet though assume that it is a lipoprotein globule, two proteinaceous subunits having the centers of binding of ATP, phosphate and sodium on the interior of a membrane, and on an outer surface — the centers of binding of potassium are a part a cut. Assume that direct transfer of ions in a sodium pompe is carried out as a result of conformational reorganizations of ionosoderzhashchy fosforilirovanny enzyme with the subsequent eliminating of ions (potassium — in a cell, and sodium — to the environment) with recovery of initial conformation of enzyme.
The system of active transport of calcium ions and structure of a globule of the calcic pump localized in membranes of a sarcoplasmic reticulum has much in common with a sodium pompe. The main component of the pump is kaltsiyzavisimy ATP-ase. The mechanism of transfer of an ion and ATF-aznoy reactions includes formation of a fosforilirovanny intermediate product (in the presence of calcium ions) and its subsequent hydrolysis. Stekhiometriya of this process measured by the relation of Ca 2+ / ATP, it is equal 2 at the beginning of transport and sharply decreases in process of accumulation of calcium ions in vesicles. In the solubilized look kaltsiyzavisimy ATP-ase represents a lipoproteid about a pier. weighing 150 000. Pier. the mass of a proteinaceous part of a molecule makes about 100 000, and from it it was succeeded to emit polypeptide about a pier. weighing 33 000, having ATF-aznoy activity. It is supposed that the lipoproteid about a pier also participates in operation of the calcic pump as a coupling factor. weighing 6000 — 12 000. The calcic pump is available also in a membrane of erythrocytes.
Active transfer of hydrogen ions (protons) is carried out in the interfaced membranes. It is also provided with energy due to functioning of ATP-ases. Though the structure of vodorodzavisimy ATP-ase is not established, is undoubted that it differs from Ca a little 2+ - and Na + -, K + - ATP-ases. H + - ATP-ase has considerably the big molecular weight and other quarternary structure; phosphorylation of enzyme, most likely, does not happen at hydrolysis of ATP H + - ATF-aznoy reactions, and transferable ion (H + ) is ATF-aznoy immediate product of reaction.
The simplest pump which is actively transferring H + through a membrane, apparently, the bacteriorhodopsin of galofilny bacteria is. This membrane protein containing a retinal about a pier. the weighing less than 27 000, forming in a membrane special sites (plaques), during the lighting transports hydrogen ions from bacteria to the environment.
Transfer of various organic substances nonelectrolytes through cellular membranes is also carried out by various mechanisms. The simplest way — simple diffusion on concentration gradients (see. Diffusion ). Often process of transfer matches in the direction free diffusion, but significantly surpasses it in speed. This process is called the facilitated diffusion. The accelerated transport of substances (amino acids, sugars, purines) on the mechanism of the facilitated diffusion shall lead to bystry alignment of transmembrane gradients. However in a cell these substances are usually quickly utilized in metabolic processes and transmembrane gradients remain. The mechanism of process of the facilitated diffusion can be presented as follows: specific recognition and binding of the transported substance a carrier, its subsequent transfer through a membrane (on a gradient of concentration), dissociation of a complex and back motion of a carrier on a gradient of its concentration. The role of carriers in this process is probably carried out by transport proteins. In general process does not need energy.
The organization of transport systems even more becomes complicated when the system is directed to strengthening of the transported substance. In this case in transport system energy shall move, but it is delivered not directly due to energy of hydrolysis of ATP, and used in the form of energy of the electrochemical gradient of ions created by ionic pumps.
Such process comes, e.g., at absorption of sugars from a gleam of intestines cells of an epithelium (fig. 1). The inner membrane of these cells contains system of active transport — the sodium pompe which is pumping out ions of sodium from a cell. The formed gradient of ions of sodium is in this case motive power of transport of a triple complex (sodium, glucose, a carrier) in an outer membrane against a concentration gradient of glucose (so-called joint transport). The transport systems constructed even more difficult when the natriyzavisimy system of joint transport is accompanied by secondary systems or systems of counter transport are known.
Transport of ions against a gradient of concentration or electrochemical potential demands big energy consumptions since often gradients reach big sizes (e.g., the concentration gradient for hydrogen ions on a plasma membrane of cells mucous a stomach makes 106; a gradient of ion concentration of calcium on a membrane of a sarcoplasmic reticulum — 104). Energy consumptions on transport processes are very big and. e.g., at the person make more than a third of all energy emitted in the course of metabolism.
Generation of bioelectric potential, carrying out excitement both on nervous and muscle cells, and in places of the synoptic terminations — one of the central functions M. Emergence of bioelectric potential is caused preferential by activity of the transport systems creating uneven distribution of ions on both sides of a membrane (see. Membrane equilibrium ), and ability to transfer excitement with high speed to considerable distances — work of specialized excitable ion channels and a special combination of electric insulation and capacity properties of membranes of nervous cells (see. Bioelectric phenomena , Excitability , Excitement ).
Processes of transformation and storage of energy take the main place in power providing live systems. They proceed in specialized biol, membranes. Two basic processes — photosynthesis (see) and breath (see) — are carried out in membranes of intracellular organellas of the higher organisms, and at bacteria — in a cellular (plasmatic) membrane.
Photosynthesizing membranes transform energy of light to energy of the recovered carbon-containing connections — sugars — the main chemical energy source for heteroorganisms. At breath energy of organic substrates is released in the course of electron transfer on the chain of oxidation-reduction carriers localized in an inner membrane of mitochondrions and utilized in the interfaced process of education adenosine triphosphoric to - you at phosphorylation adenosine diphosphoric to - you by inorganic phosphate.
Inner membranes of mitochondrions, cellular membranes of nek-ry aerobic bacteria, membranes of tilakoid of chlorolayers, chromatophores of photosynthesizing bacteria, in to-rykh the process of phosphorylation interfaced to breath is carried out, call the interfacing membranes. They are characterized by the identical thickness (7 — 9 nanometers), dominance of proteins over lipids (usually in the ratio 2:1), very low content of cholesterol and, as a rule, existence of specific lipids (e.g., cardiolipids). Among proteins the considerable share (about 30%) is made by enzymes of an electron transport chain: Tsitokhroma, negeminovy zhelezoproteida and yellow enzymes (see. Respiratory enzymes ). The interfacing membranes are characterized by very high electric resistivity and very much low-permeability for ions (in particular, for hydrogen ions).
Probable localization of components of a respiratory chain in a mitochondrial membrane and the direction of the main transport flows are provided on fig. 2. The membrane principle of the organization an electron - transport chains is observed also in photosynthesizing membranes. Besides, in these membranes the system of electronic transport structurally is also functionally accompanied by the specialized pigmentobelkovy complexes providing light absorption and effective transfer of an absorbed energy to primary links an electron trance of tailors of chains.
Metabolic functions of membranes are defined by two main factors. First, associations of a large number of enzymes and enzymatic systems with membranes and, secondly, ability of membranes to physically divide cells into compartments, separating thereby the metabolic processes proceeding in them. Metabolic systems at the same time, however, do not remain completely isolated. In the membranes dividing a cell there are special systems providing selective transfer of substrates, allocation of products the, and also cofactors and connections possessing regulatory action. Therefore, speeds of the separate metabolic processes proceeding in intracellular compartments are partially regulated by the transport systems located in the dividing membranes.
Many enzymes connected with membranes function so that their enzymatic activity is shown during the approach of substrates only on the one hand of a membrane. At the same time metabolites are allocated on the opposite side. An example of such systems are transport ATP-ases.
One more distinctiveness of the enzymes localized in membranes consists in their dependence on existence and properties of a lipidic environment. Nek-ry enzymes can be separated from a membrane, having kept their specific activity. As a rule, it is the enzymes localized on a surface of membranes. Other enzymes can be extracted only by means of the influences causing considerable disturbances of structure of membranes. Preservation of partial activity at such enzymes probably to some extent is defined by safety of their lipidic microenvironment. Enzymatic activity can be recovered, recovering a lipidic microenvironment of enzyme. At the same time high specificity to the nature of the added lipids in most cases is not observed though nek-ry enzymes «prefer» lipids of a certain chemical structure.
Influence of a lipidic environment on properties of membrane enzymes is shown, apparently, and in the nature of dependence of their activity on temperature. The coincidence of points of jump of activation energy of membrane processes to temperatures of phase changes found in some cases for lipids of biological membranes, perhaps, can demonstrate the regulating influence of a lipidic environment on membrane enzymes.
Cellular reception and intercellular interaction define interaction of a cell with the environment and formation of a metaphyte as whole. Molecular and membrane aspects of cellular reception and intercellular interactions concern first of all immunol, reactions, hormonal control of growth and metabolism, patterns of embryonic development of contact inhibition of cells and their movement, and also adhesion.
It is established that in gormonalnoretseptorny reaction of a cell the central place belongs to glycoproteins (see), localized on a cellular surface — to primary chemical receptor link. Transformation of a chemical signal in a form which can be transferred to a target cell happens to involvement of adenylatecyclase or guanylate cyclase, located on other side of a membrane and catalyzing synthesis of cyclic AMF and cyclic GMF.
The Nek-ry antigenic determinants of erythrocytes defining their group specificity are presented by the carbohydrate remains of glycolipids. Their specificity is defined, apparently, by the sequence of the carbohydrate remains connected among themselves by various types of bonds creating variety of forms of geterosakharidny components and also size of surface-bound charge.
One more way of exchange of information and substance is observed in places of intercellular contacts (see. Cell ). Depending on the carried-out functions and the ultrastructural organization intercellular contacts subdivide on dense, adhesive, slot-hole and septal. These specialized structures with a low electric resistance and high-permeability formed by cytoplasmic membranes and components of primembranny layers allow to carry out interactions by diffusion of substances through the environment or by transfer of substances, passing Wednesday.
Disturbances of structure and function of membranes at pathology. A variety of types of biological membranes, their polyfunctionality and high sensitivity to external conditions generate an unusual variety of structurally functional disturbances of the membranes arising at many adverse effects and the diseases of an organism interfaced to huge number as whole.
In a habit view it is not possible to characterize the sequence of emergence of these disturbances and in each case the detailed analysis for identifications of primary link in a chain of development of structurally functional disturbances of membranes is required. All this variety of disturbances can conditionally be subdivided on transport, functional and metabolic and structural.
Disturbance of transport functions of membranes and, in particular, increase in permeability of membranes — a universal sign of damage of a cell. Disturbance of transport functions, napr, at the person, caused more than twenty so-called «transport» diseases (a renal glycosuria, the Cystinuria, disturbance of absorption of glucose, a galactose and vitamin B 12 , hereditary spherocytosis and some other diseases). Allocate four basic processes leading to change of permeability of membranes at pathology: 1) peroxide oxidation of the remains of unsaturated fatty acids of phospholipids; 2) action of the membrane phospholipases activated by calcium ions; 3) mechanical (osmotic or hydrostatic) stretching and damage of membranes; 4) action on M. various antibodies, mediators and hormones.
The first two processes lead the main physical to change. - chemical properties M. owing to chemical modification of lipidic structure and emergence of a large number toxic, products. For interpretation of mechanisms of osmotic and hydrostatic damage of membranes often use simple mechanical analogies. Action of antibodies and other bioregulators is least studied, and is probable, accompanied by the restructurings of membranes happening in response to reactions of the specialized centers of reception.
Among functional and metabolic disturbances of cellular membranes changes of processes of biosynthesis, and also diverse deviations in power supply of live systems are central (see. Bio-energetics ). In the most habit view disturbance of structure and physical is a consequence of these processes. - chemical properties M., loss of separate links of metabolism or its perversion, and also decrease in level of the vital volatile processes (active transport of ions, processes of the interfaced transport, functioning of sokratitelny systems etc.).
Damages of the ultrastructural organization of membranes are expressed in an excessive vezikuloobrazovaniye, increase in a surface of plasma membranes due to vesiculation and shoots, merge of diverse cellular membranes, formation of micropores and local defects in structure.
Methods of studying of biological membranes
For the solution of a complex of the tasks connected with studying of structure and function M., use the drugs of membranes emitted from fabrics, separate cells and cellular organellas, the isolated fragments of membranes, artificial membranes and the reconstructed systems.
Allocation of membranes from a cell is made by means of a combination of various methods of partial destruction (a partition on fragments) cellular structures, the subsequent fractionation and concoction of membrane drugs. For M.'s fragmentation. use mechanical homogenization, influence by ultrasound, alternation of freezing and thawing, hydrodynamic blow, osmotic shock, processing by detergents and enzymes (lipases, phospholipases, proteinases). Select and concentrate cellular organellas or fragments of separate types of membranes usually by means of a sedimentation method or ultrafiltration.
Control of purity of fraction of membranes (lack of pollution by other membranes) is exercised after definition of a degree of activity of specific enzymes in drugs of membranes (marker enzymes) or by means of a submicroscopy.
The chemical structure of membranes, enzymatic properties of membrane proteins, and also the direction and speed of course biochemical, processes in membrane systems analyze by means of special methods of biochemical analysis.
At the solution of a number of the tasks connected with clarification of mechanisms of membrane transport, patterns of interaction of membranes, squirrels - lipidic interaction and specifics of course of enzymatic reactions in heterogeneous membrane systems widely use the artificial and reconstructed membrane systems. As artificial membranes use monomolecular lipidic layers on a surface water — air, an interface water — heptane, the dimolecular lipidic membranes (DLM) — flat, created on an opening in hydrophobic materials, and spherical (liposomes), and also multilayer lipidic membranes in the form of impregnirovanny a lipid of porous materials and multilayer liposomes. Most the following reconstructed systems were widely adopted: BLM + protein, liposome + protein, BLM + liposome, BLM + proteoliposoma, BLM spherical fragments biol, membranes.
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