The barrier transport function of the cell surface apparatus is ensured by the selective transfer of ions, molecules and supramolecular structures into and out of the cell. Transport through membranes ensures the delivery of nutrients and removal of final metabolic products from the cell, secretion, creation of ion gradients and transmembrane potential, maintenance of the required pH values in the cell, etc.
The mechanisms of transport of substances into and out of the cell depend on chemical nature transported substance and its concentrations on both sides of the cell membrane, as well as from sizes transported particles. Small molecules and ions are transported across the membrane by passive or active transport. The transfer of macromolecules and large particles is carried out through transport in “membrane packaging”, that is, due to the formation of vesicles surrounded by a membrane.
Passive transport is called the transfer of substances through a membrane along their concentration gradient without energy consumption. Such transport occurs through two main mechanisms: simple diffusion and facilitated diffusion.
By simple diffusion small polar and nonpolar molecules, fatty acids and other low molecular weight hydrophobic organic substances are transported. The transport of water molecules through a membrane, carried out by passive diffusion, is called osmosis. An example of simple diffusion is the transport of gases through the plasma membrane of endothelial cells of blood capillaries into the surrounding tissue fluid and back.
Hydrophilic molecules and ions that are not able to independently pass through the membrane are transported using specific membrane transport proteins. This transport mechanism is called facilitated diffusion.
There are two main classes of membrane transport proteins: carrier proteins And channel proteins. Molecules of the transported substance, binding to carrier protein cause its conformational changes, resulting in the transfer of these molecules across the membrane. Facilitated diffusion is highly selective with respect to transported substances.
Channel proteins form water-filled pores that penetrate the lipid bilayer. When these pores are open, inorganic ions or transport molecules pass through them and are thus transported across the membrane. Ion channels transport approximately 10 6 ions per second, which is more than 100 times the rate of transport carried out by carrier proteins.
Most channel proteins have "gates", which open briefly and then close. Depending on the nature of the channel, the gate may open in response to the binding of signaling molecules (ligand-gated gate channels), a change in membrane potential (voltage-gated gate channels), or mechanical stimulation.
Active transport is called the transport of substances across a membrane against their concentration gradients. It is carried out with the help of carrier proteins and requires energy, the main source of which is ATP.
An example of active transport that uses the energy of ATP hydrolysis to pump Na + and K + ions across the cell membrane is the work sodium-potassium pump, ensuring the creation of membrane potential on the plasma membrane of cells.
The pump is formed by specific adenosine triphosphatase enzyme proteins built into biological membranes, which catalyze the cleavage of phosphoric acid residues from the ATP molecule. ATPases include: an enzyme center, an ion channel and structural elements that prevent the reverse leakage of ions during pump operation. More than 1/3 of the ATP consumed by the cell is consumed to operate the sodium-potassium pump.
Depending on the ability of transport proteins to transport one or more types of molecules and ions, passive and active transport are divided into uniport and coport, or coupled transport.
Uniport - This is a transport in which the carrier protein functions only in relation to molecules or ions of one type. In coport, or coupled transport, a carrier protein is capable of transporting two or more types of molecules or ions simultaneously. These carrier proteins are called co-porters, or associated carriers. There are two types of coport: simport and antiport. When simporta molecules or ions are transported in one direction, and when antiporte - in opposite directions. For example, the sodium-potassium pump works according to the antiport principle, actively pumping Na + ions out of cells and K + ions into cells against their electrochemical gradients. An example of symport is the reabsorption of glucose and amino acids from primary urine by renal tubular cells. In primary urine, the concentration of Na + is always significantly higher than in the cytoplasm of renal tubular cells, which is ensured by the operation of the sodium-potassium pump. The binding of primary urine glucose to the conjugated carrier protein opens the Na + channel, which is accompanied by the transfer of Na + ions from primary urine into the cell along their concentration gradient, that is, by passive transport. The flow of Na + ions, in turn, causes changes in the conformation of the carrier protein, resulting in the transport of glucose in the same direction as Na + ions: from primary urine into the cell. In this case, for the transport of glucose, as can be seen, the conjugate transporter uses the energy of the Na + ion gradient created by the operation of the sodium-potassium pump. Thus, the work of the sodium-potassium pump and the associated transporter, which uses a gradient of Na + ions to transport glucose, makes it possible to reabsorb almost all glucose from primary urine and include it in the general metabolism of the body.
Thanks to the selective transport of charged ions, the plasmalemma of almost all cells carries positive charges on its outer side and negative charges on its inner cytoplasmic side. As a result, a potential difference is created between both sides of the membrane.
The formation of the transmembrane potential is achieved mainly due to the work of transport systems built into the plasmalemma: the sodium-potassium pump and protein channels for K + ions.
As noted above, during the operation of the sodium-potassium pump, for every two potassium ions absorbed by the cell, three sodium ions are removed from it. As a result, an excess of Na + ions is created outside the cells, and an excess of K + ions is created inside. However, an even more significant contribution to the creation of the transmembrane potential is made by potassium channels, which are always open in cells at rest. Due to this, K+ ions exit the cell along a concentration gradient into the extracellular environment. As a result, a potential difference of 20 to 100 mV occurs between the two sides of the membrane. The plasmalemma of excitable cells (nerve, muscle, secretory), along with K + channels, contains numerous Na + channels, which open for a short time when chemical, electrical or other signals act on the cell. The opening of Na + channels causes a change in the transmembrane potential (membrane depolarization) and a specific cell response to the signal.
Transport proteins that generate potential differences across the membrane are called electrogenic pumps. The sodium-potassium pump serves as the main electrogenic pump of cells.
Transport in membrane packaging characterized by the fact that transported substances at certain stages of transport are located inside membrane vesicles, that is, they are surrounded by a membrane. Depending on the direction in which substances are transported (into or out of the cell), transport in membrane packaging is divided into endocytosis and exocytosis.
Endocytosis is the process of absorption by a cell of macromolecules and larger particles (viruses, bacteria, cell fragments). Endocytosis is carried out by phagocytosis and pinocytosis.
Phagocytosis - the process of active capture and absorption by a cell of solid microparticles, the size of which is more than 1 micron (bacteria, cell fragments, etc.). During phagocytosis, the cell, with the help of special receptors, recognizes specific molecular groups of the phagocytosed particle.
Then, at the point of contact of the particle with the cell membrane, outgrowths of the plasmalemma are formed - pseudopodia, which envelop the microparticle from all sides. As a result of the fusion of pseudopodia, such a particle is enclosed inside a vesicle surrounded by a membrane, which is called phagosome. The formation of phagosomes is an energy-dependent process and occurs with the participation of the actomyosin system. The phagosome, plunging into the cytoplasm, can merge with a late endosome or lysosome, as a result of which the organic microparticle absorbed by the cell, for example a bacterial cell, is digested. In humans, only a few cells are capable of phagocytosis: for example, connective tissue macrophages and blood leukocytes. These cells absorb bacteria as well as a variety of particulate matter that enter the body, thereby protecting it from pathogens and foreign particles.
Pinocytosis- absorption of liquid by the cell in the form of true and colloidal solutions and suspensions. This process is in general terms similar to phagocytosis: a drop of liquid is immersed in the formed depression of the cell membrane, surrounded by it and found to be enclosed in a vesicle with a diameter of 0.07-0.02 microns, immersed in the hyaloplasm of the cell.
The mechanism of pinocytosis is very complex. This process occurs in specialized areas of the cell's surface apparatus called bordered pits, which occupy about 2% of the cell surface. Bordered pits are small invaginations of the plasmalemma, next to which there is a large amount of protein in the peripheral hyaloplasm clathrin. In the region of the bordered pits on the surface of the cells there are also numerous receptors that can specifically recognize and bind transported molecules. When the receptors bind these molecules, polymerization of clathrin occurs, and the plasmalemma invaginates. As a result, bordered bubble, carrying transportable molecules. These bubbles got their name due to the fact that clathrin on their surface looks like an uneven rim under an electron microscope. After separation from the plasmalemma, the bordered vesicles lose clathrin and acquire the ability to merge with other vesicles. The processes of polymerization and depolymerization of clathrin require energy and are blocked when there is a lack of ATP.
Pinocytosis, due to the high concentration of receptors in the bordered pits, ensures the selectivity and efficiency of the transport of specific molecules. For example, the concentration of molecules of transported substances in the bordered pits is 1000 times higher than their concentration in the environment. Pinocytosis is the main method of transport of proteins, lipids and glycoproteins into the cell. Through pinocytosis, the cell absorbs an amount of liquid equal to its volume per day.
Exocytosis- the process of removing substances from the cell. Substances to be removed from the cell are first enclosed in transport vesicles, the outer surface of which is usually coated with the protein clathrin, then such vesicles are directed to the cell membrane. Here the membrane of the vesicles merges with the plasmalemma, and their contents are poured outside the cell or, while maintaining contact with the plasmalemma, are included in the glycocalyx.
There are two types of exocytosis: constitutive (basic) and regulated.
Constitutive exocytosis occurs continuously in all cells of the body. It serves as the main mechanism for removing metabolic products from the cell and constantly restoring the cell membrane.
Regulated exocytosis carried out only in special cells that perform a secretory function. The secreted secretion accumulates in secretory vesicles, and exocytosis occurs only after the cell receives the appropriate chemical or electrical signal. For example, β-cells of the islets of Langerhans in the pancreas release their secretions into the blood only when the blood glucose concentration increases.
During exocytosis, secretory vesicles formed in the cytoplasm are usually directed to specialized areas of the surface apparatus containing a large number of fusion proteins or fusion proteins. When the fusion proteins of the plasma membrane and the secretory vesicle interact, a fusion pore is formed, connecting the cavity of the vesicle with the extracellular environment. In this case, the actomyosin system is activated, as a result of which the contents of the vesicle are poured out of it outside the cell. Thus, during inducible exocytosis, energy is required not only for the transport of secretory vesicles to the plasmalemma, but also for the secretion process.
Transcytosis, or recreation , - This is transport in which individual molecules are transferred through the cell. This type of transport is achieved through a combination of endo- and exocytosis. An example of transcytosis is the transport of substances through the cells of the vascular walls of human capillaries, which can occur in both one and the other direction.
The exchange of various substances and energy between the cell and the external environment is a vital condition for its existence.
To maintain the constancy of the chemical composition and properties of the cytoplasm in conditions where there are significant differences in the chemical composition and properties of the external environment and the cytoplasm of the cell, there must exist special transport mechanisms, selectively moving substances through.
In particular, cells must have mechanisms for delivering oxygen and nutrients from the environment and removing metabolites into it. Concentration gradients of various substances exist not only between the cell and the external environment, but also between cell organelles and the cytoplasm, and transport flows of substances are observed between different compartments of the cell.
Of particular importance for the perception and transmission of information signals is the maintenance of the transmembrane difference in the concentrations of mineral ions Na + , K + , Ca 2+. The cell spends a significant part of its metabolic energy on maintaining concentration gradients of these ions. The energy of electrochemical potentials stored in ion gradients ensures the constant readiness of the cell plasma membrane to respond to stimuli. The entry of calcium into the cytoplasm from the intercellular environment or from cellular organelles ensures the response of many cells to hormonal signals, controls the release of neurotransmitters in, and triggers.
Rice. Classification of transport types
To understand the mechanisms of transition of substances through cell membranes, it is necessary to take into account both the properties of these substances and the properties of the membranes. Transported substances differ in molecular weight, charge transfer, solubility in water, lipids, and a number of other properties. Plasma and other membranes are represented by large areas of lipids, through which fat-soluble non-polar substances easily diffuse and water and water-soluble substances of a polar nature do not pass through. For the transmembrane movement of these substances, the presence of special channels in cell membranes is necessary. The transport of molecules of polar substances becomes more difficult when their size and charge increase (in this case, additional transport mechanisms are required). The transfer of substances against concentration and other gradients also requires the participation of special carriers and energy expenditure (Fig. 1).
Rice. 1. Simple, facilitated diffusion and active transport of substances across cell membranes
For the transmembrane movement of high-molecular compounds, supramolecular particles and cell components that are not able to penetrate through membrane channels, special mechanisms are used - phagocytosis, pinocytosis, exocytosis, transport through intercellular spaces. Thus, the transmembrane movement of various substances can be carried out using different methods, which are usually divided according to the participation of special carriers in them and energy consumption. There are passive and active transport across cell membranes.
Passive transport— transfer of substances through a biomembrane along a gradient (concentration, osmotic, hydrodynamic, etc.) and without energy consumption.
Active transport- transfer of substances through a biomembrane against a gradient and with energy consumption. In humans, 30-40% of all energy generated during metabolic reactions is spent on this type of transport. In the kidneys, 70-80% of the oxygen consumed goes to active transport.
Passive transport of substances
Under passive transport understand the transfer of a substance through membranes along various gradients (electrochemical potential, concentration of a substance, electric field, osmotic pressure, etc.), which does not require direct energy expenditure for its implementation. Passive transport of substances can occur through simple and facilitated diffusion. It is known that under diffusion understand the chaotic movements of particles of matter in various environments, caused by the energy of its thermal vibrations.
If the molecule of a substance is electrically neutral, then the direction of diffusion of this substance will be determined only by the difference (gradient) in the concentrations of the substance in media separated by a membrane, for example, outside and inside the cell or between its compartments. If the molecule or ions of a substance carry an electrical charge, then diffusion will be influenced by both the concentration difference, the amount of charge of this substance, and the presence and sign of charges on both sides of the membrane. The algebraic sum of the forces of concentration and electrical gradients on the membrane determines the magnitude of the electrochemical gradient.
Simple diffusion carried out due to the presence of concentration gradients of a certain substance, electrical charge or osmotic pressure between the sides of the cell membrane. For example, the average content of Na+ ions in blood plasma is 140 mmol/l, and in erythrocytes it is approximately 12 times less. This concentration difference (gradient) creates a driving force that allows sodium to move from plasma to red blood cells. However, the rate of such a transition is low, since the membrane has very low permeability to Na + ions. The permeability of this membrane to potassium is much greater. The processes of simple diffusion do not consume the energy of cellular metabolism.
The rate of simple diffusion is described by the Fick equation:
dm/dt = -kSΔC/x,
Where dm/ dt- the amount of substance diffusing per unit time; To - diffusion coefficient characterizing the permeability of the membrane for a diffusing substance; S- diffusion surface area; ΔС— the difference in concentrations of the substance on both sides of the membrane; X— distance between diffusion points.
From the analysis of the diffusion equation, it is clear that the rate of simple diffusion is directly proportional to the concentration gradient of a substance between the sides of the membrane, the permeability of the membrane for a given substance, and the diffusion surface area.
It is obvious that the easiest substances to move through the membrane by diffusion will be those substances whose diffusion occurs along both a concentration gradient and an electric field gradient. However, an important condition for the diffusion of substances through membranes is the physical properties of the membrane and, in particular, its permeability to the substance. For example, Na+ ions, the concentration of which is higher outside the cell than inside it, and the inner surface of the plasma membrane is negatively charged, should easily diffuse into the cell. However, the rate of diffusion of Na+ ions through the plasma membrane of a cell at rest is lower than that of K+ ions, which diffuses along the concentration gradient out of the cell, since the permeability of the membrane under resting conditions for K+ ions is higher than for Na+ ions.
Since the hydrocarbon radicals of phospholipids that form the membrane bilayer have hydrophobic properties, substances of a hydrophobic nature, in particular those easily soluble in lipids (steroids, thyroid hormones, some drugs, etc.), can easily diffuse through the membrane. Low-molecular substances of a hydrophilic nature, mineral ions diffuse through passive ion channels of membranes formed by channel-forming protein molecules, and, possibly, through packing defects in the membrane of phospholipid molecules that appear and disappear in the membrane as a result of thermal fluctuations.
Diffusion of substances in tissues can occur not only through cell membranes, but also through other morphological structures, for example, from saliva into the dentin tissue of a tooth through its enamel. In this case, the conditions for diffusion remain the same as through cell membranes. For example, for the diffusion of oxygen, glucose, and mineral ions from saliva into tooth tissue, their concentration in saliva must exceed the concentration in tooth tissue.
Under normal conditions, nonpolar and small electrically neutral polar molecules can pass through the phospholipid bilayer in significant quantities through simple diffusion. Transport of significant quantities of other polar molecules is carried out by carrier proteins. If the transmembrane transition of a substance requires the participation of a carrier, then instead of the term “diffusion” the term is often used transport of a substance across a membrane.
Facilitated diffusion, just like simple “diffusion” of a substance, occurs along its concentration gradient, but unlike simple diffusion, a specific protein molecule, a carrier, is involved in the transfer of a substance through the membrane (Fig. 2).
Facilitated diffusion is a type of passive transport of ions through biological membranes, which is carried out along a concentration gradient using a carrier.
The transfer of a substance using a carrier protein (transporter) is based on the ability of this protein molecule to integrate into the membrane, penetrating it and forming channels filled with water. The carrier can reversibly bind to the transported substance and at the same time reversibly change its conformation.
It is assumed that the carrier protein is capable of being in two conformational states. For example, in a state A this protein has an affinity for the transported substance, its substance binding sites are turned inward and it forms a pore open to one side of the membrane.
Rice. 2. Facilitated diffusion. Description in the text
Having contacted the substance, the carrier protein changes its conformation and enters the state 6 . During this conformational transformation, the carrier loses its affinity for the substance being transported; it is released from its connection with the carrier and is moved to a pore on the other side of the membrane. After this, the protein returns to state a again. This transfer of a substance by a transporter protein across a membrane is called uniport.
Through facilitated diffusion, low-molecular substances such as glucose can be transported from interstitial spaces into cells, from the blood into the brain, some amino acids and glucose can be reabsorbed from primary urine into the blood in the renal tubules, and amino acids and monosaccharides can be absorbed from the intestine. The rate of transport of substances by facilitated diffusion can reach up to 10 8 particles per second through the channel.
In contrast to the rate of transfer of a substance by simple diffusion, which is directly proportional to the difference in its concentrations on both sides of the membrane, the rate of transfer of a substance during facilitated diffusion increases in proportion to the increase in the difference in concentrations of the substance up to a certain maximum value, above which it does not increase, despite the increase in the difference in concentrations of the substance along both sides of the membrane. Achieving the maximum speed (saturation) of transfer in the process of facilitated diffusion is explained by the fact that at the maximum speed all molecules of carrier proteins are involved in transfer.
Exchange diffusion- with this type of transport of substances, an exchange of molecules of the same substance located on different sides of the membrane can occur. The concentration of the substance on each side of the membrane remains unchanged.
A type of exchange diffusion is the exchange of a molecule of one substance for one or more molecules of another substance. For example, in the smooth muscle cells of blood vessels and bronchi, in the contractile myocytes of the heart, one of the ways to remove Ca 2+ ions from the cells is to exchange them for extracellular Na+ ions. For every three incoming Na+ ions, one Ca 2+ ion is removed from the cell. An interdependent (coupled) movement of Na+ and Ca2+ through the membrane in opposite directions is created (this type of transport is called antiport). Thus, the cell is freed from excess Ca 2+ ions, which is a necessary condition for the relaxation of smooth myocytes or cardiomyocytes.
Active transport of substances
Active transport substances through is the transfer of substances against their gradients, carried out with the expenditure of metabolic energy. This type of transport differs from passive transport in that transport occurs not along a gradient, but against the concentration gradients of a substance, and it uses the energy of ATP or other types of energy for the creation of which ATP was previously spent. If the direct source of this energy is ATP, then such transfer is called primary active. If energy (concentration, chemical, electrochemical gradients) previously stored due to the operation of ion pumps that consumed ATP is used for transport, then such transport is called secondary active, as well as conjugate. An example of coupled, secondary active transport is the absorption of glucose in the intestine and its reabsorption in the kidneys with the participation of Na ions and GLUT1 transporters.
Thanks to active transport, the forces of not only concentration, but also electrical, electrochemical and other gradients of a substance can be overcome. As an example of the operation of primary active transport, we can consider the operation of the Na+ -, K+ -pump.
The active transport of Na + and K + ions is ensured by a protein enzyme - Na + -, K + -ATPase, which is capable of breaking down ATP.
The Na K-ATPase protein is found in the cytoplasmic membrane of almost all cells of the body, accounting for 10% or more of the total protein content in the cell. More than 30% of the total metabolic energy of the cell is spent on the operation of this pump. Na + -, K + -ATPase can be in two conformational states - S1 and S2. In the S1 state, the protein has an affinity for Na ion and 3 Na ions are attached to three high-affinity binding sites facing the cell. The addition of the Na" ion stimulates ATPase activity, and as a result of ATP hydrolysis, Na+ -, K+ -ATPase is phosphorylated due to the transfer of a phosphate group to it and carries out a conformational transition from the S1 state to the S2 state (Fig. 3).
As a result of changes in the spatial structure of the protein, the binding sites for Na ions turn to the outer surface of the membrane. The affinity of binding sites for Na+ ions sharply decreases, and it, having been released from the bond with the protein, is transferred to the extracellular space. In the conformational state S2, the affinity of Na+ -, K-ATPase centers for K ions increases and they attach two K ions from the extracellular environment. The addition of K ions causes dephosphorylation of the protein and its reverse conformational transition from the S2 state to the S1 state. Together with the rotation of the binding centers to the inner surface of the membrane, two K ions are released from their connection with the carrier and are transferred inside. Such transfer cycles are repeated at a rate sufficient to maintain in a resting cell the unequal distribution of Na+ and K+ ions in the cell and the intercellular medium and, as a consequence, to maintain a relatively constant potential difference on the membrane of excitable cells.
Rice. 3. Schematic representation of the operation of the Na+ -, K + -pump
The substance strophanthin (ouabain), isolated from the foxglove plant, has the specific ability to block the Na + -, K + - pump. After its introduction into the body, as a result of blocking the pumping of Na+ ion from the cell, a decrease in the efficiency of the Na+ -, Ca 2 -exchange mechanism and accumulation of Ca 2+ ions in contractile cardiomyocytes are observed. This leads to increased myocardial contraction. The drug is used to treat insufficiency of the pumping function of the heart.
In addition to Na "-, K + -ATPase, there are several other types of transport ATPases, or ion pumps. Among them, a pump that transports hydrogen gases (cell mitochondria, renal tubular epithelium, parietal cells of the stomach); calcium pumps (pacemaker and contractile cells of the heart, muscle cells of striated and smooth muscles). For example, in the cells of skeletal muscles and myocardium, the Ca 2+ -ATPase protein is embedded in the membranes of the sarcoplasmic reticulum and, thanks to its work, maintains a high concentration of Ca 2+ ions in its intracellular cells. storages (cisterns, longitudinal tubules of the sarcoplasmic reticulum).
In some cells, the forces of the transmembrane electrical potential difference and the sodium concentration gradient, resulting from the operation of the Na+, Ca 2+ pump, are used to carry out secondary active types of transfer of substances across the cell membrane.
Secondary active transport characterized by the fact that the transfer of a substance across the membrane is carried out due to the concentration gradient of another substance, which was created by the mechanism of active transport with the expenditure of ATP energy. There are two types of secondary active transport: symport and antiport.
Simport called the transfer of a substance, which is associated with the simultaneous transfer of another substance in the same direction. The symport mechanism transports iodine from the extracellular space to the thyrocytes of the thyroid gland, glucose and amino acids when they are absorbed from the small intestine into enterocytes.
Antiport called the transfer of a substance, which is associated with the simultaneous transfer of another substance, but in the opposite direction. An example of an antiporter transfer mechanism is the work of the previously mentioned Na + -, Ca 2+ - exchanger in cardiomyocytes, K + -, H + - exchange mechanism in the epithelium of the renal tubules.
From the above examples it is clear that secondary active transport is carried out through the use of gradient forces of Na+ ions or K+ ions. The Na+ ion or K ion moves through the membrane towards its lower concentration and pulls another substance with it. In this case, a specific carrier protein built into the membrane is usually used. For example, the transport of amino acids and glucose when they are absorbed from the small intestine into the blood occurs due to the fact that the membrane carrier protein of the epithelium of the intestinal wall binds to the amino acid (glucose) and the Na + ion and only then changes its position in the membrane in such a way that it transports the amino acid ( glucose) and Na+ ion into the cytoplasm. To carry out such transport, it is necessary that the concentration of the Na+ ion outside the cell is much greater than inside, which is ensured by the constant work of Na+, K+ - ATPase and the expenditure of metabolic energy.
BIOPHYSICS OF SUBSTANCE TRANSPORT THROUGH THE MEMBRANE.
Self-test questions
1. What objects does the infrastructure of the motor transport complex include?
2. Name the main components of environmental pollution by the motor transport complex.
3. Name the main reasons for the formation of environmental pollution by the motor transport complex.
4. Name the sources, describe the mechanisms of formation and characterize the composition of air pollution from industrial zones and areas of road transport enterprises.
5. Give the classification of wastewater from road transport enterprises.
6. Name and characterize the main pollutants of wastewater from road transport enterprises.
7. Describe the problem of industrial waste from road transport enterprises.
8. Characterize the distribution of the mass of harmful emissions and ATK waste by their types.
9. Analyze the contribution of ATK infrastructure facilities to environmental pollution.
10. What types of standards make up the system of environmental standards. Describe each of these types of standards.
| 1. Bondarenko E.V. Environmental safety of road transport: textbook for universities / E.V. Bondarenko, A.N. Novikov, A.A. Filippov, O.V. Chekmareva, V.V. Vasilyeva, M.V. Korotkov // Orel: Orel State Technical University, 2010. – 254 p. 2. Bondarenko E.V. Road transport ecology: [Text]: textbook. allowance / E.V. Bondarenko, G.P. Dvornikov Orenburg: RIK GOU OSU, 2004. – 113 p. 3. Kaganov I.L. Handbook on sanitation and hygiene in motor transport enterprises. [Text] / I.L. Kaganov, V.D. Moroshek Mn.: Belarus, 1991. – 287 p. 4. Kartoshkin A.P. The concept of collection and processing of used lubricating oils / A.P. Kartoshkin // Chemistry and technology of fuels and oils, 2003. - No. 4. – P. 3 – 5. 5. Lukanin V.N. Industrial and transport ecology [Text] / V.N. Lukanin, Yu.V. Trofimenko M.: Higher. school, 2001. - 273 p. 6. Russian motor transport encyclopedia. Technical operation, maintenance and repair of vehicles. – T.3. – M.: RBOOIP “Prosveshcheniye”, 2001. – 456 p. | |
A cell is an open system that continuously exchanges matter and energy with the environment. Transport of substances across biological membranes is a necessary condition for life. Cell metabolic processes, bioenergetic processes, the formation of biopotentials, the generation of a nerve impulse, etc. are associated with the transfer of substances through membranes. Violation of the transport of substances through biomembranes leads to various pathologies. Treatment often involves the penetration of drugs through cell membranes. The cell membrane is a selective barrier to various substances found inside and outside the cell. There are two types of membrane transport: passive and active transport.
All types of passive transport based on the principle of diffusion. Diffusion is the result of chaotic independent movements of many particles. Diffusion gradually reduces the concentration gradient until a state of equilibrium is reached. In this case, an equal concentration will be established at each point, and diffusion in both directions will occur equally. Diffusion is passive transport, since it does not require external energy. There are several types of diffusion in the plasma membrane:
1 ) Free diffusion.
Passive transport includes simple and facilitated diffusion - processes that do not require energy. Diffusion– transport of molecules and ions through the membrane from an area with high to an area with low concentration, those. substances flow along a concentration gradient. The diffusion of water through semipermeable membranes is called by osmosis. Water is also able to pass through membrane pores formed by proteins and transport molecules and ions of substances dissolved in it. The mechanism of simple diffusion carries out the transfer of small molecules (for example, O2, H2O, CO2); this process is low specific and occurs at a rate proportional to the concentration gradient of transported molecules on both sides of the membrane.
Facilitated diffusion carried out through channels and (or) carrier proteins that have specificity for the molecules being transported. Transmembrane proteins act as ion channels, forming small water pores through which small water-soluble molecules and ions are transported along an electrochemical gradient. Transporter proteins are also transmembrane proteins that undergo reversible conformational changes that enable 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 through which the transport of molecules is carried out using carrier proteins against an electrochemical gradient. An example of a mechanism that ensures 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 K+ concentration inside the cell is 10-20 times higher than outside, and the Na concentration is the opposite. This difference in ion concentrations is ensured by the work 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. A protein in the membrane takes part in this process, performing the function of an enzyme that breaks down ATP, releasing the energy needed to operate the pump.
The participation of specific membrane proteins in passive and active transport indicates the high specificity of this process. This mechanism ensures the maintenance of constant cell volume (by regulating osmotic pressure), as well as membrane potential. Active transport of glucose into the cell is carried out by a carrier protein and is combined with unidirectional transfer of Na + ion.
Lightweight transport ion flow is mediated by special transmembrane proteins - ion channels that provide selective transport of certain ions. These channels consist of the transport system itself and a gating mechanism that opens the channel for some time in response to a change in membrane potential, (b) mechanical influence (for example, in the hair cells of the inner ear), or binding of a ligand (signal molecule or ion).
Membrane transport of substances also varies according to 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 one direction through one transporter.
- Antiport is the movement of two substances in different directions through one transporter.
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.
Simport carries out a glucose transporter located on the external (facing the intestinal lumen) side of the intestinal epithelial cells. This protein simultaneously captures a glucose molecule and a sodium ion and, changing conformation, transfers both substances into the cell. This uses the energy of the electrochemical gradient, which, in turn, is created due to the hydrolysis of ATP by sodium-potassium ATPase.
Antiport carried out, for example, by sodium-potassium ATPase (or sodium-dependent ATPase). It transports potassium ions into the cell. and from the cell - sodium ions. Initially, this transporter attaches three ions to the inner side of the membrane Na+ . These ions change the conformation of the active site of ATPase. After such activation, the ATPase is able to hydrolyze one ATP molecule, and the phosphate ion is fixed on the surface of the transporter on the inside of the membrane.
The released energy is spent on changing the conformation of ATPase, after which three ions Na+ and the ion (phosphate) end up on the outside of the membrane. Here the ions Na+ are split off and replaced by two ions K+ . Then the carrier conformation changes to the original one, and the ions K+ appear on the inside of the membrane. Here the ions K+ are split off, and the vector is ready to work again
Transport? Transmembrane movement of various high-molecular compounds, cellular components, supramolecular particles that are not able to penetrate through channels in the membrane is carried out through special mechanisms, for example, through phagocytosis, pinocytosis, exocytosis, and transport through the intercellular space. That is, the movement of substances through the membrane can occur using various mechanisms, which are divided according to the participation of specific carriers in them, as well as energy consumption. Scientists divide the transport of substances into active and passive.
Main types of transport
Passive transport is the transfer of a substance through a biological membrane along a gradient (osmotic, concentration, hydrodynamic and others), which does not require energy consumption.
It is the transfer of a substance across a biological membrane against a gradient. This consumes energy. Approximately 30 - 40% of the energy that is generated as a result of metabolic reactions in the human body is spent on active transport of substances. If we consider the functioning of human kidneys, then about 70 - 80% of the oxygen consumed is spent on active transport.
Passive transport of substances
it involves the transfer of various substances through biological membranes along a variety of channels, which can be:
- electrochemical potential gradient;
- substance concentration gradient;
- electric field gradient;
- osmotic pressure gradient and others.
The process of passive transport does not require any energy consumption. It can occur by facilitated and simple diffusion. As we know, diffusion is the chaotic movement of molecules of a substance in various environments, which is caused by the energy of thermal vibrations of the substance.
If a particle of a substance is electrically neutral, then the direction in which diffusion will occur is determined by the difference in the concentration of substances contained in the media that are separated by a membrane. For example, between the compartments of the cell, inside the cell and outside it. If the particles of a substance and its ions have an electric charge, then diffusion will depend not only on the difference in concentrations, but also on the magnitude of the charge of the substance, the presence and signs of the charge on both sides of the membrane. The magnitude of the electrochemical gradient is determined by the algebraic sum of the electrical and concentration gradients on the membrane.
What ensures transport across the membrane?
Passive membrane transport is possible due to the presence of a substance, osmotic pressure arising between different sides of the cell membrane or an electrical charge. For example, the average level of Na+ ions contained in blood plasma is about 140 mmol/l, and its content in erythrocytes is approximately 12 times higher. Such a gradient, expressed as a difference in concentrations, can create a driving force that ensures the transfer of sodium molecules into red blood cells from the blood plasma.
It should be noted that the rate of such a transition is very low due to the fact that the cell membrane is characterized by low permeability to ions of this substance. This membrane is much more permeable to potassium ions. The energy of cellular metabolism is not used to carry out the process of simple diffusion.
Diffusion rate
Active and passive transport of substances across a membrane is characterized by the rate of diffusion. It can be described using the Fick equation: dm/dt=-kSΔC/x.
In this case, dm/dt represents the amount of the substance that diffuses in one unit of time, and k is the coefficient of the diffusion process, which characterizes the permeability of the biomembrane for the diffusing substance. S equals the area over which diffusion occurs, and ΔC expresses the difference in the concentration of substances on different sides of the biological membrane, while x characterizes the distance between the diffusion points.
Obviously, those substances that diffuse simultaneously along concentration gradients and electric fields will move most easily through the membrane. An important condition for the diffusion of a substance through a membrane is the physical properties of the membrane itself, its permeability for each specific substance.
Due to the fact that the membrane bilayer is formed by hydrocarbon radicals of phospholipids, which have a nature that easily diffuses through it. In particular, this applies to substances that are easily soluble in lipids, for example, thyroid and steroid hormones, as well as some narcotic substances.
Mineral ions and low molecular weight substances of a hydrophilic nature diffuse through passive membrane ion channels, which are formed from channel-forming protein molecules, and sometimes through membrane packing defects of phospholipid molecules that arise in the cell membrane as a result of thermal fluctuations.
Passive transport through a membrane is a very interesting process. If conditions are normal, then significant amounts of a substance can penetrate the membrane bilayer only if they are non-polar and small in size. Otherwise, transfer occurs through carrier proteins. Such processes involving a carrier protein are called not diffusion, but the transport of a substance through the membrane.
Facilitated diffusion
Facilitated diffusion, like simple diffusion, occurs along a concentration gradient of a substance. The main difference is that a special protein molecule called a transporter takes part in the process of transferring a substance.
Facilitated diffusion is a type of passive transfer of substance molecules through biomembranes, carried out along a concentration gradient using a carrier.
Transport protein states
The carrier protein can be in two conformational states. For example, in state A, a given protein may have an affinity for the substance that it transports; its sites for binding to the substance are turned inward, due to which a pore is formed that is open to one side of the membrane.
After the protein has bound to the transported substance, its conformation changes and it transitions to state B. With this transformation, the carrier loses its affinity for the substance. It is released from its connection with the carrier and moves to a pore on the other side of the membrane. After the substance is transferred, the carrier protein again changes its conformation, returning to state A. Such transport of the substance through the membrane is called uniport.
Velocity under facilitated diffusion
Low molecular weight substances such as glucose can be transported across the membrane by facilitated diffusion. Such transport can occur from the blood to the brain, to cells from the interstitial spaces. The rate of substance transfer with this type of diffusion can reach up to 10 8 particles through the channel in one second.
As we already know, the rate of active and passive transport of substances during simple diffusion is proportional to the difference in the concentrations of the substance on the two sides of the membrane. In the case of facilitated diffusion, this speed increases in proportion to the increasing difference in the concentration of the substance up to a certain maximum value. Above this value, the speed does not increase, even though the difference in concentrations on different sides of the membrane continues to increase. Achieving such a maximum speed point during facilitated diffusion can be explained by the fact that the maximum speed involves the involvement of all available carrier proteins in the transfer process.
What other concepts include active and passive transport through membranes?
Exchange diffusion
This type of transport of molecules of a substance through a cell membrane is characterized by the fact that molecules of the same substance, located on different sides of the biological membrane, participate in the exchange. It is worth noting that with this transport of substances on both sides of the membrane does not change at all.
A type of exchange diffusion
One of the types of exchange diffusion is an exchange in which a molecule of one substance is exchanged for two or more molecules of another substance. For example, one of the ways in which positive calcium ions are removed from the smooth muscle cells of the bronchi and blood vessels from the contractile myocytes of the heart is by exchanging them for sodium ions located outside the cell. In this case, one sodium ion is exchanged for three calcium ions. Thus, there is a movement of sodium and calcium through the membrane, which is interdependent. This type of passive transport through the cell membrane is called antiport. This is how the cell is able to free itself from calcium ions, which are in excess. This process is necessary for smooth muscle cells and cardiomyocytes to relax.
This article examined the active and passive transport of substances through the membrane.