Passive and active transport of substances across the membrane. Transport of substances across the membrane. Facilitated diffusion properties

BIOPHYSICS OF TRANSPORT OF SUBSTANCES THROUGH A MEMBRANE.

Questions for self-examination

1. What facilities 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 atmospheric pollution by industrial zones and sections of road transport enterprises.

5. Give the classification of wastewater from road transport enterprises.

6. Name and characterize the main pollution of wastewater from road transport enterprises.

7. Describe the problem of waste production activities of road transport enterprises.

8. Give a description of the distribution of the mass of harmful emissions and wastes of ATC by their types.

9. Analyze the contribution of ATC infrastructure facilities to environmental pollution.

10. What types of regulations make up the system of environmental regulations. Describe each of these types of standards.

1. Bondarenko E.V. Environmental safety of road transport: a textbook for universities / E.V. Bondarenko, A.N. Novikov, A.A. Filippov, O.V. Chekmareva, V.V. Vasilyeva, M.V. Korotkov // Oryol: OrelGTU, 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 of sanitation and hygiene at motor transport enterprises. [Text] / I.L. Kaganov, V.D. Moroshek Minsk: Belarus, 1991. - 287 p. 4. Kartoshkin A.P. The concept of collection and processing of waste 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 motor vehicles. - T.3. - M.: RBOOIP "Enlightenment", 2001. - 456 p.

A cell is an open system that continuously exchanges matter and energy with the environment. The transport of substances across biological membranes is a necessary condition for life. The transfer of substances through membranes is associated with the processes of cell metabolism, bioenergetic processes, the formation of biopotentials, the generation of a nerve impulse, etc. Violation of the transport of substances through biomembranes leads to various pathologies. Treatment is often associated with the penetration of drugs through cell membranes. The cell membrane is a selective barrier to various substances 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 motions 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 be carried out equally. Diffusion is a passive transport, since it does not require external energy. There are several types of diffusion in the plasma membrane:

1 ) free diffusion.

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Video: Transport in Cells Diffusion and Osmosis, part - 1 Transport in cells: Diffusion and Osmosis, part - 1

Diffusion across the cell membrane is divided into two subtypes: simple diffusion and facilitated diffusion. Simple diffusion means that the kinetic movement of molecules or ions occurs through a hole in the membrane or intermolecular spaces without any interaction with membrane carrier proteins. The diffusion rate is determined by the amount of substance, the rate of kinetic movement, the number and size of holes in the membrane through which molecules or ions can move.

Video: Transport of substances in the body

Facilitated diffusion requires interaction with a carrier protein, which facilitates the transport of molecules or ions by chemically binding to them and in this form circulating through the membrane.

simple diffusion can pass through the cell membrane in two ways: (1) through the intermolecular gaps of the lipid bilayer, if the diffusible substance is fat-soluble; (2) through water-filled channels penetrating some large transport proteins, as shown in Fig.

Transport of substances across the membrane. Active and passive transport of substances across the membrane

Diffusion of fat-soluble substances through the lipid bilayer. One of the most important factors determining the rate of diffusion of a substance through a lipid bilayer is its solubility in lipids. For example, oxygen, nitrogen, carbon dioxide, and alcohols have a higher lipid solubility, so they can directly dissolve in the lipid bilayer and diffuse through the cell membrane in the same way that water-soluble substances diffuse in aqueous solutions. Obviously, the amount of diffusion of each of these substances is directly proportional to their lipid solubility. In this way, a very large amount of oxygen can be transported. Thus, oxygen can be delivered into cells almost as quickly as if the cell membrane did not exist.

Diffusion of water and other insoluble fats molecules through protein channels. Despite the fact that water does not dissolve at all in the lipids of the membrane, it easily passes through the channels in the protein molecules penetrating the membrane through. The speed with which water molecules can move through most cell membranes is amazing. For example, the total amount of water that diffuses in any direction through the erythrocyte membrane per second is about 100 times greater than the volume of the cell itself.

Through the channels provided protein pores, other lipid-insoluble molecules can also pass if they are water-soluble and small enough. However, an increase in the size of such molecules rapidly reduces their penetrating power. For example, the possibility of penetration of urea through the membrane is about 1000 times less than that of water, although the diameter of the urea molecule is only 20% larger than the diameter of the water molecule. However, given the astonishing rate of water passage, the penetrating power of urea allows it to be rapidly transported across the membrane within minutes.

Diffusion through protein channels

Computer 3D reconstruction of protein channels demonstrated the presence of tubular structures penetrating the membrane through and through - from extracellular to intracellular fluid. Therefore, substances can move through these channels by simple diffusion from one side of the membrane to the other. Protein channels are distinguished by two important features: (1) they are often selectively permeable to certain substances; (2) many channels can be opened or closed by gates.

Video: Membrane Potentials - Part 1

Electoral permeability of protein channels. Many protein channels are highly selective for the transport of one or more specific ions or molecules. This is due to the channel's own characteristics (diameter and shape), as well as to the nature of the electric charges and chemical bonds of its lining surfaces. For example, one of the most important protein channels - the so-called sodium channel - has a diameter of 0.3 to 0.5 nm, but, more importantly, the inner surfaces of this channel are highly negatively charged. These negative charges can draw small, dehydrated sodium ions into the channels, effectively pulling these ions out of the surrounding water molecules. Once in the channel, sodium ions diffuse in any direction according to the usual diffusion rules. In this regard, the sodium channel is specifically selective for the conduction of sodium ions.

These channels are somewhat smaller than sodium channels. channels, their diameter is only about 0.3 nm, but they are not negatively charged and have different chemical bonds. Consequently, there is no pronounced force pulling the ions into the channel, and potassium ions are not released from their aqueous shell. The hydrated form of the potassium ion is much smaller in size than the hydrated form of the sodium ion, because the sodium ion attracts many more water molecules than the potassium ion. Therefore, the smaller hydrated potassium ions can easily pass through this narrow channel, while the larger hydrated sodium ion is "culled", which provides selective permeability for a specific ion.

Source: http://meduniver.com
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Transport of substances: mechanisms for the penetration of substances into the cell

Passive transport

The movement of a substance (ions or small molecules) along a concentration gradient. It is carried out without energy expenditure by simple diffusion, osmosis or facilitated diffusion with the help of carrier proteins.

active transport

Transport of substances (ions or small molecules) by means of carrier proteins against a concentration gradient. It is carried out with the cost of ATP.

Endocytosis

Absorption of substances (large particles or macromolecules) by surrounding them with outgrowths of the cytoplasmic membrane with the formation of membrane-surrounded vesicles.

Exocytosis

The release of substances (large particles or macromolecules) from the cell by surrounding them with outgrowths of the cytoplasmic membrane with the formation of membrane-surrounded vesicles.

Phagocytosis and reverse phagocytosis

Absorption and release of solid and large particles. characteristic of animal and human cells.

Pinocytosis and Reverse Pinocytosis

Absorption and release of liquid and dissolved particles. characteristic of plant and animal cells.

Kirilenko A. A. Biology.

TRANSPORT OF SUBSTANCES THROUGH THE MEMBRANE

USE. Section "Molecular Biology". Theory, training tasks. 2017.

chemical nature transported substance and its concentration from sizes

Passive transport

way simple diffusion osmosis.

facilitated diffusion.

carrier proteins And channel proteins. carrier protein,

Protein channels

"gates", which open for a short time and then close.

Depending on the nature of the channel, the gates can open in response to the binding of signaling molecules (ligand-gated gate channels), changes in membrane potential (voltage-gated gate channels), or mechanical stimulation.

Active transport

sodium-potassium pump

The pump is formed by specific proteins-enzymes adenosine triphosphatases built into biological membranes, which catalyze the cleavage of phosphoric acid residues from the ATP molecule.

The composition of ATPases includes: an enzyme center, an ion channel and structural elements that prevent the back leakage of ions during the operation of the pump. The operation of the sodium-potassium pump consumes more than 1/3 of the ATP consumed by the cell.

Uniport - coporters, or associated carriers. symport antiporte - in opposite directions. For example, a sodium-potassium pump works according to the antiport principle, actively pumping Na + ions from cells, and K + ions into cells against their electrochemical gradients. An example of a symport is the reabsorption of glucose and amino acids from primary urine by renal tubular cells. In the primary urine, the concentration of Na + is always significantly higher than in the cytoplasm of the cells of the renal tubules, which is ensured by the work 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 the 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 the primary urine into the cell.

In this case, for the transport of glucose, as can be seen, the conjugated carrier uses the energy of the gradient of Na + ions created by the operation of the sodium-potassium pump. Thus, the operation of the sodium-potassium pump and the conjugated transporter, which uses a gradient of Na + ions for glucose transport, makes it possible to reabsorb almost all glucose from the primary urine and include it in the general metabolism of the body.

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 the 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 that 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 action of the signal.

electrogenic pumps.

characterized by the fact that the transported substances at certain stages of transport are located inside the membrane vesicles, that is, they are surrounded by a membrane.

22. Transport of substances through the membrane. Active and passive transport

Depending on the direction in which substances are transferred (into or out of the cell), transport in membrane packaging is divided into endocytosis and exocytosis.

Endocytosis

Phagocytosis -

pseudopodia, phagosome.

pinocytosis

bordered fossae clathrin. bordered bubble,

Exocytosis

Constitutive exocytosis

Regulated exocytosis

During exocytosis, secretory vesicles formed in the cytoplasm are usually directed to specialized areas of the surface apparatus containing a large amount of fusion proteins or fusion proteins. When the fusion proteins of the plasmalemma and the secretory vesicle interact, a fusion pore is formed that connects the cavity of the vesicle with the extracellular environment. At the same time, the actomyosin system is activated, as a result of which the contents of the vesicle pour out of it outside the cell. Thus, during induced exocytosis, energy is required not only for the transport of secretory vesicles to the plasmalemma, but also for the secretion process.

Transcytosis, or recreation , -

Methods of transport of substances through the membrane.

Most life processes, such as absorption, excretion, conduction of a nerve impulse, muscle contraction, ATP synthesis, maintaining a constant ionic composition and water content, are associated with the transfer of substances through membranes. This process in biological systems is called transport . The exchange of substances between the cell and its environment occurs constantly. The mechanisms of transport of substances into and out of the cell depend on the size of the transported particles. Small molecules and ions are transported by the cell directly across the membrane in the form of passive and active transport.

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

Diffusion – penetration of substances through the membrane along the concentration gradient (from the area where their concentration is higher to the area where their concentration is lower); this process occurs without energy expenditure due to the chaotic movement of molecules. Diffuse transport of substances (water, ions) is carried out with the participation of integral membrane proteins, in which there are molecular pores (channels through which dissolved molecules and ions pass), or with the participation of the lipid phase (for fat-soluble substances). With the help of diffusion, dissolved molecules of oxygen and carbon dioxide, as well as poisons and drugs, enter the cell.

Types of transport through the membrane.1 - simple diffusion; 2 - diffusion through membrane channels; 3 - facilitated diffusion with the help of carrier proteins; 4 - active transport.

Facilitated diffusion. The transport of substances through the lipid bilayer by simple diffusion occurs at a low rate, especially in the case of charged particles, and is almost uncontrolled. Therefore, in the process of evolution, specific membrane channels and membrane carriers appeared for some substances, which contribute to an increase in the transfer rate and, in addition, carry out selective transport.

Passive transport of substances by means of carriers is called facilitated diffusion. Special carrier proteins (permease) are built into the membrane. Permeases selectively bind to one or another ion or molecule and transfer them across the membrane. In this case, the particles move faster than with conventional diffusion.

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

Filtration — seepage of pore substances towards lower pressure values. An example of filtration in the body is the transfer of water through the walls of blood vessels, squeezing blood plasma into the renal tubules.

Rice. Movement of cations along an electrochemical gradient.

active transport. If only passive transport existed in cells, then the concentrations, pressures, and other quantities outside and inside the cell would be equal. Therefore, there is another mechanism that works in the direction against the electrochemical gradient and occurs with the expenditure of energy by the cell. The transfer of molecules and ions against the electrochemical gradient, carried out by the cell due to the energy of metabolic processes, is called active transport. It is inherent only in biological membranes. The active transfer of a substance across the membrane occurs due to free energy released during chemical reactions inside the cell. Active transport in the body creates concentration gradients, electrical potentials, pressures, i.e. maintains life in the body.

Active transport consists in the movement of substances against a concentration gradient with the help of transport proteins (porins, ATPases, etc.), which form diaphragm pumps, with the expenditure of ATP energy (potassium-sodium pump, regulation of the concentration of calcium and magnesium ions in cells, the intake of monosaccharides, nucleotides, amino acids). Three main active transport systems have been studied, which provide the transfer of Na, K, Ca, H ions through the membrane.

Mechanism. Ions K + and Na + are unevenly distributed on different sides of the membrane: the concentration of Na + outside > K + ions, and inside the cell K + > Na + . These ions diffuse through the membrane in the direction of the electrochemical gradient, which leads to its alignment. Na-K pumps are part of the cytoplasmic membranes and work due to the energy of hydrolysis of ATP molecules with the formation of ADP molecules and inorganic phosphate F n: ATP \u003d ADP + P n. The pump works reversibly: ion concentration gradients promote the synthesis of ATP molecules from the mol-l ADP and F n: ADP + F n \u003d ATP.

The Na + /K + -pump is a transmembrane protein capable of conformational changes, as a result of which it can attach both "K +" and "Na +".

Membrane transport

In one cycle of operation, the pump removes three "Na +" from the cell and starts two "K +" due to the energy of the ATP molecule. The sodium-potassium pump consumes almost a third of all the energy necessary for the life of the cell.

Not only individual molecules, but also solids can be transported through the membrane ( phagocytosis), solutions ( pinocytosis). Phagocytosis – capture and absorption of large particles(cells, cell parts, macromolecules) and pinocytosis – capturing and absorbing liquid material(solution, colloidal solution, suspension). The resulting pinocytic vacuoles range in size from 0.01 to 1-2 microns. Then the vacuole plunges into the cytoplasm and laces off. At the same time, the wall of the pinocytic vacuole completely retains the structure of the plasma membrane that gave rise to it.

If a substance is transported into the cell, then this mode of transport is called endocytosis ( transfer into the cell by direct pino or phagocytosis), if outside, then - exocytosis ( transport out of the cell by reverse pinot or phagocytosis). In the first case, an invagination is formed on the outer side of the membrane, which gradually turns into a bubble. The bubble detaches from the membrane inside the cell. Such a vesicle contains a transported substance surrounded by a bilipid membrane (vesicle). Subsequently, the vesicle merges with some cell organelle and releases its contents into it. In the case of exocytosis, the process occurs in the reverse order: the vesicle approaches the membrane from the inside of the cell, merges with it, and ejects its contents into the intercellular space.

Pinocytosis and phagocytosis are fundamentally similar processes in which four phases can be distinguished: the intake of substances by pino- or phagocytosis, their cleavage under the action of enzymes secreted by lysosomes, the transfer of cleavage products into the cytoplasm (due to changes in the permeability of vacuole membranes) and the release of metabolic products. Many protozoa and some leukocytes are capable of phagocytosis. Pinocytosis is observed in the epithelial cells of the intestine, in the endothelium of blood capillaries.

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Transport of substances across the plasma membrane

The barrier-transport function of the surface apparatus of the cell is provided by the selective transfer of ions, molecules and supramolecular structures into and out of the cell. Transport across membranes ensures the delivery of nutrients and removal of end products of metabolism from the cell, secretion, creation of ionic gradients and transmembrane potential, maintenance of the necessary 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 concentration on both sides of the cell membrane, and 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 by means of transport in a "membrane package", that is, due to the formation of bubbles surrounded by a membrane.

Passive transport The movement of substances across a membrane along their concentration gradient without the expenditure of energy is called. Such transport occurs through two main mechanisms: simple diffusion and facilitated diffusion.

way 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 the membrane, carried out by passive diffusion, is called osmosis. An example of simple diffusion is the transport of gases through the plasma membrane of the endothelial cells of blood capillaries into the surrounding tissue fluid and back.

Hydrophilic molecules and ions that are not able to pass through the membrane on their own 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 through the membrane. Facilitated diffusion is characterized by high selectivity with respect to the transported substances.

Protein channels form water-filled pores penetrating the lipid bilayer. When these pores are open, inorganic ions or molecules of transported substances pass through them and thus are transported through the membrane. Ion channels provide a transfer of 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 for a short time and then close. Depending on the nature of the channel, the gates can open in response to the binding of signaling molecules (ligand-gated gate channels), changes in membrane potential (voltage-gated gate channels), or mechanical stimulation.

Active transport is the movement of substances across a membrane against their concentration gradients. It is carried out with the help of carrier proteins and requires the expenditure of energy, the main source of which is ATP.

An example of active transport, which uses the energy of ATP hydrolysis to pump Na + and K + ions through the cell membrane, is the work sodium-potassium pump, providing the creation of a membrane potential on the plasma membrane of cells.

The pump is formed by specific proteins-enzymes adenosine triphosphatases built into biological membranes, which catalyze the cleavage of phosphoric acid residues from the ATP molecule. The composition of ATPases includes: an enzyme center, an ion channel and structural elements that prevent the back leakage of ions during the operation of the pump. The operation of the sodium-potassium pump consumes more than 1/3 of the ATP consumed by the cell.

Depending on the ability of transport proteins to carry 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 conjugated transport, a carrier protein is capable of simultaneously transporting two or more types of molecules or ions. These carrier proteins are called coporters, or associated carriers. There are two types of coport: symport and antiport. When symport molecules or ions are transported in one direction, and when antiporte - in opposite directions. For example, a sodium-potassium pump works according to the antiport principle, actively pumping Na + ions from cells, and K + ions into cells against their electrochemical gradients.

An example of a symport is the reabsorption of glucose and amino acids from primary urine by renal tubular cells. In the primary urine, the concentration of Na + is always significantly higher than in the cytoplasm of the cells of the renal tubules, which is ensured by the work 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 the 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 the primary urine into the cell. In this case, for the transport of glucose, as can be seen, the conjugated carrier uses the energy of the gradient of Na + ions created by the operation of the sodium-potassium pump. Thus, the operation of the sodium-potassium pump and the conjugated transporter, which uses a gradient of Na + ions for glucose transport, makes it possible to reabsorb almost all glucose from the primary urine and include it in the general metabolism of the body.

Due to the selective transport of charged ions, the plasmalemma of almost all cells carries positive charges on its outer side, and negative charges on the 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 plasma membrane: the sodium-potassium pump and protein channels for K + ions.

The opening of Na + channels causes a change in the transmembrane potential (membrane depolarization) and a specific cell response to the action of the signal.

Transport proteins that generate a potential difference across the membrane are called electrogenic pumps. The sodium-potassium pump serves as the main electrogenic pump of the cells.

Transport in membrane packaging characterized by the fact that the transported substances at certain stages of transport are located inside the membrane vesicles, that is, they are surrounded by a membrane. Depending on the direction in which substances are transferred (into or out of the cell), transport in membrane packaging is divided into endocytosis and exocytosis.

Endocytosis the process of absorption by a cell of macromolecules and larger particles (viruses, bacteria, cell fragments) is called. Endocytosis is carried out by phagocytosis and pinocytosis.

Phagocytosis - the process of active capture and absorption by the cell of solid microparticles, the size of which is more than 1 micron (bacteria, cell fragments, etc.). During phagocytosis, the cell recognizes specific molecular groups of the phagocytosed particle with the help of special receptors.

Then, at the point of contact of the particle with the cell membrane, outgrowths of the plasma membrane are formed - pseudopodia, which envelop the microparticle from all sides. As a result of the fusion of pseudopodia, such a particle is enclosed within a vesicle surrounded by a membrane, which is called phagosome. The formation of phagosomes is an energy-dependent process and proceeds with the participation of the actomyosin system. The phagosome, immersed in the cytoplasm, can merge with the late endosome or lysosome, as a result of which the organic microparticle absorbed by the cell, such as 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 engulf bacteria as well as a variety of solid particles that have entered the body, and thereby protect it from pathogens and foreign particles.

pinocytosis- absorption by the cell of fluid in the form of true and colloidal solutions and suspensions. This process is in general similar to phagocytosis: a drop of liquid is immersed in the formed depression of the cell membrane, surrounded by it and is enclosed in a bubble 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 is carried out in specialized areas of the cell surface apparatus, called the bordered pits, which occupy about 2% of the cell surface. bordered fossae are small invaginations of the plasmalemma, next to which there is a large amount of protein in the peripheral hyaloplasm clathrin. In the area of bordered pits on the cell surface, there are also numerous receptors that can specifically recognize and bind transported molecules. When these molecules are bound by receptors, clathrin polymerization occurs, and the plasmalemma invaginates. As a result, a bordered bubble, carrying the transported molecules. Such bubbles got their name due to the fact that clathrin on their surface under an electron microscope looks like an uneven border. After separation from the plasmalemma, the bordered vesicles lose their 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 bordered pits is 1000 times higher than their concentration in the environment. Pinocytosis is the main mode of transport of proteins, lipids and glycoproteins into the cell. Through pinocytosis, the cell absorbs an amount of fluid per day equal to its volume.

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, as a rule, is covered 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 out of the cell or, while maintaining a connection with the plasmalemma, are included in the glycocalyx.

There are two types of exocytosis: constitutive (basic) and regulated.

Constitutive exocytosis proceeds continuously in all cells of the body. It serves as the main mechanism for the removal of metabolic products from the cell and the constant restoration of the cell membrane.

Regulated exocytosis carried out only in special cells that perform a secretory function. The released secret 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 of the pancreas release their secret into the blood only when the concentration of glucose in the blood increases.

At the same time, the actomyosin system is activated, as a result of which the contents of the vesicle pour out of it outside the cell. Thus, during induced exocytosis, energy is required not only for the transport of secretory vesicles to the plasmalemma, but also for the secretion process.

Transcytosis, or recreation , - it is a transport in which individual molecules are transported 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 be carried out both in one direction and in the other.

Passive transport includes simple and facilitated diffusion - processes that do not require energy expenditure. Diffusion is the transport of molecules and ions across a membrane from an area of high concentration to an area of low concentration. Substances move along a concentration gradient. The diffusion of water across semipermeable membranes is called 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 of little specificity and proceeds at a rate proportional to the concentration gradient of transported molecules on both sides of the membrane. Facilitated diffusion occurs through channels and/or carrier proteins that are specific for the molecules being transported. The ion channels are transmembrane proteins that form small water pores through which small water-soluble molecules and ions are transported along the electrochemical gradient. Carrier proteins are also transmembrane proteins that undergo reversible conformational changes that ensure the transport of specific molecules across the plasmalemma. They function in the mechanisms of both passive and active transport.

active transport is an energy-intensive process due to which the transfer of molecules is carried out with the help of carrier proteins against an electrochemical gradient. An example of a mechanism that provides oppositely directed active transport of ions is the sodium-potassium pump (represented by the carrier protein Na + -K + -ATPase), due to which Na + ions are removed from the cytoplasm, and K + ions are simultaneously transferred into it. The concentration of K+ inside the cell is 10-20 times higher than outside, and the concentration of Na is vice versa. This difference in ion concentrations is ensured by the operation of the (Na * -K *> pump. To maintain this concentration, three Na ions are transferred from the cell for every two K * ions into the cell. This process involves a protein in the membrane that acts as an enzyme that breaks down ATP, releasing the energy needed to run the pump.
The participation of specific membrane proteins in passive and active transport indicates the high specificity of this process. This mechanism maintains the constancy of the cell volume (by regulating the osmotic pressure), as well as the membrane potential. Active transport of glucose into the cell is carried out by a carrier protein and is combined with the unidirectional transfer of the Na + ion.

Lightweight transport ions is mediated by special transmembrane proteins - ion channels that provide selective transfer of certain ions. These channels consist of the transport system itself and a gate mechanism that opens the channel for some time in response to (a) a change in membrane potential, (b) mechanical action (for example, in the hair cells of the inner ear), (c) binding of a ligand (signaling molecule or ion).

Transport across the membrane of small molecules.

Membrane transport may involve the unidirectional transport of molecules of a substance or the joint transport of two different molecules in the same or opposite directions.

Different molecules pass through it at different speeds, and the larger the size of the molecules, the lower the speed of their passage through the membrane. This property defines the plasma membrane as an osmotic barrier. Water and the gases dissolved in it have the maximum penetrating power. One of the most important properties of the plasma membrane is associated with the ability to pass various substances into or out of the cell. This is necessary to maintain the constancy of its composition (i.e. homeostasis).

Ion transport.

Unlike artificial lipid bilayer membranes, natural membranes, and primarily the plasma membrane, are still capable of transporting ions. The permeability for ions is small, and the speed of passage of different ions is not the same. Higher transmission speed for cations (K+, Na+) and much lower for anions (Cl-). The transport of ions through the plasmalemma occurs due to the participation in this process of membrane transport proteins - permeases. These proteins can transport one substance in one direction (uniport) or several substances simultaneously (symport), or, together with the import of one substance, remove another from the cell (antiport). For example, glucose can enter cells symportally together with the Na+ ion. Ion transport can take place along the concentration gradient- passively without additional energy consumption. For example, the Na+ ion enters the cell from the external environment, where its concentration is higher than in the cytoplasm.

It would seem that the presence of protein transport channels and carriers should lead to an equilibrium in the concentrations of ions and low molecular weight substances on both sides of the membrane. In fact, this is not so: the concentration of ions in the cytoplasm of cells differs sharply not only from that in the external environment, but even from the blood plasma that bathes the cells in the animal body.

It turns out that in the cytoplasm the concentration of K + is almost 50 times higher, and Na + is lower than in blood plasma. Moreover, this difference is maintained only in a living cell: if the cell is killed or the metabolic processes in it are suppressed, then after a while the ionic differences on both sides of the plasma membrane will disappear. You can simply cool the cells to +20C, and after a while the concentration of K+ and Na+ on both sides of the membrane will become the same. When the cells are heated, this difference is restored. This phenomenon is due to the fact that there are membrane protein carriers in cells that work against the concentration gradient, while expending energy due to ATP hydrolysis. This type of work is called active transport, and it is carried out with the help of protein ion pumps. The plasma membrane contains a two-subunit molecule (K + + Na +)-pump, which is also an ATPase. During operation, this pump pumps out 3 Na+ ions in one cycle and pumps 2 K+ ions into the cell against the concentration gradient. In this case, one ATP molecule is spent, which goes to ATPase phosphorylation, as a result of which Na + is transferred through the membrane from the cell, and K + gets the opportunity to bind to the protein molecule and then is transferred into the cell. As a result of active transport with the help of membrane pumps, the concentration of divalent cations Mg2+ and Ca2+ is also regulated in the cell, also with the consumption of ATP.

Thus, the active transport of glucose, which symportically (simultaneously) enters the cell along with the flow of the passively transported Na+ ion, will depend on the activity of the (K+ + Na+) pump. If this (K + -Na +) - pump is blocked, then soon the difference in the concentration of Na + on both sides of the membrane will disappear, while the diffusion of Na + into the cell will decrease, and at the same time the flow of glucose into the cell will stop. As soon as the work of (K + -Na +) -ATPase is restored and a difference in the concentration of ions is created, the diffuse flow of Na + immediately increases and at the same time the transport of glucose. Similarly, through the membrane and the flow of amino acids, which are transported by special carrier proteins that work as symport systems, simultaneously transporting ions.

The active transport of sugars and amino acids in bacterial cells is due to a gradient of hydrogen ions. In itself, the participation of special membrane proteins involved in the passive or active transport of low molecular weight compounds indicates the high specificity of this process. Even in the case of passive ion transport, proteins “recognize” a given ion, interact with it, bind

specifically, change their conformation and function. Consequently, already on the example of the transport of simple substances, membranes act as analyzers, as receptors. This receptor role is especially manifested when biopolymers are absorbed by the cell.

Passive transport is the transport of substances along a concentration gradient that does not require energy. Hydrophobic substances are passively transported through the lipid bilayer. All protein-channels and some carriers pass substances passively through themselves. Passive transport involving membrane proteins is called facilitated diffusion.

Other carrier proteins (sometimes called pump proteins) transport substances across the membrane at the expense of energy, which is usually supplied by ATP hydrolysis. This type of transport occurs against the concentration gradient of the transported substance and is called active transport.

Symport, antiport and uniport

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

1) Uniport - transport of one substance in one direction depending on the gradient

2) Symport - transport of two substances in one direction through one carrier.

3) Antiport - the movement of two substances in different directions through one carrier.

Uniport carries out, for example, a voltage-dependent sodium channel through which sodium ions move into the cell during the generation of an action potential.

Symport carries out a glucose transporter located on the outer (facing the intestinal lumen) side of the cells of the intestinal epithelium. This protein simultaneously captures a glucose molecule and a sodium ion and, changing its conformation, transfers both substances into the cell. In this case, the energy of the electrochemical gradient is used, which, in turn, is created due to the hydrolysis of ATP by sodium-potassium ATP-ase.

Antiport carries out, for example, sodium-potassium ATPase (or sodium-dependent ATPase). It transports potassium ions into the cell. and out of the cell - sodium ions.

Work of sodium-potassium atPase as an example of antiport and active transport

Initially, this carrier attaches three ions to the inside of the membrane Na+ . These ions change the conformation of the ATPase active site. After such activation, ATPase is able to hydrolyze one ATP molecule, and the phosphate ion is fixed on the surface of the carrier from the inside of the membrane.

The released energy is spent on changing the ATPase conformation, after which three ions Na+ and ion (phosphate) are on the outside of the membrane. Here the ions Na+ split off, and is replaced by two ions K+ . Then the conformation of the carrier changes to the original one, and the ions K+ appear on the inner side of the membrane. Here the ions K+ are split off, and the carrier is ready for work again.

More briefly, the actions of ATPase can be described as follows:

1) It “takes” three ions from inside the cell Na+ , then splits the ATP molecule and attaches phosphate to itself

2) "Throws out" ions Na+ and adds two ions K+ from the external environment.

3) Removes phosphate, two ions K+ throws into the cell

As a result, a high concentration of ions is created in the extracellular environment. Na+ , and inside the cell - a high concentration K+ . Job Na + , K+ - ATPase creates not only a difference in concentrations, but also a difference in charges (it works like an electrogenic pump). A positive charge is created on the outside of the membrane, and a negative charge on the inside.

AND active transport. Passive transport occurs without energy consumption along an electrochemical gradient. Passive ones include diffusion (simple and facilitated), osmosis, filtration. Active transport requires energy and occurs in spite of a concentration or electrical gradient.
active transport
This is the transport of substances in spite of the concentration or electrical gradient, which occurs with energy costs. There are primary active transport, which requires the energy of ATP, and secondary (the creation of ion concentration gradients on both sides of the membrane due to ATP, and the energy of these gradients is already used for transport).
Primary active transport is widely used in the body. It is involved in creating a difference in electrical potentials between the inner and outer sides of the cell membrane. With the help of active transport, various concentrations of Na +, K +, H +, SI "" and other ions are created in the middle of the cell and in the extracellular fluid.
The transport of Na+ and K+ - Na+,-K+-Hacoc has been studied better. This transport occurs with the participation of a globular protein with a molecular weight of about 100,000. The protein has three Na + binding sites on the inner surface and two K + binding sites on the outer surface. There is a high activity of ATPase on the inner surface of the protein. The energy generated during ATP hydrolysis leads to conformational changes in the protein and, at the same time, three Na + ions are removed from the cell and two K + ions are introduced into it. With the help of such a pump, a high concentration of Na + in the extracellular fluid and a high concentration of K + - in the cell.
Recently, Ca2+ pumps have been intensively studied, due to which the Ca2+ concentration in the cell is tens of thousands of times lower than outside it. There are Ca2 + pumps in the cell membrane and in cell organelles (sarcoplasmic reticulum, mitochondria). Ca2+ pumps also function at the expense of the carrier protein in the membranes. This protein has a high ATPase activity.
secondary active transport. Due to the primary active transport, a high concentration of Na + is created outside the cell, conditions arise for the diffusion of Na + into the cell, but together with Na +, other substances can enter it. This transport "is directed in one direction, is called symporta. Otherwise, the entry of Na + stimulates the exit of another substance from the cell, these are two flows directed in different directions - an antiport.
An example of a symport would be the transport of glucose or amino acids along with Na+. The carrier protein has two sites for Na + binding and for glucose or amino acid binding. Five separate proteins have been identified to bind five types of amino acids. Other types of symport are also known - transport of N + together with into the cell, K + and Cl- from the cell, etc.
In almost all cells, there is an antiport mechanism - Na + enters the cell, and Ca2 + leaves it, or Na + - into the cell, and H + - out of it.
Mg2 +, Fe2 +, HCO3- and many other substances are actively transported through the membrane.
Pinocytosis is one of the types of active transport. It lies in the fact that some macromolecules (mainly proteins, the macromolecules of which have a diameter of 100-200 nm) are attached to the membrane receptors. These receptors are specific for different proteins. Their attachment is accompanied by the activation of the contractile proteins of the cell - actin and myosin, which form and close the cavity with this extracellular protein and a small amount of extracellular fluid. This creates a pinocytic vesicle. It secretes enzymes that hydrolyze this protein. Hydrolysis products are absorbed by cells. Pinocytosis requires the energy of ATP and the presence of Ca2+ in the extracellular environment.
Thus, there are many modes of transport of substances across cell membranes. Different types of transport can occur on different sides of the cell (in the apical, basal, and lateral membranes). An example of this would be the processes that take place in

simple diffusion

By the path of simple diffusion, particles of a substance move through the lipid bilayer. The direction of simple diffusion is determined only by the difference in the concentrations of the substance on both sides of the membrane. Hydrophobic substances (O 2 , N 2 , benzene) and polar small molecules (CO 2 , H 2 O, urea) penetrate the cell by simple diffusion. Polar relatively large molecules (amino acids, monosaccharides), charged particles (ions) and macromolecules (DNA, proteins) do not penetrate.

Facilitated diffusion

Most substances are transported through the membrane with the help of transport proteins (carrier proteins) immersed in it. All transport proteins form a continuous protein passage across the membrane. With the help of carrier proteins, both passive and active transport of substances is carried out. Polar substances (amino acids, monosaccharides), charged particles (ions) pass through membranes with the help of facilitated diffusion, with the participation of channel proteins or carrier proteins. The participation of carrier proteins provides a higher rate of facilitated diffusion compared to simple passive diffusion. The rate of facilitated diffusion depends on a number of reasons: on the transmembrane concentration gradient of the transported substance, on the amount of carrier that binds to the transported substance, on the rate of binding of the substance by the carrier on one surface of the membrane (for example, on the outer), on the rate of conformational changes in the carrier molecule, in as a result of which the substance is transported through the membrane and released on the other side of the membrane. Facilitated diffusion does not require special energy costs due to ATP hydrolysis. This feature distinguishes facilitated diffusion from active transmembrane transport.

Carrier proteins

Carrier proteins are transmembrane proteins that specifically bind the molecule of the transported substance and, by changing the conformation, carry out the transfer of the molecule through the lipid layer of the membrane. Carrier proteins of all types have specific binding sites for the transported molecule. They can provide both passive and active membrane transport.