Active transport of substances occurs against the total (generalized) gradient. This means that the transfer of a substance occurs from places with a lower value of the electrochemical potential to places with a higher value.
Active transport cannot occur spontaneously, but only in conjunction with the process of hydrolysis of adenosine triphosphoric acid (ATP), that is, due to the expenditure of energy stored in the high-energy bonds of the ATP molecule.
Active transport of substances across biological membranes is of great importance. Due to active transport, concentration gradients, electrical potential gradients, pressure gradients, etc. are created in the body that support life processes, that is, from the point of view of thermodynamics, active transport keeps the body in a non-equilibrium state, ensuring the normal course of life processes.
To carry out active transfer, in addition to the energy source, the existence of certain structures is necessary. According to modern concepts, biological membranes contain ion pumps that operate using the energy of ATP hydrolysis or so-called transport ATPases, represented by protein complexes.
Currently, three types of electrogenic ion pumps are known that actively transport ions across the membrane. These are K + -Na + -ATPase in cytoplasmic membranes (K + -Na + -pump), Ca 2+ - ATPase (Ca 2+ -pump) and H + - ATPase in the energy-coupling membranes of mitochondria (H + - pump or proton pump ).
The transfer of ions by transport ATPases occurs due to the coupling of transfer processes with chemical reactions, due to the energy of cell metabolism.
When K + -Na + -ATPase operates, due to the energy released during the hydrolysis of each ATP molecule, two potassium ions are transferred into the cell and three sodium ions are simultaneously pumped out of the cell. This creates an increased concentration of potassium ions in the cell compared to the intercellular environment and a decreased concentration of sodium, which is of great physiological importance.
Due to the energy of ATP hydrolysis, two calcium ions are transferred to the Ca 2+ -ATPase, and two protons are transferred to the H + pump.
The molecular mechanism of operation of ion ATPases is not fully understood. However, the main stages of this complex enzymatic process can be traced. In the case of K + -Na + -ATPase (let's denote it E for brevity), there are seven stages of ion transfer associated with ATP hydrolysis. Designations E 1 and E 2 correspond to the location of the active center of the enzyme on the inner and outer surfaces of the membrane (ADP-adenosine diphosphate, P - inorganic phosphate, the asterisk indicates the activated complex):
1) E + ATP à E*ATP,
2) E*ATP + 3Naà [E*ATP]*Na 3,
3) [E*ATP]*Nа 3 à *Na 3 + ADP,
4) *Na 3 à *Na 3 ,
5) *Na 3 + 2K à *K 2 + 3Na,
6) *K 2 à *K 2,
7) *K 2 à E + P + 2K.
The diagram shows that the key stages of the enzyme are: 1) the formation of a complex of the enzyme with ATP on the inner surface of the membrane (this reaction is activated by magnesium ions); 2) binding of three sodium ions by the complex; 3) phosphorylation of the enzyme with the formation of adenosine diphosphate; 4) change in the conformation of the enzyme inside the membrane; 5) the reaction of ion exchange of sodium to potassium, occurring on the outer surface of the membrane; 6) reverse change in the conformation of the enzyme complex with the transfer of potassium ions into the cell, and 7) return of the enzyme to its original state with the release of potassium ions and inorganic phosphate. Thus, during a complete cycle, three sodium ions are released from the cell, the cytoplasm is enriched with two potassium ions, and hydrolysis of one ATP molecule occurs.
In addition to the ion pumps discussed above, similar systems are known in which the accumulation of substances is not associated with ATP hydrolysis, but with the work of redox enzymes or photosynthesis. Transport of substances in this case is secondary, mediated by membrane potential and (or) ion concentration gradient in the presence of specific carriers in the membrane. This transport mechanism is called secondary active transport. In the plasma and subcellular membranes of living cells, the simultaneous functioning of primary and secondary active transport is possible. This transfer mechanism is especially important for those metabolites for which there are no pumps (sugars, amino acids).
The joint unidirectional transport of ions involving a two-site transporter is called symport. It is assumed that the membrane may contain a carrier in complex with a cation and anion and an empty carrier. Since the membrane potential does not change in such a transfer scheme, the transfer may be caused by a difference in the concentrations of one of the ions. It is believed that the symport scheme is used to accumulate amino acids in cells.
Conclusions and conclusions.
During life, cell boundaries are crossed by a variety of substances, the flows of which are effectively regulated. This task is accomplished by the cell membrane with transport systems built into it, including ion pumps, a system of carrier molecules, and highly selective ion channels.
At first glance, such an abundance of transfer systems seems unnecessary, because the operation of only ion pumps makes it possible to provide the characteristic features of biological transport: high selectivity, transfer of substances against the forces of diffusion and electric field. The paradox, however, is that the number of flows to be regulated is infinitely large, while there are only three pumps. In this case, the mechanisms of ionic conjugation, called secondary active transport, in which diffusion processes play an important role, become of particular importance. Thus, the combination of active transport of substances with the phenomena of diffusion transfer in the cell membrane is the basis that ensures the vital activity of the cell.
Developed by the head of the department of biological and medical physics, candidate of physical and mathematical sciences, associate professor Novikova N.G.
Transport of substances in and out of the cell, as well as between the cytoplasm and various subcellular organelles (mitochondria, nucleus, etc.) is ensured by membranes. If membranes were a solid barrier, then the intracellular space would be inaccessible to nutrients, and waste products could not be removed from the cell. At the same time, with complete permeability, the accumulation of certain substances in the cell would be impossible. The transport properties of the membrane are characterized semi-permeability : some compounds can penetrate through it, while others cannot:
Membrane permeability for various substances
One of the main functions of membranes is the regulation of substance transfer. There are two ways to transport substances across a membrane: passive And active transport:
Transport of substances across membranes
Passive transport . If a substance moves through a membrane from an area of high concentration to a low concentration (i.e., along the concentration gradient of this substance) without the cell expending energy, then such transport is called passive, or diffusion . There are two types of diffusion: simple And lightweight .
Simple diffusion characteristic of small neutral molecules (H 2 O, CO 2, O 2), as well as hydrophobic low molecular weight organic substances. These molecules can pass without any interaction with membrane proteins through membrane pores or channels as long as the concentration gradient is maintained.
Facilitated diffusion . Characteristic of hydrophilic molecules that are transported through the membrane also along a concentration gradient, but with the help of special membrane proteins - carriers. Facilitated diffusion, in contrast to simple diffusion, is characterized by high selectivity, since the transporter protein has a binding center complementary to the transported substance, and the transfer is accompanied by conformational changes in the protein. One possible mechanism for facilitated diffusion could be the following: a transport protein ( translocase ) binds a substance, then approaches the opposite side of the membrane, releases this substance, takes on its original conformation and is again ready to perform the transport function. Little is known about how the protein itself moves. Another possible transport mechanism involves the participation of several transporter proteins. In this case, the initially bound compound itself moves from one protein to another, sequentially binding with one or the other protein until it ends up on the opposite side of the membrane.
Active transport occurs when transport occurs against a concentration gradient. Such transfer requires energy expenditure by the cell. Active transport serves to accumulate substances inside the cell. The energy source is often APR. For active transport, in addition to an energy source, the participation of membrane proteins is necessary. One of the active transport systems in animal cells is responsible for the transport of Na + and K + ions across the cell membrane. This system is called Na + - K + - pump. It is responsible for maintaining the composition of the intracellular environment, in which the concentration of K + is higher than Na +:
Mechanism of action of Na + , K + -ATPase
The concentration gradient of potassium and sodium is maintained by the transfer of K + into the cell and Na + out. Both transports occur against the concentration gradient. This distribution of ions determines the water content in cells, the excitability of nerve cells and muscle cells, and other properties of normal cells. The Na + ,K + -pump is a protein - transport Asia-Pacific region . The molecule of this enzyme is an oligomer and penetrates the membrane. During the full cycle of pump operation, three Na + ions are transferred from the cell to the intercellular substance, and two K + ions are transferred in the opposite direction. This uses the energy of the ATP molecule. There are transport systems for the transfer of calcium ions (Ca 2+ - ATPases), proton pumps (H + - ATPases), etc. Simport This is the active transfer of a substance across a membrane, carried out by the energy of the concentration gradient of another substance. Transport ATPase in this case has binding centers for both substances. Antiport is the movement of a substance against its concentration gradient. In this case, another substance moves in the opposite direction along its concentration gradient. Simport And antiport may occur during the absorption of amino acids from the intestine and the reabsorption of glucose from primary urine. In this case, the energy of the concentration gradient of Na + ions created by Na +, K + -ATPase is used.
TO membrane proteins These include proteins that are embedded in or associated with the cell membrane or the membrane of a cell organelle. About 25% of all proteins are membrane proteins.
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 against concentration or electrical gradients.
Active transport
This is the transport of substances contrary to concentration or electrical gradients, which occurs with the expenditure of energy. A distinction is made between primary active transport, which requires the energy of ATP, and secondary (the creation of ionic concentration gradients on both sides of the membrane due to ATP, and the energy of these gradients is used for transport).
Primary active transport is widely used in the body. It is involved in creating an electrical potential difference 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 better studied. 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. High ATPase activity is observed on the inner surface of the protein. The energy generated during the hydrolysis of ATP 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 + is created in the extracellular fluid and a high concentration of K + - in the cellular fluid.
Recently, Ca2 + pumps have been intensively studied, thanks to which the concentration of Ca2 + 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 due to a carrier protein in membranes. This protein has high ATPase activity.
Secondary active transport. Thanks to 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 and is called simport. 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 together with Na +. The carrier protein has two sites for Na + binding and for glucose or amino acid binding. Five distinct 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 + goes into the cell, and Ca2 + leaves it, or Na + goes into the cell, and H + comes 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) attach to membrane receptors. These receptors are specific for different proteins. Their attachment is accompanied by activation of the contractile proteins of the cell - actin and myosin, which form and close a cavity with this extracellular protein and a small amount of extracellular fluid. In this case, a pinocytotic vesicle is formed. It secretes enzymes that hydrolyze this protein. Hydrolysis products are absorbed by cells. Pinocytosis requires ATP energy and the presence of Ca2+ in the extracellular environment.
Thus, there are many types of transport of substances across cell membranes. Different types of transport can occur on different sides of the cell (in the apical, basal, lateral membranes). An example of this would be processes occurring in
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 antiport - 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 of the pancreas release their secretion into the blood only when the concentration of glucose in the blood 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 one or the other direction.
BIOLOGICAL MEMBRANES
Concentration gradient(from lat. grady, gradu, gradus- progress, movement, flow, approach; con- with, together, jointly + centrum- center) or concentration gradient is vector physical quantity, characterizing the magnitude and direction of the greatest change concentrations any substance in the environment. For example, if we consider two regions with different concentrations of a substance, separated by a semi-permeable membrane, then the concentration gradient will be directed from the region of lower concentration of the substance to the region with higher concentration.
Active transport- transfer of matter through cellular or intracellular membrane(transmembrane A.t.) or through a layer of cells (transcellular A.t.), flowing against concentration gradient from an area of low concentration to an area of high, i.e., with the expenditure of free energy of the body. In most cases, but not always, the source of energy is the energy of high-energy bonds ATP.
Various transport ATPases, localized in cell membranes and involved in the mechanisms of substance transfer, are the main element of molecular devices - pumps that ensure the selective absorption and pumping out of certain substances (for example, electrolytes) by the cell. Active specific transport of non-electrolytes (molecular transport) is realized using several types of molecular machines - pumps and carriers. Transport of non-electrolytes (monosaccharides, amino acids and other monomers) can be coupled with simport- transport of another substance, the movement of which against the concentration gradient is a source of energy for the first process. Symport can be provided by ion gradients (for example, sodium) without the direct participation of ATP.
Passive transport- transfer of substances through concentration gradient from an area of high concentration to an area of low, without energy expenditure (for example, diffusion, osmosis). Diffusion is the passive movement of a substance from an area of higher concentration to an area of lower concentration. Osmosis is the passive movement of certain substances through a semi-permeable membrane (usually small molecules pass through, large molecules do not pass through).
There are three types of penetration of substances into cells through membranes: simple diffusion, facilitated diffusion, active transport.
Simple diffusion
In 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. By simple diffusion they penetrate into the cell hydrophobic substances (O2, N2, benzene) and polar small molecules (CO 2, H 2 O, urea). Polar relatively large molecules (amino acids, monosaccharides), charged particles (ions) and macromolecules (DNA, proteins) do not penetrate.
Facilitated diffusion
Most substances are transported across the membrane using 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 using 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 the transporter that binds to the transported substance, on the rate of binding of the substance by the transporter on one surface of the membrane (for example, on the outer surface), on the rate of conformational changes in the transporter molecule, in as a result of which the substance is transferred 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.
Transport proteins
Transport proteins are transmembrane proteins that specifically bind the molecule of the transported substance and, by changing the conformation, transport the molecule through the lipid layer of the membrane. Transport proteins of all types have specific binding sites for the transported molecule. They can provide both passive and active membrane transport.
All living cells are separated from their environment by a surface called the cell membrane. In addition, eukaryotes are characterized by the formation of several compartments inside cells. They are represented by a number of subcellular organelles bounded by membranes, for example, the nucleus and mitochondria. Membranes are not only statically organized interfaces, but also include active biochemical systems responsible for processes such as selective transport of substances in and out of the cell, binding of hormones and other regulatory molecules, the occurrence of enzymatic reactions, transmission of impulses from the nervous system, etc. . There are different types of membranes, differing in their functions. The functions of membranes are determined by their structure.
Functions of membranes
Chemical composition
Membranes consist of lipid and protein molecules, the relative amount of which varies (from 1/5 - protein + 4/5 - lipids to 3/4 - protein + 1/4 - lipids) for different membranes. Carbohydrates are contained in the form of glycoproteins, glycolipids and make up 0.5-10% of the membrane substance.
Membrane lipids
The main part of lipids in membranes is represented by phospholipids, glycolipids and cholesterol. The structure of these lipids is shown in the figure:
Structure of membrane lipids
Membrane lipids have two distinct parts in their structure: a nonpolar hydrophobic “tail” and a polar hydrophilic “head”. This dual nature of compounds is called amphiphilic. Membrane lipids form a two-layer structure. Each layer is composed of complex lipids arranged in such a way that the nonpolar hydrophobic tails of the molecules are in close contact with each other. The hydrophilic parts of the molecules also come into contact. All interactions are non-covalent. The two monolayers are oriented tail-to-tail so that the resulting double-layer structure has an internal nonpolar part and two polar surfaces. Membrane proteins are incorporated into the lipid bilayer in two ways:
associated with the hydrophilic surface of the lipid bilayer - surface membrane proteins
immersed in the hydrophobic region of the bilayer - integral membrane proteins.
Surface proteins with their hydrophilic amino acid radicals are linked by non-covalent bonds to the hydrophilic groups of the lipid bilayer. Integral proteins differ in the degree of immersion in the hydrophobic part of the bilayer. They can be located on both sides of the membrane and are either partially immersed in the membrane or pierced through the membrane. The immersed part of integral proteins contains a large number of amino acids with hydrophobic radicals, which provide hydrophobic interaction with membrane lipids. Hydrophobic interactions maintain a specific orientation of proteins in the membrane. The hydrophilic protein overhang cannot move into the hydrophobic layer. Some membrane proteins are covalently linked to monosaccharide residues or oligosaccharide chains and are glycoproteins. Examples of the arrangement of proteins and lipids in the membrane are presented in the figure:
Structure of the plasma membrane
Membrane asymmetry
Although each monolayer is formed from lipids oriented in the same way, the lipid composition of the monolayers is nevertheless different. For example, in the plasma membrane of red blood cells, phosphatidylcholines predominate in the outer layer, and phosphatidylserines in the inner layer of the membrane. The carbohydrate portions of proteins and lipids are located on the outer part of the membrane. In addition, membrane surfaces differ in protein composition. The degree of such membrane asymmetry varies among different types of membranes and can change during the life of the cell and its aging. Mobility (hardness) and fluidity membranes also depend on its composition. Increased hardness is caused by an increase in the ratio of saturated and unsaturated fatty acids, as well as cholesterol. The physical properties of membranes depend on the location of proteins in the lipid layer. Membrane lipids are capable of diffusion within the layer parallel to the membrane surface (lateral diffusion). Proteins are also capable of lateral diffusion. Transverse diffusion in membranes is highly limited.
Membrane transport
Transport of substances in and out of the cell, as well as between the cytoplasm and various subcellular organelles (mitochondria, nucleus, etc.) is ensured by membranes. If membranes were a solid barrier, then the intracellular space would be inaccessible to nutrients, and waste products could not be removed from the cell. At the same time, with complete permeability, the accumulation of certain substances in the cell would be impossible. The transport properties of the membrane are characterized semi-permeability : some compounds can penetrate through it, while others cannot:
Membrane permeability for various substances
One of the main functions of membranes is the regulation of substance transfer. There are two ways to transport substances across a membrane: passive And active transport:
Transport of substances across membranes
Passive transport . If a substance moves through a membrane from an area of high concentration to a low concentration (i.e., along the concentration gradient of this substance) without the cell expending energy, then such transport is called passive, or diffusion . There are two types of diffusion: simple And lightweight .
Simple diffusion characteristic of small neutral molecules (H 2 O, CO 2, O 2), as well as hydrophobic low molecular weight organic substances. These molecules can pass without any interaction with membrane proteins through membrane pores or channels as long as the concentration gradient is maintained.
Facilitated diffusion . Characteristic of hydrophilic molecules that are transported through the membrane also along a concentration gradient, but with the help of special membrane proteins - carriers. Facilitated diffusion, in contrast to simple diffusion, is characterized by high selectivity, since the transporter protein has a binding center complementary to the transported substance, and the transfer is accompanied by conformational changes in the protein. One possible mechanism for facilitated diffusion could be the following: a transport protein ( translocase ) binds a substance, then approaches the opposite side of the membrane, releases this substance, takes on its original conformation and is again ready to perform the transport function. Little is known about how the protein itself moves. Another possible transport mechanism involves the participation of several transporter proteins. In this case, the initially bound compound itself moves from one protein to another, sequentially binding with one or the other protein until it ends up on the opposite side of the membrane.
Active transport occurs when transport occurs against a concentration gradient. Such transfer requires energy expenditure by the cell. Active transport serves to accumulate substances inside the cell. The energy source is often APR. For active transport, in addition to an energy source, the participation of membrane proteins is necessary. One of the active transport systems in animal cells is responsible for the transport of Na + and K + ions across the cell membrane. This system is called Na + - K + - pump. It is responsible for maintaining the composition of the intracellular environment, in which the concentration of K + is higher than Na +:
Mechanism of action of Na + , K + -ATPase
The concentration gradient of potassium and sodium is maintained by the transfer of K + into the cell and Na + out. Both transports occur against the concentration gradient. This distribution of ions determines the water content in cells, the excitability of nerve cells and muscle cells, and other properties of normal cells. The Na + ,K + -pump is a protein - transport Asia-Pacific region . The molecule of this enzyme is an oligomer and penetrates the membrane. During the full cycle of pump operation, three Na + ions are transferred from the cell to the intercellular substance, and two K + ions are transferred in the opposite direction. This uses the energy of the ATP molecule. There are transport systems for the transfer of calcium ions (Ca 2+ - ATPases), proton pumps (H + - ATPases), etc. Simport This is the active transfer of a substance across a membrane, carried out by the energy of the concentration gradient of another substance. Transport ATPase in this case has binding centers for both substances. Antiport is the movement of a substance against its concentration gradient. In this case, another substance moves in the opposite direction along its concentration gradient. Simport And antiport may occur during the absorption of amino acids from the intestine and the reabsorption of glucose from primary urine. In this case, the energy of the concentration gradient of Na + ions created by Na +, K + -ATPase is used.