What is active transport. Active transport of substances

Active transport is a process in which a molecule must move across a membrane regardless of the direction of its concentration gradient. Most often, this occurs from a region with a lower concentration to a region with a higher one and is accompanied by an increase in free energy, which is 5.71 logC2/C| kJ-mol-1.

As stated earlier, this is the process of transfer of substances from places with a lower electrochemical potential to places with a higher value.

Since active transport in the membrane is accompanied by an increase in the Gibbs energy, it cannot occur spontaneously, i.e., such a process requires coupling with some spontaneous reaction. In general, this can be accomplished in two ways: 1) in conjunction with the process of ATP hydrolysis, i.e., through the expenditure of energy stored in high-energy bonds; 2) mediated by membrane potential and/or ion concentration gradient in the presence and membrane of specific transporters.

In the first case, transport is carried out using electrogenic ion pumps, powered by the free energy of ATP hydrolysis. They are classified as special systems of integral proteins and are called transport ATPases. Currently, three types of electrogenic ion pumps are known that transport ions across the membrane: K+ - Na+ - ATPase, 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 pumped out; in Ca2+ - ATPase, due to the energy of ATP hydrolysis, two calcium ions are transferred; there are two protons in the H+ pump.

In the second case, the transport of substances is secondary, for which three schemes have been deeply studied.

Unidirectional transfer of an ion in combination with a specific carrier is called uniport. In this case, charge is transferred across the membrane either by a complex if the carrier molecule is electrically neutral, or by an empty carrier if transport is provided by a charged carrier. The result of the transfer will be the accumulation of ions due to a decrease in the membrane potential. This effect is observed when potassium ions accumulate in the presence of valinomycin in energized mitochondria.

The counter transfer of ions with the participation of a single carrier molecule is called antiport. It is assumed that the carrier molecule forms a strong complex with each of the transferred ions. The transfer occurs in two stages: first, one ion crosses the membrane from left to right, then the second ion crosses in the opposite direction. The membrane potential does not change. Apparently, the driving force in this process is the difference in the concentrations of one of the transferred ions. If initially there was no difference in the concentration of the second ion, then the result of the transfer will be the accumulation of the second ion due to a decrease in the difference in the concentrations of the first. A classic example of an antiport is the transfer of potassium and hydrogen ions across the cell membrane with the participation of the antibiotic nigiricin. It should be noted that most carrier proteins function as an antiport, i.e., the movement of a substance through the membrane becomes possible only in exchange for some rather specific substance that has the same charge, but moves in the opposite direction.

Thus, the release of any main component of the cell along a concentration gradient can control the movement of an oncoming substance against its gradient and perform “work” until both driving forces are balanced.

The joint unidirectional transfer of substances with the participation of a two-seater transporter is called symport. It is assumed that the membrane may contain two electrically neutral particles: a carrier in a complex with a cation and anion, and an empty carrier. Since the membrane potential in such a transport system does not change, the transport may be caused by a difference in the concentrations of one of the ions. It is believed that according to the symport scheme, it follows that this process must be accompanied by a significant shift in the osmotic equilibrium, since in one cycle two particles are transferred through the membrane in the same direction.

Thanks to the availability of fairly well-developed theories about the mechanisms of transport of ions and endogenous organic substances in the cell, it has become possible to interpret the data obtained in experiments with drugs (section 6.3.3).

By analogy with Fig. 6.10 active transport can be represented as shown in Fig. 6.11.

In this case, the carrier C forms a complex CA on the outer side of the membrane with the drug (L). It penetrates the membrane, splitting off L from its other side. In the case of active transport, the concentration of L on the inner side of the membrane can be much greater than the concentration on the outer side. In contrast to passive transport (Fig. 6.10), the SA complex, using the energy of ATP, is converted into the SA complex, which easily splits off L (Fig. 6.11). Considering the need for energy costs to carry out the transport of SA to the opposite side of the membrane, we can assume that /(, (cleavage constant) on the inner side is greater than K0. This is the so-called asymmetric cleavage of the drug-carrier complex.

External aqueous phase

Concentration [L]0 Activity (L)0

In living organisms, active transport mechanisms are widespread and can be considered one of the fundamental functions of the cell. For example, cells have a high concentration of potassium and a low concentration of sodium, in contrast to the extracellular space, where these ions are in an inverse relationship. The membranes are freely permeable for both ions and the asymmetric distribution is maintained by constantly “pumping” sodium out of the cell and potassium inward. .The secretion of HC1 in the stomach is a true active transport of H+ and SG. Iodine is concentrated in the thyroid gland by a similar mechanism. Sugars are transported against higher concentrations in the intestines and proximal renal tubules. Amino acids behave similarly in the intestines, kidneys, muscles and brain. The secretion of organic acids (napa-aminobenzoic, hippuric) by the renal tubules is an active transport process.

The mechanism of active transport is highly specific, since it was created by nature to satisfy the biological need of the body for essential nutrients or remove the products of their metabolism from it. As for drugs undergoing active transport, in this case they should be close in chemical structure to the natural substances of the body. The pyrimidine analogue fluorafur and iron are absorbed through active transport in the intestine. Using the same mechanism, levodopa penetrates the blood-brain barrier. The renal tubules secrete drugs belonging to organic acids and bases.

Summarizing the consideration of the mechanisms of transmembrane transport of substances, it is necessary to once again emphasize that in the process of life, the boundaries of the cell 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.

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

Facilitated diffusion. Characteristic of hydrophilic molecules that are transported through the membrane also along a concentration gradient, but with the help of special membrane proteins - carriers. Facilitated diffusion, in contrast to simple diffusion, is characterized by high selectivity, since the transporter protein has a binding center complementary to the transported substance, and the transfer is accompanied by conformational changes in the protein. One 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. 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 (Ca2+ - 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.

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Classification[edit | edit wiki text]

Membrane proteins can be classified according to topological or biochemical principles. Topological classification is based on how many times the protein crosses the lipid bilayer. In accordance with this criterion, proteins are divided into monotopic, bitopic And polytopic:

· monotopic proteins interact with one surface of the membrane and do not cross it;

· bitopic penetrate the membrane through and interact with both of its surfaces;

· polytopic penetrate the membrane several times (multiple interactions with lipids).

It is clear that the first belong to peripheral proteins, and the second and third to integral.

Various categories of polytopic proteins. Membrane binding due to (1) a single transmembrane alpha helix, (2) multiple transmembrane alpha helices, (3) a beta-sheet structure.

Various categories of integral monotopic proteins. Binding to the membrane due to (1) an amphipathic alpha helix parallel to the plane of the membrane, (2) a hydrophobic loop, (3) a covalently linked fatty acid residue, (4) electrostatic interaction (direct or calcium-mediated).

Topological classification[edit | edit wiki text]

In relation to the membrane, membrane proteins are divided into poly- and monotopic.

· Polytopic, or transmembrane, proteins completely penetrate the membrane and thus interact with both sides of the lipid bilayer. Typically, the transmembrane fragment of a protein is an alpha helix consisting of hydrophobic amino acids (possibly from 1 to 20 such fragments). Only in bacteria, as well as in mitochondria and chloroplasts, transmembrane fragments can be organized as a beta-sheet structure (from 8 to 22 turns of the polypeptide chain).

· Integral monotopic proteins permanently embedded in the lipid bilayer, but connected to the membrane on only one side, without penetrating the opposite side.

Biochemical classification[edit | edit wiki text]

According to the biochemical classification, membrane proteins are divided into integral And peripheral.

· Integral membrane proteins firmly embedded in the membrane and can be removed from the lipid environment only with the help of detergents or non-polar solvents. In relation to the lipid bilayer, integral proteins can be transmembrane polytopic or integral monotopic.

· Peripheral membrane proteins are monotopic proteins. They are either weakly bound to the lipid membrane or associate with integral proteins due to hydrophobic, electrostatic or other non-covalent forces. Thus, unlike integral proteins, they dissociate from the membrane when treated with an appropriate aqueous solution (eg, low or high pH, high salt concentration, or a chaotropic agent). This dissociation does not require membrane disruption.

Membrane proteins can be integrated into the membrane due to fatty acid or prenyl residues or glycosylphosphatidylinositol attached to the protein during their post-translational modification.

7) The carbohydrate part of the glycolipids and glycoproteins of the plasma membrane is always located on the outer surface of the membrane, in contact with the intercellular substance. Plasma membrane carbohydrates act as specific ligands for proteins. They form recognition sites to which certain proteins attach; the attached protein can change the functional state of the cell.

Functions of carbohydrates.

In the outer membrane of red blood cells, some polysaccharides contain N-acetylneuraminic acid at the ends of their chains. If erythrocytes are isolated from the blood, treated in vitro with neuraminidase, which cleaves N-acetylneuraminic acid from membrane carbohydrates, and reintroduced into the blood of the same animal, it is found that the half-life of such erythrocytes in the blood decreases several times: they are retained in the spleen and destroyed . As it turned out, the spleen cells have a receptor that recognizes carbohydrate, which has lost the terminal neuraminic acid residues. It is possible that such a mechanism ensures the selection of “aged” red blood cells by the spleen and their destruction.
It is known that in a suspension of cells isolated from any tissue, after some time cell aggregates are formed, and each aggregate, as a rule, contains cells of the same type. For example, in a suspension of cells obtained from the gastrula, three types of aggregates are formed: each of them contains cells belonging to the same germ layer - ectoderm, mesoderm or endoderm. Recognition between cells is ensured, in particular, by the interaction of membrane carbohydrates of one cell with receptor proteins of another cell (Fig. 9.39). These recognition mechanisms may be involved in processes such as histogenesis and morphogenesis. However, there are other mechanisms that ensure intercellular contacts.
Polysaccharides of the cell membrane, along with proteins, act as antigens during the development of cellular immunity, including during transplant rejection. They also serve as recognition sites when infected by pathogenic viruses and microorganisms. For example, when an influenza virus enters a cell, it first attaches to its membrane, interacting with a polysaccharide of a certain structure.

8) cell membranes have selective permeability: glucose, amino acids, fatty acids, glycerol and ions slowly diffuse through them, and the membranes themselves, to a certain extent, actively regulate this process - some substances pass through, but others do not. There are four main mechanisms for the entry of substances into the cell or their removal from the cell to the outside: diffusion, osmosis, active transport and exo- or endocytosis. The first two processes are passive in nature, that is, they do not require energy; the last two are active processes associated with energy consumption.

The selective permeability of the membrane during passive transport is due to special channels - integral proteins. They penetrate the membrane right through, forming a kind of passage. The elements K, Na and Cl have their own channels. Relative to the concentration gradient, the molecules of these elements move in and out of the cell. When irritated, the sodium ion channels open and a sudden influx of sodium ions into the cell occurs. In this case, an imbalance of membrane potential occurs. After which the membrane potential is restored. Potassium channels are always open, allowing potassium ions to slowly enter the cell.

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.

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.

Developed by the head of the department of biological and medical physics, candidate of physical and mathematical sciences, associate professor Novikova N.G.

Active transport- this is the transfer of a substance from places with a lower electrochemical potential to places with a higher value.

Active transport in the membrane is accompanied by an increase in the Gibbs energy; it 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 high-energy bonds of ATP.

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, supporting life processes, i.e. from the point of view of thermodynamics, active transfer keeps the organism in a non-equilibrium state and maintains life.

The existence of active transport of substances through biological membranes was first proven in the experiments of Ussing (1949) using the example of the transfer of sodium ions through the skin of a frog (Fig. 12).

Rice. 12. Scheme of Ussing's experiments (A - ammeter, V - voltmeter, B - battery, P - potentiometer)

Ussing's experimental chamber, filled with normal Ringer's solution, was divided into two parts with freshly isolated frog skin. In Fig. 12, on the left - the outer mucosal surface of the skin, on the right - the inner serous. Flows of sodium ions through the skin of a frog were observed: from left to right from the outer to the inner surface and from right to left from the inner to the outer surface.

From Theorell's equation, which describes passive transport, it follows Ussing-Theorell equation for the ratio of these flows in the case of passive transport:

J m,in /j m,nar = (With out /With in)×e ZF j / RT

On the skin of a frog dividing the Ringer's solution, a potential difference arises (j in - j nar) - the inner side of the skin has a positive potential in relation to the outer. The Ussing installation (Fig. 12) had a voltage compensation unit, with the help of which the potential difference on the skin of the frog was set to zero, which was controlled by a voltmeter. The same concentration of ions was maintained on the outer and inner sides: C out = C in.

Under these conditions, if the transfer of sodium through the skin of a frog was determined only by passive transport, then according to the Ussing-Theorell equation, the flows j m, in and j m, nar were equal to each other: j m, in = j m, nar

The total flux through the membrane would be zero.

Using an ammeter, it was discovered that under experimental conditions (the absence of gradients of electrical potential and concentration), an electric current I flows through the skin of the frog, therefore, a one-way transfer of charged particles occurs. It has been established that current flows through the skin from the external to the internal environment.

Experimental data irrefutably indicated that the transport of sodium ions through the skin of a frog does not obey the passive transport equation. Therefore, active transfer takes place.

Electrogenic ion pumps

According to modern concepts, biological membranes contain ion pumps, working at the expense of the free energy of ATP hydrolysis - special systems of integral proteins (transport ATPases).

Currently, three types of electrogenic ion pumps are known that actively transport ions through the membrane (Fig. 13).

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.

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.

Fig.13. Types of ion pumps: a) K + -Na + - ATPase in cytoplasmic membranes

(K + -Na + -pump); b) - Ca 2+ -ATPase (Ca 2+ -pump); c) - H + -ATPase in the energy-coupling membranes of mitochondria and chloroplasts (H + -pump, or proton pump)

The molecular mechanism of operation of ion ATPases is not fully understood. Nevertheless, the main stages of this complex enzymatic process can be traced. In the case of K + -Na + -ATPase, there are seven stages of ion transfer associated with ATP hydrolysis.

The diagram shows that the key stages of the enzyme are:

1) formation of an enzyme complex 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) revolution (flip-flop) 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 revolution of the enzyme complex with the transfer of potassium ions into the cell;

7) return of the enzyme to its original state with the release of potassium ions and inorganic phosphate (P).

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.

Secondary active ion transport.

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. The 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. This mechanism was considered in most detail by Peter Mitchell (1966) in the chemiosmotic theory of oxidative phosphorylation. In the plasma and subcellular membranes of living cells, the simultaneous functioning of primary and secondary active transport is possible. An example is the inner membrane of mitochondria. Inhibition of the ATPase in it does not deprive the particle of the ability to accumulate substances due to secondary active transport. This method of accumulation is especially important for those metabolites for which there are no pumps (sugars, amino acids).

Currently, three schemes of secondary active transport have been studied in sufficient depth. Let us consider the transport of monovalent ions with the participation of carrier molecules. This implies that the transporter, in a loaded or unloaded state, crosses the membrane equally well. The source of energy is the membrane potential and/or the concentration gradient of one of the ions. The circuits are shown in Fig. 14. Unidirectional transfer of an ion in complex with a specific carrier is called uniport . In this case, charge is transferred across the membrane either by a complex, if the carrier molecule is electrically neutral, or by an empty carrier, if the transfer is provided by a charged carrier. The result of the transfer will be the accumulation of ions due to a decrease in the membrane potential. This effect is observed when potassium ions accumulate in the presence of valinomycin in energized mitochondria.

The counter transfer of ions involving a single carrier molecule is called antiport . It is assumed that the carrier molecule forms a strong complex with each of the transferred ions. The transfer occurs in two stages: first, one ion crosses the membrane from left to right, then the second ion crosses in the opposite direction. The membrane potential does not change. What is the driving force behind this process? Obviously, the difference in the concentrations of one of the transferred ions. If initially there was no difference in the concentration of the second ion, then the result of the transfer will be the accumulation of the second ion due to a decrease in the difference in the concentrations of the first. A classic example of an antiport is the transfer of potassium and hydrogen ions across the cell membrane with the participation of the antibiotic molecule nigericin.

Joint unidirectional transport of ions involving a two-site transporter is called simport . It is assumed that the membrane may contain two electrically neutral particles: a carrier complexed 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. The potassium-sodium pump (Fig. 13) creates an initial concentration gradient of sodium ions, which then, according to the symport scheme, contribute to the accumulation of amino acids. From the symport scheme it follows that this process must be accompanied by a significant shift in osmotic equilibrium, since in one cycle two particles are transferred through the membrane in the same direction.

Fig. 14. Basic schemes of secondary active ion transport

The cell is a structural unit of all life on our planet and an open system. This means that its life requires a constant exchange of substances and energy with the environment. This exchange takes place through the membrane - the main boundary of the cell, which is designed to preserve its integrity. It is through the membrane that cellular exchange occurs and it occurs either along the concentration gradient of a substance or against it. Active transport across the cytoplasmic membrane is a complex and energy-consuming process.

Membrane - barrier and gateway

The cytoplasmic membrane is part of many cellular organelles, plastids and inclusions. Modern science is based on the fluid mosaic model of membrane structure. Active transport of substances through the membrane is possible due to its specific structure. The basis of the membranes is formed by a lipid bilayer - these are mainly phospholipids, arranged in accordance with their The main properties of the lipid bilayer are fluidity (the ability to insert and lose sections), self-assembly and asymmetry. The second component of membranes is proteins. Their functions are diverse: active transport, reception, fermentation, recognition.

Proteins are located both on the surface of the membrane and inside, and some penetrate it several times. The property of proteins in a membrane is the ability to move from one side of the membrane to the other (“flip-flop” jump). And the last component is the saccharide and polysaccharide chains of carbohydrates on the surface of the membranes. Their functions are still controversial today.

Types of active transport of substances across the membrane

Active will be the transfer of substances across the cell membrane, which is controlled, occurs with energy expenditure and goes against the concentration gradient (substances are transferred from an area of low concentration to an area of high concentration). Depending on what energy source is used, the following types of transport are distinguished:

Primary active (energy source - hydrolysis to adenosine diphosphorus ADP).
Secondarily active (provided by secondary energy created as a result of the operation of the mechanisms of primary active transport of substances).

Helper proteins

In both the first and second cases, transport is impossible without carrier proteins. These transport proteins are very specific and are designed to transport specific molecules, and sometimes even a specific type of molecule. This was proven experimentally using mutated bacterial genes, which resulted in the impossibility of active transport of a certain carbohydrate across the membrane. Transmembrane transport proteins can be carriers themselves (they interact with molecules and directly carry them through the membrane) or channel-forming proteins (they form pores in membranes that are open to specific substances).

Sodium and potassium pump

The most studied example of primary active transport of substances across a membrane is the Na+ -, K+ -pump. This mechanism ensures the difference in the concentrations of Na+ and K+ ions on both sides of the membrane, which is necessary to maintain osmotic pressure in the cell and other metabolic processes. The transmembrane transport protein, sodium-potassium ATPase, consists of three parts:

On the outside of the membrane, the protein has two receptors for potassium ions.
On the inside of the membrane there are three receptors for sodium ions.
The inner part of the protein has ATP activity.

When two potassium ions and three sodium ions bind to protein receptors on either side of the membrane, ATP activity is activated. The ATP molecule is hydrolyzed to ADP with the release of energy, which is expended on the transfer of potassium ions inward and sodium ions outward of the cytoplasmic membrane. It is estimated that the efficiency of such a pump is more than 90%, which in itself is quite surprising.

For reference: the efficiency of an internal combustion engine is about 40%, of an electric one - up to 80%. Interestingly, the pump can also work in the opposite direction and serve as a phosphate donor for ATP synthesis. Some cells (for example, neurons) typically spend up to 70% of their total energy on removing sodium from the cell and pumping potassium ions inside. Pumps for calcium, chlorine, hydrogen and some other cations (ions with a positive charge) operate on the same principle of active transport. No such pumps have been found for anions (negatively charged ions).

Cotransport of carbohydrates and amino acids

An example of secondary active transport is the transfer of glucose, amino acids, iodine, iron and uric acid into cells. As a result of the operation of the potassium-sodium pump, a gradient of sodium concentrations is created: the concentration is high outside and low inside (sometimes 10-20 times). Sodium tends to diffuse into the cell and the energy of this diffusion can be used to transport substances out. This mechanism is called cotransport or coupled active transport. In this case, the carrier protein has two receptor centers on the outside: one for sodium, and the other for the element being transported. Only after activation of both receptors does the protein undergo conformational changes, and the energy of sodium diffusion introduces the transported substance into the cell against the concentration gradient.

The importance of active transport for the cell

If the usual diffusion of substances through the membrane proceeded for any length of time, their concentrations outside and inside the cell would equalize. And this is death for cells. After all, all biochemical processes must take place in an environment of electrical potential difference. Without active, anti-transport of substances, neurons would not be able to transmit nerve impulses. And muscle cells would lose the ability to contract. The cell would not be able to maintain osmotic pressure and would collapse. And metabolic products would not be excreted. And the hormones would never enter the bloodstream. After all, even an amoeba spends energy and creates a potential difference on its membrane using the same ion pumps.