Mechanism of absorption of nutrients by roots. The mechanism of ion absorption, the role of diffusion and adsorption processes, their characteristics, the concept of free space; transport of ions across the plasma membrane, the role of the vacuole, pinocytosis Mechanisms of uptake

Rice. 7.19. Saturation curves for the absorption of two substrates by intact bacterial cells [built according to the consumption of 02 (respiratory intensity)]. Active and passive uptake of the substrate can be distinguished by the shape of the curve. Since substrate A is absorbed by active transport and accumulates in the cell, respiration reaches its maximum level even at very low substrate concentrations. Substrate B is absorbed passively, and the intensity of respiration reaches a maximum only at a relatively high concentration of such a substrate (of the order of 10–20 mM/l).
Rice. 7.20. Different kinds active transport, for which the proton potential Dr. serves as an energy source.
Membrane Ґ\ V U1 n+ O ҐN
V / IO SYMPORT and H+
ANTIPORT H* and Na+
SIMPORT B ​​and Na"
UNIPORT K*
outer side
group lies in the nature of the molecule entering the cell.
With active transport, the same molecule that was absorbed from the nutrient medium enters the cytoplasm. When a group is translocated, the transferred molecule is modified during transport, for example, it is phosphorylated.
All theories explaining active transport include the concept of the presence of specific transport proteins in the membrane. These proteins have received names indicating their function: permeases, translocases, translocator proteins, carriers. Transport processes differ from each other mainly in what serves as a source of energy for them - the proton potential Ap (Fig. 7.20), ATP or phosphoenolpyruvate (Fig. 7.18).
For the transfer of many substances, including inorganic and organic ions, as well as sugars, the energy of the proton potential is used (see pp. 243-244). Bacterial cells maintain a proton potential by continuously pumping protons and other ions (Na+) out of the cell. For this, the membrane contains specific transport proteins.
Each of these proteins has a very specific function. There is, for example, a protein that catalyzes the simultaneous and unidirectional transfer of one proton and one sugar molecule (lactose, melibiose, glucose). In such cases, one speaks of the symport of two (or several) substances. Other transport proteins catalyze the simultaneous counter transfer of two particles, for example, one proton and some other ion (Na + or an anion of an organic acid); in these cases one speaks of an antiport. When transferring sugars coupled with the transport of ions, it is likely that H+ or Na + ions are always used. In prokaryotes, symport with H + ions predominates, in eukaryotes, symport with Na + (Fig. 7.20).
That transport proteins of the type described do indeed exist in bacteria was confirmed (a) by purification and subsequent incorporation of the transport protein into protoplasts or so-called liposomes and (b) by isolating mutants lacking the corresponding protein and its specific function. With regard to transport using the energy of the proton potential, this is probably the most common mechanism for the active absorption of substrates.
The idea of ​​the participation of specific carrier proteins in ion transport is supported by data on the action of a number of antibiotics and synthetic substances. We are talking about ionophores. These are compounds with a relatively small molecular weight (500-2000), whose molecules are hydrophobic on the outside and hydrophilic on the inside. Possessing hydrophobic properties, they diffuse into the lipid membrane. Of the antibiotic ionophores, valinomycin is the best known; it diffuses into the membrane and catalyzes the transport (uniport) of K+, Cs+, Rb+ or NH4 ions. Therefore, the presence of such cations in the suspension medium leads to the equalization of the charge on both sides of the membrane (like a short circuit) and thus to a drop in the proton potential. Other ionophores form channels through which ions can pass. There are also synthetic compounds that increase the proton conductivity of membranes; the best known proton carrier is carbonyl cyanide- and trifluoromethoxyphenylhydrazone. It acts as an "uncoupler" - it breaks the conjugation of ATP synthesis with electron transport, transferring protons into the cell, bypassing ATP synthase. The study of membrane transport has led to important results that are consistent with and support the chemiosmotic theory of energy conversion.
Along with transport systems using the proton potential, there are also systems dependent on ATP. A certain role is played here by periplasmic binding proteins (Fig. 2.28). The plasma membrane of animal cells does not transport protons and does not create a proton gradient. The membrane potential is probably maintained only by ATP-dependent pumping mechanisms, such as the sodium-potassium pump, and the sodium potential, in turn, provides energy for the import of nutrients along with Na + ions.
Group translocation. In this type of transport, the molecule is chemically modified; for example, sugar as such is absorbed, and it enters the cell in a phosphorylated form. Fructose, glucose, mannitol and related substances are taken up by the phosphoenolpyruvate dependent phosphotransferase system. This system consists of non-specific and specific components. The nonspecific component is a thermostable protein, which, with the participation of enzyme I located in the cytoplasm, is phosphorylated by phosphoenolpyruvate. The second component is an inducible enzyme I located in the membrane, specific for one or another sugar; it catalyzes the transfer of phosphate from a thermostable protein (TB) to sugar during transport of the latter across the membrane;
Enzyme/
Phosphoenolpyruvate + HPg > HPg - P + Pyruvate
Enzyme I
NRg + Sugar > Sugar-I + NRg
Enzyme II probably performs the function of permease and phosphotransferase simultaneously (see Fig. 7.18).
Otherwise, the absorption of substances by cells is a very complex and still poorly understood process. Many metabolic effects of inhibition and the phenomenon of competition between simultaneously available substrates are apparently associated with the features of regulatory mechanisms that are already manifested in the processes of substance transport.
release of substances from the cell. Significantly less is known about the release of metabolites into the environment than about the mechanisms of absorption of substances by the cell. Apparently, their release from the cell also occurs as with the participation transport systems, and by uncontrolled diffusion. Substances leave the cell when, as a result of overproduction, they accumulate in it, reaching concentrations exceeding normal level. Accumulation may result from incomplete oxidation, dysregulation, or fermentation processes.
Iron transport. For the transport of this macroelement, the microbial cell has a special mechanism. Under anaerobic conditions, iron is represented by a divalent ion (Fe2 +), and its concentration can reach 10 "1 M / l, so that it does not limit the growth of microorganisms. However, under aerobic conditions at pH 7.0, iron is present in the form of hydroxide complex Fe3 +, which is almost insoluble; the concentration of ferric ions is only 10 "18 M / l. It is not surprising, therefore, that microorganisms secrete substances that convert iron into a soluble form. These substances, the so-called siderophores, bind Fe3 + ions into a complex and it is transported in this form, mainly low-molecular water-soluble substances (with a molecular weight less than 1500) that bind iron by coordination bonds with high specificity and high affinity (stability constant of the order of TO30). chemical nature these may be phenolates or hydroxamates. Enterochelin belongs to the first; it has six phenolic hydroxy groups and is secreted by some enterobacteria. Once released into the environment, it binds iron, and the resulting ferri-enterochelin is absorbed by the cell. In the cell, iron is released as a result of enzymatic hydrolysis of ferri-enterochelin (Fig. 7.21).
Many fungi form ferrichromes for the same purpose; they are classified as hydroxamate siderophores. These are cyclic hexapeptides that retain ferric iron with the help of three hydroxamate groups. They, too, are released from the cell in the form of iron-free compounds, bind iron in the nutrient medium, and are reabsorbed as ferrichromes. In the cell, iron is reduced to Fe2 +, for which ferrichromes have only a slight affinity and therefore release it. A similar function is performed by ferrioxamines (in actinomycetes), mycobactins (in mycobacteria) and exochelins (also in mycobacteria).

Rice. 7.21. Examples of the mechanisms of iron transfer into microorganism cells with the participation of siderophores. Above - the transport system with the help of enterochelin, characteristic of many bacteria; at the bottom is the ferrichromic system found in many fungi.
Microorganisms usually release siderophores into the nutrient medium only when iron limits growth. The release of siderophores is a consequence of the derepression of their synthesis. In the presence of dissolved, complex-bound iron, siderophores are synthesized only in small amounts and are retained in the cell wall. Under these conditions, they serve only to transport iron into the cell.
In this regard, it is interesting that among the natural protective devices of higher organisms, we find the "cleansing" of the internal environment from iron. There are special proteins that bind the available iron so strongly that it becomes inaccessible to microorganisms. For example, egg protein contains conalbumin, lactotransferrin in milk, lacrimal fluid and saliva, and serotransferrin in blood serum. When bacteria are seeded on chicken protein, they grow only if iron ions (in the form of citrate) are introduced simultaneously with inoculation. Thus, iron plays an important role in the antagonistic relationship between higher organisms and bacteria. The fight is won by the partner who produces a substance that binds iron more strongly.

Due to the suction force that occurs when moisture is tested through the stomata of the leaves, and the pumping action of the roots, the ions of mineral salts in the soil solution, together with the flow of water, can first enter the hollow intercellular spaces and pores of the cell membranes of young roots, and then be transported to the aerial part of the plants through the xylem - the ascending part of the vascular-conducting system, consisting of dead cells without partitions, devoid of living content.

However, inside the living cells of the root (as well as aboveground organs), which have an outer semipermeable cytoplasmic membrane, ions absorbed and transported with water can penetrate somewhat differently.

"Passive" absorption - i.e. without additional energy consumption - only along the concentration gradient - from higher to lower due to the diffusion process, or in the presence of an appropriate electric potential (negative for cations, and positive for anions) on the inner surface of the membrane relative to the outer solution.

Diffusion - the movement of molecules of gases, liquids or a solute along a concentration gradient - depends on the concentration gradient of absorbed substances and the area through which substances or ions pass. The constant passage of ions through the plasmalemma entails a continuous influx of new ions to it to equalize the concentration.

The part of the total volume of tissues of the root system, into which ions enter and from which are released due to diffusion, is called free space. It makes up about 4–6% of the total root volume and is localized in the loose primary membrane of the cell walls outside the protoplast outside the plasmalemma.

However, in plant organisms, nutrients, as a rule, are in much more high concentrations than in the surrounding nutrient solution. Moreover, the intake of individual elements and their concentration are carried out differently and do not correspond to the ratio of the concentrations of elements in the nutrient solution. This is due to the plasmolemma, which prevents the loss of substances accumulated by the cell through diffusion, while ensuring the penetration of water and mineral nutrients.

In this case, the absorption of nutrients by plants must occur against the concentration gradient and is impossible due to diffusion.

Plants simultaneously absorb both cations and anions. In this case, individual ions enter the plant in a completely different ratio than they are contained in the soil solution. Some ions are absorbed by the roots in a larger amount, others - in a smaller amount and at different rates, even at the same concentration in the surrounding solution. It is quite obvious that passive absorption, based on the phenomena of diffusion and osmosis, cannot be of significant importance in plant nutrition, which has a pronounced selective character.

Studies using labeled atoms also showed that the absorption of nutrients and their further movement in the plant occurs at a rate that is hundreds of times higher than possible due to diffusion and passive transport through the vascular-conducting system with a water current.

In addition, there is no direct dependence of the absorption of nutrients by plant roots on the intensity of transpiration, on the amount of absorbed and evaporated moisture.

All this confirms the position that the absorption of nutrients by plants is carried out not simply by passive absorption of the soil solution by the roots along with the salts contained in it, but is an active physiological process that is inextricably linked with the vital activity of the roots and aboveground organs of plants, with the processes of photosynthesis, respiration and metabolism. substances and requires energy.

Schematically, the process of supply of nutrients to the root system of plants is as follows.

To the outer surface of the cytoplasmic membrane of root hairs and the outer cells of young roots, ions of mineral salts move from the soil solution with a stream of water and due to diffusion.

The first step in the entry of ions into the cell is the absorption (adsorption) of ions on outer surface cytoplasmic membrane. It consists of two layers of phospholipids, between which protein molecules are embedded. Due to the mosaic structure, individual sections of the cytoplasmic membrane have negative and positive charges, due to which cations and anions necessary for the plant can be simultaneously adsorbed from the external environment in exchange for other ions.

The exchange fund of cations and anions in plants can be H + and OH - ions, as well as H + and HCO 3 - formed during the dissociation of carbonic acid released during respiration.

The adsorption of ions on the surface of the cytoplasmic membrane is of an exchange nature and does not require energy. Not only the ions of the soil solution take part in the exchange, but also the ions absorbed by the soil colloids. Due to the active absorption by plants of ions containing the necessary nutrients, their concentration in the zone of direct contact with the root hairs decreases. This facilitates the displacement of similar ions from the state absorbed by the soil into the soil solution (in exchange for other ions).

The transport of adsorbed ions from the outside of the cytoplasmic membrane to the inside against the concentration gradient and against the electric potential requires a mandatory expenditure of energy. The mechanism of such "active" pumping is very complicated. It is carried out with the participation of special "carriers" and the so-called ion pumps, in the functioning of which an important role belongs to proteins with ATP-ase activity. Active transport into the cell through the membrane of some ions containing nutrients necessary for plants is associated with counter transport outward of other ions that are in the cell in a functionally excessive amount.

The initial stage of nutrient uptake by plants from the soil solution - the adsorption of ions on the absorbing surface of the root - is constantly renewed, since the adsorbed ions are continuously moving inside the root cells.

The selectivity of ion absorption, an increase in their concentration inside cells, and competition between chemically close ions during absorption by root cells is explained by the theory of carriers. According to this theory, the ion crosses the membrane not in a free form, but in the form of a complex with a carrier molecule. On inside The membrane complex dissociates, releasing an ion inside the cell. The transfer of ions into cells can be carried out using carriers of various types.

The transport of substances into the root cells is stimulated by the fact that in the cytoplasm many ions are quickly involved in biosynthetic processes and, as a result of the formation organic matter the concentration of ions inside the cells falls.

Active transport of nutrients from cell to cell is carried out along plasmodesmata, which connect the cytoplasm of plant cells into a single system - the so-called symplast. When moving along the symplast, some of the ions and metabolites can be released into the intercellular space, and they move to the places of assimilation passively with an upward flow of water through the xylem. The usual speed of movement of ions, amino acids, sugars is 2 - 4 cm per hour.

There is a close relationship between the intensity of absorption of nutrients by plants and the intensity of respiration of the roots, since the process of respiration is a source of energy necessary for the active absorption of mineral nutrients. So, with deterioration in root growth and inhibition of respiration (with a lack of oxygen in conditions of poor aeration or excessive soil moisture), the absorption of nutrients is sharply limited.

For normal growth and respiration of the roots, a constant supply of energy material to them is necessary - the products of photosynthesis (carbohydrates and other organic compounds) from aboveground organs. With the weakening of photosynthesis, the formation and movement of assimilants to the roots decreases, as a result of which vital activity worsens and the absorption of nutrients from the soil decreases.

Plants assimilate ions not only from the soil solution, but also ions absorbed by colloids. Moreover, plants actively (due to the dissolving ability of root secretions, including carbonic acid, organic acids and amino acids) act on the solid phase of the soil, converting the necessary nutrients into an accessible form.

The root system of plants absorbs both water and nutrients from the soil. Both of these processes are interrelated, but are carried out on the basis of different mechanisms. The roots extract minerals from the soil solution and from the soil absorbing complex, with particles of which the root absorption zone (root hairs) is in close contact.

Cell walls are directly involved both in the absorption of substances from the soil and in the transport of mineral nutrition elements through tissues.

The main driving force of the absorption activity of the roots, as well as each cell in general, is the operation of ion pumps localized in the membranes. The radial transport of mineral substances from the root surface to the conducting system is carried out as a result of the interaction of all the main tissues of the absorption zone, and each tissue performs certain functions. Radial transport ends with the loading of minerals and their organic derivatives into tracheids and xylem vessels. Xylem sap is transported to other parts of the plant by transpiration and/or root pressure. The cells that make up various tissues and organs, in turn, absorb and metabolize the elements of mineral nutrition delivered with xylem juice. Moreover, their absorption activity depends on age and functional state.

In general, the process of mineral nutrition of a plant is a complex chain of biophysical, biochemical and physiological processes with their feedback and direct connections and the system of regulation. At present, not all links of this chain have been studied in sufficient detail.

The absorptive activity of the root is based on the mechanisms of absorptive activity inherent in any plant cell. Therefore, such general questions as the selective entry of substances into the cell, the role of the cell wall phase, and the transmembrane transport of ions will be discussed in relation to all plant cells.

In various organs of plants, an unequal amount of mineral elements accumulates, and the content of mineral substances in the cells does not correspond to the concentration of these same substances in the external environment. The content of nitrogen and potassium is ten times higher in cells. This indicates that there are mechanisms in cells not only for absorption of substances against a concentration gradient, but also for their selective accumulation. This process begins already in the cell wall and then continues with the participation of membranes.

The role of cell walls in the processes of adsorption of mineral substances. Unlike animal cells, a plant cell has a shell (wall) consisting of cellulose, hemicelluloses and pectin substances. Pectin substances (polyuronic acids) contain carboxyl groups in their composition, as a result of which the cell membranes acquire the properties of cation exchangers and can concentrate positively charged substances.

If roots (or other plant tissue) are immersed in a vessel containing a solution of 86 RbCl or a cationic dye (for example, methylene blue), then in the first 2 minutes up to 50% of rubidium (or dye) from the amount that is absorbed will disappear from the solution. per long time(rice.).

Dynamics of absorption of ions by plant cells and their release during washing with water or saline (phase I - penetration of substances into the apparent free space (CSP), phase II - accumulation of substances in cells; the dotted line indicates the extrapolation of the absorption curve in phase II to the y-axis to determine the value KSP)

In the next 10 - 30 minutes, 70% will be absorbed, and further binding of the substance by the tissues will occur very slowly (for hours). What is the reason for such a rapid movement of matter at the very beginning? If the tissue, which has been in the test solution for several hours, is transferred to water or saline solution of the same composition, but without a radioactive label (or without a dye), then the reverse picture is observed: a rapid release of a substance in the first minutes and its subsequent slow release from the tissue. Thus, two phases of the absorption of substances can be distinguished, proceeding at different speeds - high and slow, and the substance quickly absorbed by the tissue also quickly leaves it. The initial rapid absorption of substances is carried out in the cell walls and is exchange adsorption (and the rapid loss is desorption). The slow phase is associated with the functional activity of the plasmalemma (penetration of substances into the cell or exit from it). The molecular space in the cell wall, where exchange adsorption processes take place, is called the apparent free space (APS). The term "apparent" means that the amount of this free space depends on the object and the nature of the solute. The PCB includes the intermolecular space in the thickness of the cell walls and on the surface of the plasmalemma and cell walls. According to calculations, CSP occupies 5-10% of the volume in plant tissues. The absorption and release of substances in the PCB is a physicochemical passive process. It is determined by the adsorption properties of the ion exchanger and the Donnan electric potential at the interface between the aqueous medium and the cation exchanger. These factors already at the first stage ensure the selectivity of the absorption of charge-carrying substances, since the cation exchanger (cell walls) binds cations (especially divalent and trivalent ones) more actively than anions. because of high density negative fixed charges in the cell wall (1.4-1.8 meq/mg dry weight), the primary concentration of cations occurs in the space immediately adjacent to the plasmalemma.

Under specific conditions of soil nutrition, root cells (rhizoderm) are in contact with the water phase (soil solution) and with soil particles, which are also predominantly cation exchangers (soil absorbing complex). At the same time, most of the mineral nutrients are not in solution, but are adsorbed on soil particles.

Cations and anions enter the cell walls of the rhizoderm both directly from the soil solution and through contact exchange with particles of the soil absorbing complex. Both of these processes are associated with the exchange of H + ions for environmental cations and HCO 3 - (OH -) or organic acid anions for mineral anions.

Contact exchange of ions of the cell wall of the rhizoderm (H + ions) with soil particles is carried out without the transition of ions into the soil solution. Close contact is provided due to the secretion of mucus by root hairs and the absence of cuticles and other protective integumentary formations in the rhizoderm. The root absorption zone and soil particles form a single colloidal system (Fig.).

Contact ion exchange between root cells and soil particles

Since the adsorbed ions are in constant oscillatory motion and occupy a certain “oscillatory volume” (sphere of oscillations), with close contact of the surfaces, the spheres of oscillations of the two nearest adsorbed ions can overlap, resulting in ion exchange.

The capacity for exchange adsorption in general and contact exchange in particular is determined by the exchange capacity of the root. It depends on chemical composition root secretions and cell membranes and is supported by the continuous synthesis of new substances associated with the growth of the root and with the processes of renewal of its structures, as well as with the absorption of substances through the cytoplasmic membrane into the cells and their further movement into the root. The exchange capacity of the root different types plants is not the same and depends on age.

Ways of penetration of ions through biological membranes. The problem of membrane transport includes two main questions: 1) how various substances physically overcome the membrane, consisting of hydrophobic components; 2) what forces determine the movement of substances through the membrane when entering the cell or when leaving it.

It is now known that ions and various compounds cross the lipid phase biological membranes in several ways. The main ones are:

Simple diffusion through the lipid phase if the substance is lipid soluble.

Facilitated diffusion of hydrophilic substances by lipophilic carriers.

Simple diffusion through hydrophilic pores (for example, through ion channels).

Transfer of substances with the participation of active carriers (pumps).

Transfer of substances by exocytosis (vesicular secretion) and endocytosis (due to membrane invagination).

In recent years, substances have been discovered and studied that can dramatically accelerate the transport of substances through the lipid phase of membranes. For example, the antibiotic gramicidin creates channels for K + and H + ions. Molecules of another lipophilic antibiotic, valinomycin, whose properties were studied by Yu.A. Ovchinnikov et al., grouping around K + ions, form highly specific carriers for this cation. This kind of membranotropic physiologically active substances in modern biology have become a powerful and subtle instrument of experimental influence on a living cell.

Passive and active membrane transport. The second main issue in the problem of membrane transport is the elucidation of the driving forces of this process. Passive transport is the movement of substances by diffusion along the electrochemical, i.e. along the electrical and concentration gradient. This is how, for example, substances move if their concentration in the external environment is higher than in the cell. Active transport is the transmembrane movement of substances against an electrochemical gradient with the expenditure of metabolic energy, usually in the form of ATP. Examples of active transport are ion pumps: H + -ATPase, Na + , K + -ATPase, Ca 2+ -ATPase, anionic ATPase.

A special role in the plasmalemma of plant cells (and apparently also in the tonoplast) is played by the H+ pump, which creates electrical (Δψ) and chemical (ΔрН) gradients of H+ ions through these membranes.

On fig. It was shown that the electric potential of H+ ions (membrane potential) can be used for the transport of cations along the electric gradient against the concentration one. In turn, ΔрН serves as an energy basis for the transfer of Cl -, SO 4 2- and others through the membrane to symport with H + ions (i.e. in the same direction) or to pump out excess Na + into antiporte with H + (i.e. in opposite sides). In this case, H + ions move through the membrane along a concentration gradient, but this movement with the help of special carrier proteins is associated with the transport of other ions (Cl - , Na +) against their concentration gradients. This method of movement of substances through the membrane is called secondary active transport.

The appearance of ΔpH on the membrane can serve as the basis for secondary active transport and organic substances. In the plasmalemma, protein carriers of sugars and amino acids were found, which acquire a high affinity for the substrate only under conditions of protonation. Therefore, when the H + pump starts to work and the concentration of H + ions increases on the outer surface of the plasma membrane, these carrier proteins are protonated and bind sugars (amino acids). When sugar molecules are transferred to the inner side of the membrane, where there are very few H + ions, H + and sugar are released, and sugars enter the cytoplasm, and H + ions are again pumped out of the cell by the H + pump. Essentially, H + plays the role of a catalyst in this process. Similarly, in symport with H+ ions, anions can also enter the cell. In addition, anions of weak organic acids with a decrease in pH on the surface of the plasmalemma can penetrate the membrane in the form of uncharged molecules (if they are soluble in the lipid phase), since their dissociation decreases with increasing acidity.

Mechanisms of membrane transport in the plasma membrane of plant cells: K n + - cations, A - - anions, Sax - sugars, AA - amino acids.

Similarly, H + can function and HCO 3 - or OH - , the excess of which appears in the near-membrane layer of the cytoplasm during intensive operation of the H + -pump. The transport of OH - , HCO 3 - and (or) anions of organic acids outward along the electrochemical gradient can proceed in antiporte with the entry of mineral anions into the cell.

Ministry of Agriculture of the Russian Federation

FSBEI HPE "Yaroslavl State Agricultural Academy"

Department of Ecology

TEST

In the discipline "Plant Physiology"

Performed:

4th year student

Faculty of Technology

Stepanova A. Yu.

Checked:

teacher Taran T.V.

Yaroslavl 2014

1. Absorption of substances by a plant cell. Passive and active transport………………………………………………………………………

2. Transcription and its biological significance, types. Factors determining the amount of transcription……………………………………

3. Dehydrogenases, their chemical nature and nature of action………………

4. Physiology of dormancy and seed germination. Influence of internal and external conditions on the process of seed germination………………………………………..

1. Absorption of substances by a plant cell. Passive and active transport

Entry of substances into the cell wall (stage 1).

Absorption of substances by the cell begins with their interaction with the cell membrane. Even the works of D. A. Sabinin and I. I. Kolosov showed that the cell membrane is capable of rapid adsorption of ions. Moreover, this adsorption in some cases has an exchange character. Later, in experiments with isolated cell membranes, it was shown that they can be considered as an ion exchanger. On the surface of the cell membrane, H + and HC0 3 - ions are adsorbed, which in equivalent quantities change to ions located in the external environment. Ions can be partially localized in the intermicellar and intermolecular gaps of the cell wall, partially bound and fixed in the cell wall by electric charges.

The first stage of admission is characterized by high speed and reversibility. The incoming ions are easily washed out. This is a passive diffusion process that follows an electrochemical potential gradient. The cell volume available for free diffusion of ions includes cell walls and intercellular spaces, i.e., the apoplast or free space. According to calculations, free space (SP) can occupy 5-10% of the volume in plant tissues. Since the cell membrane includes amphoteric compounds (proteins), the charge of which changes when different meanings pH, then, depending on the pH value, the rate of adsorption of cations and anions can also change. Entry of substances through the membrane (stage 2). In order to penetrate the cytoplasm and be included in the metabolism of the cell, substances must pass through the membrane - the plasmalemma. The transport of substances across the membrane can be passive or active. With the passive entry of substances through the membrane, the basis of transfer in this case is also diffusion. The diffusion rate depends on the thickness of the membrane and on the solubility of the substance in the lipid phase of the membrane. Therefore, non-polar substances that dissolve in lipids (organic and fatty acids, esters) pass through the membrane more easily. However, most substances that are important for cell nutrition and metabolism cannot diffuse through the lipid layer and are transported by proteins, which facilitate the entry of water, ions, sugars, amino acids and other polar molecules into the cell. At present, the existence of three types of such transport proteins has been shown: channels, carriers, and pumps.

Three classes of transport proteins:

1 - protein channel;

2 - carrier;

3 - pump.

Channels are transmembrane proteins that act like pores. They are sometimes called selective filters. Transport through channels is generally passive. The specificity of the transported substance is determined by the properties of the pore surface. As a rule, ions move through the channels. The speed of transport depends on their size and charge. If the time is open, then the substances pass quickly. However, channels are not always open. There is a "gate" mechanism, which, under the influence of an external signal, opens or closes the channel. For a long time, the high permeability of the membrane (10 μm/s) for water, a polar substance and insoluble in lipids, seemed difficult to explain. At present, integral membrane proteins have been discovered that represent a channel through the membrane for the penetration of water - aquaporins. The ability of aquaporins to transport water is regulated by the process of phosphorylation. Attachment and donation of phosphate groups to certain aquaporin amino acids has been shown to accelerate or inhibit water entry, but does not affect the direction of transport.

Carriers are specific proteins that can bind to a carried substance. In the structure of these proteins there are groupings that are oriented in a certain way to the outer or inner surface. As a result of a change in the conformation of proteins, the substance is transferred outward or inward. Since for the transport of each individual molecule or ion, the carrier must change its configuration, the rate of transport of a substance is several times lower than the transport through channels. The presence of transport proteins was shown not only in the plasmalemma, but also in the tonoplast. Carrier transport can be active or passive. In the latter case, such transport goes in the direction of the electrochemical potential and does not require energy. This type of transport is called facilitated diffusion. Thanks to carriers, it travels at a faster rate than normal diffusion.

According to the concept of the work of carriers, the ion (M) reacts with its carrier (X) on the surface of the membrane or near it. This first reaction may involve either exchange adsorption or some kind of chemical interaction. Neither the carrier itself nor its complex with the ion can pass into external environment. However, the ion transporter complex (MX) is mobile within the membrane itself and moves to its opposite side. Here, this complex decomposes and releases an ion into the internal environment to form a carrier precursor (X 1 ). This vector precursor moves again to outside membrane, where it is again converted from a precursor to a carrier, which can combine with another ion on the membrane surface. When a substance capable of forming a stable complex with a carrier is introduced into the medium, the transfer of the substance is blocked. Experiments carried out on artificial lipid membranes have shown that ion transport can take place under the influence of certain antibiotics produced by bacteria and fungi - ionophores. Transport with the participation of carriers has the property of saturation, that is, with an increase in the concentration of substances in the surrounding solution, the rate of entry first increases and then remains constant. This is due to the limited number of carriers.

Carriers are specific, i.e., they are involved in the transfer of only certain substances and, thus, ensure the selectivity of intake.

Ionophore K complex +

This does not exclude the possibility that the same carrier can carry several ions. For example, the K + transporter, which is specific for this ion, also transports Rb + and Na + , but does not transport Cl - or uncharged sucrose molecules. A transport protein specific for neutral acids tolerates the amino acids glycine, valine, but not asparagine or lysine. Due to the diversity and specificity of proteins, their selective reaction with substances in the environment is carried out, and, as a result, their selective transfer.

Pumps (pumps) are integral transport proteins that actively supply ions. The term "pump" indicates that the flow is with the consumption of free energy and against the electrochemical gradient. The energy used for the active entry of ions is supplied by the processes of respiration and photosynthesis and is mainly accumulated in ATP. As you know, in order to use the energy contained in ATP, this compound must be hydrolyzed according to the equation ATP + HOH -> ADP + Ph n. The enzymes that hydrolyze ATP are called adenosine triphosphatases (ATPases). Various ATPases were found in cell membranes: K + - Na + - ATPase; Ca 2+ - ATPase; H + - ATPase. H + - ATPase (H + -pump or hydrogen pump) is the main mechanism of active transport in the cells of plants, fungi and bacteria. H + - ATPase functions in the plasmalemma and ensures the release of protons from the cell, which leads to the formation of an electrochemical potential difference on the membrane. H + - ATPase carries protons into the cavity of the vacuole and into the tanks of the Golgi apparatus.

The calculation shows that in order for 1 mol of salt to diffuse against the concentration gradient, it is necessary to spend about 4600 J. At the same time, 30660 J/mol is released during ATP hydrolysis. Therefore, this ATP energy should be enough to transport a few moles of salt. There is evidence showing a directly proportional relationship between ATPase activity and ion intake. The need for ATP molecules to carry out the transfer is also confirmed by the fact that inhibitors that disrupt the accumulation of respiratory energy in ATP (violation of the conjugation of oxidation and phosphorylation), in particular dinitrophenol, inhibit the flow of ions.

Pumps are divided into two groups:

1. Electrogenic, which carry out active transport of an ion of any one charge in only one direction. This process leads to the accumulation of one type of charge on one side of the membrane.

2. Electrically neutral, in which the transfer of an ion in one direction is accompanied by the movement of an ion of the same sign in the opposite direction, or the transfer of two ions with charges of the same magnitude, but different in sign, in the same direction.

The mechanism of action of transport ATPase (P - inorganic phosphate).

Thus, the transfer of ions across the membrane can be carried out in an active and passive way. In ensuring the transport function of membranes and the selectivity of absorption, transport proteins play an important role: channels, carriers, and pumps. At present, the genes for many transport proteins have been cloned. Genes encoding potassium channels have been identified. On Arabidopsis, gene mutations have been obtained that affect the transport and recovery of nitrates. It has been shown that in the plant genome, not one gene, but several, is responsible for the transport of substances through membranes. Such a multiplicity ensures the performance of a function in different parts of plants, which makes it possible to transport substances from one tissue to another.

Finally, the cell can “swallow” nutrients along with water (pinocytosis). Pinocytosis is an invagination of the surface membrane, due to which liquid droplets with solutes are swallowed. The phenomenon of pinocytosis is known for animal cells. It has now been proven that it is also characteristic of plant cells. This process can be divided into several phases: 1) adsorption of ions in a certain area of ​​the plasmalemma; 2) invagination, which occurs under the influence of charged ions; 3) the formation of vesicles with liquid that can migrate through the cytoplasm; 4) fusion of the membrane surrounding the pinocytic vesicle with the membranes of lysosomes, endoplasmic reticulum or vacuole and the inclusion of substances in metabolism. With the help of pinocytosis, not only ions, but also various soluble organic substances can enter the cells.

The action of the ATPase pump of the cytoplasmic membrane.

Transport of substances in the cytoplasm (3rd stage) and entry into the vacuole (4th stage). After passing through the membrane, the ions enter the cytoplasm, where they are included in the cell metabolism. An essential role in the process of ion binding by the cytoplasm belongs to cell organelles. Mitochondria, chloroplast, apparently, compete with each other, absorbing cations and anions that have entered the cytoplasm through the plasmalemma. In the process of accumulation of ions in various organelles of the cytoplasm and inclusion in metabolism great importance has their intracellular transport. This process is carried out, apparently, through EPR channels.

Ions enter the vacuole if the cytoplasm is already saturated with them. It is, as it were, excess nutrients that are not included in the metabolic reactions. In order to get into the vacuole, ions must overcome another barrier - the tonoplast. If in the plasmalemma the ion transport mechanism operates within relatively low concentrations, then in the tonoplast it operates at higher concentrations, when the cytoplasm is already saturated with this ion. In vacuole membranes, vacuolar channels were found that differ in opening time (fast and slow). The transfer of ions through the tonoplast is also carried out with the help of carriers and requires the expenditure of energy, which is ensured by the work of the H+ -ATPase of the tonoplast. The potential of the vacuole is positive compared to the cytoplasm, so anions flow along the electric potential gradient, while cations and sugars - in antiport with protons. The low permeability of the tonoplast for protons makes it possible to reduce the energy costs for the intake of substances. The vacuolar membrane also has a second proton pump associated with H + -pyrophosphatase. This enzyme consists of a single polypeptide chain. The energy source for the proton flux is the hydrolysis of inorganic pyrophosphate. Transport proteins were found in the tonoplast, which allow large organic molecules to penetrate into the vacuole directly due to the energy of ATP hydrolysis. This plays a role in the accumulation of pigments in the vacuole, in the formation of antimicrobial substances, and also in the neutralization of herbicides. The substances entering the vacuole provide the osmotic properties of the cell. Thus, ions penetrating through the plasmalemma are accumulated and bound by the cytoplasm, and only their excess is desorbed into the vacuole. That is why there is not and cannot be an equilibrium between the content of ions in the external solution and the cell sap. It must be emphasized once again that active intake is of great importance for the life of the cell. It is it that is responsible for the selective accumulation of ions in the cytoplasm. The absorption of nutrients by the cell is closely related to metabolism. These connections are multifaceted. Active transfer requires the synthesis of carrier proteins, the energy supplied during respiration, and the efficient operation of transport ATPases. It should also be taken into account that the faster the incoming ions are included in the metabolism, the more intense their absorption is. For a multicellular higher plant, the movement of nutrients from cell to cell is no less important. The faster this process takes place, the faster the salts will, ceteris paribus, enter the cell.

PASSIVEAndACTIVEINCOME

The uptake of nutrients by the cell can be passive or active. Passive absorption is absorption that does not require the expenditure of energy. It is associated with the diffusion process and follows the concentration gradient of a given substance. From a thermodynamic point of view, the direction of diffusion is determined by the chemical potential of the substance. The higher the concentration of a substance, the higher its chemical potential. The movement goes in the direction of lower chemical potential. It should be noted that the direction of movement of ions is determined not only by chemical, but also by electrical potential. Consequently, the passive movement of ions can follow a gradient of chemical and electrical potential. Thus, the driving force behind the passive transport of ions across membranes is the electrochemical potential.

Electric potential on the membrane - transmembrane potential can occur for various reasons:

1. If the entry of ions follows a concentration gradient (gradient-chemical potential), however, due to the different permeability of the membrane, either a cation or an anion enters at a faster rate. Because of this, a difference in electrical potentials arises on the membrane, which, in turn, leads to the diffusion of an oppositely charged ion.

2. If there are proteins on the inside of the membrane that fix certain ions, i.e., immobilize them. Due to the fixed charges, an additional possibility is created for the entry of ions of the opposite charge (Donnan equilibrium).

3. As a result of active (energy-consuming) transport of either a cation or an anion. In this case, the oppositely charged ion can move passively along the electric potential gradient. The phenomenon when the potential is generated by the active flow of ions of the same charge through the membrane is called the electrogenic pump. The term "pump" indicates that the flow comes with the consumption of free energy.

Active transport is a transport that goes against the electrochemical potential with the expenditure of energy released during metabolism.

Passive and active transport

There is some evidence for the existence of active ion transport. In particular, these are experiments on the influence of external conditions. So, it turned out that the flow of ions depends on temperature. Within certain limits, with increasing temperature, the rate of absorption of substances by the cell increases. In the absence of oxygen, in a nitrogen atmosphere, the entry of ions is drastically inhibited, and salts can even be released from the root cells to the outside. Under the influence of respiratory poisons, such as KCN, CO, the intake of ions is also inhibited. On the other hand, an increase in the ATP content enhances the absorption process. All this indicates that there is a close relationship between the absorption of salts and respiration.

Many researchers have concluded that there is a close relationship between salt intake and protein synthesis. Thus, chloramphenicol, a specific inhibitor of protein synthesis, also inhibits the absorption of salts. The active flow of ions is carried out with the help of special transport mechanisms - pumps. Pumps are divided into two groups:

1. Electrogenic (mentioned earlier), which carry out active transport of an ion of any one charge in only one direction. This process leads to the accumulation of one type of charge on one side of the membrane.

2. Electrically neutral, in which the transfer of an ion in one direction is accompanied by the movement of an ion of the same sign in the opposite direction, or the transfer of two ions with charges of the same magnitude, but different in sign, in the same direction.

The ability of the cell to selectively accumulate nutrient salts, the dependence of intake on the intensity of metabolism serve as evidence that, along with passive intake, there is also an active intake of ions. Both processes often occur simultaneously and are so closely related that it is difficult to distinguish between them.

All inorganic nutrients are absorbed in the form of ions contained in aqueous solutions. The absorption of ions by the cell begins with their entry into the apoplast and interaction with the cell wall. Ions can be partially localized in the intermicellar and interfibrillar spaces of the cell wall, partially bound and fixed in the cell wall by electric charges. The ions entering the apoplast are easily washed out. The volume of the cell available for free diffusion of ions is called free space. Free space includes intercellular spaces, cell walls, and gaps that may occur between the cell wall and the plasmalemma. It is sometimes referred to as apparent free space (APS). This term means that its calculated volume depends on the object and nature of the solute. Thus, for monovalent ions, the volume of the CSP will be larger than for divalent ones. The apparent free space occupies 5–10% of the volume in plant tissues. The absorption and release of substances in the PCB is a physicochemical passive process, independent of temperature (in the range of +15 - +35 o C) and inhibitors of energy metabolism. The cell wall has the properties of an ion exchanger, since H + and HCO -3 ions are adsorbed in it, exchanging in equivalent quantities for ions of an external solution. Due to the predominance of negative fixed charges in the cell wall, the primary concentration of cations (especially divalent and trivalent ones) occurs. The second stage of ion entry is transport through the plasmalemma. The transport of ions across the membrane can be passive or active. Passive absorption does not require energy and is carried out by diffusion along the concentration gradient of a substance for which the plasmalemma is permeable. The passive movement of ions is determined not only by the chemical potential µ, as is the case with the diffusion of uncharged particles, but also by the electric potential ε. Both potentials are combined in the form of an electrochemical potential µ:

µ = µ + nFε,

where µ – chemical, ε – electrical, µ – electrochemical potentials;

n is the ion valency; F is the Faraday constant.

Any difference in electrical potential that occurs across the membranes causes a corresponding movement of ions.

Passive transport can proceed with the participation of carriers at a higher rate than normal diffusion, and this process is called facilitated diffusion. Highly specific translocases are known - protein molecules that carry adenyl nucleotides through the inner membrane of mitochondria: Na + / Ca2 + exchanger - a protein that is part of the plasma membranes of many cells; a low molecular weight peptide of bacterial origin, valinomycin, is a specific carrier for K+ ions. Process

Facilitated diffusion has a number of features: 1) is described by the Michaelis-Menten equation and has certain Vmax and Km; 2) selective (has specificity to a certain ion); 3) is suppressed by specific

inhibitors.

Diffusion also involves the transport of ions through selective ion channels - integral protein complexes of membranes that form a hydrophilic pore. The main component of the driving force of this transport is the gradient of the electrochemical potential of the ion. Channel activity is modulated by membrane potential, pH, concentration

ions, etc. Active transport of substances is carried out against the concentration gradient and must be associated with an energy-giving process. The main source of energy for active transport is ATP. Therefore, as a rule, active transport of ions is carried out with the help of transport ATPases.

The coupling membranes contain proton pumps that act as H+-ATPases. As a result of their functioning, a difference in proton concentrations (ΔрН) and a difference in electric potentials arise on the membrane, which together form a proton electrochemical potential, denoted ΔμН+. Due to the work of H + -ATPase, an acidic environment is created in some cell organelles (for example, lysosomes). In the mitochondrial membrane, H+-ATPase works in the opposite direction, using

ΔμH+ created in the respiratory chain to form ATP. Finally, secondary active transport is widely represented in cells, during which the gradient of one substance is used to transport another. With the help of secondary active transport, cells accumulate sugars, amino acids and remove some metabolic products using the H + gradient.

After passing through the plasmalemma, the ions enter the cytoplasm, where they are included in the cell metabolism. Intracellular transport of ions is carried out due to the movement of the cytoplasm and through the channels of the endoplasmic reticulum. Ions enter the vacuole if the cytoplasm and organelles are already saturated with them, or to replenish the pool of osmotically active particles. In order to get into the vacuole, ions must overcome another barrier - the tonoplast. The transport of ions through the tonoplast is also carried out with the help of

carriers and require energy. Carriers located in the tonoplast have a lower affinity for ions and operate at higher ion concentrations than plasma membrane carriers. A specific H+-ATPase was identified in the tonoplast. It is not inhibited by diethylstilbestrol, an inhibitor of plasmalemma H+-ATPase.

The relationship between the processes of absorption of substances by the root with other functions of the plant (respiration, photosynthesis, water exchange, growth, biosynthesis, etc.); absorption of ions by leaf cells, outflow of ions from leaves, redistribution and recycling of substances in the plant.

Well-known researchers (I. Knop, Yu. Saks, D.N. Pryanishnikov and others) found that the plant's need for individual ash elements changes at different phases of its development. Higher requirements are associated with active metabolism, growth and neoplasms. With a deficiency of many mineral elements, the symptoms of starvation appear, first of all, on old organs. This is due to the fact that the regulatory systems of the plant mobilize the necessary mineral elements and they are transported to young, actively growing tissues. Nitrogen, phosphorus, potassium are very mobile. Boron and calcium are poorly or not reutilized at all. For the normal life of plants, a certain ratio of various ions in environment. Pure solutions of any one cation are poisonous. So, when placing wheat seedlings on pure solutions KCL or CaCL2 swelling first appeared on the roots, and then the roots died off. Mixed solutions of these salts did not have a toxic effect. The softening effect of one cation on the action of another is called ion antagonism. Antagonism of ions manifests itself both between different ions of the same valence, for example, between sodium and potassium ions, and between ions of different valences, for example, potassium and calcium ions. One of the reasons for the antagonism of ions is their effect on the hydration of cytoplasmic proteins. Divalent cations (calcium, magnesium) dehydrate colloids more strongly than monovalent cations (sodium, potassium). The next reason antagonism of ions is their competition for the active centers of enzymes. Thus, the activity of some respiratory enzymes is inhibited by sodium ions, but their action is removed by the addition of potassium ions. In addition, ions can compete for binding with carriers during uptake. The action of one ion can also enhance the influence of another. This phenomenon is called synergy. So, under the influence of phosphorus, the positive effect of molybdenum increases. The study of the quantitative ratios of the necessary elements made it possible to create balanced nutrient mixtures, solutions of mineral salts for growing plants. Mixtures of Knop, Pryanishnikov, Gelrigel, etc. are well known.

Research by scientists from the school of D. N. Pryanishnikov showed that each type of plant has specific requirements for quantitative combinations of individual ash elements. It was also found that the plant's need for individual ash elements changes at different phases of its development. Thus, the best nutrient solution for a plant should be considered a solution of not a constant, but a variable composition, which changes according to the change in the needs of the plant for different stages its development. This provision is of great practical importance, being the basis of a new method of artificially raising the yield. The work of D. A. Sabinin made it possible to elucidate the mechanisms of the entry of water and minerals into the cells of plant roots, antagonism and synergy in the interaction of ions.