Mechanisms of absorption of substances by plant cells. Absorption of nutrients by plants

Rice. 7.19. Saturation curves for the absorption of two substrates by intact bacterial cells [constructed based on data on O2 consumption (respiration rate)]. Active and passive substrate uptake can be distinguished by the shape of the curve. Since substrate A is absorbed by active transport and accumulates in the cell, respiration even at very low concentrations of the substrate reaches a maximum level. Substrate B is absorbed passively, and the respiration rate reaches a maximum only at a relatively high concentration of such a substrate (about 10-20 mmol/l).
Rice. 7.20. Various types of active transport, for which the source of energy is the proton potential Dr.
Membrane Ґ\ V У1 n+ O ҐN
V / ІО SIMPORT and Н+
ANTIPORT H* and Na+
SIMPORT B ​​and Na"
UNIPORT K*
External side
group lies in the nature of the molecule entering the cell.
During 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, phosphorylated.
All theories explaining active transport include the idea of ​​the presence of specific transport proteins in the membrane. These proteins received names indicating their function: permeases, translocases, translocator proteins, transporters. 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).
To transport 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 purpose, there are specific transport proteins in the membrane.
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, they speak of symport of two (or several) substances. Other transport proteins catalyze the simultaneous countertransfer of two particles, for example, one proton and some other ion (Na+ or an organic acid anion); in these cases they talk about antiport. When sugar transfer involves ion transport, H+ or Na+ ions are probably 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 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. As for transport using proton potential energy, this is probably the most common mechanism for the active uptake of substrates.
The idea of ​​the participation of specific carrier proteins in ion transport is confirmed 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), the molecules of which are hydrophobic on the outside and hydrophilic on the inside. Possessing hydrophobic properties, they diffuse into the lipid membrane. Of the aitibiotic 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 a suspension medium leads to charge equalization on both sides of the membrane (as if short circuit) and thereby 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 most well-known proton transporter is carbonyl cyanide-and-trifluoromethoxyphenylhydrazone. It acts as an “uncoupler” - it disrupts the coupling of ATP synthesis with electron transport, transferring protons into the cell, bypassing ATP synthase. The study of membrane transport has produced important results that are consistent with and support the chemiosmotic theory of energy conversion.
Along with transport systems using proton potential, there are also systems dependent on ATP. Periplasmic binding proteins play a certain role here (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 supplies energy for the symport of nutrients along with Na + ions.
Group translocation. In this type of transport, the molecule is chemically modified; For example, sugar is absorbed as such, and it enters the cell in phosphorylated form. Fructose, glucose, mannitol and related substances are absorbed by the phosphoenolpyruvate-dependent phosphotransferase system. This system consists of nonspecific 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 the inducible enzyme I, located in the membrane, specific for a particular sugar; it catalyzes the transfer of phosphate from thermostable protein (TP) to sugar during transport of the latter across the membrane;
Enzyme/
Phosphoenolpyruvate + NRg > NRg - P + Pyruvate
Enzyme I
NRg + Sugar > Sugar-I+NRg
Enzyme II probably functions as a permease and a phosphotransferase simultaneously (see Fig. 7.18).
Otherwise, the absorption of substances by cells is a very complex process and is still poorly understood. Many metabolic effects of inhibition and the phenomenon of competition between simultaneously available substrates are apparently associated with the peculiarities of regulatory mechanisms that already manifest themselves in the processes of transport of substances.
The release of substances from the cell. About the release of metabolites into environment Much less is known than about the mechanisms of absorption of substances by cells. Apparently, their release from the cell also occurs both with the participation of transport systems and through uncontrolled diffusion. Substances leave the cell when, as a result of overproduction, they accumulate in it, reaching concentrations that exceed normal levels. Accumulation may result from incomplete oxidation, dysregulation, or fermentation processes.
Iron transport. To transport 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 it does not limit the growth of microorganisms. However, under aerobic conditions at pH 7.0, iron is presented 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 in this form it is transported; we are talking mainly about low molecular weight water-soluble substances (with a molecular weight of less than 1500), which bind iron through coordination bonds with high specificity and high affinity (stability constant of the order of TO30). By their chemical nature, these can be phenolates or hydroxamates The first is enterochelin; it has six phenolic hydroxy groups, and it 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 are also released from the cell in the form of iron-free compounds, bind iron in the nutrient medium and are reabsorbed in the form of 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 mechanisms of iron transfer into microbial cells with the participation of siderophores. At the top is a transport system using enterochelin, characteristic of many bacteria; below is the ferrichrome 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 derepression of their synthesis. In the presence of dissolved, complexly bound iron, siderophores are synthesized only in small quantities 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 adaptations of higher organisms we find the “cleansing” of the internal environment from iron. There are special proteins that bind existing iron so tightly that it becomes inaccessible to microorganisms. For example, the white of a chicken egg contains conalbumin, milk, tear fluid and saliva contain lactotransferrin, and blood serum contains serotransferrin. When bacteria are inoculated onto 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.

Ministry of Agriculture of the Russian Federation

Federal State Budgetary Educational Institution of Higher Professional Education "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 the 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. The influence of internal and external conditions on the process of seed germination………………………………………………………..

1. Absorption of substances by the plant cell. Passive and active transports

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

The absorption of substances by the cell begins with their interaction with the cell membrane. The work 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 is of an exchange nature. Subsequently, 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 are replaced by ions found in the external environment. Ions can be partially localized in the intermicellar and intermolecular spaces of the cell wall, partially bound and fixed in the cell wall by electrical charges.

The first stage of entry is characterized by high speed and reversibility. The incoming ions are easily washed out. This is a passive diffusion process along 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 contains amphoteric compounds (proteins), the charge of which changes at different pH values, the rate of adsorption of cations and anions can also change depending on the pH value. Entry of substances through the membrane (2nd stage). In order to penetrate the cytoplasm and become involved in cell metabolism, substances must pass through a membrane - the plasmalemma. The transfer of substances across the membrane can be passive or active. With the passive flow of substances through the membrane, the basis of transfer in this case is diffusion. The rate of diffusion 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 with the help of proteins that facilitate the penetration of water, ions, sugars, amino acids and other polar molecules into the cell. Currently, 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 canals 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, the channels are not always open. There is a “gate” mechanism that, 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) to water, a polar and lipid-insoluble substance, seemed difficult to explain. Currently, 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. The addition and donation of phosphate groups to certain amino acids of aquaporins has been shown to accelerate or inhibit water penetration, but does not affect the direction of transport.

Transporters are specific proteins that can bind to a transported substance. The structure of these proteins contains groups that are oriented in a certain way to the outer or inner surface. As a result of changes in the conformation of proteins, the substance is transferred outward or inward. Since the transporter must change configuration to transport each individual molecule or ion, the rate of transport of the 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. Transport by vectors can be active or passive. In the latter case, such transport occurs in the direction of the electrochemical potential and does not require energy expenditure. This type of transport is called facilitated diffusion. Thanks to carriers, it occurs at a faster rate than normal diffusion.

According to the concept of how carriers work, an ion (M) reacts with its carrier (X) on or near the surface of the membrane. This first reaction may involve either exchange adsorption or some kind of chemical reaction. Neither the carrier itself nor its complex with the ion can move into the external environment. However, the transporter complex with the ion (MX) is mobile in the membrane itself and moves to its opposite side. Here this complex disintegrates and releases the ion into the internal environment to form the carrier precursor (X 1). This carrier precursor moves again to the outside of the membrane, where it is again converted from a precursor into a carrier that can bind to another ion at the membrane surface. When a substance capable of forming a strong complex with a carrier is introduced into the medium, the transfer of the substance is blocked. Experiments conducted on artificial lipid membranes have shown that ion transport can occur under the influence of certain antibiotics produced by bacteria and fungi - ionophores. Transport involving carriers has the property of saturation, i.e., 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.

Transporters are specific, i.e., they participate in the transfer of only certain substances and, thus, ensure selectivity of entry.

Ionophore K complex +

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

Pumps (pumps) are integral transport proteins that actively supply ions. The term "pump" indicates that the supply occurs with the consumption of free energy and against the electrochemical gradient. The energy used for the active supply of ions is supplied by the processes of respiration and photosynthesis and is mainly accumulated in ATP. As is known, in order to use the energy contained in ATP, this compound must be hydrolyzed according to the equation ATP + HOH -> ADP + Ph n. Enzymes that hydrolyze ATP are called adenosine triphosphatases (ATPases). Various ATPases are 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 transfers protons into the cavity of the vacuole and into the cisterns of the Golgi apparatus.

Calculations show that in order for 1 mole of salt to diffuse against the concentration gradient, it is necessary to spend about 4600 J. However, during the hydrolysis of ATP, 30660 J/mol is released. Therefore, this ATP energy should be enough to transport several moles of salt. There is data showing a directly proportional relationship that exists between ATPase activity and the supply of ions. The need for ATP molecules to carry out the transfer is also confirmed by the fact that inhibitors that disrupt the accumulation of respiration energy in ATP (violation of the coupling 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 value but different in sign in the same direction.

Mechanism of action of transport ATPase (P - inorganic phosphate).

Thus, the transfer of ions through the membrane can be carried out through active and passive pathways. Transport proteins: channels, carriers and pumps play an important role in ensuring the transport function of membranes and selectivity of absorption. Currently, the genes for many transport proteins have been cloned. Genes encoding potassium channels have been identified. Mutations of genes have been obtained in Arabidopsis that affect the transport and reduction 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. This multiplicity ensures that the function is performed in different parts of plants, which allows the transport of 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 droplets of liquid with dissolved substances are ingested. 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 on a certain section of the plasmalemma; 2) invagination, which occurs under the influence of charged ions; 3) the formation of bubbles with liquid, which can migrate throughout the cytoplasm; 4) fusion of the membrane surrounding the pinocytotic 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 cells.

Action of the ATPase pump of the cytoplasmic membrane.

Transport of substances in the cytoplasm (3rd stage) and entry into the vacuole (4th stage). Having passed through the membrane, the ions enter the cytoplasm, where they are included in the metabolism of the cell. A significant role in the process of binding ions by the cytoplasm belongs to cellular organelles. Mitochondria and chloroplasts apparently compete with each other, absorbing cations and anions that enter the cytoplasm through the plasmalemma. During the accumulation of ions in various organelles of the cytoplasm and inclusion in metabolism great importance has their intracellular transport. This process is apparently carried out through EPR channels.

Ions enter the vacuole if the cytoplasm is already saturated with them. It's like excess nutrients that are not included in metabolic reactions. In order to enter 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 a given ion. Vacuolar channels were found in the membranes of the vacuole, which differ in the opening time (fast and slow). The transfer of ions through the tonoplast is also accomplished with the help of carriers and requires 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 arrive along the electrical potential gradient, and cations and sugars enter in antiport with protons. The low permeability of tonoplast for protons makes it possible to reduce 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 flow is the hydrolysis of inorganic pyrophosphate. Transport proteins are found in the tonoplast that 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. Substances entering the vacuole provide the osmotic properties of the cell. Thus, ions penetrating through the plasmalemma accumulate and bind in the cytoplasm, and only their excess is desorbed into the vacuole. That is why there is and cannot be an equilibrium between the content of ions in the external solution and the cell sap. It is necessary to emphasize once again that active intake is of great importance for the life of the cell. It 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 transport requires the synthesis of carrier proteins, energy supplied during respiration, and efficient operation of transport ATPases. It should also be taken into account that the faster the incoming ions are included in metabolism, the more intense their absorption. 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, all other things being equal, enter the cell.

PASSIVEANDACTIVEINCOME

The absorption of nutrients by a cell can be passive or active. Passive absorption is absorption that does not require energy. It is associated with the process of diffusion 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 is 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, driving force passive transport of ions across membranes is the electrochemical potential.

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

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

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

3. As a result of active (energy-related) 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 entry of ions of the same charge through the membrane is called an electrogenic pump. The term “pump” indicates that the supply comes with the consumption of free energy.

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

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. Thus, it turned out that the supply 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 flow of ions is sharply inhibited and salts may even be released from the root cells to the outside. Under the influence of respiratory poisons, such as KCN, CO, the flow of ions is also inhibited. On the other hand, an increase in ATP content enhances the absorption process. All this indicates that there is a close connection between salt absorption and respiration.

Many researchers come to the conclusion that there is a close relationship between salt absorption and protein synthesis. Thus, chloramphenicol, a specific inhibitor of protein synthesis, also suppresses the absorption of salts. The active supply of ions is carried out using 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 value but different in sign in the same direction.

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

A2. Which scientist discovered the cell? 1) A. Leeuwenhoek 2) T. Schwann 3) R. Hooke 4) R. Virchow
A3. Contents of what chemical element predominates in cell dry matter? 1) nitrogen 2) carbon 3) hydrogen 4) oxygen
A4. Which phase of meiosis is shown in the picture? 1) Anaphase I 2) Metaphase I 3) Metaphase II 4) Anaphase II
A5. What organisms are chemotrophs? 1) animals 2) plants 3) nitrifying bacteria 4) fungi A6. The formation of a two-layer embryo occurs during the period of 1) cleavage 2) gastrulation 3) organogenesis 4) postembryonic period
A7. The totality of all the genes of an organism is called 1) genetics 2) gene pool 3) genocide 4) genotype A8. In the second generation, with monohybrid crossing and with complete dominance, a splitting of characters is observed in the ratio 1) 3:1 2) 1:2:1 3) 9:3:3:1 4) 1:1
A9. Physical mutagenic factors include 1) ultraviolet radiation 2) nitrous acid 3) viruses 4) benzopyrene
A10. In what part of the eukaryotic cell are ribosomal RNAs synthesized? 1) ribosome 2) rough ER 3) nucleolus 4) Golgi apparatus
A11. What is the term for a section of DNA that codes for one protein? 1) codon 2) anticodon 3) triplet 4) gene
A12. Name the autotrophic organism 1) boletus mushroom 2) amoeba 3) tuberculosis bacillus 4) pine
A13. What is nuclear chromatin made of? 1) karyoplasm 2) strands of RNA 3) fibrous proteins 4) DNA and proteins
A14. At what stage of meiosis does crossing over occur? 1) prophase I 2) interphase 3) prophase II 4) anaphase I
A15. What is formed from the ectoderm during organogenesis? 1) notochord 2) neural tube 3) mesoderm 4) endoderm
A16. A non-cellular form of life is 1) euglena 2) bacteriophage 3) streptococcus 4) ciliates
A17. Protein synthesis into mRNA is called 1) translation 2) transcription 3) reduplication 4) dissimilation
A18. In the light phase of photosynthesis, 1) synthesis of carbohydrates occurs 2) synthesis of chlorophyll 3) absorption of carbon dioxide 4) photolysis of water
A19. Cell division with preservation chromosome set called 1) amitosis 2) meiosis 3) gametogenesis 4) mitosis
A20. Plastic metabolism includes 1) glycolysis 2) aerobic respiration 3) assembly of an mRNA chain on DNA 4) breakdown of starch to glucose
A21. Select the incorrect statement In prokaryotes, the DNA molecule is 1) closed in a ring 2) not associated with proteins 3) contains uracil instead of thymine 4) is singular
A22. Where does the third stage of catabolism occur - complete oxidation or respiration? 1) in the stomach 2) in mitochondria 3) in lysosomes 4) in the cytoplasm
A23. Asexual reproduction includes 1) parthenocarpic formation of fruits in cucumbers 2) parthenogenesis in bees 3) reproduction of tulips by bulbs 4) self-pollination in flowering plants
A24. What organism develops without metamorphosis in the postembryonic period? 1) lizard 2) frog 3) Colorado potato beetle 4) fly
A25. The human immunodeficiency virus affects 1) gonads 2) T-lymphocytes 3) erythrocytes 4) skin and lungs
A26. Cell differentiation begins at the stage 1) blastula 2) neurula 3) zygote 4) gastrula
A27. What are protein monomers? 1) monosaccharides 2) nucleotides 3) amino acids 4) enzymes
A28. In which organelle does the accumulation of substances and the formation of secretory vesicles occur? 1) Golgi apparatus 2) rough ER 3) plastid 4) lysosome
A29. What disease is inherited in a sex-linked manner? 1) deafness 2) diabetes mellitus 3) hemophilia 4) hypertension
A30. Indicate the incorrect statement. The biological significance of meiosis is as follows: 1) the genetic diversity of organisms increases 2) the stability of the species increases when environmental conditions change 3) the possibility of recombination of traits as a result of crossing over appears 4) the probability of combinative variability of organisms decreases.

The largest systematic unit A) Kingdom B) Division C) Class D) Family 3. The cell of A) Fungi B) Bacteria C) Cyanobacteria D) Viruses is classified as eukaryotic 4. The nitrogenous base adenine, ribose and three phosphoric acid residues are part of A ) DNA B) RNA C) ATP D) protein 5. Ribosomes are A) A complex of microtubules B) A complex of two round membrane bodies C) Two membrane cylinders D) Two non-membrane mushroom-shaped subunits 6. A bacterial cell, like a plant cell, has A) Nucleus B) Golgi complex C) Endoplasmic reticulum D) Cytoplasm 7. Organelle in which the oxidation of organic substances to carbon dioxide and water occurs A) Mitochondria B) Chloroplast C) Ribosome D) Golgi complex. 8. Chloroplasts in the cell do not perform the function of A) Carbohydrate synthesis B) ATP synthesis C) Absorption of solar energy D) Glycolysis 9. Hydrogen bonds between CO and NH groups in the protein molecule give it a spiral shape, which is characteristic of the structure A) Primary B ) Secondary B) Tertiary D) Quaternary 10. Unlike tRNA, mRNA molecules A) Deliver amino acids to the site of protein synthesis B) Serve as a matrix for tRNA synthesis C) Deliver hereditary information about the primary structure of the protein from the nucleus to the ribosome D) transfer enzymes to the site assembly of protein molecules. 11. The main source of energy in the cell A) Vitamins B) Enzymes C) Fats D) Carbohydrates 12. The process of primary synthesis of glucose occurs A) In the nucleus B) In chloroplasts C) Ribosomes D) Lysosomes 13. The source of oxygen released by cells during photosynthesis , is A) Water B) Glucose C) Ribose D) Starch 14. How many cells and with what set of chromosomes are formed after meiosis? 15. The divergence of chromatids to the poles of the cell occurs in A) Anaphase B) Telophase C) Prophase D) Metaphase 16. Biological meaning of mitosis. 17. Advantages of asexual reproduction.

9. Which of the listed organs are homologous
10. The appearance of what sign in a person is classified as atavism
11. Which pair of aquatic vertebrates confirms the possibility of evolution based on convergent similarity?
12. The similarity between the functions of chloroplasts and mitochondria lies in what happens in them
13. Name the form of natural selection due to which the number of eyes and the number of fingers on the limbs of vertebrates remains constant for a long time
14. The creative nature of natural selection in evolution is manifested in
15. Name the form of natural selection that results in the loss of wings in some birds and insects
16. Which molecules contain phosphorus, which is necessary for all living organisms?
17 Paleontological evidence of evolution includes
18. The highest concentration of living matter is observed
19. What structures are missing in the skin cells of onion scales
20. Founder of scientific taxonomy (classification)
21. In a DNA molecule, the number of nucleotides with thymine is ...% of the total number. What is the percentage of nucleotides with cytosine in this molecule
22. During the process of photosynthesis, plants
23. The remainder of the third eyelid in the corner of a person’s eye - an example
24. Which cell organelles contain a wide variety of enzymes involved in the breakdown of biopolymers into monomers?
25. The distribution area of ​​reindeer in the tundra zone is a criterion
26. The small pond snail is an intermediate host
27. The highest concentration of toxic substances in an environmentally polluted ground-air environment can be found in
28. Which organelle ensures the transport of substances in the cell?
29. Non-cellular life forms include
30. The intermediate nature of inheritance of a trait manifests itself when
31 The greenhouse effect on Earth is a consequence of increased concentrations in the atmosphere
32. The most acute form of struggle for existence
33. Genetic heterogeneity of individuals in a population increases
34. The development of multicellular organisms from a zygote serves as evidence
35. Human atavisms include the appearance
36. Identify organisms that enter into competitive relationships
37.What happens during photosynthesis
38. The similarity of the structure and activity of cells of organisms of different kingdoms of living nature is one of the provisions
39. The structure and functions of the plasma membrane are determined by the molecules included in its composition
40. Establish a correspondence between the form of natural selection and its features

1) 02 and H2O 3) C02 and H20

2) C02 and H2 4) C02 and H2C03

2. The consumer of carbon dioxide in the biosphere is:

1) oak 3) earthworm

2) eagle 4) soil bacterium

3. In what case is the glucose formula written correctly:

1) CH10 O5 3) CH12 About

2) C5H220 4) C3H603

4. The energy source for ATP synthesis in chloroplasts is:

1) carbon dioxide and water 3) NADP H2

2) amino acids 4) glucose

5. During photosynthesis in plants, carbon dioxide is reduced to:

1) glycogen 3) lactose

2) cellulose 4) glucose

6. Organic substances from inorganic ones can create:

1) E. coli 3) toadstool

2) chicken 4) cornflower

7. In the light stage of photosynthesis, molecules are excited by light quanta:

1) chlorophyll 3) ATP

2) glucose 4) water

8. Autotrophs do not include:

1) chlorella and spirogyra

2) birch and pine

3) champignon and toadstool 4) blue-green algae

9.. The main suppliers of oxygen to the Earth’s atmosphere are:

1) plants 2) bacteria

3) animals 4) people

10. The following have the ability to photosynthesize:

1) protozoa 2) viruses

3) plants 4) mushrooms

11. Chemosynthetics include:

1) iron bacteria 2) influenza and measles viruses

3) cholera vibrios 4) brown algae

12. The plant absorbs during respiration:

1) carbon dioxide and releases oxygen

2) oxygen and releases carbon dioxide

3)light energy and releases carbon dioxide

4)light energy and releases oxygen

13. Photolysis of water occurs during photosynthesis:

1) during the entire process of photosynthesis

2) in the dark phase

3) in the light phase

4) in this case, carbohydrate synthesis does not occur

14. The light phase of photosynthesis occurs:

1) on the inner membrane of chloroplasts

2) on the outer membrane of chloroplasts

3) in the stroma of chloroplasts

4) in the mitochondrial matrix

15. During the dark phase of photosynthesis, the following occurs:

1) release of oxygen

2) ATP synthesis

3) synthesis of carbohydrates from carbon dioxide and water

4)excitation of chlorophyll by a photon of light

16. By type of nutrition, most plants belong to:

17. In plant cells, unlike human, animal, and fungal cells,

1) metabolism 2) aerobic respiration

3) glucose synthesis 4) protein synthesis

18. The source of hydrogen for the reduction of carbon dioxide in the process of photosynthesis is

1) water 2) glucose

3) starch 4) mineral salts

19. What happens in chloroplasts:

1) transcription of mRNA 2) formation of ribosomes

3) formation of lysosomes 4) photosynthesis

20. ATP synthesis in the cell occurs in the process:

1) glycolysis; 2) photosynthesis;

3) cellular respiration; 4)all are listed

Due to the suction force that occurs when testing moisture through the stomata of leaves, and the pumping action of the roots, ions of mineral salts present in the soil solution along 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 above-ground part of the plant along the xylem – the ascending part of the vascular conduction system, consisting of dead cells without partitions, devoid of living contents.

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

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

Diffusion - the movement of molecules of gases, liquids or dissolved substances 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 flow 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 they are released due to diffusion is called free space. It makes up about 4–6% of the total volume of the root and is localized in the loose primary membrane of the cell walls outside the protoplast outside the plasmalemma.

However, in plant organisms, nutrients are usually found in much higher concentrations than in the surrounding nutrient solution. Moreover, the supply of individual elements and their concentration is carried out differently and does not correspond to the ratio of the concentrations of elements in the nutrient solution. This occurs thanks to the plasmalemma, which prevents the loss of substances accumulated by the cell through diffusion, while simultaneously 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 greater quantities, others in smaller quantities and with at different speeds 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 is of a clearly selective nature.

Studies using labeled atoms have also shown that the absorption of nutrients and their further movement in the plant occurs at a speed that is hundreds of times higher than possible due to diffusion and passive transport through the vascular conduction system with water flow.

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 through 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 above-ground organs of plants, with the processes of photosynthesis, respiration and metabolism substances and necessarily requires energy.

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

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

The first stage in the entry of ions into the cell is the absorption (adsorption) of ions on the outer surface of the 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 the simultaneous adsorption of cations and anions necessary for the plant from the external environment can occur in exchange for other ions.

The exchange pool 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.

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

Transport of adsorbed ions from the outside of the cytoplasmic membrane to the inside against the concentration gradient and against the electrical potential requires a mandatory expenditure of energy. The mechanism of such “active” pumping is very complex. It is carried out with the participation of special “carriers” and so-called ion pumps, in the functioning of which proteins with ATPase activity play an important role. Active transport into the cell through the membrane of some ions containing nutrients necessary for plants is associated with counter transport out of other ions that are in functionally excess amounts in the cell.

The initial stage of plant absorption of nutrients from the soil solution - the adsorption of ions on the absorbent surface of the root - is constantly renewed as the adsorbed ions continuously move into the root cells.

The selectivity of ion absorption, an increase in their concentration inside cells, and competition between chemically similar 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 the inside of the membrane, the complex dissociates, releasing the ion inside the cell. The transfer of ions into cells can be carried out using various types of carriers.

The transport of substances into root cells is stimulated by the fact that in the cytoplasm many ions are quickly involved in biosynthetic processes and, due to the formation of organic substances, the concentration of ions inside the cells decreases.

Active transport of nutrients from cell to cell occurs through plasmodesmata, which connect the cytoplasm of plant cells into unified system- the so-called simplast. When moving along the symplast, some of the ions and metabolites can be released into the intercellular space, and move to the sites of absorption passively with the ascending 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 connection between the intensity of plant absorption of nutrients and the intensity of root respiration, since the respiration process is the source of energy necessary for the active absorption of mineral nutrition elements. Thus, when root growth deteriorates and respiration is inhibited (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 roots, a constant influx of energy material is required - products of photosynthesis (carbohydrates and other organic compounds) from above-ground organs. When photosynthesis is weakened, the formation and movement of assimilates into the roots decreases, as a result of which vital activity deteriorates and the absorption of nutrients from the soil decreases.

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

In a soil solution, ions are in a free state or bound to soil colloids. Elements of mineral nutrition are absorbed most often in ionic form: nitrogen as NO 3 + or NH 4 +, phosphorus as NPO 4 2- or H 2 PO 4 -, sulfur as SO 4 2-, molybdenum as MoO 4 2-; potassium, sodium, calcium, magnesium, heavy metals (iron, manganese, copper) and zinc - in the form of cations; chlorine – in the form of chloride anion; boron, probably in the form of undissociated boric acid. The plant can also absorb some organic soluble compounds, such as amino acids. However, the main source of nutrients is mineral salts.

For a long time it was believed that a highly diluted soil solution enters the roots unchanged, rises up the stem and then “condenses” in the leaves as a result of water evaporation, i.e. substances enter the plant in the same quantities and proportions as they are in the soil solution. Transpiration current was considered one of the mandatory conditions for the flow of substances from the soil into the root, and then into the above-ground organs.

However, later experiments showed that the amount of mineral substances entering the plant and accumulated in it is not proportional to the amount of water passing through it. In tropical forests, transpiration can be suppressed due to high humidity. Despite this, the trees reach very large sizes and develop a large leaf surface; therefore, they provide themselves with all the necessary elements of mineral nutrition.

A study of plants grown in water culture showed that from very dilute solutions salts are absorbed faster than water, and from concentrated solutions, on the contrary, water enters the plant faster.

So, mineral salts and water enter the plant relatively independently of each other and using significantly different mechanisms. However, the independence of these two processes does not mean that the transpiration current is of no importance. If there are a lot of salts in the soil solution, then the bulk of them moves from the roots to the above-ground organs along with water through the xylem vessels, i.e. water facilitates the transport of substances from roots to shoots (mass current). If the plant lacks salts, then they move from the roots to the shoots not along the wood, but along the bark; in this case, the transpiration current cannot affect their transport.

Not only different salts, but even the anion and cation of the same salt enter the plant from solution at different rates. So, if ammonium sulfate is used as a source of nitrogen, then the ammonium cation is absorbed by the plant more intensively than the sulfate anion, since the plant needs nitrogen in larger quantities than sulfur. As a result, as the plant grows, sulfuric acid can accumulate in the solution containing this salt, damaging the roots. If sodium nitrate is used as a nitrogen source, the anion will enter the roots faster than the cation. NaHCO 3 will accumulate in the surrounding solution. This salt, undergoing hydrolysis, forms a strong alkali NaOH and a weak acid H 2 CO 3, which causes alkalization of the nutrient solution. Ammonium nitrate is an example of a salt in which the anion and cation are absorbed at almost the same rate. The salt whose cation is absorbed faster is called physiologically acidic; in which the anion is absorbed faster, – physiologically basic, in which the anion and cation are absorbed at the same rate - physiologically neutral.

The plant absorbs substances selectively. For example, C 4 -plants absorb more potassium, iron and calcium than C 3 -species growing under the same conditions. As a result of selective absorption, the ratio of absorbed substances in cells can be completely different than in the external solution. Observations also showed that the absorption of substances occurs not only selectively, but also against the gradient of chemical potential. For such absorption, ATP energy must be expended.

Respiration energy can be used to transport ions against a gradient of chemical potential and directly, without first storing it in ATP. According to P. Mitchell's chemiosmotic theory, as a result of electron transport along the respiratory chain, hydrogen ions accumulate on the outside of the inner mitochondrial membrane. Wherein inner side membranes are charged negatively. Cations enter the organelle, being attracted to the negatively charged side of the membrane. Thus, the respiratory chain works like a proton pump. Respiratory poisons, such as dinitrophenol, which increase the permeability of the membrane to protons, also inhibit the absorption of ions.

The absorption of mineral elements into the root system of plants can be not only active, but also passive. Passive intake follows a gradient of chemical potential. However, the main role in the life of the plant is played by the active absorption of elements.

Like the supply of water, the absorption process is divided into two stages: 1st - the supply of ions from the soil or nutrient solution into the free space of the cell; 2nd – their movement from free space through the plasmalemma into the protoplast. Sometimes absorbed ions can be transported from the cytosol to a vacuole or other organelle.

Mechanisms of absorption of substances by roots. Ions enter the free space of the cell either from the soil solution, if we are talking about an epiblema, or from the free space of neighboring cells. At the first stage, the main mechanisms of absorption are diffusion and adsorption, at the second - membrane transport proteins and endocytosis. Cell walls cannot serve as a barrier to absorbed substances, because they contain interfibrillar cavities through which these substances diffuse freely.

That part of the root volume into which minerals penetrate or are released by free diffusion is called the apoplast. Anatomically, the apoplast is represented by interfibrillar cavities of cell walls and intercellular spaces. The volume of free space is 5–10% of the total volume of the root system. Free space is external to the cell protoplasts and internal to the external environment.

Due to diffusion, substances move from the soil solution into the free space of the cell (interfibrillar cavities), where their concentration is generally lower than in the surrounding solution. The rate of diffusion is small and decreases with increasing duration: in an hour the substance moves by 5 mm, in 24 hours by 25 mm, and in a year by 500 mm. Therefore, diffusion cannot play an important role in the movement of solutes over long distances, for example from a root to a leaf. So, diffusion is the main mechanism for the entry of substances into the free space of the root.

It is possible that substances pass through the cell wall not only by diffusion, but also together with water, when, moving through the membrane, it captures one or more small molecules of substances dissolved in it (mass current).

The walls of the interfibrillar cavities that make up the free space of cells have a negative electrical charge, for example, due to the carboxyl groups of pectin substances that are part of the cell walls. Since substances are absorbed mainly in ionic form, their intake will be affected by this electrical charge. Therefore, the flow of ions into the free space of the cell depends not only on the difference in concentrations, but also on the differences in electrical potentials. The existence of a negative charge on the cell wall facilitates the uptake of cations and makes the uptake of anions more difficult.

In addition, cations can be adsorbed on cell walls, so the concentration of cations near the walls of the interfibrillar cavity is higher, and the concentration of anions is lower; in the center of the cavity it is equal to the concentration of cations and anions in the outer solution. From the center of the interfibrillar cavity, ions can be released into water as a result of diffusion.

Ions adsorbed on the walls of interfibrillar cavities can be released into the saline solution by exchange, and the adsorbed ion is displaced by an ion that is contained in excess in the saline solution (exchange adsorption). For example, if a root, previously kept in a calcium solution, is transferred to a potassium solution, then calcium will be released from the root into the solution. If the root is placed in distilled water, this will not happen.

Adsorption phenomena are characterized by an extremely high initial speed; this distinguishes adsorption from diffusion. The second proof of the participation of adsorption in the absorption of substances is the existence of a saturation state. A characteristic feature of absorption is its initial low dependence on temperature. The initial absorption depends on the pH value. The concentration of hydrogen ions especially strongly affects the ratio of the amounts of absorbed cations and anions.

So, intensive absorption at the beginning, the existence of a saturation state, independence of initial absorption from temperature and complete dependence on pH - all this proves that adsorption takes part in the absorption of substances. Due to the high speed inherent in this process, the plant is able to quickly adsorb on the surface of the root and then accumulate in its cells the nutrients it needs from such dilute solutions, which are usually soil solutions.

Having entered the free space of the root through diffusion, the ions are adsorbed not only on the cell walls, but also on the plasmalemma, which also carries an electrical charge. The electrical potential difference across the plasmalemma ranges from 60 to 100, and sometimes reaches 180 mV, and the cell is negatively charged relative to the external solution.

At the second stage of absorption, the substance from the free space must penetrate into the protoplast of the cell. To do this, it needs to pass through the plasmalemma - the main barrier to the diffusion of ions and molecules into the cell. The molecules of the solute, which are in a solution in continuous chaotic motion, when colliding with this membrane, will either rebound from it, or be adsorbed on it, or pass through the plasmalemma. In the latter case, the membrane is said to be permeable to the substance. If a substance cannot pass through the plasmalemma on its own, then membrane transport proteins, as well as endocytosis, help.

Different mechanisms do not work in isolation, but in different combinations, for example, “diffusion – adsorption – carrier” or “diffusion – adsorption – ion pump”. Depending on the nature of the absorbed substance and the conditions in the cell, the specific importance of one or another mechanism changes. The body itself regulates which absorption mechanism should work.

Although the mechanisms of absorption of substances are divided into passive (diffusion, adsorption) and active (carrier proteins, ion channels, endocytosis), this division is conditional, since any mechanism requires energy to operate. For example, to maintain diffusion, the amount of substance in the protoplast must be less than in the free space of the cell. This is possible if the absorbed substance is immediately included in the metabolism, and this requires energy.

Intensive absorption of substances by the cell should lead to an equalization of concentrations and to the cessation of absorption. However, this does not happen, since incoming ions, such as nitrate, sulfate and phosphate ions, are quickly incorporated into organic compounds in the cell. The rate of inclusion of ions in metabolism, in turn, determines the rate of their absorption.

The main organ of absorption of mineral nutrition elements is the root.. Fertilizers can also be applied through the leaves (foliar feeding), but, as experiments with labeled phosphorus have shown, this causes them to quickly age and fall off. In experiments with foliar feeding, plants grew 10 times slower compared to controls that received the same dose of phosphorus through the roots.

Let us consider the structural features of the root as an organ of absorption of substances. The epiblema, which covers the outside of the root, is in direct contact with the soil solution and soil. Therefore, the primary entry of ions into the root occurs precisely through the cells of this tissue. The epiblema is the main barrier to the absorption of ions by the root. This is proven by the fact that it is in the cells of this tissue that both anions and cations accumulate in the greatest quantities. The epiblema as an absorptive tissue is heterogeneous: some of its cells turn into root hairs. There are 200–400 root hairs per 1 mm2 of root surface; as a result, the absorbent surface of the root increases hundreds of times. The root hair is the main entry point for ions into the root; Less of them enter through other cells of the epiblema. Indirect evidence of this is the largest number of plasmodesmata emerging from the root hairs into the cells of the primary cortex.

The primary cortex makes up up to 86–90% of the total volume of the root; it contains many intercellular spaces. The thicker the bark, the greater the total volume of the root and, consequently, its absorbing surface, since in the zone of root hairs the root has the shape of a cylinder.

The central cylinder of the root is separated from the cortex by the endoderm, which regulates the transition of substances from the apoplastic to symplastic pathway and vice versa. Beneath the endodermis is the pericycle. A special calculation made on electron microscopic photographs showed that while there are 45 thousand plasmodesmata in the cell walls between neighboring cells of the pericycle, only 12 thousand go from each cell of the pericycle to the cells of the central cylinder. The observation suggested that the direction changes in the pericycle movement of ions from radial to annular. As a result, ions from the cells of the pericycle enter directly into the vessels, at least in those plants in which the vessels are immersed in the pericycle. Consequently, the role of the pericycle can be compared to the role of a ring road, which allows you to change the direction of movement. The functions of the conducting tissues located in the center of the root are well known: they transport absorbed substances from the roots to the above-ground organs.

The root system is heterogeneous not only anatomically, but also physiologically. Different zones of the root, differing in different rates of growth and respiration, also absorb substances with different intensities. Cells in the elongation zone and root hair zone absorb substances most actively. Experiments with wheat seedlings have shown that root hairs not only increase the root surface, but their membranes contain transport proteins that are more active than the same proteins of other epiblema cells.

In these zones, the most active synthesis of proteins and other components of the protoplast, which are acceptors of ions absorbed from the external environment, occurs. In the suberization zone, the absorption of ions per unit of root surface decreases sharply. The zone of greatest absorption, depending on the characteristics of the plant, soil, groundwater level, irrigation and distribution of mineral nutrition elements, goes down as the plant ages. So, in each root there is a significant gradient of absorptive capacity, which decreases from the tip of the root to its basal part.

If the root absorbs substances over a period of time, their concentration near the actively absorbing areas of the root will decrease and the rate of absorption will depend on the rate of diffusion of ions in the soil. At the growing root tip the situation is different. Its cells not only absorb better, but thanks to cell division and stretching, the tip moves further and further into new areas of the soil where the reserves of the necessary substances have not yet been depleted. During the growth process, the root moves toward nutrients. Absorption in the growing part occurs much faster than in other zones of the root, near which the delivery of substances is limited. The faster the root branches, the more a given root system has stretch zones and root hair zones, i.e. zones of active absorption.

So, the absorption of substances by root zones that have completed growth depends only on the rate of diffusion of the absorbed substances; growing zones - on the rate of diffusion and on the rate of their growth.

The powerful development of plant root systems, especially their small active roots and root hairs, and their continuous distribution in more and more layers of soil is a necessary condition for the absorption of substances. The lower the mobility of a given ion in the soil, the greater the importance of the development of the root system and its distribution in a larger volume of soil. The plant has great potential for root formation. By changing the fertilizer application scheme, you can control the growth of roots in length and the speed of their branching, and therefore the yield. In addition, pruning is known to stimulate root branching, which increases the number of zones with maximum absorption.

Thus, the rapid growth of the root system, stimulating the absorption of substances, is itself one of the necessary conditions for the rapid absorption of salts.

The root zones differ from each other not only in the rate of absorption of substances, but also in the intensity of their supply to the above-ground organs. Thus, the cells of the division and elongation zone absorb substances exclusively for their own (“internal”) consumption. Not only do they not transport these substances to above-ground organs, but, on the contrary, they themselves partially consume mineral elements absorbed in the zone of root hairs. This is due both to their functions and to the peculiarities of the anatomy of the root, in the stretch zone of which conductive tissues begin to form.

Thus, a fundamental distinction is made between the zone involved in the absorption of nutrients (the zone of cell division and elongation) and the zone involved in both the absorption and supply of nutrients to above-ground organs (the zone of root hairs).

However, it would be wrong to think that powerful root development is always a prerequisite for meeting the plant's nutritional needs. For example, a huge number of sugar cane varieties with a highly developed root system have low yields, since a lot of sucrose is spent on root respiration. The development by plants of a powerful and highly branched root system is a form of adaptation to the absorption of sedentary substances, which are often very dispersed in the soil. If the root environment contains nutrients in an easily accessible form and in sufficient concentration, powerful root development is not necessary, because the root system usually does not work at full capacity.

The uneven distribution of nutrients in the soil has led to the fact that in the process of evolution, the roots have developed the ability to grow faster in the direction where the concentration of the missing element is greater. This property is called chemotropism.

So, the speed and direction of movement of the root in the soil, the size of the surface that actively absorbs salts and supplies them to other organs, the rate of inclusion of ions in metabolism are determined by the growth of the root. The consequence of this dependence is that with a lack of nutrients, the growth of shoots is primarily inhibited, and the growth of roots in length, on the contrary, is stimulated, which allows the root to quickly pass through the soil layer poor in nutrients.

The importance of studying the patterns of formation of root systems and their absorption of mineral nutrition elements is important, firstly, for solving some issues of agricultural technology (depth of the arable layer, depth of seed placement and fertilizers, choice of tillage and irrigation methods); secondly, there are certain correlations between root development and plant adaptability to drought, high humidity or pest damage.