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 at different rates, even with 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 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 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.
The root system of plants absorbs both water and mineral nutrients from the soil. Both of these processes are interrelated, but are carried out on the basis of different mechanisms. Roots extract minerals from the soil solution and from the soil absorption complex, with the 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 behind the absorption activity of roots, as well as of each individual cell in general, is the work of ion pumps localized in membranes. Radial transport of minerals from the root surface to the conducting system occurs as a result of the interaction of all the main tissues of the absorption zone, with each tissue performing specific functions. Radial transport ends with the loading of minerals and their organic derivatives into tracheids and xylem vessels. Xylem sap moves to other parts of the plant due to transpiration and (or) root pressure. The cells that make up various tissues and organs, in turn, absorb and metabolize mineral nutrition elements delivered with xylem sap. 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 its own feedback and direct connections and regulatory system. At present, not all links in this chain have been studied in sufficient detail.
The absorption activity of the root is based on the mechanisms of absorption activity inherent in any plant cell. Therefore, such general issues as the selective entry of substances into the cell, the role of the cell wall phase, and transmembrane ion transport will be discussed in relation to all plant cells.
Different plant organs accumulate unequal amounts of mineral elements, and the content of mineral substances in cells does not correspond to the concentration of these same substances in the external environment. The nitrogen and potassium content in cells is tens of times higher. This indicates that cells have mechanisms not only for the absorption of substances against a concentration gradient, but also methods for their selective accumulation. This process begins in the cell wall and then continues with the participation of membranes.
The role of cell walls in the processes of adsorption of minerals. Unlike animal cells, a plant cell has a shell (wall) consisting of cellulose, hemicelluloses and pectin substances. Pectic substances (polyuronic acids) contain carboxyl groups, as a result of which 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 the rubidium (or dye) of the amount that is absorbed will disappear from the solution over a long period of time (Fig.).
Dynamics of absorption of ions by plant cells and their release when washed with water or saline solution (phase I - penetration of substances into the apparent free space (AS), phase II - accumulation of substances in cells; the dotted line indicates extrapolation of the absorption curve in phase II to the ordinate axis to determine the value KSP)
In the next 10 - 30 minutes, 70% will be absorbed, and further binding of the substance by tissues will occur very slowly (hours). What causes such a rapid movement of matter at the very beginning? If a tissue that has been in an experimental solution for several hours is transferred to water or a saline solution of the same composition, but without a radioactive label (or without a dye), then the opposite picture is observed: a rapid release of the substance in the first minutes and its subsequent slow release from the tissue. Thus, two phases of substance absorption can be distinguished, occurring at different rates - high and slow, and a substance quickly absorbed by the tissue also leaves it just as quickly. The initial rapid absorption of substances occurs in the cell walls and is exchange adsorption (and rapid loss is desorption). The slow phase is associated with the functional activity of the plasma membrane (the penetration of substances into or out of the cell). The molecular space in the cell wall, where exchange adsorption processes occur, is called apparent free space (AFS). The term "apparent" means that the volume of this free space depends on the object and the nature of the solute. The CSP 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 CSP is a physicochemical passive process. It is determined by the adsorption properties of the ion exchanger and the Donnan electric potential at the interface of the aqueous medium and the cation exchanger. These factors already at the first stage ensure selectivity of absorption of charge-bearing substances, since the cation exchanger (cell walls) more actively binds cations (especially divalent and trivalent) compared to anions. Due to the high density of 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 soil nutrition conditions, root cells (rhizoderm) come into contact with the aqueous phase (soil solution) and with soil particles, which are also predominantly cation exchangers (soil absorption complex). In this case, 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 absorption complex. Both of these processes are associated with the exchange of H + ions for cations of the environment and HCO 3 - (OH -) or anions of organic acids for anions of mineral substances.
Contact exchange of rhizoderm cell wall ions (H+ ions) with soil particles occurs without the ions passing into the soil solution. Close contact is ensured due to the secretion of mucus by root hairs and the absence of a cuticle and other protective covering formations in the rhizoderm. The absorption zone of roots and soil particles form a single colloidal system (Fig.).
Contact ion exchange between root cells and soil particles
Since adsorbed ions are in constant oscillatory motion and occupy a certain “oscillatory volume” (sphere of oscillation), when the surfaces are in close contact, the spheres of oscillation of the two closest adsorbed ions can overlap, resulting in ion exchange.
The ability for exchange adsorption in general and contact exchange in particular is determined by the exchange capacity of the root. It depends on the chemical composition of root secretions and cell membranes and is supported by the continuous synthesis of new substances associated with the growth of the root and 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 metabolic capacity of the root varies among different plant species and depends on age.
Methods for the penetration of ions through biological membranes. The problem of membrane transport includes two main questions: 1) how various substances physically overcome a membrane consisting of hydrophobic components; 2) what forces determine the movement of substances through the membrane when entering or leaving the cell.
It is now known that ions and various compounds cross the lipid phase of biological membranes in several ways. The main ones:
Simple diffusion through the lipid phase if the substance is lipid soluble.
Facilitated diffusion of hydrophilic substances using 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, the properties of which were studied by Yu.A. Ovchinnikov et al., grouping around K + ions, form highly specific carriers for this cation. In modern biology, such membranotropic physiologically active substances have become a powerful and subtle tool for experimental influence on a living cell.
Passive and active membrane transport. The second main question in the problem of membrane transport is elucidation of the driving forces of this process. Passive transport is the movement of substances by electrochemical diffusion, i.e. by 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 also, apparently, in the tonoplast) is played by the H + pump, which creates electrical (Δψ) and chemical (ΔpH) gradients of H + ions through these membranes.
In Fig. it was shown that the electrical potential of H + ions (membrane potential) can be used to transport cations along an electrical gradient against the concentration gradient. In turn, ΔрН serves as the energy basis for the transfer of Cl -, SO 4 2-, etc. through the membrane simport with H + ions (i.e. in the same direction) or to pump out excess Na + into antiporte with H + (i.e. in opposite directions). 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 moving substances across a 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, carrier proteins for sugars and amino acids are found, which acquire high affinity for the substrate only under protonation conditions. Therefore, when the H + pump begins to work and the concentration of H + ions on the outer surface of the plasmalemma increases, 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 sugars are released, and the 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. In the same way, in symport with H + ions, anions can also enter the cell. In addition, anions of weak organic acids, when the pH on the surface of the plasmalemma decreases, 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 plasmalemma of plant cells: K n + - cations, A - anions, Sax - sugars, AK - amino acids.
HCO 3 - or OH - can function similarly to H +, the excess of which appears in the near-membrane layer of the cytoplasm during intensive operation of the H + pump. Transport of OH - ions, HCO 3 - and (or) anions of organic acids outward along an electrochemical gradient can occur in antiporte with the entry of mineral anions into the cell.
plant cell chloroplast chlorophyll flavonoid
In a living plant cell, a certain ratio of water, salts and organic substances is always maintained, regulated by the exchange of substances with the environment, without which life is impossible. In the cell, the semi-permeable partition is the surface layers of the protoplast, through which water and substances dissolved in it easily penetrate. Various substances also penetrate into the cell sap. Without this, it would be impossible for nutrients to enter the cell from the outside and move them from one cell to another, and, consequently, the existence of the plant itself. The ability of the cytoplasm to pass certain substances through itself is called cytoplasmic permeability.
The supply of nutrients to cells is the result of an active absorption process, which is subject to the laws of diffusion. But not everything can be explained by diffusion. The processes of supply of dissolved substances and the permeability of the cytoplasm are influenced by the supply of tissues with oxygen, temperature, the presence of organic substances, the content of salts in the cells, as well as the properties and concentration of solutes in the surrounding substrate.
Maintaining the permeability of the cytoplasm at a certain level is associated with maintaining the balance between the ions contained in the solutions surrounding the cell and in the cytoplasm itself; their ratio determines the degree of its viscosity. Consequently, the permeability of the cytoplasm depends on a number of conditions, namely: the nature of the substances themselves that are contained in the cell, the ratio of various ions of mineral substances, temperature and other external conditions.
All organic substances can be divided into two groups: non-polar, at whose centers of electric charges coincide, and polar, in which the centers of electric charges do not coincide. In non-polar compounds the predominant groups are --CH 3 , --C 2 H 5 , --C 4 H 9 , --C 6 H 6 , in polar compounds - OH, --COOH, --NH 2 , --SON, --CN, --CONH 2, --SH, --NCS, as well as groups that have double and triple bonds. There are also compounds of mixed type, which are both polar and non-polar. Such compounds are easily adsorbed, such as drugs. Non-polar compounds are lipids, which, together with other substances, are part of the cytoplasm. Polar substances include glucose, urea, and glycerol, which do not dissolve in lipids, but easily penetrate the cell. The penetration of ions into the cell occurs through passive non-metabolic absorption as a result of diffusion along a concentration gradient according to the saturation curve and through metabolic (active) absorption. Plasma membranes allow water to pass through well and are poorly permeable to ions, which are absorbed against the concentration gradient. Thus, the concentration of K + in the vacuole is 100 or more times higher than in the soil.
Non-metabolic absorption is a reversible process. Thus, when the root system of a plant is transferred from a nutrient solution to water, some of the elements that have diffused into the cell walls and intercellular spaces can be found in it.
Metabolic (active) absorption, unlike non-metabolic absorption, occurs slowly; absorbed ions are quickly involved in metabolism and undergo transformations:
NH 4 +, NO 3 - and SO 4 2- >amino acids,
PO 4 3- >ATP, Ca and Mg > phytin, Fe > porphyrins.
Metabolic uptake is selective for different ions. In addition, the process of ion absorption requires energy and is accompanied by an increase in respiration intensity.
Thus, the absorption of substances and nutrients is a complex process and it is carried out with the participation of physicochemical and metabolic forces. Numerous studies indicate that there is obviously no single mechanism for the absorption of substances. There may be a number of simultaneously functioning mechanisms for the absorption of substances.
The entry of substances into the cell and into the cytoplasm occurs through surface adsorption processes, diffusion, active transport and pinocytosis. In general, adsorption is expressed by the concentration of solute molecules at the interface that have surface activity. Distinguish physical, or non-polar, adsorption (van der Waals forces act), polar(adsorption of electrolytes or ions) and chemisorption(due to chemical reactions).
The movement of dispersed substances from one part of a system to another is called diffusion(from lat. diffusio - distribution, spreading). Substances dissolved in water are scattered among the solvent molecules, lose their adhesion to each other and are in continuous motion like the movement of gas particles. The solute particles are evenly distributed in the space available to them. The smaller the particles of a substance molecule, the faster they spread throughout the mass of the solvent; large particles of colloidal substances move tens of times slower than crystalloid molecules. Hence the basic law: the rate of diffusion is inversely proportional to the size of the particles.
Gases, liquids and solids are capable of diffusion. Proteins and polysaccharides have a reduced ability to diffuse.
The active transfer of molecules is carried out due to metabolic energy, which is supplied in the form of high-energy bonds (ATP) with the participation of ATPase, which breaks down and releases energy.
The theory of cellular carriers is being developed experimentally (P. Bennett-Clark, A.L. Kursanov, W. Stein, etc.). The essence of this theory is that ions that come from the environment into the semi-permeable zone of the cytoplasm are bound by special substances - cellular carriers that act as conductors of ions into the inner layers of the protoplast. Carrier substances include b-ketoglutaric acid (HOOC - COCH 2 - CH 2 - COOH) and other keto acids from the Krebs cycle, the phospholipid lecithin.
The substance “captured” by the cellular carrier enters the deeper layers of the cytoplasm and is retained there by substances that accept it.
Carrier substances can also function in the opposite direction - carrying organic and inorganic compounds out of the cell to the outside, which leads, for example, to the release of organic compounds into the environment by the root system.
The absorption of substances from the external environment by the cell can also be carried out (according to G. Holter’s hypothesis) by pinocytosis. As a result of the active movement of the cytoplasmic surface, the cell seems to swallow droplets of liquid from the solution. For a long time, pinocytosis was considered unique to animal cells. However, evidence has been discovered of the penetration of macromolecular substances, in particular ribonucleases (RNases - molecular weight 137683), into the plant cell. Thus, it is possible to transfer large molecules into the cell without their splitting, through pinocytotic invaginations on the surface membranes of the plant cell.
Electrically neutral molecules (nucleic acids, carbohydrates) do not cause pinocytosis. When an inductor - polar molecules of metal ions - is added to them, pinocytotic invaginations appear. Among organic substances, proteins are a strong inducer of pinocytosis. Substances adsorbed by the surface membranes of the cytoplasm are drawn into the inner layers, where they interact with the substance of the cytoplasm.
The absorption of substances, their transportation and transformation, and the movement of the protoplast are associated with the expenditure of energy, which is released during respiration and accumulates in substances rich in high-energy bonds. In addition, acid ions H +, HCO 3 -, formed during respiration, continuously enter the plasmalemma instead of cations and anions, which pass into the deeper layers of the cytoplasm. Thus, the processes of adsorption, desorption and respiration are closely related.
Uptake of substances by cells is relatively selective; only certain substances undergo desorption from the cytoplasm and are released from the cell into the environment. Such selectivity is one of the most important properties of living matter, which arose and became established during the development of life.
A1. What is the science of cell called? 1) citA1. What is the science of cell called? 1) cytology 2) histology 3) genetics 4) molecular biologyA2. Which scientist discovered the cell? 1) A. Leeuwenhoek 2) T. Schwann 3) R. Hooke 4) R. Virchow
A3. The content of which chemical element predominates in the dry matter of the cell? 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 of the chromosome set is 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.
8. What level of organization of living nature represents the totality of all ecosystems of the globe in their interconnection9. 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
In order for an exogenous substrate to be used by a cell, it must pass through its boundary layers. The cell wall does not serve as a significant barrier for small molecules and ions, but it retains macromolecules whose mass exceeds 600 Da. The boundary layer responsible for transporting nutrients into the cell is the plasma membrane.
The transfer of nutrients across the plasma membrane is, as a rule, specific: only those substances for which there is an appropriate transport system can be absorbed. With few exceptions, transport depends on the presence of specific permeases or translocases. We are talking about membrane proteins, the very name of which indicates that they have the properties of enzymes, i.e. can be induced by a substrate, are specific to the substrate, and are formed only under conditions in which protein synthesis is possible.
As for the mechanism of transport of substances, a number of different processes are distinguished, two of which are capable of providing only transport, but not the accumulation of substances in the cell; they can be opposed by active transport processes, leading to the accumulation of substances inside the cell (Fig. 7.18 and 7.19).
Simple diffusion. Nonspecific penetration of substances into the cell occurs through passive diffusion. For diffusion, the size of the molecules and the degree of their lipophilicity are essential. The speed of movement by diffusion is low. For sugars, such processes have not been discovered, and they are unlikely. By simple diffusion, poisons, inhibitors and other substances foreign to the cell apparently penetrate into the cell.
Facilitated diffusion. With facilitated diffusion, the substance contained in the nutrient medium is transported into the cell “down” along its concentration gradient. This process is carried out thanks to a substrate-specific permease and does not require the expenditure of metabolic energy. The rate of transport depends over a wide range on the concentration of the substrate in the medium (Fig. 7.19). A nutrient cannot accumulate in a cell against a concentration gradient.
Rice. 7.18. Scheme of four mechanisms of transport of substances into the cell. Pink circle - transported substrate; c - permease (carrier protein); with gray rectangle - energized carrier; PEP - phosphoenolpyruvate;
GB is a thermostable protein. Explanations in the text.
Active transport. Active transport and group translocation have in common with facilitated diffusion that these processes occur with the participation of substrate-specific transport proteins. However, unlike facilitated diffusion, this type of transport requires energy. When metabolic energy is used, a substance can accumulate in the cell against a concentration gradient. Main difference between active transport and translocation
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 to a 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. Much less is known about the release of metabolites into the environment 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.