In the soil solution, ions are in a free state or are associated with soil colloids. Mineral nutrition elements are absorbed most often in ionic form: nitrogen as NO 3 + or NH 4 +, phosphorus as HPO 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, mineral salts are the main source of nutrients.
For a long time, it was believed that a highly diluted soil solution enters the roots unchanged, rises along the stem, and then “thickens” in the leaves as a result of water evaporation, i.e. substances enter the plant in the same quantities and ratios in which they are in the soil solution. The transpiration current was considered one of the essential conditions for the entry of substances from the soil to the root, and then to the aboveground organs.
However, later experiments showed that the amount of mineral substances that entered the plant and accumulated in it was not proportional to the amount of water that passed through it. In tropical forests, transpiration can be suppressed due to high humidity. Despite this, the trees achieve very large sizes and develop a large leaf surface; therefore, they provide for themselves with all necessary elements 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 with the help of significantly different mechanisms. However, the independence of these two processes does not mean that the transpiration current is irrelevant. If there are many salts in the soil solution, then most of them move from the roots to the aboveground organs along with water through the xylem vessels, i.e. water facilitates the transport of substances from roots to shoots (mass flow). 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 nitrogen source, 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 a solution containing this salt, damaging the roots. If sodium nitrate is used as a source of nitrogen, then the anion will enter the roots faster than the cation. NaHCO 3 will accumulate in the surrounding solution. This salt undergoes hydrolysis to form the strong alkali NaOH and the weak acid H 2 CO 3 , which causes the nutrient solution to become alkaline. Ammonium nitrate is an example of a salt in which the anion and cation are taken up at almost the same rate. Salt, in which the 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 uptake, the ratio of absorbed substances in the cells can be quite different from that in the external solution. Observations also showed that the absorption of substances proceeds not only selectively, but also against the gradient of the chemical potential. For such absorption, it is necessary to expend the energy of ATP.
The energy of respiration can be used for the transport of ions against the gradient of the 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 outer side of the inner mitochondrial membrane. In this case, the inner side of the membrane becomes negatively charged. 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 entry follows the chemical potential gradient. However, the main role in the life of the plant is played by the active absorption of elements.
Like the influx of water, the absorption process is divided into two stages: 1st - the influx of ions from the soil or nutrient solution into the free space of the cell; 2nd - their movement from free space through the plasmalemma to the protoplast. Sometimes absorbed ions can be transported from the cytosol to the vacuole or to another organelle.
Mechanisms of absorption of substances by the root. Ions enter the free space of the cell either from the soil solution, in the case of an epiblem, or from the free space of neighboring cells. At the first stage, the main absorption mechanisms are diffusion and adsorption; at the second stage, membrane transport proteins and endocytosis. Cell walls cannot serve as a barrier to absorbed substances, because they contain interfibrillar cavities through which these substances freely diffuse.
That part of the root volume into which minerals penetrate or from which are released by free diffusion is called the apoplast. Anatomically, the apoplast is represented by interfibrillar cavities of cell walls and intercellular spaces. The amount of free space is 5-10% of the total volume of the root system. Free space is external in relation to cell protoplasts and internal in relation to the external environment.
Due to diffusion, substances enter 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 diffusion rate is low and decreases with an increase in its 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 root to 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 several 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 electric charge, for example, due to the carboxyl groups of pectin substances that make up the cell walls. Since substances are absorbed mainly in ionic form, this electrical charge will also affect their intake. Therefore, the entry of ions into the free space of the cell depends not only on the concentration difference, but also on the difference in electrical potentials. The existence of a negative charge on the cell wall facilitates the uptake of cations and makes it difficult for the uptake of anions.
In addition, cations can be adsorbed on cell walls, so the concentration of cations near the walls of the interfibrillar cavity is higher, and anions are lower; in the center of the cavity, it is equal to the concentration of cations and anions in the external 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 the 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 aged 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, then this will not happen.
The phenomena of adsorption are characterized by an extremely high initial rate, which distinguishes adsorption from diffusion. The second proof of the participation of adsorption in the absorption of substances is the existence of a saturation state. characteristic feature absorption is its small dependence initially on temperature. The initial absorption depends on the pH value. The concentration of hydrogen ions has a particularly strong effect on the ratio of the amounts of absorbed cations and anions.
Thus, intense absorption at the beginning, the existence of a saturation state, the independence of the initial absorption from temperature, and complete dependence on the pH value - 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 adsorb fairly quickly on the root surface and then accumulate in its cells the nutrients it needs from such dilute solutions as soil solutions usually are.
Having entered the free space of the root with the help of diffusion, ions are adsorbed not only on the cell walls, but also on the plasmalemma, which also carries an electric charge. The electrical potential difference across the plasma membrane ranges from 60 to 100, and sometimes reaches 180 mV, and the cell is negatively charged with respect 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, he needs to pass through the plasma membrane - the main barrier to the diffusion of ions and molecules into the cell. Molecules of the solute, which are in continuous chaotic motion in the solution, upon collision with this membrane, will either bounce off it, or be adsorbed on it, or pass through the plasmalemma. In the latter case, the membrane is said to be permeable to the given substance. If the substance itself cannot pass through the plasmalemma, then membrane transport proteins, as well as endocytosis, help.
Different mechanisms do not work in isolation, but in various 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 value 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 energy is needed for the operation of any mechanism. For example, to maintain diffusion, it is necessary that the amount of substance in the protoplast 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 uptake by the cell of substances should have led to an equalization of concentrations and to the cessation of uptake. However, this does not happen, since the incoming ions, for example, nitrate, sulfate and phosphate ions, are quickly included in the cell into organic compounds. The rate of inclusion of ions in the metabolism, in turn, determines the rate of their absorption.
The main organ of absorption of mineral nutrients is the root. Fertilizers can also be applied through the leaves (foliar feeding), but, as experiments with labeled phosphorus have shown, this causes their rapid aging and fall. In experiments with foliar feeding, the plants grew 10 times slower than the control plants that received the same dose of phosphorus through the roots.
Consider the structural features of the root as an organ for absorbing substances. Epiblema, covering the root from the outside, 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. Epiblema - the main barrier to the absorption of ions by the root. This is proved by the fact that it is in the cells of this tissue that both anions and cations accumulate in the greatest quantities. Epiblema as an absorbent tissue is heterogeneous: some of its cells turn into root hairs. There are 200–400 root hairs per 1 mm 2 of the root surface; as a result, the absorbing surface of the root increases hundreds of times. The root hair is the main entrance for ions to the root; through other cells of the epiblema, less of them enter. 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 to 86–90% of the total root volume, it contains many intercellular spaces. The thicker the cortex, 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 the symplastic pathway and vice versa. Under the endoderm is the pericycle. A calculation specially carried out on electron microscopic photographs showed that while there are 45 thousand plasmodesmata between neighboring cells of the pericycle in the cell walls, only 12 thousand go from each cell of the pericycle to the cells of the central cylinder. 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. Therefore, the role of the pericycle can be compared to the role of the ring road, allowing you to change the direction of movement. The functions of the conducting tissues located in the center of the root are well known: absorbed substances are transported through them 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 intensity. The cells of the elongation zone and the zone of root hairs absorb substances most actively. Experiments with wheat seedlings have shown that root hairs not only increase the surface of the root, but their membranes contain transport proteins that are more active than the same proteins of other cells of the epiblema.
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, takes place. In the corking zone, the absorption of ions per unit root surface sharply decreases. The zone of greatest absorption, depending on the characteristics of the plant, soil, groundwater level, irrigation and the distribution of mineral nutrition elements, descends with the age of the plant. So, in each root there is a significant gradient in absorption capacity, which decreases from the tip of the root to its basal part.
If the root absorbs substances for some time, then their concentration near the actively absorbing root areas will decrease and the rate of absorption will depend on the rate of diffusion of ions in the soil. At the growing tip of the root, 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 necessary substances have not yet been depleted. As the root grows, it moves towards nutrients. Absorption in the growing part occurs much faster than in other root zones, near which the delivery of substances is limited. The faster the root branches, the more this 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 absorbed substances; growing zones - on the rate of diffusion and on the rate of their growth.
The powerful development of the root systems of plants, especially their small active roots and root hairs, their continuous distribution in more and more new soil layers is a necessary condition for the absorption of substances. The lower the mobility of a given ion in the soil, the more important is the power of development of the root system, its distribution in a larger volume of soil. The plant has great potential for root formation. By changing the fertilizer application scheme, it is possible to control the growth of roots in length and the rate of their branching, and hence the yield. In addition, pruning is known to stimulate root branching, and this 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 rapid absorption of salts.
Root zones differ from each other not only in the rate of absorption of substances, but also in the intensity of their supply to aboveground organs. So, the cells of the zone of division and stretching absorb substances exclusively for their own (“internal”) consumption. Not only do they not transport these substances to the aboveground organs, but, on the contrary, they themselves partially use the 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, the root distinguishes 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 the above-ground organs (the zone of root hairs).
However, it would be wrong to think that the powerful development of the roots is always a prerequisite for satisfying the needs of the plant in nutrients. For example, a huge number of varieties of sugar cane with a highly developed root system are low-yielding, 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 inactive substances, moreover, often very dispersed in the soil. In the presence of nutrients in the root environment in an easily accessible form and in sufficient concentration, a powerful development of the roots is not necessary, because the root system usually does not work with full load.
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.
Thus, the speed and direction of root movement in the soil, the size of the surface that actively absorbs salts and supplies them to other organs, and the rate of inclusion of ions in metabolism are determined by root growth. The consequence of this dependence is that with a lack of nutrients, first of all, the growth of shoots is inhibited, and the growth of roots in length, on the contrary, is stimulated, which allows the root to quickly pass through the nutrient-poor soil layer.
The importance of studying the patterns of formation of root systems and the absorption of mineral nutrition elements by them is important, firstly, for solving some issues of agricultural technology (the depth of the arable layer, the depth of planting seeds and fertilizers, the choice of tillage and irrigation methods); secondly, there are certain correlations between root development and plant adaptability to drought, high humidity, or damage by pests.
A1. What is the science of the cell called? 1) citA1. What is the science of the 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. Virkhov
A3. The content of what chemical element prevails in the dry matter of the cell? 1) nitrogen 2) carbon 3) hydrogen 4) oxygen
A4. What phase of meiosis is shown in the figure? 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 1) crushing 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) A8 genotype. In the second generation, with monohybrid crossing and with complete dominance, 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) benzpyrene
A10. Where in a eukaryotic cell is ribosomal RNA synthesized? 1) ribosome 2) rough ER 3) nucleolus of the nucleus 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) tubercle bacillus 4) pine
A13. What is nuclear chromatin? 1) karyoplasm 2) RNA strands 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 during organogenesis from the ectoderm? 1) chord 2) neural tube 3) mesoderm 4) endoderm
A16. A non-cellular form of life is 1) euglena 2) bacteriophage 3) streptococcus 4) ciliate
A17. The synthesis of a protein on i-RNA is called 1) translation 2) transcription 3) reduplication 4) dissimilation
A18. In the light phase of photosynthesis, 1) synthesis of carbohydrates 2) synthesis of chlorophyll 3) absorption of carbon dioxide 4) photolysis of water occurs
A19. Cell division with the 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 the mRNA chain on DNA 4) breakdown of starch to glucose
A21. Choose the wrong statement In prokaryotes, the DNA molecule 1) is closed in a ring 2) is not associated with proteins 3) contains uracil instead of thymine 4) is in the singular
A22. Where does the third stage of catabolism take place - complete oxidation or respiration? 1) in the stomach 2) in mitochondria 3) in lysosomes 4) in cytoplasm
A23. Asexual reproduction includes 1) parthenocarpic fruit formation in cucumber 2) parthenogenesis in bees 3) reproduction of tulip bulbs 4) self-pollination in flowering plants
A24. What organism in the postembryonic period develops without metamorphosis? 1) lizard 2) frog 3) Colorado potato beetle 4) fly
A25. The human immunodeficiency virus infects 1) gonads 2) T-lymphocytes 3) erythrocytes 4) skin and lungs
A26. Cell differentiation begins at the stage of 1) blastula 2) neurula 3) zygote 4) gastrula
A27. What are protein monomers? 1) monosaccharides 2) nucleotides 3) amino acids 4) enzymes
A28. In what organelle does the accumulation of substances and the formation of secretory vesicles take place? 1) Golgi apparatus 2) rough ER 3) plastid 4) lysosome
A29. What disease is sex-linked? 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) it becomes possible to recombine traits as a result of crossing over 4) the probability of combinative variability of organisms decreases.
The largest systematic unit A) Kingdom B) Department C) Class D) Family 3. Eukaryotes include a cell A) Fungi B) Bacteria C) Cyanobacteria D) Viruses 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 rounded 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. Organoid in which organic substances are oxidized to carbon dioxide and water A) Mitochondria B) Chloroplast C) Ribosome D) Golgi complex. 8. Chloroplasts in the cell do not perform the function A) Synthesis of carbohydrates B) Synthesis of ATP 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 C) Tertiary D) Quaternary 10. Unlike tRNA, mRNA molecules A) Deliver amino acids to the site of protein synthesis B) Serve as a template 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 glucose synthesis proceeds A) In the nucleus B) In chloroplasts C) Ribosomes D) Lysosomes , is A) Water B) Glucose C) Ribose D) Starch 14. How many cells and with what set of chromosomes are formed after meiosis? 15. Divergence of chromatids to the poles of the cell occurs in A) Anaphase B) Telophase C) Prophase D) Metaphase 16. The biological meaning of mitosis. 17. Advantages of asexual reproduction.
8. What level of organization of wildlife is the totality of all ecosystems of the globe in their interconnection9. Which of the following organs are homologous
10. The appearance of what sign in a person is referred to as atavisms
11. Which pair of aquatic vertebrates confirms the possibility of evolution based on convergent similarity
12. The similarity of the functions of chloroplasts and mitochondria lies in what happens in them
13. What is 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, which 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 for evolution includes
18. The highest concentration of living matter is observed
19. What structures are absent in the cells of the skin of onion scales
20. Founder of scientific systematics (classification)
21. In a DNA molecule, the number of nucleotides with thymine is ...% of the total. What is the percentage of nucleotides with cytosine in this molecule
22. During plant photosynthesis
23. The remnant of the third eyelid in the corner of the human eye is an example
24. In which cell organelles is a wide variety of enzymes involved in the breakdown of biopolymers to monomers concentrated?
25. The distribution area of the reindeer in the tundra zone is a criterion
26. 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 provides the transport of substances in the cell
29. Non-cellular life forms include
30. The intermediate nature of the inheritance of a trait is manifested when
31 The greenhouse effect on Earth is a consequence of an increase in the concentration of
32. The most acute form of the struggle for existence
33. The genetic heterogeneity of individuals in a population is enhanced
34. The development of multicellular organisms from the zygote is evidence
35. Atavisms of a person include the appearance
36. Identify organisms that enter into competitive relationships
37. What happens during photosynthesis
38. The similarity of the structure and vital activity of the cells of organisms of different kingdoms of wildlife is one of the provisions
39. The structure and functions of the plasma membrane are determined by its constituent molecules
40. Establish a correspondence between the form of natural selection and its features
1)02iH2O 3)C02iH20
2) CO2 and H2 4) CO2 and H2CO3
2. The consumer of carbon dioxide in the biosphere is:
1) oak 3) earthworm
2) eagle 4) soil bacterium
3. In which case is the glucose formula correctly written:
1) CH10 O5 3) CH12 About
2) C5H220 4) C3H603
4. The source of energy for the synthesis of ATP in chloroplasts is:
1) carbon dioxide and water 3) NADP H2
2) amino acids 4) glucose
5. In the process of photosynthesis in plants, carbon dioxide is reduced to:
1) glycogen 3) lactose
2) cellulose 4) glucose
6. Organic substances from inorganic can create:
1) Escherichia coli 3) pale grebe
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 pale grebe 4) blue-green algae
9.. The main supplier of oxygen to the Earth's atmosphere are:
1) plants 2) bacteria
3) animals 4) people
10. The following have the ability to photosynthesis:
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 when breathing:
1) carbon dioxide and release oxygen
2) oxygen and release carbon dioxide
3)light energy and releases carbon dioxide
4) light energy and release oxygen
13. Photolysis of water occurs during photosynthesis:
1) during the whole process of photosynthesis
2) in the dark phase
3) in the light phase
4) there is no synthesis of carbohydrates
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. In 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. According to the type of nutrition, most plants belong to:
17. In plant cells, unlike human, animal, 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. In chloroplasts occurs:
1) mRNA transcription 2) formation of ribosomes
3) formation of lysosomes 4) photosynthesis
20. Synthesis of ATP in the cell occurs in the process:
1) glycolysis; 2) photosynthesis;
3) cellular respiration; 4) all listed
The root system of plants absorbs both water and nutrients from the soil. Both of these processes are interrelated, but are carried out on the basis of different mechanisms. The roots extract minerals from the soil solution and from the soil absorbing complex, with particles of which the root absorption zone (root hairs) is in close contact.
Cell walls are directly involved both in the absorption of substances from the soil and in the transport of mineral nutrition elements through tissues.
The main driving force of the absorption activity of the roots, as well as each cell in general, is the operation of ion pumps localized in the membranes. The radial transport of mineral substances from the root surface to the conducting system is carried out as a result of the interaction of all the main tissues of the absorption zone, and each tissue performs certain functions. Radial transport ends with the loading of minerals and their organic derivatives into tracheids and xylem vessels. Xylem sap is transported to other parts of the plant by transpiration and/or root pressure. The cells that make up various tissues and organs, in turn, absorb and metabolize the elements of mineral nutrition delivered with xylem juice. Moreover, their absorption activity depends on age and functional state.
In general, the process of mineral nutrition of a plant is a complex chain of biophysical, biochemical and physiological processes with their feedback and direct links and a regulatory system. At present, not all links of this chain have been studied in sufficient detail.
The absorptive activity of the root is based on the mechanisms of absorptive activity inherent in any plant cell. Therefore, such general questions as the selective entry of substances into the cell, the role of the cell wall phase, and the transmembrane transport of ions will be discussed in relation to all plant cells.
In various organs of plants, an unequal amount of mineral elements accumulates, and the content of mineral substances in the cells does not correspond to the concentration of these same substances in the external environment. The content of nitrogen and potassium is ten times higher in cells. This indicates that there are mechanisms in cells not only for absorption of substances against a concentration gradient, but also for their selective accumulation. This process begins already in the cell wall and then continues with the participation of membranes.
The role of cell walls in the processes of adsorption of mineral substances. Unlike animal cells, a plant cell has a shell (wall) consisting of cellulose, hemicelluloses and pectin substances. Pectin substances (polyuronic acids) contain carboxyl groups in their composition, as a result of which the cell membranes acquire the properties of cation exchangers and can concentrate positively charged substances.
If roots (or other plant tissue) are immersed in a vessel containing a solution of 86 RbCl or a cationic dye (for example, methylene blue), then in the first 2 minutes up to 50% of rubidium (or dye) from the amount that is absorbed will disappear from the solution. over a long period of time (Fig.).
Dynamics of absorption of ions by plant cells and their release during washing with water or saline (phase I - penetration of substances into the apparent free space (CSP), phase II - accumulation of substances in cells; the dotted line indicates the extrapolation of the absorption curve in phase II to the y-axis to determine the value KSP)
In the next 10 - 30 minutes, 70% will be absorbed, and further binding of the substance by the tissues will occur very slowly (for hours). What is the reason for such a rapid movement of matter at the very beginning? If 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 the absorption of substances can be distinguished, proceeding at different speeds - high and slow, and the substance quickly absorbed by the tissue also quickly leaves it. The initial rapid absorption of substances is carried out in the cell walls and is exchange adsorption (and the rapid loss is desorption). The slow phase is associated with the functional activity of the plasmalemma (penetration of substances into the cell or exit from it). The molecular space in the cell wall, where exchange adsorption processes take place, is called the apparent free space (APS). The term "apparent" means that the amount of this free space depends on the object and the nature of the solute. The PCB includes the intermolecular space in the thickness of the cell walls and on the surface of the plasmalemma and cell walls. According to calculations, CSP occupies 5-10% of the volume in plant tissues. The absorption and release of substances in the PCB is a physicochemical passive process. It is determined by the adsorption properties of the ion exchanger and the Donnan electric potential at the interface between the aqueous medium and the cation exchanger. These factors already at the first stage ensure the selectivity of the absorption of charge-carrying substances, since the cation exchanger (cell walls) binds cations (especially divalent and trivalent ones) more actively than anions. 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 conditions of soil nutrition, root cells (rhizoderm) are in contact with the water phase (soil solution) and with soil particles, which are also predominantly cation exchangers (soil absorbing complex). At the same time, most of the mineral nutrients are not in solution, but are adsorbed on soil particles.
Cations and anions enter the cell walls of the rhizoderm both directly from the soil solution and through contact exchange with particles of the soil absorbing complex. Both of these processes are associated with the exchange of H + ions for environmental cations and HCO 3 - (OH -) or organic acid anions for mineral anions.
Contact exchange of ions of the cell wall of the rhizoderm (H + ions) with soil particles is carried out without the transition of ions into the soil solution. Close contact is provided due to the secretion of mucus by root hairs and the absence of cuticles and other protective integumentary formations in the rhizoderm. The root absorption zone and soil particles form a single colloidal system (Fig.).
Contact ion exchange between root cells and soil particles
Since the adsorbed ions are in constant oscillatory motion and occupy a certain “oscillatory volume” (sphere of oscillations), with close contact of the surfaces, the spheres of oscillations of the two nearest adsorbed ions can overlap, resulting in ion exchange.
The capacity for exchange adsorption in general and contact exchange in particular is determined by the exchange capacity of the root. It depends on chemical composition root secretions and cell membranes and is supported by the continuous synthesis of new substances associated with the growth of the root and with the processes of renewal of its structures, as well as with the absorption of substances through the cytoplasmic membrane into the cells and their further movement into the root. The exchange capacity of the root different types plants is not the same and depends on age.
Ways of penetration of ions through biological membranes. The problem of membrane transport includes two main questions: 1) how various substances physically overcome the membrane, which consists of hydrophobic components; 2) what forces determine the movement of substances through the membrane when entering the cell or when leaving it.
It is now known that ions and various compounds cross the lipid phase of biological membranes in several ways. The main ones are:
Simple diffusion through the lipid phase if the substance is lipid soluble.
Facilitated diffusion of hydrophilic substances by lipophilic carriers.
Simple diffusion through hydrophilic pores (eg 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).
AT last years substances have been discovered and studied that can dramatically accelerate the transport of substances through the lipid phase of membranes. For example, the antibiotic gramicidin creates channels for K + and H + ions. Molecules of another lipophilic antibiotic, valinomycin, whose properties were studied by Yu.A. Ovchinnikov et al., grouping around K + ions, form highly specific carriers for this cation. Such membranotropic physiologically active substances in modern biology have become a powerful and subtle instrument of experimental influence on a living cell.
Passive and active membrane transport. The second main issue in the problem of membrane transport is the elucidation of the driving forces of this process. Passive transport is the movement of substances by diffusion along the electrochemical, i.e. along the electrical and concentration gradient. This is how, for example, substances move if their concentration in the external environment is higher than in the cell. Active transport is the transmembrane movement of substances against an electrochemical gradient with the expenditure of metabolic energy, usually in the form of ATP. Examples of active transport are ion pumps: H + -ATPase, Na + , K + -ATPase, Ca 2+ -ATPase, anionic ATPase.
A special role in the plasmalemma of plant cells (and apparently also in the tonoplast) is played by the H+ pump, which creates electrical (Δψ) and chemical (ΔрН) gradients of H+ ions through these membranes.
On fig. It was shown that the electric potential of H+ ions (membrane potential) can be used for the transport of cations along the electric gradient against the concentration one. In turn, ΔрН serves as an energy basis for the transfer of Cl -, SO 4 2- and others through the membrane to symport with H + ions (i.e. in the same direction) or to pump out excess Na + into antiporte with H + (i.e. in opposite 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 movement of substances through the membrane is called secondary active transport.
The appearance of ΔpH on the membrane can serve as the basis for secondary active transport and organic substances. In the plasmalemma, protein carriers of sugars and amino acids were found, which acquire a high affinity for the substrate only under conditions of protonation. Therefore, when the H + pump starts to work and the concentration of H + ions increases on the outer surface of the plasma membrane, these carrier proteins are protonated and bind sugars (amino acids). When sugar molecules are transferred to the inner side of the membrane, where there are very few H + ions, H + and sugar are released, and sugars enter the cytoplasm, and H + ions are again pumped out of the cell by the H + pump. Essentially, H + plays the role of a catalyst in this process. Similarly, in symport with H+ ions, anions can also enter the cell. In addition, anions of weak organic acids with a decrease in pH on the surface of the plasmalemma can penetrate the membrane in the form of uncharged molecules (if they are soluble in the lipid phase), since their dissociation decreases with increasing acidity.
Mechanisms of membrane transport in the plasma membrane of plant cells: K n + - cations, A - - anions, Sax - sugars, AA - amino acids.
Similarly, H + can function and HCO 3 - or OH - , the excess of which appears in the near-membrane layer of the cytoplasm during intensive operation of the H + -pump. The transport of OH - , HCO 3 - and (or) anions of organic acids outward along the electrochemical gradient can proceed in antiporte with the entry of mineral anions into the cell.
Ministry of Agriculture of the Russian Federation
FSBEI HPE "Yaroslavl State Agricultural Academy"
Department of Ecology
TEST
In the discipline "Plant Physiology"
Performed:
4th year student
Faculty of Technology
Stepanova A. Yu.
Checked:
teacher Taran T.V.
Yaroslavl 2014
1. Absorption of substances by a plant cell. Passive and active transport………………………………………………………………………
2. Transcription and its biological significance, types. Factors determining the amount of transcription……………………………………
3. Dehydrogenases, their chemical nature and nature of action………………
4. Physiology of dormancy and seed germination. Influence of internal and external conditions on the process of seed germination………………………………………..
1. Absorption of substances by a plant cell. Passive and active transport
Entry of substances into the cell wall (stage 1).
Absorption of substances by the cell begins with their interaction with the cell membrane. Even the works of D. A. Sabinin and I. I. Kolosov showed that the cell membrane is capable of rapid adsorption of ions. Moreover, this adsorption in some cases has an exchange character. Later, in experiments with isolated cell membranes, it was shown that they can be considered as an ion exchanger. On the surface of the cell membrane, H + and HC0 3 - ions are adsorbed, which in equivalent quantities change to ions located in the external environment. Ions can be partially localized in the intermicellar and intermolecular gaps of the cell wall, partially bound and fixed in the cell wall by electric charges.
The first stage of admission is characterized by high speed and reversibility. The incoming ions are easily washed out. This is a passive diffusion process that follows an electrochemical potential gradient. The cell volume available for free diffusion of ions includes cell walls and intercellular spaces, i.e., the apoplast or free space. According to calculations, free space (SP) can occupy 5-10% of the volume in plant tissues. Since the cell membrane includes amphoteric compounds (proteins), the charge of which changes at different pH values, the adsorption rate of cations and anions can also change depending on the pH value. Entry of substances through the membrane (stage 2). In order to penetrate the cytoplasm and be included in the metabolism of the cell, substances must pass through the membrane - the plasmalemma. The transport of substances across the membrane can be passive or active. With the passive entry of substances through the membrane, the basis of transfer in this case is also diffusion. The diffusion rate depends on the thickness of the membrane and on the solubility of the substance in the lipid phase of the membrane. Therefore, non-polar substances that dissolve in lipids (organic and fatty acids, esters) pass through the membrane more easily. However, most substances that are important for cell nutrition and metabolism cannot diffuse through the lipid layer and are transported by proteins, which facilitate the entry of water, ions, sugars, amino acids and other polar molecules into the cell. At present, the existence of three types of such transport proteins has been shown: channels, carriers, and pumps.
Three classes of transport proteins:
1 - protein channel;
2 - carrier;
3 - pump.
Channels are transmembrane proteins that act like pores. They are sometimes called selective filters. Transport through channels is generally passive. The specificity of the transported substance is determined by the properties of the pore surface. As a rule, ions move through the channels. The speed of transport depends on their size and charge. If the time is open, then the substances pass quickly. However, channels are not always open. There is a "gate" mechanism, which, under the influence of an external signal, opens or closes the channel. For a long time, the high permeability of the membrane (10 μm/s) for water, a polar substance and insoluble in lipids, seemed difficult to explain. At present, integral membrane proteins have been discovered that represent a channel through the membrane for the penetration of water - aquaporins. The ability of aquaporins to transport water is regulated by the process of phosphorylation. Attachment and donation of phosphate groups to certain aquaporin amino acids has been shown to accelerate or inhibit water entry, but does not affect the direction of transport.
Carriers are specific proteins that can bind to a carried substance. In the structure of these proteins there are groupings that are oriented in a certain way to the outer or inner surface. As a result of a change in the conformation of proteins, the substance is transferred outward or inward. Since for the transport of each individual molecule or ion, the carrier must change its configuration, the rate of transport of a substance is several times lower than the transport through channels. The presence of transport proteins was shown not only in the plasmalemma, but also in the tonoplast. Carrier transport can be active or passive. In the latter case, such transport goes in the direction of the electrochemical potential and does not require energy. This type of transport is called facilitated diffusion. Thanks to carriers, it travels at a faster rate than normal diffusion.
According to the concept of the work of carriers, the ion (M) reacts with its carrier (X) on the surface of the membrane or near it. This first reaction may involve either exchange adsorption or some kind of chemical interaction. Neither the carrier itself nor its complex with the ion can pass into the external environment. However, the ion transporter complex (MX) is mobile within the membrane itself and moves to its opposite side. Here, this complex decomposes and releases an ion into the internal environment to form a carrier precursor (X 1 ). This carrier precursor again travels to the outside of the membrane, where it is again converted from a precursor to a carrier that can combine with another ion on the membrane surface. When a substance capable of forming a stable complex with a carrier is introduced into the medium, the transfer of the substance is blocked. Experiments carried out on artificial lipid membranes have shown that ion transport can take place under the influence of certain antibiotics produced by bacteria and fungi - ionophores. Transport with the participation of carriers has the property of saturation, that is, with an increase in the concentration of substances in the surrounding solution, the rate of entry first increases and then remains constant. This is due to the limited number of carriers.
Carriers are specific, i.e., they are involved in the transfer of only certain substances and, thus, ensure the selectivity of intake.
Ionophore K complex +
This does not exclude the possibility that the same carrier can carry several ions. For example, the K + transporter, which is specific for this ion, also transports Rb + and Na + , but does not transport Cl - or uncharged sucrose molecules. A transport protein specific for neutral acids tolerates the amino acids glycine, valine, but not asparagine or lysine. Due to the diversity and specificity of proteins, their selective reaction with substances in the environment is carried out, and, as a result, their selective transfer.
Pumps (pumps) are integral transport proteins that actively supply ions. The term "pump" indicates that the flow is with the consumption of free energy and against the electrochemical gradient. The energy used for the active entry of ions is supplied by the processes of respiration and photosynthesis and is mainly accumulated in ATP. As you know, in order to use the energy contained in ATP, this compound must be hydrolyzed according to the equation ATP + HOH -> ADP + Ph n. The enzymes that hydrolyze ATP are called adenosine triphosphatases (ATPases). Various ATPases were found in cell membranes: K + - Na + - ATPase; Ca 2+ - ATPase; H + - ATPase. H + - ATPase (H + -pump or hydrogen pump) is the main mechanism of active transport in the cells of plants, fungi and bacteria. H + - ATPase functions in the plasmalemma and ensures the release of protons from the cell, which leads to the formation of an electrochemical potential difference on the membrane. H + - ATPase carries protons into the cavity of the vacuole and into the tanks of the Golgi apparatus.
The calculation shows that in order for 1 mol of salt to diffuse against the concentration gradient, it is necessary to spend about 4600 J. At the same time, 30660 J/mol is released during ATP hydrolysis. Therefore, this ATP energy should be enough to transport a few moles of salt. There is evidence showing a directly proportional relationship between ATPase activity and ion intake. The need for ATP molecules to carry out the transfer is also confirmed by the fact that inhibitors that disrupt the accumulation of respiratory energy in ATP (violation of the conjugation of oxidation and phosphorylation), in particular dinitrophenol, inhibit the flow of ions.
Pumps are divided into two groups:
1. Electrogenic, which carry out active transport of an ion of any one charge in only one direction. This process leads to the accumulation of one type of charge on one side of the membrane.
2. Electrically neutral, in which the transfer of an ion in one direction is accompanied by the movement of an ion of the same sign in the opposite direction, or the transfer of two ions with charges of the same magnitude, but different in sign, in the same direction.
The mechanism of action of transport ATPase (P - inorganic phosphate).
Thus, the transfer of ions across the membrane can be carried out in an active and passive way. In ensuring the transport function of membranes and the selectivity of absorption, transport proteins play an important role: channels, carriers, and pumps. At present, the genes for many transport proteins have been cloned. Genes encoding potassium channels have been identified. On Arabidopsis, gene mutations have been obtained that affect the transport and recovery of nitrates. It has been shown that in the plant genome, not one gene, but several, is responsible for the transport of substances through membranes. Such a multiplicity ensures the performance of a function in different parts of plants, which makes it possible to transport substances from one tissue to another.
Finally, the cell can “swallow” nutrients along with water (pinocytosis). Pinocytosis is an invagination of the surface membrane, due to which liquid droplets with solutes are swallowed. The phenomenon of pinocytosis is known for animal cells. It has now been proven that it is also characteristic of plant cells. This process can be divided into several phases: 1) adsorption of ions in a certain area of the plasmalemma; 2) invagination, which occurs under the influence of charged ions; 3) the formation of vesicles with liquid that can migrate through the cytoplasm; 4) fusion of the membrane surrounding the pinocytic vesicle with the membranes of lysosomes, endoplasmic reticulum or vacuole and the inclusion of substances in metabolism. With the help of pinocytosis, not only ions, but also various soluble organic substances can enter the cells.
The action of the ATPase pump of the cytoplasmic membrane.
Transport of substances in the cytoplasm (3rd stage) and entry into the vacuole (4th stage). After passing through the membrane, the ions enter the cytoplasm, where they are included in the cell metabolism. An essential role in the process of ion binding by the cytoplasm belongs to cell organelles. Mitochondria, chloroplast, apparently, compete with each other, absorbing cations and anions that have entered the cytoplasm through the plasmalemma. In the process of accumulation of ions in various organelles of the cytoplasm and inclusion in metabolism great importance has their intracellular transport. This process is carried out, apparently, through EPR channels.
Ions enter the vacuole if the cytoplasm is already saturated with them. It is, as it were, excess nutrients that are not included in the metabolic reactions. In order to get into the vacuole, ions must overcome another barrier - the tonoplast. If in the plasmalemma the ion transport mechanism operates within relatively low concentrations, then in the tonoplast it operates at higher concentrations, when the cytoplasm is already saturated with this ion. In vacuole membranes, vacuolar channels were found that differ in opening time (fast and slow). The transfer of ions through the tonoplast is also carried out with the help of carriers and requires the expenditure of energy, which is ensured by the work of the H+ -ATPase of the tonoplast. The potential of the vacuole is positive compared to the cytoplasm, so anions flow along the electric potential gradient, while cations and sugars - in antiport with protons. The low permeability of the tonoplast for protons makes it possible to reduce the energy costs for the intake of substances. The vacuolar membrane also has a second proton pump associated with H + -pyrophosphatase. This enzyme consists of a single polypeptide chain. The energy source for the proton flux is the hydrolysis of inorganic pyrophosphate. Transport proteins were found in the tonoplast, which allow large organic molecules to penetrate into the vacuole directly due to the energy of ATP hydrolysis. This plays a role in the accumulation of pigments in the vacuole, in the formation of antimicrobial substances, and also in the neutralization of herbicides. The substances entering the vacuole provide the osmotic properties of the cell. Thus, ions penetrating through the plasmalemma are accumulated and bound by the cytoplasm, and only their excess is desorbed into the vacuole. That is why there is not and cannot be an equilibrium between the content of ions in the external solution and the cell sap. It must be emphasized once again that active intake is of great importance for the life of the cell. It is it that is responsible for the selective accumulation of ions in the cytoplasm. The absorption of nutrients by the cell is closely related to metabolism. These connections are multifaceted. Active transfer requires the synthesis of carrier proteins, the energy supplied during respiration, and the efficient operation of transport ATPases. It should also be taken into account that the faster the incoming ions are included in the metabolism, the more intense their absorption is. For a multicellular higher plant, the movement of nutrients from cell to cell is no less important. The faster this process takes place, the faster the salts will, ceteris paribus, enter the cell.
PASSIVEAndACTIVEINCOME
The uptake of nutrients by the cell can be passive or active. Passive absorption is absorption that does not require the expenditure of energy. It is associated with the diffusion process and follows the concentration gradient of a given substance. From a thermodynamic point of view, the direction of diffusion is determined by the chemical potential of the substance. The higher the concentration of a substance, the higher its chemical potential. The movement goes in the direction of lower chemical potential. It should be noted that the direction of movement of ions is determined not only by chemical, but also by electrical potential. Consequently, the passive movement of ions can follow a gradient of chemical and electrical potential. Thus, the driving force behind the passive transport of ions across membranes is the electrochemical potential.
Electric potential on the membrane - transmembrane potential can occur for various reasons:
1. If the entry of ions follows a concentration gradient (gradient-chemical potential), however, due to the different permeability of the membrane, either a cation or an anion enters at a faster rate. Because of this, a difference in electrical potentials arises on the membrane, which, in turn, leads to the diffusion of an oppositely charged ion.
2. If there are proteins on the inside of the membrane that fix certain ions, i.e., immobilize them. Due to the fixed charges, an additional possibility is created for the entry of ions of the opposite charge (Donnan equilibrium).
3. As a result of active (energy-consuming) transport of either a cation or an anion. In this case, the oppositely charged ion can move passively along the electric potential gradient. The phenomenon when the potential is generated by the active flow of ions of the same charge through the membrane is called the electrogenic pump. The term "pump" indicates that the flow comes with the consumption of free energy.
Active transport is a transport that goes against the electrochemical potential with the expenditure of energy released during metabolism.
Passive and active transport
There is some evidence for the existence of active ion transport. In particular, these are experiments on the influence of external conditions. So, it turned out that the flow of ions depends on temperature. Within certain limits, with increasing temperature, the rate of absorption of substances by the cell increases. In the absence of oxygen, in a nitrogen atmosphere, the entry of ions is drastically inhibited, and salts can even be released from the root cells to the outside. Under the influence of respiratory poisons, such as KCN, CO, the intake of ions is also inhibited. On the other hand, an increase in the ATP content enhances the absorption process. All this indicates that there is a close relationship between the absorption of salts and respiration.
Many researchers have concluded that there is a close relationship between salt intake and protein synthesis. Thus, chloramphenicol, a specific inhibitor of protein synthesis, also inhibits the absorption of salts. The active flow of ions is carried out with the help of special transport mechanisms - pumps. Pumps are divided into two groups:
1. Electrogenic (mentioned earlier), which carry out active transport of an ion of any one charge in only one direction. This process leads to the accumulation of one type of charge on one side of the membrane.
2. Electrically neutral, in which the transfer of an ion in one direction is accompanied by the movement of an ion of the same sign in the opposite direction, or the transfer of two ions with charges of the same magnitude, but different in sign, in the same direction.
The ability of the cell to selectively accumulate nutrient salts, the dependence of intake on the intensity of metabolism serve as evidence that, along with passive intake, there is also an active intake of ions. Both processes often occur simultaneously and are so closely related that it is difficult to distinguish between them.
