The dark stage includes a group of reactions that use high-energy products of the light stage to reduce CO 2 to simple sugar, i.e. for carbon assimilation. Therefore, the dark stage is also called the synthesis stage. The term “dark stage” only means that light is not directly involved in it. Modern ideas about the mechanism of photosynthesis were formed on the basis of research conducted in the 1930–1950s. Previously, for many years, scientists were misled by the seemingly simple but incorrect hypothesis that O 2 formed from CO 2 , and the released carbon reacts with H 2 Oh, as a result of which carbohydrates are formed. In the 1930s, when it turned out that some sulfur bacteria do not produce oxygen during photosynthesis, biochemist K. van Niel suggested that the oxygen released during photosynthesis in green plants comes from water. In sulfur bacteria the reaction proceeds as follows:

Instead of O 2 these organisms produce sulfur. Van Niel came to the conclusion that all types of photosynthesis can be described by the equation

where X oxygen in photosynthesis, which occurs with the release of O 2 , and sulfur in photosynthesis of sulfur bacteria. Van Niel also suggested that this process involves two stages: a light stage and a synthesis stage.

This hypothesis was supported by the discovery of physiologist R. Hill. He discovered that destroyed or partially inactivated cells are capable of carrying out a reaction in the light in which oxygen is released, but CO 2 does not recover (it was called the Hill reaction). In order for this reaction to proceed, it was necessary to add some oxidizing agent capable of attaching electrons or hydrogen atoms given up by the oxygen of the water. One of Hill's reagents is quinone, which, by adding two hydrogen atoms, becomes dihydroquinone. Other Hill reagents contained ferric iron (ion Fe 3+ ), which, having added one electron from the oxygen of water, turned into divalent ( Fe 2+ ). Thus, it was shown that the transition of hydrogen atoms from oxygen in water to carbon can occur in the form of independent movement of electrons and hydrogen ions. It has now been established that for energy storage it is the transition of electrons from one atom to another that is important, while hydrogen ions can pass into an aqueous solution and, if necessary, be removed from it again. The Hill reaction, in which light energy is used to cause the transfer of electrons from oxygen to an oxidizing agent (electron acceptor), was the first demonstration of the conversion of light energy to chemical energy and a model for the light stage of photosynthesis.

The hypothesis that oxygen is continuously supplied from water during photosynthesis was further confirmed in experiments using water labeled with a heavy isotope of oxygen ( 18 ABOUT). Since oxygen isotopes (usual 16 Oh and heavy 18 O) in their own way chemical properties are the same, plants use H 2 18 Oh just like N 2 16 A. It turned out that the isolated oxygen contains 18 A. In another experiment, plants carried out photosynthesis with H 2 16 O and C 18 O 2 . Moreover, the oxygen released at the beginning of the experiment did not contain 18 O.

In the 1950s, plant physiologist D. Arnon and other researchers proved that photosynthesis includes light and dark stages. Preparations capable of carrying out the entire light stage were obtained from plant cells. Using them, it was possible to establish that in the light, electrons are transferred from water to the photosynthetic oxidizer, which as a result becomes an electron donor for the reduction of carbon dioxide at the next stage of photosynthesis. The electron carrier is nicotinamide adenine dinucleotide phosphate. Its oxidized form is designated NADP + , and reduced (formed after the addition of two electrons and a hydrogen ion) NADP Ch N. B NADP + the nitrogen atom is pentavalent (four bonds and one positive charge), and in NADP H H trivalent (three bonds). NADP + belongs to the so-called coenzymes. Coenzymes, together with enzymes, carry out many chemical reactions in living systems, but unlike enzymes they change during the reaction. BMost of the converted light energy stored in the light stage of photosynthesis is stored during the transfer of electrons from water to NADP + .

The resulting NADP H H does not hold electrons as tightly as oxygen in water, and can give them away in the processes of synthesis of organic compounds, spending the accumulated energy on useful chemical work. A significant amount of energy is also stored in another way, namely in the form of ATP (adenosine triphosphate). It is formed by the removal of water from the inorganic phosphate ion ( HPO 4 2 ) and organic phosphate, adenosine diphosphate (ADP), according to the following equation:

ATP is an energy-rich compound, and its formation requires energy from some source. In the reverse reaction, i.e. When ATP is broken down into ADP and phosphate, energy is released. In many cases, ATP gives its energy to others chemical compounds in a reaction in which hydrogen is replaced by phosphate. In the reaction below, sugar ( ROH ) is phosphorylated, turning into sugar phosphate:

Sugar phosphate contains more energy than non-phosphorylated sugar, so it reactivity higher.

ATP and NADP H H formed (along with O 2 ) in the light stage of photosynthesis, are then used at the stage of synthesis of carbohydrates and other organic compounds from carbon dioxide.

Checking work on the topic “Photosynthesis”

Option 1.

Task 1.

Function

Photosynthesis

Cell center

Cell division

1) EPS 2) chloroplast 3) ribosome 4) nucleus

Task 2. In the table below, there is a relationship between the positions in the first and second columns.

Function

Glucose

DNA

Nucleotide

What concept should be entered in the blank in this table?

1) amino acid 2) chitin 3) cellulose 4) RNA

Task 3. Insert into the text “Light phase of photosynthesis” the missing terms from the proposed list, using numerical notations. Write down the resulting sequence of numbers.

LIGHT PHASE OF PHOTOSYNTHESIS

It has now been established that photosynthesis occurs in two phases: light and __________ (A). In the light phase, due to solar energy, excitation of molecules __________ (B) and synthesis of molecules __________ (C) occurs. Simultaneously with this reaction, water decomposes under the influence of light, releasing free __________ (G). This process is called photolysis.

LIST OF TERMS:

1) DNA 2) dark 3) oxygen 4) ATP 5) twilight 6) hemoglobin

7) chlorophyll 8) carbon dioxide

X the concentration of carbon dioxide is plotted (in %), and along the axis at –

Which of the proposed descriptions most accurately reflects this dependence of carbon dioxide concentrations in the range of 0.03– 0.16%? Rate of photosynthesis in this interval

grows smoothly throughout the entire graph

grows sharply throughout the graph

increases smoothly at the beginning, and then does not change

Task 5. Study the graph of the dependence of the relative rate of photosynthesis on light intensity (the x-axis shows the relative light intensity in candelas, and the y-axis– relative speed photosynthesis (in arbitrary units)).

Determine at what light intensity, from those listed, the relative rate of photosynthesis will be maximum.

Option 2.

Task 1. In the table below, there is a relationship between the positions in the first and second columns.

Function

Mitochondria

Breath

Photosynthesis

What concept should be entered in the blank in this table?

1) Golgi complex 2) chloroplast 3) ribosome 4) nucleus

Task 2. In the table below, there is a relationship between the positions in the first and second columns.

Function

stroma

glucose synthesis

grains

What concept should be entered in the blank in this table?

1) protein synthesis 2) water photolysis 3) lipid synthesis 4) glycolysis

Task 3. Insert into the text “Dark phase of photosynthesis” the missing terms from the proposed list, using numerical notations. Write down the resulting sequence of numbers.

DARK PHASE OF PHOTOSYNTHESIS

It has now been established that photosynthesis occurs in two phases: __________ (A) and dark. For dark phase reactions to occur, the presence of light __________ (B). At this time, the assimilation of __________ (B) from the air occurs, its reduction by hydrogen ions and the formation of organic matter __________ (D) due to the energy accumulated in the light phase.

LIST OF TERMS

1) light 2) carbon dioxide 3) oxygen 4) protein 5) twilight 6) optional

7) glucose 8) required

Task 4. Study the graph of the dependence of the relative rate of photosynthesis on the concentration of carbon dioxide (along the axisat the relative rate of photosynthesis is plotted (in arbitrary units), and along the axis X – concentration of carbon dioxide (in %)).

Determine the concentration of carbon dioxide at which the relative rate of photosynthesis will be 24 conventional units.

0,08 % 2) 0,05 % 3) 0,03 % 4) 0,01 %

What will be the relative rate of photosynthesis if the concentration of carbon dioxide in the greenhouse air is 0.03%?

3)

oxygen

4)

glucose

PHOTOSYNTHESIS

the formation by living plant cells of organic substances, such as sugars and starch, from inorganic ones - from CO2 and water - using the energy of light absorbed by plant pigments. It is the process of food production on which all living things - plants, animals and humans - depend. All terrestrial plants and most aquatic plants release oxygen during photosynthesis. Some organisms, however, are characterized by other types of photosynthesis that occur without the release of oxygen. The main reaction of photosynthesis, which occurs with the release of oxygen, can be written in the following form:

Organic substances include all carbon compounds with the exception of its oxides and nitrides. The largest quantities of organic substances produced during photosynthesis are carbohydrates (primarily sugars and starch), amino acids (from which proteins are built) and, finally, fatty acids (which, in combination with glycerophosphate, serve as material for the synthesis of fats). Of inorganic substances, the synthesis of all these compounds requires water (H2O) and carbon dioxide (CO2). Amino acids also require nitrogen and sulfur. Plants can absorb these elements in the form of their oxides, nitrate (NO3-) and sulfate (SO42-), or in other, more reduced forms, such as ammonia (NH3) or hydrogen sulfide (hydrogen sulfide H3S). The composition of organic compounds can also include phosphorus during photosynthesis (plants absorb it in the form of phosphate) and metal ions - iron and magnesium. Manganese and some other elements are also necessary for photosynthesis, but only in trace amounts. In terrestrial plants, all these inorganic compounds, with the exception of CO2, enter through the roots. Plants obtain CO2 from atmospheric air, in which its average concentration is 0.03%. CO2 enters the leaves and O2 is released from them through small openings in the epidermis called stomata. The opening and closing of stomata is regulated by special cells - they are called guard cells - which are also green and capable of carrying out photosynthesis. When light falls on the guard cells, photosynthesis begins in them. The accumulation of its products forces these cells to stretch. In this case, the stomatal opening opens wider, and CO2 penetrates to the underlying layers of the leaf, the cells of which can now continue photosynthesis. Stomata also regulate the evaporation of water by leaves, the so-called. transpiration, since most of the water vapor passes through these openings. Aquatic plants obtain everything they need nutrients from the water in which they live. CO2 and bicarbonate ion (HCO3-) are also found in both sea and fresh water. Algae and other aquatic plants obtain them directly from water. Light in photosynthesis plays the role of not only a catalyst, but also one of the reactants. A significant part of the light energy used by plants during photosynthesis is stored in the form of chemical potential energy in the products of photosynthesis. For photosynthesis, which occurs with the release of oxygen, any visible light from violet (wavelength 400 nm) to medium red (700 nm). Some types of bacterial photosynthesis that are not accompanied by the release of O2 can effectively use light with a longer wavelength, up to the far red (900 nm). Clarification of the nature of photosynthesis began at the time of the birth of modern chemistry. The works of J. Priestley (1772), J. Ingenhaus (1780), J. Senebier (1782), as well as the chemical studies of A. Lavoisier (1775, 1781) led to the conclusion that plants convert carbon dioxide into oxygen and for this process it is necessary light. The role of water remained unknown until it was pointed out in 1808 by N. Saussure. In his very precise experiments, he measured the increase in dry weight of a plant growing in a pot of soil, and also determined the amount of carbon dioxide absorbed and oxygen released. Saussure confirmed that all the carbon incorporated into organic matter by a plant comes from carbon dioxide. At the same time, he discovered that the increase in plant dry matter was greater than the difference between the weight of carbon dioxide absorbed and the weight of oxygen released. Since the weight of the soil in the pot did not change significantly, the only possible source of weight gain was water. Thus, it was shown that one of the reactants in photosynthesis is water. The importance of photosynthesis as one of the energy conversion processes could not be appreciated until the very idea of ​​chemical energy arose. In 1845, R. Mayer came to the conclusion that during photosynthesis, light energy is converted into chemical potential energy stored in its products.

PHOTOSYNTHESIS is a process on which all life on Earth depends. It occurs only in plants. During photosynthesis, a plant produces organic substances necessary for all living things from inorganic substances. Carbon dioxide contained in the air enters the leaf through special openings in the epidermis of the leaf, which are called stomata; water and minerals come from the soil to the roots and from there are transported to the leaves through the plant's conducting system. The energy necessary for the synthesis of organic substances from inorganic ones is supplied by the Sun; this energy is absorbed by plant pigments, mainly chlorophyll. In the cell, the synthesis of organic substances occurs in chloroplasts, which contain chlorophyll. Free oxygen, also produced during photosynthesis, is released into the atmosphere.

PHOTOSYNTHESIS SCHEME

The role of photosynthesis. The total result of the chemical reactions of photosynthesis can be described for each of its products by a separate chemical equation. For the simple sugar glucose, the equation is:

The equation shows that in a green plant, due to light energy, one molecule of glucose and six molecules of oxygen are formed from six molecules of water and six molecules of carbon dioxide. Glucose is just one of many carbohydrates synthesized in plants. Below is the general equation for the formation of a carbohydrate with n carbon atoms per molecule:

The equations describing the formation of other organic compounds are not so simple. Amino acid synthesis requires additional inorganic compounds, such as the formation of cysteine:

The role of light as a reactant in the process of photosynthesis is easier to demonstrate if we turn to another chemical reaction, namely combustion. Glucose is one of the subunits of cellulose, the main component of wood. The combustion of glucose is described by the following equation:

This equation is a reversal of the equation for glucose photosynthesis, except that instead of light energy, it produces mostly heat. According to the law of conservation of energy, if energy is released during combustion, then during the reverse reaction, i.e. During photosynthesis, it must be absorbed. The biological analogue of combustion is respiration, so respiration is described by the same equation as non-biological combustion. For all living cells, with the exception of green plant cells in the light, biochemical reactions serve as a source of energy. Respiration is the main biochemical process that releases energy stored during photosynthesis, although long food chains may lie between these two processes. A constant supply of energy is necessary for any manifestation of life, and light energy, which photosynthesis converts into chemical potential energy of organic substances and uses to release free oxygen, is the only important primary source of energy for all living things. Living cells then oxidize ("burn") these organic substances with oxygen, and some of the energy released when oxygen combines with carbon, hydrogen, nitrogen and sulfur is stored for use in various life processes, such as movement or growth. Combining with the listed elements, oxygen forms their oxides - carbon dioxide, water, nitrate and sulfate. Thus the cycle ends. Why is free oxygen, the only source of which on Earth is photosynthesis, so necessary for all living things? The reason is its high reactivity. The electron cloud of a neutral oxygen atom has two fewer electrons than required for the most stable electron configuration. Therefore, oxygen atoms have a strong tendency to acquire two additional electrons, which is achieved by combining (forming two bonds) with other atoms. An oxygen atom can form two bonds with two different atoms or form a double bond with one atom. In each of these bonds, one electron is supplied by an oxygen atom, and the second electron is supplied by another atom participating in the formation of the bond. In a water molecule (H2O), for example, each of the two hydrogen atoms supplies its only electron to form a bond with oxygen, thereby satisfying the inherent desire of oxygen to acquire two additional electrons. In a CO2 molecule, each of the two oxygen atoms forms a double bond with the same carbon atom, which has four bonding electrons. Thus, in both H2O and CO2, the oxygen atom has as many electrons as necessary for a stable configuration. If, however, two oxygen atoms bond to each other, then the electron orbitals of these atoms allow only one bond to form. The need for electrons is thus only half satisfied. Therefore, the O2 molecule, compared to the CO2 and H2O molecules, is less stable and more reactive. Organic products of photosynthesis, such as carbohydrates, (CH2O)n, are quite stable, since each of the carbon, hydrogen and oxygen atoms in them receives as many electrons as necessary to form the most stable configuration. The process of photosynthesis, as a result of which carbohydrates are formed, therefore converts two very stable substances, CO2 and H2O, into one completely stable one, (CH2O)n, and one less stable one, O2. The accumulation of huge amounts of O2 in the atmosphere as a result of photosynthesis and its high reactivity determine its role as a universal oxidizing agent. When an element gives up electrons or hydrogen atoms, we say that the element is oxidized. The addition of electrons or the formation of bonds with hydrogen, as with carbon atoms in photosynthesis, is called reduction. Using these concepts, photosynthesis can be defined as the oxidation of water coupled with the reduction of carbon dioxide or other inorganic oxides.

The mechanism of photosynthesis. Light and dark stages. It has now been established that photosynthesis occurs in two stages: light and dark. The light stage is the process of using light to split water; At the same time, oxygen is released and energy-rich compounds are formed. The dark stage includes a group of reactions that use the high-energy products of the light stage to reduce CO2 to simple sugar, i.e. for carbon assimilation. Therefore, the dark stage is also called the synthesis stage. The term “dark stage” only means that light is not directly involved in it. Modern ideas about the mechanism of photosynthesis were formed on the basis of research conducted in the 1930-1950s. Previously, for many years, scientists were misled by a seemingly simple, but incorrect hypothesis, according to which O2 is formed from CO2, and the released carbon reacts with H2O, resulting in the formation of carbohydrates. In the 1930s, when it turned out that some sulfur bacteria do not produce oxygen during photosynthesis, biochemist K. van Niel suggested that the oxygen released during photosynthesis in green plants comes from water. In sulfur bacteria the reaction proceeds as follows:

Instead of O2, these organisms produce sulfur. Van Niel came to the conclusion that all types of photosynthesis can be described by the equation

where X is oxygen in photosynthesis, which occurs with the release of O2, and sulfur in the photosynthesis of sulfur bacteria. Van Niel also suggested that this process involves two stages: a light stage and a synthesis stage. This hypothesis was supported by the discovery of physiologist R. Hill. He discovered that destroyed or partially inactivated cells are capable of carrying out a reaction in the light in which oxygen is released, but CO2 is not reduced (it was called the Hill reaction). In order for this reaction to proceed, it was necessary to add some oxidizing agent capable of attaching electrons or hydrogen atoms given up by the oxygen of the water. One of Hill's reagents is quinone, which, by adding two hydrogen atoms, becomes dihydroquinone. Other Hill reagents contained ferric iron (Fe3+ ion), which, by adding one electron from the oxygen of water, was converted into divalent iron (Fe2+). Thus, it was shown that the transition of hydrogen atoms from oxygen in water to carbon can occur in the form of independent movement of electrons and hydrogen ions. It has now been established that for energy storage it is the transition of electrons from one atom to another that is important, while hydrogen ions can pass into an aqueous solution and, if necessary, be removed from it again. The Hill reaction, in which light energy is used to cause the transfer of electrons from oxygen to an oxidizing agent (electron acceptor), was the first demonstration of the conversion of light energy to chemical energy and a model for the light stage of photosynthesis. The hypothesis that oxygen is continuously supplied from water during photosynthesis was further confirmed in experiments using water labeled with a heavy isotope of oxygen (18O). Since the isotopes of oxygen (common 16O and heavy 18O) have the same chemical properties, plants use H218O in the same way as H216O. It turned out that the released oxygen contained 18O. In another experiment, plants carried out photosynthesis with H216O and C18O2. In this case, the oxygen released at the beginning of the experiment did not contain 18O. In the 1950s, plant physiologist D. Arnon and other researchers proved that photosynthesis includes light and dark stages. Preparations capable of carrying out the entire light stage were obtained from plant cells. Using them, it was possible to establish that in the light, electrons are transferred from water to the photosynthetic oxidizer, which as a result becomes an electron donor for the reduction of carbon dioxide at the next stage of photosynthesis. The electron carrier is nicotinamide adenine dinucleotide phosphate. Its oxidized form is designated NADP+, and its reduced form (formed after the addition of two electrons and a hydrogen ion) is designated NADPH. In NADP+ the nitrogen atom is pentavalent (four bonds and one positive charge), and in NADPHN it is trivalent (three bonds). NADP+ belongs to the so-called. coenzymes. Coenzymes, together with enzymes, carry out many chemical reactions in living systems, but unlike enzymes they change during the reaction. Most of the converted light energy stored in the light stage of photosynthesis is stored during the transfer of electrons from water to NADP+. The resulting NADPH does not hold electrons as tightly as water oxygen, and can give them away in the processes of synthesis of organic compounds, spending the accumulated energy on useful chemical work. A significant amount of energy is also stored in another way, namely in the form of ATP (adenosine triphosphate). It is formed by removing water from the inorganic phosphate ion (HPO42-) and the organic phosphate, adenosine diphosphate (ADP), according to the following equation:

ATP is an energy-rich compound, and its formation requires energy from some source. In the reverse reaction, i.e. When ATP is broken down into ADP and phosphate, energy is released. In many cases, ATP gives up its energy to other chemical compounds in a reaction in which hydrogen is replaced by phosphate. In the reaction below, sugar (ROH) is phosphorylated to form sugar phosphate:

Sugar phosphate contains more energy than non-phosphorylated sugar, so its reactivity is higher. ATP and NADPHN, formed (along with O2) in the light stage of photosynthesis, are then used at the stage of synthesis of carbohydrates and other organic compounds from carbon dioxide.

The structure of the photosynthetic apparatus. Light energy is absorbed by pigments (the so-called substances that absorb visible light). All plants that carry out photosynthesis have various forms of the green pigment chlorophyll, and all probably contain carotenoids, which are usually yellow in color. Higher plants contain chlorophyll a (C55H72O5N4Mg) and chlorophyll b (C55H70O6N4Mg), as well as four main carotenoids: b-carotene (C40H56), lutein (C40H55O2), violaxanthin and neoxanthin. This variety of pigments provides a wide spectrum of absorption of visible light, since each of them is “tuned” to its own region of the spectrum. Some algae have approximately the same set of pigments, but many of them have pigments that are somewhat different from those listed in their chemical nature. All these pigments, like the entire photosynthetic apparatus of the green cell, are enclosed in special organelles surrounded by a membrane, the so-called. chloroplasts. The green color of plant cells depends only on the chloroplasts; the remaining elements of the cells do not contain green pigments. The size and shape of chloroplasts vary quite widely. A typical chloroplast is shaped like a slightly curved cucumber measuring approx. 1 µm in diameter and length approx. 4 microns. Large cells of green plants, such as the leaf cells of most terrestrial species, contain many chloroplasts, but small unicellular algae, such as Chlorella pyrenoidosa, have only one chloroplast, occupying most of the cell.

Meet a very complex structure chloroplasts can be seen using an electron microscope. It makes it possible to identify much smaller structures than those visible in a conventional light microscope. In a light microscope, particles smaller than 0.5 microns cannot be distinguished. By 1961, the resolution of electron microscopes made it possible to observe particles that were a thousand times smaller (about 0.5 nm). Using an electron microscope, very thin membrane structures, the so-called, were identified in chloroplasts. thylakoids. These are flat sacs, closed at the edges and collected in stacks called grana; In the photographs, the grains look like stacks of very thin pancakes. Inside the sacs there is a space - the thylakoid cavity, and the thylakoids themselves, collected in grana, are immersed in a gel-like mass of soluble proteins that fills the internal space of the chloroplast and is called the stroma. The stroma also contains smaller and thinner thylakoids that connect individual grana to each other. All thylakoid membranes are composed of approximately equal amounts of proteins and lipids. Regardless of whether they are collected in grana or not, it is in them that the pigments are concentrated and the light stage occurs. The dark stage occurs, as is commonly believed, in the stroma.

Photosystems. Chlorophyll and carotenoids, embedded in the thylakoid membranes of chloroplasts, are assembled into functional units - photosystems, each of which contains approximately 250 pigment molecules. The structure of the photosystem is such that of all these molecules capable of absorbing light, only one specially located chlorophyll a molecule can use its energy in photochemical reactions - it is the reaction center of the photosystem. The remaining pigment molecules, absorbing light, transfer its energy to the reaction center; these light-harvesting molecules are called antenna molecules. There are two types of photosystems. In photosystem I, the specific chlorophyll a molecule, which makes up the reaction center, has an absorption optimum at a light wavelength of 700 nm (designated P700; P - pigment), and in photosystem II - at 680 nm (P680). Typically, both photosystems operate synchronously and (in light) continuously, although photosystem I can operate separately.

Transformations of light energy. Consideration of this issue should begin with photosystem II, where light energy is utilized by the reaction center P680. When light enters this photosystem, its energy excites the P680 molecule, and a pair of excited, energized electrons belonging to this molecule are detached and transferred to an acceptor molecule (probably quinone), designated by the letter Q. The situation can be imagined in such a way that the electrons as would jump from the received light “push” and the acceptor catches them in some upper position. If it were not for the acceptor, the electrons would return to their original position (to the reaction center), and the energy released during the downward movement would turn into light, i.e. would be spent on fluorescence. From this point of view, the electron acceptor can be considered as a fluorescence quencher (hence its designation Q, from the English quench - to quench).

The P680 molecule, having lost two electrons, has oxidized, and in order for the process not to stop there, it must be restored, i.e. gain two electrons from some source. Water serves as such a source: it splits into 2H+ and 1/2O2, donating two electrons to oxidized P680. This light-dependent splitting of water is called photolysis. Enzymes that carry out photolysis are located on the inner side of the thylakoid membrane, as a result of which all hydrogen ions accumulate in the thylakoid cavity. The most important cofactor for photolysis enzymes are manganese atoms. The transition of two electrons from the reaction center of the photosystem to the acceptor is an “uphill” climb, i.e. to a higher energy level, and this rise is provided by light energy. Next, in photosystem II, a pair of electrons begins a gradual “descent” from acceptor Q to photosystem I. The descent occurs along an electron transport chain, very similar in organization to the similar chain in mitochondria (see also METABOLISM). It consists of cytochromes, proteins containing iron and sulfur, copper-containing protein and other components. The gradual descent of electrons from a more energized state to a less energized state is associated with the synthesis of ATP from ADP and inorganic phosphate. As a result, light energy is not lost, but is stored in the phosphate bonds of ATP, which can be used in metabolic processes. The production of ATP during photosynthesis is called photophosphorylation. Simultaneously with the described the process is underway absorption of light in photosystem I. Here, its energy is also used to remove two electrons from the reaction center (P700) and transfer them to an acceptor - an iron-containing protein. From this acceptor, through an intermediate carrier (also a protein containing iron), both electrons go to NADP+, which as a result becomes capable of attaching hydrogen ions (formed during the photolysis of water and preserved in the thylakoids) - and turns into NADPH. As for the reaction center P700, which was oxidized at the beginning of the process, it accepts two (“descended”) electrons from photosystem II, which returns it to its original state. The total reaction of the light stage occurring during photoactivation of photosystems I and II can be represented as follows:

The total energy output of the electron flow in this case is 1 ATP molecule and 1 NADPH molecule per 2 electrons. By comparing the energy of these compounds with the energy of light that provides their synthesis, it was calculated that approximately 1/3 of the energy of absorbed light is stored in the process of photosynthesis. In some photosynthetic bacteria, photosystem I operates independently. In this case, the flow of electrons moves cyclically from the reaction center to the acceptor and - along a roundabout path - back to the reaction center. In this case, photolysis of water and the release of oxygen do not occur, NADPH is not formed, but ATP is synthesized. Such a mechanism of light reaction may also occur in higher plants under conditions when an excess of NADPH occurs in cells.

Dark reactions (synthesis stage). The synthesis of organic compounds by reduction of CO2 (as well as nitrate and sulfate) also occurs in chloroplasts. ATP and NADPH, supplied by the light reaction occurring in thylakoid membranes, serve as a source of energy and electrons for synthesis reactions. The reduction of CO2 is the result of the transfer of electrons to CO2. During this transfer, some of the C-O bonds are replaced by S-N connections, S-S and O-N. The process consists of a number of stages, some of which (15 or more) form a cycle. This cycle was discovered in 1953 by the chemist M. Calvin and his colleagues. Using a radioactive isotope of carbon instead of the usual (stable) isotope in their experiments, these researchers were able to trace the path of carbon in the reactions they were studying. In 1961, Calvin was awarded the Nobel Prize in Chemistry for this work. The Calvin cycle involves compounds with the number of carbon atoms in molecules from three to seven. All components of the cycle, with the exception of one, are sugar phosphates, i.e. sugars in which one or two OH groups are replaced by a phosphate group (-OPO3H-). An exception is 3-phosphoglyceric acid (PGA; 3-phosphoglycerate), which is a sugar acid phosphate. It is similar to phosphorylated three-carbon sugar (glycerophosphate), but differs from it in that it has a carboxyl group O=C-O-, i.e. one of its carbon atoms is connected to oxygen atoms by three bonds. It is convenient to begin the description of the cycle with ribulose monophosphate, which contains five carbon atoms (C5). ATP formed in the light stage reacts with ribulose monophosphate, converting it into ribulose diphosphate. The second phosphate group gives ribulose diphosphate additional energy, since it carries part of the energy stored in the ATP molecule. Therefore, the tendency to react with other compounds and form new bonds is more pronounced in ribulose diphosphate. It is this C5 sugar that adds CO2 to form a six-carbon compound. The latter is very unstable and under the influence of water breaks down into two fragments - two PGA molecules. If we keep in mind only the change in the number of carbon atoms in sugar molecules, then this main stage of the cycle in which the fixation (assimilation) of CO2 occurs can be represented as follows:

The enzyme that catalyzes CO2 fixation (specific carboxylase) is present in chloroplasts in very large quantities(over 16% of their total protein content); Given the enormous mass of green plants, it is probably the most abundant protein in the biosphere. The next step is that the two molecules of PGA formed in the carboxylation reaction are each reduced by one molecule of NADPH to a three-carbon sugar phosphate (triose phosphate). This reduction occurs as a result of the transfer of two electrons to the carbon of the carboxyl group of FHA. However, in this case, ATP is also needed to provide the molecule with additional chemical energy and increase its reactivity. This task is performed by an enzyme system that transfers the terminal phosphate group of ATP to one of the oxygen atoms of the carboxyl group (a group is formed), i.e. PGA is converted to diphosphoglyceric acid. Once NADPHN donates one hydrogen atom plus an electron to the carbon of the carboxyl group of this compound (equivalent to two electrons plus a hydrogen ion, H+), the C-O single bond is broken and the oxygen bound to the phosphorus is transferred to the inorganic phosphate, HPO42-, and the carboxyl group O =C-O- turns into aldehyde O=C-H. The latter is characteristic of a certain class of sugars. As a result, PGA, with the participation of ATP and NADPH, is reduced to sugar phosphate (triose phosphate). The entire process described above can be represented by the following equations: 1) Ribulose monophosphate + ATP -> Ribulose diphosphate + ADP 2) Ribulose diphosphate + CO2 -> Unstable C6 compound 3) Unstable C6 compound + H2O -> 2 PGA 4) PGA + ATP + NADPH -> ADP + H3PO42- + Triose phosphate (C3). The end result of reactions 1-4 is the formation of two molecules of triose phosphate (C3) from ribulose monophosphate and CO2 with the consumption of two molecules of NADPH and three molecules of ATP. It is in this series of reactions that the entire contribution of the light stage - in the form of ATP and NADPH - to the carbon reduction cycle is represented. Of course, the light stage must additionally supply these cofactors for the reduction of nitrate and sulfate and for the conversion of PGA and triose phosphate formed in the cycle into other organic substances - carbohydrates, proteins and fats. The significance of subsequent stages of the cycle is that they lead to the regeneration of the five-carbon compound, ribulose monophosphate, necessary to restart the cycle. This part of the loop can be written as follows:

which gives a total of 5C3 -> 3C5. Three molecules of ribulose monophosphate, formed from five molecules of triose phosphate, are converted - after the addition of CO2 (carboxylation) and reduction - into six molecules of triose phosphate. Thus, as a result of one revolution of the cycle, one molecule of carbon dioxide is included in the three-carbon organic compound; three revolutions of the cycle in total give a new molecule of the latter, and for the synthesis of a molecule of six-carbon sugar (glucose or fructose), two three-carbon molecules and, accordingly, 6 revolutions of the cycle are required. The cycle gives the increase in organic matter to reactions in which various sugars, fatty acids and amino acids are formed, i.e. "building blocks" of starch, fats and proteins. The fact that the direct products of photosynthesis are not only carbohydrates, but also amino acids, and possibly fatty acids, was also established using an isotope label - a radioactive isotope of carbon. A chloroplast is not just a particle adapted for the synthesis of starch and sugars. This is a very complex, well-organized “factory”, capable of not only producing all the materials from which it itself is built, but also supplying with reduced carbon compounds those parts of the cell and those organs of the plant that do not carry out photosynthesis themselves.

In chloroplasts. Significance photosynthesis to invigorate the biosphere... chlorophyll is called photosynthesis. Process photosynthesis appears so gloomy... vikorist in the dark phase photosynthesis. Dark phase photosynthesis or the Calvin cycle...

Fill in the missing names of scientists in the text
The involvement of microbes in infectious diseases has been proven____ Phagocytosis as a means of combating microbes has been discovered____ The first smallpox vaccine was proposed _____ The invention of a method for obtaining vaccines and therapeutic sera against various infectious diseases belongs to _____

text numbers DIGESTION IN FLAT WORMS

selected answers, and then enter the resulting sequence of numbers (according to the text) in the table below.

ORGAN SYSTEMS

An organ is a ___________ (A) that has a certain shape, structure, location and performs one or more functions. Every organ must have blood vessels and ___________ (B). Bodies jointly implementing general functions, make up organ systems. The human body has an excretory system, the main organ of which is ___________ (B). Through excretory system in external environment harmful ___________ are removed (D).

LIST OF TERMS: 1) tissue 2) body part 3) nerves 4) intestines 5) stomach 6) kidneys 7) metabolic product 8) undigested food debris

selected answers, and then enter the resulting sequence of numbers (according to the text) in the table below.

WATER EVAPORATION BY LEAF

The absorbed ___________ (A) soil solution, consisting of water and minerals, enters the leaf through special cells - ___________ (B) -. Here, part of the water is used in the process of photosynthesis, and part, having turned into a gaseous state, evaporates through ___________ (B). This process is called ___________ (D). Mineral salts remain in the leaves, accumulate and cause the annual death of leaves - defoliation.

LIST OF TERMS:

2) sieve tube

4) stem

5) transpiration

6) stomata

7) photosynthesis

8) lentils

list using digital designations. Write to text
digits of the selected answers, and then the resulting sequence of digits
(according to the text) write in the table below.
PROTEIN METABOLISM
Enzymatic breakdown of proteins supplied with food occurs
in the stomach and small intestine. Formed ___________ (A) actively
are absorbed into the intestinal villi, enter the ___________ (B) and are carried
to all cells of the body. In cells with incoming substances
two processes occur: ___________ (B) new proteins on ribosomes and
final oxidation to ammonia, which turns into
___________ (D) and in this state is excreted from the body.
LIST OF TERMS:
1)
blood
2)
glycerol
3)
amino acid
4)
lymph
5)
synthesis
6)
urea
7)
decay
8)
glucose