Photosynthesis Is a set of synthesis processes organic compounds from inorganic due to the conversion of light energy into the energy of chemical bonds. Phototrophic organisms include green plants, some prokaryotes - cyanobacteria, purple and green sulfur bacteria, plant flagellates.

Research into the process of photosynthesis began in the second half of the 18th century. An important discovery was made by the outstanding Russian scientist K.A.Timiryazev, who substantiated the doctrine of the cosmic role of green plants. Plants absorb sunlight and convert light energy into the energy of chemical bonds of organic compounds synthesized by them. Thus, they ensure the preservation and development of life on Earth. The scientist also theoretically substantiated and experimentally proved the role of chlorophyll in the absorption of light during photosynthesis.

Chlorophylls are the main photosynthetic pigment. In structure, they are similar to hemoglobin heme, but instead of iron they contain magnesium. The iron content is necessary to ensure the synthesis of chlorophyll molecules. There are several chlorophylls that differ in their chemical structure. Mandatory for all phototrophs is chlorophyll a . Chlorophyllb found in green plants, chlorophyll c - in diatoms and brown algae. Chlorophyll d characteristic of red algae.

Green and purple photosynthetic bacteria have special bacteriochlorophylls ... Bacterial photosynthesis has much in common with plant photosynthesis. It differs in that hydrogen sulphide is the donor of hydrogen in bacteria, and water in plants. Green and purple bacteria lack photosystem II. Bacterial photosynthesis is not accompanied by the release of oxygen. The overall equation of bacterial photosynthesis:

6С0 2 + 12H 2 S → C 6 H 12 O 6 + 12S + 6Н 2 0.

Photosynthesis is based on the redox process. It is associated with the transfer of electrons from compounds that provide donor electrons to compounds that accept them - acceptors. Light energy is converted into the energy of synthesized organic compounds (carbohydrates).

Chloroplast membranes have special structures - reaction centers that contain chlorophyll. In green plants and cyanobacteria, two are distinguished photo systems – first (I) and second (II) , which have different reaction centers and are interconnected through the electron transport system.

Two phases of photosynthesis

The process of photosynthesis consists of two phases: light and dark.

It occurs only when there is light on the inner membranes of mitochondria in the membranes of special structures - thylakoids ... Photosynthetic pigments capture light quanta (photons). This leads to the "excitation" of one of the electrons of the chlorophyll molecule. With the help of carrier molecules, the electron moves to the outer surface of the thylakoid membrane, acquiring a certain potential energy.

This electron in photosystem I can return to its energy level and restore it. NADP (nicotinamide adenine dinucleotide phosphate) can also be transferred. Interacting with hydrogen ions, electrons reduce this compound. Reduced NADP (NADPH) supplies hydrogen to reduce atmospheric CO2 to glucose.

Similar processes take place in photosystem II ... Excited electrons can be transferred to photosystem I and restored. The restoration of photosystem II occurs at the expense of electrons supplied by water molecules. Water molecules break down (photolysis of water) into hydrogen protons and molecular oxygen, which is released into the atmosphere. Electrons are used to restore photosystem II. Water photolysis equation:

2H 2 0 → 4H + + 0 2 + 2e.

When electrons return from outer surface the thylakoid membranes release energy to the previous energy level. It is stored in the form of chemical bonds of ATP molecules, which are synthesized during reactions in both photosystems. The synthesis of ATP with ADP and phosphoric acid is called photophosphorylation ... Some of the energy is used to evaporate water.

During the light phase of photosynthesis, energy-rich compounds are formed: ATP and NADPH. During the decay (photolysis) of a water molecule, molecular oxygen is released into the atmosphere.

The reactions take place in the internal environment of chloroplasts. They can occur with or without light. Organic substances are synthesized (CO2 is reduced to glucose) using the energy that is formed in the light phase.

Recovery process carbon dioxide is cyclic and is called Calvin cycle ... Named after the American researcher M. Calvin, who discovered this cyclical process.

The cycle begins with the reaction of atmospheric carbon dioxide with ribulezobiphosphate. Enzyme catalyzes the process carboxylase ... Ribule biphosphate is a five-carbon sugar combined with two phosphoric acid residues. A number of chemical transformations take place, each of which catalyzes its own specific enzyme. How the end product of photosynthesis is formed glucose , and also ribulezobiphosphate is restored.

The overall equation of the photosynthesis process:

6C0 2 + 6H 2 0 → C 6 H 12 O 6 + 60 2

Thanks to the process of photosynthesis, the light energy of the Sun is absorbed and is converted into the energy of chemical bonds of synthesized carbohydrates. Energy is transferred to heterotrophic organisms through food chains. In the process of photosynthesis, carbon dioxide is absorbed and oxygen is released. All atmospheric oxygen is of photosynthetic origin. Over 200 billion tons of free oxygen are released annually. Oxygen protects life on Earth from ultraviolet radiation by creating an ozone shield in the atmosphere.

The process of photosynthesis is ineffective, since only 1-2% of solar energy is transferred to the synthesized organic matter. This is due to the fact that plants do not absorb light enough, part of it is absorbed by the atmosphere, etc. Most sunlight reflected from the surface of the Earth back into space.

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

Organic substances include all carbon compounds with the exception of its oxides and nitrides. The largest quantities are formed during photosynthesis such organic substances as carbohydrates (primarily sugars and starch), amino acids (from which proteins are built) and, finally, fatty acids (which, in combination with glycerophosphate, serve as a material for the synthesis of fats). Of inorganic substances, the synthesis of all these compounds requires water (H2O) and carbon dioxide (CO2). The 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 H2S). During photosynthesis, phosphorus can also be included in the composition of organic compounds (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. CO2 of plants is obtained from atmospheric air, in which its average concentration is 0.03%. CO2 enters the leaves, and O2 is released from them through small holes in the epidermis called stomata. The opening and closing of the stomata is regulated by special cells - they are called guard cells - which are also green and capable of 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. The stomata also regulate the evaporation of water from leaves, the so-called. transpiration, since most of the water vapor passes through these holes. Aquatic plants get everything they need nutrients from the water in which they live. CO2 and bicarbonate ion (HCO3-) are also found in sea and fresh water. Algae and others aquatic plants get them directly from water. In photosynthesis, light plays the role of not only a catalyst, but also one of the reagents. A significant part of the light energy used by plants in photosynthesis is stored as chemical potential energy in the products of photosynthesis. Any visible light from violet (wavelength 400 nm) to medium red (700 nm) is more or less suitable for photosynthesis, proceeding with the release of oxygen. For some bacterial photosynthesis that is not accompanied by O2 release, light with longer wavelengths, up to far red (900 nm), can be effectively used. Elucidation 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) made it possible to conclude that plants convert carbon dioxide into oxygen and this process requires 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 dry weight gain 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 by the plant into organic matter comes from carbon dioxide. At the same time, he found that the dry matter gain of the plant was greater than the difference between the weight of the absorbed carbon dioxide and the weight of the released oxygen. Since the weight of the soil in the pot did not change significantly, the only possible source of weight gain was water. So it was shown that one of the reagents in photosynthesis is water. The importance of photosynthesis as one of the processes of energy conversion 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.

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

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

The equations describing the formation of other organic compounds are not so simple. For the synthesis of amino acids, additional inorganic compounds are required, such as in the formation of cysteine:

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

This equation is a reversal of the equation for photosynthesis of glucose, except for the fact that instead of light energy, mainly heat is released. According to the law of conservation of energy, if energy is released during combustion, then during a reverse reaction, i.e. in photosynthesis, it must be absorbed. The biological analogue of combustion is respiration, therefore respiration is described by the same equation as non-biological combustion. For all living cells, with the exception of green plant cells exposed to light, biochemical reactions serve as a source of energy. Breathing is the main biochemical process that releases energy stored during photosynthesis, although long food chains may lie between the two. A constant flow of energy is necessary for any manifestation of vital activity, 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 matter with oxygen, and some of the energy released when oxygen combines with carbon, hydrogen, nitrogen, and sulfur is stored for use in various vital processes, such as movement or growth. Combining with the listed elements, oxygen forms their oxides - carbon dioxide, water, nitrate and sulfate. This completes the cycle. Why is free oxygen, the only source of which on Earth is photosynthesis, so necessary for all living things? The reason lies in its high reactivity. In the electron cloud, the neutral oxygen atom is two less electrons than is required for the most stable electronic 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 of some atoms. In each of these bonds, one electron is supplied by the 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 a single electron to bond with oxygen, thereby satisfying the inherent tendency 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 is necessary for a stable configuration. If, however, two oxygen atoms combine with each other, then the electronic orbitals of these atoms allow the formation of only one bond. The need for electrons is thus only half satisfied. Therefore, the O2 molecule is less stable and more reactive than the CO2 and H2O molecules. Organic products of photosynthesis, such as carbohydrates, (СН2О) n, are quite stable, since each of the carbon, hydrogen and oxygen atoms in them receives as many electrons as is necessary to form the most stable configuration. The process of photosynthesis, as a result of which carbohydrates are formed, converts, therefore, two very stable substances, CO2 and H2O, into one completely stable, (CH2O) n, and one less stable, 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 oxidant. When an element gives up electrons or hydrogen atoms, we say that this element is oxidized. The attachment 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; oxygen is released and energy-rich compounds are formed. The dark stage includes a group of reactions that use high-energy products from the light stage to reduce CO2 to a 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 carried out in the 1930-1950s. Prior to that, 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 became clear that some sulfur bacteria do not release oxygen during photosynthesis, biochemist K. van Niel suggested that oxygen released during photosynthesis in green plants comes from water. In sulfur bacteria, the reaction proceeds as follows:

Instead of O2, these organisms form sulfur. Van Niel concluded that all types of photosynthesis can be described by the equation

where X is oxygen in photosynthesis, proceeding with the release of O2, and sulfur in photosynthesis of sulfur bacteria. Van Niel also suggested that this process includes two stages: light and synthesis. This hypothesis was supported by the discovery of physiologist R. Hill. He found that destroyed or partially inactivated cells are capable of carrying out a reaction in the light of which oxygen is released, but CO2 is not reduced (it was called the Hill reaction). In order for this reaction to proceed, it was required to add some kind of oxidant capable of attaching electrons or hydrogen atoms given off by the oxygen of the water. One of Hill's reagents is quinone, which, by attaching two hydrogen atoms, turns into dihydroquinone. Hill's other reagents contained ferric iron (Fe3 + ion), which, by attaching one electron from the oxygen of water, was converted into ferrous (Fe2 +). So it was shown that the transition of hydrogen atoms from water oxygen to carbon can occur in the form of an independent motion of electrons and hydrogen ions. It has now been established that it is precisely the transition of electrons from one atom to another that is important for storing energy, while hydrogen ions can pass into an aqueous solution and, if necessary, be extracted from it again. Hill's reaction, in which light energy is used to induce the transfer of electrons from oxygen to an oxidizer (electron acceptor), was the first demonstration of the transition of light energy into the chemical and model of the light stage of photosynthesis. The hypothesis, according to which oxygen during photosynthesis is continuously supplied from water, was further confirmed in experiments using water labeled with a heavy oxygen isotope (18O). Since oxygen isotopes (ordinary 16O and heavy 18O) by their chemical properties are the same, plants use H218O in the same way as H216O. It turned out that 18O is present in the released oxygen. In another experiment, the plants carried out photosynthesis with H216O and C18O2. At the same time, 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. From plant cells, preparations were obtained that were able to carry out the entire light stage. Using them, it was possible to establish that, in the light, electrons are transferred from water to a 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 the reduced form (formed after the addition of two electrons and a hydrogen ion) is designated NADPHN. 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 course of 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 NADPHN retains electrons not as firmly as water oxygen, and can give them away in the synthesis of organic compounds, spending the accumulated energy for useful chemical work. A significant amount of energy is stored in another way, namely in the form of ATP (adenosine triphosphate). It is formed by removing water from an inorganic phosphate ion (HPO42-) and an organic phosphate, adenosine diphosphate (ADP), according to the following equation:

ATP is an energy-rich compound and requires energy from some source to form. In a 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 into 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 device of the photosynthetic apparatus. Light energy is absorbed by pigments (the so-called substances that absorb visible light). All photosynthetic plants have various forms of the green pigment of chlorophyll, and probably all contain carotenoids, which are usually colored yellow. 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 a different region of the spectrum. Some algae have approximately the same set of pigments, but many of them have pigments that are slightly different from those listed in their chemical nature. All these pigments, like the entire photosynthetic apparatus of a green cell, are enclosed in special organelles surrounded by a membrane, the so-called. chloroplasts. The green coloration of plant cells depends only on chloroplasts; other elements of cells do not contain green pigments. The size and shape of chloroplasts is quite variable. A typical chloroplast resembles a slightly curved cucumber, approx. 1 μm across and approx. 4 microns. Large cells of green plants, such as leaf cells in most terrestrial species, contain many chloroplasts, while small unicellular algae, such as Chlorella pyrenoidosa, have only one chloroplast, which occupies most of the cell.
An electron microscope allows you to get acquainted with the very complex structure of chloroplasts. It makes it possible to reveal much finer structures than those that are visible in a conventional light microscope. Particles smaller than 0.5 microns cannot be distinguished in a light microscope. The resolving power of electron microscopes already by 1961 made it possible to observe particles a thousand times smaller (on the order of 0.5 nm). With the help of an electron microscope, very thin membrane structures were revealed in chloroplasts, the so-called. thylakoids. These are flat pouches, closed at the edges and collected in stacks, called granas; in the pictures, the granas look like stacks of very thin pancakes. Inside the sacs there is a space - the thylakoid cavity, and the thylakoids themselves, collected in granules, are immersed in a gel-like mass of soluble proteins that fills the inner space of the chloroplast and is called the stroma. The stroma also contains smaller and thinner thylakoids, which connect individual granules to each other. All thylakoid membranes are composed of approximately equal amounts of proteins and lipids. Regardless of whether they are collected in granules or not, it is in them that pigments are concentrated and the light stage proceeds. The dark stage occurs, as is commonly believed, in the stroma.
Photo systems. Chlorophyll and carotenoids immersed in the thylakoid membranes of chloroplasts are assembled into functional units - photosystems, each of which contains about 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 rest of the pigment molecules, absorbing light, transfer its energy to the reaction center; these light-harvesting molecules are called antennae. There are two types of photo systems. In photosystem I, a specific chlorophyll a molecule that makes up the reaction center has an absorption optimum at a light wavelength of 700 nm (denoted as P700; P is a pigment), and in photosystem II, at 680 nm (P680). Usually, both photosystems work synchronously and (in the light) continuously, although photosystem I can work separately.
Transformations of light energy. Consideration of this issue should start with photosystem II, where light energy is utilized by the P680 reaction center. When light enters this photosystem, its energy excites the P680 molecule, and a pair of excited, energized electrons belonging to this molecule is detached and transferred to an acceptor molecule (probably a quinone), denoted by the letter Q. The situation can be imagined in such a way that the electrons are 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 pass into light, i.e. would be spent on fluorescence. From this point of view, an electron acceptor can be considered as a fluorescence quencher (hence its designation Q, from English quench - to quench).
The P680 molecule, having lost two electrons, oxidized, and in order for the process not to stop at this, it must be reduced, i.e. get two electrons from any source. Water serves as such a source: it splits into 2H + and 1 / 2O2, donating two electrons to the 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. Manganese atoms are the most important cofactor of photolysis enzymes. 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 the energy of light. Further, in photosystem II, a pair of electrons begins a phased "descent" from acceptor Q to photosystem I. Descent occurs along an electron transport chain, very similar in organization to a similar chain in mitochondria (see also METABOLISM). It contains cytochromes, proteins containing iron and sulfur, copper-containing proteins 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, the energy of light 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 process, light is being absorbed in photosystem I. Here, its energy is also used to detach 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 photolysis of water and preserved in thylakoids) and turns into NADPHN. As for the P700 reaction center oxidized at the beginning of the process, it accepts two ("descended") electrons from photosystem II, which returns it to its original state. The overall reaction of the light stage that occurs during photoactivation of photosystems I and II can be represented as follows:

The total energy yield of the electron flux 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 the light that provides their synthesis, it was calculated that in the process of photosynthesis, approximately 1/3 of the energy of the absorbed light is stored. 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 bypass path - back to the reaction center. In this case, photolysis of water and oxygen evolution does not occur, NADPHN is not formed, but ATP is synthesized. Such a mechanism of light reaction can also take place in higher plants under conditions when an excess of NADPHN occurs in the cells.
Dark reactions (synthesis stage). The synthesis of organic compounds by reducing CO2 (as well as nitrate and sulfate) also occurs in chloroplasts. ATP and NADPHN, supplied by a light reaction occurring in thylakoid membranes, serve as a source of energy and electrons for fusion 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 communication C-H, C-C 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 collaborators. Using its radioactive isotope instead of the usual (stable) isotope of carbon in their experiments, these researchers were able to trace the path of carbon in the reactions being studied. In 1961 Calvin was honored for this work Nobel Prize in chemistry. The Calvin cycle involves compounds with the number of carbon atoms in molecules from three to seven. All but one of the cycle components are sugar phosphates, i.e. sugars in which one or two OH groups are replaced by a phosphate group (-OPO3H-). The exception is 3-phosphoglyceric acid (FHA; 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 linked to oxygen atoms by three bonds. It is convenient to start the description of the cycle with ribulose monophosphate containing 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 binds CO2 to form a six-carbon compound. The latter is very unstable and, under the action of water, decomposes into two fragments - two FHA molecules. If we bear 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 the total protein content in them); given the huge mass of green plants, it is probably the most abundant protein in the biosphere. The next stage consists in the fact that two FHA molecules formed in the carboxylation reaction are reduced each by one NADPHPH molecule to 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 the FHA. However, in this case, ATP is needed to supply the molecule with additional chemical energy and increase its reactivity. This task is performed by the enzyme system, which transfers the terminal phosphate group of ATP to one of the oxygen atoms of the carboxyl group (a group is formed), i.e. FHA is converted to diphosphoglyceric acid. As soon as NADPHN transfers one hydrogen atom plus an electron (which is equivalent to two electrons plus a hydrogen ion, H +) to the carbon of the carboxyl group of this compound, the single C-O link breaks up and the oxygen bound to phosphorus goes into inorganic phosphate, HPO42-, and the carboxyl group O = C-O- transforms into aldehyde O = C-H. The latter is characteristic of a certain class of sugars. As a result, FHA with the participation of ATP and NADPHN is reduced to sugar phosphate (triose phosphate). The whole 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 FGK 4) FGK + ATP + NADP -> ADP + H2PO42 - + 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 expenditure of two molecules of NADPHP 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 NADPHN - to the carbon reduction cycle is presented. Of course, the light stage must additionally supply these cofactors for the reduction of nitrate and sulfate and for the conversion of FHA and triose phosphate formed in the cycle into other organic substances - carbohydrates, proteins, and fats. The significance of the subsequent stages of the cycle is reduced to the fact that they lead to the regeneration of a five-carbon compound, ribulose monophosphate, which is necessary for the renewal of the cycle. This part of the cycle can be written as follows:

which adds up to 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 turnover of the cycle, one molecule of carbon dioxide is included in the composition of a three-carbon organic compound; three revolutions of the cycle give a total of a new molecule of the latter, and for the synthesis of a molecule of six-carbon sugar (glucose or fructose), two three-carbon molecules are required and, accordingly, 6 revolutions of the cycle. The increase in organic matter gives the cycle 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 tag - a radioactive isotope of carbon. Chloroplast is not just a particle adapted for the synthesis of starch and sugars. This is a very complex, well-organized "factory", capable not only of producing all the materials from which it is built, but also supplying with reduced carbon compounds those parts of the cell and those plant organs that do not themselves photosynthesize.
LITERATURE
Edwards J., Walker D. Photosynthesis of C3 and C4 Plants: Mechanisms and Regulation. M., 1986 Raven P., Evert R., Eichhorn S. Modern botany, vol. 1, M., 1990

Collier's Encyclopedia. - Open Society. 2000 .

Photosynthesis is the synthesis of organic compounds in the leaves of green plants from water and carbon dioxide of the atmosphere using solar (light) energy, adsorbed by chlorophyll in chloroplasts.

Thanks to photosynthesis, the energy of visible light is captured and converted into chemical energy, stored (stored) in organic substances formed during photosynthesis.

The date of the discovery of the process of photosynthesis can be considered 1771. The English scientist J. Priestley drew attention to the change in the composition of the air due to the vital activity of animals. In the presence of green plants, the air again became suitable for breathing and for burning. Later, the work of a number of scientists (J. Ingenhaus, J. Senebier, T. Saussure, J. B. Boussingault) found that green plants absorb CO2 from the air, from which organic matter is formed with the participation of water in the light. It was this process in 1877 that the German scientist W. Pfeffer called photosynthesis. The law of conservation of energy formulated by R. Mayer was of great importance for the disclosure of the essence of photosynthesis. In 1845, R. Mayer put forward the assumption that the energy used by plants is the energy of the Sun, which plants convert into chemical energy during photosynthesis. This position was developed and experimentally confirmed in the studies of the remarkable Russian scientist K.A. Timiryazev.

The main role of photosynthetic organisms:

1) transformation of the energy of sunlight into the energy of chemical bonds of organic compounds;

2) saturation of the atmosphere with oxygen;

As a result of photosynthesis on the Earth, 150 billion tons of organic matter is formed and about 200 billion tons of free oxygen are released per year. It prevents an increase in the concentration of CO2 in the atmosphere, preventing overheating of the Earth (greenhouse effect).

The atmosphere created by photosynthesis protects living things from destructive short-wave UV radiation (oxygen-ozone screen of the atmosphere).

Only 1-2% of solar energy is transferred to the crop of agricultural plants; losses are caused by incomplete absorption of light. Therefore, there is a great prospect for increasing yields due to the selection of varieties with high photosynthetic efficiency, the creation of a crop structure favorable for light absorption. In this regard, the development of the theoretical foundations of photosynthesis control becomes especially urgent.

The importance of photosynthesis is enormous. Let's just note that it supplies fuel (energy) and atmospheric oxygen, which are necessary for the existence of all living things. Hence, the role of photosynthesis is planetary.

The planetary nature of photosynthesis is also determined by the fact that due to the circulation of oxygen and carbon (mainly) modern composition atmosphere, which in turn determines the further maintenance of life on Earth. We can say further that the energy that is stored in the products of photosynthesis is essentially the main source of energy that mankind now has.

Total reaction of photosynthesis

CO 2 + H 2 O = (CH 2 O) + O 2 .

The chemistry of photosynthesis is described by the following equations:

Photosynthesis - 2 groups of reactions:

light stage (depends on illumination)

dark stage (depends on temperature).

Both groups of reactions proceed simultaneously

Photosynthesis takes place in the chloroplasts of green plants.

Photosynthesis begins with the capture and absorption of light by the pigment chlorophyll, which is contained in the chloroplasts of green plant cells.

This is enough to shift the absorption spectrum of the molecule.

The chlorophyll molecule absorbs photons in the violet and blue, and then in the red part of the spectrum, and does not interact with photons in the green and yellow part of the spectrum.

Therefore, chlorophyll and plants look green - they simply cannot take advantage of the green rays and leave them walking in the white light (thereby making it greener).

Photosynthetic pigments are located on the inner side of the thylakoid membrane.

The pigments are organized in photo systems(antenna fields for capturing light) - containing 250-400 molecules of different pigments.

The photo system consists of:

reaction center photosystems (chlorophyll molecule a),

antenna molecules

All pigments in the photosystem are capable of transferring excited state energy to each other. The photon energy absorbed by one or another pigment molecule is transferred to a neighboring molecule until it reaches the reaction center. When the resonance system of the reaction center goes into an excited state, it transfers two excited electrons to the acceptor molecule and thereby oxidizes and acquires a positive charge.

In plants:

photosystem 1(maximum light absorption at a wavelength of 700 nm - P700)

photosystem 2(maximum absorption of light at a wavelength of 680 nm - P680

The differences in absorption optima are due to small differences in the structure of the pigments.

The two systems work in conjunction, like a two-piece conveyor called non-cyclic photophosphorylation .

The summary equation for non-cyclic photophosphorylation:

F - symbol phosphoric acid residue

The cycle begins with photosystem 2.

1) antenna molecules capture a photon and transfer excitation to a molecule of the P680 active center;

2) the excited P680 molecule gives up two electrons to the cofactor Q, while it is oxidized and acquires a positive charge;

Cofactor(cofactor). Coenzyme or any other substance required for an enzyme to function properly

Coenzymes (coenzymes)[from lat. co (cum) - together and enzymes], organic compounds of non-protein nature, participating in the enzymatic reaction as acceptors of individual atoms or atomic groups, cleaved by the enzyme from the substrate molecule, i.e. for the implementation of the catalytic action of enzymes. These substances, in contrast to the protein component of the enzyme (apoenzyme), have a relatively low molecular weight and, as a rule, are thermostable. Sometimes coenzymes mean any low-molecular substances, the participation of which is necessary for the manifestation of the catalytic action of the enzyme, including ions, for example. K +, Mg 2+ and Mn 2+. The offers are located. in the active center of the enzyme and together with the substrate and functional groups of the active center form an activated complex.

For the manifestation of catalytic activity, most enzymes require the presence of a coenzyme. The exception is hydrolytic enzymes (for example, proteases, lipases, ribonuclease), which perform their function in the absence of a coenzyme.

The molecule is reduced by P680 (by the action of enzymes). In this case, water dissociates into protons and molecular oxygen, those. water is an electron donor that provides electron replenishment in P 680.

PHOTOLYSIS WATER- splitting of a water molecule, in particular during photosynthesis. As a result of photolysis of water, oxygen is formed, which is released by green plants in the light.

Photosynthesis is a set of processes for the formation of light energy into the energy of chemical bonds of organic substances with the participation of photosynthetic dyes.

This type of nutrition is typical for plants, prokaryotes, and some species of unicellular eukaryotes.

Natural synthesis converts carbon and water into glucose and free oxygen in interaction with light:

6CO2 + 6H2O + light energy → C6H12O6 + 6O2

Modern plant physiology understands the concept of photosynthesis as a photoautotrophic function, which is a combination of processes of absorption, transformation and use of quanta of light energy in various non-spontaneous reactions, including the conversion of carbon dioxide into organic matter.

Phases

Photosynthesis in plants occurs in the leaves through chloroplasts- semi-autonomous two-membrane organelles belonging to the class of plastids. The flat shape of the sheet plates ensures high-quality absorption and full utilization of light energy and carbon dioxide. The water needed for natural synthesis comes from the roots through the conductive tissue. Gas exchange occurs by diffusion through the stomata and partly through the cuticle.

Chloroplasts are filled with colorless stroma and permeated with lamellae, which, when combined with each other, form thylakoids. It is in them that photosynthesis takes place. Cyanobacteria themselves are chloroplasts; therefore, the apparatus for natural synthesis in them is not isolated into a separate organelle.

Photosynthesis proceeds with the participation of pigments, which are usually chlorophylls. Some organisms contain a different pigment, carotenoid or phycobilin. Prokaryotes possess the pigment bacteriochlorophyll, and these organisms do not emit oxygen upon completion of natural synthesis.

Photosynthesis goes through two phases - light and dark. Each of them is characterized by certain reactions and interacting substances. Let us consider in more detail the process of the phases of photosynthesis.

Luminous

The first phase of photosynthesis characterized by the formation of high-energy products, which are ATP, a cellular energy source, and NADP, a reducing agent. At the end of the stage, oxygen is generated as a by-product. The light stage necessarily occurs with sunlight.

The process of photosynthesis takes place in the membranes of thylakoids with the participation of electron transport proteins, ATP synthetase and chlorophyll (or other pigment).

The functioning of electrochemical circuits, through which the transfer of electrons and partially hydrogen protons occurs, is formed in complex complexes formed by pigments and enzymes.

Light phase process description:

1. When sunlight hits the leaf plates of plant organisms, chlorophyll electrons are excited in the structure of the plates;
2. In the active state, the particles leave the pigment molecule and fall on the negatively charged outer side of the thylakoid. This occurs simultaneously with the oxidation and subsequent reduction of chlorophyll molecules, which take the next electrons from the water that has entered the leaves;
3. Then, photolysis of water occurs with the formation of ions, which donate electrons and are converted into OH radicals that can participate in reactions in the future;
4. These radicals then combine to form water molecules and free oxygen, which is released into the atmosphere;
5. The thylakoid membrane acquires on the one hand a positive charge due to the hydrogen ion, and on the other - negative due to electrons;
6. When a difference of 200 mV between the sides of the membrane is reached, protons pass through the enzyme ATP synthetase, which leads to the conversion of ADP into ATP (phosphorylation process);
7. With the atomic hydrogen released from water, NADP + is reduced to NADPH2;

While free oxygen is released into the atmosphere during the reactions, ATP and NADPH2 participate in the dark phase of natural synthesis.

Dark

An obligatory component for this stage is carbon dioxide., which plants constantly absorb from the external environment through the stomata in the leaves. The dark phase processes take place in the chloroplast stroma. Since at this stage a lot of solar energy is not required and there will be enough ATP and NADPH2 produced during the light phase, reactions in organisms can proceed both day and night. Processes at this stage are faster than at the previous one.

The totality of all processes occurring in the dark phase is presented in the form of a kind of chain of successive transformations of carbon dioxide received from the external environment:

1. The first reaction in such a chain is carbon dioxide fixation. The presence of the enzyme RuBP-carboxylase contributes to the rapid and smooth course of the reaction, as a result of which a six-carbon compound is formed, which decomposes into 2 molecules of phosphoglyceric acid;
2. Then a rather complex cycle occurs, which includes a certain number of reactions, at the end of which phosphoglyceric acid is converted into a natural sugar - glucose. This process is called the Calvin cycle;

Along with sugar, fatty acids, amino acids, glycerol and nucleotides are also formed.

The essence of photosynthesis

From the table of comparisons of the light and dark phases of natural synthesis, you can briefly describe the essence of each of them. The light phase occurs in the chloroplast grains with the obligatory inclusion of light energy in the reactions. The reactions involve such components as proteins that carry electrons, ATP synthetase and chlorophyll, which, when interacting with water, form free oxygen, ATP and NADPH2. For the dark phase occurring in the chloroplast stroma, sunlight is not necessary. The ATP and NADPH2 obtained at the last stage, when interacting with carbon dioxide, form natural sugar (glucose).

As can be seen from the above, photosynthesis appears to be a rather complex and multistep phenomenon, including many reactions in which various substances are involved. As a result of natural synthesis, oxygen is obtained, which is necessary for the respiration of living organisms and their protection from ultraviolet radiation by means of the formation of the ozone layer.

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Tasks: To form knowledge about the reactions of plastic and energy metabolism and their relationship; remember the structural features of chloroplasts. To characterize the light and dark phases of photosynthesis. Show the importance of photosynthesis as a process that ensures the synthesis of organic substances, the absorption of carbon dioxide and the release of oxygen into the atmosphere.

Lesson type: lecture.

Equipment:

1. Visual aids: tables on general biology;
2. TSO: computer; multimedia projector.

Lecture plan:

1. The history of the study of the process.
2. Experiments on photosynthesis.
3. Photosynthesis as an anabolic process.
4. Chlorophyll and its properties.
5. Photo systems.
6. Light phase of photosynthesis.
7. The dark phase of photosynthesis.
8. Limiting factors of photosynthesis.

Lecture progress

History of the study of photosynthesis

1630 year the beginning of the study of photosynthesis ... Van Helmont proved that plants form organic matter, and do not get them from the soil. Weighing a pot with earth and willow, and separately the tree itself, he showed that after 5 years the weight of the tree increased by 74 kg, while the soil lost only 57 g. He decided that the tree gets its food from water. We currently know that carbon dioxide is used.

V 1804 Saussure found that in the process of photosynthesis, water is of great importance.

V 1887 year chemosynthetic bacteria are discovered.

V 1905 Blackman found that photosynthesis consists of two phases: fast - light and a series of successive slow reactions of the dark phase.

Experiments on photosynthesis

1 experience proves the value of sunlight (fig. 1.)	2 experience proves the importance of carbon dioxide for photosynthesis (Fig. 2.)
3 experience proves the importance of photosynthesis (Fig. 3.)

Photosynthesis as an anabolic process

1. Photosynthesis produces 150 billion tons of organic matter and 200 billion tons of free oxygen annually.
2. The cycle of oxygen, carbon and other elements involved in photosynthesis. Supports the modern atmosphere required for existence modern forms life.
3. Photosynthesis prevents an increase in the concentration of carbon dioxide, preventing the Earth from overheating due to the greenhouse effect.
4. Photosynthesis is the basis of all food chains on Earth.
5. Energy stored in food is the main source of energy for humanity.

The essence of photosynthesis consists in converting the light energy of the sun's ray into chemical energy in the form of ATP and NADPH 2.

The overall equation of photosynthesis:

6CO 2 + 6H 2 O→C 6 H 12 O 6 + 6O 2

There are two main types of photosynthesis:

Chlorophyll and its properties

Chlorophyll species

Chlorophyll has modifications a, b, c, d. They differ in structural structure and light absorption spectrum. For example: chlorophyll b contains one more oxygen atom and two less hydrogen atoms than chlorophyll a.

All plants and oxyphotobacteria have yellow-green chlorophyll a as the main pigment, and chlorophyll b as an additional pigment.

Other plant pigments

Some other pigments are able to absorb solar energy and transfer it to chlorophyll, thereby involving it in photosynthesis.

Most plants have a dark orange pigment - carotene, which in the animal body turns into vitamin A and yellow pigment - xanthophyll.

Phycocyanin and phycoerythrin- contain red and blue-green algae. In red algae, these pigments are more actively involved in the process of photosynthesis than chlorophyll.

Chlorophyll minimally absorbs light in the blue-green part of the spectrum. Chlorophyll a, b - in the violet region of the spectrum, where the wavelength is 440 nm. Chlorophyll's unique function consists in the fact that it intensively absorbs solar energy and transfers it to other molecules.

Pigments absorb a certain wavelength, non-absorbed portions of the solar spectrum are reflected, which ensures the color of the pigment. Green light is not absorbed, so chlorophyll is green.

Pigments Are chemical compounds that absorb visible light, which causes electrons to become excited. The shorter the wavelength, the greater the energy of the light and the greater its ability to transfer electrons to an excited state. This state is unstable and soon the entire molecule returns to its normal low-energy state, losing its excitation energy. This energy can be used for fluorescence.

Photo systems

Pigments of plants participating in photosynthesis are “packed” in chloroplast thylakoids in the form of functional photosynthetic units - photosynthetic systems: photosystem I and photosystem II.

Each system consists of a set of auxiliary pigments (from 250 to 400 molecules) that transfer energy to one molecule of the main pigment and it is called reactionary center... It uses the energy of the sun for photochemical reactions.

The light phase takes place necessarily with the participation of light, the dark phase both in the light and in the dark. The light process takes place in the thylakoids of chloroplasts, the dark process takes place in the stroma, i.e. these processes are spatially separated.

Light phase of photosynthesis

V 1958 Arnon and his collaborators studied the light phase of photosynthesis. They found that the source of energy in photosynthesis is light, and since chlorophyll is exposed to light in chlorophyll, synthesis from ADP + ph. → ATP, then this process is called phosphorylation. It is associated with the transfer of electrons in membranes.

The role of light reactions: 1. ATP synthesis - phosphorylation. 2. Synthesis of NADP.H 2.

The electron transport path is called Z-scheme.

Z-diagram. Non-cyclic and cyclic photophosphorylation(fig. 6.)

During the cyclic transport of electrons, the formation of NADPH 2 and photodecomposition of H 2 O do not occur, and therefore the release of O 2. This pathway is used when there is an excess of NADPH 2 in the cell, but additional ATP is required.

All these processes are related to the light phase of photosynthesis. Subsequently, the energy of ATP and NADP.H 2 is used for the synthesis of glucose. No light is needed for this process. These are reactions of the dark phase of photosynthesis.

Dark Phase of Photosynthesis or Calvin Cycle

The synthesis of glucose occurs in a cyclical process, which is named after the scientist Melvin Calvin, who discovered it, and was awarded the Nobel Prize.

Rice. 8. Calvin cycle

Each reaction of the Calvin cycle is carried out by a different enzyme. For the formation of glucose are used: CO 2, protons and electrons from NADP.H 2, the energy of ATP and NADP.H 2. The process takes place in the chloroplast stroma. The initial and final compound of the Calvin cycle, to which with the help of an enzyme ribulose diphosphate carboxylase joins CO2, is a five-carbon sugar - ribulose biphosphate containing two phosphate groups. As a result, a six-carbon compound is formed, which immediately decays into two three-carbon molecules phosphoglyceric acid which are then restored to phosphoglycerol aldehyde... At the same time, part of the formed phosphoglyceric aldehyde is used for the regeneration of ribulose biphosphate, and thus the cycle is resumed again (5C 3 → 3C 5), and part is used for the synthesis of glucose and other organic compounds (2C 3 → C 6 → C 6 H 12 O 6).

For the formation of one glucose molecule, 6 revolutions of the cycle are required and 12 NADP.H 2 and 18 ATP are required. From the total reaction equation it is obtained:

6CO 2 + 6H 2 O → C 6 H 12 O 6 + 6O 2

From the above equation, it can be seen that C and O atoms entered glucose from CO 2, and hydrogen atoms from H 2 O. Glucose can later be used both for the synthesis of complex carbohydrates (cellulose, starch) and for the formation of proteins and lipids.

(С 4 - photosynthesis. In 1965 it was proved that in sugar cane - the first products of photosynthesis are acids containing four carbon atoms (malic, oxaloacetic, aspartic). C 4 plants include corn, sorghum, millet).

Limiting factors of photosynthesis

The rate of photosynthesis is the most important factor affecting the yield of agricultural crops. So, for the dark phases of photosynthesis, NADP.H 2 and ATP are needed, and therefore the rate of dark reactions depends on light reactions. In low light, the rate of formation of organic matter will be low. Therefore, light is the limiting factor.

Of all the factors simultaneously affecting the process of photosynthesis limiting there will be one that is closer to the minimum level. It installed Blackman in 1905... Various factors can be limiting, but one of them is the main one.

The cosmic role of plants(described K. A. Timiryazev) lies in the fact that plants are the only organisms that assimilate solar energy and accumulate it in the form of potential chemical energy of organic compounds. The released O 2 supports the vital activity of all aerobic organisms. Ozone is formed from oxygen, which protects all living things from ultraviolet rays. Plants used an enormous amount of CO 2 from the atmosphere, the excess of which created a "greenhouse effect", and the planet's temperature dropped to its present values.