Photosynthesis is also called. The process of photosynthesis in plant leaves

Photosynthesis is the process of formation of organic substances in green plants. Photosynthesis created the entire mass of plants on Earth and saturated the atmosphere with oxygen.

How does the plant feed?

Previously, people were sure that plants took all the substances for their nutrition from the soil. But one experience has shown that this is not so.

A tree was planted in a pot of soil. At the same time, the mass of both the earth and the tree was measured. When, a few years later, both were weighed again, it turned out that the mass of the earth had decreased by only a few grams, and the mass of the plant had increased by many kilograms.

Only water was added to the soil. Where did these kilograms of plant mass come from?

From the air. All organic matter in plants is created from atmospheric carbon dioxide and soil water.

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Energy

Animals and humans eat plants to obtain energy for life. This energy is contained in the chemical bonds of organic substances. Where is she from?

It is known that a plant cannot grow normally without light. Light is the energy with which a plant builds the organic substances of its body.

It doesn’t matter what kind of light it is, solar or electric. Any ray of light carries energy, which becomes the energy of chemical bonds and, like glue, holds atoms in large molecules of organic substances.

Where does photosynthesis take place?

Photosynthesis takes place only in the green parts of plants, or more precisely, in special organs of plant cells - chloroplasts.

Rice. 1. Chloroplasts under a microscope.

Chloroplasts are a type of plastid. They are always green, because they contain a green substance - chlorophyll.

The chloroplast is separated from the rest of the cell by a membrane and has the appearance of a grain. The interior of the chloroplast is called the stroma. This is where the processes of photosynthesis begin.

Rice. 2. Internal structure of the chloroplast.

Chloroplasts are like a factory that receives raw materials:

• carbon dioxide (formula – CO₂);
• water (H₂O).

Water comes from the roots, and carbon dioxide comes from the atmosphere through special holes in the leaves. Light is the energy for the operation of the factory, and the resulting organic substances are the products.

First, carbohydrates (glucose) are produced, but subsequently they form many substances of various smells and tastes that animals and people love so much.

From the chloroplasts, the resulting substances are transported to various organs of the plant, where they are stored or used.

Photosynthesis reaction

In general, the photosynthesis equation looks like this:

CO₂ + H₂O = organic matter + O₂ (oxygen)

Green plants belong to the group of autotrophs (translated as “I feed myself”) - organisms that do not need other organisms to obtain energy.

The main function of photosynthesis is the creation of organic substances from which the plant body is built.

The release of oxygen is a side effect of the process.

The meaning of photosynthesis

The role of photosynthesis in nature is extremely large. Thanks to him, the entire plant world of the planet was created.

Rice. 3. Photosynthesis.

Thanks to photosynthesis, plants:

• are a source of oxygen for the atmosphere;
• convert the sun's energy into a form accessible to animals and humans.

Life on Earth became possible with the accumulation of sufficient oxygen in the atmosphere. Neither man nor animals could have lived in those distant times when he was not there, or there was little of him.

What science studies the process of photosynthesis?

Photosynthesis is studied in various sciences, but most of all in botany and plant physiology.

Botany is the science of plants and therefore studies it as an important life process of plants.

Plant physiology studies photosynthesis in the most detail. Physiological scientists have determined that this process is complex and has stages:

• light;
• dark

This means that photosynthesis begins in the light but ends in the dark.

What have we learned?

Having studied this topic in grade 5 biology, you can briefly and clearly explain photosynthesis as the process of formation of organic substances from inorganic substances (CO₂ and H₂O) in plants. Its features: it takes place in green plastids (chloroplasts), is accompanied by the release of oxygen, and is carried out under the influence of light.

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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 H2S). 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 - 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 all the nutrients they need 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) is more or less suitable. 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.

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, which produces carbohydrates, therefore converts two very stable substances, CO2 and H2O, into one completely stable substance, (CH2O)n, and one less stable substance, 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 NADPHN 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 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 become 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.
An electron microscope allows you to get acquainted with the very complex structure of chloroplasts. 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 pouches, 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, as is commonly believed, occurs 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), denoted 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 reduced, 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 one 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 formation of ATP during photosynthesis is called photophosphorylation. Simultaneously with the described process, light is absorbed in photosystem I. Here, its energy is also used to separate 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 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. This mechanism of light reaction can also occur in higher plants under conditions when an excess of NADPH occurs in the 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 C-H, C-C, and O-H bonds. 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 being studied. 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 FHA 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 + 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 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 plant organs that do not carry out photosynthesis themselves.
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 .

Non-chlorophyll photosynthesis

Spatial localization

Plant photosynthesis occurs in chloroplasts: isolated double-membrane organelles of the cell. Chloroplasts can be found in the cells of fruits and stems, but the main organ of photosynthesis, anatomically adapted for its conduct, is the leaf. In the leaf, the palisade parenchyma tissue is richest in chloroplasts. In some succulents with degenerate leaves (such as cacti), the main photosynthetic activity is associated with the stem.

Light for photosynthesis is more fully captured due to the flat leaf shape, which provides a high surface to volume ratio. Water is delivered from the root through a developed network of vessels (leaf veins). Carbon dioxide enters partly by diffusion through the cuticle and epidermis, but most of it diffuses into the leaf through the stomata and through the leaf through the intercellular space. Plants that carry out CAM photosynthesis have developed special mechanisms for the active assimilation of carbon dioxide.

The internal space of the chloroplast is filled with colorless contents (stroma) and is penetrated by membranes (lamellae), which, when connected to each other, form thylakoids, which in turn are grouped into stacks called grana. The intrathylakoid space is separated and does not communicate with the rest of the stroma; it is also assumed that the internal space of all thylakoids communicates with each other. The light stages of photosynthesis are confined to membranes; autotrophic fixation of CO 2 occurs in the stroma.

Chloroplasts have their own DNA, RNA, ribosomes (70s type), and protein synthesis occurs (although this process is controlled from the nucleus). They are not synthesized again, but are formed by dividing the previous ones. All this made it possible to consider them the descendants of free cyanobacteria that became part of the eukaryotic cell during the process of symbiogenesis.

Photosystem I

Light-harvesting complex I contains approximately 200 chlorophyll molecules.

In the reaction center of the first photosystem there is a dimer of chlorophyll a with an absorption maximum at 700 nm (P700). After excitation by a light quantum, it restores the primary acceptor - chlorophyll a, which restores the secondary acceptor (vitamin K 1 or phylloquinone), after which the electron is transferred to ferredoxin, which reduces NADP using the enzyme ferredoxin-NADP reductase.

The plastocyanin protein, reduced in the b 6 f complex, is transported to the reaction center of the first photosystem from the side of the intrathylakoid space and transfers an electron to the oxidized P700.

Cyclic and pseudocyclic electron transport

In addition to the complete non-cyclic electron path described above, a cyclic and pseudo-cyclic path has been discovered.

The essence of the cyclic pathway is that ferredoxin, instead of NADP, reduces plastoquinone, which transfers it back to the b 6 f complex. This results in a larger proton gradient and more ATP, but no NADPH.

In the pseudocyclic pathway, ferredoxin reduces oxygen, which is further converted into water and can be used in photosystem II. In this case, NADPH is also not formed.

Dark stage

In the dark stage, with the participation of ATP and NADPH, CO 2 is reduced to glucose (C 6 H 12 O 6). Although light is not required for this process, it is involved in its regulation.

C 3 photosynthesis, Calvin cycle

The third stage involves 5 PHA molecules, which, through the formation of 4-, 5-, 6- and 7-carbon compounds, are combined into 3 5-carbon ribulose-1,5-biphosphate, which requires 3ATP.

Finally, two PHAs are required for glucose synthesis. To form one of its molecules, 6 cycle revolutions, 6 CO 2, 12 NADPH and 18 ATP are required.

C 4 photosynthesis

Main articles: Hatch-Slack-Karpilov cycle, C4 photosynthesis

At a low concentration of CO 2 dissolved in the stroma, ribulose biphosphate carboxylase catalyzes the oxidation reaction of ribulose-1,5-biphosphate and its breakdown into 3-phosphoglyceric acid and phosphoglycolic acid, which is forced to be used in the process of photorespiration.

To increase CO2 concentration, type 4 C plants changed their leaf anatomy. The Calvin cycle is localized in the sheath cells of the vascular bundle; in the mesophyll cells, under the action of PEP carboxylase, phosphoenolpyruvate is carboxylated to form oxaloacetic acid, which is converted into malate or aspartate and transported to the sheath cells, where it is decarboxylated to form pyruvate, which is returned to the mesophyll cells.

With 4, photosynthesis is practically not accompanied by losses of ribulose-1,5-biphosphate from the Calvin cycle, and therefore is more efficient. However, it requires not 18, but 30 ATP for the synthesis of 1 glucose molecule. This is justified in the tropics, where the hot climate requires keeping the stomata closed, which prevents the entry of CO 2 into the leaf, as well as with a ruderal life strategy.

photosynthesis itself

Later it was found that in addition to releasing oxygen, plants absorb carbon dioxide and, with the participation of water, synthesize organic matter in the light. Based on the law of conservation of energy, Robert Mayer postulated that plants convert the energy of sunlight into the energy of chemical bonds. W. Pfeffer called this process photosynthesis.

Chlorophylls were first isolated by P. J. Peltier and J. Caventou. M. S. Tsvet managed to separate the pigments and study them separately using the chromatography method he created. The absorption spectra of chlorophyll were studied by K. A. Timiryazev, who, developing Mayer’s principles, showed that it is the absorbed rays that make it possible to increase the energy of the system, creating high-energy C-C bonds instead of weak C-O and O-H bonds (before that it was believed that in photosynthesis uses yellow rays that are not absorbed by leaf pigments). This was done thanks to the method he created for accounting for photosynthesis based on absorbed CO 2: during experiments on illuminating a plant with light of different wavelengths (different colors), it turned out that the intensity of photosynthesis coincides with the absorption spectrum of chlorophyll.

The redox nature of photosynthesis (both oxygenic and anoxygenic) was postulated by Cornelis van Niel. This meant that oxygen in photosynthesis is formed entirely from water, which was experimentally confirmed by A.P. Vinogradov in experiments with an isotope label. Robert Hill found that the process of water oxidation (and oxygen release) and CO 2 assimilation can be separated. W. D. Arnon established the mechanism of the light stages of photosynthesis, and the essence of the CO 2 assimilation process was revealed by Melvin Calvin using carbon isotopes in the late 1940s, for which he was awarded the Nobel Prize.

Other facts

see also

Literature

• Hall D., Rao K. Photosynthesis: Transl. from English - M.: Mir, 1983.
• Plant physiology / ed. prof. Ermakova I. P. - M.: Academy, 2007
• Molecular biology of cells / Albertis B., Bray D. et al. In 3 vols. - M.: Mir, 1994
• Rubin A. B. Biophysics. In 2 vols. - M.: Publishing house. Moscow University and Science, 2004.
• Chernavskaya N. M.,

The process of converting radiant energy from the Sun into chemical energy using the latter in the synthesis of carbohydrates from carbon dioxide. This is the only way to capture solar energy and use it for life on our planet.

The capture and transformation of solar energy is carried out by a variety of photosynthetic organisms (photoautotrophs). These include multicellular organisms (higher green plants and their lower forms - green, brown and red algae) and unicellular organisms (euglena, dinoflagellates and diatoms). A large group of photosynthetic organisms are prokaryotes - blue-green algae, green and purple bacteria. About half of the work of photosynthesis on Earth is carried out by higher green plants, and the remaining half is carried out mainly by single-celled algae.

The first ideas about photosynthesis were formed in the 17th century. Subsequently, as new data became available, these ideas changed many times. [show] .

Development of ideas about photosynthesis

The study of photosynthesis began in 1630, when van Helmont showed that plants themselves form organic substances and do not obtain them from the soil. By weighing the pot of soil in which the willow grew and the tree itself, he showed that over the course of 5 years the mass of the tree increased by 74 kg, while the soil lost only 57 g. Van Helmont concluded that the plant received the rest of its food from water that was used to water the tree. Now we know that the main material for synthesis is carbon dioxide, extracted by the plant from the air.

In 1772, Joseph Priestley showed that mint sprouts "corrected" air "tainted" by a burning candle. Seven years later, Jan Ingenhuis discovered that plants can “correct” bad air only by being in the light, and the ability of plants to “correct” air is proportional to the clarity of the day and the length of time the plants remain in the sun. In the dark, plants emit air that is “harmful to animals.”

The next important step in the development of knowledge about photosynthesis were the experiments of Saussure, conducted in 1804. By weighing the air and plants before and after photosynthesis, Saussure established that the increase in the dry mass of the plant exceeded the mass of carbon dioxide absorbed from the air. Saussure concluded that another substance involved in the increase in mass was water. Thus, 160 years ago the process of photosynthesis was imagined as follows:

H 2 O + CO 2 + hv -> C 6 H 12 O 6 + O 2

Water + Carbon Dioxide + Solar Energy ----> Organic Matter + Oxygen

Ingenhues proposed that the role of light in photosynthesis is to break down carbon dioxide; in this case, oxygen is released, and the released “carbon” is used to build plant tissue. On this basis, living organisms were divided into green plants, which can use solar energy to “assimilate” carbon dioxide, and other organisms that do not contain chlorophyll, which cannot use light energy and are not able to assimilate CO 2.

This principle of division of the living world was violated when S. N. Winogradsky in 1887 discovered chemosynthetic bacteria - chlorophyll-free organisms capable of assimilating (i.e. converting into organic compounds) carbon dioxide in the dark. It was also disrupted when, in 1883, Engelmann discovered purple bacteria that carry out a kind of photosynthesis that is not accompanied by the release of oxygen. At one time this fact was not adequately appreciated; Meanwhile, the discovery of chemosynthetic bacteria that assimilate carbon dioxide in the dark shows that the assimilation of carbon dioxide cannot be considered a specific feature of photosynthesis alone.

After 1940, thanks to the use of labeled carbon, it was established that all cells - plant, bacterial and animal - are capable of assimilating carbon dioxide, that is, incorporating it into the molecules of organic substances; Only the sources from which they draw the energy necessary for this are different.

Another major contribution to the study of photosynthesis was made in 1905 by Blackman, who discovered that photosynthesis consists of two sequential reactions: a fast light reaction and a series of slower, light-independent stages, which he called the rate reaction. Using high-intensity light, Blackman showed that photosynthesis proceeds at the same rate under intermittent light with flashes lasting only a fraction of a second as under continuous light, despite the fact that in the first case the photosynthetic system receives half as much energy. The intensity of photosynthesis decreased only with a significant increase in the dark period. In further studies, it was found that the rate of the dark reaction increases significantly with increasing temperature.

The next hypothesis regarding the chemical basis of photosynthesis was put forward by van Niel, who in 1931 experimentally showed that photosynthesis in bacteria can occur under anaerobic conditions, without the release of oxygen. Van Niel suggested that, in principle, the process of photosynthesis is similar in bacteria and in green plants. In the latter, light energy is used for photolysis of water (H 2 0) with the formation of a reducing agent (H), determined by participating in the assimilation of carbon dioxide, and an oxidizing agent (OH), a hypothetical precursor of molecular oxygen. In bacteria, photosynthesis proceeds in generally the same way, but the hydrogen donor is H 2 S or molecular hydrogen, and therefore oxygen is not released.

Modern ideas about photosynthesis

According to modern concepts, the essence of photosynthesis is the conversion of the radiant energy of sunlight into chemical energy in the form of ATP and reduced nicotinamide adenine dinucleotide phosphate (NADP · N).

Currently, it is generally accepted that the process of photosynthesis consists of two stages in which photosynthetic structures take an active part [show] and photosensitive cell pigments.

Photosynthetic structures

In bacteria photosynthetic structures are presented in the form of invaginations of the cell membrane, forming lamellar organelles of the mesosome. Isolated mesosomes obtained from the destruction of bacteria are called chromatophores; the light-sensitive apparatus is concentrated in them.

In eukaryotes The photosynthetic apparatus is located in special intracellular organelles - chloroplasts, containing the green pigment chlorophyll, which gives the plant its green color and plays a crucial role in photosynthesis, capturing the energy of sunlight. Chloroplasts, like mitochondria, also contain DNA, RNA and an apparatus for protein synthesis, i.e., they have the potential ability to reproduce themselves. Chloroplasts are several times larger in size than mitochondria. The number of chloroplasts ranges from one in algae to 40 per cell in higher plants.

In addition to chloroplasts, the cells of green plants also contain mitochondria, which are used to produce energy at night through respiration, as in heterotrophic cells.

Chloroplasts have a spherical or flattened shape. They are surrounded by two membranes - outer and inner (Fig. 1). The inner membrane is arranged in the form of stacks of flattened bubble-like disks. This stack is called a grana.

Each grain consists of individual layers arranged like columns of coins. Layers of protein molecules alternate with layers containing chlorophyll, carotenes and other pigments, as well as special forms of lipids (containing galactose or sulfur, but only one fatty acid). These surfactant lipids appear to be adsorbed between individual layers of molecules and serve to stabilize the structure, which consists of alternating layers of protein and pigments. This layered (lamellar) structure of the grana most likely facilitates the transfer of energy during photosynthesis from one molecule to a nearby one.

In algae there is no more than one grain in each chloroplast, and in higher plants there are up to 50 grains, which are interconnected by membrane bridges. The aqueous environment between the grana is the stroma of the chloroplast, which contains enzymes that carry out “dark reactions”

The vesicle-like structures that make up the grana are called thylactoids. There are from 10 to 20 thylactoids in the grana.

The elementary structural and functional unit of thylactoid membrane photosynthesis, containing the necessary light-trapping pigments and components of the energy transformation apparatus, is called the quantosome, consisting of approximately 230 chlorophyll molecules. This particle has a mass of about 2 x 10 6 daltons and dimensions of about 17.5 nm.

Land plants absorb the water necessary for photosynthesis through their roots, while aquatic plants receive it by diffusion from the environment. Carbon dioxide, necessary for photosynthesis, diffuses into the plant through small holes on the surface of the leaves - stomata. Since carbon dioxide is consumed during photosynthesis, its concentration in the cell is usually slightly lower than in the atmosphere. Oxygen released during photosynthesis diffuses out of the cell and then out of the plant through the stomata. Sugars produced during photosynthesis also diffuse to those parts of the plant where their concentration is lower.

To carry out photosynthesis, plants need a lot of air, since it contains only 0.03% carbon dioxide. Consequently, from 10,000 m 3 of air, 3 m 3 of carbon dioxide can be obtained, from which about 110 g of glucose is formed during photosynthesis. Plants generally grow better with higher levels of carbon dioxide in the air. Therefore, in some greenhouses the CO 2 content in the air is adjusted to 1-5%.

Solar energy and various pigments take part in the implementation of the photochemical function of photosynthesis: green - chlorophylls a and b, yellow - carotenoids and red or blue - phycobilins. Among this complex of pigments, only chlorophyll a is photochemically active. The remaining pigments play a supporting role, being only collectors of light quanta (a kind of light-collecting lenses) and their conductors to the photochemical center.

Based on the ability of chlorophyll to effectively absorb solar energy of a certain wavelength, functional photochemical centers or photosystems were identified in thylactoid membranes (Fig. 3):

• photosystem I (chlorophyll A) - contains pigment 700 (P 700) that absorbs light with a wavelength of about 700 nm, plays a major role in the formation of the products of the light stage of photosynthesis: ATP and NADP · H 2
• photosystem II (chlorophyll b) - contains pigment 680 (P 680), which absorbs light with a wavelength of 680 nm, plays an auxiliary role by replenishing electrons lost by photosystem I through photolysis of water

For every 300-400 molecules of light-harvesting pigments in photosystems I and II, there is only one molecule of photochemically active pigment - chlorophyll a.

Light quantum absorbed by a plant

• transfers pigment P 700 from the ground state to the excited state - P * 700, in which it easily loses an electron with the formation of a positive electron hole in the form of P 700 + according to the scheme:

  P 700 ---> P * 700 ---> P + 700 + e -

  After which the pigment molecule that has lost an electron can serve as an electron acceptor (capable of accepting an electron) and transform into a reduced form

• causes decomposition (photooxidation) of water in the photochemical center P 680 of photosystem II according to the scheme

  H 2 O ---> 2H + + 2e - + 1/2O 2

  Photolysis of water is called the Hill reaction. Electrons produced during the decomposition of water are initially accepted by a substance designated Q (sometimes called cytochrome C 550 due to its maximum absorption, although it is not a cytochrome). Then, from substance Q, through a chain of carriers similar in composition to the mitochondrial one, electrons are supplied to photosystem I to fill the electron hole formed as a result of the absorption of light quanta by the system and restore pigment P + 700

If such a molecule simply receives back the same electron, then light energy will be released in the form of heat and fluorescence (this is due to the fluorescence of pure chlorophyll). However, in most cases, the released negatively charged electron is accepted by special iron-sulfur proteins (FeS center), and then

1. or is transported along one of the carrier chains back to P+700, filling the electron hole
2. or along another chain of transporters through ferredoxin and flavoprotein to a permanent acceptor - NADP · H 2

In the first case, closed cyclic electron transport occurs, and in the second case, non-cyclic transport occurs.

Both processes are catalyzed by the same electron transport chain. However, during cyclic photophosphorylation, electrons are returned from chlorophyll A back to chlorophyll A, whereas in non-cyclic photophosphorylation electrons are transferred from chlorophyll b to chlorophyll A.

Thus, the electrons lost by P 700 are replenished by electrons from water decomposed under the influence of light in photosystem II.

A+ to the ground state, are apparently formed upon excitation of chlorophyll b. These high-energy electrons pass to ferredoxin and then through flavoprotein and cytochromes to chlorophyll A. At the last stage, phosphorylation of ADP to ATP occurs (Fig. 5).

Electrons needed to return chlorophyll V its ground state are probably supplied by OH - ions formed during the dissociation of water. Some of the water molecules dissociate into H + and OH - ions. As a result of the loss of electrons, OH - ions are converted into radicals (OH), which subsequently produce molecules of water and gaseous oxygen (Fig. 6).

This aspect of the theory is confirmed by the results of experiments with water and CO 2 labeled with 18 0 [show] .

According to these results, all the oxygen gas released during photosynthesis comes from water and not from CO 2 . The reactions of water splitting have not yet been studied in detail. It is clear, however, that the implementation of all sequential reactions of non-cyclic photophosphorylation (Fig. 5), including the excitation of one chlorophyll molecule A and one chlorophyll molecule b, should lead to the formation of one NADP molecule · H, two or more ATP molecules from ADP and Pn and to the release of one oxygen atom. This requires at least four quanta of light - two for each chlorophyll molecule.

Non-cyclic flow of electrons from H 2 O to NADP · H2, which occurs during the interaction of two photosystems and the electron transport chains connecting them, is observed contrary to the values ​​of redox potentials: E° for 1/2O2/H2O = +0.81 V, and E° for NADP/NADP · H = -0.32 V. Light energy reverses the flow of electrons. It is significant that when transferred from photosystem II to photosystem I, part of the electron energy is accumulated in the form of proton potential on the thylactoid membrane, and then into ATP energy.

The mechanism of formation of the proton potential in the electron transport chain and its use for the formation of ATP in chloroplasts is similar to that in mitochondria. However, there are some peculiarities in the photophosphorylation mechanism. Thylactoids are like mitochondria turned inside out, so the direction of electron and proton transfer through the membrane is opposite to the direction in the mitochondrial membrane (Fig. 6). Electrons move to the outside, and protons concentrate inside the thylactoid matrix. The matrix is ​​charged positively, and the outer membrane of the thylactoid is charged negatively, i.e., the direction of the proton gradient is opposite to its direction in the mitochondria.

Another feature is the significantly larger proportion of pH in the proton potential compared to mitochondria. The thylactoid matrix is ​​highly acidified, so Δ pH can reach 0.1-0.2 V, while Δ Ψ is about 0.1 V. The overall value of Δ μ H+ > 0.25 V.

H + -ATP synthetase, designated in chloroplasts as the “CF 1 + F 0” complex, is also oriented in the opposite direction. Its head (F 1) looks outward, towards the stroma of the chloroplast. Protons are pushed out through CF 0 + F 1 from the matrix, and ATP is formed in the active center of F 1 due to the energy of the proton potential.

Unlike the mitochondrial chain, the thylactoid chain apparently has only two conjugation sites, so the synthesis of one ATP molecule requires three protons instead of two, i.e., a ratio of 3 H + /1 mol of ATP.

So, at the first stage of photosynthesis, during light reactions, ATP and NADP are formed in the stroma of the chloroplast · H - products necessary for dark reactions.

Dark reactions of photosynthesis are the process of incorporating carbon dioxide into organic matter to form carbohydrates (photosynthesis of glucose from CO 2). Reactions occur in the stroma of the chloroplast with the participation of the products of the light stage of photosynthesis - ATP and NADP · H2.

The assimilation of carbon dioxide (photochemical carboxylation) is a cyclic process, also called the pentose phosphate photosynthetic cycle or the Calvin cycle (Fig. 7). There are three main phases in it:

• carboxylation (fixation of CO 2 with ribulose diphosphate)
• reduction (formation of triose phosphates during reduction of 3-phosphoglycerate)
• regeneration of ribulose diphosphate

Ribulose 5-phosphate (a sugar containing 5 carbon atoms with a phosphate moiety at carbon 5) undergoes phosphorylation by ATP, resulting in the formation of ribulose diphosphate. This latter substance is carboxylated by the addition of CO 2 , apparently to a six-carbon intermediate, which, however, is immediately cleaved by the addition of a molecule of water, forming two molecules of phosphoglyceric acid. Phosphoglyceric acid is then reduced through an enzymatic reaction that requires the presence of ATP and NADP. · H with the formation of phosphoglyceraldehyde (three-carbon sugar - triose). As a result of the condensation of two such trioses, a hexose molecule is formed, which can be included in a starch molecule and thus stored as a reserve.

To complete this phase of the cycle, photosynthesis absorbs 1 molecule of CO2 and uses 3 molecules of ATP and 4 H atoms (attached to 2 molecules of NAD · N). From hexose phosphate, through certain reactions of the pentose phosphate cycle (Fig. 8), ribulose phosphate is regenerated, which can again attach another carbon dioxide molecule to itself.

None of the described reactions - carboxylation, reduction or regeneration - can be considered specific only to the photosynthetic cell. The only difference they found was that the reduction reaction that converts phosphoglyceric acid to phosphoglyceraldehyde requires NADP. · N, not OVER · N, as usual.

The fixation of CO 2 by ribulose diphosphate is catalyzed by the enzyme ribulose diphosphate carboxylase: Ribulose diphosphate + CO 2 --> 3-Phosphoglycerate Next, 3-phosphoglycerate is reduced with the help of NADP · H 2 and ATP to glyceraldehyde 3-phosphate. This reaction is catalyzed by the enzyme glyceraldehyde-3-phosphate dehydrogenase. Glyceraldehyde 3-phosphate readily isomerizes to dihydroxyacetone phosphate. Both triose phosphates are used in the formation of fructose bisphosphate (the reverse reaction catalyzed by fructose bisphosphate aldolase). Part of the molecules of the resulting fructose bisphosphate participates, together with triose phosphates, in the regeneration of ribulose bisphosphate (closing the cycle), and the other part is used to store carbohydrates in photosynthetic cells, as shown in the diagram.

It is estimated that the synthesis of one molecule of glucose from CO 2 in the Calvin cycle requires 12 NADP · H + H + and 18 ATP (12 ATP molecules are spent on the reduction of 3-phosphoglycerate, and 6 molecules are used in the regeneration reactions of ribulose diphosphate). Minimum ratio - 3 ATP: 2 NADP · N 2.

One can notice the commonality of the principles underlying photosynthetic and oxidative phosphorylation, and photophosphorylation is, as it were, reversed oxidative phosphorylation:

Light energy is the driving force behind phosphorylation and synthesis of organic substances (S-H 2) during photosynthesis and, conversely, the energy of oxidation of organic substances during oxidative phosphorylation. Therefore, it is plants that provide life for animals and other heterotrophic organisms:

Carbohydrates produced during photosynthesis serve to build the carbon skeletons of numerous organic plant substances. Organonitrogen substances are absorbed by photosynthetic organisms by reducing inorganic nitrates or atmospheric nitrogen, and sulfur is absorbed by reducing sulfates to sulfhydryl groups of amino acids. Photosynthesis ultimately ensures the construction of not only proteins, nucleic acids, carbohydrates, lipids, cofactors essential for life, but also numerous secondary synthesis products that are valuable medicinal substances (alkaloids, flavonoids, polyphenols, terpenes, steroids, organic acids, etc. .).

Non-chlorophyll photosynthesis

Non-chlorophyll photosynthesis is found in salt-loving bacteria that have a violet light-sensitive pigment. This pigment turned out to be the protein bacteriorhodopsin, which contains, like the visual purple of the retina - rhodopsin, a derivative of vitamin A - retinal. Bacteriorhodopsin, built into the membrane of salt-loving bacteria, forms a proton potential on this membrane in response to the absorption of light by retinal, which is converted into ATP. Thus, bacteriorhodopsin is a chlorophyll-free converter of light energy.

Photosynthesis and the external environment

Photosynthesis is possible only in the presence of light, water and carbon dioxide. The efficiency of photosynthesis is no more than 20% in cultivated plant species, and usually it does not exceed 6-7%. In the atmosphere there is approximately 0.03% (vol.) CO 2, when its content increases to 0.1%, the intensity of photosynthesis and plant productivity increase, so it is advisable to feed plants with bicarbonates. However, CO 2 content in the air above 1.0% has a harmful effect on photosynthesis. In a year, terrestrial plants alone absorb 3% of the total CO 2 of the Earth’s atmosphere, i.e., about 20 billion tons. Up to 4 × 10 18 kJ of light energy is accumulated in carbohydrates synthesized from CO 2. This corresponds to a power plant capacity of 40 billion kW. A byproduct of photosynthesis, oxygen, is vital for higher organisms and aerobic microorganisms. Preserving vegetation means preserving life on Earth.

Efficiency of photosynthesis

The efficiency of photosynthesis in terms of biomass production can be assessed through the proportion of total solar radiation falling on a certain area over a certain time that is stored in the organic matter of the crop. The productivity of the system can be assessed by the amount of organic dry matter obtained per unit area per year, and expressed in units of mass (kg) or energy (mJ) of production obtained per hectare per year.

The biomass yield thus depends on the area of ​​the solar energy collector (leaves) operating during the year and the number of days per year with such lighting conditions when photosynthesis is possible at the maximum rate, which determines the efficiency of the entire process. The results of determining the proportion of solar radiation (in %) available to plants (photosynthetically active radiation, PAR), and knowledge of the basic photochemical and biochemical processes and their thermodynamic efficiency make it possible to calculate the probable maximum rates of formation of organic substances in terms of carbohydrates.

Plants use light with a wavelength from 400 to 700 nm, i.e. photosynthetically active radiation accounts for 50% of all sunlight. This corresponds to an intensity on the Earth's surface of 800-1000 W/m2 for a typical sunny day (on average). The average maximum efficiency of energy conversion during photosynthesis in practice is 5-6%. These estimates are obtained based on studies of the process of CO 2 binding, as well as associated physiological and physical losses. One mole of bound CO 2 in the form of carbohydrate corresponds to an energy of 0.47 MJ, and the energy of a mole of red light quanta with a wavelength of 680 nm (the most energy-poor light used in photosynthesis) is 0.176 MJ. Thus, the minimum number of moles of red light quanta required to bind 1 mole of CO 2 is 0.47:0.176 = 2.7. However, since the transfer of four electrons from water to fix one CO 2 molecule requires at least eight quanta of light, the theoretical binding efficiency is 2.7:8 = 33%. These calculations are made for red light; It is clear that for white light this value will be correspondingly lower.

Under the best field conditions, the fixation efficiency in plants reaches 3%, but this is only possible during short periods of growth and, if calculated over the entire year, it will be somewhere between 1 and 3%.

In practice, the average annual efficiency of photosynthetic energy conversion in temperate zones is usually 0.5-1.3%, and for subtropical crops - 0.5-2.5%. The yield that can be expected at a given level of sunlight intensity and different photosynthetic efficiency can be easily estimated from the graphs shown in Fig. 9.

The meaning of photosynthesis

• The process of photosynthesis is the basis of nutrition for all living things, and also supplies humanity with fuel, fiber and countless useful chemical compounds.
• About 90-95% of the dry weight of the crop is formed from carbon dioxide and water combined from the air during photosynthesis.
• Humans use about 7% of photosynthetic products as food, animal feed, fuel and building materials.

Photosynthesis occurs in plants (mainly in their leaves) in the light. This is a process in which the organic substance glucose (one of the types of sugars) is formed from carbon dioxide and water. Next, glucose in the cells is converted into a more complex substance, starch. Both glucose and starch are carbohydrates.

The process of photosynthesis not only produces organic matter, but also produces oxygen as a by-product.

Carbon dioxide and water are inorganic substances, while glucose and starch are organic. Therefore, it is often said that photosynthesis is the process of formation of organic substances from inorganic substances in the light. Only plants, some single-celled eukaryotes, and some bacteria are capable of photosynthesis. There is no such process in the cells of animals and fungi, so they are forced to absorb organic substances from the environment. In this regard, plants are called autotrophs, and animals and fungi are called heterotrophs.

The process of photosynthesis in plants occurs in chloroplasts, which contain the green pigment chlorophyll.

So, for photosynthesis to occur, you need:

chlorophyll,

carbon dioxide.

During the process of photosynthesis the following are formed:

organic matter,

oxygen.

Plants are adapted to capture light. In many herbaceous plants, the leaves are collected in a so-called basal rosette, when the leaves do not shade each other. Trees are characterized by a leaf mosaic, in which the leaves grow in such a way as to shade each other as little as possible. In plants, leaf blades can turn towards the light due to the bending of the leaf petioles. With all this, there are shade-loving plants that can only grow in the shade.

Water for photosynthesis enters the leaves from the roots along the stem. Therefore, it is important that the plant receives enough moisture. With a lack of water and certain minerals, the process of photosynthesis is inhibited.

Carbon dioxide for photosynthesis is taken directly from the air by the leaves. Oxygen, which is produced by the plant during photosynthesis, on the contrary, is released into the air. Gas exchange is facilitated by intercellular spaces (spaces between cells).

Organic substances formed during the process of photosynthesis are partly used in the leaves themselves, but mainly flow into all other organs and are converted into other organic substances, used in energy metabolism, and converted into reserve nutrients.