home biology tutor General equation of plant photosynthesis. pathways and energetics of glucose photosynthesis from CO2. starch and cellulose. Calvin cycle. General equation of photosynthesis. The meaning of photosynthesis, its scope. Peculiarities of bacterial photosynthesis Overall reaction equation phot

General equation of plant photosynthesis. pathways and energetics of glucose photosynthesis from CO2. starch and cellulose. Calvin cycle. General equation of photosynthesis. The meaning of photosynthesis, its scope. Peculiarities of bacterial photosynthesis Overall reaction equation phot

Photosynthesis- a biological process that carries out the transfer of electrons along the electron transport chain from one redox system to another.

In the photosynthesis of plants from carbon dioxide and water form carbohydrates:

(total reaction of photosynthesis).

The role of a donor of electrons or hydrogen atoms for the subsequent reduction of CO2 in the process of photosynthesis in plants is played by water. Therefore, the equation describing photosynthesis can be rewritten as

In a comparative study of photosynthesis, it was found that in photosynthetic cells in the role of an electron acceptor

(or hydrogen atoms), in addition to CO 2, in some cases, the nitrate ion, molecular nitrogen, or even hydrogen ions act. In the role of donors of electrons or hydrogen atoms, in addition to water, hydrogen sulfide, isopropyl alcohol, and any other possible donor, depending on the type of photosynthetic cells, can act.

For the implementation of the total photosynthesis reaction, it is necessary to spend energy 2872 kJ / mol. In other words, it is necessary to have a reducing agent with a sufficiently low redox potential. In plant photosynthesis, NADPH + serves as such a reducing agent.

Photosynthesis reactions take place in chloroplast* green plant cells - intracellular organelles similar to mitochondria and also having their own DNA. Internal membrane structures in chloroplasts - thylakoids - contain chlorophyll(a pigment that captures light), as well as all electron carriers. The thylakoid-free space within the chloroplast is called stroma.

In the light-dependent part of photosynthesis, the "light reaction", H 2 0 molecules break down to form protons, electrons, and an oxygen atom. Electrons "excited" by the energy of light reach an energy level sufficient to reduce NADP+. The resulting NADP + H + , as opposed to H 2 0 , is a suitable reducing agent for converting carbon dioxide into an organic compound. If NADPH + H + , ATP and the corresponding enzymes are present in the system, CO 2 fixation can also proceed in the dark; such a process is called tempo reaction.

There are three types of complexes in the thylakoid membrane (Fig. 16.2). The first two are connected by a diffusing electron carrier - plastoquinone (Q), similar in structure to ubiquinone, and the third - a small water-soluble protein - plastocyanin (Rs), also involved in electron transfer. It contains a copper atom, which serves either as a donor or as an electron acceptor (alternately in the Cu + or Cu 2+ state). These three types of complexes are called respectively photosystem II (FS II), cytochrome Y complex/(cyt b/f), consisting of two cytochromes and an iron-sulfur center and carrying out electron transfer from reduced plastoquinone to plastocyanin, and photosystem I (FS I). The numbering of photosystems reflects the order in which they were discovered, and not the order in which they entered the transfer chain.

Rice. 16.2.

The function of this entire apparatus is to carry out the overall reaction

The reaction is accompanied by a large increase in the Gibbs energy entering the system in the form of sunlight: the formation of each NADPH molecule consumes the energy of two absorbed photons.

The energy of photons is directly proportional to the frequency of the incident light and can be calculated from the Einstein formula, which determines the energy E one "mole" of light quanta, equal to 6.023-10 23 quanta (1 Einstein):

Here N- Avogadro's number (6.023-10 23 1/mol); h- Planck's constant (6.626-10 34 J/s); v is the frequency of the incident light, numerically equal to the ratio s/X, where c is the speed of light in vacuum (3.0-10 8 m/s); X- wavelength of light, m; E- energy, J.

When a photon is absorbed, an atom or molecule goes into an excited state with a higher energy. Only photons with a certain wavelength can excite an atom or molecule, since the excitation process is discrete (quantum) in nature. The excited state is extremely unstable, the return to the ground state is accompanied by a loss of energy.

Chlorophyll is the receptor for light absorption in plants. a, the chemical structure of which is shown below.

Chlorophyll is a tetrapyrrole, similar in structure to heme. Unlike heme, the central atom of chlorophyll is magnesium, and one of the side chains contains a long hydrophobic hydrocarbon chain, which “anchor” holds chlorophyll in the lipid bilayer of the thylakoid membrane. Like heme, chlorophyll has a system of conjugated double bonds that determine the appearance of intense color. In green plants, chlorophyll molecules are packed into photosystems consisting of light-trapping chlorophyll molecules, a reaction center, and an electron transport chain.

Chlorophyll in the composition of PS II is designated P 680, and in PS I - P 7 oo (from English, pigment- pigment; number corresponds to the wavelength of maximum absorption of light in nm). Chlorophyll molecules that pump energy into such centers are called antenna. The combination of absorption of these two wavelengths of light by chlorophyll molecules gives a higher rate of photosynthesis than when light is absorbed by each of these wavelengths separately. Photosynthesis in chloroplasts is described by the so-called Z-scheme (from fr. zigzag).

Chlorophyll P 6 8o in the reaction centers of PS II in the dark is in the ground state, without showing any reducing properties. When P 680 receives photon energy from antenna chlorophyll, it goes into an excited state and tends to donate an electron that is in the upper energy level. As a result, this electron acquires the PS II electron carrier, pheophytin (Ph), a pigment similar in structure to chlorophyll, but without Mg 2+ .

Two reduced pheophytin molecules successively donate the resulting electrons to the reduction of plastoquinone, a lipid-soluble electron carrier from PS II to the cytochrome b/f complex.

In the reaction center of PS I, the photon energy captured by antenna chlorophyll also flows onto chlorophyll P700. At the same time, P700 becomes a powerful reducing agent. An electron from excited chlorophyll P 7 oo is transferred along a short chain to ferredoxin(Fd) is a water-soluble stromal protein containing an electron-withdrawing cluster of iron atoms. Ferredoxin via FAD-Dependent Enzyme ferredox-syn-NADP*-reductases restores NADP+ to NADPH.

To return to its original (ground) state, P 7 oo acquires an electron from the reduced plastocyanin:

In PSII, P680+ returns to its original state, receiving an electron from water, since its electron affinity is higher than that of oxygen.

Photosynthesis differs from other biochemical processes in that NADP+ reduction and ATP synthesis occur at the expense of light energy. All further chemical transformations, during which glucose and other carbohydrates are formed, do not fundamentally differ from enzymatic reactions.

The key metabolite is 3-phosphoglycerate, from which carbohydrates are further synthesized in the same way as in the liver, with the only difference that NADPH, and not NADH, serves as a reducing agent in these processes.

The synthesis of 3-phosphoglycerate from carbon dioxide is carried out using the enzyme - ribulose diphosphate carboxylase/oxygenase:

Carboxylase cleaves ribulose-1,5-diphosphate into two molecules of 3-phosphoglycerate and, in doing so, adds one molecule of carbon dioxide.

The addition (fixation) of carbon dioxide occurs in a cyclic process called the Calvin cycle.

The total reaction of the cycle:

During catabolism, this reaction goes in the opposite direction (see Chap. 12).

The sequence of reactions of the Calvin cycle can be represented as follows:

At the 15th stage, the cycle ends and 6-ribulose-1,5-diphosphate enters the 1st stage.

So, during photosynthesis in plants, carbon dioxide enters the carbon skeleton of glucose as a result of a dark reaction with ribulose-1,5-phosphate with the formation of 3-phosphoglycerate (1st stage of the cycle).

AT flora carbohydrates accumulate in large quantities as a reserve nutrient (starch). The polysaccharide starch is formed as a result of the polymerization of glucose obtained in the 8th stage.

The process of converting the radiant energy of 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.

Capturing and converting solar energy is carried out by diverse 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. Approximately half of the work of photosynthesis on Earth is carried out by higher green plants, and the remaining half is mainly by unicellular algae.

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

Development of ideas about photosynthesis

The beginning of the study of photosynthesis was laid in 1630, when van Helmont showed that plants themselves form organic matter rather than getting them from the soil. Weighing the pot of earth in which the willow grew and the tree itself, he showed that within 5 years the mass of the tree increased by 74 kg, while the soil lost only 57 g. Van Helmont came to the conclusion that the plant received the rest of the food from water that was watered on the tree. Now we know that the main material for synthesis is carbon dioxide, which is extracted by the plant from the air.

In 1772, Joseph Priestley showed that the mint shoot "corrects" the air "spoiled" by a burning candle. Seven years later, Jan Ingenhuis discovered that plants can only "correct" bad air when they are in the light, and the ability of plants to "correct" the air is proportional to the clarity of the day and the length of time the plants stay 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 was the experiments of Saussure, carried out in 1804. By weighing the air and plants before and after photosynthesis, Saussure found that the increase in the dry mass of a plant exceeded the mass of carbon dioxide absorbed by it from the air. Saussure came to the conclusion that the other 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

Ingenhus suggested that the role of light in photosynthesis is the breakdown of carbon dioxide; in this case, oxygen is released, and the released "carbon" is used to build plant tissues. 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 dividing the living world was violated when S. N. Vinogradsky in 1887 discovered chemosynthetic bacteria - chlorophyll-free organisms that can assimilate (i.e., convert into organic compounds) carbon dioxide in the dark. It was also violated when, in 1883, Engelman discovered purple bacteria that carry out a kind of photosynthesis that is not accompanied by the release of oxygen. At the time, this fact was not properly 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 found that all cells - plant, bacterial and animal - are able to assimilate carbon dioxide, that is, include it in 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 the process of photosynthesis was made in 1905 by Blackman, who discovered that photosynthesis consists of two successive reactions: a fast light reaction and a series of slower, light-independent steps, which he called the tempo reaction. Using high-intensity light, Blackman showed that photosynthesis proceeds at the same rate under intermittent illumination with flashes of only a fraction of a second, and under continuous illumination, 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 being accompanied by 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 the photolysis of water (H 2 0) with the formation of a reducing agent (H), which participates in the assimilation of carbon dioxide in a certain way, and an oxidizing agent (OH), a hypothetical precursor of molecular oxygen. In bacteria, photosynthesis proceeds in general the same way, but H 2 S or molecular hydrogen serves as a hydrogen donor, and therefore oxygen is not released.

Modern ideas about photosynthesis

By modern ideas 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 an invagination cell membrane, forming lamellar organelles of the mesosome. Isolated mesosomes obtained by the destruction of bacteria are called chromatophores, they contain a light-sensitive apparatus.

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

In the cells of green plants, in addition to chloroplasts, there are also mitochondria, which are used to generate energy at night due to respiration, as in heterotrophic cells.

Chloroplasts are spherical or flattened. They are surrounded by two membranes - outer and inner (Fig. 1). The inner membrane is stacked in the form of stacks of flattened bubble-shaped discs. This stack is called a facet.

Each grana consists of separate 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 seem to be adsorbed between individual layers of molecules and serve to stabilize the structure, which consists of alternating layers of protein and pigments. Such a layered (lamellar) grana structure 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 - up to 50 grains, which are interconnected by membrane bridges. The aqueous medium 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 10 to 20 thylactoids in a grana.

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

	Stages of photosynthesis
	Light stage (or energy)	Dark stage (or metabolic)
Location of the reaction	In the quantosomes of thylactic membranes, it proceeds in the light.	It is carried out outside the thylactoids, in the aquatic environment of the stroma.
Starting products	Light energy, water (H 2 O), ADP, chlorophyll	CO 2, ribulose diphosphate, ATP, NADPH 2
The essence of the process	Photolysis of water, phosphorylation In the light stage of photosynthesis, light energy is transformed into the chemical energy of ATP, and energy-poor water electrons are converted into energy-rich NADP electrons. · H 2 . The by-product formed during the light stage is oxygen. The reactions of the light stage are called "light reactions".	Carboxylation, hydrogenation, dephosphorylation In the dark stage of photosynthesis, "dark reactions" occur in which the reductive synthesis of glucose from CO 2 is observed. Without the energy of the light stage, the dark stage is impossible.
end products	O 2, ATP, NADPH 2 Energy-rich products of the light reaction - ATP and NADP · H 2 is further used in the dark stage of photosynthesis.
The relationship between the light and dark stages can be expressed by the scheme The process of photosynthesis is endergonic, i.e. is accompanied by an increase in free energy, therefore, it requires a significant amount of energy supplied from outside. The overall photosynthesis equation is: 6CO 2 + 12H 2 O ---> C 6 H 12 O 62 + 6H 2 O + 6O 2 + 2861 kJ / mol.

Terrestrial plants absorb the water needed for photosynthesis through their roots, while aquatic plants obtain it by diffusion from the environment. The carbon dioxide necessary for photosynthesis diffuses into the plant through small holes on the surface of the leaves - stomata. Since carbon dioxide is consumed in the process of photosynthesis, its concentration in the cell is usually somewhat lower than in the atmosphere. The oxygen released during photosynthesis diffuses out of the cell, and then out of the plant through the stomata. Sugars formed during photosynthesis also diffuse into those parts of the plant where their concentration is lower.

For 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 content of CO 2 in the air is adjusted to 1-5%.

The mechanism of the light (photochemical) stage of photosynthesis

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. Only chlorophyll a is photochemically active among this complex of pigments. The remaining pigments play an auxiliary 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 thylactic membranes (Fig. 3):

photosystem I (chlorophyll a) - contains pigment 700 (P 700) absorbing light with a wavelength of about 700 nm, plays a major role in the formation of 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 due to water photolysis

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

Light quantum absorbed by a plant

transfers the P 700 pigment 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 that, the pigment molecule, which has lost an electron, can serve as an electron acceptor (capable of accepting an electron) and go into the 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
The photolysis of water is called the Hill reaction. The electrons produced by the decomposition of water are initially accepted by a substance designated Q (sometimes called cytochrome C 550 because of its absorption maximum, although it is not a cytochrome). Then, from substance Q, through a chain of carriers similar in composition to the mitochondrial, 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 the 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 the reason for 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

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

In the first case, there is a closed cyclic electron transport, and in the second - non-cyclic.

Both processes are catalyzed by the same electron carrier chain. However, in cyclic photophosphorylation, electrons are returned from chlorophyll a back to chlorophyll a, whereas in acyclic photophosphorylation, electrons are transferred from chlorophyll b to chlorophyll a.

Cyclic (photosynthetic) phosphorylation

Non-cyclic phosphorylation

As a result of cyclic phosphorylation, the formation of ATP molecules occurs. The process is associated with the return of excited electrons through a series of successive stages to P 700 . The return of excited electrons to P 700 leads to the release of energy (during the transition from a high to a low energy level), which, with the participation of the phosphorylating enzyme system, accumulates in the phosphate bonds of ATP, and does not dissipate in the form of fluorescence and heat (Fig. 4.). This process is called photosynthetic phosphorylation (as opposed to oxidative phosphorylation carried out by mitochondria);

Photosynthetic phosphorylation- the primary reaction of photosynthesis - the mechanism of formation chemical energy(synthesis of ATP from ADP and inorganic phosphate) on the membrane of thylactoids of chloroplasts using the energy of sunlight. Necessary for the dark reaction of CO 2 assimilation

As a result of non-cyclic phosphorylation, NADP + is reduced with the formation of NADP · N. The process is associated with the transfer of an electron to ferredoxin, its reduction and its further transition to NADP +, followed by its reduction to NADP · H

Both processes occur in thylactics, although the second is more complex. It is associated (interrelated) with the work of photosystem II.

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

a+ into the ground state, are apparently formed upon excitation of chlorophyll b. These high energy electrons go to ferredoxin and then through flavoprotein and cytochromes to chlorophyll a. At the last stage, ADP is phosphorylated to ATP (Fig. 5).

Electrons needed to return chlorophyll in its ground state is 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 later give water molecules 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 gaseous oxygen released during photosynthesis comes from water, and not from CO 2 . Water splitting reactions have not yet been studied in detail. It is clear, however, that the implementation of all successive 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 F n and to the release of one oxygen atom. This requires at least four quanta of light - two for each chlorophyll molecule.

Non-cyclic electron flow from H 2 O to NADP · H 2 that occurs during the interaction of two photosystems and the electron transport chains connecting them, is observed despite the values of redox potentials: E ° for 1 / 2O 2 /H 2 O \u003d +0.81 V, and E ° for NADP / NADP · H \u003d -0.32 V. The energy of light reverses the flow of electrons. It is essential that during the transfer from photosystem II to photosystem I, part of the electron energy is accumulated in the form of a proton potential on the thylactoid membrane, and then into the energy of ATP.

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 mechanism of photophosphorylation. Thylactoids are like mitochondria turned inside out, so the direction of electron and proton transfer through the membrane is opposite to its direction in the mitochondrial membrane (Fig. 6). The electrons move to the outside, and the protons are concentrated inside the thylactic matrix. The matrix is charged positively, and the outer membrane of the thylactoide is negatively charged, i.e., the direction of the proton gradient is opposite to its direction in mitochondria.

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

H + -ATP synthetase, designated in chloroplasts as the "СF 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 of the matrix through СF 0 +F 1, and ATP is formed in the active center of F 1 due to the energy of the proton potential.

In contrast to the mitochondrial chain, the thylactoid chain apparently has only two conjugation sites; therefore, the synthesis of one ATP molecule requires three protons instead of two, i.e., the ratio 3 H + / 1 mol 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 the implementation of dark reactions.

Mechanism of the dark stage of photosynthesis

Dark reactions of photosynthesis is the process of incorporating carbon dioxide into organic substances with the formation of carbohydrates (glucose photosynthesis 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, which is also called the pentose phosphate photosynthetic cycle or the Calvin cycle (Fig. 7). It can be divided into three main phases:

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

Ribulose 5-phosphate (a 5-carbon sugar with a phosphate residue at carbon 5) is phosphorylated by ATP to form ribulose diphosphate. This last substance is carboxylated by the addition of CO 2 , apparently to an intermediate six-carbon product, which, however, is immediately cleaved with the addition of a water molecule, forming two molecules of phosphoglyceric acid. Phosphoglyceric acid is then reduced in 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 the starch molecule and thus deposited in reserve.

To complete this phase of the cycle, photosynthesis consumes 1 CO 2 molecule and uses 3 ATP and 4 H atoms (attached to 2 NAD molecules). · N). From hexose phosphate, by 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 for the photosynthetic cell. The only difference found between them is that NADP is required for the reduction reaction, during which phosphoglyceric acid is converted to phosphoglyceraldehyde. · H, not OVER · N, as usual.

The fixation of CO 2 with ribulose diphosphate is catalyzed by the enzyme ribulose diphosphate carboxylase: Ribulose diphosphate + CO 2 --> 3-Phosphoglycerate Further, 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 (a reverse reaction catalyzed by fructose bisphosphate aldolase). Some of the molecules of the resulting fructose bisphosphate are involved, together with triose phosphates, in the regeneration of ribulose diphosphate (they close the cycle), and the other part is used to store carbohydrates in photosynthetic cells, as shown in the diagram.

It is estimated that 12 NADP is required to synthesize one molecule of glucose from CO2 in the Calvin cycle. · H + H + and 18 ATP (12 ATP molecules are spent on the reduction of 3-phosphoglycerate, and 6 molecules in the regeneration reactions of ribulose diphosphate). Minimum ratio - 3 ATP: 2 NADP · H 2 .

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

The energy of light is the driving force of 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 to animals and other heterotrophic organisms:

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

Chlorophilic photosynthesis

Chlorophilic photosynthesis was found in salt-loving bacteria that have a violet light-sensitive pigment. This pigment turned out to be the protein bacteriorhodopsin, which, like the visual purple of the retina - rhodopsin, contains a derivative of vitamin A - retinal. Bacteriorhodopsin, embedded in 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 light energy converter.

Photosynthesis and the environment

Photosynthesis is possible only in the presence of light, water and carbon dioxide. The efficiency of photosynthesis is not more than 20% for cultural species plants, and usually it does not exceed 6-7%. In an atmosphere of about 0.03% (vol.) CO 2, with an increase in its content to 0.1%, the intensity of photosynthesis and plant productivity increase, so it is advisable to feed plants with hydrocarbons. However, the content of CO 2 in the air above 1.0% has a harmful effect on photosynthesis. In a year, only terrestrial plants assimilate 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 the composition of carbohydrates synthesized from CO 2. This corresponds to a power plant capacity of 40 billion kW. A by-product of photosynthesis - oxygen - is vital for higher organisms and aerobic microorganisms. Preserving vegetation means preserving life on Earth.

Photosynthesis efficiency

The efficiency of photosynthesis in terms of biomass production can be estimated through the proportion of total solar radiation falling on a certain area in a certain time, which is stored in the organic matter of the crop. The productivity of the system can be estimated 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 light conditions when photosynthesis is possible at the maximum rate, which determines the efficiency of the entire process. The results of determining the share of solar radiation (in %) available to plants (photosynthetically active radiation, PAR), and knowledge of the main photochemical and biochemical processes and their thermodynamic efficiency, make it possible to calculate the probable limiting rates of formation of organic substances in terms of carbohydrates.

Plants use light with a wavelength of 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 / m 2 for a typical sunny day (on average). The average maximum efficiency of energy conversion during photosynthesis in practice is 5-6%. These estimates are based on the study of the process of CO 2 binding, as well as the accompanying physiological and physical losses. One mole of bound CO 2 in the form of a 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 photons 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, fixation efficiency in plants reaches 3%, but this is only possible in short periods of growth and, if calculated for the whole year, it will be somewhere between 1 and 3%.

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

The Importance of Photosynthesis

The process of photosynthesis is the basis of nutrition for all living beings, and also supplies mankind with fuel, fibers and countless useful chemical compounds.
From the carbon dioxide and water bound from the air during photosynthesis, about 90-95% of the dry weight of the crop is formed.
Man uses about 7% of the products of photosynthesis for food, animal feed, fuel and building materials.

Photosynthesis is a set of synthesis processes organic compounds from inorganic due to the conversion of light energy into the energy of chemical bonds. Green plants belong to phototrophic organisms, 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 Sun rays 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 pigments. They are similar in structure to the heme of hemoglobin, but contain magnesium instead of iron. 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 . The photosynthesis of bacteria has much in common with the photosynthesis of plants. It differs in that in bacteria hydrogen sulfide is the donor, and in plants it is water. Green and purple bacteria do not have photosystem II. Bacterial photosynthesis is not accompanied by the release of oxygen. The overall equation for bacterial photosynthesis is:

6C0 2 + 12H 2 S → C 6 H 12 O 6 + 12S + 6H 2 0.

Photosynthesis is based on a redox process. It is associated with the transfer of electrons from compounds-suppliers of electron-donors to compounds that perceive 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 photosystems – first (I) and second (II) , which have different reaction centers and are interconnected through an electron transport system.

Two phases of photosynthesis

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

Occurs only in the presence of light on the inner membranes of mitochondria in the membranes of special structures - thylakoids . Photosynthetic pigments capture light quanta (photons). This leads to "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 is photosystem I can return to its energy level and restore it. NADP (nicotinamide adenine dinucleotide phosphate) can also be transmitted. Interacting with hydrogen ions, electrons restore this compound. Reduced NADP (NADP H) supplies hydrogen to reduce atmospheric CO 2 to glucose.

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

2Н 2 0 → 4Н + + 0 2 + 2е.

When electrons return from the outer surface of the thylakoid membrane to the previous energy level, energy is released. It is stored in the form of chemical bonds of ATP molecules, which are synthesized during reactions in both photosystems. The process of ATP synthesis 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 NADP H. During the decay (photolysis) of a water molecule, molecular oxygen is released into the atmosphere.

Reactions take place in the internal environment of chloroplasts. They can occur with or without light. Organic substances are synthesized (CO 2 is reduced to glucose) using the energy that was formed in the light phase.

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

The cycle begins with the reaction of atmospheric carbon dioxide with ribulose biphosphate. Enzyme catalyzes the process carboxylase . Ribulose biphosphate is a five-carbon sugar combined with two phosphoric acid residues. There are a number of chemical transformations, each of which catalyzes its own specific enzyme. How is the end product of photosynthesis formed? glucose , and ribulose biphosphate is also reduced.

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 converted into the energy of chemical bonds of synthesized carbohydrates. Energy is transferred along the food chains to heterotrophic organisms. During photosynthesis, carbon dioxide is taken in and oxygen is released. All atmospheric oxygen is of photosynthetic origin. More than 200 billion tons of free oxygen are released annually. Oxygen protects life on Earth from ultraviolet radiation, creating an ozone shield of the atmosphere.

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

Photosynthetic phosphorylation was discovered by D. Arnon et al. and other researchers in experiments with isolated chloroplasts of higher plants and with cell-free preparations from various photosynthetic bacteria and algae. There are two types of photosynthetic phosphorylation during photosynthesis: cyclic and non-cyclic. In both types of photophosphorylation, ATP synthesis from ADP and inorganic phosphate occurs at the stage of electron transfer from cytochrome b6 to cytochrome f.

Synthesis of ATP is carried out with the participation of the ATP-ase complex, "built-in" into the protein-lipid membrane of the thylakoid from its outer side. According to Mitchell's theory, just as in the case of oxidative phosphorylation in mitochondria, the electron transport chain located in the thylakoid membrane functions as a "proton pump", creating a proton concentration gradient. However, in this case, the electron transfer that occurs when light is absorbed causes them to move from outside to inside the thylakoid, and the resulting transmembrane potential (between the inner and outer surfaces of the membrane) is the opposite of that formed in the mitochondrial membrane. Electrostatic energy and proton gradient energy are used for ATP synthesis by ATP synthetase.

In acyclic photophosphorylation, the electrons that came from water and compound Z to photosystem 2, and then to photosystem 1, are directed to the intermediate compound X, and then used to reduce NADP+ to NADPH; their journey ends here. During cyclic photophosphorylation, the electrons that came from photosystem 1 to compound X are sent again to cytochrome b6 and from it further to cytochrome Y, participating at this last stage of their journey in the synthesis of ATP from ADP and inorganic phosphate. Thus, during acyclic photophosphorylation, the movement of electrons is accompanied by the synthesis of ATP and NADPH. In cyclic photophosphorylation, only ATP is synthesized, and NADPH is not formed. ATP formed in the process of photophosphorylation and respiration is used not only in the reduction of phosphoglyceric acid to carbohydrate, but also in other synthetic reactions - in the synthesis of starch, proteins, lipids, nucleic acids and pigments. It also serves as an energy source for the processes of movement, transport of metabolites, maintaining ionic balance, etc.

The role of plastoquinones in photosynthesis

In chloroplasts, five forms of plastoquinones, denoted by the letters A, B, C, D, and E, are derivatives of benzoquinone. For example, plastoquinone A is 2,3-dimethyl-5-solanesylbenzoquinone. Plastoquinones are very similar in structure to ubiquinones (coenzymes Q), which play an important role in the process of electron transfer during respiration. The important role of plastoquinones in the process of photosynthesis follows from the fact that if they are extracted from chloroplasts with petroleum ether, water photolysis and photophosphorylation stop, but resume after the addition of plastoquinones. What are the details of the functional relationship of various pigments and electron carriers involved in the process of photosynthesis - cytochromes, ferredoxin, plastocyanin and plastoquinones - should be shown by further research. In any case, whatever the details of this process, it is now clear that the light phase of photosynthesis leads to the formation of three specific products: NADPH, ATP, and molecular oxygen.

What compounds are formed as a result of the third, dark stage of photosynthesis?

Significant results shedding light on the nature of the primary products formed during photosynthesis have been obtained using the isotope technique. In these studies, barley plants, as well as unicellular green algae Chlorella and Scenedesmus, received carbon dioxide containing labeled radioactive carbon 14C as a carbon source. After extremely short exposure of experimental plants, which ruled out the possibility of secondary reactions, the distribution of isotopic carbon in various products of photosynthesis was studied. It was found that the first product of photosynthesis is phosphoglyceric acid; at the same time, during a very short-term irradiation of plants, along with phosphoglyceric acid, an insignificant amount of phosphoenolpyruvic and malic acids is formed. For example, in experiments with the single-celled green alga Sceriedesmus, after photosynthesis lasting five seconds, 87% of the isotopic carbon was found in phosphoglyceric acid, 10% in phosphoenolpyruvic acid, and 3% in malic acid. Apparently, phosphoenolpyruvic acid is a product of the secondary conversion of phosphoglyceric acid. With longer photosynthesis, lasting 15-60 seconds, radioactive carbon 14C is also found in glycolic acid, triose phosphates, sucrose, aspartic acid, alanine, serine, glycocol, and also in proteins. Later, labeled carbon is found in glucose, fructose, succinic, fumaric and citric acids, as well as in some amino acids and amides (threonine, phenylalanine, tyrosine, glutamine, asparagine). Thus, experiments with the assimilation of carbon dioxide containing labeled carbon by plants showed that the first product of photosynthesis is phosphoglyceric acid.

What substance is carbon dioxide added to during photosynthesis?

The work of M. Calvin, carried out with the help of radioactive carbon 14C, showed that in most plants the compound to which CO2 is attached is ribulose diphosphate. By adding CO2, it gives two molecules of phosphoglyceric acid. The latter is phosphorylated with the participation of ATP with the formation of diphosphoglyceric acid, which, with the participation of NADPH, is reduced and forms phosphoglyceraldehyde, which is partially converted into phosphodioxyacetone. Due to the synthetic action of the enzyme aldolase, phosphoglyceraldehyde and phosphodioxyacetone, when combined, form a molecule of fructose diphosphate, from which sucrose and various polysaccharides are further synthesized. Ribulose diphosphate, a CO2 acceptor, is formed as a result of a series of enzymatic transformations of phosphoglyceraldehyde, phosphodioxyacetone, and fructose diphosphate. Erythrose phosphate, sedoheptulose phosphate, xylulose phosphate, ribose phosphate and ribulose phosphate appear as intermediate products. Enzyme systems that catalyze all these transformations have been found in chlorella cells, in spinach leaves, and in other plants. According to M. Calvin, the process of formation of phosphoglyceric acid from ribulose diphosphate and CO2 is cyclic. The assimilation of carbon dioxide with the formation of phosphoglyceric acid occurs without the participation of light and chlorophyll and is a dark process. The hydrogen in the water is ultimately used to reduce phosphoglyceric acid to phosphoglyceraldehyde. This process is catalyzed by the enzyme phosphoglyceraldehyde dehydrogenase and requires the participation of NADPH as a source of hydrogen. Since this process immediately stops in the dark, it is obvious that the reduction of NADP is carried out by hydrogen formed during the photolysis of water.

Calvin's equation for photosynthesis

The overall equation of the Calvin cycle has the following form:

6CO2 + 12NADPH + 12H+ + 18ATP + 11H2O = fructose-b-phosphate + 12NADP+ + 18ADP + 17P inorg

Thus, for the synthesis of one hexose molecule, six CO2 molecules are required. For the conversion of one CO2 molecule, two NADPH molecules and three ATP molecules (1: 1.5) are needed. Since the ratio of NADPH:ATP formed during non-cyclic photophosphorylation is 1:1, the additional required amount of ATP is synthesized during cyclic photophosphorylation.

The path of carbon in photosynthesis was studied by Calvin at relatively high concentrations of CO2. At lower concentrations approaching atmospheric (0.03%), a significant amount of phosphoglycolic acid is formed in the chloroplast under the action of ribulose diphosphate carboxylase. The latter, in the process of transport through the chloroplast membrane, is hydrolyzed by a specific phosphatase, and the resulting glycolic acid moves from the chloroplast to the associated subcellular structures - peroxisomes, where, under the action of the glycolate oxidase enzyme, it is oxidized to glyoxylic acid HOC-COOH. The latter, by transamination, forms glycine, which, moving into the mitochondria, turns here into serine.

This transformation is accompanied by the formation of CO2 and NH3: 2 glycine + H2O = serine + CO2 + NH3 + 2H+ + 2e-.

However, ammonia is not released into the environment, but is bound in the form of glutamine. Thus, peroxisomes and mitochondria take part in the process of so-called photorespiration, a light-stimulated process of oxygen uptake and CO2 release. This process is associated with the transformation of glycolic acid and its oxidation to CO2. As a result of intense photorespiration, plant productivity can significantly (up to 30%) decrease.

Other possibilities of CO2 assimilation in the process of photosynthesis

Assimilation of CO2 during photosynthesis occurs not only by carboxylation of ribulose diphosphate, but also by carboxylation of other compounds. For example, it has been shown that in sugarcane, corn, sorghum, millet, and a number of other plants, the enzyme phosphoenolpyruvate carboxylase, which synthesizes oxaloacetic acid from phosphoenolpyruvate, CO2, and water, plays a particularly important role in the process of photosynthetic fixation. Plants in which the first product of CO2 fixation is phosphoglyceric acid are commonly called C3 plants, and those in which oxaloacetic acid is synthesized are called C4 plants. The process of photorespiration mentioned above is characteristic of C3 plants and is a consequence of the inhibitory effect of oxygen on ribulose diphosphate carboxylase.

photosynthesis in bacteria

In photosynthetic bacteria, CO2 fixation occurs with the participation of ferredoxin. So, from the photosynthetic bacterium Chromatium, an enzyme system was isolated and partially purified, which, with the participation of ferredoxin, catalyzes the reductive synthesis of pyruvic acid from CO2 and acetylcoenzyme A:

Acetyl-CoA + CO2 + ferredoxin restored. = pyruvate + ferredoxin oxidized. + COA

Similarly, with the participation of ferredoxin in cell-free enzyme preparations isolated from a photosynthetic bacterium Chlorobium thiosulfatophilum, α-ketoglutaric acid is synthesized by carboxylation of succinic acid:

Succinyl-CoA + CO2 + ferredoxin reduced. \u003d a-ketoglutarate + CoA + ferredoxin is oxidized.

In some microorganisms containing bacteriochlorophyll, the so-called purple sulfur bacteria, the process of photosynthesis also occurs in the light. However, in contrast to the photosynthesis of higher plants, in this case, the reduction of carbon dioxide is carried out by hydrogen sulfide. The overall equation for photosynthesis in purple bacteria can be represented as follows:

Light, bacteriochlorophyll: CO2 + 2H2S = CH2O + H2O + 2S

Thus, in this case, too, photosynthesis is a conjugated redox process that occurs under the influence of light energy absorbed by bacteriochlorophyll. From the above equation, it can be seen that as a result of photosynthesis, purple bacteria release free sulfur, which accumulates in them in the form of granules.

Studies using isotopic techniques with the anaerobic photosynthetic purple bacterium Chromatium have shown that at very short photosynthesis times (30 seconds), about 45% of CO2 carbon is incorporated into aspartic acid, and about 28% into phosphoglyceric acid. Apparently, the formation of phosphoglyceric acid precedes the formation aspartic acid, and the earliest product of photosynthesis in Chromatium, as well as in higher plants and unicellular green algae, is ribulose diphosphate. The latter, under the action of ribulose diphosphate carboxylase, adds CO2 to form phosphoglyceric acid. This acid in Chromatium, according to Calvin's scheme, can be partly converted into phosphorylated sugars, and mainly converted into aspartic acid. The formation of aspartic acid occurs by converting phosphoglyceric acid into phosphoenolpyruvic acid, which, undergoing carboxylation, gives oxaloacetic acid; the latter gives aspartic acid by transamination.

Photosynthesis - the source of organic matter on Earth

The process of photosynthesis, which takes place with the participation of chlorophyll, is currently - main source formation of organic matter on Earth.

Photosynthesis to produce hydrogen

It should be noted that unicellular photosynthetic algae under anaerobic conditions release hydrogen gas. Isolated chloroplasts of higher plants, illuminated in the presence of the hydrogenase enzyme catalyzing the reaction 2H+ + 2e- = H2, also release hydrogen. Thus, photosynthetic production of hydrogen as a fuel is possible. This issue, especially in the conditions of the energy crisis, attracts a lot of attention.

A new kind of photosynthesis

W. Stockenius discovered in principle the new kind photosynthesis. It turned out that the bacteria Halobacterium halobium living in concentrated solutions of sodium chloride, the protein-lipid membrane surrounding the protoplasm contains the chromoprotein bacteriorhodopsin, similar to rhodopsin, the visual purple of the animal eye. In bacteriorhodopsin, retinal (the aldehyde form of vitamin A) is bound to a protein with a molecular weight of 26,534 and consists of 247 amino acid residues. By absorbing light, bacteriorhodopsin is involved in the process of converting light energy into chemical energy of high-energy ATP bonds. Thus, an organism that does not contain chlorophyll is able, with the help of bacteriorhodopsin, to use light energy to synthesize ATP and provide the cell with energy.

Parameter name	Meaning
Article subject:	The overall photosynthesis equation
Rubric (thematic category)	Education

Photosynthesis - ϶ᴛᴏ the process of transforming the energy of light absorbed by the body into the chemical energy of organic (and inorganic) compounds.

The process of photosynthesis is expressed by the overall equation:

6CO 2 + 6H 2 O ® C 6 H 12 O 6 + 6O 2.

In the light, in a green plant, organic substances are formed from extremely oxidized substances - carbon dioxide and water, and molecular oxygen is released. In the process of photosynthesis, not only CO 2 is reduced, but also nitrates or sulfates, and the energy must be directed to various endergonic processes, incl. for the transport of substances.

General Equation photosynthesis should be represented as:

12 H 2 O → 12 [H 2] + 6 O 2 ( light reaction)

6 CO 2 + 12 [H 2] → C 6 H 12 O 6 + 6 H 2 O (dark reaction)

6 CO 2 + 12 H 2 O → C 6 H 12 O 6 + 6 H 2 O + 6 O 2

or in terms of 1 mol of CO 2:

CO 2 + H 2 O CH 2 O + O 2

All the oxygen released during photosynthesis comes from water. The water on the right side of the equation cannot be reduced because its oxygen comes from CO 2 . Using the methods of labeled atoms, it was obtained that H 2 O in chloroplasts is heterogeneous and consists of water coming from the external environment and water formed during photosynthesis. Both types of water are used in the process of photosynthesis. Evidence of the formation of O 2 in the process of photosynthesis is the work of the Dutch microbiologist Van Niel, who studied bacterial photosynthesis, and came to the conclusion that the primary photochemical reaction of photosynthesis is the dissociation of H 2 O, and not the decomposition of CO 2. Bacteria (except cyanobacteria) capable of photosynthetic assimilation of CO 2 use H 2 S, H 2, CH 3 and others as reducing agents, and do not emit O 2. This type of photosynthesis is called photoreduction:

CO 2 + H 2 S → [CH 2 O] + H 2 O + S 2 or

CO 2 + H 2 A → [CH 2 O] + H 2 O + 2A,

where H 2 A - oxidizes the substrate, a hydrogen donor (in higher plants - ϶ᴛᴏ H 2 O), and 2A - ϶ᴛᴏ O 2. Then the primary photochemical act in plant photosynthesis should be the decomposition of water into an oxidizing agent [OH] and a reducing agent [H]. [H] restores CO 2, and [OH] participates in the reactions of the release of O 2 and the formation of H 2 O.

Solar energy, with the participation of green plants and photosynthetic bacteria, is converted into free energy of organic compounds. To carry out this unique process, during evolution, a photosynthetic apparatus was created containing: I) a set of photoactive pigments capable of absorbing electromagnetic radiation certain areas of the spectrum and store this energy in the form of electronic excitation energy, and 2) a special apparatus for converting electronic excitation energy into various forms of chemical energy. First of all, this redox energy , associated with the formation of highly reduced compounds, electrochemical potential energy, due to the formation of electrical and proton gradients on the conjugating membrane (Δμ H +), phosphate bond energy of ATP and other macroergic compounds, which is then converted into free energy of organic molecules.

All these types of chemical energy are used in the process of life for the absorption and transmembrane transport of ions and in most metabolic reactions, ᴛ.ᴇ. in a constructive exchange.

The ability to use solar energy and introduce it into biospheric processes determines the ʼʼʼʼʼʼ role of green plants, about which the great Russian physiologist K.A. Timiryazev.

The process of photosynthesis is a very complex system of spatial and temporal organization. The use of high-speed methods of pulsed analysis made it possible to establish that the process of photosynthesis includes reactions of different rates - from 10 -15 s (energy absorption and migration processes occur in the femtosecond time interval) to 10 4 s (formation of photosynthesis products). The photosynthetic apparatus includes structures with sizes from 10 -27 m 3 on the lower molecular level up to 10 5 m 3 at the level of crops.

Concept of photosynthesis. The whole complex set of reactions that make up the process of photosynthesis should be represented by a schematic diagram, which displays the main stages of photosynthesis and their essence. In the modern scheme of photosynthesis, four stages can be distinguished, which differ in the nature and rate of reactions, as well as in the meaning and essence of the processes occurring at each stage:

Stage I - physical. It includes reactions of photophysical nature of the absorption of energy by pigments (P), its storage in the form of electronic excitation energy (P *) and migration to the reaction center (RC). All reactions are extremely fast and proceed at a rate of 10 -15 - 10 -9 s. Primary reactions of energy absorption are localized in light-harvesting antenna complexes (LSCs).

Stage II - photochemical. Reactions are localized in reaction centers and proceed at a rate of 10 -9 s. At this stage of photosynthesis, the energy of the electronic excitation of the pigment (P (RC)) of the reaction center is used to separate charges. In this case, an electron with a high energy potential is transferred to the primary acceptor A, and the resulting system with separated charges (P (RC) - A) contains a certain amount of energy already in chemical form. The oxidized pigment P (RC) restores its structure due to the oxidation of the donor (D).

The transformation of one type of energy into another occurring in the reaction center is the central event of the photosynthesis process, which requires severe conditions for the structural organization of the system. Today, molecular models of reaction centers in plants and bacteria are mostly known. Their similarity in structural organization was established, which indicates a high degree of conservatism of the primary processes of photosynthesis.

The primary products formed at the photochemical stage (P * , A -) are very labile, and the electron can return to the oxidized pigment P * (recombination process) with a useless loss of energy. For this reason, rapid further stabilization of the formed reduced products with a high energy potential is necessary, which is carried out at the next, III stage of photosynthesis.

Stage III - electron transport reactions. A chain of carriers with different redox potential (E n ) forms the so-called electron transport chain (ETC). The redox components of ETC are organized in chloroplasts in the form of three basic functional complexes - photosystem I (PSI), photosystem II (PSII), cytochrome b 6 f-complex, which provides a high speed of the electron flow and the possibility of its regulation. As a result of the work of the ETC, highly reduced products are formed: reduced ferredoxin (PD restore) and NADPH, as well as energy-rich ATP molecules, which are used in the dark reactions of CO 2 reduction that make up the IV stage of photosynthesis.

Stage IV - "dark" reactions of absorption and reduction of carbon dioxide. The reactions take place with the formation of carbohydrates, the end products of photosynthesis, in the form of which the solar energy is stored, absorbed and converted in the "light" reactions of photosynthesis. The speed of ʼʼdarkʼʼ enzymatic reactions is 10 -2 - 10 4 s.

Τᴀᴋᴎᴍ ᴏϬᴩᴀᴈᴏᴍ, the entire course of photosynthesis is carried out through the interaction of three flows - energy flow, electron flow and carbon flow. The conjugation of the three streams requires precise coordination and regulation of their constituent reactions.

The total equation of photosynthesis - the concept and types. Classification and features of the category "Total equation of photosynthesis" 2017, 2018.