Photosynthesis. The meaning of photosynthesis. Light and Dark phases of photosynthesis. Summary equation of photosynthesis Schematic diagram of photosynthesis

1. Give definitions of concepts.
Photosynthesis- the process of formation of organic substances from carbon dioxide and water in the light with the participation of photosynthetic pigments.
Autotrophs- organisms that synthesize organic substances from inorganic ones.
Heterotrophs are organisms that are not capable of synthesizing organic substances from inorganic ones through photosynthesis or chemosynthesis.
Mixotrophs- organisms capable of using various sources of carbon and energy.

2. Fill out the table.

3. Fill out the table.

4. Explain the essence of the statement of the great Russian scientist K. A. Timiryazev: “Log is a canned product of solar energy.”
A log is a part of a tree, its tissues consist of accumulated organic compounds (cellulose, sugar, etc.) that were formed during the process of photosynthesis.

5. Write the overall equation for photosynthesis. Do not forget to indicate the required conditions for the reactions to occur.

12. Choose a term and explain how it is modern meaning corresponds to the original meaning of its roots.
The chosen term is mixotrophs.
Correspondence. The term is clarified; this is the name given to organisms with a mixed type of nutrition that are capable of using different sources of carbon and energy.

13. Formulate and write down the main ideas of § 3.3.
According to the type of nutrition, all living organisms are divided into:
Autotrophs that synthesize organic substances from inorganic ones.
Heterotrophs that feed on ready-made organic substances.
Mixotrophs with mixed nutrition.
Photosynthesis is the process of formation of organic substances from carbon dioxide and water in the light with the participation of photosynthetic pigments by phototrophs.
It is divided into a light phase (water and H+ molecules necessary for the dark phase are formed, and oxygen is also released) and a dark phase (glucose is formed). The overall equation for photosynthesis is: 6CO2 + 6H2O → C6H12O6 + 6O2. It occurs in the light in the presence of chlorophyll. This is how light energy turns into
energy chemical bonds, and plants produce glucose and sugar for themselves.

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

The process of photosynthesis is expressed by the summary equation:

6СО 2 + 6Н 2 О ® С 6 Н 12 О 6 + 6О 2 .

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

The general equation for photosynthesis should be presented 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 per 1 mole of CO 2:

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

All the oxygen released during photosynthesis comes from water. Water on the right side of the equation cannot be reduced because its oxygen comes from CO 2 . Using labeled atom methods, it was found that H2O in chloroplasts is heterogeneous and consists of water coming from 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 comes from the work of the Dutch microbiologist Van Niel, who studied bacterial photosynthesis and came to the conclusion that the primary photochemical reaction of photosynthesis consists of 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 release O 2. This type of photosynthesis is commonly called photo reduction:

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] reduces CO 2, and [OH] participates in the reactions of O 2 release and H 2 O formation.

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, a photosynthetic apparatus was created during evolution, containing: I) a set of photoactive pigments capable of absorbing electromagnetic radiation certain regions 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 different shapes chemical energy. First of all this redox energy , associated with the formation of highly reduced compounds, electrochemical potential energy, caused by the formation of electrical and proton gradients on the coupling membrane (Δμ H +), ATP phosphate bond energy and other high-energy 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 biosphere processes determines the “cosmic” role of green plants, which the great Russian physiologist K.A. wrote about. Timiryazev.

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

Schematic diagram of photosynthesis. The entire complex set of reactions that make up the process of photosynthesis should be represented by a schematic diagram that shows 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. Includes photophysical in nature reactions of energy absorption by pigments (R), its storage in the form of electronic excitation energy (R*) and migration to the reaction center (RC). All reactions are extremely fast and proceed at a speed of 10 -15 - 10 -9 s. Primary energy absorption reactions are localized in light-harvesting antenna complexes (LACs).

Stage II - photochemical. The reactions are localized in reaction centers and proceed at a speed of 10 -9 s. At this stage of photosynthesis, the energy of 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. Oxidized pigment P (RC) restores its structure due to oxidation of the donor (D).

The conversion of one type of energy into another occurring in the reaction center is the central event of the photosynthesis process, requiring stringent conditions structural organization systems. Today, molecular models of the reaction centers of plants and bacteria are largely known. Their similarity in structural organization has been established, which indicates a high degree of conservatism of the primary processes of photosynthesis.

The primary products (P *, A -) formed at the photochemical stage 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 high energy potential, which occurs at the next, stage III of photosynthesis.

Stage III - electron transport reactions. A chain of carriers with different redox potentials (E n ) forms the so-called electron transport chain (ETC). The redox components of the 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 electron flow and the possibility of its regulation. As a result of the operation of the ETC, highly reduced products are formed: reduced ferredoxin (FD reduced) and NADPH, as well as energy-rich ATP molecules, which are used in the dark reactions of CO 2 reduction, which make up the fourth stage of photosynthesis.

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

However, the entire course of photosynthesis occurs through the interaction of three flows - the flow of energy, the flow of electrons and the flow of carbon. The coupling of the three flows requires clear coordination and regulation of their constituent reactions.

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

Photosynthesis is a set of processes for the synthesis of organic compounds from inorganic ones due to the conversion of light energy into the energy of chemical bonds. Phototrophic organisms include green plants, some prokaryotes - cyanobacteria, purple and green sulfur bacteria, and 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 the organic compounds they synthesize. 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 hemoglobin, but contain magnesium instead of iron. Iron content is necessary to ensure the synthesis of chlorophyll molecules. There are several chlorophylls that differ in their chemical structure. Mandatory for all phototrophs is chlorophyll a . Chlorophyllb found in green plants chlorophyll c – in diatoms and brown algae. Chlorophyll d characteristic of red algae.

Green and purple photosynthetic bacteria have special bacteriochlorophylls . Bacterial photosynthesis has much in common with plant photosynthesis. It differs in that in bacteria the hydrogen donor is hydrogen sulfide, 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 the redox process. It is associated with the transfer of electrons from compounds that supply electrons-donors to compounds that accept them - acceptors. Light energy is converted into the energy of synthesized organic compounds (carbohydrates).

There are special structures on the membranes of chloroplasts - reaction centers that contain chlorophyll. In green plants and cyanobacteria there are two photosystems – first (I) And second (II) , which have different reaction centers and are interconnected through an electron transfer 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 internal membranes of mitochondria in the membranes of special structures - thylakoids . Photosynthetic pigments capture light quanta (photons). This leads to the “excitation” of one of the electrons of the chlorophyll molecule. With the help of carrier molecules, the electron moves to the outer surface of the thylakoid membrane, acquiring a certain potential energy.

This electron in photosystem I can return to its energy level and restore it. NADP (nicotinamide adenine dinucleotide phosphate) may also be transmitted. By interacting with hydrogen ions, electrons restore this compound. Reduced NADP (NADP H) supplies hydrogen to reduce atmospheric CO 2 to glucose.

Similar processes occur in photosystem II . Excited electrons can be transferred to photosystem I and restore it. The restoration of photosystem II occurs due to electrons supplied by water molecules. Water molecules split (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 from the outer surface of the thylakoid membrane return 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 breakdown (photolysis) of water molecules, molecular oxygen is released into the atmosphere.

Reactions take place in internal environment chloroplasts. They can occur both in the presence of light and without it. Organic substances are synthesized (C0 2 is reduced to glucose) using the energy that was formed in the light phase.

The process of carbon dioxide reduction is cyclical 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. The process is catalyzed by an enzyme carboxylase . Ribulose biphosphate is a five-carbon sugar combined with two phosphoric acid units. A number of chemical transformations occur, each of which is catalyzed by its own specific enzyme. How is the end product of photosynthesis formed? glucose , and ribulose biphosphate is also reduced.

The overall equation for the process of photosynthesis is:

6C0 2 + 6H 2 0 → C 6 H 12 O 6 + 60 2

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

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

Photosynthesis- a biological process that transfers electrons along the electron transport chain from one redox system to another.

During plant photosynthesis, carbohydrates are formed from carbon dioxide and water:

(total reaction of photosynthesis).

Water plays the role of a donor of electrons or hydrogen atoms for the subsequent reduction of CO2 in the process of photosynthesis in plants. Therefore, the equation describing photosynthesis can be rewritten as

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

(or hydrogen atoms), in addition to C0 2, in some cases there are nitrate ions, molecular nitrogen, or even hydrogen ions. In addition to water, hydrogen sulfide, isopropyl alcohol and any other possible donor, depending on the type of photosynthetic cells, can act as donors of electrons or hydrogen atoms.

To carry out the total reaction of photosynthesis, it is necessary to expend energy of 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 occur in chloroplast* green plant cells - intracellular organelles similar to mitochondria and also having their own DNA. Internal membrane structures of chloroplasts - thylakoids - contain chlorophyll(light-trapping pigment), as well as all electron carriers. The space inside the chloroplast free of thylakoids is called stroma.

In the light-dependent part of photosynthesis, the “light reaction,” the splitting of H 2 0 molecules occurs to form protons, electrons and an oxygen atom. Electrons "excited" by light energy reach energy levels 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 occur in the dark; this process is called tempo reaction.

The thylakoid membrane contains three types of complexes (Fig. 16.2). The first two are connected by a diffusible 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 acceptor of electrons (alternately in the Cu + or Cu 2+ state). These three types of complexes are called respectively photosystem II (FS II), cytochrome b complex(cit b/f), consisting of two cytochromes and an iron-sulfur center and transferring electrons from reduced plastoquinone to plastocyanin, and photosystem I (FS I). The numbering of photosystems reflects the order of their discovery, and not the order of entry into 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 energy of two absorbed quanta is spent on the formation of each NADPH molecule.

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

Here N- Avogadro 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 agricultural, where c is the speed of light in vacuum (3.0-10 8 m/sec); X- light wavelength, m; E- energy, J.

When a photon is absorbed, an atom or molecule goes into an excited state with 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; returning to the ground state is accompanied by a loss of energy.

In plants, the receptor that absorbs light is the chlorophyll molecule. A, the chemical structure of which is given 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 that “anchors” 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 packaged into photosystems consisting of light-trapping chlorophyll molecules, a reaction center, and an electron transport chain.

Chlorophyll in PS II is designated P 680, and in PS I it is P 7 oo (from English, pigment- pigment; the number corresponds to the wavelength of maximum light absorption in nm). Chlorophyll molecules that pump energy into such centers are called antennas. The combination of absorption of light of these two wavelengths by chlorophyll molecules gives a higher rate of photosynthesis than when light of each of these wavelengths is absorbed separately. Photosynthesis in chloroplasts is described by the so-called Z-scheme (from the French. 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 the antenna chlorophyll, it goes into an excited state and tends to give away an electron that is in the upper energy level. As a result, this electron is acquired by the PSII electron carrier - pheophytin (Ph), a pigment similar in structure to chlorophyll, but not containing Mg 2+.

Two reduced pheophytin molecules sequentially 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, photon energy captured by the antenna chlorophyll also flows onto chlorophyll P700. In this case, 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 by FAD-dependent enzyme ferredoxin-NADP*-reductase reduces NADP+ to NADPH.

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

In PS II, Pb80+ 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 the reduction of NADP + and the synthesis of ATP occur due to light energy. All further chemical transformations, during which glucose and other carbohydrates are formed, are not fundamentally different 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 the reducing agent in these processes.

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

Carboxylase breaks down ribulose-1,5-bisphosphate into two molecules of 3-phosphoglycerate and in the process adds one molecule of carbon dioxide.

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

Total cycle reaction:

During catabolism, this reaction occurs in the opposite direction (see Chapter 12).

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

At the 15th stage, the cycle is completed 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 to form 3-phosphoglycerate (1st stage of the cycle).

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

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 found 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, i.e., 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 following hypothesis is regarding chemical basis photosynthesis was put forward by van Niel, who in 1931 experimentally showed that in bacteria photosynthesis 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

By modern ideas The essence of photosynthesis is the conversion of radiant energy from 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 vital role in photosynthesis, capturing energy from 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 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.

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 last substance carboxylates by the addition of CO 2 , apparently to a six-carbon intermediate, which, however, is immediately cleaved by the addition of a water molecule, forming two molecules of phosphoglyceric acid. Phosphoglyceric acid is then reduced during enzymatic reaction, the implementation of which 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 driving force 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 on this membrane in response to the absorption of light by retinal proton potential, converted to 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 share of total solar radiation, falling on a certain area in a certain time, which is stored in organic matter harvest. 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.