Light and dark phases. The process of photosynthesis: brief and understandable for children. Photosynthesis: light and dark phases. What is the importance of water for plants?

Photosynthesis- synthesis of organic compounds from inorganic ones using light energy (hv). The overall equation for photosynthesis is:

6CO 2 + 6H 2 O → C 6 H 12 O 6 + 6O 2

Photosynthesis occurs with the participation of photosynthetic pigments, which have the unique property of converting the energy of sunlight into chemical bond energy in the form of ATP. Photosynthetic pigments are protein-like substances. The most important of them is the pigment chlorophyll. In eukaryotes, photosynthetic pigments are embedded in the inner membrane of plastids; in prokaryotes, they are embedded in invaginations of the cytoplasmic membrane.

The structure of the chloroplast is very similar to the structure of the mitochondrion. The inner membrane of the grana thylakoids contains photosynthetic pigments, as well as electron transport chain proteins and ATP synthetase enzyme molecules.

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

Light phase Photosynthesis occurs only in the light in the grana thylakoid membrane. In this phase, chlorophyll absorbs light quanta, produces an ATP molecule, and photolysis of water.

Under the influence of a light quantum (hv), chlorophyll loses electrons, passing into an excited state:

Chl → Chl + e -

These electrons are transferred by carriers to the outside, i.e. the surface of the thylakoid membrane facing the matrix, where they accumulate.

At the same time, photolysis of water occurs inside the thylakoids, i.e. its decomposition under the influence of light

2H 2 O → O 2 +4H + + 4e —

The resulting electrons are transferred by carriers to chlorophyll molecules and restore them: the chlorophyll molecules return to a stable state.

Hydrogen protons formed during photolysis of water accumulate inside the thylakoid, creating an H + reservoir. As a result, the inner surface of the thylakoid membrane is charged positively (due to H +), and the outer surface is charged negatively (due to e -). As oppositely charged particles accumulate on both sides of the membrane, the potential difference increases. When the potential difference reaches a critical value, the electric field force begins to push protons through the ATP synthetase channel. The energy released in this case is used to phosphorylate ADP molecules:

ADP + P → ATP

The formation of ATP during photosynthesis under the influence of light energy is called photophosphorylation.

Hydrogen ions, once on the outer surface of the thylakoid membrane, meet electrons there and form atomic hydrogen, which binds to the hydrogen carrier molecule NADP (nicotinamide adenine dinucleotide phosphate):

2H + + 4e - + NADP + → NADP H 2

Thus, during the light phase of photosynthesis, three processes occur: the formation of oxygen due to the decomposition of water, the synthesis of ATP, and the formation of hydrogen atoms in the form of NADP H2. Oxygen diffuses into the atmosphere, ATP and NADP H2 participate in the processes of the dark phase.

Dark phase photosynthesis occurs in the chloroplast matrix both in the light and in the dark and represents a series of sequential transformations of CO 2 coming from the air in the Calvin cycle. Dark phase reactions are carried out using the energy of ATP. In the Calvin cycle, CO 2 bonds with hydrogen from NADP H 2 to form glucose.

In the process of photosynthesis, in addition to monosaccharides (glucose, etc.), monomers of other organic compounds are synthesized - amino acids, glycerol and fatty acids. Thus, thanks to photosynthesis, plants provide themselves and all living things on Earth with the necessary organic substances and oxygen.

Comparative characteristics photosynthesis and respiration of eukaryotes is given in the table:

• occurs only with the participation of sunlight;
• in prokaryotes, the light phase occurs in the cytoplasm; in eukaryotes, reactions occur on the membranes of the chloroplast granules, where chlorophyll is located;
• Due to the energy of sunlight, ATP molecules (adenosine triphosphate) are formed, in which it is stored.

Reactions occurring in the light phase

A necessary condition for the light phase of photosynthesis to begin is the presence of sunlight. It all starts with the fact that a photon of light hits chlorophyll (in chloroplasts) and transfers its molecules to an excited state. This happens because an electron in the pigment, having caught a photon of light, moves to a higher energy level.

Then this electron, passing through a chain of carriers (they are proteins located in the membranes of the chloroplast), gives off excess energy to the reaction of ATP synthesis.

ATP is a very convenient molecule for storing energy. It refers to high-energy compounds - these are substances whose hydrolysis releases a large number of energy.

The ATP molecule is also convenient in that energy can be released from it in two stages: separating one phosphoric acid residue at a time, each time receiving a portion of energy. It goes further to meet any needs of the cell and the body as a whole.

Water splitting

The light phase of photosynthesis allows us to obtain energy from sunlight. She goes not only to ATP formation, and also for water splitting:

This process is also called photolysis (photo - light, lysis - split). As you can see, oxygen is eventually released, which allows all animals and plants to breathe.

Protons are used to form NADP-H, which will be used in the dark phase as a source of these same protons.

And the electrons formed during photolysis of water will compensate chlorophyll for its losses at the very beginning of the chain. Thus, everything falls into place and the system is again ready to absorb the next photon of light.

Light phase value

Plants are autotrophs - organisms that are able to obtain energy not from the breakdown of finished substances, but create it independently, using only light, carbon dioxide and water. That is why they are producers in the food chain. Animals, unlike plants, cannot perform photosynthesis in their cells.

The mechanism of photosynthesis - video

Photosynthesis consists of two phases - light and dark.

In the light phase, light quanta (photons) interact with chlorophyll molecules, as a result of which these molecules move into a more energy-rich “excited” state for a very short time. The excess energy of some of the “excited” molecules is then converted into heat or emitted as light. Another part of it is transferred to hydrogen ions, which are always present in an aqueous solution due to the dissociation of water. The resulting hydrogen atoms are loosely combined with organic molecules - hydrogen carriers. Hydroxide ions "OH" give up their electrons to other molecules and turn into free radicals OH. OH radicals interact with each other, resulting in the formation of water and molecular oxygen:

4OH = O2 + 2H2O Thus, the source of molecular oxygen formed during photosynthesis and released into the atmosphere is photolysis - the decomposition of water under the influence of light. In addition to photolysis of water, the energy of solar radiation is used in the light phase for the synthesis of ATP and ADP and phosphate without the participation of oxygen. This is a very efficient process: chloroplasts produce 30 times more ATP than in the mitochondria of the same plants with the participation of oxygen. In this way, the energy necessary for processes in the dark phase of photosynthesis is accumulated.

In complex chemical reactions In the dark phase, for which light is not required, the key place is occupied by the binding of CO2. These reactions involve ATP molecules synthesized during the light phase and hydrogen atoms formed during the photolysis of water and associated with carrier molecules:

6СО2 + 24Н -» С6Н12О6 + 6НО

This is how the energy of sunlight is converted into the energy of chemical bonds of complex organic compounds.

87. The importance of photosynthesis for plants and for the planet.

Photosynthesis is the main source of biological energy; photosynthetic autotrophs use it to synthesize organic substances from inorganic ones; heterotrophs exist at the expense of the energy stored by autotrophs in the form of chemical bonds, releasing it in the processes of respiration and fermentation. The energy obtained by humanity by burning fossil fuels (coal, oil, natural gas, peat) is also stored in the process of photosynthesis.

Photosynthesis is the main input of inorganic carbon into the biological cycle. All free oxygen in the atmosphere is of biogenic origin and is a by-product of photosynthesis. The formation of an oxidizing atmosphere (oxygen catastrophe) completely changed the state of the earth's surface, made the appearance of respiration possible, and later, after the formation of the ozone layer, allowed life to reach land. The process of photosynthesis is the basis of nutrition for all living things, and also supplies humanity with fuel (wood, coal, oil), fiber (cellulose) 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. The remaining 5-10% comes from mineral salts and nitrogen obtained from the soil.

Humans use about 7% of the products of photosynthesis as food, as animal feed, and in the form of fuel and building materials.

Photosynthesis, which is one of the most common processes on Earth, determines the natural cycles of carbon, oxygen and other elements and provides the material and energy basis for life on our planet. Photosynthesis is the only source of atmospheric oxygen.

Photosynthesis is one of the most common processes on Earth; it determines the cycle of carbon, O2 and other elements in nature. It forms the material and energetic basis of all life on the planet. Every year, as a result of photosynthesis, about 8 1010 tons of carbon are bound in the form of organic matter, and up to 1011 tons of cellulose are formed. Thanks to photosynthesis, land plants produce about 1.8 1011 tons of dry biomass per year; approximately the same amount of plant biomass is formed annually in the oceans. Tropical forest contributes up to 29% to the total photosynthetic production of land, and the contribution of forests of all types is 68%. Photosynthesis of higher plants and algae is the only source of atmospheric O2. The emergence on Earth about 2.8 billion years ago of the mechanism of water oxidation with the formation of O2 is the most important event in biological evolution, making the light of the Sun the main source of free energy in the biosphere, and water an almost unlimited source of hydrogen for the synthesis of substances in living organisms. As a result, an atmosphere of modern composition was formed, O2 became available for the oxidation of food, and this led to the emergence of highly organized heterotrophic organisms (using exogenous organic substances as a carbon source). The total storage of solar radiation energy in the form of photosynthesis products is about 1.6 1021 kJ per year, which is approximately 10 times higher than the modern energy consumption of humanity. Approximately half of the solar radiation energy is in the visible region of the spectrum (wavelength l from 400 to 700 nm), which is used for photosynthesis (physiologically active radiation, or PAR). IR radiation is not suitable for photosynthesis of oxygen-producing organisms (higher plants and algae), but is used by some photosynthetic bacteria.

Discovery of the chemosynthesis process by S.N. Vinogradsky. Characteristics of the process.

Chemosynthesis is the process of synthesis of organic substances from carbon dioxide, which occurs due to the energy released during the oxidation of ammonia, hydrogen sulfide and other chemicals during the life of microorganisms. Chemosynthesis also has another name - chemolithoautotrophy. The discovery of chemosynthesis by S. N. Vinogradovsky in 1887 radically changed the understanding of science about the types of metabolism that are basic for living organisms. Chemosynthesis is the only type of nutrition for many microorganisms, since they are able to assimilate carbon dioxide as the only source of carbon. Unlike photosynthesis, chemosynthesis uses energy that is generated as a result of redox reactions instead of light energy.

This energy should be sufficient for the synthesis of adenosine triphosphoric acid (ATP), and its amount should exceed 10 kcal/mol. Some of the oxidized substances donate their electrons to the chain already at the cytochrome level, and thus additional energy consumption is created for the synthesis of the reducing agent. During chemosynthesis, the biosynthesis of organic compounds occurs due to the autotrophic assimilation of carbon dioxide, that is, in exactly the same way as during photosynthesis. As a result of the transfer of electrons through the chain of bacterial respiratory enzymes, which are built into the cell membrane, energy is obtained in the form of ATP. Due to the very high energy consumption, all chemosynthesizing bacteria, except for hydrogen ones, form quite a small amount of biomass, but at the same time they oxidize a large volume of inorganic substances. Hydrogen bacteria are used by scientists to produce protein and clean the atmosphere from carbon dioxide, especially necessary in closed ecological systems. There is a great variety of chemosynthetic bacteria, most of them belong to pseudomonads, they are also found among filamentous and budding bacteria, leptospira, spirilla and corynebacteria.

Examples of the use of chemosynthesis by prokaryotes.

The essence of chemosynthesis (a process discovered by Russian researcher Sergei Nikolaevich Vinogradsky) is the body’s production of energy through redox reactions carried out by the body itself with simple (inorganic) substances. Examples of such reactions can be the oxidation of ammonium to nitrite, or divalent iron to ferric, hydrogen sulfide to sulfur, etc. Only certain groups of prokaryotes (bacteria in the broad sense of the word) are capable of chemosynthesis. Due to chemosynthesis, currently there are only ecosystems of some hydrothermal sites (places on the ocean floor where there are outlets of hot underground waters rich in reduced substances - hydrogen, hydrogen sulfide, iron sulfide, etc.), as well as extremely simple ones, consisting only of bacteria , ecosystems found at great depths in rock faults on land.

Bacteria are chemosynthetics, destroy rocks, purify wastewater, and participate in the formation of minerals.

Topic 3 Stages of photosynthesis

Section 3 Photosynthesis

1. Light phase of photosynthesis

2. Photosynthetic phosphorylation

3.Ways of CO 2 fixation during photosynthesis

4.Photobreathing

The essence of the light phase of photosynthesis is the absorption of radiant energy and its transformation into assimilative force (ATP and NADP-H), necessary for the reduction of carbon in dark reactions. The complexity of the processes of converting light energy into chemical energy requires their strict membrane organization. The light phase of photosynthesis occurs in the grana of the chloroplast.

Thus, the photosynthetic membrane carries out a very important reaction: it converts the energy of absorbed light quanta into the redox potential of NADP-H and into the potential of the reaction of transfer of the phosphoryl group in the ATP molecule. In this case, energy is converted from a very short-lived form to a fairly long-lived form. The stabilized energy can later be used in the biochemical reactions of the plant cell, including reactions leading to the reduction of carbon dioxide.

Five major polypeptide complexes are embedded in the inner membranes of chloroplasts: photosystem I complex (PSI), photosystem II complex (PSII), light harvesting complex II (LHCII), cytochrome b 6 f complex And ATP synthase (CF 0 – CF 1 complex). The PSI, PSII and CCKII complexes contain pigments (chlorophylls, carotenoids), most of which function as antenna pigments that collect energy for the pigments of the PSI and PSII reaction centers. PSI and PSII complexes, as well as cytochrome b 6 f-complex contain redox cofactors and participate in photosynthetic electron transport. The proteins of these complexes are distinguished by a high content of hydrophobic amino acids, which ensures their integration into the membrane. ATP synthase ( CF 0 – CF 1-complex) carries out the synthesis of ATP. In addition to large polypeptide complexes, thylakoid membranes contain small protein components - plastocyanin, ferredoxin And ferredoxin-NADP oxidoreductase, located on the surface of the membranes. They are part of the electron transport system of photosynthesis.

The following processes occur in the light cycle of photosynthesis: 1) photoexcitation of photosynthetic pigment molecules; 2) migration of energy from the antenna to the reaction center; 3) photo-oxidation of a water molecule and the release of oxygen; 4) photoreduction of NADP to NADP-H; 5) photosynthetic phosphorylation, ATP formation.

Chloroplast pigments are combined into functional complexes - pigment systems, in which the reaction center is chlorophyll A, Carrying out photosensitization, it is connected by energy transfer processes with an antenna consisting of light-harvesting pigments. The modern scheme of photosynthesis in higher plants includes two photochemical reactions carried out with the participation of two different photosystems. The assumption of their existence was made by R. Emerson in 1957 based on the effect he discovered of enhancing the action of long-wave red light (700 nm) by combined illumination with shorter-wave rays (650 nm). Subsequently, it was found that photosystem II absorbs shorter wavelength rays compared to PSI. Photosynthesis occurs efficiently only when they function together, which explains the Emerson amplification effect.

PSI contains a chlorophyll dimer as a reaction center and with maximum light absorption of 700 nm (P 700), as well as chlorophylls A 675-695, playing the role of an antenna component. The primary electron acceptor in this system is the monomeric form of chlorophyll A 695, secondary acceptors are iron-sulfur proteins (-FeS). The PSI complex, under the influence of light, reduces the iron-containing protein - ferredoxin (Fd) and oxidizes the copper-containing protein - plastocyanin (Pc).

PSII includes a reaction center containing chlorophyll A(P 680) and antenna pigments - chlorophylls A 670-683. The primary electron acceptor is pheophytin (Ph), which transfers electrons to plastoquinone. PSII also includes the S-system protein complex, which oxidizes water, and the electron transporter Z. This complex functions with the participation of manganese, chlorine and magnesium. PSII reduces plastoquinone (PQ) and oxidizes water, releasing O2 and protons.

The link between PSII and PSI is the plastoquinone fund, a protein cytochrome complex b 6 f and plastocyanin.

In plant chloroplasts, each reaction center contains approximately 300 pigment molecules, which are part of the antenna or light-harvesting complexes. A light-harvesting protein complex containing chlorophylls has been isolated from chloroplast lamellae A And b and carotenoids (CCC), closely related to PSP, and antenna complexes directly included in PSI and PSII (focusing antenna components of photosystems). Half of the thylakoid protein and about 60% of the chlorophyll are localized in the SSC. Each SSC contains from 120 to 240 chlorophyll molecules.

The antenna protein complex PS1 contains 110 chlorophyll molecules a 680-695 for one R 700 , of these, 60 molecules are components of the antenna complex, which can be considered as the SSC PSI. The PSI antenna complex also contains b-carotene.

The PSII antenna protein complex contains 40 chlorophyll molecules A with an absorption maximum of 670-683 nm per P 680 and b-carotene.

Chromoproteins of antenna complexes do not have photochemical activity. Their role is to absorb and transfer quantum energy to a small number of molecules of the reaction centers P 700 and P 680, each of which is associated with an electron transport chain and carries out a photochemical reaction. The organization of electron transport chains (ETC) for all chlorophyll molecules is irrational, since even in direct sunlight, light quanta hit the pigment molecule no more than once every 0.1 s.

Physical mechanisms processes of absorption, storage and migration of energy chlorophyll molecules have been studied quite well. Photon absorption(hν) is due to the transition of the system to various energy states. In a molecule, unlike an atom, electronic, vibrational and rotational movements are possible, and the total energy of the molecule is equal to the sum of these types of energies. The main indicator of the energy of an absorbing system is the level of its electronic energy, determined by the energy of external electrons in orbit. According to the Pauli principle, there are two electrons with oppositely directed spins in the outer orbit, resulting in the formation of a stable system of paired electrons. The absorption of light energy is accompanied by the transition of one of the electrons to a higher orbit with the storage of the absorbed energy in the form of electronic excitation energy. The most important characteristic of absorbing systems is the selectivity of absorption, determined by the electronic configuration of the molecule. In a complex organic molecule there is a certain set of free orbits into which an electron can transfer when absorbing light quanta. According to Bohr's "frequency rule", the frequency of absorbed or emitted radiation v must strictly correspond to the energy difference between the levels:

ν = (E 2 – E 1)/h,

where h is Planck's constant.

Each electronic transition corresponds to a specific absorption band. Thus, the electronic structure of the molecule determines the nature of the electronic vibrational spectra.

Storage of absorbed energy associated with the appearance of electronically excited states of pigments. The physical regularities of the excited states of Mg-porphyrins can be considered based on an analysis of the electronic transition scheme of these pigments (figure).

There are two main types of excited states - singlet and triplet. They differ in energy and electron spin state. In a singlet excited state, the electron spins at the ground and excited levels remain antiparallel; upon transition to the triplet state, the spin of the excited electron rotates with the formation of a biradical system. When a photon is absorbed, the chlorophyll molecule passes from the ground state (S 0) to one of the excited singlet states - S 1 or S 2 , which is accompanied by the transition of an electron to an excited level with a higher energy. The excited state of S2 is very unstable. The electron quickly (within 10 -12 s) loses some of its energy in the form of heat and falls to the lower vibrational level S 1, where it can remain for 10 -9 s. In the S 1 state, an electron spin reversal can occur and a transition to the T 1 triplet state, the energy of which is lower than S 1 .

There are several possible ways to deactivate excited states:

· emission of a photon with the transition of the system to the ground state (fluorescence or phosphorescence);

transfer of energy to another molecule;

· use of excitation energy in a photochemical reaction.

Energy Migration between pigment molecules can occur through the following mechanisms. Inductive resonance mechanism(Förster mechanism) is possible provided that the electron transition is optically allowed and energy exchange is carried out according to exciton mechanism. The concept of “exciton” means an electronically excited state of a molecule, where the excited electron remains bound to the pigment molecule and charge separation does not occur. Energy transfer from an excited pigment molecule to another molecule is carried out by non-radiative transfer of excitation energy. An electron in an excited state is an oscillating dipole. The resulting alternating electric field can cause similar vibrations of an electron in another pigment molecule if resonance conditions are met (equality of energy between the ground and excited levels) and induction conditions that determine a sufficiently strong interaction between molecules (distance no more than 10 nm).

Exchange resonance mechanism of Terenin-Dexter energy migration occurs when the transition is optically forbidden and a dipole is not formed upon excitation of the pigment. For its implementation, close contact of molecules (about 1 nm) with overlapping external orbitals is required. Under these conditions, the exchange of electrons located in both singlet and triplet levels is possible.

In photochemistry there is a concept of quantum flow process. In relation to photosynthesis, this indicator of the efficiency of converting light energy into chemical energy shows how many quanta of light are absorbed in order for one O 2 molecule to be released. It should be borne in mind that each molecule of a photoactive substance simultaneously absorbs only one quantum of light. This energy is enough to cause certain changes in the photoactive substance molecule.

The reciprocal of the quantum flow rate is called quantum yield: the number of oxygen molecules released or carbon dioxide molecules absorbed per quantum of light. This figure is less than one. So, if 8 quanta of light are consumed to assimilate one CO 2 molecule, then the quantum yield is 0.125.

Structure of the electron transport chain of photosynthesis and characteristics of its components. The electron transport chain of photosynthesis includes a fairly large number of components located in the membrane structures of chloroplasts. Almost all components, except quinones, are proteins containing functional groups capable of reversible redox changes and acting as carriers of electrons or electrons together with protons. A number of ETC transporters include metals (iron, copper, manganese). The following groups of compounds can be noted as the most important components of electron transfer in photosynthesis: cytochromes, quinones, pyridine nucleotides, flavoproteins, as well as iron proteins, copper proteins and manganese proteins. The location of these groups in the ETC is determined primarily by the value of their redox potential.

Ideas about photosynthesis, during which oxygen is released, were formed under the influence of the Z-scheme of electron transport by R. Hill and F. Bendell. This scheme was presented based on measurements of the redox potentials of cytochromes in chloroplasts. The electron transport chain is the site of conversion of physical electron energy into chemical bond energy and includes PS I and PS II. The Z-scheme is based on the sequential functioning and integration of PSII with PSI.

P 700 is the primary electron donor, is chlorophyll (according to some sources, a dimer of chlorophyll a), transfers an electron to an intermediate acceptor and can be oxidized photochemically. A 0 - an intermediate electron acceptor - is a dimer of chlorophyll a.

Secondary electron acceptors are bound iron-sulfur centers A and B. The structural element of iron-sulfur proteins is a lattice of interconnected iron and sulfur atoms, which is called an iron-sulfur cluster.

Ferredoxin, an iron protein soluble in the stromal phase of the chloroplast located outside the membrane, transfers electrons from the PSI reaction center to NADP, resulting in the formation of NADP-H, which is necessary for CO 2 fixation. All soluble ferredoxins from photosynthetic oxygen-producing organisms (including cyanobacteria) are of the 2Fe-2S type.

The electron transfer component is also membrane-bound cytochrome f. The electron acceptor for membrane-bound cytochrome f and the direct donor for the chlorophyll-protein complex of the reaction center is a copper-containing protein, which is called the “distribution carrier,” plastocyanin.

Chloroplasts also contain cytochromes b 6 and b 559. Cytochrome b 6, which is a polypeptide with a molecular weight of 18 kDa, is involved in cyclic electron transfer.

The b 6 /f complex is an integral membrane complex of polypeptides containing cytochromes type b and f. The cytochrome b 6 /f complex catalyzes electron transport between two photosystems.

The cytochrome b 6 /f complex restores a small pool of water-soluble metalloprotein - plastocyanin (Pc), which serves to transfer reducing equivalents to the PS I complex. Plastocyanin is a small hydrophobic metalloprotein that includes copper atoms.

Participants in the primary reactions in the PS II reaction center are the primary electron donor P 680, the intermediate acceptor pheophytin, and two plastoquinones (usually designated Q and B), located close to Fe 2+. The primary electron donor is one of the forms of chlorophyll a, called P 680, since a significant change in light absorption was observed at 680 nm.

The primary electron acceptor in PS II is plastoquinone. It is assumed that Q is an iron-quinone complex. The secondary electron acceptor in PS II is also plastoquinone, designated B, and functioning in series with Q. The plastoquinone/plastoquinone system simultaneously transfers two more protons with two electrons and is therefore a two-electron redox system. As two electrons are transferred along the ETC through the plastoquinone/plastoquinone system, two protons are transferred across the thylakoid membrane. It is believed that the proton concentration gradient that arises is the driving force behind the process of ATP synthesis. The consequence of this is an increase in the concentration of protons inside the thylakoids and the emergence of a significant pH gradient between the outer and inner sides of the thylakoid membrane: from the inside the environment is more acidic than from the outside.

2. Photosynthetic phosphorylation

Water serves as an electron donor for PS-2. Water molecules, giving up electrons, disintegrate into free hydroxyl OH and proton H +. Free hydroxyl radicals react with each other to produce H2O and O2. It is assumed that manganese and chlorine ions take part as cofactors in the photooxidation of water.

In the process of photolysis of water, the essence of the photochemical work carried out during photosynthesis is revealed. But the oxidation of water occurs under the condition that the electron knocked out of the P 680 molecule is transferred to the acceptor and further into the electron transport chain (ETC). In the ETC of photosystem-2, electron carriers are plastoquinone, cytochromes, plastocyanin (copper-containing protein), FAD, NADP, etc.

The electron knocked out of the P 700 molecule is captured by a protein containing iron and sulfur and transferred to ferredoxin. In the future, the path of this electron can be twofold. One of these pathways consists of sequential electron transfer from ferredoxin through a series of carriers back to P 700. Then the light quantum knocks out the next electron from the P 700 molecule. This electron reaches ferredoxin and returns to the chlorophyll molecule. The cyclical nature of the process is clearly visible. When an electron is transferred from ferredoxin, the electronic excitation energy goes into the formation of ATP from ADP and H3PO4. This type of photophosphorylation was named by R. Arnon cyclical . Cyclic photophosphorylation can theoretically occur even with closed stomata, since exchange with the atmosphere is not necessary for it.

Non-cyclic photophosphorylation occurs with the participation of both photosystems. In this case, the electrons and proton H + knocked out from P 700 reach ferredoxin and are transferred through a number of carriers (FAD, etc.) to NADP with the formation of reduced NADP·H 2. The latter, as a strong reducing agent, is used in dark reactions of photosynthesis. At the same time, the chlorophyll P 680 molecule, having absorbed a light quantum, also goes into an excited state, giving up one electron. Having passed through a number of carriers, the electron compensates for the electron deficiency in the P 700 molecule. The electron “hole” of chlorophyll P 680 is replenished by an electron from the OH ion - one of the products of water photolysis. The energy of an electron knocked out of P 680 by a light quantum, when passing through the electron transport chain to photosystem 1, goes to photophosphorylation. During non-cyclic electron transport, as can be seen from the diagram, photolysis of water occurs and free oxygen is released.

Electron transfer is the basis of the considered photophosphorylation mechanism. The English biochemist P. Mitchell put forward the theory of photophosphorylation, called the chemiosmotic theory. The ETC of chloroplasts is known to be located in the thylakoid membrane. One of the electron carriers in the ETC (plastoquinone), according to P. Mitchell’s hypothesis, transports not only electrons, but also protons (H +), moving them through the thylakoid membrane in the direction from outside to inside. Inside the thylakoid membrane, with the accumulation of protons, the environment becomes acidic and, as a result, a pH gradient arises: the outer side becomes less acidic than the inner. This gradient also increases due to the supply of protons - products of water photolysis.

The pH difference between the outside of the membrane and the inside creates a significant source of energy. With the help of this energy, protons are thrown out through special channels in special mushroom-shaped projections on the outer side of the thylakoid membrane. These channels contain a coupling factor (a special protein) that can take part in photophosphorylation. It is assumed that such a protein is the enzyme ATPase, which catalyzes the reaction of ATP breakdown, but in the presence of energy of protons flowing through the membrane - and its synthesis. As long as there is a pH gradient and, therefore, as long as electrons move along the chain of carriers in photosystems, ATP synthesis will also occur. It is calculated that for every two electrons that pass through the ETC inside the thylakoid, four protons are accumulated, and for every three protons released with the participation of the conjugation factor from the membrane to the outside, one ATP molecule is synthesized.

Thus, as a result of the light phase, due to light energy, ATP and NADPH 2 are formed, used in the dark phase, and the product of photolysis of water O 2 is released into the atmosphere. The overall equation for the light phase of photosynthesis can be expressed as follows:

2H 2 O + 2NADP + 2 ADP + 2 H 3 PO 4 → 2 NADPH 2 + 2 ATP + O 2

Question 1. How much glucose is synthesized during photosynthesis for each of the 4 billion inhabitants of the Earth per year?
If we take into account that in a year all the planet’s vegetation produces about 130,000 million tons of sugars, then per one inhabitant of the Earth (assuming that the Earth’s population is 4 billion inhabitants) there are 32.5 million tons (130,000/4 = 32.5) .

Question 2. Where does the oxygen released during photosynthesis come from?
Oxygen entering the atmosphere during the process of photosynthesis is formed during the reaction of photolysis - the decomposition of water under the influence of the energy of sunlight (2H 2 O + light energy = 2H 2 + O 2).

Question 3. What is the meaning of the light phase of photosynthesis; dark phase?
Photosynthesis is the process of synthesis of organic substances from inorganic ones under the influence of the energy of sunlight.
Photosynthesis in plant cells occurs in chloroplasts. Total formula:
6CO 2 + 6H 2 O + light energy = C 6 H 12 O 6 + 6O 2.
The light phase of photosynthesis occurs only in the light: a light quantum knocks out an electron from a chlorophyll molecule lying in the thylakoid membrane.; the knocked out electron either returns back or ends up in a chain of enzymes that oxidize each other. A chain of enzymes transfers an electron to the outside of the thylakoid membrane to an electron transporter. The membrane is charged negatively from the outside. The positively charged chlorophyll molecule lying in the center of the membrane oxidizes enzymes containing manganese ions lying on the inner side of the membrane. These enzymes participate in water photolysis reactions, which result in the formation of H +; Hydrogen protons are released onto the inner surface of the thylakoid membrane, and a positive charge appears on this surface. When the potential difference across the thylakoid membrane reaches 200 mV, protons begin to flow through the ATP synthetase channel. ATP is synthesized.
In the dark phase, glucose is synthesized from CO 2 and atomic hydrogen bound to carriers using the energy of ATP. Glucose synthesis occurs in the stroma of chloroplasts using enzyme systems. Total reaction of the dark stage:
6CO 2 + 24H = C 6 H 12 O 6 + 6H 2 O.
Photosynthesis is very productive, but leaf chloroplasts capture only 1 light quantum out of 10,000 to participate in this process. Nevertheless, this is enough for a green plant to synthesize 1 g of glucose per hour from a leaf surface area of ​​1 m2.

Question 4. Why do higher plants need the presence of chemosynthetic bacteria in the soil?
Plants need mineral salts containing elements such as nitrogen, phosphorus, and potassium for normal growth and development. Many species of bacteria that are capable of synthesizing the organic compounds they need from inorganic ones using the energy of chemical oxidation reactions occurring in the cell are classified as chemotrophs. The substances captured by the bacterium are oxidized, and the resulting energy is used for the synthesis of complex organic molecules from CO 2 and H 2 O. This process is called chemosynthesis.
The most important group of chemosynthetic organisms are nitrifying bacteria. Investigating them, S.N. Winogradsky discovered the process in 1887 chemosynthesis. Nitrifying bacteria living in the soil oxidize ammonia, formed during the decay of organic residues, to nitrous acid:
2MN 3 + ZO 2 = 2HNO 2 + 2H 2 O + 635 kJ.
Then bacteria of other species of this group oxidize nitrous acid to nitric acid:
2HNO 2 + O 2 = 2HNO 3 + 151.1 kJ.
Interacting with soil minerals, nitrous and nitric acids form salts, which are the most important components of the mineral nutrition of higher plants. Under the influence of other types of bacteria in the soil, phosphates are formed, which are also used by higher plants.
Thus, chemosynthesis is the process of synthesis of organic substances from inorganic ones using the energy of chemical oxidation reactions occurring in the cell.