The largest achievement in the field of photobiology was the discovery by A. A. Krasnovsky in 1948 of the reversible photoreduction reaction of chlorophyll in an evacuated pyridine solution in the presence of hydrogen donors (ascorbic acid) with the formation of a pink form of pigment with an absorption maximum at 525 nm. It is now generally accepted that the primary photochemical reaction of photosynthesis is the reversible redox transformation of chlorophyll. The quantum yield of the chlorophyll photoreduction reaction is in ethanol-water-pyridine and in water-pyridine solutions. The Krasnovsky reaction occurs in several stages:
In the presence of oxygen and other oxidizing agents, a reverse reaction occurs, as a result of which the original green color of the solution is restored:
In the radical anion, the electron is delocalized along the orbits of the system of conjugated bonds of the chromophore nucleus. Two hydrogen atoms can attach to chlorophyll,
apparently different ways depending on environmental conditions, as evidenced by the identification of several forms differing in absorption and fluorescence spectra. In addition to ascorbic acid, the following compounds can serve as electron and proton donors: phenylhydrazine, cysteine, benzylnicotinamide, NADH, cytochrome c; acceptors, in addition to oxygen, are various azo dyes and quinones, riboflavin, NAD, NADP, viologens and other substances.
The reversible photoreduction reaction is characteristic not only of chlorophyll a, but also of other chlorophylls and related compounds (chlorophyll bis, bacteriochlorophylls a and pheophytins a and protochlorophylls, various porphyrins). It is not observed for phycoerythrin and phycocyanin.
It was later found that chlorophyll is also capable of reversible photooxidation in the presence of electron acceptors in alcohol solutions. This reaction occurs, for example, in the presence of -quinone in viscous media at temperature. In contrast to the destructive oxidation of chlorophyll under intense light, in this case the addition of reducing agents is accompanied by partial regeneration of the original pigment. The initial stages of photooxidation are described by the following scheme:
It is extremely important that the ability of chlorophyll in an excited state to accept or donate an electron ensures its transfer against the thermodynamic potential in the ternary molecular system acceptor - chlorophyll - donor, i.e., the energy of a light quantum with the help of chlorophyll as an intermediary is consumed and stored when the electron rises with more " low” energy level in the donor to a “higher” energy level in the acceptor. The work of chlorophyll as an “electron pump” has been demonstrated in numerous model systems. The effect of sensitized “upward” electron transport is observed regardless of whether this process begins with photoreduction of chlorophyll
(excited chlorophyll accepts an electron from the donor to the ground level, previously occupied by the “excited” photoelectron, and then the “excited” electron is transferred to the acceptor) or from its photo-oxidation (the “excited” electron of chlorophyll is given to the acceptor, then the donor electron passes to the ground level of chlorophyll). These relationships can be illustrated by V.B. Evstigneev’s diagram (Fig. 14).
Rice. 14. Electron phototransfer in the ternary system chlorophyll - oxidizer - reducer (Evstigneev V.B., 1966): a, b - ground and excited electronic levels of chlorophyll; electronic levels of oxidizer and reducer (I - before illumination, II - after absorption of light by the pigment, III - after electron transitions); indices 1 (2) and 2 (1) indicate the time sequence of electron transitions during which chlorophyll is oxidized and reduced, respectively
What will be primary - the reduction or oxidation of chlorophyll is determined by the nature of the medium and the redox properties of electron acceptors and donors. Indeed, depending on the conditions of acid-base equilibrium in the ternary system, either the electron-donor or electron-acceptor properties of chlorophyll predominate.
It is thanks to its ability to undergo a reversible oxidation-reduction reaction that chlorophyll can serve as an energy photocatalyst during photosynthesis, promoting the primary storage of light energy in the form of reduced intermediates.
Although indisputable evidence of the reaction of reversible photoreduction - photooxidation of chlorophyll itself directly in photosynthetic organisms has not yet been obtained, rapid, reversible changes in the absorption spectra of pigments, for example, in the absorption band of 890 nm, may be associated with this reaction
one of the forms of bacteriochlorophyll and in the absorption band of chlorophyll a, observed when irradiated with laser pulses.
Electron paramagnetic resonance (EPR) data also support the formation of radical cations of chlorophyll pigments in the cell. The EPR photosignal in Rhodospirillum rubrurn is attributed to the bacteriochlorophyll radical ion based, firstly, on the correspondence between the kinetics of the EPR signal and changes in absorption at both room and low (77 K, 4 K) temperatures; secondly, the closeness of the redox potentials of free radicals and bacteriochlorophyll a.
From Parson's work it follows that the primary reaction in bacterial photosynthesis is:
where A? is the primary electron acceptor of unknown nature, representing the oxidation of bacteriochlorophyll.
Valuable information about the kinetics of redox transformations in the reaction centers of photosynthetic objects was obtained using laser technology. It was discovered, for example, that the rate of electron transfer in the donor–photochemically active chlorophyll–acceptor system lies in the microsecond range. Thus, for purple bacteria, the time of electron transfer from the pigment to the acceptor is 0.5, and for higher plants it is 2 μs. At such a speed, electron transfer from chlorophyll to the acceptor occurs even at a liquid nitrogen temperature of -196 ° C), which indicates its physical nature. The reduction reaction of photochemically active chlorophyll (donor pigment) occurs at a lower rate: for bacteria and higher plants, respectively. In the reduction reaction, it was possible to isolate two parallel paths, one of which depends and the other does not depend on temperature. It is assumed that the reduction of photochemically active chlorophyll occurs predominantly during the second reaction.
So, chlorophyll pigments in photosynthetic
organisms control electron flows due to their direct oxidation - reduction, and not due to physical sensitization (energy migration) of intermediate redox intermediates (I) with the transfer of an electron to a carrier (P) according to the scheme
Moreover, the literature suggests that in one of the two photochemical systems photoreduction is primary, in the other, photooxidation of chlorophyll. For example, it has been established that the primary photochemical reaction in the first photosystem (PS I) of red algae is the oxidation of the active form of chlorophyll
Thus, the interface between the photophysics of chlorophyll and the storage of absorbed light energy in organic matter is provided by its redox transformations. As a result, the “cold” electron of water turns into an energy-rich “hot” electron of NADPH, the molecule of which retains solar energy:
According to modern concepts, the removal of an electron from water and its transfer to NADPH is not a one-step process, but a multi-stage process of successive redox reactions in a chain (electron cascade) of specialized substances - electron carriers.
To transfer electrons from water to NAD, plants have evolved a mechanism that uses two quanta in series for a non-cyclic flow of electrons. Two independent photochemical acts occur one after another, and for each of them in a certain area of the cell there is a special photointhetic apparatus. Of course, these two areas must be appropriately connected.
Graphs of this double process in its possible variants are called -schemes. It is believed that first, the electron “rises” into photosystem II, caused by light energy, where it is accepted by a specific acceptor. The electron then undergoes a series of spontaneous (dark) reactions. At the same time, it is sequentially transmitted along a long chain of redox compounds with ever-decreasing negative potentials, i.e.
to increasingly weaker reducing agents. In the end, the electron fills a hole in the photosystem; this hole appeared earlier when one electron was removed from the system. Only after this, a second quantum of light energy is applied to the electron, now in system I, and the electron is accepted by the acceptor, which in this case becomes a much stronger reducing agent than acceptor system II.
Rice. 12.1. Simplified -ychema for non-cyclic electron flow. The y-axis shows the standard potentials of redox compounds. Their numerical values are given approximately.
(Less simplifying and closer to reality, we can say that in any this moment in each of the two photosystems there is a certain number of holes distributed randomly.) The result is the final product - reduced ferredoxin, which, through an enzyme containing flavin, transfers electrons to
The first scheme was proposed by Hill and Bendoll. Other authors have also expressed or accepted the idea that photosynthesis is based on a two-step process; For the detailed development of the scheme, enormous efforts were made by numerous researchers. Then it was necessary to identify the members of the electron transport chain and arrange them in the appropriate order. Among the most effective methods we can mention the study of action spectra, reaction kinetics using pulse spectrometry and the study of defective mutations. In Fig. 12.1 shows a simplified new form of the diagram (as modified by).
As far as is known, the scheme applies to all plants
Although different groups of plants differ greatly in their supporting photosensitive compounds. One problem is the order of the compounds: cytochrome - plastodianin - photosystem; the order shown here is based on the results of Knaff and Arnon, as well as Zidov et al.
Rice. 12.2. Chlorella spectrum for spectrally pure light. The ordinate axis shows the amount of oxygen released per quantum:
The starting point for the creation of the two-quanta hypothesis was the observation that in plants, when suitable light quanta of different wavelengths acted on them together, an increase in the yield of photosynthesis was observed. The action spectrum for monochromatic light is shown in FIG. 12.2, but in the long wavelength range two different quanta acting synergistically give a greater yield than the calculated sum of the two separate yields (Fig. 12.3). For example, with a mixture of “red” and “far-red” quanta, an increase in photosynthesis by 30% was found. The enhancement is due to differences between the action spectra of the two systems. As Myers writes: “The best idea of the enhancement effect is given by the following thought experiment: when a plant is irradiated with light of one wavelength and another light with the right wavelength, the rate of photosynthesis is higher than the sum of the intensities obtained by separate irradiation. An even clearer way to describe gain is as the increase in quantum yield, measured at wavelength , when a second (unmeasured) beam of the correct wavelength is added
In plants in photosystem I, the photosensitive substance of the active center is . In photosystem II, the active center absorbs at a shorter wavelength - about. Apparently, both active centers consist of modified chlorophylls.
Rice. 12.3. Enhancing effect of chlorella. The action of far red light of a certain wavelength is complemented by (near) short wave red light. I - with additional light; II - without additional light.
Duysens called the light active in photosystem I “light I,” and in photosystem “.” Photosystems I and II can be partially separated by preparative methods. In this case, either mechanical forces or detergents are used.
No enhancement effect was found in photosynthetic bacteria.
Of all the photochemical processes known in nature highest value It has photosynthesis. The founder of the theory of photosynthesis is K. A. Timiryazev. Photosynthesis is the basis for the existence of all life on earth. Photosynthesis of green plants is the only primary source of accumulation of organic matter on Earth, which serves to feed humans and animals. All the vegetation of the globe creates about 120 billion tons of organic matter annually, of which approximately 10 billion tons are produced by humans, growing food and fodder plants on an area of about 2.5 billion hectares.
In the green leaf of a plant, under the influence of solar radiation, a whole complex of photochemical processes occurs, as a result of which starch, fiber, proteins, fats and other complex organic substances are formed from water, carbon dioxide and mineral salts. The process of photosynthesis is very complex. It is carried out with the direct participation of the most important natural photocatalyst, chlorophyll, and is accompanied by a whole cycle of chemical transformations that do not depend on solar radiation. A large number of different biocatalysts - enzymes - are involved in these transformations. The overall equation for photosynthesis is usually expressed as the reaction that converts carbon dioxide and water into hexose:
6CO 2 + 6H 2 O = C 6 H 12 O 6 + 6O 2
However, this equation, like most summary equations in biology, does not express the main features of the process.
The most important merit of K. A. Timiryazev is the materialistic scientific basis photosynthesis. Timiryazev was the first to show that photosynthesis obeys the law of conservation and transformation of energy. Thus, idealistic views on the process of photosynthesis, which explained it by the action of an intangible “life force,” were refuted.
An equally important achievement of Timiryazev is the discovery of the role of chlorophyll as a sensitizer of photochemical reactions occurring during photosynthesis. He experimentally established that photosynthesis occurs predominantly in the red and blue rays of the visible spectrum. Timiryazev conducted the following experiment. A series of glass tubes filled with a mixture of air and carbon dioxide and containing one identical green leaf were exposed to sunlight spread out using a triangular prism so that there was one tube in each part of the solar spectrum. Every few hours the carbon dioxide content in the tubes was determined. It turned out that the absorption of CO 2 occurs only in those rays that are absorbed by chlorophyll, that is, in the red, orange and yellow parts of the spectrum.
Thus, Timiryazev showed that it is chlorophyll that is the absorber of light in green plants and that this pigment, absorbing light quanta, has the ability to transfer them further to the molecules of substances that are the starting materials for photosynthesis.
In these reactions, chlorophyll undergoes a reversible redox transformation. The structure of chlorophyll is based on a porphyrin core called chlorin. It consists of four CH-bridged pyrrole residues, which are connected by two main and two coordination bonds to the central magnesium atom. In addition, the chlorophyll molecule contains the remainder of the molecule of the high molecular weight unsaturated alcohol phytol. Currently, at least five types of chlorophyll are known, which differ from each other in the structure of the molecule.
In addition to chlorophyll, which is the main type of photosynthetic pigments, the green leaf (in the so-called chloroplasts, which are complex specialized biological structures) also contains other pigments - carotenoids and phycobelins, which are usually called auxiliary. These pigments, according to modern ideas, take a certain part in photosynthesis and also protect chlorophyll from photo-oxidation. In addition to pigments, the main components of chloroplasts, in which, in fact, the entire process of photosynthesis takes place, are lipoid substances and proteins, which contain a large number of enzymes necessary for the subsequent stages of photosynthesis, not associated with exposure to solar radiation.
Many issues of photosynthesis, despite the rapid development of science, remain poorly studied to this day. As mentioned earlier, the process of photosynthesis consists of two stages - light and dark, and both of these stages are closely related to each other.
Since the initial process of photosynthesis is the absorption of light by chlorophyll, photosynthesis can be approximately represented as the following diagram.
In the light stage, chlorophyll, having absorbed a light quantum, goes into an excited state and in this form, through a series of intermediate processes, causes the decomposition of a water molecule into a hydrogen atom H and an OH radical according to the scheme
where the symbol X conventionally denotes a chlorophyll molecule; X* is the same molecule in the active state.
Next, the chlorophyll molecule, attaching a hydrogen atom, is reduced. OH radicals, combining in pairs, form a molecule of hydrogen peroxide H 2 O 2, which, as a fragile compound, breaks down into water and oxygen:
4OH = 2H 2 O 2
2H 2 O 2 = 2H 2 O + O 2
After the completion of these reactions, the dark stage of the photosynthesis process begins, the essence of which is the transfer of hydrogen from the reduced chlorophyll molecule to the chlorophyll molecule. CO 2 with education organic compounds type of carbohydrates. This process is carried out under the action of appropriate enzymes according to the scheme: 4H + CO 2 = CH 2 O + H 2 O
As a result, due to polymerization, the final product of photosynthesis is obtained - hexose C 6 H 12 O 6.
The fact that the oxygen released during photosynthesis belongs to water, and not carbon dioxide, was proven by A.P. Vinogradov (1946) using the method of labeled atoms. Thus, when using water H 2 18 O, all of its oxygen 18 O was found after photosynthesis in free molecular oxygen, and when working with C 18 O 2 and H 2 16 0, free oxygen 16 O is released, while oxygen 18 O was included in composition of organic compounds. The establishment of this fact was essential for the theory of photosynthesis, since previously many scientists believed that molecular oxygen is formed by light decomposition or photolysis of CO 2.
The above diagram of photosynthesis is only an approximate one and does not reflect all the details of this extremely complex phenomenon. IN last years It was found that the reduction of one CO 2 molecule to carbon requires not one, but 8-12 quanta of energy. This indicates that during the process of photosynthesis at least eight primary photochemical reactions occur, which occur in a certain order with other (non-photochemical) reactions.
It is known that not every molecule of chlorophyll or other pigment that absorbs light and retains a sufficient amount of energy for a photochemical reaction is the center of such a reaction. In fact, photochemical activity, i.e. direct connection with a photochemical reaction, is carried out by only about one molecule out of 200-250 chlorophyll molecules. About this phenomenon A.G. Pasynsky writes: “...There could be a misconception that the bulk of chlorophyll is photochemically inactive and plays the role of a reserve substance in the leaf, as was sometimes assumed in the literature.
In reality, this situation is a necessary consequence of the quantum nature of the active light. The absorption of light by a given chlorophyll molecule does not occur in a continuous stream; Light quanta falling like raindrops are absorbed all the time by different chlorophyll molecules.
According to Rabinovich, even in direct sunlight, each chlorophyll molecule absorbs a quantum of light only once every 0.1 s, and under less favorable conditions - much less often. Meanwhile, the rate of subsequent enzymatic reactions is extremely high. If, under these conditions, each chlorophyll molecule were an independent center of a photochemical reaction associated with the necessary auxiliary enzymes, then such a device would be as impractical as if each section of the roof on which a separate drop of rain falls was equipped with an independent drain. There simply wouldn't be enough space on the sheet for such a device, not to mention the fact that it would only be used a fraction of the time.
On the contrary, the connection of a large group (200-250) of chlorophyll molecules with one center of the photochemical reaction ensures its continuous operation, just as the connection of one drain to a sufficiently large surface of the roof makes it possible to obtain from individual drops continuous flow water. It is clear that in this case the entire mass of chlorophyll molecules is actively involved in the beneficial process, although it is associated with only one center for converting absorbed radiant energy into chemical energy.”
All this once again confirms the extreme complexity of the photosynthesis process, each stage of which requires not only certain environmental conditions, but also a very complex system of auxiliary substances, as well as a strictly defined internal structure intracellular contents. The importance of structural factors is indicated by the fact that a green leaf that has been subjected to mechanical stress (for example, if it is rolled on glass with a thick glass rod) loses its ability to photosynthesize.
The study of photosynthesis processes is very important not only from a purely theoretical point of view, but also from the point of view of obtaining high and sustainable yields. Understanding these processes and learning to manage them are the tasks to which the efforts of an entire army of domestic and foreign scientists are currently directed.
What occurs under the influence of UV light is very important dimerization reactions of nitrogenous bases in DNA and RNA. The main chromophores (a chromophore is a part of a molecule that absorbs light and determines the color of a substance) of DNA molecules are the nitrogenous bases of nucleotides. Absorption of UV light quanta by nitrogenous bases leads to the formation of electronically excited singlet states resulting from P → P* transitions.
In an electronically excited state, pyridine bases enter into a dimerization reaction, which consists of combining two nitrogenous bases with a 5,6 double carbon bond to form a cyclobutane ring between the residues of the nitrogenous base molecules. Thus, individual nucleotides are connected not only through phosphoric acid residues, but also through nitrogenous bases. For this reaction, the quantum yield is γ = 2 ∙10 -2. This reaction causes so-called "point" mutations; 80% of all lethal mutations associated with exposure to UV radiation are consequences of thymine dimerization.
At low radiation intensity, beneficial point mutations occur. As a result of irradiation of parent forms with UV light and selection of useful traits, the wheat variety Erythrospermum-103 was created.
Dark reactions that occur in the stroma do not require light. The reduction of CO 2 occurs due to energy (ATP) and reducing force (NADPH 2) generated during light reactions. Dark reactions are controlled by enzymes. The sequence of these reactions was determined in the USA by Calvin, Benson and Bassem between 1946 and 1953; in 1961, Calvin was awarded the Nobel Prize for this work.
Calvin's experiments
Calvin's work was based on the use of the radioactive isotope of carbon 14 C (half-life 5570 years, see Appendix 1.3), which became available to researchers only in 1945. In addition, Calvin used paper chromatography, which at that time was relatively new, not yet a little common method. Cultures of unicellular green algae Chlorella (Chlorella) were grown in a special apparatus (Fig. 9.17). The culture was kept at 14 CO 2 for various periods of time, then the cells were quickly fixed by pouring the suspension into hot methanol. Soluble photosynthetic products were extracted, concentrated and separated using two-dimensional paper chromatography(Fig. 9.18 and Appendix 1.8.2). The goal was to trace the path by which labeled carbon enters (via a series of intermediate products) into final products photosynthesis. The position of radioactive compounds on paper was determined using autoradiography: to do this, photographic film sensitive to 14 C radiation was placed on the chromatogram, and it was exposed, that is, blackened, in those places where radioactive substances were located (Fig. 9.18). Within just one minute of incubation with 14 CO 2, many sugars and organic acids, including various amino acids, were synthesized. However, Calvin was able, using very short exposures - for 5 seconds or less - to identify the first product of photosynthesis and establish that it was an acid containing three carbon atoms, namely phosphoglyceric acid(FGK). He then figured out the entire chain of intermediates through which the fixed carbon is transmitted; these stages will be discussed later. Since then these reactions have been called Calvin cycle(or the Calvin-Benson-Bassem cycle).
Rice. 9.18. A. Fixation of 14 CO 2 in algae under short-term illumination. Determination of fixation products using paper chromatography and autoradiography. B. Autoradiographs of photosynthesis products obtained after short-term illumination of algae in the presence of 14 CO 2
9.18. What are the advantages of using long-lived radioactive isotopes in biological research?
9.19. What benefits can you get by taking chlorella instead of a higher plant?
9.20. Why is the Calvin apparatus vessel flat and not spherical?
Carbon Pathway Stages
Carbon dioxide fixation:
The CO 2 acceptor is a five-carbon sugar (pentose) ribulose bisphosphate(i.e., ribulose with two phosphate groups; this compound was previously called ribulose diphosphate). The addition of CO 2 to a particular substance is called carboxylation, and the enzyme that catalyzes such a reaction is carboxylase. The resulting six-carbon product is unstable and immediately breaks down into two molecules phosphoglyceric acid(FGK), which is the first product of photosynthesis. The enzyme ribulose bisphosphate carboxylase is found in the stroma of chloroplasts in large quantities is actually the most abundant protein in the world.
Recovery phase:
FHA contains three carbon atoms and has an acidic carboxyl group (-COOH). TP is triose phosphate, or glyceraldehyde phosphate (three-carbon sugar); it has an aldehyde group (-CHO).
To remove oxygen from PGA (i.e., to restore it), the reducing force of NADPH 2 and the energy of ATP are used. The reaction proceeds in two stages: first, part of the ATP formed during light reactions is consumed, and then all of the NADP·H 2, also obtained in light, is used. The overall result is the reduction of the carboxyl group of the acid (-COOH) to the aldehyde group (-CHO). The reaction product is triose phosphate, that is, a three-carbon sugar with a phosphate group attached to it. There is more in this connection chemical energy, than in FHA, and it is the first carbohydrate that is formed during photosynthesis.
Regeneration of the acceptor for CO 2 - ribulose bisphosphate. Part of the triose phosphate (TP) must be spent on the regeneration of ribulose bisphosphate, which is used in the first reaction. This process is a complex cycle that involves sugar phosphates with 3, 4, 5, 6, 7 carbon atoms. This is where the rest of the ATP is used up. All dark reactions are summarized in Fig. 9.19. In this figure, the Calvin cycle is depicted as a “black box”, into which CO 2 and H 2 O enter on one side, and triose phosphate exits on the other side. As can be seen from this diagram, the ATP residue is used to phosphorylate ribulose bisphosphate, but the details of this complex chain of reactions are not shown.
From Fig. 9.19 we can derive the following summary equation:
It is important to note here that the formation of two molecules of triose phosphate requires six molecules of CO 2. The equation can be simplified by dividing all coefficients by 6:
9.21. Redraw the figure. 9.19, indicating only the number of carbon atoms participating in the reactions; for example, instead of 6 RiBF write "6 × 5C", etc.
Basic information about the process of photosynthesis is summarized in Table. 9.6.
| Light reactions | Dark reactions | |
| Localization in chloroplasts | Thylakoids | Stroma |
| Reactions | Photochemical, i.e. require light. Light energy causes the transfer of electrons from electron "donors" to their "acceptors" in either a non-cyclic or cyclic path. Two photosystems are involved - Ι and ΙΙ. They contain chlorophyll molecules, which, when absorbing light energy, emit electrons. Water serves as an electron donor for the non-cyclic pathway. Electron transfer leads to the formation of ATP (photophosphorylation) and NADPH 2 (see also Table 9.5). | They don't require light. CO 2 is fixed when it binds to a five-carbon acceptor, ribulose bisphosphate (RiBP); in this case, two molecules of the three-carbon compound phosphoglyceric acid (PGA), the first product of photosynthesis, are formed. A number of reactions occur, collectively called the Calvin cycle; in this case, the acceptor for CO 2 -RiBP is regenerated, and FGA is reduced, turning into sugar (see also Fig. 9.19). |
| Combined equations |
The light phase of photosynthesis depends on the entry of light radiation (photons) into the cell. In nature, photosynthesis is stimulated by sunlight.
Contained in chloroplasts plant cells chlorophylls and other pigments detect radiation of specific wavelengths. Photon energy transfers pigment electrons to higher energy level. Instead of returning to the previous energy level with the return energy emission, the electrons are captured by acceptors and transferred along the electron transport chain embedded in the thylakoid membrane of the chloroplasts.
Along the path of electrons, their energy is partially lost and partially spent for ATP synthesis and NADP reduction. Thus solar energy is converted into energy chemical bonds, which is then used in the dark phase for synthesis organic matter . In this sense, the light phase of photosynthesis can be called preparatory.
The electron transport chain consists of pigments, enzymes and coenzymes. Some are localized in the membrane almost motionless, others move, acting as carriers of electrons and protons.
However, the light reactions of photosynthesis occur not only on the thylakoid membrane. Also photons of light launch photolysis of water. As a result of photolysis, water breaks down into hydrogen protons (H +), electrons (e -) and oxygen atoms (O). The latter, combining in pairs, are released from the cell in the form of molecular oxygen (O 2).
The reason for the need for photolysis becomes clear with a more detailed examination of the light phase reactions occurring on the thylakoid membrane.
There are two photosystems functioning here. These are the so-called photosystem I And photosystem II. Each of them captures light energy, and excited electrons come off from each and are accepted by their acceptors. In photosystems they are formed electron holes, i.e. lack of electrons. The chlorophylls of the reaction centers of photosystems become positively charged. In order for the system to work again, these holes must be eliminated due to the influx of electrons from outside.
In plants, the light phase of photosynthesis is organized in such a way that Photosystem I fills holes with electrons transported from photosystem II. And it receives electrons that are formed during photolysis of water.
Electrons leaving the first photosystem, passing through the electron transport chain, reach NADP. This coenzyme is reduced and charged negatively. After this, it attracts hydrogen protons, turning into NADP H 2. Thus, photolysis of water is necessary to obtain protons and electrons.
Along the path of electrons from the second photosystem to the first, ATP synthesis occurs due to the accumulated electrochemical gradient- charge differences on different sides of the membrane.
Let's take a closer look simplified diagram of the light phase of photosynthesis:
In addition to light energy, photolysis of water also requires an enzyme, which is marked in the diagram as “ water-oxidizing complex" It is built into the photosystem. The resulting protons remain in the lumen, and the electrons go to photosystem II (PSII). The flow of electrons is shown by the blue dotted arrow.
The inscriptions P680 and P700 in photosystems indicate the wavelengths of light that are preferentially absorbed by the PS reaction centers. The photosystems themselves have complex structure. In addition to emitting electrons reaction center, they also include light-harvesting complex.
From PSII electrons are transferred to the coenzyme plastoquinone. Charged negatively, it attaches protons from the stroma. The proton flux is shown by the red dotted arrow. Plastoquinone transports electrons and protons to the enzymatic complex cytochrome b 6 f. The latter oxidizes plastoquinone.
Cytochrome b 6 f pumps protons into the lumen, and transfers electrons to the next coenzyme carrier - plastocyanin.
At this time, due to protons transferred from the stroma and formed as a result of photolysis of water, a sufficient positive charge accumulates in the lumen for the enzyme to “work” ATP synthase. Through its channels, protons rush to the outer side of the thylakoid membrane. This energy is used by ATP synthase for ATP synthesis from ADP and phosphoric acid.
Plastocyanin transports electrons to PSI, reducing it. From here, as a result of the action of light, electrons are transferred to ferredoxin. Under the action of an enzyme ferredoxin-NADP reductase it restores NADP. This also uses protons located in the stroma of the chloroplast. They entered here, among other things, through ATP synthase channels.
The light phase reactions considered are non-cyclic electron transport . However, this stage of photosynthesis can also occur cyclical path. In this case, ferrodoxin reduces not NADP, but plastoquinone. This way PSI gets its electrons back. In the case of cyclic electron transport, the synthesis of NADP·H2 does not occur; the light phase produces only ATP.
Non-cyclic (ordinary) electron transport is also called the Z-electron transfer scheme. If you depict the flow of electrons taking into account the gradual decrease in their energy, you will get a diagram similar to the letter Z rotated by 90°.
