What is the meaning of the dark phase of photosynthesis. Photosynthesis: light and dark phases. Functions of plant cell parts
The explanation of such a voluminous material as photosynthesis is best done in two paired lessons - then the integrity of the perception of the topic is not lost. The lesson must begin with the history of the study of photosynthesis, the structure of chloroplasts and laboratory work for the study of leaf chloroplasts. After that, it is necessary to proceed to the study of the light and dark phases of photosynthesis. When explaining the reactions occurring in these phases, it is necessary to draw up a general scheme:
In the course of the explanation it is necessary to draw diagram of the light phase of photosynthesis.
1. The absorption of a quantum of light by a chlorophyll molecule, which is located in the membranes of the thylakoids of the grana, leads to the loss of one electron by it and transfers it to an excited state. Electrons are transferred along the electron transport chain, which leads to the reduction of NADP + to NADP H.
2. The place of the released electrons in the chlorophyll molecules is occupied by the electrons of the water molecules - this is how water undergoes decomposition (photolysis) under the action of light. The resulting OH– hydroxyls become radicals and combine in the reaction 4 OH – → 2 H 2 O + O 2 , leading to the release of free oxygen into the atmosphere.
3. Hydrogen ions H+ do not penetrate the thylakoid membrane and accumulate inside, charging it positively, which leads to an increase in the electric potential difference (EPD) on the thylakoid membrane.
4. When the critical REB is reached, the protons rush outward through the proton channel. This flow of positively charged particles is used to generate chemical energy using a special enzyme complex. The resulting ATP molecules pass into the stroma, where they participate in carbon fixation reactions.
5. Hydrogen ions that have come to the surface of the thylakoid membrane combine with electrons, forming atomic hydrogen, which is used to reduce the NADP + carrier.
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After considering this issue, having analyzed it again according to the drawn up scheme, we invite students to fill in the table.
Table. Reactions of light and dark phases of photosynthesis
After filling in the first part of the table, you can proceed to the analysis dark phase of photosynthesis.
In the stroma of the chloroplast, pentoses are constantly present - carbohydrates, which are five-carbon compounds that are formed in the Calvin cycle (carbon dioxide fixation cycle).
1. Carbon dioxide is added to pentose, an unstable six-carbon compound is formed, which decomposes into two molecules of 3-phosphoglyceric acid (PGA).
2. FGK molecules take one phosphate group from ATP and are enriched with energy.
3. Each FGC adds one hydrogen atom from two carriers, turning into a triose. Trioses combine to form glucose and then starch.
4. Triose molecules, combining in different combinations, form pentoses and are again included in the cycle.
Total reaction of photosynthesis:
Scheme. Photosynthesis process
Test
1. Photosynthesis is carried out in organelles:
a) mitochondria;
b) ribosomes;
c) chloroplasts;
d) chromoplasts.
2. The chlorophyll pigment is concentrated in:
a) the membrane of the chloroplast;
b) stroma;
c) grains.
3. Chlorophyll absorbs light in the region of the spectrum:
a) red;
b) green;
c) purple;
d) all over the region.
4. Free oxygen during photosynthesis is released during splitting:
a) carbon dioxide;
b) ATP;
c) NADP;
d) water.
5. Free oxygen is formed in:
a) dark phase;
b) light phase.
6. In the light phase of ATP photosynthesis:
a) synthesized;
b) splits.
7. In the chloroplast, the primary carbohydrate is formed in:
a) light phase;
b) dark phase.
8. NADP in the chloroplast is required:
1) as a trap for electrons;
2) as an enzyme for the formation of starch;
3) as an integral part of the chloroplast membrane;
4) as an enzyme for water photolysis.
9. Photolysis of water is:
1) accumulation of water under the action of light;
2) dissociation of water into ions under the action of light;
3) release of water vapor through stomata;
4) injection of water into the leaves under the action of light.
10. Under the influence of light quanta:
1) chlorophyll is converted to NADP;
2) the electron leaves the chlorophyll molecule;
3) the chloroplast increases in volume;
4) chlorophyll is converted to ATP.
LITERATURE
Bogdanova T.P., Solodova E.A. Biology. Handbook for high school students and university applicants. - M .: LLC "AST-Press school", 2007.
Photosynthesis is a set of processes for the formation of light energy into energy chemical bonds organic substances with the participation of photosynthetic dyes.
This type of nutrition is typical for plants, prokaryotes and some types of unicellular eukaryotes.
In natural synthesis, carbon and water, in interaction with light, are converted into glucose and free oxygen:
6CO2 + 6H2O + light energy → C6H12O6 + 6O2
Modern plant physiology under the concept of photosynthesis understands the photoautotrophic function, which is a set of processes of absorption, transformation and use of light energy quanta in various non-spontaneous reactions, including the conversion of carbon dioxide into organic matter.
Phases
Photosynthesis in plants occurs in leaves via chloroplasts- semi-autonomous two-membrane organelles belonging to the plastid class. With a flat shape of the sheet plates, high-quality absorption and full use of light energy and carbon dioxide are ensured. The water needed for natural synthesis comes from the roots through the water-conducting tissue. Gas exchange occurs by diffusion through the stomata and partly through the cuticle.
Chloroplasts are filled with a colorless stroma and permeated with lamellae, which, when combined with each other, form thylakoids. This is where photosynthesis takes place. Cyanobacteria themselves are chloroplasts, so the apparatus for natural synthesis in them is not isolated into a separate organelle.
Photosynthesis proceeds with the participation of pigments which are usually chlorophylls. Some organisms contain another pigment - a carotenoid or phycobilin. Prokaryotes possess the pigment bacteriochlorophyll, and these organisms do not release oxygen upon completion of natural synthesis.
Photosynthesis goes through two phases - light and dark. Each of them is characterized by certain reactions and interacting substances. Let us consider in more detail the process of the phases of photosynthesis.
Luminous
First phase of photosynthesis characterized by the formation of high-energy products, which are ATP, cell source energy, and NADP, a reducing agent. At the end of the stage, oxygen is formed as a by-product. The light stage occurs necessarily with sunlight.
The process of photosynthesis takes place in thylakoid membranes with the participation of electron carrier proteins, ATP synthetase and chlorophyll (or other pigment).
The functioning of electrochemical circuits, through which the transfer of electrons and partially hydrogen protons, is formed in complex complexes formed by pigments and enzymes.
Description of the light phase process:
- On hit sunlight on the leaf plates of plant organisms, chlorophyll electrons are excited in the structure of the plates;
- In the active state, the particles leave the pigment molecule and enter the outer side of the thylakoid, which is negatively charged. This occurs simultaneously with the oxidation and subsequent reduction of chlorophyll molecules, which take the next electrons from the water that has entered the leaves;
- Then photolysis of water occurs with the formation of ions that donate electrons and are converted into OH radicals that can participate in reactions in the future;
- These radicals then combine to form water molecules and free oxygen escaping into the atmosphere;
- The thylakoid membrane acquires, on the one hand, a positive charge due to the hydrogen ion, and on the other, a negative charge due to electrons;
- With a difference of 200 mV between the sides of the membrane, protons pass through the enzyme ATP synthetase, which leads to the conversion of ADP to ATP (phosphorylation process);
- With atomic hydrogen released from water, NADP + is reduced to NADP H2;
While free oxygen is released into the atmosphere during the reactions, ATP and NADP H2 participate in the dark phase of natural synthesis.
Dark
A mandatory component for this stage is carbon dioxide., which plants constantly absorb from the external environment through the stomata in the leaves. The processes of the dark phase take place in the stroma of the chloroplast. Since at this stage a lot of solar energy is not required and there will be enough ATP and NADP H2 obtained during the light phase, reactions in organisms can proceed both day and night. Processes at this stage are faster than at the previous one.
The totality of all processes occurring in the dark phase is presented as a kind of chain of successive transformations of carbon dioxide coming from the external environment:
- The first reaction in such a chain is the fixation of carbon dioxide. The presence of the enzyme RiBP-carboxylase contributes to the rapid and smooth flow of the reaction, which results in the formation of a six-carbon compound, decomposing into 2 molecules of phosphoglyceric acid;
- Then a rather complex cycle occurs, including a certain number of reactions, after which phosphoglyceric acid is converted into natural sugar - glucose. This process is called the Calvin cycle;
Together with sugar, the formation of fatty acids, amino acids, glycerol and nucleotides also occurs.
The essence of photosynthesis
From the table of comparisons of the light and dark phases of natural synthesis, one can briefly describe the essence of each of them. The light phase occurs in the grains of the chloroplast with the obligatory inclusion of light energy in the reactions. The reactions involve such components as electron-carrying proteins, ATP synthetase and chlorophyll, which, when interacting with water, form free oxygen, ATP and NADP H2. For the dark phase occurring in the stroma of the chloroplast, sunlight is not essential. The ATP and NADP H2 obtained at the last stage, when interacting with carbon dioxide, form natural sugar (glucose).
As can be seen from the above, photosynthesis appears to be a rather complex and multi-stage phenomenon, including many reactions in which different substances are involved. As a result of natural synthesis, oxygen is obtained, which is necessary for the respiration of living organisms and their protection from ultraviolet radiation through the formation of the ozone layer.
Topic 3 Stages of photosynthesis
Section 3 Photosynthesis
1. Light phase of photosynthesis
2.Photosynthetic phosphorylation
3. Ways of fixing CO 2 during photosynthesis
4. Photorespiration
The essence of the light phase of photosynthesis is the absorption of radiant energy and its transformation into an assimilation 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 grains of the chloroplast.
Thus, the photosynthetic membrane performs a very important reaction: it converts the energy of the absorbed light quanta into the redox potential of NADP-H and into the reaction potential of the transfer of the phosphoryl group in the ATP molecule. In this case, energy is converted from its very short-lived form into a form that is quite long-lived. The stabilized energy can later be used in the biochemical reactions of the plant cell, including those leading to the reduction of carbon dioxide.
Five major polypeptide complexes are embedded in the inner membranes of chloroplasts: photosystem complex I (PS I), photosystem complex II (PSII), light-harvesting complex II (CCII), 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 are involved in photosynthetic electron transport. The proteins of these complexes are characterized by a high content of hydrophobic amino acids, which ensures their incorporation into the membrane. ATP synthase ( CF0 - CF1-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 molecules of photosynthetic pigments; 2) energy migration from the antenna to the reaction center; 3) photooxidation of a water molecule and release of oxygen; 4) photoreduction of NADP to NADP-H; 5) photosynthetic phosphorylation, the formation of ATP.
Chloroplast pigments are combined into functional complexes - pigment systems in which the reaction center is chlorophyll a, carrying out photosensitization, is associated with 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 about their existence was made by R. Emerson in 1957 on the basis of the effect he discovered of enhancing the action of long-wavelength red light (700 nm) by joint illumination with shorter-wavelength rays (650 nm). Subsequently, it was found that photosystem II absorbs shorter wavelengths compared to PSI. Photosynthesis is efficient only when they work together, which explains the Emerson amplification effect.
PSI contains chlorophyll dimer as a reaction center a c maximum absorption of light 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 FSI complex under the action of light restores 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 (Pf), which donates electrons to plastoquinone. PSII also includes the protein complex of the S-system, which oxidizes water, and the electron carrier Z. This complex functions with the participation of manganese, chlorine, and magnesium. PSII reduces plastoquinone (PQ) and oxidizes water with the release of O 2 and protons.
The connecting link between PSII and FSI is the plastoquinone fund, the protein cytochrome complex b 6 f and plastocyanin.
In plant chloroplasts, each reaction center accounts for approximately 300 pigment molecules, which are part of antenna or light-harvesting complexes. Light-harvesting protein complex containing chlorophylls isolated from chloroplast lamellae a and b and carotenoids (CCK), closely associated with PS, and antenna complexes that are directly part of PSI and PSII (focusing antenna components of photosystems). Half of the thylakoid protein and about 60% of chlorophyll are localized in the CSC. Each SSC contains from 120 to 240 chlorophyll molecules.
The PS1 antenna protein complex contains 110 chlorophyll molecules a 680-695 for one P 700 , of these, 60 molecules are components of the antenna complex, which can be considered as SSC PSI. The FSI 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 the energy of quanta to a small number of molecules of the P 700 and P 680 reaction centers, 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 a pigment molecule no more than once every 0.1 s.
Physical mechanisms of the processes of absorption, storage and migration of energy chlorophyll molecules are well studied. Photon absorption(hν) is due to the transition of the system to different energy states. In a molecule, unlike an atom, electronic, vibrational and rotational motions are possible, and the total energy of a 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, which is determined by the energy of external electrons in orbit. According to the Pauli principle, two electrons with oppositely directed spins are in the outer orbit, as a result of which a stable system of paired electrons is formed. 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, which is determined by the electronic configuration of the molecule. In a complex organic molecule there is a certain set of free orbits to which an electron can pass when absorbing light quanta. According to Bohr's "frequency rule", the frequency of the absorbed or emitted radiation v must strictly correspond to the energy difference between the levels:
ν \u003d (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 a molecule determines the character of the electronic-vibrational spectra.
Absorbed energy storage associated with the appearance of electronically excited states of pigments. The physical regularities of the excited states of Mg-porphyrins can be considered on the basis of an analysis of the scheme of electronic transitions of these pigments (figure).
There are two main types of excited states - singlet and triplet. They differ in energy and electron spin state. In the excited singlet state, the electron spins at the ground and excited levels remain antiparallel; upon transition to the triplet state, the excited electron spin rotates to form a biradical system. When a photon is absorbed, the chlorophyll molecule passes from the ground (S 0) to one of the excited singlet states - S 1 or S 2 , which is accompanied by the transition of the electron to an excited level with a higher energy. The excited state S 2 is very unstable. The electron quickly (within 10 -12 s) loses part of its energy in the form of heat and descends to the lower vibrational level S 1 , where it can stay for 10 -9 s. In the S 1 state, the spin of the electron can be reversed and the transition to the triplet state T 1 can occur, the energy of which is lower than S 1 .
There are several ways to deactivate excited states:
photon emission 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 be carried out by the following mechanisms. Inductive resonance mechanism(Förster mechanism) is possible under the condition that the electron transition is optically allowed and the energy exchange is carried out according to exciton mechanism. The term "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. The transfer of energy from an excited pigment molecule to another molecule is carried out by nonradiative transfer of excitation energy. An excited electron is an oscillating dipole. The resulting alternating electric field can cause similar oscillations of an electron in another pigment molecule under the conditions of resonance (energy equality between the ground and excited levels) and induction conditions that determine a sufficiently strong interaction between molecules (a distance of no more than 10 nm).
Exchange-resonance mechanism of Terenin-Dexter energy migration occurs when the transition is optically forbidden and no dipole is formed upon excitation of the pigment. Its implementation requires close contact of molecules (about 1 nm) with overlapping outer orbitals. Under these conditions, the exchange of electrons located both at the singlet and at the triplet levels is possible.
In photochemistry there is a concept of quantum consumption process. In relation to photosynthesis, this indicator of the efficiency of converting light energy into chemical energy shows how many photons of light are absorbed in order to release one O 2 molecule. It should be borne in mind that each molecule of a photoactive substance absorbs only one quantum of light at a time. This energy is sufficient to cause certain changes in the molecule of the photoactive substance.
The reciprocal of the quantum flow is called quantum yield: the number of released oxygen molecules or absorbed carbon dioxide molecules per quantum of light. This indicator is less than one. So, if 8 light quanta are spent on the assimilation of one CO 2 molecule, then the quantum yield is 0.125.
The structure of the electron transport chain of photosynthesis and the 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 for quinones, are proteins containing functional groups, capable of reversible redox changes, and performing the functions of electron carriers or electrons together with protons. A number of ETC carriers 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.
The concept of photosynthesis, during which oxygen is released, was formed under the influence of the Z-scheme of electron transport by R. Hill and F. Bendell. This scheme was presented based on the measurement of redox potentials of cytochromes in chloroplasts. The electron transport chain is the site of the transformation of the physical energy of an electron into the chemical energy of bonds and includes PS I and PS II. The Z-scheme comes from the sequential functioning and association 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 by photochemical means. A 0 - an intermediate electron acceptor - is a dimer of chlorophyll a.
The secondary electron acceptors are the bound iron-sulfur centers A and B. The structural element of the iron-sulfur proteins is a lattice of interconnected iron and sulfur atoms, which is called the 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 of oxygen-producing photosynthetic organisms (including cyanobacteria) are of the 2Fe-2S type.
The electron-carrying 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 b and f. The cytochrome b 6 /f complex catalyzes electron transport between two photosystems.
The cytochrome b 6 /f complex reduces a small pool of the water-soluble metalloprotein plastocyanin (Pc), which serves to transfer reducing equivalents to the PS I complex. Plastocyanin is a small hydrophobic metalloprotein containing copper atoms.
The participants in the primary reactions in the reaction center of PS II 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. Q is believed to be an iron-quinone complex. The secondary electron acceptor in PSII is also plastoquinone, denoted B, and functioning in series with Q. The plastoquinone/plastoquinone system transfers two more protons simultaneously with two electrons and, therefore, is 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 occurs in this case 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 appearance 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, decompose into free OH hydroxyl and H + proton. Free hydroxyl radicals, reacting with each other, give H 2 O and O 2. It is assumed that manganese and chlorine ions take part in the photooxidation of water as cofactors.
In the process of photolysis of water, the essence of the photochemical work carried out during photosynthesis is manifested. 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 to the electron transport chain (ETC). In the ETC of photosystem-2, electron carriers are plastoquinone, cytochromes, plastocyanin (a protein containing copper), FAD, NADP, etc.
An 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 again returns to the chlorophyll molecule. The process is clearly cyclical. When an electron is transferred from ferredoxin, the energy of electronic excitation goes to the formation of ATP from ADP and H 3 P0 4. This type of photophosphorylation is named by R. Arnon cyclical . Cyclic photophosphorylation can theoretically proceed 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 knocked out of P 700 and the H + proton 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 the dark reactions of photosynthesis. At the same time, the chlorophyll P 680 molecule, having absorbed a quantum of light, also goes into an excited state, giving up one electron. Having passed through a number of carriers, the electron makes up for the electron deficiency in the P 700 molecule. The electronic "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 by a light quantum from P 680, when passing through the electron transport chain to photosystem 1, is used to carry out photophosphorylation. In the case of noncyclic 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 mechanism of photophosphorylation. 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 the hypothesis of P. Mitchell, carries 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 medium is acidified and, as a result, a pH gradient arises: the outer side becomes less acidic than the inner one. This gradient also increases due to the influx of protons, the 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 ejected through special tubules in special mushroom-shaped outgrowths on the outer side of the thylakoid membrane. In these channels there is a conjugation factor (a special protein) that is able to take part in photophosphorylation. It is assumed that such a protein is the enzyme ATPase, which catalyzes the reaction of ATP decomposition, 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 carrier chain 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 ejected 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 the energy of light, ATP and NADPH 2 are formed, which are used in the dark phase, and the product of water photolysis 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 RO 4 → 2 NADPH 2 + 2 ATP + O 2
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 for a very short time pass into a more energy-rich "excited" state. Then the excess energy of a part of the "excited" molecules is converted into heat or emitted in the form of 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 formed hydrogen atoms are loosely connected with organic molecules - carriers of hydrogen. OH hydroxide ions "donate their electrons to other molecules and turn into free OH radicals. OH radicals interact with each other, resulting in the formation of water and molecular oxygen:
4OH \u003d 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: 30 times more ATP is formed in chloroplasts than in the mitochondria of the same plants with the participation of oxygen. In this way, the energy necessary for the processes in the dark phase of photosynthesis is accumulated.
In complex chemical reactions In the dark phase, for which light is not necessary, the key place is occupied by CO2 binding. These reactions involve ATP molecules synthesized during the light phase and hydrogen atoms formed during the photolysis of water and associated with carrier molecules:
6CO2 + 24H - "C6H12O6 + 6NEO
So 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 due to the energy stored by autotrophs in the form of chemical bonds, releasing it in the processes of respiration and fermentation. The energy received by humanity from the combustion of 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 possible the appearance of respiration, and later, after the formation of the ozone layer, allowed life to come to land. The process of photosynthesis is the basis of nutrition for all living beings, and also supplies mankind with fuel (wood, coal, oil), fibers (cellulose) and countless useful chemical compounds. From the carbon dioxide and water bound from the air during photosynthesis, about 90-95% of the dry weight of the crop is formed. The remaining 5-10% are mineral salts and nitrogen obtained from the soil.
Man uses about 7% of the products of photosynthesis for food, as animal feed and as 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, which determines the cycle of carbon, O2 and other elements in nature. It is the material and energy 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. Due to photosynthesis, land plants form about 1.8 1011 tons of dry biomass per year; approximately the same amount of plant biomass is formed annually in the oceans. The rainforest contributes up to 29% to the total production of photosynthesis on 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, which made the light of the Sun the main source - free energy of 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 food oxidation, and this led to the emergence of highly organized heterotrophic organisms (exogenous organic substances are used 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 about 10 times higher than the current energy consumption of mankind. Approximately half of the energy of solar radiation falls on 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. Process characteristic.
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 others. chemical substances during the life of microorganisms. Chemosynthesis also has another name - chemolithoautotrophy. The discovery of chemosynthesis by S. N. Vinogradovsky in 1887 radically changed the ideas of science about the types of metabolism that are basic for living organisms. Chemosynthesis for many microorganisms is the only type nutrition, as they are able to absorb carbon dioxide as the only source of carbon. Unlike photosynthesis, chemosynthesis uses energy instead of light energy, which is formed as a result of redox reactions.
This energy should be sufficient for the synthesis of adenosine triphosphoric acid (ATP), and its amount should exceed 10 kcal/mol. Some of the oxidizable substances donate their electrons to the chain already at the level of cytochrome, and thus an additional energy consumption is created for the synthesis of the reducing agent. In chemosynthesis, the biosynthesis of organic compounds occurs due to the autotrophic assimilation of carbon dioxide, that is, in exactly the same way as in photosynthesis. As a result of the transfer of electrons along the chain of respiratory enzymes of bacteria, which are built into the cell membrane, energy is obtained in the form of ATP. Due to the very high energy consumption, all chemosynthetic bacteria, except for hydrogen ones, form rather little biomass, but at the same time they oxidize a large amount of inorganic substances. Hydrogen bacteria are used by scientists to produce protein and clean the atmosphere of carbon dioxide, especially in closed ecological systems. There is a great variety of chemosynthetic bacteria, most of them belong to Pseudomonas, they are also found among filamentous and budding bacteria, leptospira, spirillum and corynebacteria.
Examples of the use of chemosynthesis by prokaryotes.
The essence of chemosynthesis (a process discovered by the Russian researcher Sergei Nikolaevich Vinogradsky) is the body's obtaining energy through redox reactions carried out by this organism itself with simple (inorganic) substances. Examples of such reactions can be the oxidation of ammonium to nitrite, or ferrous 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, only the ecosystems of some hydrothermals (places on the ocean floor where there are outlets of hot groundwater rich in reduced substances - hydrogen, hydrogen sulfide, iron sulfide, etc.) currently exist, as well as extremely simple ones, consisting only of bacteria , ecosystems found at great depths in rock faults on land.
Bacteria - chemosynthetics, destroy rocks, purify wastewater, participate in the formation of minerals.
Basic concepts and key terms: photosynthesis. Chlorophyll. light phase. dark phase.
Remember! What is plastic exchange?
Think!
The green color is quite often mentioned in the verses of poets. So, Bogdan-Igor Anto-nich has the lines: "... poetry seething and wise, like greens", "... a blizzard of greens, a fire of greens",
"...vegetable rivers rises green flood." Green is the color of renewal, a symbol of youth, tranquility, the color of nature.
Why are plants green?
What are the conditions for photosynthesis?
Photosynthesis (from the Greek photo - light, synthesis - combination) is an extremely complex set of plastic exchange processes. Scientists distinguish three types of photosynthesis: oxygenic (with the release of molecular oxygen in plants and cyanobacteria), anoxic (with the participation of bacteriochlorophyll under anaerobic conditions without oxygen release in photobacteria) and chlorophyll-free (with the participation of bacteriorhodopsins in archaea). At a depth of 2.4 km, green sulfur bacteria GSB1 were found, which use the weak rays of black smokers instead of sunlight. But, as K. Swenson wrote in a monograph on cells: "The primary source of energy for wildlife is the energy of visible light."
The most common in living nature is oxygen photosynthesis, which requires light energy, carbon dioxide, water, enzymes and chlorophyll. Light for photosynthesis is absorbed by chlorophyll, water is delivered to the cells through the pores of the cell wall, carbon dioxide enters the cells by diffusion.
The main photosynthetic pigments are chlorophylls. Chlorophils (from the Greek chloros - green and phylon - leaf) are green pigments of plants, with the participation of which photosynthesis occurs. The green color of chlorophyll is a device for absorbing blue rays and partially red ones. And green rays are reflected from the body of plants, fall on the retina of the human eye, irritate the cones and cause color visual sensations. That's why plants are green!
In addition to chlorophylls, plants have auxiliary carotenoids, cyanobacteria and red algae have phycobilins. Green
and purple bacteria contain bacteriochlorophylls that absorb blue, violet, and even infrared rays.
Photosynthesis occurs in higher plants, algae, cyanobacteria, some archaea, that is, in organisms known as photo-autotrophs. Photosynthesis in plants is carried out in chloroplasts, in cyanobacteria and photobacteria - on internal invaginations of membranes with photopigments.
So, PHOTOSYNTHESIS is the process of formation of organic compounds from inorganic ones using light energy and with the participation of photosynthetic pigments.
What are the features of the light and dark phases of photosynthesis?
In the process of photosynthesis, two stages are distinguished - the light and dark phases (Fig. 49).
The light phase of photosynthesis occurs in the grana of chloroplasts with the participation of light. This stage begins from the moment of absorption of light quanta by the chlorophyll molecule. In this case, the electrons of the magnesium atom in the chlorophyll molecule move to a higher energy level, accumulating potential energy. A significant part of the excited electrons transfers it to others chemical compounds for the formation of ATP and the reduction of NADP (nicotinamide adenine dinucleotide phosphate). This compound with such a long name is the universal biological carrier of hydrogen in the cell. Under the influence of light, the process of decomposition of water - photolysis occurs. This produces electrons (e“), protons (H +) and, as a by-product, molecular oxygen. Hydrogen protons H +, attaching electrons with high energy level, are converted into atomic hydrogen, which is used to reduce NADP+ to NADP. N. Thus, the main processes of the light phase are: 1) photolysis of water (splitting of water under the action of light with the formation of oxygen); 2) reduction of NADP (addition of a hydrogen atom to NADP); 3) photophosphorylation (formation of ATP from ADP).
So, the light phase is a set of processes that ensure the formation of molecular oxygen, atomic hydrogen and ATP due to light energy.
The dark phase of photosynthesis occurs in the stroma of chloroplasts. Its processes do not depend on light and can proceed both in the light and in the dark, depending on the needs of the cell for glucose. The basis of the dark phase is a cyclic reaction called the carbon dioxide fixation cycle, or Calvin cycle. This process was first studied by the American biochemist Melvin Calvin (1911 - 1997), laureate Nobel Prize in Chemistry (1961). In the dark phase, glucose is synthesized from carbon dioxide, hydrogen from NADP and the energy of ATP. CO2 fixation reactions are catalyzed by ribulose bisphosphate carboxylase (Rubisco), the most common enzyme on Earth.
So, the dark phase is a set of cyclic reactions that, thanks to the chemical energy of ATP, provide the formation of glucose using carbon dioxide, which is a source of carbon, and water, a source of hydrogen.
What is the planetary role of photosynthesis?
The importance of photosynthesis for the biosphere cannot be overestimated. It is through this process that the light energy of the Sun is converted by photo-autotrophs into the chemical energy of carbohydrates, which generally give primary organic matter. Food chains begin with it, along which energy is transferred to heterotrophic organisms. Plants serve as food for herbivores, which receive the necessary nutrients through this. Then herbivores become food for predators, they also need energy, without which life is impossible.
Only a small part of the Sun's energy is captured by plants and used for photosynthesis. The energy of the Sun is mainly used to evaporate and maintain the temperature regime of the earth's surface. So, only about 40 - 50% of the solar energy penetrates into the biosphere, and only 1 - 2% of the solar energy is converted into synthesized organic matter.
Green plants and cyanobacteria affect the gas composition of the atmosphere. All oxygen in the modern atmosphere is a product of photosynthesis. The formation of the atmosphere completely changed the state of the earth's surface, made possible the emergence of aerobic respiration. Later in the process of evolution, after the formation of the ozone layer, living organisms made landfall. In addition, photosynthesis prevents the accumulation of CO 2 and protects the planet from overheating.
So, photosynthesis is of planetary importance, ensuring the existence of the living nature of planet Earth.
ACTIVITY Match task
Using the table, compare photosynthesis with aerobic respiration and draw a conclusion about the relationship between plastic and energy metabolism.
COMPARATIVE CHARACTERISTICS OF PHOTOSYNTHESIS AND AEROBIC RESPIRIT
Knowledge Application Task
Recognize and name the levels of organization of the process of photosynthesis in plants. Name the adaptations of a plant organism for photosynthesis at different levels of its organization.
ATTITUDE Biology + Literature
K. A. Timiryazev (1843 - 1920), one of the most famous researchers of photosynthesis, wrote: “The microscopic green grain of chlorophyll is a focus, a point in the world space, into which the energy of the Sun flows from one end, and all manifestations of life originate from the other on the ground. It is the real Prometheus, who stole fire from the sky. The ray of sun stolen by him burns both in the shimmering abyss and in the dazzling spark of electricity. The ray of the sun sets in motion both the flywheel of a giant steam engine, and the artist's brush, and the poet's pen. Apply your knowledge and prove the statement that the ray of the Sun sets the poet's pen in motion.
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Tasks for self-control |
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1. What is photosynthesis? 2. What is chlorophyll? 3. What is light phase photosynthesis? 4. What is the dark phase of photosynthesis? 5. What is primary organic matter? 6. How does photosynthesis determine the aerobic respiration of organisms? |
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7. What are the conditions for photosynthesis? 8. What are the features of the light and dark phases of photosynthesis? 9. What is the planetary role of photosynthesis? |
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10. What are the similarities and differences between photosynthesis and aerobic respiration? |
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