Processes occurring during the light stage of photosynthesis. The concept of photosynthesis, where and what happens during the light phase of photosynthesis

With or without the use of light energy. It is characteristic of plants. Let us next consider what the dark and light phases of photosynthesis are.

General information

The organ of photosynthesis in higher plants is the leaf. Chloroplasts act as organelles. Photosynthetic pigments are present in the membranes of their thylakoids. They are carotenoids and chlorophylls. The latter exist in several forms (a, c, b, d). The main one is a-chlorophyll. Its molecule contains a porphyrin “head” with a magnesium atom located in the center, as well as a phytol “tail”. The first element is presented as a flat structure. The “head” is hydrophilic, therefore it is located on that part of the membrane that is directed towards aquatic environment. The phytol "tail" is hydrophobic. Due to this, it retains the chlorophyll molecule in the membrane. Chlorophylls absorb blue-violet and red light. They also reflect green, giving plants their characteristic color. In thylactoid membranes, chlorophyll molecules are organized into photosystems. Blue-green algae and plants are characterized by systems 1 and 2. Photosynthetic bacteria have only the first. The second system can decompose H 2 O and release oxygen.

Light phase of photosynthesis

The processes occurring in plants are complex and multi-stage. In particular, two groups of reactions are distinguished. They are the dark and light phases of photosynthesis. The latter occurs with the participation of the enzyme ATP, electron transfer proteins, and chlorophyll. The light phase of photosynthesis occurs in thylactoid membranes. Chlorophyll electrons become excited and leave the molecule. After this, they end up on the outer surface of the thylactoid membrane. It, in turn, becomes negatively charged. After oxidation, the reduction of chlorophyll molecules begins. They take electrons from water, which is present in the intralacoid space. Thus, the light phase of photosynthesis occurs in the membrane during decay (photolysis): H 2 O + Q light → H + + OH -

Hydroxyl ions turn into reactive radicals, donating their electrons:

OH - → .OH + e -

OH radicals combine to form free oxygen and water:

4NO. → 2H 2 O + O 2.

In this case, oxygen is removed into the surrounding (external) environment, and protons accumulate inside the thylactoid in a special “reservoir”. As a result, where the light phase of photosynthesis occurs, the thylactoid membrane receives a positive charge due to H + on one side. At the same time, due to electrons, it is charged negatively.

Phosphyrylation of ADP

Where the light phase of photosynthesis occurs, there is a potential difference between the inner and outer surfaces of the membrane. When it reaches 200 mV, protons begin to be pushed through the channels of ATP synthetase. Thus, the light phase of photosynthesis occurs in the membrane when ADP is phosphorylated to ATP. In this case, atomic hydrogen is sent to restore the special carrier nicotinamide adenine dinucleotide phosphate NADP+ to NADP.H2:

2Н + + 2е — + NADP → NADP.Н 2

The light phase of photosynthesis thus includes the photolysis of water. It, in turn, is accompanied by three most important reactions:

1. ATP synthesis.
2. Formation of NADP.H 2.
3. Formation of oxygen.

The light phase of photosynthesis is accompanied by the release of the latter into the atmosphere. NADP.H2 and ATP move into the stroma of the chloroplast. This completes the light phase of photosynthesis.

Another group of reactions

The dark phase of photosynthesis does not require light energy. It goes in the stroma of the chloroplast. The reactions are presented in the form of a chain of sequential transformations of carbon dioxide coming from the air. As a result, glucose and other organic substances are formed. The first reaction is fixation. Ribulose biphosphate (five-carbon sugar) RiBP acts as a carbon dioxide acceptor. The catalyst in the reaction is ribulose biphosphate carboxylase (enzyme). As a result of carboxylation of RiBP, a six-carbon unstable compound is formed. It almost instantly breaks down into two molecules of PGA (phosphoglyceric acid). After this, a cycle of reactions occurs where it is transformed into glucose through several intermediate products. They use the energy of NADP.H 2 and ATP, which were converted during the light phase of photosynthesis. The cycle of these reactions is called the “Calvin cycle”. It can be represented as follows:

6CO 2 + 24H+ + ATP → C 6 H 12 O 6 + 6H 2 O

In addition to glucose, other monomers of organic (complex) compounds are formed during photosynthesis. These include, in particular, fatty acids, glycerol, amino acids and nucleotides.

C3 reactions

They are a type of photosynthesis that produces three-carbon compounds as the first product. It is this that is described above as the Calvin cycle. As characteristic features C3 photosynthesis is performed by:

1. RiBP is an acceptor for carbon dioxide.
2. The carboxylation reaction is catalyzed by RiBP carboxylase.
3. A six-carbon substance is formed, which subsequently breaks down into 2 FHA.

Phosphoglyceric acid is reduced to TP (triose phosphates). Some of them are used for the regeneration of ribulose biphosphate, and the rest is converted into glucose.

C4 reactions

This type of photosynthesis is characterized by the appearance of four-carbon compounds as the first product. In 1965, it was discovered that C4 substances appear first in some plants. For example, this has been established for millet, sorghum, sugar cane, and corn. These crops became known as C4 plants. The next year, 1966, Slack and Hatch (Australian scientists) discovered that they almost completely lack photorespiration. It was also found that such C4 plants absorb carbon dioxide much more efficiently. As a result, the pathway of carbon transformation in such crops began to be called the Hatch-Slack pathway.

Conclusion

The importance of photosynthesis is very great. Thanks to it, carbon dioxide is absorbed from the atmosphere in huge volumes (billions of tons) every year. Instead, no less oxygen is released. Photosynthesis acts as the main source of formation organic compounds. Oxygen is involved in the formation of the ozone layer, which protects living organisms from the effects of short-wave UV radiation. During photosynthesis, a leaf absorbs only 1% of the total energy of light falling on it. Its productivity is within 1 g of organic compound per 1 sq. m of surface per hour.

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 energy conversion 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 inner surface The thylakoid membrane is charged positively (due to H +), and the outer membrane 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:

Photosynthesis is a set of processes of forming light energy into the energy of chemical bonds of organic substances with the participation of photosynthetic dyes.

This type of nutrition is characteristic of plants, prokaryotes and some types of unicellular eukaryotes.

During 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 understands the concept of photosynthesis as a 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 through chloroplasts- semi-autonomous double-membrane organelles belonging to the class of plastids. The flat shape of the sheet plates ensures high-quality absorption and full use of light energy and carbon dioxide. The water needed for natural synthesis comes from the roots through water-conducting tissue. Gas exchange occurs by diffusion through the stomata and partly through the cuticle.

Chloroplasts are filled with colorless stroma and penetrated by lamellae, which, when connected to each other, form thylakoids. It is in them that photosynthesis occurs. Cyanobacteria themselves are chloroplasts, so the apparatus for natural synthesis in them is not separated 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 have the pigment bacteriochlorophyll, and these organisms do not release oxygen after natural synthesis is completed.

Photosynthesis goes through two phases - light and dark. Each of them is characterized by certain reactions and interacting substances. Let's take a closer look at the process of the phases of photosynthesis.

Light

First phase of photosynthesis characterized by the formation of high-energy products, which are ATP, the cellular energy source, and NADP, the reducing agent. At the end of the stage, oxygen is produced as a by-product. The light stage necessarily occurs with sunlight.

The process of photosynthesis occurs in thylakoid membranes with the participation of electron transport proteins, ATP synthetase and chlorophyll (or other pigment).

The functioning of electrochemical chains, through which electrons and partially hydrogen protons are transferred, is formed in complex complexes formed by pigments and enzymes.

Description of the light phase process:

1. When sunlight hits the leaf blades of plant organisms, chlorophyll electrons in the structure of the plates are excited;
2. In the active state, the particles leave the pigment molecule and land on the outer side of the thylakoid, which is negatively charged. This occurs simultaneously with the oxidation and subsequent reduction of chlorophyll molecules, which take away the next electrons from the water entering the leaves;
3. Then photolysis of water occurs with the formation of ions, which donate electrons and are converted into OH radicals that can participate in further reactions;
4. These radicals then combine to form water molecules and free oxygen released into the atmosphere;
5. The thylakoid membrane acquires a positive charge on one side due to the hydrogen ion, and on the other side a negative charge due to electrons;
6. When a difference of 200 mV is reached between the sides of the membrane, protons pass through the enzyme ATP synthetase, which leads to the conversion of ADP to ATP (phosphorylation process);
7. With the atomic hydrogen released from water, NADP + is reduced to NADP H2;

While free oxygen is released into the atmosphere during 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 external environment through stomata in leaves. The dark phase processes 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 produced during the light phase, reactions in organisms can occur both day and night. Processes at this stage occur faster than at the previous one.

The totality of all processes occurring in the dark phase is presented in the form of a unique chain of sequential transformations of carbon dioxide coming from the external environment:

1. 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 course of the reaction, which results in the formation of a six-carbon compound that breaks down into 2 molecules of phosphoglyceric acid;
2. Then a rather complex cycle occurs, including a certain number of reactions, upon completion of which phosphoglyceric acid is converted into natural sugar - glucose. This process is called the Calvin cycle;

Along with sugar, the formation of fatty acids, amino acids, glycerol and nucleotides also occurs.

The essence of photosynthesis

From the table comparing the light and dark phases of natural synthesis, you can briefly describe the essence of each of them. The light phase occurs in the grana of the chloroplast with the obligatory inclusion of light energy in the reaction. The reactions involve components such as electron transfer proteins, ATP synthetase and chlorophyll, which, when interacting with water, form free oxygen, ATP and NADP H2. For the dark phase, which occurs in the stroma of the chloroplast, sunlight is not necessary. The ATP and NADP H2 obtained at the previous 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 that involve different substances. 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.

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 are very a short time move into a more energy-rich “excited” state. 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, always present in aqueous solution due to water dissociation. 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, solar radiation energy is used in the light phase for the synthesis of ATP and ADP and phosphate without the participation of oxygen. This is 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 the complex of chemical reactions of the dark phase, for the course of which light is not necessary, key place occupies 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 during 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 most important event in biological evolution, which made 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 level of cytochrome, and thus are created for the synthesis of a reducing agent additional expense energy. 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 diversity of chemosynthetic bacteria, their most of belongs 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 capable of chemosynthesis certain groups prokaryotes (bacteria in in a broad sense words). 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.

It is better to explain such a voluminous material as photosynthesis 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 on the study of leaf chloroplasts. After this, it is necessary to move on 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 diagram:

As you explain, you need 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 grana thylakoid membranes, leads to the loss of one electron and transfers it to an excited state. Electrons are transferred along the electron transport chain, resulting in the reduction of NADP + to NADP H.

2. The place of the released electrons in chlorophyll molecules is taken by the electrons of water molecules - this is how water undergoes decomposition (photolysis) under the influence of light. The resulting hydroxyls OH– 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 electrical potential difference (EPD) across the thylakoid membrane.

4. When the critical REF is reached, protons rush out through the proton channel. This stream of positively charged particles is used to produce chemical energy using a special enzyme complex. The resulting ATP molecules move into the stroma, where they participate in carbon fixation reactions.

5. Hydrogen ions released to the surface of the thylakoid membrane combine with electrons, forming atomic hydrogen, which is used to restore the NADP + transporter.

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After consideration this issue Having analyzed it again according to the diagram, we invite students to fill out the table.

Table. Reactions of light and dark phases of photosynthesis

After filling out 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, forming an unstable six-carbon compound, which breaks down into two molecules of 3-phosphoglyceric acid (PGA).

2. PGA molecules accept one phosphate group from ATP and are enriched with energy.

3. Each of the FHAs attaches one hydrogen atom from two carriers, turning into a triose. Trioses combine to form glucose and then starch.

4. Triose molecules combining to form different combinations, form pentoses and re-enter the cycle.

Total reaction of photosynthesis:

Scheme. Photosynthesis process

Test

1. Photosynthesis occurs in organelles:

a) mitochondria;
b) ribosomes;
c) chloroplasts;
d) chromoplasts.

2. The chlorophyll pigment is concentrated in:

a) chloroplast shell;
b) stroma;
c) grains.

3. Chlorophyll absorbs light in the spectrum region:

a) red;
b) green;
c) purple;
d) throughout the region.

4. Free oxygen during photosynthesis is released during the breakdown of:

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 photosynthesis, ATP:

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 necessary:

1) as a trap for electrons;
2) as an enzyme for the formation of starch;
3) how component chloroplast membranes;
4) as an enzyme for photolysis of water.

9. Photolysis of water is:

1) accumulation of water under the influence of light;
2) dissociation of water into ions under the influence of light;
3) release of water vapor through stomata;
4) injection of water into the leaves under the influence of light.

10. Under the influence of light quanta:

1) chlorophyll is converted into NADP;
2) an electron leaves the chlorophyll molecule;
3) the chloroplast increases in volume;
4) chlorophyll is converted into ATP.

LITERATURE

Bogdanova T.P., Solodova E.A. Biology. Handbook for high school students and applicants to universities. – M.: LLC “AST-Press School”, 2007.