The intensity of photosynthesis depends on a number of factors. First, on the wavelength of light. The process proceeds most effectively under the action of the waves of the blue-violet and red parts of the spectrum. In addition, the rate of photosynthesis is affected by the degree of illumination, and up to a certain point the rate of the process increases in proportion to the amount of light, then the note is no longer dependent on it.

Another factor is the concentration of carbon dioxide. The higher it is, the more intense the process of photosynthesis. Under normal conditions, the lack of carbon dioxide is the main limiting factor, since it contains a small percentage in the atmospheric air. However, under greenhouse conditions, this deficiency can be eliminated, which will favorably affect the rate of photosynthesis and the growth rate of plants.

An important factor in the intensity of photosynthesis is temperature. All photosynthesis reactions are catalyzed by enzymes, for which the optimal temperature range is 25-30 ° C. At more low temperatures the rate of action of enzymes is sharply reduced.

Water is an important factor influencing photosynthesis. However, it is impossible to quantify this factor, since water is involved in many other metabolic processes occurring in the plant cell.

The Importance of Photosynthesis. Photosynthesis is a fundamental process in living nature. Thanks to him, from inorganic substances - carbon dioxide and water - with the participation of the energy of sunlight, green plants synthesize organic substances necessary for the life of all life on Earth. The primary synthesis of these substances ensures the implementation of the processes of assimilation and dissimilation in all organisms.

The products of photosynthesis - organic substances - are used by organisms:

• to build cells;
• as a source of energy for life processes.

Man uses substances created by plants:

• as food (fruits, seeds, etc.);
• as an energy source (coal, peat, wood);
• as a building material.

Mankind owes its existence to photosynthesis. All fuels on Earth are products of photosynthesis. Using fossil fuels, we get the energy stored as a result of photosynthesis by ancient plants that existed in past geological epochs.

Simultaneously with the synthesis of organic substances, a by-product of photosynthesis, oxygen, is released into the Earth's atmosphere, which is necessary for the respiration of organisms. Without oxygen, life on our planet is impossible. Its reserves are constantly spent on products of combustion, oxidation, respiration occurring in nature. According to scientists, without photosynthesis, the entire supply of oxygen would be used up within 3,000 years. Therefore, photosynthesis is of the greatest importance for life on Earth.

For many centuries scientists biologists tried to unravel the mystery of the green leaf. For a long time it was believed that plants create nutrients from water and minerals. This belief is connected with the experiment of the Dutch researcher Anna van Helmont, conducted back in the 17th century. He planted a willow tree in a tub, accurately measuring the mass of the plant (2.3 kg) and dry soil (90.8 kg). For five years, he only watered the plant, adding nothing to the soil. After five years, the mass of the tree increased by 74 kg, while the mass of the soil decreased by only 0.06 kg. The scientist concluded that the plant forms all substances from water. Thus, one substance was established that the plant absorbs during photosynthesis.

The first attempt to scientifically determine the function of a green leaf was made in 1667 by the Italian naturalist Marcello Malpighi. He noticed that if the first germinal leaves are torn off from pumpkin seedlings, then the plant stops developing. Studying the structure of plants, he made an assumption: under the influence of sunlight, some transformations occur in the leaves of the plant and water evaporates. However, these assumptions were ignored at the time.

After 100 years, the Swiss scientist Charles Bonnet conducted several experiments by placing a leaf of a plant in water and lighting it with sunlight. Only he made an incorrect conclusion, believing that the plant does not participate in the formation of bubbles.

The discovery of the role of the green leaf belongs to the English chemist Joseph Priestley. In 1772, while studying the importance of air for burning substances and breathing, he set up an experiment and found out that plants improve the air and make it suitable for breathing and burning. After a series of experiments, Priestley noticed that plants improve the air in the light. He was the first to suggest the role of light in the life of plants.

In 1800, the Swiss scientist Jean Senebier scientifically explained the essence of this process (by that time Lavoisier had already discovered oxygen and studied its properties): plant leaves decompose carbon dioxide and release oxygen only under the action of sunlight.

In the second half of the 19th century, an alcohol extract was obtained from the leaves of green plants. This substance is called chlorophyll.

German naturalist Robert Mayer discovered that plants absorb sunlight and turn it into energy. chemical bonds organic substances (the amount of carbon stored in the plant in the form of organic substances directly depends on the amount of light falling on the plant).

Kliment Arkadyevich Timiryazev, a Russian scientist, studied the influence of various parts of the sunlight spectrum on the process of photosynthesis. He managed to establish that it is in the red rays that photosynthesis proceeds most efficiently, and to prove that the intensity of this process corresponds to the absorption of light by chlorophyll.

K.A. Timiryazev emphasized that by assimilating carbon, the plant also assimilates sunlight, converting its energy into the energy of organic substances.

Endogenous mechanisms of regulation of photosynthesis.

The implementation of the photosynthetic function of the plant as a whole is determined, on the one hand, by the significant autonomy of chloroplasts, and, on the other hand, by a complex system of links between photosynthesis and all plant functions. In the course of ontogenesis, the plant organism always contains attracting zones(zones that attract nutrients). In attracting centers, either neoplasm and growth of structures occurs, or an intensive unidirectional synthesis of reserve substances (tubers, fruits, etc.) occurs. In both cases, the state of attracting centers determines the amount of "request" for photosynthesis. If external conditions do not limit photosynthesis, then the leading role belongs to attracting centers. The more powerful the centers that attract assimilates, the more intense photosynthesis.

Second. An important mechanism for the regulation of photosynthesis is associated with phytohormones and endogenous inhibitors of growth and metabolism. Phytohormones are formed in different parts of the plant, including chloroplasts, and act on the processes of photosynthesis both remotely and directly at the level of chloroplasts. Remote influence is carried out due to the regulating influence of phytohormones on the processes of growth and development, on the deposition of substances in the reserve, on the transport of assimilates, etc., i.e. on the formation and activity of attracting centers. On the other hand, phytohormones have a direct effect on the functional activity of chloroplasts through a change in the state of membranes, enzyme activity, and the generation of a transmembrane potential. The role of phytohormones, in particular cytokinin, in the biogenesis of chloroplasts, the synthesis of chlorophylls, enzymes of Calvin has also been proven.

The intensity of photosynthesis is influenced by such environmental factors as: the intensity and quality of light, the concentration of carbon dioxide, temperature, the water regime of plant tissues, mineral nutrition, etc.

Intensity and spectral composition of light .

The leaves of higher plants absorb light in the red and blue regions of the spectrum - the rays that are most effective for photosynthesis .. The leaves reflect green rays. Most (60%) of the solar radiation falling on the leaf cannot participate in photochemical processes, since it has a wavelength that is not absorbed by the leaf pigments. Part of the light is reflected by the leaf surface, dissipated in the form of heat, spent on processes not related to photosynthesis, and only 1.5-5% is spent on photosynthesis (photosynthetically active radiation - PAR).

The dependence of the rate of photosynthesis on light intensity has the form of a logarithmic curve. At low illumination, a point can be distinguished on the light curve when the amount of carbon dioxide absorbed during photosynthesis and released during respiration are equal. This point is called light compensation point (fig.). An increase in illumination above the light compensation point causes a gradual increase in the intensity of photosynthesis. With a further increase in intensity, the curve reaches a plateau, which indicates saturation of the process of carbon dioxide binding. Under these conditions, the process of photosynthesis is already limited only by the content of carbon dioxide. In light-loving species, saturation occurs at higher illumination (10-40 thousand lux) than in shade-tolerant species (1000 lux).

The activity of photosynthesis in the region of saturating light intensity is limited by the CO2 concentration and depends on the power of the carbon dioxide absorption and reduction system. The higher the ability of the plant to restore CO 2, the higher the light curve of photosynthesis passes

Rice. Change in the intensity of photosynthesis in the quinoa Atriplex triangularis, grown under various lighting conditions.

Therefore, in C 3 plants, saturation occurs at lower illumination than in C 4 plants, which bind carbon dioxide more efficiently.

CO 2 is the main substrate of photosynthesis. The dependence of photosynthesis on the concentration of carbon dioxide is described by a logarithmic curve (Fig.). At a concentration of 0.036%, the intensity of photosynthesis is only 50% and reaches a maximum at 0.3%.

Rice. The dependence of the intensity of photosynthesis on partial pressure CO 2

Many biological processes, in which gases (carbon dioxide, oxygen) are involved, are determined not by concentration, but by partial pressure. For example, if the atmospheric pressure is 0.1 MPa, then the partial pressure of carbon dioxide will be 36 Pa (it is calculated by multiplying the molar content of gas by the total atmospheric pressure of 0.036x0.1 MPa).

In C 3 -plants at low concentrations of carbon dioxide, the amount of CO 2 fixed during photosynthesis is less than the amount of CO 2 released during respiration. With an increase in CO 2, you can fix the point at which the total absorption of carbon dioxide in photosynthesis is 0. This concentration of CO 2 is called carbon dioxide compensation point. This parameter characterizes the ratio between the processes of photosynthesis and respiration, depending on the content of CO 2 in the atmosphere.

The process of photosynthesis is usually carried out under aerobic conditions. At an oxygen concentration of 21%. An increase in the content or lack of oxygen for photosynthesis is unfavorable.

High oxygen concentrations reduce the intensity of photosynthesis for the following reasons: 1) an increase in partial pressure activates the photorespiration process (Calvin's RBF carboxylase works as an oxygenase); 20 oxygen oxidizes the primary reduced products of photosynthesis.

Temperature

The dependence of the intensity of photosynthesis on temperature has the form of a parabola with a maximum of 25 o -35 o C. However, if the concentration of carbon dioxide in the air is higher, then the temperature optimum will shift to 35-38 o C. This is explained by the fact that it is at such temperatures that active enzymatic reactions (dark phase of photosynthesis) (Fig.).

Rice. The dependence of the intensity of photosynthesis on temperature: 1 - at a high content of carbon dioxide; 2 - at 0.036%

Water regime

Water is directly involved in photosynthesis as a substrate for oxidation and a source of oxygen. On the other hand, the amount of water content in the tissues determines the degree of stomata opening and, consequently, the influx of CO2 into the leaf. When the leaf is completely saturated with water, the stomata close, which reduces the intensity of photosynthesis. Therefore, a slight water deficit is favorable for photosynthesis. In drought conditions, the stomata close under the influence of abscisic acid, which accumulates in the leaves. Prolonged water deficiency leads to inhibition of non-cyclic and cyclic electron transport and photophosphorylation.

mineral nutrition

For the normal functioning of the photosynthetic apparatus, the plant must be provided with the whole complex of macro- and microelements. The dependence of photosynthesis on mineral nutrition elements is determined by their necessity for the formation of the photosynthetic apparatus (pigments, ETC components, structural and transport proteins).

Magnesium is a part of chlorophylls, participates in the activity of conjugating proteins in the synthesis of ATP, affects the activity of carboxylation reactions and reduction of NADP+.

Iron is necessary for the functioning of cytochromes, ferredoxin (components of the ETC). Iron deficiency disrupts the functioning of cyclic and non-cyclic photophosphorylation, the synthesis of pigments, and disrupts the structure of chloroplasts.

Manganese and chlorine are essential for water photolysis.

Copper is part of plastocyanin.

Nitrogen is part of chlorophylls, amino acids. Its deficiency affects the activity of photosynthesis in general.

Phosphorus is essential for the photochemical and dark reactions of photosynthesis. Both a lack and an excess of it have a negative effect (membrane permeability is disturbed)

Potassium is necessary for the formation of the faceted structure of chloroplasts, the work of stomata, and the absorption of water by cells. With a lack of potassium, all photosynthesis processes are disrupted.

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Green leaf is the source of life on our planet. If it were not for green plants, there would be neither animals nor people on Earth. One way or another, plants serve as a source of food for the entire animal world.

A person uses the energy not only of the sun's rays falling on the earth now, but also of those that fell on it tens and hundreds of millions of years ago. After all, coal, oil, and peat are chemically altered remains of plants and animals that lived in those distant times.

In recent decades, the attention of leading specialists in a number of branches of natural science has been riveted to the problem of photosynthesis, its various aspects are being comprehensively and deeply studied in many laboratories around the world. Interest is determined primarily by the fact that photosynthesis is the basis of the energy exchange of the entire biosphere.

The intensity of photosynthesis depends on many factors. light intensity , necessary for the greatest efficiency of photosynthesis, in various plants different. In shade-tolerant plants, the maximum activity of photosynthesis is reached at about half of full sunlight, and in photophilous plants - almost at full sunlight.

Many shade-tolerant plants do not develop palisade (columnar) parenchyma in the leaves, and there is only spongy (lily of the valley, hoof). In addition, these plants have larger leaves and larger chloroplasts.

Also affects the rate of photosynthesis temperature environment . The highest intensity of photosynthesis is observed at a temperature of 20–28 °C. With a further increase in temperature, the intensity of photosynthesis decreases, and the intensity of respiration increases. When the rates of photosynthesis and respiration coincide, they speak of compensation point.

The compensation point changes depending on the intensity of the light, the rise and fall of the temperature. For example, in cold-resistant brown algae, it corresponds to a temperature of about 10 ° C. Temperature affects, first of all, chloroplasts, in which the structure changes depending on temperature, which is clearly visible in an electron microscope.

Highly great importance for photosynthesis carbon dioxide content in the air surrounding the plant. The average concentration of carbon dioxide in the air is 0.03% (by volume). A decrease in carbon dioxide content adversely affects the yield, and its increase, for example, to 0.04%, can increase the yield by almost 2 times. A more significant increase in concentration is harmful to many plants: for example, at a carbon dioxide content of about 0.1%, tomato plants get sick, their leaves begin to curl. In greenhouses and greenhouses, you can increase the carbon dioxide content by releasing it from special cylinders or letting dry carbon dioxide evaporate.

Light of different wavelengths also affects the intensity of photosynthesis in different ways. For the first time, the intensity of photosynthesis in different rays of the spectrum was studied by the physicist W. Daubeny, who showed in 1836 that the rate of photosynthesis in a green leaf depends on the nature of the rays. Methodical errors during the experiment led him to wrong conclusions. The scientist placed a segment of an elodea shoot in a test tube with water cut up, illuminated the test tube by passing sunlight through colored glasses or colored solutions, and took into account the intensity of photosynthesis by the number of oxygen bubbles coming off the cut surface per unit time. Daubeny came to the conclusion that the intensity of photosynthesis is proportional to the brightness of the light, and the brightest rays at that time were considered yellow. John Draper (1811-1882), who studied the intensity of photosynthesis in various beams of the spectrum emitted by a spectroscope, adhered to the same point of view.

The role of chlorophyll in the process of photosynthesis was proved by the outstanding Russian botanist and plant physiologist K.A. Timiryazev. Having spent in 1871-1875. a series of experiments, he found that green plants most intensively absorb the rays of the red and blue parts of the solar spectrum, and not yellow, as was thought before him. Absorbing the red and blue part of the spectrum, chlorophyll reflects green rays, which is why it appears green.

Based on these data, the German plant physiologist Theodor Wilhelm Engelmann in 1883 developed bacterial method study of carbon dioxide assimilation by plants.

He suggested that if you place a cell of a green plant together with aerobic bacteria in a drop of water and illuminate them with differently colored rays, then the bacteria should concentrate in those parts of the cell in which carbon dioxide is most decomposed and oxygen is released. To test this, Engelman somewhat improved the light microscope by mounting a prism above the mirror, which decomposed the sunlight into separate components of the spectrum. As a green plant, Engelman used the green alga Spirogyra, whose large cells contain long spiral chromatophores.

Having placed a piece of algae in a drop of water on a glass slide, Engelman introduced some aerobic bacteria there, after which he examined the preparation under a microscope. It turned out that in the absence of a prism, the prepared preparation was illuminated with even white light, and the bacteria were evenly distributed along the entire area of ​​the algae. In the presence of a prism, the beam of light reflected from the mirror was refracted, illuminating the area of ​​the algae under the microscope with light of different wavelengths. After a few minutes, the bacteria concentrated on those areas that were illuminated with red and blue light. Based on this, Engelman concluded that the decomposition of carbon dioxide (and, hence, the release of oxygen) in green plants is observed in additional to the main color (i.e. green) rays - red and blue.

Data received on modern equipment, fully confirm the results obtained by Engelman more than 120 years ago.

The light energy absorbed by chlorophyll takes part in the reactions of the first and second stages of photosynthesis; reactions of the third stage are dark; occurs without the participation of light. Measurements have shown that the process of reducing one oxygen molecule requires a minimum of eight quanta of light energy. Thus, the maximum quantum yield of photosynthesis, i.e. the number of oxygen molecules corresponding to one quantum of light energy absorbed by the plant is 1/8, or 12.5%.

R. Emerson and colleagues determined the quantum yield of photosynthesis when plants are illuminated with monochromatic light of various wavelengths. It was found that the yield remains constant at 12% in most of the visible spectrum, but decreases sharply near the far red region. This decrease in green plants begins at a wavelength of 680 nm. At lengths greater than 660 nm, only chlorophyll absorbs light. a; chlorophyll b has a maximum absorption of light at 650 nm, and at 680 nm practically does not absorb light. At a wavelength greater than 680 nm, the quantum yield of photosynthesis can be increased to a maximum value of 12%, provided that the plant is also illuminated with light at a wavelength of 650 nm at the same time. In other words, if the light absorbed by chlorophyll a supplemented by light absorbed by chlorophyll b, then the quantum yield of photosynthesis reaches a normal value.

The increase in the intensity of photosynthesis during simultaneous illumination of a plant with two beams of monochromatic light of different wavelengths compared to its intensity observed under separate illumination by the same beams is called Emerson effect. Experiments with various combinations of far red light and light of shorter wavelength over green, red, blue-green and brown algae have shown that the greatest increase in photosynthesis is observed if the second beam with a shorter wavelength is absorbed by auxiliary pigments.

In green plants, such auxiliary pigments are carotenoids and chlorophyll. b, in red algae - carotenoids and phycoerythrin, in blue-green algae - carotenoids and phycocyanin, in brown algae - carotenoids and fucoxanthin.

Further study of the process of photosynthesis led to the conclusion that auxiliary pigments transfer from 80 to 100% of the light energy absorbed by them to chlorophyll. a. Thus, chlorophyll a accumulates light energy absorbed by a plant cell and then uses it in a photo chemical reactions photosynthesis.

It was later discovered that chlorophyll a is present in a living cell in the form of forms with different absorption spectra and different photochemical functions. One form of chlorophyll a, whose absorption maximum corresponds to a wavelength of 700 nm, belongs to the pigment system, called photosystem I, the second form of chlorophyll a with an absorption maximum of 680 nm, belongs to photosystem II.

So, a photoactive pigment system was discovered in plants, which absorbs light especially strongly in the red region of the spectrum. It begins to act even in low light. In addition, another regulatory system is known that selectively absorbs and uses for photosynthesis Blue colour. This system works in sufficiently strong light.

It has also been established that the photosynthetic apparatus of some plants largely uses red light for photosynthesis, while others use blue light.

To determine the intensity of photosynthesis of aquatic plants, you can use the method of counting oxygen bubbles. In the light, the process of photosynthesis takes place in the leaves, the product of which is oxygen, which accumulates in the intercellular spaces. When cutting the stem, excess gas begins to be released from the cut surface in the form of a continuous flow of bubbles, the rate of formation of which depends on the intensity of photosynthesis. This method does not differ in great accuracy, but it is simple and gives a visual representation of the dependence of the photosynthesis process on external conditions.

Experience 1. Dependence of photosynthesis productivity on light intensity

Materials and equipment: elodea; aqueous solutions NaHCO 3 , (NH 4) 2 CO 3 or mineral water; settled tap water; glass rod; threads; scissors; 200 W electric lamp; watch; thermometer.

1. For the experiment, healthy shoots of elodea about 8 cm long of intense green color with an intact tip were selected. They were cut under water, tied with a thread to a glass rod and lowered upside down into a glass of water at room temperature (the water temperature should remain constant).

2. For the experiment, we took settled tap water enriched with CO 2 by adding NaHCO 3 or (NH 4) 2 CO 3, or mineral water, and exposed a glass with an aquatic plant to a bright light. We observed the appearance of air bubbles from the cut of the plant.

3. When the bubble flow became uniform, the number of bubbles released in 1 min was counted. The counting was carried out 3 times with a break of 1 min, the data were recorded in a table, and the average result was determined.

4. The glass with the plant was removed from the light source by 50–60 cm and the steps indicated in paragraph 3 were repeated.

5. The results of the experiments were compared and a conclusion was drawn about the different intensity of photosynthesis in bright and weak light.

The results of the experiments are presented in table 1.

Conclusion: at the used light intensities, the intensity of photosynthesis increases with increasing light intensity, i.e. the more light, the better photosynthesis goes.

Table 1. Dependence of photosynthesis on light intensity

Experience 2. Dependence of the productivity of photosynthesis on the spectral composition of light

Materials and equipment: elodea; a set of light filters (blue, orange, green); seven tall wide-mouth jars; settled tap water; scissors; 200 W electric lamp; watch; thermometer; test tubes.

1. The test tube was filled to 2/3 of the volume with settled tap water and an aquatic plant was placed in it with the top down. The stem was cut under water.

2. A blue light filter (circular) was placed in a high wide-mouth jar, a test tube with a plant was placed under the filter, and the jar was exposed to bright light so that it fell on the plant, passing through the light filter. We observed the appearance of air bubbles from the cut of the plant stem.

3. When the bubble flow became uniform, the number of bubbles released in 1 min was counted. The calculation was carried out 3 times with a break of 1 min, the average result was determined, the data were entered into the table.

4. The blue light filter was replaced with a red one and the steps indicated in paragraph 3 were repeated, making sure that the distance from the light source and the water temperature remained constant.

5. The results of the experiments were compared and a conclusion was made about the dependence of the intensity of photosynthesis on the spectral composition of light.

The results of the experiment are presented in table 2.

Conclusion: the process of photosynthesis in orange light is very intensive, in blue it slows down, and in green it practically does not go.

Table 2. Dependence of the productivity of photosynthesis on the spectral composition of light

experience number	light filter	First dimension	Second dimension	third dimension	Mean
	Orange

Experience 3. The dependence of the intensity of photosynthesis on temperature

Materials and equipment: elodea; three tall wide-mouth jars; settled tap water; scissors; test tubes; 200 W electric lamp; watch; thermometer.

1. A 2/3 test tube was filled with settled tap water and an aquatic plant was placed in it with the top down. The stem was cut off under water.

2. Settled tap water of different temperatures (from 14°C to 45°C) was poured into three wide-mouth jars, a test tube with a plant was placed in a jar of medium temperature water (for example, 25°C), and the device was exposed to bright light. We observed the appearance of air bubbles from the cut of the plant stem.

3. After 5 min, the number of bubbles released in 1 min was counted. The calculation was carried out 3 times with a break of 1 min, the average result was determined, the data were entered into the table.

4. The test tube with the plant was transferred to a jar with water of a different temperature and the steps indicated in paragraph 3 were repeated, making sure that the distance from the light source and the water temperature remained constant.

5. The results of the experiments were compared and a written conclusion was made about the effect of temperature on the intensity of photosynthesis.

The results of the experiment are presented in table 3.

Conclusion: in the studied temperature range, the intensity of photosynthesis depends on temperature: the higher it is, the better photosynthesis proceeds.

Table 3. Temperature dependence of photosynthesis

As a result of our study, we made the following conclusions.

1. The photoactive pigment system absorbs light especially strongly in the red region of the spectrum. Blue rays are quite well absorbed by chlorophyll and very little green, which explains the green color of plants.

2. Our experiment with a branch of elodea convincingly proves that the maximum intensity of photosynthesis is observed when illuminated with red light.

3. The rate of photosynthesis depends on temperature.

4. Photosynthesis depends on the intensity of light. The more light, the better photosynthesis goes.

The results of such work may be of practical importance. In greenhouses with artificial lighting, by selecting the spectral composition of light, you can increase the yield. At the Agrophysical Institute in Leningrad in the late 1980s. in the laboratory of B.S. Moshkov, using special lighting modes, 6 tomato crops per year (180 kg / m 2) were obtained.

Plants require light rays of all colors. How, when, in what sequence and proportion to supply it with radiant energy is a whole science. The prospects for light culture are very great: from laboratory experiments, it can turn into an industrial year-round production of vegetable, green, ornamental and medicinal crops.

LITERATURE

1. Genkel P.A. Plant Physiology: Proc. allowance for an optional course for the 9th grade. - M: Education, 1985. - 175 p., ill.
2. Kretovich V.L. Biochemistry of plants: Textbook for biol. faculties of universities. - M .: Higher School, 1980. - 445 p., ill.
3. Raven P., Evert R., Eichhorn S. Modern botany: In 2 volumes: Per. from English. - M.: Mir, 1990. - 344 p., ill.
4. Salamatova T.S. Plant cell physiology: Tutorial. - L .: Publishing House of Leningrad University, 1983. - 232 p.
5. Taylor D., Green N., Stout W. Biology: In 3 volumes: Per. from English / Ed. R. Sopera - M .: Mir, 2006. - 454 p., ill.
6. http://sc.nios.ru (drawings and diagrams)

Photosynthesis- The formation of complex organic substances by higher plants from simple compounds - carbon dioxide and water - due to light energy absorbed by chlorophyll. Organic substances created in the process of photosynthesis are necessary for plants to build their organs and maintain life.

The initial substances for photosynthesis - carbon dioxide entering the leaves from the air, and water - are products of the complete oxidation of carbon (CO 2) and hydrogen (H 2 O). In organic substances formed during photosynthesis, carbon is in a reduced state. During photosynthesis, the CO 2 - H 2 O system, which consists of oxidized substances and is at a low energy level, is reduced to a less stable CH 2 O - O 2 system, which is at a higher energy level.

It can be seen from the equation that in order to obtain one gram - a molecule of glucose (C 6 HO 6), light energy is consumed in the amount of 2872.14 kJ, which is stored in the form of chemical energy. This releases free oxygen into the atmosphere.

The above equation gives a concrete idea of ​​the initial and final substances involved in photosynthesis, but it does not reveal the essence of a very complex biochemical process.

The history of the doctrine of carbon nutrition of plants has more than 200 years. In the treatise "The Word on Air Phenomena" M.V. Lomonosov in 1753 wrote that a plant builds its body from the air around it, absorbed with the help of leaves. However, the discovery of photosynthesis is associated with the name of the English chemist J. Priestley, who in 1771 discovered that in the light green plants "correct" the air "spoiled" by combustion.

The subsequent works of the Dutch scientist J. Ingenhaus (1779, 1798), the Swiss J. Senebier (1782, 1783) and

N. Saussure (1804) found that in the light green plants absorb carbon dioxide from the surrounding atmosphere and release oxygen.

An important role in the study of photosynthesis was played by the work of K. A. Timiryazev, who showed that light is an energy source for the synthesis of organic substances from carbon dioxide and water, and established the absorption maximum of chlorophyll in the red and blue-violet regions of the spectrum. Further research by many scientists using modern methods made it possible to reveal many links in the complex chain of transformations of substances in the plant body.

It was found that photosynthesis proceeds in two phases. The first of them is light, the second is dark. The first phase proceeds only in the light, while the second - with equal success both in the dark and in the light. The light phase takes place in the green fraction of the chloroplast - grana, and all transformations of the dark phase take place in its colorless fraction - the cytoplasmic matrix. The light phase is characteristic only of photosynthetic cells, while most of the reactions that make up the process of carbon dioxide fixation in the dark phase are characteristic not only of photosynthetic cells.

The light phase of photosynthesis begins with the absorption of light by pigments. In the chemical reactions of the light phase, only chlorophyll a molecules are involved, which are in an activated (due to the absorption of light energy) state. The remaining pigments - chlorophyll b and carotenoids - capture light using special systems, transfer the received energy to chlorophyll a molecules.

The most important role of the light phase is to build the ATP (adenosine triphosphate) molecule, in which energy is stored. The process of formation of ATP in chloroplasts with the expenditure of solar energy is called cyclic phosphorylation. The breakdown of ATP to ADP (adenosine diphosphate) releases about 40 kJ of energy.

To restore the NADP (nicotinamide adenine dinucleotide phosphate) molecule, two hydrogen atoms are required, which is obtained from water using light. Chlorophyll, activated by light, spends its energy on the decomposition of water, turns into an inactivated form, and four hydrogen atoms are released, which are used in reducing reactions, and two oxygen atoms entering the atmosphere.

Thus, the first stable chemical products of the light reaction in plants are NADP - H 2 and ATP.

During the dark phase, amino acids and proteins are formed in the cytoplasm.

The dark phase of photosynthesis is a continuation of the light phase. In the dark phase, with the participation of ATP and NADP - H 2, various organic substances are built from carbon dioxide. In this case, NADP - H 2 performs the role of a reducing agent in the dark phase, and ATP serves as an energy source. The reducing agent is oxidized to NADP, and one residue of phosphoric acid (H 3 PO 4) is cleaved from ATP and ADP is obtained. NADP and ADP again return from the matrix to the grana, where in the light phase they are again converted into NADP - H 2 and ATP and everything starts all over again.

The sequence of reactions on the way of converting CO 2 into sugar was clarified thanks to the use of radioactive carbon 14C. It was found that in the process of photosynthesis a large number of compounds are formed in a few minutes. However, when the time allotted for photosynthesis was reduced to 0.5 s, only a three-carbon phosphorylated compound, triphosphoglyceric acid (PGA), was found. Therefore, FHA is the first stable product formed from CO 2 during photosynthesis. It turned out that the first substance that combines with CO 2 (CO 2 acceptor) is a five-carbon phosphorylated compound - ribulose diphosphate (RDP), which decomposes after the addition of CO 2 into two FHA molecules. The enzyme that catalyzes this reaction, RDF - carboxylase, occupies the first place in quantitative terms among the proteins contained in the protein tissue.

Phosphoglyceric acid is reduced to the level of aldehyde due to the reduction potential of NADP - H 2 and the energy of ATP.

Phosphoglyceraldehyde, which is a phosphorylated sugar compound, contains only three carbon atoms, while the simplest sugars contain six carbon atoms. In order to form hexose (the simplest sugar), two molecules of phosphoglyceraldehyde must combine and the resulting product - hexose diphosphate - must undergo dephosphorylation.

The resulting hexose can be directed either to the synthesis of sucrose and polysaccharides, or to the construction of any other organic compounds of the cell. Thus, the sugar formed in the process of photosynthesis from CO 2 is the main organic matter, which in the cells of higher plants serves as a source of both energy and building proteins necessary for the cell.

factors affecting photosynthesis

Carbon dioxide. The rate of photosynthesis depends on the amount of carbon dioxide in the air. Typically, atmospheric air contains 0.03% CO 2 . An increase in its content contributes to an increase in yield, which is used when growing plants in greenhouses, greenhouses, greenhouses. Determined that best conditions for photosynthesis are created at a CO 2 content of about 1.0%. An increase in the content of CO 2 to 5.0% contributes to an increase in the intensity of photosynthesis, but in this case it is necessary to increase the illumination.

The amount of CO 2 absorbed per unit time per unit mass of chlorophyll is called the assimilation number. The number of milligrams of CO 2 absorbed in 1 hour per 1 dm 2 of the leaf surface is called the intensity of photosynthesis. The rate of photosynthesis in various kinds plants is not the same, it also changes with the age of the plants.

Light. Plants absorb 85 - 90% of the light energy that falls on them, but only 1 - 5% of the absorbed light energy goes to photosynthesis. The rest of the energy is used to heat the plant and transpiration.

All plants in their relation to the intensity of illumination can be divided into two groups - light-loving and shade-loving. Light-loving ones require more illumination, shade-tolerant - less.

Water. The provision of plants with water is important. Insufficient saturation of cells with water causes the stomata to close and, consequently, reduces the supply of carbon dioxide to plants. Dehydration of cells disrupts the activity of enzymes.

Temperature regime. Best temperature regime for most plants, at which photosynthesis is most intensive, 20 - 30 ° C. As the temperature rises or falls, photosynthesis slows down. Chlorophyll in plant cells is formed at a temperature of 2 to 40 °C.

With a favorable combination of all the factors necessary for photosynthesis, plants most actively accumulate organic matter and release oxygen. The products of photosynthesis formed in excess - sugars - are immediately converted into a high-polymer reserve compound - starch, which is deposited in the form of starch grains in chloroplasts and leukoplasts. At the same time, some part of the sugars is removed from the plastids and moved to other parts of the plant. Starch can be broken down again into sugars, which, oxidized during respiration, provide the cell with energy.

Thus, by artificially regulating the gas composition of the atmosphere, providing plants with light, water, and heat, it is possible to increase the intensity of photosynthesis and, consequently, increase plant productivity. This is precisely what agrotechnical practices are aimed at cultivating crops: soil enrichment with organic substances, tillage, irrigation, mulching, regulation of crop density, etc.
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Of all the factors simultaneously affecting the process of photosynthesis limiting will be the one that is closer to the minimum level. It installed Blackman in 1905. Miscellaneous factors may be limited, but one of them is the main one.

1. In low light, the rate of photosynthesis is directly proportional to the light intensity. Light is the limiting factor in low light conditions. At high light intensity, chlorophyll becomes discolored and photosynthesis slows down. Under such conditions in nature, plants are usually protected (thick cuticle, pubescent leaves, scales).

1. The dark reactions of photosynthesis require carbon dioxide, which is included in organic matter, is a limiting factor in the field. The concentration of CO 2 in the atmosphere varies from 0.03-0.04%, but if you increase it, you can increase the rate of photosynthesis. Some greenhouse crops are now grown with increased CO 2 content.
2. temperature factor. Dark and some light reactions of photosynthesis are controlled by enzymes, and their action depends on temperature. The optimum temperature for plants in the temperate zone is 25 °C. With each increase in temperature by 10 °C (up to 35 °C), the reaction rate doubles, but due to the influence of a number of other factors, plants grow better at 25 °C.
3. Water- source material for photosynthesis. Lack of water affects many processes in cells. But even temporary wilting leads to serious crop losses. Reasons: when withering, the stomata of plants close, and this interferes with the free access of CO 2 for photosynthesis; with a lack of water in the leaves of some plants accumulates abscisic acid. It is a plant hormone - a growth inhibitor. In laboratory conditions, it is used to study the inhibition of the growth process.
4. Chlorophyll concentration. The amount of chlorophyll may decrease with diseases powdery mildew, rust, viral diseases, lack of minerals and age (with normal aging). When the leaves turn yellow, chlorotic phenomena or chlorosis. The reason may be a lack of minerals. For the synthesis of chlorophyll, Fe, Mg, N and K are needed.
5. Oxygen. A high concentration of oxygen in the atmosphere (21%) inhibits photosynthesis. Oxygen competes with carbon dioxide for the active site of the enzyme involved in CO 2 fixation, which reduces the rate of photosynthesis.
6. Specific inhibitors. The best way to kill a plant is to suppress photosynthesis. To do this, scientists have developed inhibitors - herbicides- dioxins. For example: DHMM - dichlorophenyldimethylurea- inhibits the light reactions of photosynthesis. Successfully used to study the light reactions of photosynthesis.
7. Environmental pollution. Gases of industrial origin, ozone and sulfur dioxide, even in small concentrations, severely damage the leaves of a number of plants. Lichens are very sensitive to sulfur dioxide. Therefore, there is a method lichen indications– determination of environmental pollution by lichens. Soot clogs the stomata and reduces the transparency of the leaf epidermis, which reduces the rate of photosynthesis.

6. Plant life factors, heat, light, air, water- Plants throughout their lives are constantly in interaction with the external environment. Plant requirements for life factors are determined by the heredity of plants, and they are different not only for each species, but also for each variety of a particular crop. That is why a deep knowledge of these requirements makes it possible to correctly establish the structure of sown areas, the rotation of crops, the placement crop rotations.
For normal life, plants need light, heat, water, nutrients, including carbon dioxide and air.
The main source of light for plants is solar radiation. Although this source is beyond human influence, the degree of use of the sun's light energy for photosynthesis depends on the level of agricultural technology: sowing methods (rows directed from north to south or from east to west), differentiated seeding rates, tillage, etc.
Timely thinning of plants and the destruction of weeds improve the illumination of plants.
Heat in plant life, along with light, represents the main factor in plant life and a necessary condition for biological, chemical and physical processes in the soil. Each plant at various phases and stages of development makes certain, but unequal requirements for heat, the study of which is one of the tasks of plant physiology and scientific agriculture. heat in plant life affects the rate of development in each stage of growth. The task of agriculture also includes the study thermal regime soil and methods of its regulation.
Water in plant life and nutrients, with the exception of carbon dioxide coming from both the soil and the atmosphere, are the soil factors of plant life. Therefore, water and nutrients are called elements of soil fertility.
Air in plant life(atmospheric and soil) is necessary as a source of oxygen for the respiration of plants and soil microorganisms, as well as a source of carbon that the plant absorbs during photosynthesis. In addition, Air in the life of plants is necessary for microbiological processes in the soil, as a result of which the organic matter of the soil is decomposed by aerobic microorganisms with the formation of soluble mineral compounds of nitrogen, phosphorus, potassium and other plant nutrients.

7 . Indicators of photosynthetic productivity of crops

A crop is created in the process of photosynthesis, when organic matter is formed in green plants from carbon dioxide, water and minerals. The energy of the sun's beam is converted into the energy of plant biomass. The efficiency of this process and ultimately the yield depend on the functioning of the crop as a photosynthetic system. In field conditions, sowing (cenosis) as a set of plants per unit area is a complex dynamic self-regulating photosynthetic system. This system includes many components that can be considered as subsystems; it is dynamic, as it constantly changes its parameters over time; self-regulating, since, despite various influences, sowing changes its parameters in a certain way, maintaining homeostasis.

Indicators of photosynthetic activity of crops. Sowing is optical system, in which the leaves absorb PAR. In the initial period of plant development, the assimilation surface is small and a significant part of the PAR passes by the leaves and is not captured by them. With an increase in the area of ​​leaves, their absorption of solar energy also increases. When the leaf surface index* is 4...5, i.e. the area of ​​leaves in the crop is 40...50 thousand m 2 /ha, the absorption of PAR by the leaves of the crop reaches a maximum value - 75...80% of the visible, 40% of total radiation. With a further increase in leaf area, PAR absorption does not increase. In crops where the course of formation of the leaf area is optimal, the absorption of PAR can be on average 50...60% of the incident radiation during the growing season. PAR absorbed by the plant cover is the energy basis for photosynthesis. However, only part of this energy is accumulated in the crop. The PAR utilization factor is usually determined in relation to the PAR incident on the vegetation cover. If in the biomass crop in middle lane Russia has accumulated 2...3% of PAR sowing, then the dry weight of all plant organs will be 10...15 t/ha, and the possible yield will be 4...6 t of grain per 1 ha. In sparse crops, the PAR utilization factor is only 0.5...1.0%.

Considering a crop as a photosynthetic system, the dry biomass yield generated during a growing season, or its growth over a given period, depends on the average leaf area, the length of the period, and the net photosynthesis productivity for that period.

Y \u003d FP NPF,

where Y is the yield of dry biomass, t/ha;

FP - photosynthetic potential, thousand m 2 - days / ha;

NPP - net productivity of photosynthesis, g/(m2 - days).

Photosynthetic potential is calculated by the formula

where Sc is the average leaf area for the period, thousand m 2 /ha;

T is the duration of the period, days.

The main indicators for the cenosis, as well as the yield, are determined per unit area - 1 m 2 or 1 ha. So, the leaf area is measured in thousand m 2 / ha. In addition, they use such an indicator as the leaf surface index. The main part of the assimilation surface is made up of leaves, it is in them that photosynthesis takes place. Photosynthesis can also occur in other green parts of plants - stems, awns, green fruits, etc., but the contribution of these organs to total photosynthesis is usually small. It is customary to compare crops with each other, as well as different states of one crop in dynamics in terms of leaf area, identifying it with the concept of "assimilation surface". The dynamics of the area of ​​leaves in the crop follows a certain regularity. After germination, the leaf area slowly increases, then the growth rate increases. By the time the formation of lateral shoots ceases and the growth of plants in height, the leaf area reaches its maximum value during the growing season, then it begins to gradually decrease due to the yellowing and death of the lower leaves. By the end of the growing season in the crops of many crops (cereals, legumes), green leaves on the plants are absent. The leaf area of ​​various agricultural plants can vary greatly during the growing season depending on the conditions of water supply, nutrition, and agricultural practices. The maximum leaf area in arid conditions reaches only 5...10 thousand m 2 /ha, and with excessive moisture and nitrogen nutrition, it can exceed 70 thousand m 2 /ha. It is believed that with a leaf surface index of 4...5, sowing as an optical photosynthesizing system operates in the optimal mode, absorbing the largest amount of PAR. With a smaller area of ​​leaves, a part of the PAR is not captured by the leaves. If the leaf area is more than 50000 m2/ha, then the upper leaves shade the lower ones, and their share in photosynthesis sharply decreases. Moreover, the upper leaves “feed” the lower ones, which is unfavorable for the formation of fruits, seeds, tubers, etc. The dynamics of the leaf area shows that at different stages of the growing season, sowing as a photosynthetic system functions differently (Fig. 3). During the first 20...30 days of vegetation, when the average leaf area is 3...7 thousand m 2 /ha, most of the PAR is not captured by the leaves, and therefore the PAR utilization factor cannot be high. Further, the area of ​​leaves begins to increase rapidly, reaching a maximum. As a rule, this occurs in bluegrasses in the phase of the milky state of the grain, in cereal legumes in the phase of full seed filling in the middle layer, and in perennial grasses in the flowering phase. Then the leaf area begins to decrease rapidly. At this time, the redistribution and outflow of substances from vegetative organs into generative ones. The duration of these periods and their ratio is influenced by various factors, including agrotechnical ones. With their help, it is possible to regulate the process of increasing the area of ​​leaves and the duration of periods. In arid conditions, the density of plants, and hence the area of ​​leaves, is deliberately reduced, since with a large area of ​​leaves, transpiration increases, plants suffer more from a lack of moisture, and yields decrease.