photosynthesis called the process of converting light energy into chemical bond energy organic compounds with chlorophyll.
As a result of photosynthesis, about 150 billion tons of organic matter and approximately 200 billion tons of oxygen are produced annually. This process ensures the circulation of carbon in the biosphere, preventing the accumulation of carbon dioxide and thereby preventing the occurrence of the greenhouse effect and overheating of the Earth. The organic substances formed as a result of photosynthesis are not completely consumed by other organisms, a significant part of them formed mineral deposits (hard and brown coal, oil) over millions of years. Recently, rapeseed oil (“biodiesel”) and alcohol obtained from plant residues have also been used as fuel. From oxygen, under the action of electrical discharges, ozone is formed, which forms an ozone shield that protects all life on Earth from the harmful effects of ultraviolet rays.
Our compatriot, the outstanding plant physiologist K. A. Timiryazev (1843-1920) called the role of photosynthesis “cosmic”, since it connects the Earth with the Sun (space), providing an influx of energy to the planet.
Phases of photosynthesis. Light and dark reactions of photosynthesis, their relationship
In 1905, the English plant physiologist F. Blackman discovered that the rate of photosynthesis cannot increase indefinitely, some factor limits it. Based on this, he proposed the existence of two phases of photosynthesis: light And dark. At low light intensity, the speed of light reactions increases in proportion to the increase in light intensity, and, in addition, these reactions do not depend on temperature, since enzymes are not needed for their occurrence. Light reactions occur on thylakoid membranes.
The rate of dark reactions, on the contrary, increases with increasing temperature; however, upon reaching a temperature threshold of 30°C, this growth stops, which indicates the enzymatic nature of these transformations occurring in the stroma. It should be noted that light also has a certain effect on dark reactions, despite the fact that they are called dark.
The light phase of photosynthesis (Fig. 2.44) proceeds on the membranes of thylakoids, which carry several types of protein complexes, the main of which are photosystems I and II, as well as ATP synthase. The composition of photosystems includes pigment complexes, in which, in addition to chlorophyll, there are also carotenoids. Carotenoids trap light in those regions of the spectrum in which chlorophyll does not, and also protect chlorophyll from destruction by high-intensity light.
In addition to pigment complexes, photosystems also include a number of electron acceptor proteins that successively transfer electrons from chlorophyll molecules to each other. The sequence of these proteins is called chloroplast electron transport chain.
A special complex of proteins is also associated with photosystem II, which ensures the release of oxygen during photosynthesis. This oxygen-evolving complex contains manganese and chlorine ions.
IN light phase light quanta, or photons, falling on chlorophyll molecules located on thylakoid membranes, transfer them to an excited state characterized by a higher electron energy. At the same time, excited electrons from the chlorophyll of photosystem I are transferred through a chain of intermediaries to the hydrogen carrier NADP, which adds hydrogen protons, which are always present in an aqueous solution:
NADP+ 2e-+ 2H + → NADPH + H + .
The recovered NADPH + H + will subsequently be used in the dark stage. Electrons from the chlorophyll of photosystem II are also transferred along the electron transport chain, but they fill the "electron holes" of the chlorophyll of photosystem I. The lack of electrons in the chlorophyll of photosystem II is filled by depriving water molecules from water molecules, which occurs with the participation of the oxygen-releasing complex already mentioned above. As a result of the decomposition of water molecules, which is called photolysis, hydrogen protons are formed and molecular oxygen is released, which is a by-product of photosynthesis:
H 2 0 → 2H + + 2e- + 1 / 2O 2
Hydrogen protons accumulated in the cavity of the thylakoid as a result of water photolysis and injection during the transfer of electrons along the electron transport chain flow out of the thylakoid through a channel in the membrane protein - ATP synthase, while ATP is synthesized from ADP. This process is called photophosphorylation. It does not require the participation of oxygen, but is very effective, as it provides 30 times more ATP than mitochondria in the process of oxidation. The ATP formed in light reactions will subsequently be used in dark reactions.
The overall reaction equation for the light phase of photosynthesis can be written as follows:
2H 2 0 + 2NADP + 3ADP + ZN 3 P0 4 → 2NADPH + H + + 3ATP.
During dark reactions photosynthesis (Fig. 2.45), CO 2 molecules are bound in the form of carbohydrates, for which ATP molecules and NADPH + H + synthesized in light reactions:
6C0 2 + 12 NADPH + H + + 18ATP → C 6 H 12 0 6 + 6H 2 0 + 12 NADP + 18ADP + 18H 3 P0 4.
The process of carbon dioxide binding is a complex chain of transformations called Calvin cycle in honor of its discoverer. Dark reactions take place in the stroma of chloroplasts. Their flow requires a constant influx of carbon dioxide from the outside through the stomata, and then through the system of intercellular spaces.
The first to form in the process of carbon dioxide fixation are three-carbon sugars, which are the primary products of photosynthesis, while the later formed glucose, which is used for starch synthesis and other life processes, is called the end product of photosynthesis.
Thus, in the process of photosynthesis, the energy of sunlight is converted into the energy of chemical bonds of complex organic compounds not without the participation of chlorophyll. The overall photosynthesis equation can be written as follows:
6C0 2 + 12H 2 0 → C 6 H 12 0 6 + 60 2 + 6H 2 0, or
6C0 2 + 6H 2 0 → C 6 H 12 0 6 + 60 2.
The reactions of the light and dark phases of photosynthesis are interrelated, since an increase in the rate of only one group of reactions affects the intensity of the entire photosynthesis process only up to a certain point, until the second group of reactions acts as a limiting factor, and there is a need to accelerate the reactions of the second group in order to the first occurred without restriction.
The light stage occurring in the thylakoids provides energy storage for the formation of ATP and hydrogen carriers. At the second stage, dark, the energy products of the first stage are used to reduce carbon dioxide, and this happens in the stroma compartments of chloroplasts.
Various factors influence the rate of photosynthesis. environment: illumination, concentration of carbon dioxide in the atmosphere, air and soil temperature, water availability, etc.
To characterize photosynthesis, the concept of its productivity is used.
Photosynthesis productivity- this is the mass of glucose synthesized in 1 hour per 1 dm 2 of the leaf surface. This rate of photosynthesis is maximum under optimal conditions.
Photosynthesis is inherent not only in green plants, but also in many bacteria, including cyanobacteria, green and purple bacteria, but in the latter it may have some differences, in particular, bacteria may not release oxygen during photosynthesis (this does not apply to cyanobacteria).
The meaning and role of photosynthesis
Main source of energy
The word photosynthesis literally means making or assembling something under the influence of light. Usually, when talking about photosynthesis, they mean the process by which plants in sunlight synthesize organic compounds from inorganic raw materials. All life forms in the universe need energy to grow and sustain life. Algae, higher plants and some types of bacteria directly capture the energy of solar radiation and use it to synthesize basic nutrients. Animals do not know how to use sunlight directly as a source of energy, they get energy by eating plants or other animals that eat plants. So, ultimately, the source of energy for all metabolic processes on our planet is the Sun, and the process of photosynthesis is necessary to maintain all forms of life on Earth.
We use fossil fuels - coal, natural gas, oil, etc. All these fuels are nothing but the decay products of terrestrial and marine plants or animals, and the energy stored in them was obtained from sunlight millions of years ago. Wind and rain also owe their origin to solar energy, and therefore the energy of windmills and hydroelectric power plants is ultimately also due to solar radiation.
The most important way chemical reactions Photosynthesis is the conversion of carbon dioxide and water into carbon and oxygen. The overall reaction can be described by the equation CO2 + H20? [CH20]+02
The carbohydrates formed in this reaction contain more energy than the original substances, i.e. CO2 and H20. Thus, due to the energy of the Sun, energy substances (CO2 and H20) are converted into energy-rich products - carbohydrates and oxygen. The energy levels of the various reactions described by the overall equation can be characterized by redox potentials measured in volts. Potential values show how much energy is stored or wasted in each reaction. So, photosynthesis can be considered as the process of formation of the radiant energy of the Sun into the chemical energy of plant tissues.
The content of CO2 in the atmosphere remains almost complete, despite the fact that carbon dioxide is consumed in the process of photosynthesis. The fact is that all plants and animals breathe. In the process of respiration in mitochondria, oxygen absorbed from the atmosphere by living tissues is used to oxidize carbohydrates and other tissue components, ultimately forming carbon dioxide and water and with the concomitant release of energy. The released energy is stored in high-energy compounds - adenosine triphosphate (ATP), which is used by the body to perform all vital functions. Thus, respiration leads to the consumption of organic matter and oxygen and increases the content of CO2 on the planet. For the processes of respiration in all living organisms and for the combustion of all types of fuel containing carbon, in the aggregate, about 10,000 tons of 02 per second are consumed on an average scale of the Earth. At this rate of consumption, all of the oxygen in the atmosphere should run out in about 3,000 years. Fortunately for us, the consumption of organic matter and atomic oxygen is balanced by the creation of carbohydrates and oxygen through photosynthesis. Under ideal conditions, the rate of photosynthesis in green plant tissues is about 30 times higher than the rate of respiration in the same tissues, thus photosynthesis is an important factor regulating the content of 02 on Earth.
The history of the discovery of photosynthesis
At the beginning of the XVII century. Flemish doctor Van Helmont grew a tree in a tub of earth, which he watered only with rainwater. He noticed that after five years, the tree had grown to a large size, although the amount of land in the tub had not practically decreased. Van Helmont naturally concluded that the material from which the tree was formed came from the water used for irrigation. In 1777, the English botanist Stephen Hales published a book in which he reported that plants mainly use air as a nutrient necessary for growth. In the same period, the famous English chemist Joseph Priestley (he was one of the discoverers of oxygen) conducted a series of experiments on combustion and respiration and came to the conclusion that green plants are capable of performing all those respiratory processes that were found in animal tissues. Priestley burned a candle in a closed volume of air, and found that the resulting air could no longer support combustion. A mouse placed in such a vessel would die. However, the sprig of mint continued to live in the air for weeks. In conclusion, Priestley discovered that in the air, restored by a sprig of mint, the candle began to burn again, the mouse could breathe. We now know that the candle consumed oxygen from the closed volume of air when it burned out, but then the air was again saturated with oxygen due to photosynthesis that took place in the left sprig of mint. A few years later, the Dutch physician Ingenhaus discovered that plants oxidize oxygen only in sunlight and that only their green parts provide oxygen. Jean Senebier, who served as minister, confirmed Ingenhaus's data and continued the study, showing that plants use carbon dioxide dissolved in water as a nutrient. At the beginning of the 19th century, another Swiss researcher, de Sausedi, studied the quantitative relationships between carbon dioxide absorbed by a plant, on the one hand, and synthesized organic substances and oxygen, on the other. As a result of his experiments, he came to the conclusion that water is also consumed by the plant during the assimilation of CO2. In 1817, two French chemists, Pelletier and Cavantoux, isolated a green substance from leaves and named it chlorophyll. The next important milestone in the history of the study of photosynthesis was the statement made in 1845 by the German physicist Robert Mayer that green plants convert the energy of sunlight into chemical energy. The ideas about photosynthesis that had developed by the middle of the last century can be expressed by the following relationship:
green plant
CO2 + H2 O + Light? O2 + org. substances + chemical energy
The ratio of the amount of CO2 absorbed during photosynthesis to the amount of 02 released was accurately measured by the French plant physiologist Busengo. In 1864 he discovered that the photosynthetic ratio, i.e. the ratio of the volume of released 02 to the volume of absorbed CO2 is almost equal to unity. In the same year, the German botanist Sachs (who also discovered respiration in plants) demonstrated the formation of starch grains during photosynthesis. Zaks placed green leaves for several hours in the dark so that they used up the starch accumulated in them. Then he brought the leaves into the light, but at the same time illuminated only half of each leaf, leaving the other half of the leaf in darkness. After some time, the entire leaf was treated with iodine vapor. As a result, the illuminated part of the leaf became dark purple, indicating the formation of a starch-iodine complex, while the color of the other half of the leaf did not change. A direct connection between the release of oxygen and chloroplasts in green leaves, as well as the correspondence between the action spectrum of photosynthesis and the spectrum absorbed by chloroplasts, was established in 1880 by Engelman. He placed a filamentous green algae with spirally twisted chloroplasts on a glass slide, illuminating it with a narrow and wide beam of white light. Together with algae, a suspension of cells of motile bacteria sensitive to oxygen concentration was applied to a glass slide. The glass slide was placed in a chamber without air and illuminated. Under these conditions, motile bacteria should have migrated to the part where the 02 concentration was higher. After some time, the sample was examined under a microscope and the distribution of the bacteriopopulation was calculated. It turned out that the bacteria were concentrated around the green stripes in the filamentous algae. In another series of experiments, Engelman illuminated algae with rays of different spectral composition, placing a prism between the light source and the microscope stage. In this case, the greatest number of bacteria accumulated around those parts of the alga that were illuminated by the blue and red regions of the spectrum. Chlorophyll found in algae absorbs blue and red light. Since by that time it was already known that photosynthesis requires the absorption of light, Engelman concluded that chlorophylls participate in synthesis as pigments that are active photoreceptors. The level of knowledge about photosynthesis at the beginning of our century can be represented as follows.
CO2 + H2O + Light -O2 + Starch + Chemical Energy
So, by the beginning of our century, the total reaction of photosynthesis was already known. However, biochemistry was not at such a high level to fully reveal the mechanisms of reduction of carbon dioxide to carbohydrates. Unfortunately, it must be admitted that even now some aspects of photosynthesis are still rather poorly studied. Since ancient times, attempts have been made to investigate the influence of light intensity, temperature, carbon dioxide concentration, etc. to the total yield of photosynthesis. And although in these works plants of various species were studied, most of the measurements were performed on unicellular green algae and on the unicellular flagella algae Euglena. Single-celled organisms are more convenient for qualitative research, since they can be grown in all laboratories under quite standard conditions. They can be evenly suspended, i.e., suspended in aqueous buffer solutions, and the required volume of such a suspension, or suspension, can be taken at such a dosage, just like when working with ordinary plants. Chloroplasts for experiments are best isolated from the leaves of higher plants. Spinach is the most commonly used because it is easy to grow and the fresh leaves are good for research; sometimes pea leaves and lettuce are used.
Since CO2 is highly soluble in water, and O2 is relatively insoluble in water, during photosynthesis in a closed system, the gas pressure in this system can change. Therefore, the effect of light on photosynthetic systems is often studied using a Warburg respirator, which makes it possible to register threshold changes in the O2 volume in the system. The Warburg respirator was first used in relation to photosynthesis in 1920. To measure the consumption or release of oxygen during the reaction, it is more convenient to use another device - an oxygen electrode. This device is based on the use of the polarographic method. The oxygen electrode is sensitive enough to detect concentrations as low as 0.01 mmol per liter. The device consists of a rather thin platinum wire cathode hermetically pressed into the anode plate, which is a ring of silver wire immersed in a saturated solution. The electrodes are separated from the mixture in which the reaction proceeds by a membrane permeable to 02. The reaction system is located in a plastic or glass vessel and is constantly stirred by a rotating bar magnet. When a voltage is applied to the electrodes, the platinum electrode becomes negative with respect to the standard electrode, the oxygen in the solution is electrolytically reduced. At a voltage of 0.5 to 0.8 V, the magnitude of the electric current depends linearly on the partial pressure of oxygen in the solution. Typically, the oxygen electrode is operated at a voltage of about 0.6 V. The electrical current is measured by connecting the electrode to a suitable recording system. The electrode together with the reaction mixture is irrigated with a stream of water from a thermostat. Using an oxygen electrode, the effect of light and various chemicals on photosynthesis is measured. The advantage of the oxygen electrode over the Warburg apparatus is that the oxygen electrode makes it possible to quickly and continuously record changes in the O2 content in the system. On the other hand, up to 20 samples with different reaction mixtures can be simultaneously analyzed in the Warburg instrument, while when working with an oxygen electrode, the samples have to be analyzed one by one.
Until about the early 1930s, many researchers in this field believed that the primary reaction of photosynthesis was the breakdown of carbon dioxide by the action of light into carbon and oxygen, followed by the reduction of carbon to carbohydrates using water in several successive reactions. The point of view changed in the 1930s as a result of two important discoveries. First, varieties of bacteria were described that are able to assimilate and synthesize carbohydrates without using light energy for this. Then, the Dutch microbiologist Van Neel compared the processes of photosynthesis in bacteria and showed that some bacteria can assimilate CO2 in the light without releasing oxygen. Such bacteria are capable of photosynthesis only in the presence of a suitable hydrogen donor substrate. Van Neel suggested that the photosynthesis of green plants and algae is a special case when oxygen in photosynthesis comes from water, and not from carbon dioxide.
The second important discovery was made in 1937 by R. Hill at the University of Cambridge. Using differential centrifugation of a leaf tissue homogenate, he separated photosynthetic particles (chloroplasts) from respiratory particles. The chloroplasts obtained by Hill did not themselves release oxygen when illuminated (possibly due to the fact that they were damaged during separation). However, they began to release oxygen in the presence of light if suitable electron acceptors (oxidizers), such as potassium ferrioxalate or potassium ferricyanide, were added to the suspension. During the isolation of one 02 molecule, four equivalents of the oxidizing agent were photochemically reduced. Later it was found that many quinones and dyes are reduced by chloroplasts in the light. However, chloroplasts could not recover CO2, a natural electron acceptor during photosynthesis. This phenomenon, now known as the Hill reaction, is the light-induced transfer of electrons from water to non-physiological oxidants (Hill's reagents) against a chemical potential gradient. The significance of the Hill reaction lies in the fact that it demonstrated the possibility of separating two processes - the photochemical release of oxygen and the reduction of carbon dioxide during photosynthesis.
The decomposition of water, leading to the release of free oxygen during photosynthesis, was established by Reuben and Kamen, in California in 1941. They placed photosynthetic cells in water enriched with an oxygen isotope having a mass of 18 atomic units 180. The isotopic composition of oxygen released by the cells corresponded to the composition water, but not CO2. In addition, Kamen and Ruben discovered the radioactive isotope 18O, which was subsequently successfully used by Bassat and Benson Wien, who studied the pathway of carbon dioxide conversion during photosynthesis. Calvin and his collaborators found that the reduction of carbon dioxide to sugars occurs as a result of dark enzymatic processes, and the reduction of one molecule of carbon dioxide requires two molecules of reduced ADP and three molecules of ATP. By that time, the role of ATP and pyridine nucleotides in tissue respiration had been established. The possibility of photosynthetic reduction of ADP to ATP by isolated chlorophylls was proved in 1951 in three different laboratories. In 1954, Arnon and Allen demonstrated photosynthesis - they observed the assimilation of CO2 and O2 by isolated spinach chloroplasts. Over the next decade, it was possible to isolate from chloroplasts proteins involved in the transfer of electrons in the synthesis - ferredoxin, plastocyanin, ferroATP reductase, cytochromes, etc.
Thus, in healthy green leaves, under the action of light, ADP and ATP are formed and the energy of hydrobonds is used to reduce CO2 to carbohydrates in the presence of enzymes, and the activity of enzymes is regulated by light.
Limiting factors
The intensity or speed of the photosynthesis process in a plant depends on a number of internal and external factors. Of the internal factors, the most important are the structure of the leaf and the content of chlorophyll in it, the rate of accumulation of photosynthesis products in chloroplasts, the influence of enzymes, and the presence of low concentrations of essential inorganic substances. External parameters are the quantity and quality of light falling on the leaves, the ambient temperature, the concentration of carbon dioxide and oxygen in the atmosphere near the plant.
The rate of photosynthesis increases linearly, or in direct proportion to the increase in light intensity. As the light intensity increases further, the increase in photosynthesis becomes less and less pronounced, and finally stops when the illumination reaches a certain level of 10,000 lux. A further increase in light intensity no longer affects the rate of photosynthesis. The region of stable rate of photosynthesis is called the region of light saturation. If you want to increase the rate of photosynthesis in this area, you should not change the light intensity, but some other factors. The intensity of sunlight falling on the surface of the earth on a clear summer day in many places on our planet is approximately 100,000 lux. Consequently, for plants, with the exception of those that grow in dense forests and in the shade, the incident sunlight is enough to saturate their photosynthetic activity (the energy of quanta corresponding to the extreme parts of the visible range - violet and red, differs only two times, and all photons of this range are, in principle, capable of triggering photosynthesis).
In the case of low light intensities, the rate of photosynthesis at 15 and 25°C is the same. Reactions occurring at such light intensities that correspond to the light limiting region, like true photochemical reactions, are not sensitive to temperatures. However, at higher intensities, the rate of photosynthesis at 25°C is much higher than at 15°C. Consequently, in the region of light saturation, the level of photosynthesis depends not only on the absorption of photons, but also on other factors. Most plants in temperate climates function well in the temperature range from 10 to 35°C, the most favorable conditions are temperatures around 25°C.
In the light-limited region, the rate of photosynthesis does not change with decreasing CO2 concentration. From this we can conclude that CO2 is directly involved in the photochemical reaction. At the same time, at higher illumination intensities that lie outside the limiting region, photosynthesis increases significantly with increasing CO2 concentration. In some grain crops, photosynthesis increased linearly with an increase in CO2 concentration to 0.5%. (These measurements were carried out in short-term experiments, since long-term exposure to high concentrations of CO2 damages the sheets). The rate of photosynthesis reaches high values at a CO2 content of about 0.1%. The average concentration of carbon dioxide in the atmosphere is from 0.03%. Therefore, under normal conditions, plants do not have enough CO2 in order to use the sunlight falling on them with maximum efficiency. If a plant placed in a closed volume is illuminated with light of saturating intensity, then the concentration of CO2 in the air volume will gradually decrease and reach a constant level, known as the "CO2 compensation point". At this point, the appearance of CO2 during photosynthesis is balanced by the release of O2 as a result of respiration (dark and light). In plants of different species, the positions of compensation points are different.
Light and dark reactions.
Back in 1905, the English plant physiologist F.F. Blackman, interpreting the shape of the photosynthesis light saturation curve, suggested that photosynthesis is a two-stage process that includes photochemical, i.e. a photosensitive reaction and a non-photochemical, i.e. dark, reaction. The dark reaction, being enzymatic, proceeds more slowly than the light reaction, and therefore, at high light intensities, the rate of photosynthesis is completely determined by the rate of the dark reaction. The light reaction either does not depend on temperature at all, or this dependence is very weakly expressed, then the dark reaction, like all enzymatic processes, depends on temperature to a rather significant degree. It should be clearly understood that the reaction called dark can proceed both in the dark and in the light. Light and dark reactions can be separated using flashes of light lasting brief fractions of a second. Flashes of light with a duration of less than one millisecond (10-3 s) can be obtained either using a mechanical device, placing a rotating disk with a slot in the path of a constant light beam, or electrically, by charging a capacitor and discharging it through a vacuum or gas discharge lamp. Ruby lasers with a wavelength of 694 nm are also used as light sources. In 1932, Emerson and Arnold illuminated a cell suspension with flashes of light from a gas-discharge lamp with a duration of about 10-3 s. They measured the rate of oxygen release as a function of the energy of the flashes, the duration of the dark interval between flashes, and the temperature of the cell suspension. With an increase in the intensity of flashes, saturation of photosynthesis in normal cells occurred when one O2 molecule per 2500 chlorophyll molecules was released. Emerson and Arnold concluded that the maximum yield of photosynthesis is determined not by the number of chlorophyll molecules that absorb light, but by the number of enzyme molecules that catalyze the dark reaction. They also found that when the dark intervals between successive flashes increased beyond 0.06 s, the oxygen output per flash no longer depended on the duration of the dark interval, while at shorter intervals it increased with increasing duration of the dark interval (from 0 to 0.06 s). Thus, the dark reaction, which determines the level of saturation of photosynthesis, is completed in about 0.06 s. Based on these data, it was calculated that the average time characterizing the reaction rate was about 0.02 s at 25°C.
Structural and biochemical organization of the apparatus of photosynthesis
Modern ideas about the structural and functional organization of the photosynthetic apparatus include a wide range of issues related to the characteristics of the chemical composition of plastids, the specifics of their structural organization, the physiological and genetic patterns of the biogenesis of these organelles, and their relationships with other functional structures of the cell. In terrestrial plants, the leaf serves as a special organ of photosynthetic activity, where specialized cell structures are localized - chloroplasts containing pigments and other components necessary for the processes of absorption and conversion of light energy into chemical potential. In addition to the leaf, functionally active chloroplasts are present in plant stems, petioles, awns, and spike scales, and even in the illuminated roots of a number of plants. However, it was the leaf that was formed in the course of a long evolution as a special organ to perform the main function of a green plant - photosynthesis, therefore, the anatomy of the leaf, the location of chlorophyll-containing cells and tissues, their relationship with other elements of the morphemic structure of the leaf are subject to the most efficient course of the photosynthesis process, and they are most degrees are subject to intense changes under environmental stress.
In this regard, it is advisable to consider the problem of the structural and functional organization of the photosynthetic apparatus at two main levels - at the level of the leaf as an organ of photosynthesis and chloroplasts, where the entire mechanism of photosynthesis is concentrated.
The organization of the photosynthetic apparatus at the leaf level can be considered on the basis of an analysis of its mesostructure. The concept of "mesostructure" was proposed in 1975. According to the ideas about the structural and functional features of the photosynthetic apparatus with a characteristic of the chemical composition, structural organization, physiological and genetic features of the biogenesis of these organelles and their relationships with other functional structures, a leaf is a special organ of the photosynthetic process, where specialized formations are localized - chloroplasts containing pigments necessary for processes of absorption and conversion of light into chemical potential. In addition, active chloroplasts are present in the stems, awns, and scales of the ear, and even in the illuminated parts of the roots of some plants. However, it was the leaf that was formed by the entire course of evolution as a special organ for performing the main function of a green plant - photosynthesis.
The mesostructure includes a system of morphophysiological characteristics of the photosynthetic apparatus of the leaf, chlorenchyma, and clesophyll. The main indicators of the mesostructure of photosynthetic
tic apparatus (according to A. T. Mokronosov) include: area, number of cells, chlorophyll, protein, cell volume, number of chloroplasts in a cell, chloroplast volume, chloroplast cross-sectional area and its surface. An analysis of the mesostructure and functional activity of the photosynthetic apparatus in many plant species helps to determine the most common values of the studied parameters and the limits of variation of individual characteristics. According to these data, the main indicators of the mesostructure of the photosynthetic apparatus (Mokronosov, 19V1):
I - sheet area;
II - the number of cells per 1 cm2,
III - chlorophyll per 1 dm2, key enzymes per 1 dm2, cell volume, thousand µm2, number of chloroplasts per cell;
IV - chloroplast volume, chloroplast projection area, µm2, chloroplast surface, µm2.
The average number of chloroplasts in a leaf that has finished growing usually reaches 10-30, in some species it exceeds 400. This corresponds to millions of chloroplasts per 1 cm2 of leaf area. Chloroplasts are concentrated in the cells of various tissues in the amount of 15 - 80 pieces per cell. The average volume of a chloroplast is one µm2. In most plants, the total volume of all chloroplasts is 10-20%, in woody plants - up to 35% of the cell volume. The ratio of the total surface of chloroplasts to leaf area is in the range of 3-8. One chloroplast contains a different number of chlorophyll molecules; in shade-loving species, their number increases. The above indicators can vary significantly depending on the physiological state and environmental conditions of plant growth. According to A. T. Mokronosov, in a young leaf, the activation of photosynthesis when 50-80% of the leaf is removed is ensured by an increase in the number of chloroplasts in the cell without changing their individual activity, while in a leaf that has completed growth, the increase in photosynthesis after defoliation occurs due to an increase in activity of each chloroplast without changing their number. Analysis of the mesostructure showed that adaptation to lighting conditions causes a rearrangement that optimizes the light absorbing properties of the sheet.
Chloroplasts have the highest degree of organization of internal membrane structures compared to other cell organelles. In terms of the degree of structure ordering, chloroplasts can only be compared with the receptor cells of the retina, which also perform the function of converting light energy. A high degree of organization of the internal structure of the chloroplast is determined by a number of points:
1) the need for spatial separation of reduced and oxidized photoproducts resulting from primary acts of charge separation in the reaction center;
2) the need for strict ordering of the components of the reaction center, where fast photophysiological and slower enzymatic reactions are coupled: energy conversion of a photoexcited pigment molecule requires its specific orientation with respect to the chemical energy acceptor, which implies the presence of certain structures where the pigment and acceptor are rigidly oriented relative to each other ;
3) the spatial organization of the electron transport chain requires a consistent and strictly oriented organization of carriers in the membrane, which provides the possibility of fast and controlled transport of electrons and protons;
4) for conjugation of electron transport and ATP synthesis, a system of chloroplasts is organized in a certain way.
Lipoprotein membranes as the structural basis of energy processes arise at the earliest stages of evolution; it is assumed that the main lipid components of membranes - phospholipids - were formed under certain biological conditions. The formation of lipid complexes made it possible to include various compounds in them, which, apparently, was the basis of the primary catalytic functions of these structures.
Held in last years electron microscope studies have found organized membrane structures in organisms at the lowest stage of evolution. In some bacteria, the membrane photosynthesizing cell structures of closely packed organelles are located at the cell periphery and are associated with cytoplasmic membranes; in addition, in the cells of green algae, the process of photosynthesis is associated with a system of double closed membranes - thylakoids, localized in the peripheral part of the cell. In this group of photosynthetic organisms, chlorophyll first appears, and the formation of specialized organelles - chloroplasts occurs in cryptophyte algae. They contain two chloroplasts containing from one to several thylakoids. A similar structure of the photosynthetic apparatus also occurs in other groups of algae: red, brown, etc. In the process of evolution, the membrane structure of the photosynthetic process becomes more complicated.
Microscopic studies of the chloroplast, the technique of cryoscopy made it possible to formulate a spatial model of the volumetric organization of chloroplasts. The best known is the granular-lattice model by J. Heslop-Harrison (1964).
Thus, photosynthesis is a complex process of converting light energy into the energy of chemical bonds of organic substances necessary for the life of both the photosynthetic organisms themselves and other organisms that are not capable of independent synthesis of organic substances.
The study of the problems of photosynthesis, in addition to general biological ones, also has applied significance. In particular, the problems of nutrition, the creation of life support systems in space research, the use of photosynthetic organisms to create various biotechnical devices are directly related to photosynthesis.
Bibliography
1. D. Hull, K. Rao "Photosynthesis". M., 1983
2. Mokronosov A.G. "Photosynthetic reaction and integrity of the plant organism". M., 1983
3. Mokronosov A.G., Gavrilenko V.F. "Photosynthesis: physiological - ecological and biochemical aspects" M., 1992
4. "Physiology of photosynthesis", ed. Nichiporovich A.A., M., 1982
5. Evening A.S. "Plant plastids"
6. Vinogradov A.P. "Oxygen isotopes and photosynthesis"
7. Godnev T.N. "Chlorophyll and its structure".
8. Gurinovich G.P., Sevchenko A.N., Soloviev K.N. "Chlorophyll Spectroscopy"
9. Krasnovsky A.A. "Conversion of light energy during photosynthesis"
Photosynthesis is the life process of green plants, the only one in the biosphere associated with the accumulation of solar energy. Its significance lies in the versatile provision of life on Earth.
Biomass formation
Living things - plants, fungi, bacteria and animals - are composed of organic substances. The entire mass of organic matter is initially formed in the process of photosynthesis, which takes place in autotrophic organisms - plants and some bacteria.
Rice. 1. Auto- and heterotrophic organisms.
Heterotrophic organisms, eating plants, only modify organic matter without increasing the total biomass of the planet. The uniqueness of photosynthesis is that during the synthesis of organic substances, the energy of the sun is stored in their chemical bonds. In fact, photosynthetic organisms “tether” solar energy to the Earth.
Life support
Photosynthesis constantly forms organic substances from carbon dioxide and water, which are food and habitat for various animals and humans.
All the energy used in the life of living organisms is originally solar. Photosynthesis captures this energy on Earth and transfers it to all the inhabitants of the planet.
The matter and energy stored during photosynthesis are widely used by humans:
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who read along with this- fossil energy resources;
- wood;
- wild plants as a raw material and aesthetic resource;
- food and technical crop production.
1 hectare of forest or park absorbs 8 kg of carbon dioxide in 1 hour in summer. This amount is allocated for the same time by two hundred people.
Atmosphere
The composition of the atmosphere changed precisely due to the process of photosynthesis. The amount of oxygen gradually increased, increasing the ability of organisms to survive. Initially, the first role in the formation of oxygen belonged to green algae, and now forests.
Rice. 2. Graph of changes in the O₂ content in the atmosphere during evolution.
One of the consequences of increasing the oxygen content in the atmosphere is the formation of an ozone layer that protects living organisms from harmful solar radiation.
It is believed that it was after the formation of the ozone layer that life on land became possible.
Photosynthesis is both a primary source and a factor in the development of life on Earth.
The significance of photosynthesis at the present stage has acquired a new aspect. Photosynthesis inhibits the growth of CO₂ concentration in the air, which is due to the combustion of fuel in transport and industry. This reduces the greenhouse effect. The intensity of photosynthesis increases with an increase in the concentration of CO₂ up to a certain limit.
Rice. 3. Graph of the dependence of photosynthesis on the content of CO₂ in the air.
What have we learned?
To understand the importance of photosynthesis in nature, it is necessary to assess the scale of the biomass formed on Earth and the role of oxygen for the life of all organisms. Photosynthesis is one of the forces that created the modern appearance of the planet and constantly provides the vital processes of nutrition and respiration.
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The importance of photosynthesis in nature has long been not entirely accurate. At the initial stage of the study, many scientists believed that plants emit as much oxygen as they absorb. In fact, careful research has shown that the work done by plants is on a grandiose scale. Despite their relatively small size, green spaces perform a number of useful functions that are aimed at supporting life on Earth.
The most important value of photosynthesis is providing energy to all living beings on the planet, including humans. In the green parts of plants, under the influence of sunlight, oxygen and a huge amount of energy begin to form. This energy is used by plants for their own needs only partially, and the unused potential accumulates. Then the plants go to feed herbivores, which receive the necessary ones without which their development will be impossible. Then herbivores become food for predators, they also need energy, without which life will simply stop.
A little away from this is man, so for him the true meaning of photosynthesis does not appear immediately. It's just that many people are trying to prove to themselves that they are not part of the animal world of our planet. Unfortunately, such a denial will not lead to anything, since all living organisms depend to some extent on each other. It is worth disappearing several species of animals or plants - and the balance in nature will be greatly disturbed. To adapt to new living conditions, other living organisms will be forced to look for alternative food sources. True, there are cases when the extinction of some species leads to the extinction of others.
The value of photosynthesis lies not only in the production of energy, but also in protection from destruction. Scientists have long tried to figure out how life originated on our planet - and created a fairly plausible theory. It turned out that the diversity of living organisms became possible only due to the presence of a protective atmosphere, which was formed due to the intensive work of a huge number of plants. Of course, with the size of modern forests and individual plants, one cannot believe in such a miracle, but the ancient plants were of gigantic size.
The old giants of the plant world have died, but even after death they benefit all of humanity. The energy that has accumulated in them now enters our homes in the form of coal. Today, the role of this type of fuel has significantly decreased, but for a long time mankind was saved from the cold with its help.
Also, do not forget that ancient plants have passed their baton to modern trees and flowers, which maintain the safety of the atmosphere. The more green spaces on our planet, the cleaner the air we breathe. The destruction and increase of harmful ones led to the fact that holes appeared in the ozone layer. If humanity does not realize the true role of photosynthesis, it will lead itself to self-destruction. We simply cannot survive without oxygen and protection, and the number of tropical forests continues to rapidly decrease.
If people really want to save life on their planet, they must fully understand the meaning of photosynthesis. When each individual recognizes the importance of plants, when we stop mindlessly cutting down forests, then life on Earth will become better and cleaner. Otherwise, people will have to learn how to withstand the scorching rays of the sun, breathe smog, harmful emissions and get energy from alternative sources.
It is only from us that what our future will be is curled - and I want to believe that people will make the right choice.
- synthesis of organic substances from carbon dioxide and water with the obligatory use of light energy:
6CO 2 + 6H 2 O + Q light → C 6 H 12 O 6 + 6O 2.
In higher plants, the organ of photosynthesis is the leaf, the organelles of photosynthesis are chloroplasts (the structure of chloroplasts is lecture No. 7). The thylakoid membranes of chloroplasts contain photosynthetic pigments: chlorophylls and carotenoids. There are several different types of chlorophyll ( a, b, c, d), the main one is chlorophyll a. In the chlorophyll molecule, a porphyrin “head” with a magnesium atom in the center and a phytol “tail” can be distinguished. The porphyrin “head” is a flat structure, is hydrophilic, and therefore lies on the surface of the membrane that faces the aquatic environment of the stroma. The phytol "tail" is hydrophobic and thus keeps the chlorophyll molecule in the membrane.
Chlorophyll absorbs red and blue-violet light, reflects green and therefore gives plants their characteristic green color. Chlorophyll molecules in thylakoid membranes are organized into photosystems. Plants and blue-green algae have photosystem-1 and photosystem-2; photosynthetic bacteria have photosystem-1. Only photosystem-2 can decompose water with the release of oxygen and take electrons from the hydrogen of water.
Photosynthesis is a complex multi-stage process; photosynthesis reactions are divided into two groups: reactions light phase and reactions dark phase.
light phase
This phase occurs only in the presence of light in thylakoid membranes with the participation of chlorophyll, electron carrier proteins, and the enzyme ATP synthetase. Under the action of a quantum of light, chlorophyll electrons are excited, leave the molecule and enter the outer side of the thylakoid membrane, which eventually becomes negatively charged. Oxidized chlorophyll molecules are restored by taking electrons from the water located in the intrathylakoid space. This leads to the decomposition or photolysis of water:
H 2 O + Q light → H + + OH -.
Hydroxyl ions donate their electrons, turning into reactive radicals. OH:
OH - → .OH + e - .
Radicals.OH combine to form water and free oxygen:
4NO. → 2H 2 O + O 2.
In this case, oxygen is removed to the external environment, and protons accumulate inside the thylakoid in the "proton reservoir". As a result, the thylakoid membrane, on the one hand, is positively charged due to H +, on the other, negatively due to electrons. When the potential difference between the outer and inner sides of the thylakoid membrane reaches 200 mV, protons are pushed through the channels of ATP synthetase and ADP is phosphorylated to ATP; atomic hydrogen is used to restore the specific carrier NADP + (nicotinamide adenine dinucleotide phosphate) to NADP H 2:
2H + + 2e - + NADP → NADP H 2.
Thus, in light phase photolysis of water occurs, which is accompanied by three critical processes: 1) ATP synthesis; 2) the formation of NADP·H 2; 3) the formation of oxygen. Oxygen diffuses into the atmosphere, ATP and NADP·H 2 are transported to the stroma of the chloroplast and participate in the processes of the dark phase.
1 - stroma of the chloroplast; 2 - grana thylakoid.
dark phase
This phase takes place in the stroma of the chloroplast. Its reactions do not require the energy of light, so they occur not only in the light, but also in the dark. The reactions of the dark phase are a chain of successive transformations of carbon dioxide (comes from the air), leading to the formation of glucose and other organic substances.
The first reaction in this chain is carbon dioxide fixation; carbon dioxide acceptor is a five-carbon sugar ribulose bisphosphate(RiBF); enzyme catalyzes the reaction ribulose bisphosphate carboxylase(RiBP-carboxylase). As a result of carboxylation of ribulose bisphosphate, an unstable six-carbon compound is formed, which immediately decomposes into two molecules phosphoglyceric acid(FGK). Then there is a cycle of reactions in which, through a series of intermediate products, phosphoglyceric acid is converted to glucose. These reactions use the energies of ATP and NADP·H 2 formed in the light phase; The cycle of these reactions is called the Calvin cycle:
6CO 2 + 24H + + ATP → C 6 H 12 O 6 + 6H 2 O.
In addition to glucose, other monomers of complex organic compounds are formed during photosynthesis - amino acids, glycerol and fatty acids, nucleotides. Currently, there are two types of photosynthesis: C 3 - and C 4 -photosynthesis.
C 3 -photosynthesis
This is a type of photosynthesis in which three-carbon (C3) compounds are the first product. C 3 -photosynthesis was discovered before C 4 -photosynthesis (M. Calvin). It is C 3 -photosynthesis that is described above, under the heading "Dark phase". Characteristic features of C 3 photosynthesis: 1) RiBP is an acceptor of carbon dioxide, 2) RiBP carboxylase catalyzes the RiBP carboxylation reaction, 3) as a result of RiBP carboxylation, a six-carbon compound is formed, which decomposes into two FHAs. FHA is restored to triose phosphates(TF). Part of TF is used for regeneration of RiBP, part is converted into glucose.
1 - chloroplast; 2 - peroxisome; 3 - mitochondrion.
This is the light-dependent uptake of oxygen and the release of carbon dioxide. Even at the beginning of the last century, it was found that oxygen inhibits photosynthesis. As it turned out, not only carbon dioxide, but also oxygen can be a substrate for RiBP carboxylase:
O 2 + RiBP → phosphoglycolate (2С) + FHA (3С).
The enzyme is called RiBP-oxygenase. Oxygen is a competitive inhibitor of carbon dioxide fixation. The phosphate group is cleaved off and the phosphoglycolate becomes glycolate, which the plant must utilize. It enters the peroxisomes, where it is oxidized to glycine. Glycine enters the mitochondria, where it is oxidized to serine, with the loss of already fixed carbon in the form of CO 2. As a result, two molecules of glycolate (2C + 2C) are converted into one FHA (3C) and CO 2. Photorespiration leads to a decrease in the yield of C 3 -plants by 30-40% ( C 3 -plants- plants that are characterized by C 3 -photosynthesis).
C 4 -photosynthesis - photosynthesis, in which the first product is four-carbon (C 4) compounds. In 1965, it was found that in some plants (sugarcane, corn, sorghum, millet) the first products of photosynthesis are four-carbon acids. Such plants are called With 4 plants. In 1966, the Australian scientists Hatch and Slack showed that C 4 plants have practically no photorespiration and absorb carbon dioxide much more efficiently. The path of carbon transformations in C 4 plants began to be called by Hatch-Slack.
C 4 plants are characterized by a special anatomical structure of the leaf. All conducting bundles are surrounded by a double layer of cells: the outer one is mesophyll cells, the inner one is lining cells. Carbon dioxide is fixed in the cytoplasm of mesophyll cells, the acceptor is phosphoenolpyruvate(PEP, 3C), as a result of PEP carboxylation, oxaloacetate (4C) is formed. The process is catalyzed PEP carboxylase. In contrast to RiBP carboxylase, PEP carboxylase has a high affinity for CO 2 and, most importantly, does not interact with O 2 . In mesophyll chloroplasts, there are many granae, where reactions of the light phase are actively taking place. In the chloroplasts of the sheath cells, reactions of the dark phase take place.
Oxaloacetate (4C) is converted to malate, which is transported through plasmodesmata to the lining cells. Here it is decarboxylated and dehydrated to form pyruvate, CO 2 and NADP·H 2 .
Pyruvate returns to mesophyll cells and regenerates at the expense of ATP energy in PEP. CO 2 is again fixed by RiBP carboxylase with the formation of FHA. The regeneration of PEP requires the energy of ATP, so almost twice as much energy is needed as with C 3 photosynthesis.
The Importance of Photosynthesis
Thanks to photosynthesis, billions of tons of carbon dioxide are absorbed from the atmosphere every year, billions of tons of oxygen are released; photosynthesis is the main source of the formation of organic substances. The ozone layer is formed from oxygen, which protects living organisms from short-wave ultraviolet radiation.
During photosynthesis, a green leaf uses only about 1% of the solar energy falling on it, the productivity is about 1 g of organic matter per 1 m 2 of surface per hour.
Chemosynthesis
The synthesis of organic compounds from carbon dioxide and water, carried out not at the expense of light energy, but at the expense of the oxidation energy of inorganic substances, is called chemosynthesis. Chemosynthetic organisms include some types of bacteria.
Nitrifying bacteria oxidize ammonia to nitrous, and then to nitric acid (NH 3 → HNO 2 → HNO 3).
iron bacteria convert ferrous iron to oxide (Fe 2+ → Fe 3+).
Sulfur bacteria oxidize hydrogen sulfide to sulfur or sulfuric acid (H 2 S + ½O 2 → S + H 2 O, H 2 S + 2O 2 → H 2 SO 4).
As a result of the oxidation reactions of inorganic substances, energy is released, which is stored by bacteria in the form of high-energy bonds of ATP. ATP is used for the synthesis of organic substances, which proceeds similarly to the reactions of the dark phase of photosynthesis.
Chemosynthetic bacteria contribute to the accumulation of minerals in the soil, improve soil fertility, promote wastewater treatment, etc.
Go to lectures №11“The concept of metabolism. Biosynthesis of proteins"
Go to lectures №13"Methods of division of eukaryotic cells: mitosis, meiosis, amitosis"