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photosynthesis(fō'tōsĭn`thəsĭs), process in which green plants, algae, and cyanobacteria utilize the energy of sunlight to manufacture carbohydrates from carbon dioxide and water in the presence of chlorophyll. Some of the plants that lack chlorophyll, e.g., the Indian pipeIndian pipe,
common name for the genus Monotropa and for the family Monotropaceae, low flowering plants of north temperate zones. They are chlorophylless saprophytes with a funguslike appearance.
..... Click the link for more information. , secure their nutrients from organic material, as do animals, and a few bacteria manufacture their own carbohydrates with hydrogen and energy obtained from inorganic compounds (e.g., hydrogen sulfide) in a process called chemosynthesischemosynthesis,
process in which carbohydrates are manufactured from carbon dioxide and water using chemical nutrients as the energy source, rather than the sunlight used for energy in photosynthesis. Much life on earth is fueled directly or indirectly by sunlight.
..... Click the link for more information. . However, the vast majority of plants contain chlorophyll—concentrated, in the higher land plants, in the leaves.
In these plants water is absorbed by the roots and carried to the leaves by the xylem, and carbon dioxide is obtained from air that enters the leaves through the stomata and diffuses to the cells containing chlorophyll. The green pigment chlorophyllchlorophyll
, green pigment that gives most plants their color and enables them to carry on the process of photosynthesis. Chemically, chlorophyll has several similar forms, each containing a complex ring structure and a long hydrocarbon tail.
..... Click the link for more information. is uniquely capable of converting the active energy of light into a latent form that can be stored (in food) and used when needed.
The Photosynthetic Process
The initial process in photosynthesis is the decomposition of water (H2O) into oxygen, which is released, and hydrogen; direct light is required for this process. The hydrogen and the carbon and oxygen of carbon dioxide (CO2) are then converted into a series of increasingly complex compounds that result finally in a stable organic compound, glucose (C6H12O6), and water. This phase of photosynthesis utilizes stored energy and therefore can proceed in the dark. The simplified equation used to represent this overall process is 6CO2+12H2O+energy=C6H12O6+6O2+6H2O. In general, the results of this process are the reverse of those in respiration, in which carbohydrates are oxidized to release energy, with the production of carbon dioxide and water.
The intermediary reactions before glucose is formed involve several enzymes, which react with the coenzyme ATP (see adenosine triphosphateadenosine triphosphate
(ATP) , organic compound composed of adenine, the sugar ribose, and three phosphate groups. ATP serves as the major energy source within the cell to drive a number of biological processes such as photosynthesis, muscle contraction, and the synthesis of
..... Click the link for more information. ) to produce various molecules. Studies using radioactive carbon have indicated that among the intermediate products are three-carbon molecules from which acids and amino acids, as well as glucose, are derived. This suggests that fats and proteins are also products of photosynthesis. The main product, glucose, is the fundamental building block of carbohydrates (e.g., sugars, starches, and cellulose). The water-soluble sugars (e.g., sucrose and maltose) are used for immediate energy. The insoluble starches are stored as tiny granules in various parts of the plant—chiefly the leaves, roots (including tubers), and fruits—and can be broken down again when energy is needed. Cellulose is used to build the rigid cell walls that are the principal supporting structure of plants.
Importance of Photosynthesis
Animals and plants both synthesize fats and proteins from carbohydrates; thus glucose is a basic energy source for all living organisms. The oxygen released (with water vapor, in transpiration) as a photosynthetic byproduct, principally of phytoplankton, provides most of the atmospheric oxygen vital to respiration in plants and animals, and animals in turn produce carbon dioxide necessary to plants. Photosynthesis can therefore be considered the ultimate source of life for nearly all plants and animals by providing the source of energy that drives all their metabolic processes.
See I. Asimov, Photosynthesis (1969); R. M. Devlin and A. V. Barker, Photosynthesis (1972); O. Morton, Eating the Sun (2009).
The manufacture in light of organic compounds (primarily certain carbohydrates) from inorganic materials by chlorophyll- or bacteriochlorophyll-containing cells. This process requires a supply of energy in the form of light. In chlorophyll-containing plant cells and in cyanobacteria, photosynthesis involves oxidation of water (H2O) to oxygen molecules, which are released into the environment. In contrast, bacterial photosynthesis does not involve O2 evolution—instead of H2O, other electron donors, such as H2S, are used. This article will focus on photosynthesis in plants. See Bacterial physiology and metabolism, Chlorophyll, Plant respiration
The light energy absorbed by the pigments of photosynthesizing cells, especially by the pigment chlorophyll or bacteriochlorophyll, is efficiently converted into stored chemical energy. Together, the two aspects of photosynthesis—the conversion of inorganic into organic matter, and the conversion of light energy into chemical energy—make it the fundamental process of life on Earth: it is the ultimate source of all living matter and of all life energy.
The net overall chemical reaction of plant photosynthesis is shown in the equation below,
The photochemical reaction in photosynthesis belongs to the type known as oxidation-reduction, with CO2 acting as the oxidant (hydrogen or electron acceptor) and water as the reductant (hydrogen or electron donor). The unique characteristic of this particular oxidation-reduction is that it goes “in the wrong direction” energetically; that is, it converts chemically stable materials into chemically unstable products. Light energy is used to make this “uphill” reaction possible.
Photosynthesis is a complex, multistage process. Its main parts are (1) the primary photochemical process in which light energy absorbed by chlorophyll is converted into chemical energy, in the form of some energy-rich intermediate products; and (2) the enzyme-catalyzed “dark” (that is, not photochemical) reactions by which these intermediates are converted into the final products—carbohydrates and free oxygen.
Experiments suggest that plants contain two pigment systems. One (called photosystem I, or PS I, sensitizing reaction I) contains the major part of chlorophyll a; the other (called photosystem II, or PS II, sensitizing reaction II) contains some chlorophyll a and the major part of chlorophyll b or other auxiliary pigments (for example, the red and blue pigments, called phycobilins, in red and blue-green algae, and the brown pigment fucoxanthol in brown algae and diatoms). It appears that efficient photosynthesis requires the absorption of an equal number of light quanta in PS I and in PS II; and that within both systems excitation energy undergoes resonance migration from one pigment to another until it ends in special molecules of chlorophyll a called the reaction centers. The latter molecules then enter into a series of chemical reactions that result in the oxidation of water to produce O2 and the reduction of nicotinamide adenine dinucleotide phosphate (NADP+). Chromatophores from photosynthetic bacteria and chloroplasts from green plants, when illuminated in the presence of adenosine diphosphate (ADP) and inorganic phosphate, also use light energy to synthesize adenosine triphosphate (ATP); this photophosphorylation could be associated with some energy-releasing step in photosynthesis.
The light-dependent conversion of radiant energy into chemical energy as ATP and reduced nicotinamide adenine dinucleotide phosphate (NADPH) serves as a prelude to the utilization of these compounds for the reductive fixation of CO2 into organic molecules. Such molecules, broadly designated as photosynthates, are usually but not invariably in the form of carbohydrates such as glucose polymers or sucrose, and form the base for the nutrition of all living things. Collectively, the biochemical processes by which CO2 is assimilated into organic molecules are known as the photosynthetic dark reactions, not because they must occur in darkness, but because light—in contrast to the photosynthetic light reactions—is not required.
The essential details of C3 photosynthesis can be seen in Fig. 1. Three molecules of CO2 combine with three molecules of the five-carbon compound ribulose bisphosphate (RuBP) in a reaction catalyzed by RuBP carboxylase to form three molecules of an enzyme-bound six-carbon compound. These are hydrolyzed into six molecules of the three-carbon compound phosphoglyceric acid (PGA), which are phosphorylated by the conversion of six molecules of ATP (releasing ADP for photophosphorylation via the light reactions). The resulting compounds are reduced by the NADPH formed in photosynthetic light reactions to form six molecules of the three-carbon compound phosphoglyceraldehyde (PGAL). One molecule of PGAL is made available for combination with another three-carbon compound, dihydroxyacetone phosphate, which is isomerized from a second PGAL (requiring a second “turn” of the Calvin-cycle wheel) to form a six-carbon sugar. The other five PGAL molecules, through a complex series of enzymatic reactions, are rearranged into three molecules of RuBP, which can again be carboxylated with CO2 to start the cycle turning again. The net product of two “turns” of the cycle, a six-carbon sugar (glucose-6-phosphate) is formed either within the chloroplast in a pathway leading to starch (a polymer of many glucose molecules), or externally in the cytoplasm in a pathway leading to sucrose (condensed from two six-carbon sugars, glucose and fructose).
Initially, the C3 cycle was thought to be the only route for CO2 assimilation, although it was recognized by plant anatomists that some rapidly growing plants (such as maize, sugarcane, and sorghum) possessed an unusual organization of the photosynthetic tissues in their leaves (Kranz morphology). It was then demonstrated that plants having the Kranz anatomy utilized an additional CO2 assimilation route now known as the C4-dicarboxylic acid pathway (Fig. 2). Carbon dioxide enters a mesophyll cell, where it combines with the three-carbon compound phosphoenolpyruvate (PEP) to form a four-carbon acid, oxaloacetic acid, which is reduced to malic acid or transaminated to aspartic acid. The four-carbon acid moves into bundle sheath cells, where the acid is decarboxylated, the CO2 assimilated via the C3 cycle, and the resulting three-carbon compound, pyruvic acid, moves back into the mesophyll cell and is transformed into PEP, which can be carboxylated again. The two cell types, mesophyll and bundle sheath, are not necessarily adjacent, but in all documented cases of C4 photosynthesis the organism had two distinct types of green cells. C4 metabolism is classified into three types, depending on the decarboxylation reaction used with the four-carbon acid in the bundle sheath cells:
1. NADP-ME type (sorghum):
2. NAD-ME type (Atriplex species):
3. PCK type (Panicum species):
Under arid and desert conditions, where soil water is in short supply, transpiration during the day when temperatures are high and humidity is low may rapidly deplete the plant of water, leading to desiccation and death. By keeping stomata closed during the day, water can be conserved, but the uptake of CO2, which occurs entirely through the stomata, is prevented. Desert plants in the Crassulaceae, Cactaceae, Euphorbiaceae, and 15 other families evolved, apparently independently of C4 plants, an almost identical strategy of assimilating CO2 by which the CO2 is taken in at night when the stomata open; water loss is low because of the reduced temperatures and correspondingly higher humidities. First studied in plants of the Crassulaceae, the process has been called crassulacean acid metabolism (CAM).
In contrast to C4, where two cell types cooperate, the entire process occurs within an individual cell; the separation of C4 and C3 is thus temporal rather than spatial. At night, CO2 combines with PEP through the action of PEP carboxylase, resulting in the formation of oxaloacetic acid and its conversion into malic acid. The PEP is formed from starch or sugar via the glycolytic route of respiration. Thus, there is a daily reciprocal relationship between starch (a storage product of C3 photosynthesis) and the accumulation of malic acid (the terminal product of nighttime CO2 assimilation).
the process in which higher plants, algae, and photosynthesizing bacteria utilize light energy absorbed by chlorophyll and other photosynthetic pigments to manufacture complex organic substances necessary for the life processes of the plants themselves and all other organisms from simple compounds, such as carbon dioxide and water. It is one of the most important biological processes, occurring continuously on an enormous scale on our planet. As a result of photosynthesis, the earth’s vegetation annually produces more than 100 billion tons of organic matter, about one-half of which is produced by aquatic plants, using up in the process about 200 billion tons of CO2 and liberating about 145 billion tons of free oxygen. It is conjectured that all the oxygen in our atmosphere is formed through photosynthesis.
Photosynthesis is the only biological process that occurs with an increase in the free energy of the system; all other processes, except chemosynthesis, utilize the potential energy stored in the products of photosynthesis. The amount of energy bound annually by marine and terrestrial photosynthesizing organisms (about 3 × 1021 joules) is many times greater than the amount of energy used by man (about 3 × 1020 joules).
Historical survey. The earliest research on photosynthesis can be traced to J. Priestley, J. Senebier, N. de Saussure, J. Ingen-Housz, and J. von Mayer, in whose works it gradually became clear that plants in the presence of light take up carbon dioxide from the air, liberate oxygen, and as a result form organic substances, storing solar energy in the substances. In the second half of the 19th century, K. A. Timiriazev demonstrated that solar energy is brought into the chain of photosynthetic processes through the green pigment in plants—chlorophyll. He also showed that the spectrum of photosynthetic activity corresponds to the spectrum of light absorption by chlorophyll and that the intensity of photosynthesis increases with the intensity of the light.
In 1905 the British scientist F. Blackmann discovered that photosynthesis consists of a light reaction, which is rapid, and a slower dark reaction. However, the biochemical proof of the existence of these light and dark phases was obtained only in 1937, by the British scientist R. Hill. Important contributions to the study of the dark and light phases were also made by the German biochemist and physiologist O. Warburg and the American biochemist H. Gaffron. In 1931 the American microbiologist C. Van Niel showed that photosynthesizing bacteria effect photosynthesis without liberating O2, since they oxidize hydrogen sulfide, thiosulfate, and other substrates upon assimilating CO2. This led to the notion that photosynthesis is an oxidation-reduction process, in which the reduction of CO2 is effected with the simultaneous oxidation of the hydrogen donor. In 1941 the Soviet scientists A. P. Vinogradov and M. V. Teits and the American scientist S. Ruben, among others, established that the oxygen emitted by higher plants and algae in photosynthesis originates in water, not in CO2 as was previously believed.
In the first quarter of the 20th century, important research on the physiology and ecology of photosynthesis was conducted by V. V. Sapozhnikov, S. P. Kostychev, V. N. Liubimenko, A. A. Nichiporovich, and O. V. Zalenskii. In the mid–20th century, the advent of new research techniques, such as gas-diffraction analysis, isotope methods, spectroscopy, and electron microscopy, proved useful in the study of photosynthesis. These methods made it possible to elucidate the subtle mechanisms of chlorophyll participation in photosynthesis (the Soviet scientists A. N. Terenin and A. A. Krasnovskii, the American scientists E. Rabinowitch, B. Kok, W. Arnold, R. Clayton, and J. Franck, and the French scientist J. Lavorel), as well as the oxidation-reduction reactions of photosynthesis and the existence of two photochemical reactions in photosynthesis (the English plant physiologist R. Hill and British scientist S. Ochoa, the American scientists W. Vishniac, R. Emerson, and C. S. French, and the Dutch scientist L. Duysens). The new methods also helped in the study of photophosphorylation (D. Arnon), the pathways of carbon conversion (the American scientists M. Calvin, J. Bassham, and A. Benson, and the Australian scientists M. Hatch and C. Stack), and the mechanism of water decomposition (B. Kok, the French scientists A. Joliot and P. Joliot, and the Soviet scientist V. M. Kutiurin).
Characteristic features of photosynthesis of higher green plants, algae, and photosynthesizing bacteria. In the photosynthetic processes of higher green plants and algae, both multicellular algae (green, brown, and red algae) and unicellular algae (Euglena, dinoflagellates, diatoms), the hydrogen donor and the source of the liberated oxygen is water, while the principal acceptor of hydrogen atoms and the source of carbon is carbon dioxide. When only CO2 and H2O are used in photosynthesis, carbohydrates are formed. However, in addition to carbohydrates plants also produce nitrogen- and sulfur-containing amino acids, proteins, pigments, and other compounds during photosynthesis. In this case, the acceptors of hydrogen atoms, along with CO2, and the sources of nitrogen and sulfur are nitrates (NO3–) and sulfates (SO42–). Photosynthesizing bacteria do not liberate and do not use molecular oxygen (most of them are obligate anaerobes). Instead of water, they use as electron donors either inorganic compounds, such as hydrogen sulfide, thiosulfate, and hydrogen gas, or organic compounds, such as lactic acid and isopropyl alcohol. In most cases, the source of carbon is also CO2, along with some other organic compounds, such as acetate. Thus, in different organisms photosynthesis may occur with the use of various electron and hydrogen donors (DH2) and acceptors (A); it may be represented by the following general equation:
where AH2 are the products of photosynthesis.
Structural features of the photosynthetic apparatus. The high effectiveness of photosynthesis in higher green plants is ensured by a sophisticated photosynthetic apparatus, whose primary structural elements are the intracellular organelles known as chloroplasts (a green-leaf cell contains anywhere from 20 to 100 chloroplasts). The chloroplasts are surrounded by a membrane consisting of two layers. The inner layer consists of tiny flattened sacs or spheres called thylakoids, which are often stacked, forming grana, joined to one another by single inter-grana thylakoids. The thylakoids consist mainly of photosynthetic membranes proper. The membranes, in turn, consist of biomolecular lipoidic layers impregnated with a mosaic of lipoproteidic pigment complexes that form photochemically active centers. They also contain special components that participate in the transfer of electrons and the formation of adenosine triphosphate (ATP). Some parts of the chloroplast, namely the stroma, which are located between the thylakoids, contain enzymes that catalyze the dark reactions of photosynthesis, such as the conversion of carbon, nitrogen, and sulfur, as well as the biosynthesis of carbohydrates and proteins. Starch formed during photosynthesis is deposited in the stroma. The chloroplasts have their own DNA, RNA, and ribosomes that synthesize proteins; they have some measure of genetic autonomy, although they are under the general control of the nucleus.
Photosynthesizing bacteria and most algae do not have chloroplasts. The photosynthetic apparatus in most algae comprises specialized intracellular organelles called chromatophores, while photosynthesizing bacteria and blue-green algae have thylakoids (their membranes contain either the pigment bacteriochlorophyll or the pigment bacterioviridin, as well as other photosynthetic components) embedded in the peripheral layers of the cytoplasm.
Phase of primary conversions and energy storage in photosynthesis. The basis of photosynthesis in plants is the oxidation-reduction process, in which four electrons and protons are raised from the level of the oxidation-reduction potential of water (+ 0.8 volt) to the level corresponding to the reduction of CO2 with the formation of carbohydrates (–0.4 volt). At the same time, the increase in the free energy in the course of the reduction of CO2 to the carbohydrate level is 120 kilocalories per mole (kcal/mole); the overall equation of photosynthesis is expressed as
The energy of one mole of quanta (einstein) from the red region of the spectrum is about 40 kcal/mole. Thus, for photosynthesis that proceeds in accordance with the above equation, the absorption of three quanta of energy per molecule of CO2 (or per molecule of liberated O2) would be sufficient. However, in the oxidation-reduction reaction, four electrons must be transferred from water to CO2, and the transfer of each is effected in two successive photochemical reactions. Consequently, the quantum requirement under optimal conditions is eight to 12 quanta per molecule of O2, and the maximum effectiveness of the conversion of the energy of red light is about 30 percent. As a result of a number of factors, such as the incomplete absorption of light, the energy requirements of respiration, and the limitedness of the growing period, the effective utilization of sunlight under natural conditions by agricultural plants in temperate latitudes does not exceed 0.5 to 1.3 percent. A comparison of these figures with the theoretical maximum value indicates the existence of substantial reserves that can be used in the future. For certain agricultural crops the effective utilization of light energy has been increased under special conditions to 5–6 percent and even higher (in the cultivation of algae up to 7–10 percent).
Neither CO2 nor water absorbs light directly. The interaction of these compounds with quanta of light is mediated by chlorophyll a, a constituent of chloroplasts and chromatophores that forms the functional photosynthetic unit, consisting of several hundred molecules of pigment and reaction centers. The major part of the accessory pigments (for example, chlorophyll b, carotenoids, phycobilins, and short-wave forms of chlorophyll a) function as light-collecting antennas. Upon absorption of quanta of light, the molecules of these pigments are transformed into an excited state; through migration, the excitation energy is transferred to the chlorophyll a molecule, located in the reaction center. The effectiveness of the energy transfer is conditioned by the close spatial arrangement of molecules and by the presence of several aggregates of chlorophyll a, which participate in the formation of the reaction centers and form a descending ladder of energy levels. Semiconductor transfer of the electron along the aggregated pigment is possible.
The reaction center is the site of the primary reaction of photosynthesis—the separation of the charges with the subsequent formation of the primary oxidizing agent and the primary reducing agent. There are two types of reaction centers (Figure 1), one of which is included in pigment photosystem I and one in pigment photosystem II. System II participates in the photoreaction associated with the decomposition of water: chlorophyll a serves as the center’s pigment, with a maximum absorption of 680 nanometers (nm); the hypothetical primary reducing agent is Q (probably cytochrome); and the primary oxidizing agent is the intricate complex Z. The excitation of the reaction center’s pigment molecule, in this case P680, is accompanied by the separation of charges and the formation of oxidized Z+, which
participates in the oxidation of water and the liberation of O2. It is theorized that unknown enzymes and manganous and bicarbonate ions take part in the water decomposition phase, which has been little studied thus far. The primary reducing agent Q (whose existence is inferred from the excitation of fluorescence) of photosystem II transfers an electron to the carriers—cytochromes b and f, plastoquinone, and plastocyanin—of the photosynthetic electron transport chain to the reaction center of photosystem I. The pigment of this center is chlorophyll a, with a maximum absorption of 700 nm, and the primary reducing agent is the unknown substance X. The reduced X transfers an electron to ferredoxin, an iron-containing protein that reduces nicotinamide adenine dinucleotide phosphate (NADP). The reduced form— NADPH—stores the principal portion of light energy. The remaining energy of the electron flow is stored in the form of ATP, produced as a result of photophosphorylation during the descending stage of the electron transfer from photosystem II to photosystem I (noncyclic phosphorylation) or with circular closing of the flow in photosystem I (cyclic phosphorylation).
Phosphorylation possibly occurs through a chemosmotic mechanism due to the electrical potential and the gradient of H+ concentration, which arise when light induces an electron flow in the membranous structures of the thylakoids. It has been discovered experimentally that illumination induces an electrical potential in the chloroplast membrane.
The aforementioned successive operation of photoreactions I and II is most probable, although a parallel operation of reactions is also a possibility. It is supposed that photosynthesizing bacteria effect photosynthesis with the participation of only a single pigment photosystem, but this problem has yet to be resolved.
The light-induced physical process and photochemical stages end in 10–12 to 10–8 second with the separation of charges and subsequent formation of the primary oxidizing and reducing agents. The boundary of the primary biophysical and biochemical processes is usually considered to be the formation of the first chemically stable products—NADPH and ATP. These substances (the “reducing force”) are then used in the dark reaction of CO2 reduction.
Assimilation of carbon dioxide. Carbon dioxide assimilation occurs in the course of dark reactions. It is not free CO2 that undergoes reduction during photosynthesis, but the CO2 that is incorporated into a certain organic compound. In most cases the acceptor of CO2 is a twice phosphorylated sugar with five carbons, ribulose diphosphate (RUDP). Upon the introduction of CO2, RUDP splits into two molecules of phosphoglyceric acid (PGA). The carbon of CO2 included in the PGA molecule, then, is the final link in the chain toward which the electrons mobilized by chlorophyll are directed. Having acquired an electron, PGA is reduced to phosphoglyceraldehyde (ATP and NADPH participate in this process), which can be considered the first stable product of photosynthesis containing carbon in reduced (organic) form. Subsequent conversions occur in the pentose phosphate cycle and are completed, on the one hand, by the formation of RUDP—that is, regeneration of the primary CO2 acceptor occurs (which makes the cycle continuous in the presence of light and CO2)—and, on the other hand, by the formation of the products of photosynthesis—carbohydrates.
The preceding discussion applies to C3 plants, which are plants that absorb carbon in photosynthesis through the Calvin cycle (Figure 2) and attach CO2 to RUDP by means of RUDP carboxylase, forming the first three-carbon products of photosynthesis—phosphoglyceric acid and phosphoglyceraldehyde. Certain herbaceous plants, mainly of tropical origin, such as sugarcane, corn, and sorghum, form as the first products of photosynthesis not three-carbon but four-carbon compounds— oxalacetic, malic, and aspartic acids. The pathway of autotrophic assimilation of CO2 through phosphoenolpyruvic acid or phosphoenol pyruvate (PEPA), with the formation of C4-dicarboxylic acids, is called the C4 pathway of carbon assimilation, and the organisms are called C4 plants. The leaves of such plants have two types of photosynthesizing cells, and photosynthesis proceeds in two stages. The primary acceptance of CO2 by PEPA occurs in the mesophyllic cells of the leaves of C4 plants, with the participation of PEPA-carboxylase, which effects the carboxylation reaction even in the presence of very low concentrations of CO2 in the atmosphere. As a result of carboxylation, oxalacetic, malic, and aspartic acids are formed. The last two are transferred to the parietal cells of the vascular bundles of the leaf, where they undergo decarboxylation and create within the cells a high concentration of CO2 already assimilated through RUDP carboxylase in the Calvin cycle. This is advantageous because it facilitates the introduction of CO2 in the Calvin cycle through the carboxylation of RUDP by means of RUDP carboxylase, which is less active than PEPA carboxylase and requires higher concentrations of CO2 for optimal operation. Moreover, the high concentration of CO2 in the parietal cells diminishes respiration and the energy losses associated with it. In this way, a high-intensity “cooperative” photosynthesis occurs, which is free of excessive losses as a result of respiration and oxygen inhibition and is well adapted to the execution of photosynthesis in the presence of low concentrations of CO2 and high concentrations of O2.
There are other pathways for the conversion of CO2 in photosynthesis, as a result of which various organic acids, proteins, and other substances are formed in different ratios. The ratios between these groups of compounds in the plant depend on the intensity and quality of the light, the species of plant, and the conditions of the plant’s development (for example, root nourishment and conditions of illumination). By regulating a plant’s growth environment, it is possible to direct the composition of the products of photosynthesis and, consequently, the chemism of the plant as a whole.
Role in the biosphere. Along with photosynthesis, other processes occur on the earth that are approximately equal in magnitude but opposite in direction. These processes include the oxidation of organic matter and reduction of carbon that occur when fuels, such as coal, petroleum, gas, peat, and wood, are burned and organic matter is used up by living organisms in the course of life processes (respiration, fermentation), as a result of which completely oxidized compounds—carbon dioxide and water—are produced and energy is liberated. Then, by means of the solar energy, carbon dioxide and water are again involved in photosynthesis. Thus, the solar energy used in photosynthesis serves as the moving force of an immense cycle on the earth of such elements as carbon, hydrogen, and oxygen, a cycle that also includes many other elements, such as N, S, P, Mg, and Ca. Over the course of the earth’s existence, the most important elements and substances on the earth have passed through many thousands of complete cycles as a result of photosynthesis.
In preceding geological periods, conditions for photosynthesis on the earth were more favorable owing to the preponderance of reducing processes over oxidizing ones. Gradually, enormous quantities of reduced carbon in organic remains came to be buried deep within the earth, forming vast deposits of fossil fuels. As a result, the relative content of carbon dioxide in the atmosphere has decreased considerably (to 0.03 percent of volume) and the oxygen content has increased, which has substantially worsened the conditions for photosynthesis.
The appearance on the earth of photosynthesizing plants and the continuous formation of large quantities of new energy-rich organic substances by the plants engendered the development of heterotrophic organisms, such as bacteria, fungi, animals, and man, which utilize these substances and their energy. As a result, in the process of respiration, fermentation, decay, and combustion, organic compounds began to oxidize and undergo decomposition in the same quantities in which higher plants, algae, and bacteria form them. A cycle of matter was established on the earth, in which the sum total of life on our planet is determined by the scale of photosynthesis.
In the current (Anthropogenic) geological period, the dimensions of photosynthetic productivity on the earth have probably stabilized. However, owing to the rapidly increasing use of the products of photosynthesis by man, it has become necessary to think about the eventual exhaustion of the supply of fossil fuels, food and forest resources, and the like. The photosynthetic capacity of the existing vegetation is insufficient to regenerate the atmosphere: the earth’s vegetation is incapable of completely assimilating all the carbon dioxide (its relative content in the atmosphere in the last 100 years has slowly but inexorably increased) that enters the atmosphere as a result of excessive mining and burning of fossil fuels.
At the same time, the potential photosynthetic activity of plants is far from being used up completely. The problem of maintaining, increasing, and best using the photosynthetic productivity of plants is one of the most important problems in modern natural science and in the practical activity of man.
Photosynthesis and harvest yield. One way to increase the total productivity of plants is to intensify their photosynthetic activity. For example, in order to obtain a wheat harvest of 40 quintals/ hectare, which would yield 100 tons of total dry biomass, the plants must assimilate about 20 tons of CO2, photochemically decompose about 7.3 tons of H2O, and liberate about 13 tons of O2. During the growing period in the middle latitudes, which lasts about three to four months, about 2 × 109 kilocalories of photosynthetically active radiation (in the 380- to 720-nm region of the spectrum) usually comes to the earth’s surface. In a harvest with a biomass of 10 tons, about 40 × 106 kilocalories, or 2 percent of the photosynthetically active radiation, are stored. A small part of the remaining energy is reflected. Most of the energy is converted to heat and induces the evaporation of enormous quantities of H2O. Thus, in order to intensify photosynthetic activity in plants, it is necessary to increase the plant’s effective utilization of solar radiation. This is attained by increasing leaf surface in plants, prolonging the periods of the active functioning of leaves, and regulating the spatial density of plantings. Also of importance in helping intensify photosynthetic activity are the method of distributing plants in a given area (correct rates of sowing seeds) and the provision of sufficient CO2 in the air, water, and nutrient elements in the soil.
In addition to external conditions, the functional activity of the photosynthetic apparatus of plants is also determined by the anatomical structure of the leaf, the activity of the enzymatic systems, and the type of carbon metabolism. A large role is played by plant selection—the creation of varieties that assimilate CO2 intensely—and the management of the processes associated with the effective use of the organic substances produced in photosynthesis. An important property of highly productive varieties is the capacity to use a large proportion of the assimilates for the formation of economically valuable organs, such as grains in cereals, tubers in potatoes, and roots in root crops.
The elucidation of the laws and foundations of photosynthetic productivity in plants and the development of the principles for optimizing and increasing photosynthesis are important tasks of modern science.
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