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CARBON CYCLE, carbon cycle - the cyclic movement of carbon between the world of living beings and the inorganic world of the atmosphere, seas, fresh waters, soil and rocks. This is one of the most important biogeochemical cycles, involving many complex reactions during which carbon moves from the air and aquatic environment into the tissues of plants and animals, and then returns to the atmosphere, water and soil, becoming again available for use by organisms. Since carbon is necessary to support any form of life, any interference with the cycle of this element affects the number and diversity of living organisms that can exist on Earth.
Sources and reserves of carbon.
The main source of carbon for living organisms is the Earth's atmosphere, where this element is present in the form of carbon dioxide (carbon dioxide, CO 2). For many millions of years, the concentration of CO 2 in the atmosphere apparently did not change significantly, amounting to ca. 0.03% of the weight of dry air at sea level. Although the proportion of CO 2 is small, its absolute amount is truly huge - approx. 750 billion tons. In the atmosphere, CO 2 is transported by winds in both vertical and horizontal directions.
Carbon dioxide is present in water, where it dissolves easily, forming weak carbonic acid H 2 CO 3. This acid reacts with calcium and other elements to form minerals called carbonates. Carbonate rocks, such as limestone, are in equilibrium with carbon dioxide, which is contained in the water that comes into contact with them. Likewise, the amount of CO 2 dissolved in oceans and fresh waters is determined by its concentration in the atmosphere. The total amount of dissolved and sedimentary carbonaceous matter is estimated at approximately 1.8 trillion. T.
Carbon, combined with hydrogen and other elements, is one of the main components of plant and animal cells. For example, in the human body it is approx. 18% body weight. The large number and very wide distribution of living organisms do not allow a satisfactory assessment of the total carbon content in them. However, it is possible to approximately estimate the total amount of carbon fixed by plants, as well as released during the respiration of plants, animals and microorganisms. It has been established that green plants absorb approx. 220 billion tons of CO 2 . Almost the same amount of this substance is released into the inorganic environment during the respiration of all living organisms, as well as as a result of the decomposition and combustion of organic substances.
Under certain conditions, decomposition and combustion of substances created by living organisms does not occur, which leads to the accumulation of carbon-containing compounds. For example, the wood of living trees can be reliably protected from microbial decomposition and from fire for 3-4 millennia by bark that can resist the action of microbes and fire. Wood that ends up in a peat bog lasts even longer. In both cases, the carbon bound in it appears to be trapped and is removed from the cycle for a long time. When organic matter is buried and isolated from exposure to air, it decomposes only partially and the carbon it contains is retained. If these organic remains are subsequently pressed by overlying sediments and heated by the earth's heat over millions of years, much of it is converted into fossil fuels such as coal or oil. Fossil fuels form a natural carbon reserve. Despite intensive burning since the 1700s, approximately 4.5 trillion still remain unspent. T.
Photosynthesis.
The main way through which carbon moves from the inorganic world to the living world is through photosynthesis carried out by green plants. This process is a chain of reactions during which plants absorb carbon dioxide from the atmosphere or water, binding its molecules with the molecules of a special substance - a CO 2 acceptor. In the course of other reactions that occur with the consumption of solar (light) energy, water molecules are split and the released hydrogen ions and bound CO 2 are used in the synthesis of carbon-rich organic substances, including the CO 2 acceptor.
For every molecule of CO 2 that a plant absorbs to synthesize organic matter, an oxygen molecule is released, formed by the splitting of water. It is assumed that this is how all the free oxygen in the atmosphere was formed. If the process of photosynthesis on Earth suddenly stopped and the carbon cycle was disrupted, then, according to available calculations, all free oxygen would disappear from the atmosphere in about 2000 years.
Other reactions.
A green plant uses the carbon in the organic matter it produces in a variety of ways. For example, it can accumulate in starch, stored in cells, or cellulose, the main structural material of plants and a nutrient for many other organisms. Both starch and cellulose are digestible as food only after being broken down into their constituent 6-carbon sugars (i.e., sugars containing six carbon atoms per molecule). Unlike starch, an insoluble high-molecular compound, 6-carbon sugars are easily soluble and, moving throughout the plant, serve as a source of energy and material for cell growth and renewal, as well as for their restoration in case of damage. Sprouts, for example, break down the starch and fats stored in the seed, obtaining simpler organic substances from them, used in the process of cellular respiration (to release their energy) and for growth.
In animals, ingested food undergoes a similar process of digestion. Before its main components can be absorbed, they must be converted: carbohydrates into 6-carbon sugars, fats into glycerol and fatty acids, and proteins into amino acids. These digestion products serve as sources of energy for the animal, released during respiration, as well as building blocks necessary for the growth of the body and the renewal of its components. Like plants, animals are able to convert nutrients into a form suitable for storage. The animal analogue of starch is glycogen, formed from excess 6-carbon sugars and stored as an energy reserve in the liver and muscle cells. Excess sugar can also be converted into fatty acids and glycerol, which, together with the same substances coming from food, are used for the synthesis of fats accumulated in the tissue. Thus, synthesis processes ensure the storage of substances rich in carbon and associated energy, which allows the body to survive during periods of food shortage.
After their death, plants and animals become food for the so-called. decomposers - organisms that decompose organic matter. Most of the decomposers are represented by bacteria and fungi, the cells of which secrete out into their immediate environment small amounts of digestive fluid that breaks down the substrate, and then consume the products of this “digestion.” As a rule, decomposers have a limited set of enzymes and, accordingly, use only a few types of organic substances as food and a source of energy. Conventional yeast, for example, processes only the 6- and 12-carbon sugars found in the broken down cells of overripe fruits or in the thick (pulpy) juice obtained by crushing them. However, if exposed to a variety of decomposers for a sufficient duration, all carbon-containing substances of plants or animals are eventually broken down into carbon dioxide and water, and the released energy is used by the organisms that carry out the decomposition. Many artificially synthesized organic compounds are also subject to biological destruction (biodegradation) - a process during which decomposers receive energy and the necessary building material, and carbon is released into the atmosphere in the form of carbon dioxide.
Organic matter consists of 45% carbon. Therefore, the question of the source of carbon nutrition for organisms is extremely important. All organisms are divided into autotrophic and heterotrophic. Autotrophic organisms are characterized by the ability to use its mineral forms as a source of carbon, that is, to synthesize organic matter from inorganic compounds. Heterotrophic organisms build the organic matter of their body from already existing ready-made organic compounds, that is, they use organic compounds as a source of carbon. In order to carry out the synthesis of organic matter, energy is needed. Depending on the compound used, as well as on the energy sources, the following main types of carbon nutrition and the construction of organic substances are distinguished.
Types of carbon nutrition of organisms
Of all the listed types of carbon nutrition, photosynthesis of green plants, in which the construction of organic compounds occurs through simple inorganic substances (CO 2 and H 2 O) using the energy of sunlight, occupies a very special place. The general equation for photosynthesis is:
6CO 2 + 12H 2 O = C 6 H 12 O 6 + 6O 2 + 6H 2 O
Photosynthesis is the process by which energy from sunlight is converted into chemical energy. In its most general form, this can be represented as follows: a light quantum (hv) is absorbed by chlorophyll, the molecule of which goes into an excited state, and the electron moves to a higher energy level. In photoautotrophic cells, during the process of evolution, a mechanism has been developed in which the energy of an electron returning to the main energy level is converted into chemical energy.
During the process of photosynthesis, various organic substances are built from simple inorganic compounds (CO 2, H 2 O). As a result, a restructuring of chemical bonds occurs: instead of C–O and H–O bonds, C–C and C–H bonds appear, in which electrons occupy a higher energy level. Thus, energy-rich organic substances on which animals and humans feed and receive energy (through respiration) are initially created in the green leaf. We can say that almost all living matter on Earth is the result of photosynthetic activity.
Almost all oxygen in the atmosphere is of photosynthetic origin. The processes of respiration and combustion became possible only after photosynthesis arose. Aerobic organisms emerged that were able to absorb oxygen. On the surface of the Earth, processes took on a biogeochemical nature; oxidation of compounds of iron, sulfur, and manganese occurred. The composition of the atmosphere has changed: the content of CO 2 and ammonia has decreased, and oxygen and nitrogen have increased. The emergence of an ozone screen, which blocks ultraviolet radiation dangerous to living organisms, is also a consequence of the appearance of oxygen.
In order for the process of photosynthesis to proceed normally, CO 2 must enter the chloroplasts. The main supplier is the atmosphere, where the amount of CO 2 is 0.03%. To form 1 g of sugar, 1.47 g of CO 2 is required - this amount is contained in 2500 liters of air.
Carbon dioxide enters the plant leaf through the stomata. Some CO 2 enters directly through the cuticle. When the stomata are closed, the diffusion of CO 2 into the leaf is sharply reduced.
The most primitive organization of the photosynthetic apparatus is found in green bacteria and cyanobacteria. In these organisms, the function of photosynthesis is performed by intracytoplasmic membranes or special structures - chlorosomes, phycobilisomes. Algae have already evolved organelles (chromatophores) in which pigments are concentrated; they are varied in shape (spiral, ribbon-like, lamellar, star-shaped). Higher plants are characterized by a fully formed type of plastid in the shape of a disk or biconvex lens. Having taken the form of a disk, chloroplasts become a universal photosynthetic apparatus. Photosynthesis occurs in green plastids - chloroplasts. In leucoplasts, starch is synthesized and deposited as storage, while carotenoids accumulate in chromoplasts.
The size of disc-shaped chloroplasts of higher plants ranges from 4 to 10 microns. The number of chloroplasts is usually from 20 to 100 per cell. The chemical composition of chloroplasts is quite complex and can be characterized by the following average data (% on dry weight): protein - 35-55; lipids – 20-30; carbohydrates – 10; RNA – 2-3; DNA – up to 0.5; chlorophyll – 9; carotenoids – 4.5.
Chloroplasts contain enzymes that take part in the process of photosynthesis (redox enzymes, synthetases, hydrolases). Chloroplasts, like mitochondria, have their own protein synthesizing system. Many of the enzymes localized in chloroplasts are two-component. In many cases, the prosthetic group of enzymes is various vitamins. Many vitamins and their derivatives (vitamins B, K, E, D) are concentrated in chloroplasts. Chloroplasts contain 80% Fe, 70% Zn, and about 50% Cu of the total amount of these elements in the leaf.
Chloroplasts are surrounded by a double membrane. The thickness of each membrane is 7.5-10 nm, the distance between them is 10-30 nm. The internal space of chloroplasts is filled with colorless contents - stroma and is penetrated by membranes. The membranes connected to each other form flat closed cavities (vesicles) - thylakoids (Greek “thylakoides” - sac-shaped). Chloroplasts contain two types of thylakoids. Short thylakoids are collected in packs and arranged one above the other, resembling a stack of coins. These stacks are called grana, and their constituent thylakoids are called grana thylakoids. Between the grana, long thylakoids, stromal thylakoids, are located parallel to each other. There are narrow gaps between individual thylakoids in grana stacks. Thylakoid membranes contain a large number of proteins involved in photosynthesis. Integral membrane proteins contain many hydrophobic amino acids. This creates an anhydrous environment and makes the membranes more stable.
In order for light energy to be used in the process of photosynthesis, it must be absorbed by photoreceptors - pigments. Photosynthetic pigments are substances that absorb light of a specific wavelength. Unabsorbed areas of the solar spectrum are reflected, which determines the color of the pigments. Thus, the green pigment chlorophyll absorbs red and blue rays, while green rays are mainly reflected. The visible part of the solar spectrum includes wavelengths from 400 to 700 nm.
The composition of pigments depends on the systematic position of the group of organisms. Photosynthetic bacteria and algae have a diverse pigment composition (chlorophylls, bacteriochlorophylls, bacteriorhodopsin, carotenoids, phycobilins). Their set and ratio are specific to different groups of organisms. Pigments concentrated in plastids can be divided into three groups: chlorophylls, carotenoids, phycobilins.
The most important role in the process of photosynthesis is played by green pigments – chlorophylls. French scientists P.Zh. Pelletier and J. Caventou (1818) isolated a green substance from leaves and called it chlorophyll (from the Greek “chloros” - green and “phyllon” - leaf). Currently, about ten chlorophylls are known. They differ in chemical structure, color, and distribution among groups of organisms. All higher plants contain chlorophylls a and b. Chlorophyllc is found in diatoms, chlorophylld is found in red algae. In addition, bacteriochlorophylls (a, b, c, d) are known to be contained in the cells of photosynthetic bacteria. The cells of green bacteria contain bacteriochlorophylls id, and the cells of purple bacteria contain bacteriochlorophylls aib. The main pigments, without which photosynthesis does not occur, are chlorophyll for green higher plants and algae, and bacteriochlorophylls for bacteria.
For the first time, an accurate idea of the pigments of green leaves of higher plants was obtained thanks to the work of the largest Russian botanist M.S. Colors (1872-1919). He developed a new chromatographic method for separating substances and isolated the leaf pigments in their pure form. It turned out that the leaves of higher plants contain chlorophyll a and chlorophyll b, as well as carotenoids (carotene, xanthophyll). Chlorophylls, like carotenoids, are insoluble in water, but highly soluble in organic solvents. Chlorophyllsaib differ in color: chlorophylla has a blue-green tint, chlorophyllb is yellow-green. The content of chlorophylla in the leaf is approximately 3 times higher compared to chlorophyllb. According to the chemical structure, chlorophylls are esters of a dicarboxylic organic acid - chlorophyllin and two residues of alcohols - phytol (C 20 H 39 OH) and methyl (CH 3 OH). The empirical formula of chlorophyll is C 55 H 72 O 5 N 4 Mg( rice. 5.1).
Organic dicarboxylic acid chlorophyllin is a nitrogen-containing organometallic compound related to magnesium porphyrins: (COOH) 2 = C 32 H 30 ON 4 Mg.
In chlorophyll, the hydrogen of the carboxyl groups is replaced by the residues of two alcohols - methyl CH 3 OH and phytol C 20 H 39 OH, therefore chlorophyll is an ester.
Rice. 5.1. Structural formula of chlorophyll a.
Chlorophyll differs in that it contains two fewer hydrogen atoms and one more oxygen atom (instead of the CH 3 group, the CHO group). In this regard, the molecular weight of chlorophylla is 893 and chlorophyllb is 907.
At the center of the chlorophyll molecule is a magnesium atom, which is connected to four nitrogen atoms of pyrrole groups. The pyrrole groups of chlorophyll have a system of alternating double and single bonds. This is the chromophore group of chlorophyll, which determines the absorption of certain rays of the solar spectrum and its color.
Also K.A. Timiryazev drew attention to the similarity of the chemical structure of two important pigments: green - leaf chlorophyll and red - blood hemin. Indeed, if chlorophyll belongs to magnesium porphyrins, then hemin belongs to iron porphyrins. This similarity serves as another proof of the unity of the entire organic world.
The chlorophyll molecule is polar, its porphyrin core has hydrophilic properties, and its phytol end has hydrophobic properties. This property of the chlorophyll molecule determines its specific location in the membranes of chloroplasts. The porphyrin part of the molecule is associated with protein, and the phytol chain is immersed in the lipid layer.
Chlorophyll is capable of selective absorption of light. The absorption spectrum is determined by its ability to absorb light of a certain wavelength (of a certain color). In order to obtain the absorption spectrum, K.A. Timiryazev passed a beam of light through a chlorophyll solution. It has been shown that chlorophyll at the same concentration as in the leaf has two main absorption lines in red and blue-violet rays. In this case, chlorophyll a in solution has an absorption maximum at 429 and 660 nm, while chlorophyll b – at 453 and 642 nm (Fig. 5.2).
Rice. 5.2. Absorption spectra of chlorophyll a and chlorophyll b
Along with green pigments, chloroplasts and chromatophores contain pigments belonging to the group of carotenoids. Carotenoids are yellow and orange pigments of an aliphatic structure, derivatives of isoprene. Carotenoids are found in all higher plants and many microorganisms. These are the most common pigments with a variety of functions. Carotenoids containing oxygen are called xanthophylls. The main representatives of carotenoids in higher plants are two pigments - beta-carotene (orange) C 40 H 56 and xanthophyll (yellow) C 40 H 56 O 2. Carotene consists of 8 isoprene residues. When the carbon chain is broken in half and an alcohol group is formed at the end, carotene is converted into 2 molecules of vitamin A.
Beta-carotene has two absorption maxima, corresponding to wavelengths of 482 and 452 nm. Unlike chlorophylls, carotenoids do not absorb red rays and do not fluoresce. Like chlorophyll, carotenoids in chloroplasts and chromatophores are found in the form of water-insoluble complexes with proteins. Carotenoids are always present in chloroplasts; they take part in the process of photosynthesis. By absorbing light energy in certain parts of the solar spectrum, they transfer the energy of these rays to chlorophyll molecules. Thus, they contribute to the use of rays that are not absorbed by chlorophyll. The physiological role of carotenoids is not limited to their participation in the transfer of energy to chlorophyll molecules. Carotenoids perform a protective function, protecting chlorophyll molecules from destruction in light during the process of photo-oxidation ( rice. 5.3).
Rice. 5.3. Structural formula of beta carotene
Phycobilins are red and blue pigments found in cyanobacteria and red algae. The chemical structure of phycobilins is based on 4 pyrrole groups. Unlike chlorophyll, phycobilins have pyrrole groups arranged in an open chain ( rice. 5.4).
Rice. 5.4. Structural formula of the chromophore group of phycoerythrins
Phycobilins are represented by pigments: phycocyanin, phycoerythrin and allophycocyanin. Phycoerythrin is an oxidized phycocyanin. Red algae mainly contain phycoerythrin, and cyanobacteria mainly contain phycocyanin. Phycobilins form strong compounds with proteins (phycobilin proteins). Unlike chlorophylls and carotenoids, which are located in membranes, phycobilins are concentrated in special granules (phycobilisomes), closely associated with thylakoid membranes. Phycobilins absorb rays in the green and yellow parts of the solar spectrum. This is the part of the spectrum that lies between the two main absorption lines of chlorophyll. Phycoerythrin absorbs rays with a wavelength of 495-565 nm, and phycocyanin - 550-615 nm. A comparison of the absorption spectra of phycobilins with the spectral composition of light in which photosynthesis occurs in cyanobacteria and red algae shows that they are very close. This suggests that phycobilins absorb light energy and, like carotenoids, transfer it to the chlorophyll molecule, after which it is used in the process of photosynthesis. The presence of phycobilins in algae is an example of the adaptation of organisms in the process of evolution to the use of areas of the solar spectrum that penetrate through the thickness of sea water (chromatic adaptation).
Photosynthesis is a complex multi-stage redox process in which carbon dioxide is reduced to carbohydrates and water is oxidized to oxygen. During the process of photosynthesis, not only reactions occur that use light energy, but also dark reactions that do not require the direct participation of light energy. The following evidence can be given for the existence of dark reactions in the process of photosynthesis: photosynthesis accelerates with increasing temperature. It directly follows from this that some stages of this process are not directly related to the use of light energy. The process of photosynthesis includes the following stages: 1) photophysical; 2) photochemical (light); 3) enzymatic (dark).
According to the laws of photochemistry, when a quantum of light is absorbed by an atom or molecule of any substance, the electron moves to another, more distant orbital, that is, to a higher energy level (Fig. 5.5).
Rice. 5.5. Transitions between excited states of chlorophyll after absorption of blue and red light quanta
The electron that is distant from the nucleus of the atom and located at a sufficiently large distance from it has the greatest energy. Each electron moves to a higher energy level under the influence of one quantum of light if the energy of this quantum is equal to the difference between these energy levels. All photosynthetic organisms contain some type of chlorophyll. There are two levels of excitation in the chlorophyll molecule. This is precisely why it has two main absorption lines. The first level of excitation is due to the transition to a higher energy level of an electron in the system of conjugated double bonds, and the second is due to the excitation of unpaired electrons of nitrogen and oxygen atoms in the porphyrin core. When light is absorbed, electrons undergo vibrational motion and move to the next orbital with a higher energy level.
The highest energy level is the second singlet level. The electron transfers to it under the influence of blue-violet rays, the quanta of which contain more energy.
Electrons can move to the first excited state by absorbing smaller quanta of red light. The lifetime at the second level is 10 -12 s. This time is so short that during its duration the electronic excitation energy cannot be used. After this short period of time, the electron returns to the first singlet state (without changing the spin direction). The transition from the second singlet state to the first is accompanied by some loss of energy (100 kJ) in the form of heat. The lifetime in the first singlet state is slightly longer (10 -9 or 10 -8 s). The triplet state has the longest lifetime (10 -2 s). The transition to the triplet level occurs with a change in the spin of the electron.
From the excited, first singlet and triplet states, the chlorophyll molecule can also transition to the ground state. In this case, its deactivation (loss of energy) can occur:
1) by releasing energy in the form of light (fluorescence and phosphorescence) or in the form of heat;
2) by transferring energy to another pigment molecule;
3) by spending energy on photochemical processes (loss of an electron and its addition to an acceptor).
In any of these cases, the pigment molecule is deactivated and goes to the main energy level.
Chlorophyll has two functions - absorption and transmission of energy. Moreover, the main part of chlorophyll molecules - more than 90% of the total chlorophyll of chloroplasts is part of the light harvesting complex (LHC). The light-harvesting complex acts as an antenna that effectively absorbs light and transfers excitation energy to the reaction center. In addition to a large number (up to several hundred) chlorophyll molecules, SSC contains carotenoids, and in some algae and cyanobacteria, phycobilins, which increase the efficiency of light absorption.
In the process of evolution, plants have developed a mechanism that allows them to make full use of light quanta falling on a leaf like raindrops. This mechanism is that the energy of light quanta is captured by 200-400 molecules of chlorophyll and CCK carotenoids and transferred to one molecule - the reaction center. Calculations have shown that there are up to 1 billion chlorophyll molecules in one chloroplast. Shade-tolerant plants, as a rule, have a larger SSC size compared to plants growing in high light conditions. In reaction centers, as a result of photochemical reactions, primary reducing agents and oxidizing agents are formed. They then trigger a chain of successive redox reactions. As a result, energy is stored in the form of reduced nicotinamide adenine dinucleotide phosphate (NADP H+) and adenosine triphosphate (ATP), which is synthesized from adenosine diphosphate (ADP) and inorganic phosphoric acid through the reaction of photosynthetic phosphorylation. Consequently, NADP H+ and ATP are the main products of the light phase of photosynthesis. Thus, in the primary processes of photosynthesis associated with the absorption of light quantum by the chlorophyll molecule, energy transfer processes play an important role. The photophysical stage of photosynthesis consists in the fact that light quanta are absorbed and transfer pigment molecules to an excited state. This energy is then transferred to the reaction center, which carries out the primary photochemical reactions: charge separation. The further conversion of light energy into chemical energy goes through a series of stages, starting with the redox transformations of chlorophyll and including both photochemical (light) and enzymatic (dark) reactions.
That is, photosynthesis includes the transformation of energy (a phenomenon called the light process) and the transformation of matter (the dark process). The light process occurs in the thylakoids, the dark process in the stroma of chloroplasts. The two processes of photosynthesis are expressed by separate equations:
12H 2 O = 12H 2 + 6O 2 + ATP energy (light process).
From this equation it is clear that the oxygen released during photosynthesis is formed during the decomposition of water molecules. In addition, light energy is used to synthesize adenosine triphosphoric acid (ATP) during photophosphorylation.
6CO 2 + 12H 2 + ATP energy = C 6 H 12 O 6 + H 2 O (dark process)
Dark reactions use products accumulated in the light phase. The essence of dark reactions comes down to the fixation of CO 2 and its inclusion in the sugar molecule. This process was called the Calvin cycle after the American biochemist who studied in detail the sequence of dark reactions. The use of water as a source of hydrogen for the synthesis of organic molecules has given plants a great evolutionary advantage due to its ubiquitous presence (water is the most abundant mineral on Earth).
Since all the photosynthetic oxygen is released from the water, the resulting equation becomes:
6CO 2 + 12H 2 O +hv= C 6 H 12 O 6 + 6O 2 + 6H 2 O
Water on the right side of the equation cannot be reduced, since its oxygen has a different origin (from CO 2). Therefore, photosynthesis is a redox process in which water is oxidized to molecular oxygen (O2), and carbon dioxide is reduced by hydrogen in water to carbohydrates.
At the completion of each cycle, the final product is formed: one molecule of sugar, which forms the basis of the primary organic matter formed during photosynthesis.
Carbon dioxide enters plants from the air, transforming with the help of radiant energy from the sun into complex, high-energy organic compounds that feed the animal world. Animals, using the potential energy of organic substances, again release carbon dioxide. According to modern concepts, the above equation of photosynthesis can be depicted in the form of a diagram:
Consequently, photosynthesis consists of two coupled systems of reactions: the oxidation of water to oxygen and the reduction of carbon dioxide by hydrogen of water to polysaccharides.
The leaf is covered above and below with a colorless skin and a cuticle that is poorly permeable to gases. Carbon dioxide, which is absorbed during photosynthesis, enters the leaf through the stomata. On 1 cm 2 of the leaf surface, the share of stomata is only 1 mm 2, the remaining area is the impenetrable cuticle. The diffusion of CO 2 into the leaf occurs very intensively. For example, 1. cm 2 of a catalpa leaf surface absorbs 0.07 cm 3 CO 2 in 1 hour, and the same surface of an alkali solution absorbs 0.12-0.15 cm 3, or 2 times more.
The percentage of light energy absorbed by the sheet is spent on various types of work.
The structural features of the leaf are important for the process of photosynthesis. Adjacent to the upper side of the leaf is palisade tissue, the cells of which are arranged perpendicularly, tightly touch each other and are rich in chloroplasts. Palisade parenchyma is predominantly an assimilation tissue. Adjacent to the lower epidermis is spongy parenchyma with loosely arranged cells and intercellular spaces. This adaptation in plants is important for better penetration of gases into cells (Fig. 1).
In order for the process of photosynthesis to proceed continuously, the cells must be sufficiently saturated with water. Under these conditions, the stomata are open to a certain extent. In this case, transpiration and gas exchange will take place, the leaves will be sufficiently supplied with carbon dioxide, i.e. the photosynthesis process will proceed normally.
The leaf is permeated with conductive bundles that ensure the outflow of assimilation products from it, which is very important for the normal course of the photosynthesis process, since in cells overflowing with assimilation products, in particular starch, photosynthesis is inhibited and may stop completely.
Growing plants under artificial light. Conditions for the best use of electric light.
Research has shown that plant development is significantly influenced by the intensity and spectral composition of light. In this regard, the experiments of V.I. are of great interest. Razumov, who proved that red light acts as natural daylight, and blue light is perceived by the plant as darkness. If you illuminate short-day plants with red light at night, they will not bloom; Long-day plants bloom earlier under these conditions than under normal conditions. Illuminating plants at night with blue light does not interfere with the effects of darkness. Therefore, long-wavelength light is perceived as daylight, and short-wavelength light as darkness. Thus, the qualitative composition of light affects the development of the plant.
However, there is another view, namely, that all light rays, if they are intense enough, are perceived by the plant as daylight. It is believed that the spectral composition of light is almost the same throughout the day. Only its intensity changes significantly - it is the least in the morning and evening and the greatest at noon.
It has been established that the spectral composition of the light from fluorescent lamps is similar to sunlight, so these lamps are used to grow plants under artificial lighting.
Luminaires with fluorescent lamps are preferably placed in rows, preferably parallel to a wall with windows or the long side of a narrow room. But in rooms intended for plants, the optimal arrangement of lamps is such that the direction of light approaches the direction of natural light.
It must be remembered that excess light has a detrimental effect on plants, the process of photosynthesis is suspended, the plants weaken and are less able to tolerate unfavorable conditions. Beans tolerate the longest daylight hours - up to 12 hours.
31. In a test tube with a chlorophyll solution, photosynthesis does not occur, since this process requires a set of enzymes located on the +
A. Cristach of mitochondria B. Granach of chloroplasts
B. Endoplasmic reticulum D. Plasma membrane
32. What buds develop on the leaves and roots of flowering plants?+
A. Accessory B. Apical C. Axillary D. Lateral
33. The source of carbon used by plants in the process of photosynthesis is the molecule +
A. Carbonic acid B. Hydrocarbon
B. Polysaccharide G. Carbon dioxide
34. To improve the respiration of the roots of cultivated plants it is necessary +
A. Weeding
B. Systematically water the plants
B. Periodically loosen the soil around the plant
D. Periodically feed plants with mineral fertilizers
35. Adaptation of plants to reduce water evaporation - presence-G
A. Stomata on the upper side of the leaf
B. Large number of leaf blades
B. Wide leaf blades
G. Waxy coating on leaves
36. A modified underground shoot of perennial plants with a thickened stem, buds, adventitious roots and scale-like leaves is +
A. Main root B. Rhizome
B. Lateral root D. Root tuber
37. An underground shoot differs from a root in that it has +
A. Vegetative buds
B. Venue areas
B. Suction zones
G. root hairs
38. What fertilizers enhance the growth of green mass of plants?-B
A. Organic B. Nitrogen
B. Potash D. Phosphorus
39. The property of plant organs to bend under the influence of gravity is called +
A. Hydrotropism B. Phototropism
B. Geotropism D. Chemotropism
40. An external signal stimulating the onset of leaf fall in plants is +
A. Increase in environmental humidity
B. Reducing the length of daylight hours
B. Reducing environmental humidity
D. Increase in ambient temperature
41. Flooding of wheat fields with melt water in early spring sometimes leads to the death of seedlings, since this disrupts the process+
A. Photosynthesis due to lack of oxygen
B. Breathing due to lack of oxygen
B. Absorption of water from the soil
D. Water evaporation
Part B
Q1(choose several correct answers out of six)
Transpiration value+3
A. regulates the gas composition inside the sheet
B. promotes the movement of water
V. ensures the attraction of pollinators
G. improves carbohydrate transport
D. regulates leaf temperature
E. reduces the specific gravity of foliage
Q2(choose several correct answers out of six)
The root cap performs the functions +3
A. provides negative geotropism
B. provides positive geotropism
V. facilitates root penetration into the soil
G. stores nutrients
D. protects actively dividing cells
E. participates in the transport of substances
AT 3. Choose multiple correct answers
What is the significance of photosynthesis?+2
A. in providing all living things with organic substances
B. in the breakdown of biopolymers into monomers
B. in the oxidation of organic substances to carbon dioxide and water
G. in providing all living things with energy
D. enriching the atmosphere with oxygen necessary for breathing
E. in enriching the soil with nitrogen salts
AT 4. Establish a correspondence between the most important processes and phases of photosynthesis+6
AT 5. Establish the correct sequence of photosynthesis processes+5
A. stimulation of chlorophyll 1
B. glucose synthesis 5
B. connection of electrons with NADP + and H + 3
D. carbon dioxide fixation 4
D. photolysis of water 2
AT 6. Choose multiple correct answers
Select the processes occurring during the light phase of photosynthesis+3
A. photolysis of water B. synthesis of carbohydrates
B. carbon dioxide fixation D. ATP synthesis
D. release of oxygen E. hydrolysis of ATP
AT 7. Choose multiple correct answers +3
In the dark phase of photosynthesis, unlike the light phase,
A. photolysis of water
B. reduction of carbon dioxide to glucose
B. synthesis of ATP molecules using the energy of sunlight
D. hydrogen connection with the NADP + transporter
D. use of the energy of ATP molecules for the synthesis of carbohydrates
E. formation of starch molecules from glucose
AT 8. Choose several correct answers -
What processes does the energy of sunlight cause in a leaf?
A. Formation of molecular oxygen as a result of the decomposition of water
B. Oxidation of pyruvic acid to carbon dioxide and water
B. Synthesis of ATP molecules
D. Breakdown of biopolymers to monomers
D. Breakdown of glucose to pyruvic acid
E. Formation of atomic hydrogen due to the removal of an electron from a water molecule by chlorophyll
AT 9. Choose several correct answers.
What functions does a leaf perform in a plant organism?+3
A. Absorption of water and minerals
B. Synthesis of organic substances from minerals
B. Gas exchange with the environment
D. Plant growth in length and thickness
D. Formation of tissues and organs
E. Transpiration
Part C
C1. (detailed response)
Prove that the rhizome of plants is a modified shoot
A plant rhizome is a modified shoot because it has adventitious roots.
C2(short answer)
Why are there no plants or very sparse plants on forest paths?
Because people and animals moving along the paths trample the plants.
C3(short answer)
For what purpose do you pinch off the tip of the root when transplanting cabbage seedlings?
The ends of the cabbage roots are pinched to increase the growth of the cabbage adventitious roots.
C4(short answer)
Why is it necessary to loosen the soil when growing plants?
When growing plants, it is necessary to loosen the soil in order to provide the roots with good access to moisture and oxygen.
C5(detailed answer)
What role do stomata play in plant life?
With the help of stomata, gas exchange occurs between the leaf and the environment.
C6 (detailed answer)
Leaf fall is very important in the life of plants. What is it?
It consists in the adaptation of plants to climate change. Water evaporation decreases, chlorophyll is destroyed. The dropped leaves rot, forming fertilizer for the trees.
C7 (detailed answer) The annual rings are usually clearly visible on the cut of the trunk of a woody plant. What can be determined from them?
By looking at the growth rings, you can determine how many growing seasons a tree has had and how the climate has changed during its existence.
C8. (detailed response) What is the significance of the process of photosynthesis for life on Earth?
As a result of photosynthesis, oxygen is produced, which is necessary for the life of people and animals.
C9. (short answer) The process of photosynthesis occurs intensively in the leaves of plants. Does it occur in ripe and unripe fruits? Explain your answer.
C10. In the 17th century, the Dutch scientist Van Helmont conducted an experiment. He planted a small willow tree in a tub of soil, after weighing the plant and soil, and only watered it for several years. After 5 years, the scientist weighed the plant again. Its weight increased by 63.7 kg, the weight of the soil decreased by only 0.06 kg. Explain why the increase in plant mass occurred, what substances from the external environment ensured this increase.
C11.(long answer) Why does plowing the soil improve the living conditions of cultivated plants?
C12. (short answer) What processes ensure the movement of water and minerals throughout the plant? Explain your answer.
C13.(short answer) Gardeners, when picking cabbage seedlings, pinch the top of the main root, and when propagating currant bushes, they use stem cuttings on which adventitious roots develop. Both of these flowering plants belong to the class of dicotyledons. Explain what type of root system will be in cabbage grown from this seedling, and what type of root system will be in currants grown from a stem cutting.
Answers
| question | answer | question | answer | question | answer | question | answer | question | answer |
| B | G | B | B | B | |||||
| B | A | A | A | ||||||
| G | G | A | G | ||||||
| IN | G | B | IN | ||||||
| B | B | G | G | ||||||
| IN | G | A | B | ||||||
| A | IN | B | A | ||||||
| B | A | B | B | ||||||
| G | IN | G | IN | ||||||
| IN | A | G | B |
In part
| question | answer |
| IN 1 | DBA |
| AT 2 | BVD |
| AT 3 | AGD |
| AT 4 | AAAAAA |
| AT 5 | ADVGB |
| AT 6 | AGD |
| AT 7 | BDE |
| AT 8 | AVE |
| AT 9 | BEE |
C1.. 1. the rhizome has nodes in which rudimentary leaves and buds are located;
2. at the top of the rhizome there is an apical bud that determines the growth of the shoot;
3. adventitious roots extend from the rhizome;
4. the internal anatomical structure of the rhizome is similar to the stem;
C2. Constant trampling leads to soil compaction (disturbance of the water and air regime of the roots) and oppression of plants
C3. To increase the number of lateral roots, which leads to an increase in plant nutrition area
C4. To improve root respiration and reduce water evaporation from the soil.
C5. The stomata is a highly specialized formation of the plant epidermis, consisting of two guard cells and an intercellular space (stomatal fissure) between them. Transpiration and gas exchange occur through the stomata. Transpiration is the evaporation of water by a plant. Transpiration regulates the water and temperature regime of the plant
C6. 1. provides savings in water and nutrients necessary to survive the winter period
2. Protects the plant from mechanical damage in winter
3. releases metabolic end products accumulated in the leaves.
C7. 1. Approximate age of the plant
2. Growing conditions at different periods of life
3. Location of cardinal directions
C8. 1. Release of free oxygen necessary for the respiration of all living organisms
2. Formation of organic substances necessary for all living organisms
3. Conversion of solar energy into the energy of chemical bonds, available to all living organisms.
4. Creation of an ozone layer that protects against the harmful effects of UV rays
C9. 1. Photosynthesis occurs in unripe fruits (while they are green), since they contain chloroplasts
2. As they mature, chloroplasts turn into chromoplasts, in which photosynthesis does not occur
C10. 1. the mass of plants increases due to organic substances formed during photosynthesis
2. The process of photosynthesis uses water and carbon dioxide, which come from the external environment
C11. 1. Promotes the destruction of weeds and reduces competition with cultivated plants.
2. Helps supply plants with water and minerals
3. Increases oxygen supply to roots
C12. 1. From the root to the leaves, water and mineral salts dissolved in it move through the vessels due to transpiration, which results in suction force. 2. The upward flow of the plant is facilitated by root pressure, which arises as a result of the constant flow of water into the root due to the difference in the concentration of substances in the cells and the environment
C13. 1. The type of root system is initially taproot in cabbage and currants (dicots). 2. When picking cabbage, after pinching, the main root stops growing in length (since the zones of division and growth are removed) and the development of lateral and adventitious roots begins. When currant stem cuttings root, adventitious roots develop. Thus, the root system in both cases will become similar to the fibrous one (predominant development of lateral and adventitious roots)
Organisms that live off an inorganic carbon source (carbon dioxide) are called autotrophic (autotrophs)(Greek autos - himself), and organisms using an organic carbon source - heterotrophic (heterotrophic)(Greek heteros - different). Unlike heterotrophs, autotrophs satisfy all their needs for organic substances, synthesizing them from simple inorganic compounds.
In table Figure 9.1 presents both of these classifications - by energy source and by carbon source. Their relationship is clearly visible. In addition, another very important principle is revealed, namely that chemotrophic organisms are entirely dependent on phototrophic organisms, which supply them with energy, and heterotrophic organisms are completely dependent on autotrophs, which supply them with carbon compounds.
Table 9.1. Classification of living organisms according to the main source of carbon and energy *
* (Most organisms are photoautotrophs or chemoheterotrophs.)
The most important groups are photoautotrophs (which include all green plants) and chemoheterotrophs (all animals and fungi). If we ignore some bacteria for the moment, the situation becomes even simpler, and we can say that heterotrophic organisms ultimately depend on green plants to supply them with energy and carbon. Sometimes photoautotrophic organisms are called holophytic(Greek holos - whole, complete, phyton - plant).
9.1. Define what photoautotrophic nutrition and chemoheterotrophic nutrition are.
Ignoring for now the two smaller groups (see Table 9.1), we must, however, immediately note that the vital activity of chemosynthetic organisms is also very important - we will see this in section. 9.10 and 9.11.
Several organisms cannot be entirely classified into any one of the four groups. For example, Euglena usually behaves as an autotroph, but some species can live as heterotrophs in the dark if a source of organic carbon is available. The relationship between the two main categories is further illustrated in Fig. 9.1; It also shows how energy and carbon flows are included in the general cycle between living organisms and the environment. These issues have important ecological implications (Chapter 12).
Carbon is released during respiration in the form of CO 2, and CO 2 is then converted back into organic compounds through photosynthesis. The carbon cycle is presented in more detail in Fig. 9.2, which shows the role played by chemosynthetic organisms in this process.
Rice. 9.2. Carbon cycle. Bold arrows show the predominant path (of two possible ones). According to some rough estimates, the actual amount of carbon is: In the ocean: (mainly in phytoplankton): 40·10 12 kg of carbon per year is fixed in the process of photosynthesis in the form of CO 2. Most of it is then released through breathing. On land: 35·10 12 kg of carbon per year is fixed during photosynthesis in the form of CO 2; 10·10 12 kg of carbon per year is released during the respiration of plants and animals; 25·10 12 kg of carbon per year is released during the respiration of decomposers; 5·10 12 kg of carbon per year is released by burning fossil fuels; this amount is quite enough to gradually increase the concentration of carbon dioxide in the atmosphere and oceans
9.2. Look at Fig. 9.2. What types of food are presented here a) on a gray background and b) on a white background?