To date, two types of pigments have been discovered in living organisms that can perform the function of photosynthetic antennas. These pigments absorb visible light quanta and provide further storage of radiation energy in the form of the energy of the electrochemical H + gradient on biological membranes. Less common is the case in which the vitamin A derivative, retinal, serves as the antenna; In the vast majority of organisms, chlorophylls play the role of antennas. In accordance with this, non-chlorophyll and chlorophyll photosynthesis are distinguished.
Non-chlorophyll photosynthesis
The system of chlorophyll-free photosynthesis is characterized by significant simplicity of organization, and therefore is assumed to be the evolutionarily primary mechanism for storing the energy of electromagnetic radiation. The efficiency of chlorophyll-free photosynthesis as an energy conversion mechanism is relatively low (only one H + is transferred per absorbed quantum).
Discovery in halophilic archaea
Dieter Oesterhelt and Walther Stoeckenius identified a representative of halophilic archaea in the “purple membranes” Halobacterium salinarium(former name N. halobium) a protein that was later named bacteriorhodopsin. Evidence was soon accumulated indicating that bacteriorhodopsin is a light-dependent generator of a proton gradient. In particular, photophosphorylation was demonstrated on artificial vesicles containing bacteriorhodopsin and mitochondrial ATP synthase, photophosphorylation in intact cells H. salinarium, a light-induced drop in pH of the environment and suppression of respiration, all of these effects correlated with the absorption spectrum of bacteriorhodopsin. Thus, irrefutable evidence of the existence of chlorophyll-free photosynthesis was obtained.
Mechanism
The photosynthetic apparatus of extreme halobacteria is the most primitive currently known; it lacks an electron transport chain. Cytoplasmic membrane halobacteria is a coupling membrane containing two main components: a light-dependent proton pump (bacteriorhodopsin) and ATP synthase. The operation of such a photosynthetic apparatus is based on the following energy transformations:
- The chromophore of bacteriorhodopsin, retinal, absorbs light quanta, which leads to conformational changes in the structure of bacteriorhodopsin and proton transport from the cytoplasm to the periplasmic space. In addition, an additional contribution to the electrical component of the gradient is made by the active light-dependent import of the chloride anion, which is provided by halorhodopsin [ ] . Thus, as a result of the work of bacteriorhodopsin, the energy solar radiation is transformed into the energy of the electrochemical gradient of protons on the membrane.
- During the operation of ATP synthase, the energy of the transmembrane gradient is transformed into the energy of ATP chemical bonds. Thus, chemiosmotic coupling occurs.
With the chlorophyll-free type of photosynthesis (as well as with the implementation of cyclic flows in electron transport chains), the formation of reducing equivalents (reduced ferredoxin or NAD(P)H) necessary for assimilation does not occur carbon dioxide. Therefore, during chlorophyll-free photosynthesis, there is no assimilation of carbon dioxide, but only the storage of solar energy in the form of ATP (photophosphorylation).
Meaning
The main way for halobacteria to obtain energy is the aerobic oxidation of organic compounds (carbohydrates and amino acids are used during cultivation). In case of oxygen deficiency, in addition to non-chlorophyll photosynthesis, anaerobic nitrate respiration or fermentation of arginine and citrulline can serve as energy sources for halobacteria. However, the experiment showed that chlorophyll-free photosynthesis can also serve as the only source of energy under anaerobic conditions when anaerobic respiration and fermentation are suppressed at mandatory condition that retinal is introduced into the medium, the synthesis of which requires oxygen.
Chlorophyll photosynthesis
Chlorophyll photosynthesis differs from bacteriorhodopsin photosynthesis by its significantly greater efficiency of energy storage. For each absorbed quantum of radiation, at least one H + is transferred against the gradient, and in some cases the energy is stored in the form of reduced compounds (ferredoxin, NADP).
Anoxygenic
Anoxygenic (or oxygen-free) photosynthesis occurs without the release of oxygen. Purple and green bacteria, as well as heliobacteria, are capable of anoxygenic photosynthesis.
With anoxygenic photosynthesis, it is possible to:
- Light-dependent cyclic electron transport, not accompanied by the formation of reducing equivalents and leading exclusively to the storage of light energy in the form of ATP. With cyclic light-dependent electron transport, there is no need for exogenous electron donors. The need for reducing equivalents is met non-photochemically, usually through exogenous organic compounds.
- Light-dependent non-cyclic electron transport, accompanied by the formation of reducing equivalents and the synthesis of ADP. In this case, there is a need for exogenous electron donors, which are necessary to fill the electron vacancy in the reaction center. Both organic and inorganic reducing agents can be used as exogenous electron donors. Among inorganic compounds, the most commonly used are various reduced forms of sulfur (hydrogen sulfide, molecular sulfur, sulfites, thiosulfates, tetrathionates, thioglycolates), and molecular hydrogen can also be used.
Oxygenic
Oxygenic (or oxygenic) photosynthesis is accompanied by the release of oxygen as a by-product. In oxygenic photosynthesis, noncyclic electron transport occurs, although under certain physiological conditions, exclusively cyclic electron transport occurs. An extremely weak electron donor - water - is used as an electron donor in a non-cyclic flow.
Oxygenic photosynthesis is much more widespread. Characteristic of higher plants, algae, many protists and cyanobacteria.
Stages
Photosynthesis is a process with an extremely complex spatiotemporal organization.
Scatter of characteristic times various stages photosynthesis is 19 orders of magnitude: the rate of absorption of light quanta and energy migration is measured in the femtosecond interval (10−15 s), the rate of electron transport has characteristic times of 10−10−10−2 s, and processes associated with plant growth are measured in days ( 10 5 −10 7 s).
Also, a large variation in size is characteristic of structures that ensure photosynthesis occurs: from molecular level(10 −27 m 3) to the level of phytocenoses (10 5 m 3).
In photosynthesis we can distinguish individual stages, differing in nature and characteristic rates of processes:
- Photophysical;
- Photochemical;
- Chemical:
- Electron transport reactions;
- "Dark" reactions or carbon cycles during photosynthesis.
At the first stage, light quanta are absorbed by pigments, their transition to an excited state and energy transfer to other molecules of the photosystem. At the second stage, charges are separated in the reaction center, electrons are transferred along the photosynthetic electron transport chain, which ends in the synthesis of ATP and NADPH. The first two stages are collectively called the light-dependent stage of photosynthesis. The third stage occurs without the mandatory participation of light and includes biochemical reactions of the synthesis of organic substances using the energy accumulated in the light-dependent stage. Most often, such reactions are considered to be the Calvin cycle and gluconeogenesis, the formation of sugars and starch from carbon dioxide in the air.
Spatial localization
Sheet
Plant photosynthesis occurs in chloroplasts: semi-autonomous double-membrane organelles belonging to the class of plastids. Chloroplasts can be contained in the cells of stems, fruits, and sepals, but the main organ of photosynthesis is the leaf. The leaf has formed during evolution and is anatomically adapted to absorb light energy and assimilate carbon dioxide. Flat shape The sheet provides a large surface-to-volume ratio, allowing for more complete use of the energy of sunlight. The water necessary to maintain turgor and photosynthesis is delivered to the leaves from the root system through the xylem of a developed network of conducting bundles (leaf veins) and the stem. Loss of water through evaporation through the stomata and, to a lesser extent, through the cuticle (transpiration) serves as the driving force for vascular transport. However, excess transpiration is undesirable and, during the course of evolution, plants have developed various adaptations aimed at reducing water loss. The outflow of assimilates, necessary for the functioning of the Calvin cycle, occurs through the phloem of vascular bundles (veins) and the phloem of the stem. During intense photosynthesis, carbohydrates can polymerize and at the same time starch grains are formed in chloroplasts. Gas exchange (intake of carbon dioxide and release of oxygen) is carried out by diffusion through the stomata, some of the gases move through the cuticle.
Since carbon dioxide deficiency significantly increases the loss of assimilates during photorespiration, it is necessary to maintain a high concentration of carbon dioxide in the intercellular space, which is possible with open stomata. However, maintaining stomata open when high temperature leads to an increase in transpiration water losses - water losses by evaporation, which leads to water deficiency and also reduces the productivity of photosynthesis. This conflict is resolved in accordance with the principle of adaptive compromise. In addition, the primary absorption of carbon dioxide at night, at low temperatures, in plants with CAM photosynthesis allows one to avoid high transpiration losses of water.
Photosynthesis at the tissue level
At the tissue level, photosynthesis in higher plants is provided by specialized tissue - chlorenchis And my . Chlorenchyma is located near the surface of the plant body, where it receives a sufficient amount of light energy. Usually chlorenchyma is located directly under the epidermis. In plants growing in conditions of increased insolation, one or two layers of transparent cells (hypodermis) may be located between the epidermis and chlorenchyma, providing light scattering. Some shade-loving plants The epidermis is also rich in chloroplasts (for example, wood sorrel). Often the mesophyll chlorenchyma of a leaf is differentiated into palisade (columnar) and spongy, but can also consist of homogeneous cells. Subject to differentiation, palisade chlorenchyma is richest in chloroplasts.
Chloroplasts
The internal space of the chloroplast is filled with colorless contents (stroma) and permeated by membranes (lamellae), which, connecting with each other, form thylakoids, which, in turn, are grouped into stacks called grana. The intrathylakoid space is separated and does not communicate with the rest of the stroma, it is also assumed that inner space all thylakoids communicate with each other. The light stages of photosynthesis are confined to membranes; autotrophic fixation of CO 2 occurs in the stroma.
Chloroplasts have their own DNA, RNA, ribosomes (70s type), and protein synthesis occurs (although this process is controlled from the nucleus). They are not synthesized again, but are formed by dividing the previous ones. All this made it possible to consider them the descendants of free cyanobacteria that became part of the eukaryotic cell during the process of symbiogenesis.
Photosynthetic membranes of prokaryotes
Photochemical essence of the process
Photosystem I
Light-harvesting complex I contains approximately 200 chlorophyll molecules.
In the reaction center of the first photosystem there is a dimer of chlorophyll a with an absorption maximum at 700 nm (P 700). After excitation by a light quantum, it restores the primary acceptor - chlorophyll a, which is the secondary acceptor (vitamin K 1 or phylloquinone), after which the electron is transferred to ferredoxin, which restores NADP using the enzyme ferredoxin-NADP reductase.
The plastocyanin protein, reduced in the b 6 f complex, is transported to the reaction center of the first photosystem from the side of the intrathylakoid space and transfers an electron to the oxidized P 700.
Cyclic and pseudocyclic electron transport
In addition to the complete non-cyclic electron path described above, a cyclic and pseudo-cyclic path has been discovered.
The essence of the cyclic pathway is that ferredoxin, instead of NADP, reduces plastoquinone, which transfers it back to the b 6 f complex. This results in a larger proton gradient and more ATP, but no NADPH.
In the pseudocyclic pathway, ferredoxin reduces oxygen, which is further converted into water and can be used in photosystem II. In this case, NADPH is also not formed.
Dark phase
In the dark stage, with the participation of ATP and NADP, CO 2 is reduced to glucose (C 6 H 12 O 6). Although light is not required for this process, it is involved in its regulation.
C 3 photosynthesis, Calvin cycle
In the second stage, FHA is restored in two stages. First, it is phosphorylated by ATP under the action of phosphoroglycerokinase with the formation of 1,3-diphosphoglyceric acid (DPGA), then, under the influence of triosephosphate dehydrogenase and NADPH, the acyl-phosphate group of DPGA is dephosphorylated and reduced to an aldehyde and glyceraldehyde-3-phosphate - phosphorylated carbohydrate (PHA) is formed.
The third stage involves 5 PHA molecules, which, through the formation of 4-, 5-, 6- and 7-carbon compounds, are combined into 3 5-carbon ribulose-1,5-biphosphate, which requires 3ATP.
Finally, two PHAs are required for glucose synthesis. To form one of its molecules, 6 cycle revolutions, 6 CO 2, 12 NADPH and 18 ATP are required.
C 4 photosynthesis
The difference between this mechanism of photosynthesis and the usual one is that the fixation of carbon dioxide and its use are divided in space, between different cells of the plant.
At a low concentration of CO 2 dissolved in the stroma, ribulose biphosphate carboxylase catalyzes the oxidation reaction of ribulose-1,5-biphosphate and its breakdown into 3-phosphoglyceric acid and phosphoglycolic acid, which is forced to be used in the process of photorespiration.
To increase CO2 concentration, type 4 C plants changed their leaf anatomy. The Calvin cycle is localized in the sheath cells of the vascular bundle; in the mesophyll cells, under the action of PEP carboxylase, phosphoenolpyruvate is carboxylated to form oxaloacetic acid, which is converted into malate or aspartate and transported to the sheath cells, where it is decarboxylated to form pyruvate, which is returned to the mesophyll cells.
With 4, photosynthesis is practically not accompanied by losses of ribulose-1,5-biphosphate from the Calvin cycle, and therefore is more efficient. However, it requires not 18, but 30 ATP for the synthesis of 1 glucose molecule. This is justified in the tropics, where the hot climate requires keeping the stomata closed, which prevents the entry of CO 2 into the leaf, as well as with a ruderal life strategy.
Photosynthesis via the C4 pathway is carried out by about 7,600 species of plants, all of which are flowering plants: many Cereals (61% of species, including cultivated crops - corn, sugar cane and sorghum, etc.), Cloves (the largest share in the Chenopoaceae families - 40% of species, Amaranthaceae - 25%), some Sedgeaceae, Asteraceae, Brassicas, Euphorbiaceae.
photosynthesis itself
The energy obtained by humanity by burning fossil fuels (coal, oil, natural gas, peat) is also stored in the process of photosynthesis.
Photosynthesis serves as the main input of inorganic carbon into the biogeochemical cycle.
Photosynthesis is the basis for the productivity of agriculturally important plants.
Most of the free oxygen in the atmosphere is of biogenic origin and is a by-product of photosynthesis. The formation of an oxidizing atmosphere (oxygen catastrophe) completely changed the state earth's surface, made possible the emergence of respiration, and later, after the formation of the ozone layer, allowed life to exist on land.
History of the study
The first experiments in the study of photosynthesis were carried out by Joseph Priestley in the 1780s, when he drew attention to the “spoilage” of air in a sealed vessel with a burning candle (the air ceased to support combustion, and the animals placed in it suffocated) and its “correction” by plants. Priestley concluded that plants produce oxygen, which is necessary for respiration and combustion, but did not notice that plants need light for this. This was soon shown by Jan Ingenhaus.
Later it was found that in addition to releasing oxygen, plants absorb carbon dioxide and, with the participation of water, synthesize organic matter in the light. Based on the law of conservation of energy, Robert Mayer postulated that plants convert the energy of sunlight into the energy of chemical bonds. V. Pfeffer called this process photosynthesis.
Chlorophylls were first isolated in P. J. Peltier and J. Caventou. M. S. Tsvet managed to separate the pigments and study them separately using the chromatography method he created. The absorption spectra of chlorophyll were studied by K. A. Timiryazev, who, developing Mayer’s principles, showed that it is the absorbed rays that make it possible to increase the energy of the system, creating instead of weak ones C-O connections and O-H high-energy C-C (before this, it was believed that photosynthesis uses yellow rays that are not absorbed by leaf pigments). This was done thanks to the method he created for accounting for photosynthesis based on absorbed CO 2: during experiments on illuminating a plant with light different lengths waves ( different color) it turned out that the intensity of photosynthesis coincides with the absorption spectrum of chlorophyll.
The redox essence of photosynthesis (both oxygenic and anoxygenic) was postulated by Cornelis van Niel, who in 1931 proved that purple bacteria and green sulfur bacteria carry out anoxygenic photosynthesis. The redox nature of photosynthesis meant that oxygen in oxygenic photosynthesis is formed entirely from water, which was experimentally confirmed in A.P. Vinogradov in experiments with an isotope label. Robert Hill found that the process of water oxidation (and oxygen release) and CO 2 assimilation can be separated. In - gg.
As the name implies, photosynthesis is essentially the natural synthesis of organic substances, converting CO2 from the atmosphere and water into glucose and free oxygen.
This requires the presence of solar energy.
The chemical equation for the process of photosynthesis can generally be represented as follows:
Photosynthesis has two phases: dark and light. The chemical reactions of the dark phase of photosynthesis differ significantly from the reactions of the light phase, but the dark and light phases of photosynthesis depend on each other.
The light phase can occur in plant leaves exclusively in sunlight. For dark, the presence of carbon dioxide is necessary, which is why the plant must constantly absorb it from the atmosphere. All comparative characteristics The dark and light phases of photosynthesis will be provided below. For this purpose it was created comparison table"Phases of photosynthesis."
Light phase of photosynthesis
The main processes in the light phase of photosynthesis occur in the thylakoid membranes. It involves chlorophyll, electron transport proteins, ATP synthetase (an enzyme that accelerates the reaction) and sunlight.
Further, the reaction mechanism can be described as follows: when sunlight hits the green leaves of plants, chlorophyll electrons (negative charge) are excited in their structure, which, having passed into an active state, leave the pigment molecule and end up on outside thylakoid, the membrane of which is also negatively charged. At the same time, chlorophyll molecules are oxidized and the already oxidized ones are reduced, thus taking electrons from the water that is in the leaf structure.
This process leads to the fact that water molecules disintegrate, and the ions created as a result of photolysis of water give up their electrons and turn into OH radicals that are capable of carrying out further reactions. These reactive OH radicals then combine to create full-fledged water molecules and oxygen. In this case, free oxygen escapes into the external environment.
As a result of all these reactions and transformations, the leaf thylakoid membrane on one side is charged positively (due to the H+ ion), and on the other - negatively (due to electrons). When the difference between these charges on the two sides of the membrane reaches more than 200 mV, protons pass through special channels of the ATP synthetase enzyme and due to this, ADP is converted to ATP (as a result of the phosphorylation process). And atomic hydrogen, which is released from water, restores the specific carrier NADP+ to NADP·H2. As we can see, as a result of the light phase of photosynthesis, three main processes occur:
- ATP synthesis;
- creation of NADP H2;
- formation of free oxygen.
The latter is released into the atmosphere, and NADP H2 and ATP take part in the dark phase of photosynthesis.
Dark phase of photosynthesis
The dark and light phases of photosynthesis are characterized by at great expense energy from the plant, but the dark phase proceeds faster and requires less energy. Dark phase reactions do not require sunlight, so they can occur both day and night.
All the main processes of this phase occur in the stroma of the plant chloroplast and represent a unique chain of successive transformations of carbon dioxide from the atmosphere. The first reaction in such a chain is the fixation of carbon dioxide. To make it happen more smoothly and faster, nature provided the enzyme RiBP-carboxylase, which catalyzes the fixation of CO2.
Next, a whole cycle of reactions occurs, the completion of which is the conversion of phosphoglyceric acid into glucose (natural sugar). All these reactions use the energy of ATP and NADP H2, which were created in the light phase of photosynthesis. In addition to glucose, photosynthesis also produces other substances. Among them are various amino acids, fatty acids, glycerol, and nucleotides.
Phases of photosynthesis: comparison table
| Comparison criteria | Light phase | Dark phase |
| sunlight | Required | Not required |
| Place of reaction | Chloroplast grana | Chloroplast stroma |
| Dependence on energy source | Depends on sunlight | Depends on ATP and NADP H2 formed in the light phase and on the amount of CO2 from the atmosphere |
| Starting materials | Chlorophyll, electron transport proteins, ATP synthetase | Carbon dioxide |
| The essence of the phase and what is formed | Free O2 is released, ATP and NADP H2 are formed | Formation of natural sugar (glucose) and absorption of CO2 from the atmosphere |
Photosynthesis - video
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Tasks: To generate knowledge about the reactions of plastic and energy metabolism and their relationship; recall the structural features of chloroplasts. Describe the light and dark phases of photosynthesis. Show the importance of photosynthesis as a process that ensures the synthesis of organic substances, the absorption of carbon dioxide and the release of oxygen into the atmosphere.
Lesson type: lecture.
Equipment:
- Visual aids: tables by general biology;
- TCO: computer; multimedia projector.
Lecture outline:
- History of the study of the process.
- Experiments on photosynthesis.
- Photosynthesis as an anabolic process.
- Chlorophyll and its properties.
- Photosystems.
- Light phase of photosynthesis.
- Dark phase of photosynthesis.
- Limiting factors of photosynthesis.
Progress of the lecture
History of the study of photosynthesis
1630 year the study of photosynthesis began . Van Helmont proved that plants form organic substances and do not obtain them from the soil. Weighing a pot with soil and willow, and separately the tree itself, he showed that after 5 years the mass of the tree increased by 74 kg, while the soil lost only 57 g. He decided that the tree gets its food from water. Currently we know that carbon dioxide is used.
IN 1804 Saussure found that water is important in the process of photosynthesis.
IN 1887 Chemosynthetic bacteria were discovered.
IN 1905 Blackman established that photosynthesis consists of two phases: fast - light and a series of successive slow reactions of the dark phase.
Photosynthesis experiments
| 1 experiment proves the importance of sunlight (Fig. 1.) | Experiment 2 proves the importance of carbon dioxide for photosynthesis (Fig. 2.) |
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Experience 3 proves the importance of photosynthesis (Fig. 3.) |
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Photosynthesis as an anabolic process
- Every year, as a result of photosynthesis, 150 billion tons of organic matter and 200 billion tons of free oxygen are formed.
- The cycle of oxygen, carbon and other elements involved in photosynthesis. Maintains the modern atmospheric composition necessary for existence modern forms life.
- Photosynthesis prevents the increase in carbon dioxide concentrations, preventing the Earth from overheating due to the greenhouse effect.
- Photosynthesis is the basis of all food chains on Earth.
- The energy stored in products is the main source of energy for humanity.
The essence of photosynthesis consists of converting the light energy of a solar ray into chemical energy in the form of ATP and NADPH 2 .
The overall equation for photosynthesis is:
6CO 2 + 6H 2 O→C 6 H 12 O 6 + 6 O 2
There are two main types of photosynthesis:
Chlorophyll and its properties
Types of chlorophyll
Chlorophyll has modifications a, b, c, d. They differ in their structural structure and light absorption spectrum. For example: chlorophyll b contains one more oxygen atom and two less hydrogen atoms than chlorophyll a.
All plants and oxyphotobacteria have yellow-green chlorophyll a as their main pigment and chlorophyll b as an additional pigment.
Other plant pigments
Some other pigments are capable of absorbing solar energy and transfer it to chlorophyll, thereby involving it in photosynthesis.
Most plants have a dark orange pigment - carotene, which in the animal body is converted into vitamin A and yellow pigment - xanthophyll.
Phycocyanin And phycoerythrin– contain red and blue-green algae. In red algae, these pigments take a more active part in the process of photosynthesis than chlorophyll.
Chlorophyll absorbs light minimally in the blue-green part of the spectrum. Chlorophyll a, b - in the violet region of the spectrum, where the wavelength is 440 nm. Unique function of chlorophyll is that it intensively absorbs solar energy and transfers it to other molecules.
Pigments absorb a certain wavelength; unabsorbed parts of the solar spectrum are reflected, which provides the color of the pigment. Green light is not absorbed, so chlorophyll is green.
Pigments- This chemical compounds, which absorb visible light, which leads electrons to an excited state. The shorter the wavelength, the greater the energy of the light and the greater its ability to convert electrons into an excited state. This state is unstable and soon the entire molecule returns to its normal low-energy state, losing its excitation energy. This energy can be used for fluorescence.
Photosystems
Plant pigments involved in photosynthesis are “packed” into the thylakoids of chloroplasts in the form of functional photosynthetic units - photosynthetic systems: photosystem I and photosystem II.
Each system consists of a set of auxiliary pigments (from 250 to 400 molecules) that transfer energy to one molecule of the main pigment and is called reaction center. It uses solar energy for photochemical reactions.
The light phase necessarily occurs with the participation of light, the dark phase in both light and darkness. The light process occurs in the thylakoids of chloroplasts, the dark process occurs in the stroma, i.e. these processes are spatially separated.
Light phase of photosynthesis
IN 1958 Arnon and his staff studied light phase photosynthesis. They established that the source of energy during photosynthesis is light, and since in light, chlorophyll undergoes synthesis from ADP + Ph.c. → ATP, this process is called phosphorylation. It is associated with the transfer of electrons in membranes.
The role of light reactions: 1. ATP synthesis - phosphorylation. 2. Synthesis of NADP.H 2.
The electron transfer path is called Z-scheme.
Z-scheme. Non-cyclic and cyclic photophosphorylation(Fig. 6.)
During the cyclic transport of electrons, there is no formation of NADP.H 2 and photodecomposition of H 2 O, hence the release of O 2. This pathway is used when there is an excess of NADP.H 2 in the cell, but additional ATP is required.
All these processes belong to the light phase of photosynthesis. Subsequently, the energy of ATP and NADP.H 2 is used for the synthesis of glucose. This process does not require light. These are reactions of the dark phase of photosynthesis.
Dark phase of photosynthesis or Calvin cycle
Glucose synthesis occurs through a cyclic process, which is named after the scientist Melvin Calvin, who discovered it and was awarded the Nobel Prize.
Rice. 8. Calvin cycle
Each reaction in the Calvin cycle is carried out by its own enzyme. For the formation of glucose, the following are used: CO 2, protons and electrons from NADP.H 2, energy from ATP and NADP.H 2. The process occurs in the stroma of the chloroplast. The initial and final connection of the Calvin cycle, to which with the help of an enzyme ribulose diphosphate carboxylase CO2 is added, is a five-carbon sugar - ribulose biphosphate, containing two phosphate groups. The result is a six-carbon compound that immediately breaks down into two three-carbon molecules phosphoglyceric acid, which are then restored to phosphoglyceraldehyde. At the same time, part of the resulting phosphoglyceraldehyde is used to regenerate ribulose biphosphate, and thus the cycle resumes again (5C 3 → 3C 5), and part is used for the synthesis of glucose and other organic compounds (2C 3 → C 6 → C 6 H 12 O 6).
To form one molecule of glucose, 6 revolutions of the cycle are required and 12 NADPH.H 2 and 18 ATP are required. From the overall reaction equation we get:
6CO 2 + 6H 2 O → C 6 H 12 O 6 + 6O 2
From the above equation it is clear that C and O atoms entered glucose from CO 2, and hydrogen atoms from H 2 O. Glucose can later be used both for the synthesis of complex carbohydrates (cellulose, starch) and for the formation of proteins and lipids.
(C 4 - photosynthesis. In 1965, it was proven that in sugar cane, the first products of photosynthesis are acids containing four carbon atoms (malic, oxaloacetic, aspartic). C 4 plants include corn, sorghum, millet).
Limiting factors of photosynthesis
The rate of photosynthesis is the most important factor affecting the yield of agricultural crops. So, for the dark phases of photosynthesis, NADP.H 2 and ATP are needed, and therefore the rate of dark reactions depends on light reactions. In low light conditions, the rate of formation of organic substances will be low. Therefore, light is a limiting factor.
Of all the factors simultaneously influencing the process of photosynthesis limiting will be the one that is closer to the minimum level. This is installed Blackman in 1905. Various factors may be limiting, but one of them is the main one.
The cosmic role of plants(described K. A. Timiryazev) lies in the fact that plants are the only organisms that absorb solar energy and accumulate it in the form of potential chemical energy of organic compounds. The released O2 supports the vital activity of all aerobic organisms. Ozone is formed from oxygen, which protects all living things from ultraviolet rays. Plants used a huge amount of CO 2 from the atmosphere, the excess of which created “ Greenhouse effect", and the temperature of the planet dropped to its current values.
Implemented the process of photosynthesis in plant leaves. Photosynthesis is characteristic only of green plants.
This most important aspect of leaf activity is most fully characterized by K. A. Timiryazev:
It can be said that the life of the leaf expresses the very essence of plant life. All organic substances, no matter how diverse they are, wherever they are found - whether in a plant, an animal or a person - passed through the leaf, originating from substances produced by the leaf.
The structure of plant leaves
Plant leaves They are characterized by great diversity in their anatomical structure, which depends on both the type of plant and the conditions of their growth. The leaf is covered above and below with epidermis - a covering tissue with numerous openings called stomata. Under the upper epidermis there is a palisade, or columnar parenchyma, called assimilation.
Beneath it there is looser tissue - spongy parenchyma, followed by the lower epidermis. The entire leaf is penetrated by a network of veins consisting of conductive bundles through which water, minerals and organic substances pass.
The columnar and spongy tissue of the leaf contains green plastids - chloroplasts containing pigments. The presence of chloroplasts and the green pigments they contain (chlorophylls) explains the color of plants.
Huge leaf surface, reaching 30,000 - 50,000 sq. m per 1 ha different plants, is well adapted for the successful absorption of CO 2 from the air during photosynthesis.
Carbon dioxide penetrates into the plant leaf through stomata located in the epidermis, enters the intercellular spaces and, penetrating through the cell membrane, enters the cytoplasm, and then into the chloroplasts, where the assimilation process takes place.
The oxygen formed in this process diffuses from the surface of chloroplasts in a free state.
Thus, through the stomata, gas exchange between the leaves and the external environment occurs - the intake of carbon dioxide and the release of oxygen during photosynthesis, the release of carbon dioxide and the absorption of oxygen during respiration. In addition, stomata serve to release water vapor.
Despite the fact that the total area of the stomatal openings is only 1-2% of the entire leaf surface, nevertheless, when the stomata are open, carbon dioxide penetrates into the leaves at a rate 50 times higher than its absorption by alkali. The number of stomata is very large - from several dozen to 1500 per 1 square. mm.
Chloroplasts
Chloroplasts- green plastids in which the process of photosynthesis occurs. They are located in the cytoplasm. In higher plants, chloroplasts are disc-shaped or lens-shaped; in lower plants they are more diverse.
The size of chloroplasts in higher plants is quite constant, averaging 1-10 microns. Usually the cage contains a large number of chloroplasts, on average 20-50, and sometimes more. They are located mainly in the leaves, and there are many of them in unripe fruits. In a plant, the total number of chloroplasts is huge; in an adult oak tree, for example, their area is 2 hectares.
Chloroplast has a membrane structure. It is separated from the cytoplasm by a double membrane membrane. The chloroplast contains lamellae, protein-lipoid plates, collected in bundles and called grana.
Chlorophyll is located in the lamellae in the form of a monomolecular layer. Between the lamellae there is a watery protein fluid - the stroma; it contains starch grains and drops of oil.
The structure of the chloroplast is well adapted to photosynthesis, since the division of the chlorophyll-bearing apparatus into small plates significantly increases the active surface of the chloroplast, which facilitates the access of energy and its transfer to chemical systems involved in photosynthesis.
Data from A. A. Tabentsky show that chloroplasts change all the time during plant ontogenesis. In young leaves, a fine-granular structure of chloroplasts is observed, while in leaves that have completed growth, a coarse-grained structure is observed.
In old leaves, the breakdown of chloroplasts is already observed. The dry matter of chloroplasts contains 20-45% proteins, 20-40% lipoids, 10-12% carbohydrates and other reserve substances, 10% mineral elements, 5-10% green pigments (chlorophyll A and chlorophyll b), 1-2% carotenoids, as well as small amounts of RNA and DNA. The water content reaches 75%.
Chloroplasts contain a large set of hydrolytic and redox enzymes. Research by N. M. Sissakyan has shown that the synthesis of many enzymes also occurs in chloroplasts. Thanks to this, they take part in the entire complex complex of plant life processes.
Pigments, their properties and conditions of formation
Pigments can be extracted from plant leaves with alcohol or acetone. The extract contains the following pigments: green - chlorophyll A and chlorophyll b; yellow - carotene and xanthophyll (carotenoids).
Chlorophyll
Chlorophyll represents
one of the most interesting substances on the earth's surface
(C. Darwin),
since thanks to it, the synthesis of organic substances from inorganic CO 2 and H 2 O is possible.
Chlorophyll is insoluble in water and easily changes under the influence of salts, acids and alkalis, so it was very difficult to establish it chemical composition. Commonly used to extract chlorophyll ethanol or acetone.
Chlorophyll has the following summary formulas: chlorophyll A- C 55 H 72 O 5 N 4 Mg, chlorophyll b- C 55 H 70 O 6 N 4 Mg.
In chlorophyll A 2 more hydrogen atoms and 1 less oxygen atom than chlorophyll b. The formulas for chlorophyll can be represented as follows:
Chlorophyll formulas A And b
The central place in the chlorophyll molecule is occupied by Mg; it can be displaced by acting on the alcohol extract of chlorophyll hydrochloric acid. The green pigment turns into a brown one, called pheophytin, in which Mg is replaced by two H atoms from hydrochloric acid.
Restore green color extracts are very easy by introducing magnesium or another metal into the pheophytin molecule. Therefore, the green color of chlorophyll is associated with the presence of metal in its composition.
When an alcoholic extract of chlorophyll is exposed to alkali, alcohol groups (phytol and methyl alcohol) are eliminated; in this case, the green color of chlorophyll is retained, indicating that the core of the chlorophyll molecule is preserved during this reaction.
The chemical composition of chlorophyll is the same in all plants. The content of chlorophyll a is always greater (about 3 times) than chlorophyll b. The total amount of chlorophyll is small and amounts to about 1% of the dry matter of the leaf.
In its chemical nature, chlorophyll is close to the coloring substance in the blood - hemoglobin, the central place in the molecule of which is occupied not by magnesium, but by iron. In accordance with this, their physiological functions also differ: chlorophyll takes part in the most important regenerative process in a plant - photosynthesis, and hemoglobin - in the process of respiration of animal organisms, carrying oxygen.
Optical properties of pigments
Chlorophyll absorbs solar energy and directs it to chemical reactions that cannot occur without energy received from the outside. A chlorophyll solution in transmitted light is green, but with increasing layer thickness or chlorophyll concentration it becomes red.
Chlorophyll absorbs light not completely, but selectively. When white light is passed through a prism, it produces a spectrum consisting of seven visible colors, which gradually transform into each other.
When passing white light through a prism and a chlorophyll solution, the most intense absorption in the resulting spectrum will be in red and blue-violet rays. Green rays are little absorbed, so thin layer chlorophyll has a green color in transmitted light.
However, with increasing chlorophyll concentration, the absorption bands expand (a significant part of the green rays is also absorbed) and only part of the extreme red rays passes through without absorption. Absorption spectra of chlorophyll A And b very close.
In reflected light, chlorophyll appears cherry red because it emits absorbed light with a change in its wavelength. This property of chlorophyll is called fluorescence.
Carotene and xanthophyll
Carotene and xanthophyll have absorption bands only in blue and violet rays. Their spectra are close to each other.
Absorption spectra of chlorophyll A And b
The energy absorbed by these pigments is transferred to chlorophyll A, which is a direct participant in photosynthesis. Carotene is considered provitamin A, since its breakdown produces 2 molecules of vitamin A. The formula of carotene is C 40 H 56, xanthophyll is C 40 H 54 (OH) 2.
Conditions for the formation of chlorophyll
Chlorophyll formation carried out in 2 phases: the first phase is dark, during which the precursor of chlorophyll, protochlorophyll, is formed, and the second is light, during which chlorophyll is formed from protochlorophyll in the light.
The formation of chlorophyll depends both on the type of plant and on a number of external conditions. Some plants, such as coniferous seedlings, can turn green even without light, in the dark, but in most plants, chlorophyll is formed from protochlorophyll only in the light.
In the absence of light, etiolated plants are obtained that have a thin, weak, highly elongated stem and very small pale yellow leaves. If etiolated plants are exposed to light, the leaves will quickly turn green. This is explained by the fact that the leaves already contain protochlorophyll, which under the influence of light is easily converted into chlorophyll.
Temperature has a great influence on the formation of chlorophyll; In a cold spring, the leaves of some shrubs do not turn green until warm weather sets in: when the temperature drops, the formation of protochlorophyll is suppressed.
The minimum temperature at which the formation of chlorophyll begins is 2°, the maximum at which the formation of chlorophyll does not occur is 40°. In addition to a certain temperature, the formation of chlorophyll requires elements mineral nutrition, especially iron.
In its absence, plants experience a disease called chlorosis. Apparently, iron is a catalyst in the synthesis of protochlorophyll, since it is not part of the chlorophyll molecule. The formation of chlorophyll also requires nitrogen and magnesium, which are part of its molecule. An important condition is also the presence in the leaf cells of plastids capable of greening.
In their absence, the plant leaves remain white, the plant is not capable of photosynthesis and can live only until it uses up its seed reserves. This phenomenon is called albinism. It is associated with a change in the hereditary nature of a given plant.
Quantitative relationships between chlorophyll and assimilable carbon dioxide
With higher content chlorophyll In a plant, the process of photosynthesis begins at lower light intensity and even at lower temperature. With an increase in chlorophyll content in leaves, photosynthesis increases, but to a certain limit. Consequently, there is no direct relationship between the chlorophyll content and the intensity of CO 2 absorption.
The amount of CO 2 assimilated by the leaf per hour, calculated per unit of chlorophyll contained in the leaf, is higher, the less chlorophyll. R. Willstetter and A. Stohl proposed a unit that characterizes the relationship between the amount of chlorophyll and absorbed carbon dioxide.
They called the amount of carbon dioxide decomposed per unit time per unit weight of chlorophyll assimilation number.
The assimilation number is not constant: it is higher when the chlorophyll content is low and lower when its content in the leaves is high. Consequently, the chlorophyll molecule is used more productively when its content in the leaf is low, and the productivity of chlorophyll decreases with increasing its amount. The data is entered into the table.
Table
Assimilation number depending on chlorophyll content
(according to R. Willstetter and A. Stohl)
| Plants |
at 10 leaves (mg) |
Assimilation number |
|
green race yellow race |
||
| Lilac | 16,2 | 5,8 |
| Etiolated bean sprouts after illumination for: |
The published table shows that there is no direct relationship between the content of chlorophyll and the amount of absorbed CO 2. Chlorophyll in plants is always found in excess and, obviously, not all is involved in photosynthesis. This is explained by the fact that during photosynthesis, along with photochemical processes that are carried out with the participation of chlorophyll, there are purely chemical processes that do not need light.
Dark reactions in plants proceed much slower than light reactions. The speed of the light reaction is 0.00001 seconds, the dark reaction is 0.04 seconds. Dark reactions in the process of photosynthesis were first discovered by F. Blackman.
He found that the dark reaction depends on temperature, and as it increases, the rate of dark processes increases. The duration of light reactions is negligible, therefore the rate of photosynthesis is determined mainly by the duration of dark processes.
Human life, like all living things on Earth, is impossible without breathing. We inhale oxygen from the air and exhale carbon dioxide. But why doesn't the oxygen run out? It turns out that the air in the atmosphere is continuously supplied with oxygen. And this saturation occurs precisely thanks to photosynthesis.
Photosynthesis - simple and clear!
Every person must understand what photosynthesis is. You don't need to write at all for this. complex formulas, it is enough to understand the importance and magic of this process.
The main role in the process of photosynthesis is played by plants - grass, trees, shrubs. It is in the leaves of plants that, over millions of years, the amazing transformation of carbon dioxide into oxygen occurs, which is so necessary for life for those who like to breathe. Let's try to analyze the entire process of photosynthesis in order.
1. Plants take water from the soil with minerals dissolved in it - nitrogen, phosphorus, manganese, potassium, various salts - more than 50 different ones in total chemical elements. Plants need this for nutrition. But plants receive only 1/5 of the necessary substances from the ground. The remaining 4/5 they get out of thin air!
2. Plants absorb carbon dioxide from the air. The same carbon dioxide that we exhale every second. Plants breathe carbon dioxide, just as we breathe oxygen. But this is not enough.
3. An irreplaceable component in a natural laboratory is sunlight. The sun's rays in the leaves of plants awaken an extraordinary chemical reaction. How does this happen?
4. There is an amazing substance in the leaves of plants - chlorophyll. Chlorophyll is able to capture streams of sunlight and tirelessly process the resulting water, microelements, and carbon dioxide into organic substances necessary for every living creature on our planet. At this moment, plants release oxygen into the atmosphere! It is this work of chlorophyll that scientists call compound word – photosynthesis.
A presentation on the topic Photosynthesis can be downloaded on the educational portal
So why is the grass green?
Now that we know that plant cells contain chlorophyll, this question is very easy to answer. No wonder chlorophyll is translated from ancient Greek as “ green leaf" For photosynthesis, chlorophyll uses all rays of sunlight except green. We see grass and plant leaves green precisely because chlorophyll turns out green.
The meaning of photosynthesis.
The importance of photosynthesis cannot be overestimated - without photosynthesis, too much carbon dioxide would accumulate in the atmosphere of our planet, most living organisms simply would not be able to breathe and would die. Our Earth would turn into a lifeless planet. In order to prevent this, every person on planet Earth must remember that we are very much indebted to plants.
This is why it is so important to create as many parks and green spaces in cities as possible. Protect the taiga and jungle from destruction. Or just plant a tree next to your house. Or don't break branches. Only the participation of every person on planet Earth will help preserve life on our home planet.
But the importance of photosynthesis goes beyond converting carbon dioxide into oxygen. It was as a result of photosynthesis that the ozone layer in the atmosphere, protecting the planet from harmful ultraviolet rays. Plants are food for most living things on Earth. Food is necessary and healthy. The nutritional value of plants is also the result of photosynthesis.
Recently, chlorophyll has been actively used in medicine. People have long known that sick animals instinctively eat green leaves to heal. Scientists have found that chlorophyll is similar to a substance in human blood cells and can work real miracles.