What is the correct equation for the process of photosynthesis. General and partial equations of photosynthesis. General Calvin cycle equation
Photosynthesis is the process of transforming the energy of light absorbed by the body into the chemical energy of organic (and inorganic) compounds.
The process of photosynthesis is expressed by the overall equation:
6CO 2 + 6H 2 O ® C 6 H 12 O 6 + 6O 2.
In the light in a green plant, organic substances are formed from extremely oxidized substances - carbon dioxide and water, and molecular oxygen is released. In the process of photosynthesis, not only CO 2 is reduced, but also nitrates or sulfates, and energy can be directed to various endergonic processes, including the transport of substances.
The general equation for photosynthesis can be represented as:
12 H 2 O → 12 [H 2] + 6 O 2 (light reaction)
6 CO 2 + 12 [H 2] → C 6 H 12 O 6 + 6 H 2 O (dark reaction)
6 CO 2 + 12 H 2 O → C 6 H 12 O 6 + 6 H 2 O + 6 O 2
or in terms of 1 mol of CO 2:
CO 2 + H 2 O CH 2 O + O 2
All the oxygen released during photosynthesis comes from water. The water on the right side of the equation cannot be reduced because its oxygen comes from CO 2 . Using the methods of labeled atoms, it was obtained that H 2 O in chloroplasts is heterogeneous and consists of water coming from the external environment and water formed during photosynthesis. Both types of water are used in the process of photosynthesis. Evidence of the formation of O 2 in the process of photosynthesis is the work of the Dutch microbiologist Van Niel, who studied bacterial photosynthesis, and came to the conclusion that the primary photochemical reaction of photosynthesis is the dissociation of H 2 O, and not the decomposition of CO 2. Capable of photosynthetic assimilation of CO 2 bacteria (except cyanobacteria) are used as reducing agents H 2 S, H 2 , CH 3 and others, and do not emit O 2 . This type of photosynthesis is called photoreduction:
CO 2 + H 2 S → [CH 2 O] + H 2 O + S 2 or
CO 2 + H 2 A → [CH 2 O] + H 2 O + 2A,
where H 2 A - oxidizes the substrate, a hydrogen donor (in higher plants it is H 2 O), and 2A is O 2. Then the primary photochemical act in plant photosynthesis should be the decomposition of water into an oxidizing agent [OH] and a reducing agent [H]. [H] restores CO 2, and [OH] participates in the reactions of the release of O 2 and the formation of H 2 O.
Solar energy with the participation of green plants and photosynthetic bacteria is converted into free energy of organic compounds. To implement this unique process, during evolution, a photosynthetic apparatus was created containing: I) a set of photoactive pigments capable of absorbing electromagnetic radiation of certain spectral regions and storing this energy in the form of electronic excitation energy, and 2) a special apparatus for converting electronic excitation energy into various forms chemical energy. First of all, this redox energy , associated with the formation of highly reduced compounds, electrochemical potential energy, due to the formation of electrical and proton gradients on the conjugating membrane (Δμ H +), phosphate bond energy of ATP and other macroergic compounds, which is then converted into free energy of organic molecules.
All these types of chemical energy can be used in the process of life for the absorption and transmembrane transport of ions and in most metabolic reactions, i.e. in a constructive exchange.
The ability to use solar energy and introduce it into biospheric processes determines the "cosmic" role of green plants, which was written about by the great Russian physiologist K.A. Timiryazev.
The process of photosynthesis is a very complex system of spatial and temporal organization. The use of high-speed methods of pulse analysis made it possible to establish that the process of photosynthesis includes reactions of different rates - from 10 -15 s (energy absorption and migration processes occur in the femtosecond time interval) to 10 4 s (formation of photosynthesis products). The photosynthetic apparatus includes structures with sizes from 10 -27 m 3 at the lowest molecular level to 10 5 m 3 at the crop level.
Concept of photosynthesis. The entire complex set of reactions that make up the process of photosynthesis can be represented by a schematic diagram, which displays the main stages of photosynthesis and their essence. In the modern scheme of photosynthesis, four stages can be distinguished, which differ in the nature and rate of reactions, as well as in the meaning and essence of the processes occurring at each stage:
* - SSC - light harvesting antenna complex of photosynthesis - a set of photosynthetic pigments - chlorophylls and carotenoids; RC - photosynthesis reaction center - chlorophyll dimer a; ETC - the electron transport chain of photosynthesis - is localized in the membranes of chloroplast thylakoids (conjugated membranes), includes quinones, cytochromes, iron-sulfur cluster proteins and other electron carriers.
Stage I - physical. It includes reactions of photophysical nature of the absorption of energy by pigments (P), its storage in the form of electronic excitation energy (P *) and migration to the reaction center (RC). All reactions are extremely fast and proceed at a rate of 10 -15 - 10 -9 s. Primary reactions of energy absorption are localized in light-harvesting antenna complexes (LSCs).
Stage II - photochemical. Reactions are localized in reaction centers and proceed at a rate of 10 -9 s. At this stage of photosynthesis, the energy of electronic excitation of the pigment of the reaction center (P (RC)) is used for charge separation. In this case, an electron with a high energy potential is transferred to the primary acceptor A, and the resulting system with separated charges (P (RC) - A) contains a certain amount of energy already in chemical form. The oxidized pigment P (RC) restores its structure due to the oxidation of the donor (D).
The transformation of one type of energy into another occurring in the reaction center is the central event of the photosynthesis process, which requires severe conditions for the structural organization of the system. At present, molecular models of reaction centers in plants and bacteria are generally known. Their similarity in structural organization was established, which indicates a high degree of conservatism of the primary processes of photosynthesis.
The primary products formed at the photochemical stage (P * , A -) are very labile, and the electron can return to the oxidized pigment P * (recombination process) with a useless loss of energy. Therefore, a rapid further stabilization of the formed reduced products with a high energy potential is necessary, which is carried out at the next, III stage of photosynthesis.
Stage III - electron transport reactions. A chain of carriers with different redox potential (E n ) forms the so-called electron transport chain (ETC). The redox components of ETC are organized in chloroplasts in the form of three main functional complexes - photosystem I (PSI), photosystem II (PSII), cytochrome b 6 f-complex, which provides a high speed of the electron flow and the possibility of its regulation. As a result of the work of the ETC, highly reduced products are formed: reduced ferredoxin (PD restore) and NADPH, as well as energy-rich ATP molecules, which are used in the dark reactions of CO 2 reduction that make up the IV stage of photosynthesis.
Stage IV - "dark" reactions of absorption and reduction of carbon dioxide. The reactions take place with the formation of carbohydrates, the end products of photosynthesis, in the form of which the solar energy is stored, absorbed and converted in the "light" reactions of photosynthesis. The speed of "dark" enzymatic reactions is 10 -2 - 10 4 s.
Thus, the entire course of photosynthesis is carried out with the interaction of three flows - the flow of energy, the flow of electrons and the flow of carbon. The conjugation of the three streams requires precise coordination and regulation of their constituent reactions.
The planetary role of photosynthesis
Photosynthesis, having arisen at the first stages of the evolution of life, remains the most important process of the biosphere. It is green plants through photosynthesis that provide the cosmic connection of life on Earth with the Universe and determine the ecological well-being of the biosphere up to the possibility of the existence of human civilization. Photosynthesis is not only a source of food resources and minerals, but also a factor in the balance of biospheric processes on Earth, including the constancy of the content of oxygen and carbon dioxide in the atmosphere, the state of the ozone screen, the content of humus in the soil, the greenhouse effect, etc.
The global net productivity of photosynthesis is 7–8·10 8 tons of carbon per year, of which 7% is directly used for food, fuel and building materials. At present, the consumption of fossil fuels is approximately equal to the formation of biomass on the planet. Every year, during photosynthesis, 70–120 billion tons of oxygen enters the atmosphere, which ensures the respiration of all organisms. One of the most important consequences of oxygen release is the formation of an ozone screen in the upper atmosphere at an altitude of 25 km. Ozone (O 3) is formed as a result of photodissociation of O 2 molecules under the action of solar radiation and traps most of the ultraviolet rays that have a detrimental effect on all living things.
Stabilization of CO 2 content in the atmosphere is also an essential factor in photosynthesis. At present, the content of CO 2 is 0.03–0.04% by volume of air, or 711 billion tons in terms of carbon. The respiration of organisms, the World Ocean, in the waters of which 60 times more CO 2 is dissolved than is in the atmosphere, the production activities of people, on the one hand, photosynthesis, on the other, maintain a relatively constant level of CO 2 in the atmosphere. Carbon dioxide in the atmosphere, as well as water, absorb infrared rays and retain a significant amount of heat on Earth, providing the necessary conditions for life.
However, over the past decades, due to increasing human burning of fossil fuels, deforestation and decomposition of humus, a situation has developed where technological progress has made the balance of atmospheric phenomena negative. The situation is aggravated by demographic problems: every day 200 thousand people are born on Earth, who need to be provided with vital resources. These circumstances put the study of photosynthesis in all its manifestations, from the molecular organization of the process to biospheric phenomena, into the rank of the leading problems of modern natural science. The most important tasks are to increase the photosynthetic productivity of agricultural crops and plantations, as well as to create effective biotechnologies for phototrophic syntheses.
K.A. Timiryazev was the first to study space role green plants. Photosynthesis is the only process on Earth that takes place on a grandiose scale and is associated with the conversion of the energy of sunlight into the energy of chemical compounds. This cosmic energy, stored by green plants, forms the basis of the vital activity of all other heterotrophic organisms on Earth, from bacteria to humans. There are 5 main aspects of space and planetary activity of green plants.
1. Accumulation of organic matter. In the process of photosynthesis, land plants form 100-172 billion tons. biomass per year (in terms of dry matter), and plants of the seas and oceans - 60-70 billion tons. The total mass of plants on Earth is currently 2402.7 billion tons, and 90% of this mass is cellulose. About 2402.5 billion tons. accounted for by terrestrial plants and 0.2 billion tons. - on plants of the hydrosphere (lack of light!). The total mass of animals and microorganisms on Earth is 23 billion tons, that is, 1% of the mass of plants. Of this amount, ~ 20 billion tons. accounts for the inhabitants of the land and ~ 3 billion tons. - on the inhabitants of the hydrosphere. During the existence of life on Earth, the organic remains of plants and animals accumulated and modified (litter, humus, peat, and in the lithosphere - coal; in the seas and oceans - sedimentary rocks). When descending into deeper regions of the lithosphere, gas and oil were formed from these remains under the action of microorganisms, elevated temperatures and pressure. The mass of organic matter in the litter is ~ 194 billion tons; peat - 220 billion tons; humus ~ 2500 billion tons. Oil and gas - 10,000 - 12,000 billion tons. The content of organic matter in sedimentary rocks in terms of carbon is ~ 2 10 16 t. Especially intensive accumulation of organic matter occurred in Paleozoic(~ 300 million years ago). The stored organic matter is intensively used by man (wood, minerals).
2. Ensuring the constancy of the content of CO 2 in the atmosphere. The formation of humus, sedimentary rocks, combustible minerals removed significant amounts of CO 2 from the carbon cycle. In the Earth's atmosphere, CO 2 became less and less, and at present its content is ~ 0.03–0.04% by volume, or ~ 711 billion tons. in terms of carbon. In the Cenozoic era, the content of CO 2 in the atmosphere stabilized and experienced only daily, seasonal and geochemical fluctuations (plant stabilization at the modern level). Stabilization of the content of CO 2 in the atmosphere is achieved by balanced binding and release of CO 2 on a global scale. The binding of CO 2 in photosynthesis and the formation of carbonates (sedimentary rocks) is compensated by the release of CO 2 due to other processes: Annual intake of CO 2 into the atmosphere (in terms of carbon) is due to: plant respiration - ~ 10 billion tons; ~ 25 billion tons; breathing of humans and animals - ~ 1.6 billion tons. economic activities of people ~ 5 billion tons; geochemical processes ~ 0.05 billion tons. Total ~ 41.65 billion tons If CO 2 did not enter the atmosphere, its entire available supply would be bound in 6–7 years. The World Ocean is a powerful reserve of CO 2, 60 times more CO 2 is dissolved in its waters than it is in the atmosphere. So, photosynthesis, respiration and the carbonate system of the ocean maintain a relatively constant level of CO 2 in the atmosphere. Due to human economic activity (burning of combustible minerals, deforestation, decomposition of humus), the content of CO 2 in the atmosphere began to increase by ~ 0.23% per year. This circumstance may have global consequences, since the content of CO 2 in the atmosphere affects the thermal regime of the planet.
3. Greenhouse effect. The Earth's surface receives heat mainly from the Sun. Some of this heat is returned in the form of infrared rays. CO 2 and H 2 O contained in the atmosphere absorb infrared rays and thus retain a significant amount of heat on Earth (greenhouse effect). Microorganisms and plants in the process of respiration or fermentation supply ~ 85% of the total amount of CO 2 that enters the atmosphere annually and, as a result, affect the thermal regime of the planet. The upward trend in CO 2 content in the atmosphere can lead to an increase in the average temperature on the Earth's surface, melting of glaciers (mountains and polar ice) and flooding of coastal zones. However, it is possible that an increase in the concentration of CO 2 in the atmosphere will enhance plant photosynthesis, which will lead to the fixation of excess CO 2.
4. Accumulation of O 2 in the atmosphere. Initially, O 2 was present in the Earth's atmosphere in trace amounts. It is currently ~21% by air volume. The appearance and accumulation of O 2 in the atmosphere is associated with the vital activity of green plants. Every year ~ 70–120 billion tons enter the atmosphere. O 2 formed in photosynthesis. Forests play a special role in this: 1 hectare of forest in 1 hour gives O 2, enough for 200 people to breathe.
5. Ozone shield formation at an altitude of ~ 25 km. O 3 is formed during the dissociation of O 2 under the action of solar radiation. The O 3 layer retains most of the UV (240-290 nm), which is detrimental to living things. The destruction of the planet's ozone screen is one of the global problems of our time.
Photosynthesis is the conversion of light energy into chemical bond energy. organic compounds.
Photosynthesis is characteristic of plants, including all algae, a number of prokaryotes, including cyanobacteria, and some unicellular eukaryotes.
In most cases, photosynthesis produces oxygen (O2) as a by-product. However, this is not always the case as there are several different pathways for photosynthesis. In the case of oxygen release, its source is water, from which hydrogen atoms are split off for the needs of photosynthesis.
Photosynthesis consists of many reactions in which various pigments, enzymes, coenzymes, etc. participate. The main pigments are chlorophylls, in addition to them, carotenoids and phycobilins.
In nature, two ways of plant photosynthesis are common: C 3 and C 4. Other organisms have their own specific reactions. What unites these different processes under the term “photosynthesis” is that in all of them, in total, the conversion of photon energy into a chemical bond occurs. For comparison: during chemosynthesis, the energy of the chemical bond of some compounds (inorganic) is converted into others - organic.
There are two phases of photosynthesis - light and dark. The first depends on the light radiation (hν), which is necessary for the reactions to proceed. The dark phase is light independent.
In plants, photosynthesis takes place in chloroplasts. As a result of all reactions, primary organic substances are formed, from which carbohydrates, amino acids, fatty acids, etc. are then synthesized. Usually, the total reaction of photosynthesis is written in relation to glucose - the most common product of photosynthesis:
6CO 2 + 6H 2 O → C 6 H 12 O 6 + 6O 2
The oxygen atoms that make up the O 2 molecule are not taken from carbon dioxide, but from water. Carbon dioxide is a source of carbon which is more important. Due to its binding, plants have the opportunity to synthesize organic matter.
The chemical reaction presented above is a generalized and total. It is far from the essence of the process. So glucose is not formed from six individual molecules of carbon dioxide. The binding of CO 2 occurs in one molecule, which first attaches to an already existing five-carbon sugar.
Prokaryotes have their own characteristics of photosynthesis. So in bacteria, the main pigment is bacteriochlorophyll, and oxygen is not released, since hydrogen is not taken from water, but often from hydrogen sulfide or other substances. In blue-green algae, the main pigment is chlorophyll, and oxygen is released during photosynthesis.
Light phase of photosynthesis
In the light phase of photosynthesis, ATP and NADP·H 2 are synthesized due to radiant energy. It happens on the thylakoids of chloroplasts, where pigments and enzymes form complex complexes for the functioning of electrochemical circuits, through which electrons and partly hydrogen protons are transferred.
The electrons end up at the coenzyme NADP, which, being negatively charged, attracts some of the protons to itself and turns into NADP H 2 . Also, the accumulation of protons on one side of the thylakoid membrane and electrons on the other creates an electrochemical gradient, the potential of which is used by the enzyme ATP synthetase to synthesize ATP from ADP and phosphoric acid.
The main pigments of photosynthesis are various chlorophylls. Their molecules capture the radiation of certain, partly different spectra of light. In this case, some electrons of chlorophyll molecules move to a higher energy level. This is an unstable state, and, in theory, electrons, by means of the same radiation, should give the energy received from outside into space and return to the previous level. However, in photosynthetic cells, excited electrons are captured by acceptors and, with a gradual decrease in their energy, are transferred along the chain of carriers.
On thylakoid membranes, there are two types of photosystems that emit electrons when exposed to light. Photosystems are a complex complex of mostly chlorophyll pigments with a reaction center from which electrons are torn off. In a photosystem, sunlight catches a lot of molecules, but all the energy is collected in the reaction center.
The electrons of photosystem I, having passed through the chain of carriers, restore NADP.
The energy of the electrons detached from photosystem II is used to synthesize ATP. And the electrons of photosystem II fill the electron holes of photosystem I.
The holes of the second photosystem are filled with electrons formed as a result of water photolysis. Photolysis also occurs with the participation of light and consists in the decomposition of H 2 O into protons, electrons and oxygen. It is as a result of the photolysis of water that free oxygen is formed. Protons are involved in the creation of an electrochemical gradient and the reduction of NADP. Electrons are received by the chlorophyll of photosystem II.
Approximate summary equation of the light phase of photosynthesis:
H 2 O + NADP + 2ADP + 2P → ½O 2 + NADP H 2 + 2ATP
Cyclic electron transport
The so-called non-cyclic light phase of photosynthesis. Is there some more cyclic electron transport when NADP reduction does not occur. In this case, electrons from photosystem I go to the carrier chain, where ATP is synthesized. That is, this electron transport chain receives electrons from photosystem I, not II. The first photosystem, as it were, implements a cycle: the emitted electrons return to it. On the way, they spend part of their energy on the synthesis of ATP.
Photophosphorylation and oxidative phosphorylation
The light phase of photosynthesis can be compared with the stage of cellular respiration - oxidative phosphorylation, which occurs on the mitochondrial cristae. There, too, ATP synthesis occurs due to the transfer of electrons and protons along the carrier chain. However, in the case of photosynthesis, energy is stored in ATP not for the needs of the cell, but mainly for the needs of the dark phase of photosynthesis. And if during respiration organic substances serve as the initial source of energy, then during photosynthesis it is sunlight. The synthesis of ATP during photosynthesis is called photophosphorylation rather than oxidative phosphorylation.
Dark phase of photosynthesis
For the first time the dark phase of photosynthesis was studied in detail by Calvin, Benson, Bassem. The cycle of reactions discovered by them was later called the Calvin cycle, or C 3 -photosynthesis. In certain groups of plants, a modified photosynthesis pathway is observed - C 4, also called the Hatch-Slack cycle.
In the dark reactions of photosynthesis, CO 2 is fixed. The dark phase takes place in the stroma of the chloroplast.
Recovery of CO 2 occurs due to the energy of ATP and the reducing power of NADP·H 2 formed in light reactions. Without them, carbon fixation does not occur. Therefore, although the dark phase does not directly depend on light, it usually also proceeds in light.
Calvin cycle
The first reaction of the dark phase is the addition of CO 2 ( carboxylatione) to 1,5-ribulose biphosphate ( ribulose 1,5-diphosphate) – RiBF. The latter is a doubly phosphorylated ribose. This reaction is catalyzed by the enzyme ribulose-1,5-diphosphate carboxylase, also called rubisco.
As a result of carboxylation, an unstable six-carbon compound is formed, which, as a result of hydrolysis, decomposes into two three-carbon molecules phosphoglyceric acid (PGA) is the first product of photosynthesis. FHA is also called phosphoglycerate.
RiBP + CO 2 + H 2 O → 2FGK
FHA contains three carbon atoms, one of which is part of the acidic carboxyl group (-COOH):
FHA is converted into a three-carbon sugar (glyceraldehyde phosphate) triose phosphate (TF), which already includes an aldehyde group (-CHO):
FHA (3-acid) → TF (3-sugar)
This reaction consumes the energy of ATP and the reducing power of NADP · H 2 . TF is the first carbohydrate of photosynthesis.
After that, most of the triose phosphate is spent on the regeneration of ribulose bisphosphate (RiBP), which is again used to bind CO 2 . Regeneration involves a series of ATP-consuming reactions involving sugar phosphates with 3 to 7 carbon atoms.
It is in this cycle of RiBF that the Calvin cycle is concluded.
A smaller part of the TF formed in it leaves the Calvin cycle. In terms of 6 bound molecules of carbon dioxide, the yield is 2 molecules of triose phosphate. The total reaction of the cycle with input and output products:
6CO 2 + 6H 2 O → 2TF
At the same time, 6 RiBP molecules participate in the binding and 12 FHA molecules are formed, which are converted into 12 TF, of which 10 molecules remain in the cycle and are converted into 6 RiBP molecules. Since TF is a three-carbon sugar, and RiBP is a five-carbon one, in relation to carbon atoms we have: 10 * 3 = 6 * 5. The number of carbon atoms that provide the cycle does not change, all the necessary RiBP is regenerated. And six molecules of carbon dioxide included in the cycle are spent on the formation of two molecules of triose phosphate leaving the cycle.
The Calvin cycle, based on 6 bound CO 2 molecules, consumes 18 ATP molecules and 12 NADP · H 2 molecules, which were synthesized in the reactions of the light phase of photosynthesis.
The calculation is carried out for two triose phosphate molecules leaving the cycle, since the glucose molecule formed later includes 6 carbon atoms.
Triose phosphate (TF) is the end product of the Calvin cycle, but it can hardly be called the end product of photosynthesis, since it almost does not accumulate, but, reacting with other substances, turns into glucose, sucrose, starch, fats, fatty acids, amino acids. In addition to TF, FHA plays an important role. However, such reactions occur not only in photosynthetic organisms. In this sense, the dark phase of photosynthesis is the same as the Calvin cycle.
PHA is converted into a six-carbon sugar by stepwise enzymatic catalysis. fructose-6-phosphate, which turns into glucose. In plants, glucose can be polymerized into starch and cellulose. The synthesis of carbohydrates is similar to the reverse process of glycolysis.
photorespiration
Oxygen inhibits photosynthesis. The more O 2 in the environment, the less efficient the CO 2 sequestration process. The fact is that the enzyme ribulose bisphosphate carboxylase (rubisco) can react not only with carbon dioxide, but also with oxygen. In this case, the dark reactions are somewhat different.
Phosphoglycolate is phosphoglycolic acid. The phosphate group is immediately cleaved from it, and it turns into glycolic acid (glycolate). For its "utilization" oxygen is needed again. Therefore, the more oxygen in the atmosphere, the more it will stimulate photorespiration and the more oxygen the plant will need to get rid of the reaction products.
Photorespiration is the light-dependent consumption of oxygen and the release of carbon dioxide. That is, the exchange of gases occurs as during respiration, but takes place in chloroplasts and depends on light radiation. Photorespiration depends on light only because ribulose biphosphate is formed only during photosynthesis.
During photorespiration, carbon atoms are returned from glycolate to the Calvin cycle in the form of phosphoglyceric acid (phosphoglycerate).
2 Glycolate (C 2) → 2 Glyoxylate (C 2) → 2 Glycine (C 2) - CO 2 → Serine (C 3) → Hydroxypyruvate (C 3) → Glycerate (C 3) → FGK (C 3)
As you can see, the return is not complete, since one carbon atom is lost when two molecules of glycine are converted into one molecule of the amino acid serine, while carbon dioxide is released.
Oxygen is needed at the stages of conversion of glycolate to glyoxylate and glycine to serine.
The conversion of glycolate to glyoxylate and then to glycine occurs in peroxisomes, and serine is synthesized in mitochondria. Serine again enters the peroxisomes, where it first produces hydroxypyruvate, and then glycerate. Glycerate already enters the chloroplasts, where FHA is synthesized from it.
Photorespiration is typical mainly for plants with C3-type photosynthesis. It can be considered harmful, since energy is wasted on the conversion of glycolate into FHA. Apparently, photorespiration arose due to the fact that ancient plants were not ready for a large amount of oxygen in the atmosphere. Initially, their evolution took place in an atmosphere rich in carbon dioxide, and it was he who mainly captured the reaction center of the rubisco enzyme.
C 4 -photosynthesis, or the Hatch-Slack cycle
If in C 3 photosynthesis the first product of the dark phase is phosphoglyceric acid, which includes three carbon atoms, then in the C 4 pathway, the first products are acids containing four carbon atoms: malic, oxaloacetic, aspartic.
C 4 -photosynthesis is observed in many tropical plants, for example, sugar cane, corn.
C 4 -plants absorb carbon monoxide more efficiently, they have almost no photorespiration.
Plants in which the dark phase of photosynthesis proceeds along the C 4 pathway have a special leaf structure. In it, the conducting bundles are surrounded by a double layer of cells. The inner layer is the lining of the conducting beam. The outer layer is mesophyll cells. Chloroplast cell layers differ from each other.
Mesophilic chloroplasts are characterized by large grains, high activity of photosystems, absence of the enzyme RiBP carboxylase (rubisco) and starch. That is, the chloroplasts of these cells are adapted mainly for the light phase of photosynthesis.
In the chloroplasts of the cells of the conducting bundle, the grana are almost not developed, but the concentration of RiBP carboxylase is high. These chloroplasts are adapted for the dark phase of photosynthesis.
Carbon dioxide first enters the mesophyll cells, binds with organic acids, is transported in this form to the sheath cells, is released, and then binds in the same way as in C3 plants. That is, the C 4 -path complements rather than replaces C 3 .
In the mesophyll, CO 2 is added to phosphoenolpyruvate (PEP) to form oxaloacetate (acid), which includes four carbon atoms:
The reaction takes place with the participation of the PEP-carboxylase enzyme, which has a higher affinity for CO 2 than rubisco. In addition, PEP-carboxylase does not interact with oxygen, and therefore is not spent on photorespiration. Thus, the advantage of C4 photosynthesis is more efficient fixation of carbon dioxide, an increase in its concentration in the sheath cells, and, consequently, more efficient operation of RiBP carboxylase, which is almost not consumed for photorespiration.
Oxaloacetate is converted into a 4-carbon dicarboxylic acid (malate or aspartate), which is transported to the chloroplasts of the cells lining the vascular bundles. Here, the acid is decarboxylated (removal of CO2), oxidized (removal of hydrogen) and converted to pyruvate. Hydrogen restores NADP. Pyruvate returns to the mesophyll, where PEP is regenerated from it with the consumption of ATP.
The torn off CO 2 in the chloroplasts of the lining cells goes to the usual C 3 path of the dark phase of photosynthesis, i.e., to the Calvin cycle.
Photosynthesis along the Hatch-Slack pathway requires more energy.
It is believed that the C 4 pathway evolved later than the C 3 pathway and is in many ways an adaptation against photorespiration.
Photosynthesis is the process of synthesizing organic substances from inorganic substances using light energy. In the vast majority of cases, photosynthesis is carried out by plants using cell organelles such as chloroplasts containing green pigment chlorophyll.
If plants were not capable of synthesizing organic matter, then almost all other organisms on Earth would have nothing to eat, since animals, fungi and many bacteria cannot synthesize organic substances from inorganic ones. They only absorb ready-made ones, split them into simpler ones, from which they again assemble complex ones, but already characteristic of their body.
This is the case if we talk about photosynthesis and its role very briefly. To understand photosynthesis, you need to say more: what specific inorganic substances are used, how does synthesis occur?
Photosynthesis requires two inorganic substances - carbon dioxide (CO 2) and water (H 2 O). The first is absorbed from the air by the aerial parts of plants mainly through the stomata. Water - from the soil, from where it is delivered to the photosynthetic cells by the conducting system of plants. Photosynthesis also requires the energy of photons (hν), but they cannot be attributed to matter.
In total, as a result of photosynthesis, organic matter and oxygen (O 2) are formed. Usually, under organic matter, glucose (C 6 H 12 O 6) is most often meant.
Organic compounds are mostly made up of carbon, hydrogen and oxygen atoms. They are found in carbon dioxide and water. However, photosynthesis releases oxygen. Its atoms come from water.
Briefly and generally, the equation for the reaction of photosynthesis is usually written as follows:
6CO 2 + 6H 2 O → C 6 H 12 O 6 + 6O 2
But this equation does not reflect the essence of photosynthesis, does not make it understandable. Look, although the equation is balanced, it has a total of 12 atoms in free oxygen. But we said that they come from water, and there are only 6 of them.
In fact, photosynthesis occurs in two phases. The first is called light, second - dark. Such names are due to the fact that light is needed only for the light phase, the dark phase is independent of its presence, but this does not mean that it goes in the dark. The light phase flows on the membranes of the thylakoids of the chloroplast, the dark phase - in the stroma of the chloroplast.
In the light phase, CO 2 binding does not occur. There is only the capture of solar energy by chlorophyll complexes, its storage in ATP, the use of energy for the reduction of NADP to NADP * H 2. The flow of energy from chlorophyll excited by light is provided by electrons transmitted through the electron transport chain of enzymes built into thylakoid membranes.
Hydrogen for NADP is taken from water, which, under the action of sunlight, decomposes into oxygen atoms, hydrogen protons and electrons. This process is called photolysis. Oxygen from water is not needed for photosynthesis. The oxygen atoms from two water molecules combine to form molecular oxygen. The reaction equation for the light phase of photosynthesis briefly looks like this:
H 2 O + (ADP + F) + NADP → ATP + NADP * H 2 + ½O 2
Thus, the release of oxygen occurs in the light phase of photosynthesis. The number of ATP molecules synthesized from ADP and phosphoric acid per photolysis of one water molecule can be different: one or two.
So, ATP and NADP * H 2 enter the dark phase from the light phase. Here, the energy of the first and the restorative force of the second are spent on the binding of carbon dioxide. This step of photosynthesis cannot be explained simply and briefly, because it does not proceed in such a way that six CO 2 molecules combine with hydrogen released from NADP * H 2 molecules and glucose is formed:
6CO 2 + 6NADP * H 2 → C 6 H 12 O 6 + 6NADP
(the reaction takes place with the expenditure of energy from ATP, which breaks down into ADP and phosphoric acid).
The above reaction is just a simplification for ease of understanding. In fact, carbon dioxide molecules bind one at a time, joining the already prepared five-carbon organic matter. An unstable six-carbon organic substance is formed, which breaks down into three-carbon carbohydrate molecules. Some of these molecules are used for the resynthesis of the initial five-carbon substance for CO 2 binding. This resynthesis is provided Calvin cycle. A smaller part of the carbohydrate molecules, which includes three carbon atoms, leaves the cycle. Already from them and other substances, all other organic substances (carbohydrates, fats, proteins) are synthesized.
That is, in fact, three-carbon sugars, and not glucose, come out of the dark phase of photosynthesis.
1. Give definitions of concepts.
Photosynthesis- the process of formation of organic substances from carbon dioxide and water in the light with the participation of photosynthetic pigments.
Autotrophs organisms that synthesize organic substances from inorganic substances.
Heterotrophs are organisms that are unable to synthesize organic substances from inorganic substances by photosynthesis or chemosynthesis.
Mixotrophs- organisms that can use various sources of carbon and energy.
2. Fill in the table.
3. Fill in the table.
4. Explain the essence of the statement of the great Russian scientist K. A. Timiryazev: "A log is a canned solar energy."
A log is a part of a tree, its tissues consist of accumulated organic compounds (cellulose, sugar, etc.), which were formed during photosynthesis.
5. Write the overall photosynthesis equation. Do not forget to specify the required conditions for the reactions to take place.
12. Choose a term and explain how its modern meaning corresponds to the original meaning of its roots.
The chosen term is mixotrophs.
Conformity. The term is specified, as organisms with a mixed type of nutrition are called, which are able to use various sources of carbon and energy.
13. Formulate and write down the main ideas of § 3.3.
According to the type of nutrition, all living organisms are divided into:
Autotrophs that synthesize organic substances from inorganic substances.
Heterotrophs that feed on ready-made organic matter.
Mixotrophs with mixed nutrition.
Photosynthesis is the process of formation of organic substances from carbon dioxide and water in the light with the participation of photosynthetic pigments by phototrophs.
It is divided into a light phase (water and H+ molecules are formed, which are necessary for the dark phase, and oxygen is also released) and dark (glucose is formed). The total photosynthesis equation: 6CO2 + 6H2O → C6H12O6 + 6O2. It flows in the light in the presence of chlorophyll. Thus, light energy is converted into
the energy of chemical bonds, and plants form for themselves glucose and sugars.
The chemical equation of the photosynthesis process can be generally represented as follows:
6CO 2 + 6H 2 O + Qlight → C 6 H 12 O 6 + 6O 2.
Photosynthesis is a process in which the electromagnetic energy of the sun is absorbed by chlorophyll and auxiliary pigments and converted into chemical energy, the absorption of carbon dioxide from the atmosphere, its reduction into organic compounds and the return of oxygen to the atmosphere.
In the process of photosynthesis, various organic compounds are built from simple inorganic compounds (CO 2, H 2 O). As a result, chemical bonds are rearranged: instead of C - O and H - O bonds, C - C and C - H bonds arise, in which electrons occupy a higher energy level. Thus, energy-rich organic substances that animals and humans feed on and receive energy from (during respiration) are initially created in a green leaf. We can say that almost all living matter on Earth is the result of photosynthetic activity.
The date of the discovery of the process of photosynthesis can be considered 1771. The English scientist J. Priestley drew attention to the change in the composition of the air due to the vital activity of animals. In the presence of green plants, the air again became suitable for both breathing and combustion. Later, the work of a number of scientists (J. Ingengauz, J. Senebier, T. Saussure, J. B. Bussengo) found that green plants absorb CO 2 from the air, from which organic matter is formed with the participation of water in the light. It was this process that in 1877 the German scientist W. Pfeffer called photosynthesis. Of great importance for revealing the essence of photosynthesis was the law of conservation of energy, formulated by R. Mayer. In 1845, R. Mayer suggested that the energy used by plants is the energy of the Sun, which plants convert into chemical energy during photosynthesis. This position was developed and experimentally confirmed in the studies of the remarkable Russian scientist K.A. Timiryazev.
Photosynthesis involves both light and dark reactions. A number of experiments were carried out proving that in the process of photosynthesis, not only reactions that take place with the use of light energy, but also dark reactions that do not require the direct participation of light energy take place. We can cite the following evidence for the existence of dark reactions in the process of photosynthesis:
1) 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 dependence of photosynthesis on temperature is especially pronounced at high light intensities. Apparently, in this case, the rate of photosynthesis is limited precisely by dark reactions;
2) the efficiency of the use of light energy in the process of photosynthesis turned out to be higher with intermittent illumination. At the same time, for more efficient use of light energy, the duration of dark intervals should significantly exceed the duration of light intervals.
photosynthesis pigments
In order for light to have an effect on the plant organism and, in particular, to be used in the process of photosynthesis, it must be absorbed by photoreceptor pigments. Pigments are colored substances. Pigments absorb light of a certain wavelength. Unabsorbed parts 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. Substances that absorb the entire visible spectrum appear black.
Pigments concentrated in plastids can be divided into three groups: chlorophylls, carotenoids, phycobilins.
To the group chlorophylls include organic compounds that contain 4 pyrrole rings connected by magnesium atoms and have a green color.
Currently, about ten chlorophylls are known. They differ in chemical structure, color, distribution among living organisms. All higher plants contain chlorophylls a and b. Chlorophyll c is found in diatoms, chlorophyll d is found in red algae.
The main pigments without which photosynthesis does not proceed are chlorophyll a for green plants and bacteriochlorophylls for bacteria. For the first time, an accurate idea of the pigments of the green leaf 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 leaf pigments in their pure form.
The chromatographic method for separating substances is based on their different adsorption capacities. This method has been widely used. M.S. The color passed the extract from the leaf through a glass tube filled with powder - chalk or sucrose (chromatographic column). The individual components of the pigment mixture differed in the degree of adsorption and moved at different speeds, as a result of which they were concentrated in different zones of the column. By dividing the column into separate parts (zones) and using the appropriate solvent system, it was possible to isolate each pigment. It turned out that the leaves of higher plants contain chlorophyll a and chlorophyll b, as well as carotenoids (carotene, xanthophyll, etc.). Chlorophylls, like carotenoids, are insoluble in water, but readily soluble in organic solvents. Chlorophyll a and b differ in color: chlorophyll a is blue-green, and chlorophyll b is yellow-green. The content of chlorophyll a in the leaf is about three times greater than that of chlorophyll b.
Carotenoids are yellow and orange pigments of aliphatic structure, derivatives of isoprene. Carotenoids are found in all higher plants and in 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 - carotene (orange) and xanthophyll (yellow). Unlike chlorophylls, carotenoids do not absorb red rays, and also do not have the ability to fluoresce. Like chlorophyll, carotenoids in chloroplasts and chromatophores are in the form of water-insoluble complexes with proteins. Carotenoids, absorbing certain parts of the solar spectrum, transfer the energy of these rays to chlorophyll molecules. Thus, they contribute to the use of rays that are not absorbed by chlorophyll.
Phycobilins- red and blue pigments found in cyanobacteria and some algae. Studies have shown that red algae and cyanobacteria, along with chlorophyll a, contain phycobilins. The chemical structure of phycobilins is based on four pyrrole groups.
Phycobilins are represented by pigments: phycocyanin, phycoerythrin and allophycocyanin. Phycoerythrin is an oxidized phycocyanin. Phycobilins form strong compounds with proteins (phycobilin proteins). The connection between phycobilins and proteins is destroyed only by acid.
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. Comparison of the absorption spectra of phycobilins with the spectral composition of light in which photosynthesis takes place 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 parts of the solar spectrum that penetrate the sea water column (chromatic adaptation). As is known, red rays corresponding to the main absorption line of chlorophyll are absorbed when passing through the water column. The green rays penetrate most deeply, which are absorbed not by chlorophyll, but by phycobilins.
Properties of chlorophyll
All chlorophylls are magnesium salts of pyrrole. In the center of the chlorophyll molecule are magnesium and four pyrrole rings connected to each other by methane bridges.
According to the chemical structure, chlorophylls are esters of a dicarboxylic organic acid - chlorophyllin and two alcohol residues - phytol and methyl.
The most important part of the chlorophyll molecule is the central nucleus. It consists of four pyrrole five-membered rings connected by carbon bridges and forming a large porphyrin core with nitrogen atoms in the middle, associated with a magnesium atom. The chlorophyll molecule has an additional cyclopentanone ring, which contains carbonyl and carboxyl groups linked by an ether bond with methyl alcohol. The presence in the porphyrin core of a system of ten double bonds conjugated in a circle and magnesium determines the green color characteristic of chlorophyll.
Chlorophyll c differs from chlorophyll a only in that instead of a methyl group in the second pyrrole ring it has an aldehyde COH group. Chlorophyll is blue-green, while chlorophyll b is light green. They are adsorbed in different layers of the chromatogram, which indicates different chemical and physical properties. According to modern concepts, the biosynthesis of chlorophyll B proceeds through chlorophyll a.
Fluorescence is a property of many bodies under the influence of incident light, in turn, to emit light: the wavelength of the emitted light is usually greater than the wavelength of the exciting light. One of the most important properties of chlorophylls is their pronounced ability to fluorescence, which is intense in solution and suppressed in chlorophyll contained in leaf tissues, in plastids. If you look at a solution of chlorophyll in the rays of light passing through it, then it seems emerald green, but if you look at it in the rays of reflected light, then it becomes red - this is the phenomenon of fluorescence.
Chlorophylls differ in absorption spectra, while in chlorophyll b, compared to chlorophyll a, the absorption band in the red region of the spectrum is somewhat shifted towards short-wavelength rays, and in the blue-violet region, the absorption maximum is shifted towards long-wavelength (red) rays.