- synthesis of organic substances from carbon dioxide and water with the mandatory use of light energy:
6CO 2 + 6H 2 O + Q light → C 6 H 12 O 6 + 6O 2.
In higher plants, the organ of photosynthesis is the leaf, and the organelles of photosynthesis are the chloroplasts (structure of chloroplasts - lecture No. 7). The membranes of chloroplast thylakoids contain photosynthetic pigments: chlorophylls and carotenoids. There are several different types chlorophyll ( a, b, c, d), the main one is chlorophyll a. In the chlorophyll molecule, a porphyrin “head” with a magnesium atom in the center and a phytol “tail” can be distinguished. The porphyrin “head” is a flat structure, is hydrophilic and therefore lies on the surface of the membrane that faces the aqueous environment of the stroma. The phytol “tail” is hydrophobic and due to this retains the chlorophyll molecule in the membrane.
Chlorophylls absorb red and blue-violet light, reflect green and therefore give plants their characteristic green color. Chlorophyll molecules in thylakoid membranes are organized into photosystems. Plants and blue-green algae have photosystem-1 and photosystem-2, and photosynthetic bacteria have photosystem-1. Only photosystem-2 can decompose water to release oxygen and take electrons from the hydrogen of water.
Photosynthesis is a complex multi-step process; photosynthesis reactions are divided into two groups: reactions light phase and reactions dark phase.
Light phase
This phase occurs only in the presence of light in thylakoid membranes with the participation of chlorophyll, electron transport proteins and the enzyme ATP synthetase. Under the influence of a quantum of light, chlorophyll electrons are excited, leave the molecule and enter the outer side of the thylakoid membrane, which ultimately becomes negatively charged. Oxidized chlorophyll molecules are reduced, taking electrons from water located in the intrathylakoid space. This leads to the breakdown or photolysis of water:
H 2 O + Q light → H + + OH - .
Hydroxyl ions give up their electrons, becoming reactive radicals.OH:
OH - → .OH + e - .
OH radicals combine to form water and free oxygen:
4NO. → 2H 2 O + O 2.
In this case, oxygen is removed to the external environment, and protons accumulate inside the thylakoid in the “proton reservoir”. As a result, the thylakoid membrane, on the one hand, is charged positively due to H +, and on the other, due to electrons, it is charged negatively. When the potential difference between the outer and inner sides of the thylakoid membrane reaches 200 mV, protons are pushed through the ATP synthetase channels and ADP is phosphorylated to ATP; Atomic hydrogen is used to restore the specific carrier NADP + (nicotinamide adenine dinucleotide phosphate) to NADPH 2:
2H + + 2e - + NADP → NADPH 2.
Thus, in the light phase, photolysis of water occurs, which is accompanied by three important processes: 1) ATP synthesis; 2) the formation of NADPH 2; 3) the formation of oxygen. Oxygen diffuses into the atmosphere, ATP and NADPH 2 are transported into the stroma of the chloroplast and participate in the processes of the dark phase.
1 - chloroplast stroma; 2 - grana thylakoid.
Dark phase
This phase occurs in the stroma of the chloroplast. Its reactions do not require light energy, so they occur not only in the light, but also in the dark. Dark phase reactions are a chain of successive transformations of carbon dioxide (coming from the air), leading to the formation of glucose and other organic substances.
The first reaction in this chain is the fixation of carbon dioxide; The carbon dioxide acceptor is a five-carbon sugar. ribulose biphosphate(RiBF); enzyme catalyzes the reaction Ribulose biphosphate carboxylase(RiBP carboxylase). As a result of carboxylation of ribulose bisphosphate, an unstable six-carbon compound is formed, which immediately breaks down into two molecules phosphoglyceric acid(FGK). A cycle of reactions then occurs in which phosphoglyceric acid is converted to glucose through a series of intermediates. These reactions use the energy of ATP and NADPH 2 formed in the light phase; The cycle of these reactions is called the “Calvin cycle”:
6CO 2 + 24H + + ATP → C 6 H 12 O 6 + 6H 2 O.
In addition to glucose, other monomers of complex organic compounds are formed during photosynthesis - amino acids, glycerol and fatty acids, nucleotides. Currently, there are two types of photosynthesis: C 3 - and C 4 photosynthesis.
C 3-photosynthesis
This is a type of photosynthesis in which the first product is three-carbon (C3) compounds. C 3 photosynthesis was discovered before C 4 photosynthesis (M. Calvin). It is C 3 photosynthesis that is described above, under the heading “Dark phase”. Characteristic features of C 3 photosynthesis: 1) the carbon dioxide acceptor is RiBP, 2) the carboxylation reaction of RiBP is catalyzed by RiBP carboxylase, 3) as a result of carboxylation of RiBP, a six-carbon compound is formed, which decomposes into two PGAs. FGK is restored to triose phosphates(TF). Some of the TF is used for the regeneration of RiBP, and some is converted into glucose.
1 - chloroplast; 2 - peroxisome; 3 - mitochondria.
This is a light-dependent absorption of oxygen and release of carbon dioxide. At the beginning of the last century, it was established that oxygen suppresses photosynthesis. As it turned out, for RiBP carboxylase the substrate can be not only carbon dioxide, but also oxygen:
O 2 + RiBP → phosphoglycolate (2C) + PGA (3C).
The enzyme is called RiBP oxygenase. Oxygen is a competitive inhibitor of carbon dioxide fixation. The phosphate group is split off and the phosphoglycolate becomes glycolate, which the plant must utilize. It enters peroxisomes, where it is oxidized to glycine. Glycine enters the mitochondria, where it is oxidized to serine, with the loss of already fixed carbon in the form of CO 2. As a result, two glycolate molecules (2C + 2C) are converted into one PGA (3C) and CO 2. Photorespiration leads to a decrease in the yield of C3 plants by 30-40% ( With 3 plants- plants characterized by C 3 photosynthesis).
C 4 photosynthesis is photosynthesis in which the first product is four-carbon (C 4) compounds. In 1965, it was found that in some plants (sugar cane, corn, sorghum, millet) the first products of photosynthesis are four-carbon acids. These plants were called With 4 plants. In 1966, Australian scientists Hatch and Slack showed that C4 plants have virtually no photorespiration and absorb carbon dioxide much more efficiently. The pathway of carbon transformations in C 4 plants began to be called by Hatch-Slack.
C 4 plants are characterized by a special anatomical structure leaf. All vascular bundles are surrounded by a double layer of cells: the outer layer is mesophyll cells, the inner layer is sheath cells. Carbon dioxide is fixed in the cytoplasm of mesophyll cells, the acceptor is phosphoenolpyruvate(PEP, 3C), as a result of carboxylation of PEP, oxaloacetate (4C) is formed. The process is catalyzed PEP carboxylase. Unlike RiBP carboxylase, PEP carboxylase has a greater affinity for CO 2 and, most importantly, does not interact with O 2 . Mesophyll chloroplasts have many grains where light phase reactions actively occur. Dark phase reactions occur in the chloroplasts of the sheath cells.
Oxaloacetate (4C) is converted to malate, which is transported through plasmodesmata into the sheath cells. Here it is decarboxylated and dehydrogenated to form pyruvate, CO 2 and NADPH 2 .
Pyruvate returns to the mesophyll cells and is regenerated using the energy of ATP in PEP. CO 2 is again fixed by RiBP carboxylase to form PGA. PEP regeneration requires ATP energy, so it requires almost twice as much energy as C 3 photosynthesis.
The meaning of photosynthesis
Thanks to photosynthesis, billions of tons of carbon dioxide are absorbed from the atmosphere every year and billions of tons of oxygen are released; photosynthesis is the main source of the formation of organic substances. Oxygen forms the ozone layer, which protects living organisms from short-wave ultraviolet radiation.
During photosynthesis, a green leaf uses only about 1% of the solar energy falling on it; productivity is about 1 g of organic matter per 1 m2 of surface per hour.
Chemosynthesis
The synthesis of organic compounds from carbon dioxide and water, carried out not due to the energy of light, but due to the energy of oxidation of inorganic substances, is called chemosynthesis. Chemosynthetic organisms include some types of bacteria.
Nitrifying bacteria ammonia is oxidized to nitrous and then to nitric acid (NH 3 → HNO 2 → HNO 3).
Iron bacteria convert ferrous iron into oxide iron (Fe 2+ → Fe 3+).
Sulfur bacteria oxidize hydrogen sulfide to sulfur or sulfuric acid (H 2 S + ½O 2 → S + H 2 O, H 2 S + 2O 2 → H 2 SO 4).
As a result of oxidation reactions of inorganic substances, energy is released, which is stored by bacteria in the form of high-energy ATP bonds. ATP is used for the synthesis of organic substances, which proceeds similarly to the reactions of the dark phase of photosynthesis.
Chemosynthetic bacteria contribute to the accumulation in soil minerals, improve soil fertility, promote cleaning waste water etc.
Go to lectures No. 11“The concept of metabolism. Biosynthesis of proteins"
Go to lectures No. 13“Methods of division of eukaryotic cells: mitosis, meiosis, amitosis”
Life on Earth is possible thanks to light, mainly solar energy. This energy is converted into the energy of chemical bonds of organic substances formed during photosynthesis.
All plants and some prokaryotes (photosynthetic bacteria and blue-green algae) engage in photosynthesis. Such organisms are called phototrophs . The energy for photosynthesis comes from light, which is captured by special molecules called photosynthetic pigments. Since only a certain wavelength of light is absorbed, some of the light waves are not absorbed but reflected. Depending on the spectral composition of the reflected light, the pigments acquire color - green, yellow, red, etc.
There are three types of photosynthetic pigments - chlorophylls, carotenoids and phycobilins . The most important pigment is chlorophyll. The base is a flat porphyrin core formed by four pyrrole rings connected by methyl bridges, with a magnesium atom in the center. There are various type-a chlorophylls. Higher plants, green and euglena algae have chlorophyll-B, which is formed from chlorophyll-A. Brown and diatom algae contain chlorophyll-C instead of chlorophyll-B, and red algae contain chlorophyll-D. Another group of pigments is formed by carotenoids, which range in color from yellow to red. They are found in all colored plastids (chloroplasts, chromoplasts) of plants. Moreover, in the green parts of plants, chlorophyll masks carotenoids, making them invisible until the onset of cold weather. In autumn, the green pigments are destroyed and carotenoids become clearly visible. Carotenoids are synthesized by phototrophic bacteria and fungi. Phycobilins are present in red algae and cyanobacteria.
Light stage of photosynthesis
Chlorophylls and other pigments in chloroplasts form specific light-harvesting complexes . Using electromagnetic resonance, they transfer the collected energy to special chlorophyll molecules. These molecules, under the influence of excitation energy, give electrons to molecules of other substances - vectors , and then take away electrons from proteins and then from water. The splitting of water during photosynthesis is called photolysis . This occurs in the thylakoid cavities. Protons pass through special channels into the stroma. This releases the energy necessary for ATP synthesis:
2H 2 O = 4e + 4H + + O 2
ADP + P = ATP
The participation of light energy here is a prerequisite, therefore this stage is called the light stage. Oxygen produced as a by-product is removed outside and used by the cell for respiration.
Dark stage of photosynthesis
The following reactions take place in the stroma of the chloroplast. Monosaccharides are formed from carbon dioxide and water. This process itself contradicts the laws of thermodynamics, but since ATP molecules are involved, due to this energy, glucose synthesis is a real process. Later, polysaccharides are created from its molecules - cellulose, starch and other complex organic molecules. The overall equation for photosynthesis can be represented as follows:
6CO 2 + 6H 2 O = C 6 H 12 O 6 + 6O 2
Especially a lot of starch is deposited in chloroplasts during the day during intense photosynthetic processes; at night, starch is broken down into soluble forms and used by the plant.
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With or without the use of light energy. It is characteristic of plants. Let us next consider what the dark and light phases of photosynthesis are.
General information
The organ of photosynthesis in higher plants is the leaf. Chloroplasts act as organelles. Photosynthetic pigments are present in the membranes of their thylakoids. They are carotenoids and chlorophylls. The latter exist in several forms (a, c, b, d). The main one is a-chlorophyll. Its molecule contains a porphyrin “head” with a magnesium atom located in the center, as well as a phytol “tail”. The first element is presented as a flat structure. The “head” is hydrophilic, therefore it is located on that part of the membrane that is directed towards the aqueous environment. The phytol "tail" is hydrophobic. Due to this, it retains the chlorophyll molecule in the membrane. Chlorophylls absorb blue-violet and red light. They also reflect green, giving plants their characteristic color. In thylactoid membranes, chlorophyll molecules are organized into photosystems. Blue-green algae and plants are characterized by systems 1 and 2. Photosynthetic bacteria have only the first. The second system can decompose H 2 O and release oxygen.
Light phase of photosynthesis
The processes occurring in plants are complex and multi-stage. In particular, two groups of reactions are distinguished. They are the dark and light phases of photosynthesis. The latter occurs with the participation of the enzyme ATP, electron transfer proteins, and chlorophyll. The light phase of photosynthesis occurs in thylactoid membranes. Chlorophyll electrons become excited and leave the molecule. After this, they end up on the outer surface of the thylactoid membrane. It, in turn, becomes negatively charged. After oxidation, the reduction of chlorophyll molecules begins. They take electrons from water, which is present in the intralacoid space. Thus, the light phase of photosynthesis occurs in the membrane during decay (photolysis): H 2 O + Q light → H + + OH -
Hydroxyl ions turn into reactive radicals, donating their electrons:
OH - → .OH + e -
OH radicals combine to form free oxygen and water:
4NO. → 2H 2 O + O 2.
In this case, oxygen is removed into the surrounding (external) environment, and protons accumulate inside the thylactoid in a special “reservoir”. As a result, where the light phase of photosynthesis occurs, the thylactoid membrane receives a positive charge due to H + on one side. At the same time, due to electrons, it is charged negatively.
Phosphyrylation of ADP
Where the light phase of photosynthesis occurs, there is a potential difference between the inner and outer surfaces of the membrane. When it reaches 200 mV, protons begin to be pushed through the channels of ATP synthetase. Thus, the light phase of photosynthesis occurs in the membrane when ADP is phosphorylated to ATP. In this case, atomic hydrogen is sent to restore the special carrier nicotinamide adenine dinucleotide phosphate NADP+ to NADP.H2:
2Н + + 2е — + NADP → NADP.Н 2
The light phase of photosynthesis thus includes the photolysis of water. It, in turn, is accompanied by three most important reactions:
- ATP synthesis.
- Formation of NADP.H 2.
- Formation of oxygen.
The light phase of photosynthesis is accompanied by the release of the latter into the atmosphere. NADP.H2 and ATP move into the stroma of the chloroplast. This completes the light phase of photosynthesis.
Another group of reactions
The dark phase of photosynthesis does not require light energy. It goes in the stroma of the chloroplast. The reactions are presented in the form of a chain of sequential transformations of carbon dioxide coming from the air. As a result, glucose and other organic substances are formed. The first reaction is fixation. Ribulose biphosphate (five-carbon sugar) RiBP acts as a carbon dioxide acceptor. The catalyst in the reaction is ribulose biphosphate carboxylase (enzyme). As a result of carboxylation of RiBP, a six-carbon unstable compound is formed. It almost instantly breaks down into two molecules of PGA (phosphoglyceric acid). After this, a cycle of reactions occurs where it is transformed into glucose through several intermediate products. They use the energy of NADP.H 2 and ATP, which were converted during the light phase of photosynthesis. The cycle of these reactions is called the “Calvin cycle”. It can be represented as follows:
6CO 2 + 24H+ + ATP → C 6 H 12 O 6 + 6H 2 O
In addition to glucose, other monomers of organic (complex) compounds are formed during photosynthesis. These include, in particular, fatty acids, glycerol, amino acids and nucleotides.
C3 reactions
They are a type of photosynthesis that produces three-carbon compounds as the first product. It is this that is described above as the Calvin cycle. The characteristic features of C3 photosynthesis are:
- RiBP is an acceptor for carbon dioxide.
- The carboxylation reaction is catalyzed by RiBP carboxylase.
- A six-carbon substance is formed, which subsequently breaks down into 2 FHA.
Phosphoglyceric acid is reduced to TP (triose phosphates). Some of them are used for the regeneration of ribulose biphosphate, and the rest is converted into glucose.
C4 reactions
This type of photosynthesis is characterized by the appearance of four-carbon compounds as the first product. In 1965, it was discovered that C4 substances appear first in some plants. For example, this has been established for millet, sorghum, sugar cane, and corn. These crops became known as C4 plants. The next year, 1966, Slack and Hatch (Australian scientists) discovered that they almost completely lack photorespiration. It was also found that such C4 plants absorb carbon dioxide much more efficiently. As a result, the pathway of carbon transformation in such crops began to be called the Hatch-Slack pathway.
Conclusion
The importance of photosynthesis is very great. Thanks to it, carbon dioxide is absorbed from the atmosphere in huge volumes (billions of tons) every year. Instead, no less oxygen is released. Photosynthesis acts as the main source of the formation of organic compounds. Oxygen is involved in the formation of the ozone layer, which protects living organisms from the effects of short-wave UV radiation. During photosynthesis, a leaf absorbs only 1% of the total energy of light falling on it. Its productivity is within 1 g of organic compound per 1 sq. m of surface per hour.
The process of photosynthesis is completed by dark phase reactions, during which carbohydrates are formed. To carry out these reactions, energy and substances stored during the light phase are used: for the discovery of this cycle of reactions in 1961, the Nobel Prize. We will try to talk briefly and clearly about the dark phase of photosynthesis.
Localization and conditions
Dark phase reactions take place in the stroma (matrix) of chloroplasts. They do not depend on the presence of light, since the energy they require is already stored in the form of ATP.
For the synthesis of carbohydrates, hydrogen obtained from the photolysis of water and bound in NADPH₂ molecules is used. The presence of sugars is also necessary, to which a carbon atom from the CO₂ molecule will be attached.
The source of sugars for germinating plants is the endosperm - reserve substances that are found in the seed and obtained from the parent plant.
Studying
The set of chemical reactions of the dark phase of photosynthesis leading to the formation of glucose was discovered by M. Calvin and his collaborators.
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Rice. 1. Melvin Calvin in the laboratory.
The first step of the phase is to obtain compounds with three carbon atoms.
For some plants, the first step will be the formation of organic acids with 4 carbon atoms. This path was discovered by Australian scientists M. Hatch and S. Slack and is called C₄ - photosynthesis.
The result of C₄ photosynthesis is also glucose and other sugars.
CO₂ binding
Due to the energy of ATP obtained in the light phase, ribulose phosphate molecules are activated in the stroma. It is converted to the highly reactive compound ribulose diphosphate (RDP), which has 5 carbon atoms.
Rice. 2. Scheme of connecting CO₂ to the RDF.
Two molecules of phosphoglyceric acid (PGA), which has three carbon atoms, are formed. In the next step, PGA reacts with ATP and forms diphosphoglyceric acid. DiPHA reacts with NADPH₂ and is reduced to phosphoglyceraldehyde (PGA).
All reactions occur only under the influence of appropriate enzymes.
PHA forms phosphodioxyacetone.
Hexose formation
At the next stage, by condensation of PHA and phosphodioxyacetone, fructose diphosphate is formed, which contains 6 carbon atoms and is the starting material for the formation of sucrose and polysaccharides.
Rice. 3. Scheme of the dark phase of photosynthesis.
Fructose diphosphate can react with PHA and other dark phase products, giving rise to chains of 4-, 5-, 6-, and 7-carbon sugars. One of the stable products of photosynthesis is ribulose phosphate, which is again included in the reaction cycle, interacting with ATP. To obtain a glucose molecule, it undergoes 6 cycles of dark phase reactions.
Carbohydrates are the main product of photosynthesis, but amino acids, fatty acids, and glycolipids are also formed from intermediate products of the Calvin cycle.
Thus, in the plant body, many functions depend on what happens in the dark phase of photosynthesis. Substances obtained in this phase are used in the biosynthesis of proteins, fats, respiration and other intracellular processes.Evaluation of the report
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Photosynthesis is the process of synthesis of organic substances from inorganic ones using light energy. In the vast majority of cases, photosynthesis is carried out by plants using cellular 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 matter from inorganic. They only absorb ready-made ones, break them down 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, we 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 above-ground parts of plants mainly through stomata. Water comes from the soil, from where it is delivered to photosynthetic cells by the plant's conducting system. Also, photosynthesis requires the energy of photons (hν), but they cannot be attributed to matter.
In total, photosynthesis produces organic matter and oxygen (O2). Typically, organic matter most often means glucose (C 6 H 12 O 6).
Organic compounds mostly consist of carbon, hydrogen and oxygen atoms. They are found in carbon dioxide and water. However, during photosynthesis, oxygen is released. Its atoms are taken 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 and does not make it understandable. Look, although the equation is balanced, in it the total number of atoms in free oxygen is 12. But we said that they come from water, and there are only 6 of them.
In fact, photosynthesis occurs in two phases. The first one 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 occurs in the dark. The light phase occurs on the membranes of the thylakoids of the chloroplast, and the dark phase occurs in the stroma of the chloroplast.
During the light phase, CO 2 binding does not occur. All that happens is that solar energy is captured by chlorophyll complexes, stored in ATP, and energy used to reduce NADP to NADP*H 2 . The flow of energy from light-excited chlorophyll is provided by electrons transmitted along the electron transport chain of enzymes built into the thylakoid membranes.
The hydrogen for NADP comes from water, which is decomposed by sunlight into oxygen atoms, hydrogen protons and electrons. This process is called photolysis. Oxygen from water is not needed for photosynthesis. 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+P) + NADP → ATP + NADP*H 2 + ½O 2
Thus, the release of oxygen occurs during 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 come from the light phase to the dark phase. Here, the energy of the first and the reducing power of the second are spent on the binding of carbon dioxide. This stage of photosynthesis cannot be explained simply and concisely because it does not proceed in the way that six CO 2 molecules combine with hydrogen released from NADP*H 2 molecules to form glucose:
6CO 2 + 6NADP*H 2 →C 6 H 12 O 6 + 6NADP
(the reaction occurs with the expenditure of energy ATP, which breaks down into ADP and phosphoric acid).
The given reaction is just a simplification to make it easier to understand. In fact, carbon dioxide molecules bind one at a time, joining the already prepared five-carbon organic substance. An unstable six-carbon organic substance is formed, which breaks down into three-carbon carbohydrate molecules. Some of these molecules are used to resynthesize the original five-carbon substance to bind CO 2 . This resynthesis is ensured Calvin cycle. A minority of carbohydrate molecules containing three carbon atoms exit the cycle. All other organic substances (carbohydrates, fats, proteins) are synthesized from them and other substances.
That is, in fact, three-carbon sugars, not glucose, come out of the dark phase of photosynthesis.
