Photosynthesis is a set of processes for the synthesis of organic compounds from inorganic ones due to the conversion of light energy into energy chemical bonds. Phototrophic organisms include green plants, some prokaryotes - cyanobacteria, purple and green sulfur bacteria, and plant flagellates.
Research into the process of photosynthesis began in the second half of the 18th century. Important discovery made by the outstanding Russian scientist K. A. Timiryazev, who substantiated the doctrine of the cosmic role of green plants. Plants absorb sunlight and convert light energy into the energy of chemical bonds of organic compounds synthesized by them. Thus, they ensure the preservation and development of life on Earth. The scientist also theoretically substantiated and experimentally proved the role of chlorophyll in the absorption of light during photosynthesis.
Chlorophylls are the main photosynthetic pigments. They are similar in structure to hemoglobin, but contain magnesium instead of iron. Iron content is necessary to ensure the synthesis of chlorophyll molecules. There are several chlorophylls that differ in their chemical structure. Mandatory for all phototrophs is chlorophyll a . Chlorophyllb found in green plants chlorophyll c - in diatoms and brown algae. Chlorophyll d characteristic of red algae.
Green and purple photosynthetic bacteria have special bacteriochlorophylls . Bacterial photosynthesis has much in common with plant photosynthesis. It differs in that in bacteria the hydrogen donor is hydrogen sulfide, and in plants it is water. Green and purple bacteria do not have photosystem II. Bacterial photosynthesis is not accompanied by the release of oxygen. The overall equation for bacterial photosynthesis is:
6C0 2 + 12H 2 S → C 6 H 12 O 6 + 12S + 6H 2 0.
Photosynthesis is based on the redox process. It is associated with the transfer of electrons from compounds that supply electrons-donors to compounds that accept them - acceptors. Light energy is converted into the energy of synthesized organic compounds (carbohydrates).
There are special structures on the membranes of chloroplasts - reaction centers that contain chlorophyll. In green plants and cyanobacteria there are two photosystems – first (I) And second (II) , which have different reaction centers and are interconnected through an electron transfer system.
Two phases of photosynthesis
The process of photosynthesis consists of two phases: light and dark.
Occurs only in the presence of light on the internal membranes of mitochondria in the membranes of special structures - thylakoids . Photosynthetic pigments capture light quanta (photons). This leads to the “excitation” of one of the electrons of the chlorophyll molecule. With the help of carrier molecules, the electron moves to the outer surface of the thylakoid membrane, acquiring a certain potential energy.
This electron in photosystem I can return to his energy level and restore it. NADP (nicotinamide adenine dinucleotide phosphate) may also be transmitted. By interacting with hydrogen ions, electrons restore this compound. Reduced NADP (NADP H) supplies hydrogen to reduce atmospheric CO 2 to glucose.
Similar processes occur in photosystem II . Excited electrons can be transferred to photosystem I and restore it. The restoration of photosystem II occurs due to electrons supplied by water molecules. Water molecules split (photolysis of water) into hydrogen protons and molecular oxygen, which is released into the atmosphere. The electrons are used to restore photosystem II. Water photolysis equation:
2Н 2 0 → 4Н + + 0 2 + 2е.
When electrons from the outer surface of the thylakoid membrane return to the previous energy level, energy is released. It is stored in the form of chemical bonds of ATP molecules, which are synthesized during reactions in both photosystems. The process of ATP synthesis with ADP and phosphoric acid is called photophosphorylation . Some of the energy is used to evaporate water.
During the light phase of photosynthesis, energy-rich compounds are formed: ATP and NADP H. During the breakdown (photolysis) of water molecules, molecular oxygen is released into the atmosphere.
Reactions take place in internal environment chloroplasts. They can occur both in the presence of light and without it. Organic substances are synthesized (C0 2 is reduced to glucose) using the energy that was formed in the light phase.
Recovery process carbon dioxide is cyclic and is called Calvin cycle . Named after the American researcher M. Calvin, who discovered this cyclic process.
The cycle begins with the reaction of atmospheric carbon dioxide with ribulose biphosphate. The process is catalyzed by an enzyme carboxylase . Ribulose biphosphate is a five-carbon sugar combined with two phosphoric acid units. Happening whole line chemical transformations, each of which catalyzes its own specific enzyme. How is the end product of photosynthesis formed? glucose , and ribulose biphosphate is also reduced.
The overall equation for the process of photosynthesis is:
6C0 2 + 6H 2 0 → C 6 H 12 O 6 + 60 2
Thanks to the process of photosynthesis, light energy from the Sun is absorbed and converted into the energy of chemical bonds of synthesized carbohydrates. Energy is transferred through food chains to heterotrophic organisms. During photosynthesis, carbon dioxide is absorbed and oxygen is released. All atmospheric oxygen is of photosynthetic origin. Over 200 billion tons of free oxygen are released annually. Oxygen protects life on Earth from ultraviolet radiation by creating an ozone shield in the atmosphere.
The process of photosynthesis is ineffective, since only 1-2% is converted into synthesized organic matter. solar energy. This is due to the fact that plants do not absorb light enough, part of it is absorbed by the atmosphere, etc. Most of the sunlight is reflected from the surface of the Earth back into space.
- synthesis organic matter 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.
U higher plants the organ of photosynthesis is the leaf, 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 aquatic environment 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 light 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, while 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, the electrons of chlorophyll are excited, leave the molecule and enter the outside thylakoid membrane, which eventually 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.
Oxygen is removed in external environment, and protons accumulate inside the thylakoid in a “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 outside and internal sides thylakoid membrane reaches 200 mV, protons are pushed through 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, during the light phase, photolysis of water occurs, which is accompanied by three the most 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. The dark phase reactions are a chain successive transformations carbon dioxide (comes 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 through a series of intermediates to glucose. 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”. Characteristics 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 of the 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. It is formed from oxygen ozone layer, protecting 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 oxidize ammonia to nitrogen, 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 wastewater treatment, 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”
Photosynthesis consists of two phases - light and dark.
In the light phase, light quanta (photons) interact with chlorophyll molecules, as a result of which these molecules are very a short time move into a more energy-rich “excited” state. The excess energy of some of the “excited” molecules is then converted into heat or emitted as light. Another part of it is transferred to hydrogen ions, always present in aqueous solution due to water dissociation. The resulting hydrogen atoms bond loosely with organic molecules- hydrogen carriers. Hydroxide ions "OH" give up their electrons to other molecules and turn into free radicals OH. OH radicals interact with each other, resulting in the formation of water and molecular oxygen:
4OH = O2 + 2H2O Thus, the source of molecular oxygen formed during photosynthesis and released into the atmosphere is photolysis - the decomposition of water under the influence of light. In addition to water photolysis, energy solar radiation used in the light phase for the synthesis of ATP and ADP and phosphate without the participation of oxygen. This is very efficient process: chloroplasts produce 30 times more ATP than in the mitochondria of the same plants with the participation of oxygen. In this way, the energy necessary for processes in the dark phase of photosynthesis is accumulated.
In complex chemical reactions dark phase, for which light is not required, key place occupies the binding of CO2. These reactions involve ATP molecules synthesized during the light phase and hydrogen atoms formed during the photolysis of water and associated with carrier molecules:
6СО2 + 24Н -» С6Н12О6 + 6НО
This is how the energy of sunlight is converted into the energy of chemical bonds of complex organic compounds.
87. The importance of photosynthesis for plants and for the planet.
Photosynthesis is the main source of biological energy; photosynthetic autotrophs use it to synthesize organic substances from inorganic ones; heterotrophs exist at the expense of the energy stored by autotrophs in the form of chemical bonds, releasing it in the processes of respiration and fermentation. The energy obtained by humanity by burning fossil fuels (coal, oil, natural gas, peat) is also stored during photosynthesis.
Photosynthesis is the main input inorganic carbon into the biological cycle. All free oxygen in the atmosphere is of biogenic origin and is a by-product of photosynthesis. Formation of an oxidizing atmosphere ( oxygen catastrophe) completely changed the state earth's surface, did possible appearance breathing, and later, after the formation of the ozone layer, allowed life to reach land. The process of photosynthesis is the basis of nutrition for all living things, and also supplies humanity with fuel (wood, coal, oil), fiber (cellulose) and countless useful chemical compounds. About 90-95% of the dry weight of the crop is formed from carbon dioxide and water combined from the air during photosynthesis. The remaining 5-10% comes from mineral salts and nitrogen obtained from the soil.
Humans use about 7% of the products of photosynthesis as food, as animal feed, and in the form of fuel and building materials.
Photosynthesis, which is one of the most common processes on Earth, causes natural cycles carbon, oxygen and other elements and provides the material and energy basis for life on our planet. Photosynthesis is the only source of atmospheric oxygen.
Photosynthesis is one of the most common processes on Earth; it determines the cycle of carbon, O2 and other elements in nature. It forms the material and energetic basis of all life on the planet. Every year, as a result of photosynthesis, about 8 1010 tons of carbon are bound in the form of organic matter, and up to 1011 tons of cellulose are formed. Thanks to photosynthesis, land plants produce about 1.8 1011 tons of dry biomass per year; approximately the same amount of plant biomass is formed annually in the oceans. A tropical forest contributes up to 29% to the total photosynthetic production of land, and the contribution of forests of all types is 68%. Photosynthesis of higher plants and algae is the only source of atmospheric O2. The emergence on Earth about 2.8 billion years ago of the mechanism of water oxidation with the formation of O2 is most important event V biological evolution, which made the light of the Sun the main source of free energy in the biosphere, and water an almost unlimited source of hydrogen for the synthesis of substances in living organisms. The result was an atmosphere modern composition, O2 became available for the oxidation of food, and this led to the emergence of highly organized heterotrophic organisms (using exogenous organic substances as a carbon source). The total storage of solar radiation energy in the form of photosynthesis products is about 1.6 1021 kJ per year, which is approximately 10 times higher than the modern energy consumption of humanity. Approximately half of solar radiation energy comes from visible area spectrum (wavelength l from 400 to 700 nm), which is used for photosynthesis (physiologically active radiation, or PAR). IR radiation is not suitable for photosynthesis of oxygen-producing organisms (higher plants and algae), but is used by some photosynthetic bacteria.
Discovery of the chemosynthesis process by S.N. Vinogradsky. Characteristics of the process.
Chemosynthesis is the process of synthesis of organic substances from carbon dioxide, which occurs due to the energy released during the oxidation of ammonia, hydrogen sulfide and others chemical substances, during the life of microorganisms. Chemosynthesis also has another name - chemolithoautotrophy. The discovery of chemosynthesis by S. N. Vinogradovsky in 1887 radically changed the understanding of science about the types of metabolism that are basic for living organisms. Chemosynthesis is the only type of nutrition for many microorganisms, since they are able to assimilate carbon dioxide as the only source of carbon. Unlike photosynthesis, chemosynthesis uses energy that is generated as a result of redox reactions instead of light energy.
This energy should be sufficient for the synthesis of adenosine triphosphoric acid (ATP), and its amount should exceed 10 kcal/mol. Some of the oxidized substances donate their electrons to the chain already at the cytochrome level, and thus additional energy consumption is created for the synthesis of the reducing agent. During chemosynthesis, the biosynthesis of organic compounds occurs due to the autotrophic assimilation of carbon dioxide, that is, in exactly the same way as during photosynthesis. As a result of the transfer of electrons along the chain of bacterial respiratory enzymes, which are built into cell membrane, energy is obtained in the form of ATP. Due to the very high energy consumption, all chemosynthesizing bacteria, except for hydrogen ones, form rather little biomass, but at the same time they oxidize a large volume of inorganic substances. Hydrogen bacteria are used by scientists to produce protein and clean the atmosphere from carbon dioxide, especially necessary in closed ecological systems. There is a great diversity of chemosynthetic bacteria, their most of belongs to pseudomonads, they are also found among filamentous and budding bacteria, leptospira, spirillum and corynebacteria.
Examples of the use of chemosynthesis by prokaryotes.
The essence of chemosynthesis (a process discovered by Russian researcher Sergei Nikolaevich Vinogradsky) is the body’s production of energy through redox reactions carried out by the body itself with simple (inorganic) substances. Examples of such reactions include the oxidation of ammonium to nitrite, or ferrous iron to trivalent, hydrogen sulfide to sulfur, etc.. Capable of chemosynthesis only certain groups prokaryotes (bacteria in in a broad sense words). Due to chemosynthesis, currently only the ecosystems of some hydrothermal sites exist (places on the ocean floor where there are outlets of hot groundwater, rich in reduced substances - hydrogen, hydrogen sulfide, iron sulfide, etc.), as well as extremely simple ecosystems, consisting only of bacteria, found at great depths in rock faults on land.
Bacteria are chemosynthetics and destroy rocks, clean wastewater, participate in the formation of minerals.
The history of the discovery of an amazing and vitally important phenomenon such as photosynthesis is deeply rooted in the past. More than four centuries ago, in 1600, the Belgian scientist Jan Van Helmont performed a simple experiment. He placed a willow twig in a bag containing 80 kg of earth. The scientist recorded the initial weight of the willow, and then watered the plant exclusively with rainwater for five years. Imagine Jan Van Helmont's surprise when he re-weighed the willow. The weight of the plant increased by 65 kg, and the mass of the earth decreased by only 50 grams! Where did the plant get 64 kg 950 g nutrients remained a mystery to the scientist!
The next significant experiment on the path to the discovery of photosynthesis belonged to the English chemist Joseph Priestley. The scientist put a mouse under the hood, and five hours later the rodent died. When Priestley placed a sprig of mint with the mouse and also covered the rodent with a cap, the mouse remained alive. This experiment led the scientist to the idea that there is a process opposite to breathing. Jan Ingenhouse in 1779 established the fact that only green parts of plants are capable of releasing oxygen. Three years later, the Swiss scientist Jean Senebier proved that carbon dioxide, under the influence sun rays, decomposes in green plant organelles. Just five years later, French scientist Jacques Boussingault, conducting laboratory research, discovered the fact that the absorption of water by plants also occurs during the synthesis of organic substances. The epochal discovery was made in 1864 by the German botanist Julius Sachs. He was able to prove that the volume of carbon dioxide consumed and oxygen released occurs in a 1:1 ratio.
Photosynthesis is one of the most significant biological processes
Speaking scientific language, photosynthesis (from ancient Greek φῶς - light and σύνθεσις - connection, binding) is a process in which organic substances are formed from carbon dioxide and water in the light. The main role in this process belongs to photosynthetic segments.
Speaking figuratively, a plant leaf can be compared to a laboratory, the windows of which face the sunny side. It is in it that the formation of organic substances occurs. This process is the basis for the existence of all life on Earth.
Many will reasonably ask the question: what do people who live in a city breathe, where you can’t even find a tree or a blade of grass during the day with fire? The answer is very simple. The fact is that the share land plants accounts for only 20% of the oxygen released by plants. The leading role in the production of oxygen into the atmosphere is played by seaweed. They account for 80% of the oxygen produced. Speaking in the language of numbers, both plants and algae annually release 145 billion tons (!) of oxygen into the atmosphere! It’s not for nothing that the world’s oceans are called “the lungs of the planet.”
General formula photosynthesis looks like in the following way:
Water + Carbon dioxide + Light → Carbohydrates + Oxygen
Why do plants need photosynthesis?
As we have learned, photosynthesis is necessary condition existence of man on Earth. However, this is not the only reason why photosynthetic organisms actively produce oxygen into the atmosphere. The fact is that both algae and plants annually form more than 100 billion organic substances (!), which form the basis of their life activity. Remembering the experiment of Jan Van Helmont, we understand that photosynthesis is the basis of plant nutrition. It has been scientifically proven that 95% of the harvest is determined by the organic substances obtained by the plant during the process of photosynthesis, and 5% by the mineral fertilizers that the gardener applies to the soil.
Modern summer residents pay the main attention to soil nutrition of plants, forgetting about its air nutrition. It is unknown what kind of harvest gardeners could get if they were careful about the process of photosynthesis.
However, neither plants nor algae could produce oxygen and carbohydrates so actively if they did not have an amazing green pigment - chlorophyll.
The Mystery of the Green Pigment
The main difference between plant cells and the cells of other living organisms is the presence of chlorophyll. By the way, it is he who is responsible for the fact that plant leaves are colored precisely green color. It's complicated organic compound has one amazing property: it is capable of absorbing sunlight! Thanks to chlorophyll, the process of photosynthesis also becomes possible.
Two stages of photosynthesis
Speaking in simple language, photosynthesis is a process in which water and carbon dioxide absorbed by a plant in the light with the help of chlorophyll form sugar and oxygen. Thus, inorganic substances miraculously transform into organic. The sugar obtained as a result of conversion is a source of energy for plants.
Photosynthesis has two stages: light and dark.
Light phase of photosynthesis
It is carried out on thylakoid membranes.
Thylakoids are membrane-bounded structures. They are located in the stroma of the chloroplast.
The order of events in the light stage of photosynthesis is:
- Light hits the chlorophyll molecule, which is then absorbed by the green pigment and causes it to become excited. An electron that is part of a molecule is transferred to more high level, participates in the synthesis process.
- Water splits, during which protons are converted into hydrogen atoms under the influence of electrons. Subsequently, they are spent on the synthesis of carbohydrates.
- At the final stage of the light stage, ATP (Adenosine triphosphate) is synthesized. This is an organic substance that plays the role of a universal energy accumulator in biological systems.
Dark phase of photosynthesis
The place where the dark phase occurs is the stroma of chloroplasts. It is during the dark phase that oxygen is released and glucose is synthesized. Many will think that this phase received this name because the process occurring within this stage occurs exclusively at night. In fact, this is not entirely true. Glucose synthesis occurs around the clock. The point is that it is on at this stage light energy is no longer consumed, which means it is simply not needed.
The importance of photosynthesis for plants
We have already determined the fact that plants need photoynthesis no less than we do. It is very easy to talk about the scale of photosynthesis in terms of numbers. Scientists have calculated that land plants alone store as much solar energy as could be consumed by 100 megacities within 100 years!
Plant respiration is the opposite process of photosynthesis. The meaning of plant respiration is to release energy during the process of photosynthesis and direct it to the needs of plants. In simple terms, yield is the difference between photosynthesis and respiration. The more photosynthesis and the lower the respiration, the greater the harvest, and vice versa!
Photosynthesis is an amazing process that makes possible life on the ground!
And NADP·H 2 obtained in the light phase. More precisely: in the dark phase, carbon dioxide (CO 2) is bound.
This process is multi-stage; in nature there are two main paths: C 3 -photosynthesis and C 4 -photosynthesis. Latin letter C denotes a carbon atom, the number after it is the number of carbon atoms in the primary organic product of the dark phase of photosynthesis. Thus, in the case of the C 3 pathway, the primary product is considered to be three-carbon phosphoglyceric acid, designated as PGA. In the case of the C4 pathway, the first organic substance to bind carbon dioxide is four-carbon oxaloacetic acid (oxaloacetate).
C 3 photosynthesis is also called the Calvin cycle after the scientist who studied it. C 4 photosynthesis includes the Calvin cycle, but it does not consist only of it and is called the Hatch-Slack cycle. In temperate latitudes, C3 plants are common, in tropical latitudes - C4 plants.
The dark reactions of photosynthesis take place in the stroma of the chloroplast.
Calvin cycle
The first reaction of the Calvin cycle is the carboxylation of ribulose-1,5-bisphosphate (RiBP). Carboxylation- this is the addition of a CO 2 molecule, resulting in the formation of a carboxyl group -COOH. RiBP is ribose (a five-carbon sugar) with phosphate groups (formed by phosphoric acid) attached to the terminal carbon atoms:
Chemical formula RiBF
The reaction is catalyzed by the enzyme ribulose-1,5-bisphosphate carboxylase oxygenase ( RubisKO). It can catalyze not only the binding of carbon dioxide, but also oxygen, as indicated by the word “oxygenase” in its name. If RuBisCO catalyzes the reaction of oxygen addition to the substrate, then the dark phase of photosynthesis no longer follows the path of the Calvin cycle, but along the path photorespiration, which is basically harmful to the plant.
Catalysis of the reaction of adding CO 2 to RiBP occurs in several steps. As a result, an unstable six-carbon organic compound is formed, which immediately breaks down into two three-carbon molecules phosphoglyceric acid
Chemical formula of phosphoglyceric acid
Next FGK for a few enzymatic reactions, occurring with the expenditure of ATP energy and the reducing power of NADP H 2, is converted into phosphoglyceraldehyde (PGA), also called triose phosphate.
A smaller portion of PHA leaves the Calvin cycle and is used for the synthesis of more complex organic substances, such as glucose. This, in turn, can polymerize to starch. Other substances (amino acids, fatty acids) are formed with the participation of various starting substances. Such reactions are observed not only in plant cells. Therefore, if we consider photosynthesis as unique phenomenon cells containing chlorophyll, it ends with the synthesis of PHA, not glucose.
Most of the PHA molecules remain in the Calvin cycle. A series of transformations occur with it, as a result of which PHA turns into RiBP. This also uses ATP energy. Thus, RiBP is regenerated to bind new carbon dioxide molecules.
Hatch-Slack cycle
In many plants in hot habitats, the dark phase of photosynthesis is somewhat more complex. In the process of evolution, C 4 photosynthesis arose as a more effective method binding carbon dioxide when the amount of oxygen in the atmosphere increased, and RuBisCO began to be spent on ineffective photorespiration.
In C4 plants there are two types of photosynthetic cells. In the chloroplasts of the mesophyll of leaves occurs light phase photosynthesis and part of the dark, namely the binding of CO 2 with phosphoenolpyruvate(FEP). As a result, a four-carbon organic acid is formed. This acid is then transported to the chloroplasts of the vascular bundle sheath cells. Here, a CO 2 molecule is enzymatically split off from it, which then enters the Calvin cycle. The three-carbon acid remaining after decarboxylation is pyruvic- returns to mesophyll cells, where it is again converted into PEP.
Although the Hatch-Slack cycle is a more energy-consuming version of the dark phase of photosynthesis, the enzyme that binds CO 2 and PEP is a more effective catalyst than RuBisCO. In addition, it does not react with oxygen. CO 2 transport using organic acid into deeper cells, to which the flow of oxygen is difficult, leads to the fact that the concentration of carbon dioxide here increases, and RuBisCO is almost not spent on binding molecular oxygen.