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 the plant got 64 kg 950 grams of nutrients remains 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, Swiss scientist Jean Senebier proved that carbon dioxide, under the influence of sunlight, 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
In scientific terms, 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 terrestrial plants account for only 20% of the oxygen released by plants. Seaweed plays a leading role in the production of oxygen in the atmosphere. 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.”
The general formula for photosynthesis is as follows:
Water + Carbon dioxide + Light → Carbohydrates + Oxygen
Why do plants need photosynthesis?
As we have learned, photosynthesis is a necessary condition for human existence 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 green. This complex organic compound has one amazing property: it can absorb sunlight! Thanks to chlorophyll, the process of photosynthesis also becomes possible.
Two stages of photosynthesis
In simple terms, 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. In this way, inorganic substances are surprisingly transformed into organic ones. 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. The electron included in the molecule moves to a higher level and 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 fact is that it is at this stage that 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 life on Earth possible!
Photosynthesis- the process of synthesis of organic substances using light energy. Organisms that are capable of synthesizing organic substances from inorganic compounds are called autotrophic. Photosynthesis is characteristic only of cells of autotrophic organisms. Heterotrophic organisms are not capable of synthesizing organic substances from inorganic compounds.
The cells of green plants and some bacteria have special structures and complexes of chemicals that allow them to capture energy from sunlight.
The role of chloroplasts in photosynthesis
Plant cells contain microscopic formations - chloroplasts. These are organelles in which energy and light are absorbed and converted into the energy of ATP and other molecules - energy carriers. The grana of chloroplasts contain chlorophyll, a complex organic substance. Chlorophyll captures light energy for use in the biosynthesis of glucose and other organic substances. The enzymes necessary for the synthesis of glucose are also located in chloroplasts.
Light phase of photosynthesis
A quantum of red light absorbed by chlorophyll transfers the electron to an excited state. An electron excited by light acquires a large supply of energy, as a result of which it moves to a higher energy level. An electron excited by light can be compared to a stone raised to a height, which also acquires potential energy. He loses it, falling from a height. The excited electron, as if in steps, moves along a chain of complex organic compounds built into the chloroplast. Moving from one step to another, the electron loses energy, which is used for the synthesis of ATP. The electron that wasted energy returns to chlorophyll. A new portion of light energy again excites the chlorophyll electron. It again follows the same path, spending energy on the formation of ATP molecules.
Hydrogen ions and electrons, necessary for the restoration of energy-carrying molecules, are formed by the splitting of water molecules. The breakdown of water molecules in chloroplasts is carried out by a special protein under the influence of light. This process is called photolysis of water.
Thus, the energy of sunlight is directly used by the plant cell to:
1. excitation of chlorophyll electrons, the energy of which is further spent on the formation of ATP and other energy carrier molecules;
2. photolysis of water, supplying hydrogen ions and electrons to the light phase of photosynthesis.
This releases oxygen as a by-product of photolysis reactions. The stage during which, due to the energy of light, energy-rich compounds are formed - ATP and energy-carrying molecules, called light phase of photosynthesis.
Dark phase of photosynthesis
Chloroplasts contain five-carbon sugars, one of which ribulose diphosphate, is a carbon dioxide acceptor. A special enzyme binds five-carbon sugar with carbon dioxide in the air. In this case, compounds are formed that, using the energy of ATP and other energy carrier molecules, are reduced to a six-carbon glucose molecule. Thus, the light energy converted during the light phase into the energy of ATP and other energy carrier molecules is used for the synthesis of glucose. These processes can take place in the dark.
It was possible to isolate chloroplasts from plant cells, which in a test tube, under the influence of light, carried out photosynthesis - they formed new glucose molecules and absorbed carbon dioxide. If the illumination of the chloroplasts was stopped, the synthesis of glucose also stopped. However, if ATP and reduced energy carrier molecules were added to the chloroplasts, then glucose synthesis resumed and could proceed in the dark. This means that light is really only needed to synthesize ATP and charge energy-carrying molecules. Absorption of carbon dioxide and formation of glucose in plants called dark phase of photosynthesis because she can walk in the dark.
Intense lighting and increased carbon dioxide content in the air lead to increased photosynthesis activity.
Two types of pigments have been discovered in living organisms that can perform the function of photosynthetic antennas. These pigments absorb visible light quanta and provide further storage of radiation energy in the form of the energy of the electrochemical H + gradient on biological membranes. In the vast majority of organisms, chlorophylls play the role of antennas; A less common case is in which the vitamin A derivative, retinal, serves as the antenna. In accordance with this, chlorophyll and non-chlorophyll photosynthesis are distinguished.
Non-chlorophyll photosynthesis
The system of chlorophyll-free photosynthesis is characterized by significant simplicity of organization, and therefore is assumed to be the evolutionarily primary mechanism for storing the energy of electromagnetic radiation. The efficiency of chlorophyll-free photosynthesis as an energy conversion mechanism is relatively low (only one H + is transferred per absorbed quantum).
Discovery in halophilic archaea
Dieter Oesterhelt and Walther Stoeckenius identified a representative of halophilic archaea in the “purple membranes” Halobacterium salinarium(former name N. halobium) a protein that was later named bacteriorhodopsin. Evidence was soon accumulated indicating that bacteriorhodopsin is a light-dependent generator of a proton gradient. In particular, photophosphorylation was demonstrated on artificial vesicles containing bacteriorhodopsin and mitochondrial ATP synthase, photophosphorylation in intact cells H. salinarium, a light-induced drop in pH of the environment and suppression of respiration, all of these effects correlated with the absorption spectrum of bacteriorhodopsin. Thus, irrefutable evidence of the existence of chlorophyll-free photosynthesis was obtained.
Mechanism
The photosynthetic apparatus of extreme halobacteria is the most primitive currently known; it lacks an electron transport chain. Cytoplasmic membrane halobacteria is a coupling membrane containing two main components: a light-dependent proton pump (bacteriorhodopsin) and ATP synthase. The operation of such a photosynthetic apparatus is based on the following energy transformations:
- The chromophore of bacteriorhodopsin, retinal, absorbs light quanta, which leads to conformational changes in the structure of bacteriorhodopsin and proton transport from the cytoplasm to the periplasmic space. In addition, an additional contribution to the electrical component of the gradient is made by the active light-dependent import of chloride anion, which is provided by halorhodopsin [ ] . Thus, as a result of the work of bacteriorhodopsin, the energy of solar radiation is transformed into the energy of the electrochemical gradient of protons on the membrane.
- During the operation of ATP synthase, the energy of the transmembrane gradient is transformed into the energy of ATP chemical bonds. Thus, chemiosmotic coupling occurs.
With the chlorophyll-free type of photosynthesis (as well as with the implementation of cyclic flows in electron transport chains), the formation of reducing equivalents (reduced ferredoxin or NAD(P)H) necessary for the assimilation of carbon dioxide does not occur. Therefore, during chlorophyll-free photosynthesis, there is no assimilation of carbon dioxide, but only the storage of solar energy in the form of ATP (photophosphorylation).
Meaning
The main way for halobacteria to obtain energy is the aerobic oxidation of organic compounds (carbohydrates and amino acids are used during cultivation). In case of oxygen deficiency, in addition to non-chlorophyll photosynthesis, anaerobic nitrate respiration or fermentation of arginine and citrulline can serve as energy sources for halobacteria. However, the experiment showed that chlorophyll-free photosynthesis can also serve as the only source of energy under anaerobic conditions when anaerobic respiration and fermentation are suppressed, provided that retinal is added to the medium, the synthesis of which requires oxygen.
Chlorophyll photosynthesis
Chlorophyll photosynthesis differs from bacteriorhodopsin photosynthesis by its significantly greater efficiency of energy storage. For each absorbed quantum of radiation, at least one H + is transferred against the gradient, and in some cases the energy is stored in the form of reduced compounds (ferredoxin, NADP).
Anoxygenic
Anoxygenic (or oxygen-free) photosynthesis occurs without the release of oxygen. Purple and green bacteria, as well as heliobacteria, are capable of anoxygenic photosynthesis.
With anoxygenic photosynthesis, it is possible to:
- Light-dependent cyclic electron transport, not accompanied by the formation of reducing equivalents and leading exclusively to the storage of light energy in the form of ATP. With cyclic light-dependent electron transport, there is no need for exogenous electron donors. The need for reducing equivalents is met non-photochemically, usually through exogenous organic compounds.
- Light-dependent non-cyclic electron transport, accompanied by the formation of reducing equivalents and the synthesis of ADP. In this case, there is a need for exogenous electron donors, which are necessary to fill the electron vacancy in the reaction center. Both organic and inorganic reducing agents can be used as exogenous electron donors. Among inorganic compounds, the most commonly used are various reduced forms of sulfur (hydrogen sulfide, molecular sulfur, sulfites, thiosulfates, tetrathionates, thioglycolates), and molecular hydrogen can also be used.
Oxygenic
Oxygenic (or oxygenic) photosynthesis is accompanied by the release of oxygen as a by-product. In oxygenic photosynthesis, noncyclic electron transport occurs, although under certain physiological conditions, exclusively cyclic electron transport occurs. An extremely weak electron donor - water - is used as an electron donor in a non-cyclic flow.
Oxygenic photosynthesis is much more widespread. Characteristic of higher plants, algae, many protists and cyanobacteria.
Stages
Photosynthesis is a process with an extremely complex spatiotemporal organization.
The spread of characteristic times of various stages of photosynthesis is 19 orders of magnitude: the rate of absorption of light quanta and energy migration is measured in the femtosecond interval (10−15 s), the rate of electron transport has characteristic times of 10−10−10−2 s, and processes associated with growth plants are measured in days (10 5 −10 7 s).
Also, a large variation in size is characteristic of structures that ensure photosynthesis occurs: from the molecular level (10 −27 m 3) to the level of phytocenoses (10 5 m 3).
In photosynthesis, individual stages can be distinguished, differing in nature and characteristic rates of processes:
- Photophysical;
- Photochemical;
- Chemical:
- Electron transport reactions;
- "Dark" reactions or carbon cycles during photosynthesis.
At the first stage, light quanta are absorbed by pigments, their transition to an excited state and energy transfer to other molecules of the photosystem. At the second stage, charges are separated in the reaction center, electrons are transferred along the photosynthetic electron transport chain, which ends in the synthesis of ATP and NADPH. The first two stages are collectively called the light-dependent stage of photosynthesis. The third stage occurs without the mandatory participation of light and includes biochemical reactions of the synthesis of organic substances using the energy accumulated in the light-dependent stage. Most often, such reactions are considered to be the Calvin cycle and gluconeogenesis, the formation of sugars and starch from carbon dioxide in the air.
Spatial localization
Sheet
Plant photosynthesis occurs in chloroplasts - semi-autonomous double-membrane organelles belonging to the class of plastids. Chloroplasts can be contained in the cells of stems, fruits, and sepals, but the main organ of photosynthesis is the leaf. It is anatomically adapted to absorb light energy and assimilate carbon dioxide. The flat shape of the sheet, providing a large surface-to-volume ratio, allows for more complete use of the energy of sunlight. The water necessary to maintain turgor and photosynthesis is delivered to the leaves from the root system through the xylem, one of the conducting tissues of the plant. Loss of water through evaporation through the stomata and, to a lesser extent, through the cuticle (transpiration) serves as the driving force for vascular transport. However, excess transpiration is undesirable, and plants have evolved various adaptations aimed at reducing water loss. The outflow of assimilates, necessary for the functioning of the Calvin cycle, occurs through the phloem. With intense photosynthesis, carbohydrates can polymerize, and at the same time starch grains are formed in chloroplasts. Gas exchange (intake of carbon dioxide and release of oxygen) is carried out by diffusion through the stomata (some of the gases move through the cuticle).
Since carbon dioxide deficiency significantly increases the loss of assimilates during photorespiration, it is necessary to maintain a high concentration of carbon dioxide in the intercellular space, which is possible with open stomata. However, keeping stomata open at high temperatures leads to increased evaporation of water, which leads to water deficiency and also reduces the productivity of photosynthesis. This conflict is resolved in accordance with the principle of adaptive compromise. In addition, the primary absorption of carbon dioxide at night, at low temperatures, in plants with CAM photosynthesis allows one to avoid high transpiration losses of water.
Photosynthesis at the tissue level
At the tissue level, photosynthesis in higher plants is provided by specialized tissue - chlorenchyma. It is located near the surface of the plant body, where it receives enough light energy. Typically, chlorenchyma is found directly beneath the epidermis. In plants growing in conditions of increased insolation, one or two layers of transparent cells (hypodermis) may be located between the epidermis and chlorenchyma, providing light scattering. In some shade-loving plants, the epidermis is also rich in chloroplasts (for example, wood sorrel). Often the chlorenchyma of the leaf mesophyll is differentiated into palisade (columnar) and spongy, but can also consist of homogeneous cells. In the case of differentiation, palisade chlorenchyma is richest in chloroplasts.
Chloroplasts
The internal space of the chloroplast is filled with colorless contents (stroma) and permeated by membranes (lamellae), which, connecting with each other, form thylakoids, which, in turn, are grouped into stacks called grana. The intrathylakoid space is separated and does not communicate with the rest of the stroma; it is also assumed that the internal space of all thylakoids communicates with each other. The light stages of photosynthesis are confined to membranes; autotrophic fixation of CO 2 occurs in the stroma.
Chloroplasts have their own DNA, RNA, ribosomes (type 70s), and protein synthesis occurs (although this process is controlled from the nucleus). They are not synthesized again, but are formed by dividing the previous ones. All this made it possible to consider them the descendants of free cyanobacteria that became part of the eukaryotic cell during the process of symbiogenesis.
Photosynthetic membranes of prokaryotes
Photochemical essence of the process
Photosystem I
Light-harvesting complex I contains approximately 200 chlorophyll molecules.
In the reaction center of the first photosystem there is a dimer of chlorophyll a with an absorption maximum at 700 nm (P 700). After excitation by a light quantum, it restores the primary acceptor - chlorophyll a, which is the secondary acceptor (vitamin K 1 or phylloquinone), after which the electron is transferred to ferredoxin, which restores NADP using the enzyme ferredoxin-NADP reductase.
The plastocyanin protein, reduced in the b 6 f-complex, is transported to the reaction center of the first photosystem from the side of the intrathylakoid space and transfers an electron to the oxidized P 700.
Cyclic and pseudocyclic electron transport
In addition to the complete non-cyclic electron path described above, a cyclic and pseudo-cyclic path has been discovered.
The essence of the cyclic pathway is that ferredoxin, instead of NADP, reduces plastoquinone, which transfers it back to the b 6 f complex. This results in a larger proton gradient and more ATP, but no NADPH.
In the pseudocyclic pathway, ferredoxin reduces oxygen, which is further converted into water and can be used in photosystem II. In this case, NADPH is also not formed.
Dark phase
In the dark stage, with the participation of ATP and NADP, CO 2 is reduced to glucose (C 6 H 12 O 6). Although light is not required for this process, it is involved in its regulation.
C 3 photosynthesis, Calvin cycle
In the second stage, FHA is restored in two stages. First, it is phosphorylated by ATP under the action of phosphoroglycerokinase with the formation of 1,3-diphosphoglyceric acid (DPGA), then, under the influence of triosephosphate dehydrogenase and NADPH, the acyl-phosphate group of DPGA is dephosphorylated and reduced to an aldehyde and glyceraldehyde-3-phosphate - phosphorylated carbohydrate (PHA) is formed.
The third stage involves 5 PHA molecules, which, through the formation of 4-, 5-, 6- and 7-carbon compounds, are combined into 3 5-carbon ribulose-1,5-biphosphate, which requires 3ATP.
Finally, two PHAs are required for glucose synthesis. To form one of its molecules, 6 cycle revolutions, 6 CO 2, 12 NADPH and 18 ATP are required.
C 4 photosynthesis
The difference between this mechanism of photosynthesis and the usual one is that the fixation of carbon dioxide and its use are divided in space, between different cells of the plant.
At a low concentration of CO 2 dissolved in the stroma, ribulose biphosphate carboxylase catalyzes the oxidation reaction of ribulose-1,5-biphosphate and its breakdown into 3-phosphoglyceric acid and phosphoglycolic acid, which is forced to be used in the process of photorespiration.
To increase CO2 concentration, type 4 C plants changed their leaf anatomy. In them, the Calvin cycle is localized in the sheath cells of the vascular bundle; in the mesophyll cells, under the action of PEP carboxylase, phosphoenolpyruvate is carboxylated to form oxaloacetic acid, which is converted into malate or aspartate and transported to the sheath cells, where it is decarboxylated to form pyruvate, which is returned to the mesophyll cells.
C 4 -photosynthesis is practically not accompanied by losses of ribulose-1,5-biphosphate from the Calvin cycle, and therefore is more efficient. However, it requires not 18, but 30 ATP for the synthesis of 1 glucose molecule. This is justified in the tropics, where the hot climate requires keeping the stomata closed, which prevents the entry of CO 2 into the leaf, as well as with a ruderal life strategy.
About 7,600 plant species carry out photosynthesis via the C4 pathway. All of them belong to the flowering family: many Cereals (61% of species, including cultivated ones - corn, sugar cane and sorghum, etc.), Carnationaceae (the largest share in the families Chenopoaceae - 40% of species, Amaranthaceae - 25%), some Sedgeaceae, Asteraceae, Brassicas, Euphorbiaceae.
CAM photosynthesis
The emergence on Earth more than 3 billion years ago of a mechanism for the splitting of a water molecule by quanta of sunlight with the formation of O 2 is the most important event in biological evolution, which made the light of the Sun the main source of energy in the biosphere.
The energy obtained by humanity by burning fossil fuels (coal, oil, natural gas, peat) is also stored in the process of photosynthesis.
Photosynthesis serves as the main input of inorganic carbon into the biogeochemical cycle.
Photosynthesis is the basis for the productivity of agriculturally important plants.
Most of the free oxygen in the atmosphere is of biogenic origin and is a by-product of photosynthesis. The formation of an oxidizing atmosphere (oxygen catastrophe) completely changed the state of the earth's surface, made the appearance of respiration possible, and later, after the formation of the ozone layer, allowed life to exist on land.
History of the study
The first experiments in the study of photosynthesis were carried out by Joseph Priestley in the 1780s, when he drew attention to the “spoilage” of air in a sealed vessel with a burning candle (the air ceased to support combustion, and the animals placed in it suffocated) and its “correction” by plants. Priestley concluded that plants produce oxygen, which is necessary for respiration and combustion, but did not notice that plants need light for this. This was soon shown by Jan Ingenhaus.
Later it was found that in addition to releasing oxygen, plants absorb carbon dioxide and, with the participation of water, synthesize organic matter in the light. Based on the law of conservation of energy, Robert Mayer postulated that plants convert the energy of sunlight into the energy of chemical bonds. In W. Pfeffer called this process photosynthesis.
Chlorophylls were first isolated by P. J. Pelletier and J. Cavanto. M. S. Tsvet managed to separate the pigments and study them separately using the chromatography method he created. The absorption spectra of chlorophyll were studied by K. A. Timiryazev, who, developing Mayer’s principles, showed that it is the absorbed rays that make it possible to increase the energy of the system, creating high-energy C-C bonds instead of weak C-O and O-H bonds (before that it was believed that in photosynthesis uses yellow rays that are not absorbed by leaf pigments). This was done thanks to the method he created for accounting for photosynthesis based on absorbed CO 2: during experiments on illuminating a plant with light of different wavelengths (different colors), it turned out that the intensity of photosynthesis coincides with the absorption spectrum of chlorophyll.
The redox essence of photosynthesis (both oxygenic and anoxygenic) was postulated by Cornelis van Niel, who in 1931 proved that purple bacteria and green sulfur bacteria carry out anoxygenic photosynthesis. The redox nature of photosynthesis meant that oxygen in oxygenic photosynthesis is formed entirely from water, which was experimentally confirmed in A.P. Vinogradov in experiments with an isotope label. IN
Concept from school photosynthesis associated with the color green. This is the color of a pigment called chlorophyll. Without its accumulation in the leaves photosynthesis process not possible. How does the white sequoia survive?
Plant photosynthesis based on 0.4% of light rays. Half of them do not reach the surface of the planet. Of the remaining, only 1/8 is suitable for photosynthesis. There are restrictions on the wavelength of light. Plants take 0.4% from suitable rays.
If converted into energy, this is 1% of its total amount. The usual course of photosynthesis occurs under the influence of sunlight. However, plants have also learned to use artificial rays.
Light photosynthesis comes down to the production of glucose. She goes for food. The byproduct of the reaction is oxygen. It is released by flora into the external environment, replenishing the Earth's atmosphere.
Oxygen and glucose are produced during the reaction between carbon dioxide and water. Chlorophyll is a kind of catalyst in this interaction. Without it, the reaction is not possible.
Interestingly, chlorophyll is found only in plants. The functions assigned to the pigment are reminiscent of the work of blood in the body of animals. Chlorophyll is similar to the hemoglobin molecule, but with magnesium in the center.
Iron is used in human blood cells. However, chlorophyll has an effect on human bodies similar to hemoglobin, namely, it increases the level of oxygen in the blood and accelerates nitrogen metabolism.
Photosynthesis reaction may proceed quickly or slowly. It all depends on environmental conditions. Important: the intensity of the light flux, air temperature, its saturation with carbon dioxide and oxygen. The ideal is to reach the compensation point. This is the name given to the coincidence of the plant’s respiration rates and the release of oxygen.
If light enters the chloroplast cells, in which chlorophyll accumulates, from above, then water for the plant reaction is pumped out of the soil. This is why watering plants is needed. Lack of moisture inhibits photosynthesis reactions. As a result, the plant turns yellow, that is, it loses chlorophyll.
Field representative of the flora at this moment, the leaves will not turn green. Chlorophyll also helps pump water out of the soil. It turns out to be a vicious circle. No watering - no chlorophyll, no chlorophyll - no water delivery to the plant.
Now, let's turn our attention to glucose. Since greens produce it from water and carbon dioxide, it means that organics are obtained from inorganic. By adding phosphorus, sulfur, or nitrogen to sugar, plants produce vitamins, fats, proteins, and starches. Grass and trees supplement glucose from the soil. The elements come dissolved in water.
Phases of photosynthesis
Phases of photosynthesis- This is a division of the process into photolysis and a reduction reaction. The first occurs in the light and boils down to the release of hydrogen. Oxygen is a by-product of the reaction, but it is also needed by the plant. It uses gas during the breathing process.
Light phase of photosynthesis stimulates chlorophyll. Due to an excess of energy, its electron breaks away and begins to move along the chain of organic compounds. During the journey, the particle promotes the synthesis of adenosine diphosphoric acid from adenosine triphosphoric acid.
The energy given to the electron is spent on this. ADP is needed for the plant to form nucleotides. They are included in nucleic acids, without which the metabolism of flora representatives is not possible.
Having wasted energy, the electron returns to the chlorophyll molecule. This photosynthesis cell recaptures a quantum of light. The electron, tired from work, is reinforced by it, again going to work. This is the light phase of the process. However, he doesn't stop in the dark.
Dark photosynthesis is aimed at capturing carbon dioxide from the external environment. Together with hydrogen, it participates in the formation of 6-carbon sugar. This is glucose. This result of photosynthesis is also accompanied by the formation of substances that help capture new portions of carbon dioxide.
They are captured again by chloroplasts. They spend the energy accumulated during the day. The resource is used to bind carbon dioxide with ribulose bisphosphate. This is a 5-carbon sugar. The reaction produces two molecules of phosphoglyceric acid.
Each of them has 3 carbon atoms. This is one of the stages of the Calvin cycle. It occurs in the stroma, that is, the lining of the chloroplasts. The cycle consists of three reactions. First, carbon dioxide attaches to rubulose 1,5-bisphosphate.
The reaction requires the presence of rubulose biphosphate carboxylase. This is an enzyme. In his presence, hexose is born. Phosphoglyceric acid molecules are obtained from it.
Once the phosphoglycerol compound is produced, the plant reduces it to glyceraldehyde-3-phosphate. Its molecules go in two “directions”. The first produces glucose, and the second produces rubulose-1,5-diphosphate. As we remember, he picks up the gas carbonic.
Photosynthesis at both stages it occurs actively in plants, since they have adapted to capture the maximum amount of sun energy during the day. Let's remember school classes. Photosynthesis Several botany lessons are devoted.
Teachers explain why most plants have flat and wide leaves. This is how flora representatives increase the area for capturing light quanta. It’s not for nothing that people made solar panels wide but flat.
Photosynthesis of carbon dioxide
Carbon dioxide enters plants through stomata. These are similar to pores in leaves and trunks. The process of absorbing gas and then releasing oxygen through the same ostia is reminiscent of breathing in humans.
The only difference is the alternation of stages. People inhale oxygen and exhale carbon dioxide. In plants the opposite is true. This is how the planet maintains a balance of two gases in the atmosphere.
Products of photosynthesis carry the energy of the sun. Animals do not know how to process it. Eating plants is the only way to “recharge” from the daylight.
By processing carbon dioxide, plants are able to give people and animals twice as much. Representatives of the flora work with 0.03% of gas in the atmosphere. As you can see, carbon dioxide is not the predominant one in it.
Under artificial conditions, scientists brought the percentage of carbon dioxide in the air to 0.05%. Cucumbers, at the same time, produced 2 times more fruit. They reacted to changes in the same way.
Scientists increased the level of carbon dioxide by burning sawdust and other waste from the wood processing industry in greenhouses. Interestingly, at a gas concentration of 0.1%, the plants were no longer happy.
Many species began to get sick. In tomatoes, for example, in an atmosphere with an excess of carbon dioxide, the leaves began to turn yellow and curl. This is another confirmation of the danger of atmospheric oversaturation with CO 2. By continuing deforestation and industrial development, people risk putting the remaining plants in conditions unsuitable for them.
It is possible to increase the level of carbon dioxide to the optimal level not only by burning wood waste, but also by adding fertilizers to the soil. They provoke the proliferation of bacteria.
Many microorganisms produce carbon dioxide compounds. Concentrating near the ground, it is immediately captured by plants, benefiting the flora and the entire population of the Earth.
The meaning of photosynthesis
If we allow the level of carbon dioxide in the lower atmosphere to increase everywhere, and not just in experimental greenhouses, a greenhouse effect will occur. This is the same global warming that is either already approaching or is not “shine.”
Scientists do not agree. If we talk about facts that speak in favor of the greenhouse effect, we recall the melting of Antarctic ice. Polar bears live there. For several years now they have been included in .
Part of the life of bears has historically been crossing water latitudes on the way to new glaciers. Rushing towards them, the animals are increasingly exhausted, never reaching their goal. The expanses of water are increasing.
It is becoming increasingly difficult to swim to patches of land. Sometimes bears die on the way. Sometimes, predators from the Red Book reach the ground, but exhausted. There is no strength left for hunting or walking on solid ground.
From the above, we conclude: without photosynthesis or with a reduction in its share, the level of carbon dioxide in the atmosphere will provoke a greenhouse effect. Not only the climate of the planet will change, but also the composition of its inhabitants, their appearance, and adaptations to the environment.
This will happen until the proportion of carbon dioxide in the air reaches a critical 1%. Further, the question itself arises photosynthesis. Water The world's oceans may remain its only source. Algae also “breathe”. The cells that store chlorophyll are different.
However, the essence of the process of photosynthesis in terrestrial and aquatic plants is the same. The concentration of carbon dioxide in the atmosphere is not necessarily transferred to the aquatic environment. It can maintain balance.
Some scientists suggest that with a gradual increase in the proportion of carbon dioxide in the air, representatives of the flora will be able to adapt to new conditions. Tomatoes will not fold their leaves, capitulating to the realities of the future.
Perhaps plants are evolving to process more CO 2 . The scientists' guess falls into the "better not to check" category. Too risky.
The meaning of photosynthesis is associated not only with maintaining the life of the plants themselves and saturating the Earth’s atmosphere with oxygen. Scientists are struggling with artificial reactions.
Water split into hydrogen and oxygen under the influence of solar radiation is a source of energy. This energy, unlike that obtained from petroleum products and coal, is environmentally friendly and safe.
Where does photosynthesis occur?- doesn't matter. The energy that he carries with him is important. So far, a person receives a resource only by absorbing plant foods. The question arises, how do carnivores survive? It is not for nothing that they hunt herbivores, and not their own kind. The meat of animals that eat grass and leaves retains some of their energy.
In addition to the energy of photosynthesis, its products are also important. Oxygen, for example, is used not only for the respiration of animals, but also for the formation of the ozone layer. It is located in the Earth's stratosphere, on the border with space.
Ozone is one of the modifications of oxygen that it takes when rising to thousands of kilometers in height. Here the element fights the radiation of the Sun. Without the ozone layer, the sun's radiation would reach the surface of the planet in doses dangerous to all living things.
Interestingly, some invertebrates can help maintain the balance of gases on the planet. The slug Elisia Chloroti, for example, has learned to assimilate algal chloroplasts.
The sea dweller eats them, “taming” the cells with chlorophyll in the mucous membrane of his stomach. The slug genome encodes proteins needed for green pigment for photosynthesis.
The produced substances are supplied to the chloroplasts and they “feed” the invertebrate with sweet glucose. People can survive on it for some time. Suffice it to recall hospitals where glucose is administered intravenously to the weakened.
Sugar is the main source of energy and, most importantly, fast. The chain of conversion of glucose into clean energy is shorter than the chain of conversion of fats and proteins. Of course, they learned to synthesize sugar artificially.
But many scientists are inclined to believe that glucose from plants, fruits and vegetables is more beneficial for the body. This is similar to the effect of vitamins. Synthetic and natural have the same composition, but the position of the atoms is slightly different. Experiments prove that pharmacy vitamin C provides questionable benefits, but the same substance from lemon or cabbage provides undeniable benefits.
The benefits of photosynthesis are also undeniable. It is familiar and, at the same time, still keeps many secrets. Get to know them in order to ensure a happy future for yourself and the planet as a whole.
Photosynthesis is the conversion of light energy into the energy of chemical bonds 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 byproduct. 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. are involved. The main pigments are chlorophylls, in addition to them - carotenoids and phycobilins.
In nature, two pathways of plant photosynthesis are common: C 3 and C 4. Other organisms have their own specific reactions. All these different processes are united under the term "photosynthesis" - in all of them, in total, the energy of photons is converted into a chemical bond. 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 light radiation (hν), which is necessary for reactions to occur. The dark phase is light-independent.
In plants, photosynthesis occurs in chloroplasts. As a result of all reactions, primary organic substances are formed, from which carbohydrates, amino acids, fatty acids, etc. are then synthesized. The total reaction of photosynthesis is usually 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 included in the O 2 molecule are taken not from carbon dioxide, but from water. Carbon dioxide - source of carbon, which is more important. Thanks to its binding, plants have the opportunity to synthesize organic matter.
The chemical reaction presented above is generalized and total. It is far from the essence of the process. So glucose is not formed from six separate molecules of carbon dioxide. CO 2 binding occurs one molecule at a time, which first attaches to an 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 chloroplast thylakoids, where pigments and enzymes form complex complexes for the functioning of electrochemical circuits through which electrons and partly hydrogen protons are transmitted.
The electrons ultimately end up with the coenzyme NADP, which, when charged negatively, attracts some protons 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, through the same radiation, should release into space the energy received from outside 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 a chain of carriers.
There are two types of photosystems on thylakoid membranes that emit electrons when exposed to light. Photosystems are a complex complex of mostly chlorophyll pigments with a reaction center from which electrons are removed. In a photosystem, sunlight catches many molecules, but all the energy is collected in the reaction center.
Electrons from photosystem I, passing through the chain of transporters, reduce NADP.
The energy of electrons released from photosystem II is used for the synthesis of ATP. And the electrons of photosystem II themselves fill the electron holes of photosystem I.
The holes of the second photosystem are filled with electrons resulting from photolysis of water. Photolysis also occurs with the participation of light and consists of the decomposition of H 2 O into protons, electrons and oxygen. It is as a result of photolysis of water that free oxygen is formed. Protons are involved in creating an electrochemical gradient and reducing NADP. Electrons are received by chlorophyll of photosystem II.
An approximate summary equation for the light phase of photosynthesis:
H 2 O + NADP + 2ADP + 2P → ½O 2 + NADP H 2 + 2ATP
Cyclic electron transport
The so-called non-cyclical 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 transporter chain, where ATP synthesis occurs. That is, this electron transport chain receives electrons from photosystem I, not II. The first photosystem, as it were, implements a cycle: the electrons emitted by it are returned to it. Along the way, they spend part of their energy on ATP synthesis.
Photophosphorylation and oxidative phosphorylation
The light phase of photosynthesis can be compared with the stage of cellular respiration - oxidative phosphorylation, which occurs on the cristae of mitochondria. ATP synthesis also occurs there due to the transfer of electrons and protons through a chain of carriers. 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 the initial source of energy is organic substances, 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, and Bassem. The reaction cycle they discovered was later called the Calvin cycle, or C 3 photosynthesis. In certain groups of plants, a modified photosynthetic pathway is observed - C 4, also called the Hatch-Slack cycle.
In the dark reactions of photosynthesis, CO 2 is fixed. The dark phase occurs in the stroma of the chloroplast.
The reduction of CO 2 occurs due to the energy of ATP and the reducing force 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 occurs 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-bisphosphate) – 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, breaks down into two three-carbon molecules phosphoglyceric acid (PGA)- the first product of photosynthesis. PGA 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):
Three-carbon sugar (glyceraldehyde phosphate) is formed from PGA triose phosphate (TP), already including an aldehyde group (-CHO):
FHA (3-acid) → TF (3-sugar)
This reaction requires the energy of ATP and the reducing power of NADP H2. TF is the first carbohydrate of photosynthesis.
After this, most of the triose phosphate is spent on the regeneration of ribulose biphosphate (RiBP), which is again used to fix CO 2. Regeneration includes a series of ATP-consuming reactions involving sugar phosphates with a number of carbon atoms from 3 to 7.
This cycle of RiBF is the Calvin cycle.
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 → 2TP
In this case, 6 molecules of RiBP participate in the binding and 12 molecules of PGA are formed, which are converted into 12 TF, of which 10 molecules remain in the cycle and are converted into 6 molecules of RiBP. Since TP is a three-carbon sugar, and RiBP is a five-carbon sugar, then in relation to carbon atoms we have: 10 * 3 = 6 * 5. The number of carbon atoms providing the cycle does not change, all the necessary RiBP is regenerated. And six carbon dioxide molecules entering the cycle are spent on the formation of two triose phosphate molecules leaving the cycle.
The Calvin cycle, per 6 bound CO 2 molecules, requires 18 ATP molecules and 12 NADP H 2 molecules, which were synthesized in the reactions of the light phase of photosynthesis.
The calculation is based on two triose phosphate molecules leaving the cycle, since the subsequently formed glucose molecule includes 6 carbon atoms.
Triose phosphate (TP) is the final product of the Calvin cycle, but it can hardly be called the final product of photosynthesis, since it almost does not accumulate, but, reacting with other substances, is converted into glucose, sucrose, starch, fats, fatty acids, and amino acids. In addition to TF, FGK 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.
Six-carbon sugar is formed from FHA by stepwise enzymatic catalysis fructose 6-phosphate, which turns into glucose. In plants, glucose can polymerize into starch and cellulose. Carbohydrate synthesis 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 biphosphate 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 split off from it, and it turns into glycolic acid (glycolate). To “recycle” it, oxygen is again needed. Therefore, the more oxygen in the atmosphere, the more it will stimulate photorespiration and the more oxygen the plant will require to get rid of reaction products.
Photorespiration is the light-dependent consumption of oxygen and the release of carbon dioxide. That is, gas exchange occurs as during respiration, but occurs in chloroplasts and depends on light radiation. Photorespiration depends on light only because ribulose biphosphate is formed only during photosynthesis.
During photorespiration, carbon atoms from glycolate are returned 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) → FHA (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, and carbon dioxide is released.
Oxygen is required during the conversion of glycolate to glyoxylate and glycine to serine.
The transformation of glycolate into glyoxylate and then into glycine occurs in peroxisomes, and the synthesis of serine in mitochondria. Serine again enters the peroxisomes, where it is first converted into hydroxypyruvate and then glycerate. Glycerate already enters the chloroplasts, where PGA is synthesized from it.
Photorespiration is characteristic mainly of plants with the C 3 type of photosynthesis. It can be considered harmful, since energy is wasted on the conversion of glycolate to PGA. Apparently photorespiration arose due to the fact that ancient plants were not prepared for a large amount of oxygen in the atmosphere. Initially, their evolution took place in an atmosphere rich in carbon dioxide, and it was this that mainly captured the reaction center of the rubisco enzyme.
C 4 photosynthesis, or the Hatch-Slack cycle
If during C 3 -photosynthesis the first product of the dark phase is phosphoglyceric acid, which contains three carbon atoms, then during 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 and corn.
C4 plants absorb carbon monoxide more efficiently and have almost no photorespiration.
Plants in which the dark phase of photosynthesis proceeds along the C4 pathway have a special leaf structure. In it, the vascular bundles are surrounded by a double layer of cells. The inner layer is the lining of the conductive bundle. The outer layer is mesophyll cells. The chloroplasts of the cell layers are different from each other.
Mesophilic chloroplasts are characterized by large grana, high activity of photosystems, and the absence of the enzyme RiBP-carboxylase (rubisco) and starch. That is, the chloroplasts of these cells are adapted primarily for the light phase of photosynthesis.
In the chloroplasts of the vascular bundle cells, grana are almost undeveloped, 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 to organic acids, in this form is transported to the sheath cells, released and further bound in the same way as in C 3 plants. That is, the C 4 path complements, rather than replaces C 3 .
In the mesophyll, CO2 combines with phosphoenolpyruvate (PEP) to form oxaloacetate (an acid) containing four carbon atoms:
The reaction occurs with the participation of the enzyme PEP carboxylase, which has a higher affinity for CO 2 than rubisco. In addition, PEP carboxylase does not interact with oxygen, which means it is not spent on photorespiration. Thus, the advantage of C 4 photosynthesis is a more efficient fixation of carbon dioxide, an increase in its concentration in the sheath cells and, consequently, a more efficient operation of RiBP carboxylase, which is almost not spent on photorespiration.
Oxaloacetate is converted to a 4-carbon dicarboxylic acid (malate or aspartate), which is transported into the chloroplasts of bundle sheath cells. Here the acid is decarboxylated (removal of CO2), oxidized (removal of hydrogen) and converted to pyruvate. Hydrogen reduces NADP. Pyruvate returns to the mesophyll, where PEP is regenerated from it with the consumption of ATP.
The separated CO 2 in the chloroplasts of the sheath cells goes to the usual C 3 pathway of the dark phase of photosynthesis, i.e., to the Calvin cycle.
Photosynthesis via the Hatch-Slack pathway requires more energy.
It is believed that the C4 pathway arose later in evolution than the C3 pathway and is largely an adaptation against photorespiration.
