As the name implies, photosynthesis is essentially the natural synthesis of organic substances, converting CO2 from the atmosphere and water into glucose and free oxygen.
This requires the presence of solar energy.
The chemical equation for the process of photosynthesis can generally be represented as follows:
Photosynthesis has two phases: dark and light. The chemical reactions of the dark phase of photosynthesis differ significantly from the reactions of the light phase, but the dark and light phases of photosynthesis depend on each other.
Light phase can occur in plant leaves exclusively in sunlight. For dark, the presence of carbon dioxide is necessary, which is why the plant must constantly absorb it from the atmosphere. All comparative characteristics The dark and light phases of photosynthesis will be provided below. For this purpose it was created comparison table"Phases of photosynthesis."
Light phase of photosynthesis
The main processes in the light phase of photosynthesis occur in the thylakoid membranes. It involves chlorophyll, electron transport proteins, ATP synthetase (an enzyme that accelerates the reaction) and sunlight.
Further, the reaction mechanism can be described as follows: when sunlight hits the green leaves of plants, chlorophyll electrons (negative charge) are excited in their structure, which, having passed into an active state, leave the pigment molecule and end up on the outside of the thylakoid, the membrane of which is also negatively charged. At the same time, chlorophyll molecules are oxidized and the already oxidized ones are reduced, thus taking electrons from the water that is in the leaf structure.
This process leads to the fact that water molecules disintegrate, and the ions created as a result of photolysis of water give up their electrons and turn into OH radicals that are capable of carrying out further reactions. These reactive OH radicals then combine to create full-fledged water molecules and oxygen. In this case, free oxygen escapes into the external environment.
As a result of all these reactions and transformations, the leaf thylakoid membrane on one side is charged positively (due to the H+ ion), and on the other - negatively (due to electrons). When the difference between these charges on the two sides of the membrane reaches more than 200 mV, protons pass through special channels of the ATP synthetase enzyme and due to this, ADP is converted to ATP (as a result of the phosphorylation process). And atomic hydrogen, which is released from water, restores the specific carrier NADP+ to NADP·H2. As we can see, as a result of the light phase of photosynthesis, three main processes occur:
- ATP synthesis;
- creation of NADP H2;
- formation of free oxygen.
The latter is released into the atmosphere, and NADP H2 and ATP take part in the dark phase of photosynthesis.
Dark phase of photosynthesis
The dark and light phases of photosynthesis are characterized by at great expense energy from the plant, but the dark phase proceeds faster and requires less energy. Dark phase reactions do not require sunlight, so they can occur both day and night.
All the main processes of this phase occur in the stroma of the plant chloroplast and represent a unique chain of successive transformations of carbon dioxide from the atmosphere. The first reaction in such a chain is the fixation of carbon dioxide. To make it happen more smoothly and faster, nature provided the enzyme RiBP-carboxylase, which catalyzes the fixation of CO2.
Next, a whole cycle of reactions occurs, the completion of which is the conversion of phosphoglyceric acid into glucose (natural sugar). All these reactions use the energy of ATP and NADP H2, which were created in the light phase of photosynthesis. In addition to glucose, photosynthesis also produces other substances. Among them are various amino acids, fatty acids, glycerol, and nucleotides.
Phases of photosynthesis: comparison table
| Comparison criteria | Light phase | Dark phase |
| sunlight | Required | Not required |
| Place of reaction | Chloroplast grana | Chloroplast stroma |
| Dependence on energy source | Depends on sunlight | Depends on ATP and NADP H2 formed in the light phase and on the amount of CO2 from the atmosphere |
| Starting materials | Chlorophyll, electron transport proteins, ATP synthetase | Carbon dioxide |
| The essence of the phase and what is formed | Free O2 is released, ATP and NADP H2 are formed | Formation of natural sugar (glucose) and absorption of CO2 from the atmosphere |
Photosynthesis - video
Plants convert sunlight into stored chemical energy in two steps: first, they capture the energy from sunlight and then use it to fix carbon to form organic molecules.
Green plants - biologists call them autotrophs- the basis of life on the planet. Almost all food chains begin with plants. They convert the energy that falls on them in the form of sunlight into energy stored in carbohydrates ( cm. Biological molecules), of which the most important is the six-carbon sugar glucose. This energy conversion process is called photosynthesis. Other living organisms access this energy by eating plants. This creates a food chain that supports the planetary ecosystem.
In addition, the air we breathe is saturated with oxygen thanks to photosynthesis. The overall equation for photosynthesis looks like this:
water + carbon dioxide + light → carbohydrates + oxygen
Plants absorb carbon dioxide produced during respiration and release oxygen, a waste product of plants ( cm. Glycolysis and respiration). In addition, photosynthesis plays vital role in the carbon cycle in nature.
It seems surprising that, despite the importance of photosynthesis, scientists did not begin to study it for so long. After Van Helmont's experiment, carried out in the 17th century, there was a lull, and only in 1905 the English plant physiologist Frederick Blackman (1866-1947) conducted research and established the basic processes of photosynthesis. He showed that photosynthesis begins in low light, that the rate of photosynthesis increases with increasing luminous flux, but, starting from a certain level, further intensification of illumination no longer leads to an increase in photosynthesis activity. Blackman showed that increasing temperature under low light conditions had no effect on the rate of photosynthesis, but that when temperature and light were increased simultaneously, the rate of photosynthesis increased significantly more than with increased light alone.
From these experiments, Blackman concluded that two processes were occurring: one was highly dependent on light level but not temperature, while the other was strongly influenced by temperature regardless of light level. This insight formed the basis of modern ideas about photosynthesis. The two processes are sometimes called “light” and “dark” reactions, which is not entirely correct, since it turned out that although the reactions of the “dark” phase occur in the absence of light, they require products of the “light” phase.
Photosynthesis begins when photons emitted by the sun enter special pigment molecules found in the leaf - molecules chlorophyll. Chlorophyll is found in leaf cells and in the membranes of cellular organelles chloroplasts(they are the ones who give the leaf its green color). The process of energy capture consists of two stages and is carried out in separate clusters of molecules - these clusters are usually called Photosystem I And Photosystem II. The cluster numbers reflect the order in which these processes were discovered, and this is one of the funny scientific oddities, since in the leaf the reactions in Photosystem II occur first, and only then in Photosystem I.
When a photon collides with 250-400 molecules of Photosystem II, the energy increases abruptly and is transferred to the chlorophyll molecule. At this point, two chemical reactions occur: the chlorophyll molecule loses two electrons (which are accepted by another molecule, called an electron acceptor) and the water molecule splits. The electrons of the two hydrogen atoms that were part of the water molecule replace the two electrons lost by chlorophyll.
After this, the high-energy (“fast”) electron is transferred to each other like a hot potato by the molecular carriers assembled in a chain. In this case, part of the energy goes to the formation of the adenosine triphosphate (ATP) molecule, one of the main energy carriers in the cell ( cm. Biological molecules). Meanwhile, a slightly different Photosystem I chlorophyll molecule absorbs the photon's energy and donates an electron to another acceptor molecule. This electron is replaced in chlorophyll by an electron that arrived along the chain of carriers from Photosystem II. The energy of the electron from Photosystem I and the hydrogen ions previously formed during the splitting of a water molecule are used to form NADP-H, another carrier molecule.
As a result of the process of light capture, the energy of two photons is stored in the molecules used by the cell to carry out reactions, and an additional oxygen molecule is formed. (I note that as a result of another, significantly less efficient process with the participation of Photosystem I alone, ATP molecules are also formed.) After solar energy is absorbed and stored, it is the turn of carbohydrates to be formed. The basic mechanism for the synthesis of carbohydrates in plants was discovered by Melvin Calvin, who carried out a series of experiments in the 1940s that have now become classic. Calvin and his collaborators grew the algae in the presence of carbon dioxide containing radioactive carbon-14. They were able to establish the chemical reactions of the dark phase by interrupting photosynthesis at different stages.
Transformation cycle solar energy into carbohydrates - the so-called Calvin cycle - is similar to the Krebs cycle ( cm. Glycolysis and respiration: It also consists of a series of chemical reactions that begin with the combination of an incoming molecule with a “helper” molecule, followed by the initiation of other chemical reactions. These reactions lead to the formation of the final product and at the same time reproduce the “helper” molecule, and the cycle begins again. In the Calvin cycle, the role of such a “helper” molecule is played by the five-carbon sugar ribulose diphosphate (RDP). The Calvin cycle begins with carbon dioxide molecules combining with RDP. Due to the energy of sunlight stored in the form of ATP and NADP-H, chemical reactions of carbon fixation first occur to form carbohydrates, and then reactions of the reconstruction of ribulose diphosphate occur. During the six turns of the cycle, six carbon atoms are incorporated into the molecules of the precursors of glucose and other carbohydrates. This cycle of chemical reactions will continue as long as energy is supplied. Thanks to this cycle, the energy of sunlight becomes available to living organisms.
In most plants, the Calvin cycle described above occurs, in which carbon dioxide, directly participating in reactions, binds to ribulose diphosphate. These plants are called C 3 plants because the carbon dioxide-ribulose diphosphate complex is broken down into two smaller molecules, each consisting of three carbon atoms. Some plants (such as corn and sugarcane, and many tropical grasses, including creeping weed) operate differently. The fact is that carbon dioxide normally penetrates through holes in the surface of the sheet, called stomata. At high temperatures, the stomata close, protecting the plant from excessive moisture loss. In C 3 plants, when the stomata are closed, the supply of carbon dioxide also stops, which leads to a slowdown in photosynthesis and a change in photosynthetic reactions. In the case of corn, carbon dioxide attaches to a three-carbon molecule on the surface of the leaf, then moves to the interior of the leaf, where the carbon dioxide is released and the Calvin cycle begins. Thanks to this rather complex process, photosynthesis in corn occurs even in very hot, dry weather. We call plants in which this process occurs C 4 plants, since carbon dioxide is transported as a four-carbon molecule at the beginning of the cycle. C 3 plants are mostly plants. temperate climate, and C 4 plants mainly grow in the tropics.
Van Niel hypothesis
The process of photosynthesis is described by the following chemical reaction:
CO 2 + H 2 O + light → carbohydrate + O 2
At the beginning of the 20th century, it was believed that the oxygen released during photosynthesis was formed as a result of the breakdown of carbon dioxide. This point of view was refuted in the 1930s by Cornelis Bernardus Van Niel (1897-1986), then a graduate student at Stanford University in California. He studied the purple sulfur bacterium (pictured), which requires hydrogen sulfide (H 2 S) for photosynthesis and releases atomic sulfur as a by-product. For such bacteria, the photosynthesis equation looks like this:
CO 2 + H 2 S + light → carbohydrate + 2S.
Based on the similarity of these two processes, Van Niel suggested that in ordinary photosynthesis the source of oxygen is not carbon dioxide, but water, since in sulfur bacteria, which metabolize sulfur instead of oxygen, photosynthesis returns this sulfur, which is a by-product of photosynthetic reactions. Modern detailed explanation photosynthesis confirms this guess: the first stage of the photosynthesis process (carried out in Photosystem II) is the splitting of a water molecule.
Have you ever wondered how many living organisms there are on the planet?! And after all, they all need to inhale oxygen to generate energy and exhale carbon dioxide. This is precisely the main reason for such a phenomenon as stuffiness in the room. It occurs when there are many people in it, and the room is not ventilated for a long time. In addition, production facilities, private automobiles and public transport fill the air with toxic substances.
Taking into account the above, a completely logical question arises: how come we haven’t suffocated yet, if all living things are a source of poisonous carbon dioxide? The savior of all living beings in this situation is photosynthesis. What is this process and why is it necessary?
Its result is the regulation of the balance of carbon dioxide and saturation of the air with oxygen. This process is known only to representatives of the world of flora, that is, plants, since it occurs only in their cells.
Photosynthesis itself is an extremely complex procedure, depending on certain conditions and occurring in several stages.
Definition of the concept
According to the scientific definition, they are converted to organic at the cellular level in autotrophic organisms due to exposure to light from the sun.
In more understandable terms, photosynthesis is a process in which the following occurs:
- The plant is saturated with moisture. The source of moisture can be water from the soil or humid tropical air.
- There is a reaction of chlorophyll (a special substance found in the plant) to the influence of solar energy.
- The formation of food necessary for the representatives of the flora, which they are not able to obtain on their own in a heterotrophic way, but they themselves are its producer. In other words, plants eat what they themselves produce. This is the result of photosynthesis.
Stage one
Almost every plant contains a green substance that allows it to absorb light. This substance is nothing more than chlorophyll. Its location is chloroplasts. But chloroplasts are located in the stem part of the plant and its fruits. But leaf photosynthesis is especially common in nature. Since the latter is quite simple in structure and has a relatively large surface, which means that the amount of energy required for the savior process to occur will be much greater.
When light is absorbed by chlorophyll, the latter is in a state of excitement and transmits its energy messages to other organic molecules of the plant. Largest quantity This energy goes to participants in the process of photosynthesis.
Stage two
The formation of photosynthesis at the second stage does not require the participation of light. It consists of forming chemical bonds using poisonous carbon dioxide produced from air masses and water. There is also a synthesis of many substances that ensure the vital activity of flora representatives. These are starch and glucose.
In plants, such organic elements act as a source of nutrition for individual parts plants, while ensuring the normal functioning of vital processes. Such substances are also obtained by representatives of the fauna that eat plants. The human body is saturated with these substances through food, which is included in the daily diet.
What? Where? When?
In order for organic matter to become organic, proper conditions for photosynthesis must be provided. For the process under consideration, light is required first of all. We are talking about both artificial and sunlight. In nature, plant activity is usually characterized by intensity in spring and summer, that is, when there is a need for food supply. large quantity solar energy. What can't you say about autumn time, when there is less and less light, the days get shorter. As a result, the foliage turns yellow and then falls off completely. But as soon as the first spring rays of the sun shine, green grass will sprout, chlorophylls will immediately resume their activity, and the active production of oxygen and other nutrients which are of vital importance.
The conditions for photosynthesis include not only the availability of light. There should also be enough moisture. After all, the plant first absorbs moisture, and then a reaction begins with the participation of solar energy. The result of this process is plant food.
Only in the presence of green matter does photosynthesis occur. we have already described above. They act as a kind of conductor between light or solar energy and the plant itself, ensuring the proper course of their life and activity. Green substances have the ability to absorb a lot of sunlight.
Oxygen also plays a significant role. For the process of photosynthesis to be successful, plants need a lot of it, since it contains only 0.03% carbonic acid. This means that from 20,000 m 3 of air, 6 m 3 of acid can be obtained. It is the last substance that is the main raw material for glucose, which, in turn, is a substance necessary for life.
There are two stages of photosynthesis. The first is called light, the second is dark.
What is the mechanism of the light stage?
The light stage of photosynthesis has another name - photochemical. The main participants at this stage are:
- energy of sun;
- various pigments.
Everything is clear with the first component, it is sunlight. But not everyone knows what pigments are. They come in green, yellow, red or blue. The green ones include chlorophylls of groups “A” and “B”, the yellow and red/blue ones include phycobilins, respectively. Among the participants in this stage of the process, only chlorophylls “A” exhibit photochemical activity. The rest play a complementary role, the essence of which is the collection of light quanta and their transportation to the photochemical center.
Since chlorophyll is endowed with the ability to efficiently absorb solar energy at a specific wavelength, the following photochemical systems have been identified:
Photochemical center 1 (green substances of group “A”) - the composition includes pigment 700, which absorbs light rays with a length of approximately 700 nm. This pigment plays a fundamental role in the creation of products of the light stage of photosynthesis.
Photochemical center 2 (green substances of group “B”) - the composition includes pigment 680, which absorbs light rays with a length of 680 nm. It plays a supporting role, consisting in the function of replenishing electrons lost by photochemical center 1. Achieved through hydrolysis of the liquid.
For 350-400 molecules of pigments that concentrate light fluxes in photosystems 1 and 2, there is only one molecule of pigment that is photochemically active - chlorophyll of group “A”.
What's happening?
1. The light energy absorbed by the plant affects the pigment 700 contained in it, which changes from a normal state to an excited state. The pigment loses an electron, resulting in the formation of a so-called electron hole. Next, the pigment molecule that has lost an electron can act as its acceptor, that is, the side that accepts the electron, and return to its shape.
2. The process of decomposition of liquid in the photochemical center of the light-absorbing pigment 680 of photosystem 2. During the decomposition of water, electrons are formed, which are initially accepted by a substance such as cytochrome C550 and are designated by the letter Q. Then, from the cytochrome, electrons enter the transport chain and are transported to photochemical center 1 for filling the electron hole, which was the result of the penetration of light quanta and the reduction process of pigment 700.
There are cases when such a molecule receives back an electron identical to the previous one. This will result in the release of light energy in the form of heat. But almost always an electron having negative charge, combines with special iron-sulfur proteins and is transferred along one of the chains to pigment 700 or enters another chain of transporters and is reunited with a permanent acceptor.
In the first option, cyclic electron transport of a closed type takes place, in the second - non-cyclic.
Both processes are catalyzed at the first stage of photosynthesis by the same chain of electron transporters. But it is worth noting that with cyclic photophosphorylation, the initial and at the same time the final point of transport is chlorophyll, while non-cyclic transport involves the transition of the green substance of group “B” to chlorophyll “A”.
Features of cyclic transportation
Cyclic phosphorylation is also called photosynthetic. As a result of this process, ATP molecules are formed. This transport is based on the return of electrons in an excited state to pigment 700 through several successive stages, as a result of which energy is released, which takes part in the work of the phosphorylating enzyme system for the purpose of further accumulation in the phosphate bonds of ATP. That is, energy is not dissipated.
Cyclic phosphorylation is the primary reaction of photosynthesis, which is based on the technology of generating chemical energy on the membrane surfaces of chloroplast thylactoids through the use of solar energy.
Without photosynthetic phosphorylation, assimilation reactions are impossible.
Nuances of non-cyclic transportation
The process involves the reduction of NADP+ and the formation of NADP*H. The mechanism is based on the transfer of an electron to ferredoxin, its reduction reaction and subsequent transition to NADP+ with further reduction to NADP*H.
As a result, the electrons that were lost by pigment 700 are replenished thanks to the electrons of water, which decomposes under light rays in photosystem 2.
The non-cyclic path of electrons, the course of which also involves light photosynthesis, is carried out through the interaction of both photosystems with each other, connecting their electron transport chains. Light energy directs the flow of electrons back. When transported from photochemical center 1 to center 2, electrons lose part of their energy due to accumulation as proton potential on the membrane surface of thylactoids.
In the dark phase of photosynthesis, the process of creating a proton-type potential in the electron transport chain and its operation for the formation of ATP in chloroplasts is almost completely identical to the same process in mitochondria. But the features are still present. In this situation, thylactoids are mitochondria that are turned inside out. This is what it is main reason the fact that electrons and protons move through the membrane in the opposite direction relative to the flow of transport in the mitochondrial membrane. Electrons are transported to the outside, and protons accumulate in the inside of the thylactoid matrix. The latter accepts only a positive charge, while the outer membrane of the thylactoid receives a negative charge. It follows that the proton gradient pathway is opposite to its pathway in mitochondria.
The next feature is the high pH level in the proton potential.
The third feature is the presence of only two conjugation sites in the thylactoid chain and, as a result, the ratio of the ATP molecule to protons is 1:3.
Conclusion
In the first stage, photosynthesis is the interaction of light energy (artificial and non-artificial) with the plant. Green substances - chlorophylls, most of which are found in leaves - react to rays.
The formation of ATP and NADP*H is the result of such a reaction. These products are necessary for dark reactions to occur. Consequently, the light stage is an obligatory process, without which the second stage, the dark stage, will not take place.
Dark stage: essence and features
Dark photosynthesis and its reactions are the process of converting carbon dioxide into substances of organic origin to produce carbohydrates. Such reactions occur in the stroma of the chloroplast and the products of the first stage of photosynthesis - light - take an active part in them.
The mechanism of the dark stage of photosynthesis is based on the process of assimilation (also called photochemical carboxylation, the Calvin cycle), which is characterized by cyclicity. Consists of three phases:
- Carboxylation - addition of CO 2.
- Recovery phase.
- Ribulose diphosphate regeneration phase.
Ribulophosphate - a sugar with five carbon atoms - is subject to phosphorylation by ATP, resulting in the formation of ribulose diphosphate, which is further carboxylated by combining with CO 2 a product with six carbons, which instantly decompose when interacting with a water molecule, creating two molecular particles of phosphoglyceric acid . Then this acid goes through a course full recovery when carrying out an enzymatic reaction, for which the presence of ATP and NADP is required to form a sugar with three carbons - three-carbon sugar, triose or phosphoglyceraldehyde. When two such trioses condense, a hexose molecule is produced, which can become integral part starch molecules and debug in reserve.
This phase ends with the absorption of one molecule of CO 2 during the process of photosynthesis and the use of three molecules of ATP and four atoms of H. Hexose phosphate is amenable to the reactions of the pentose phosphate cycle, resulting in the regeneration of ribulose phosphate, which can recombine with another molecule of carbon acid.
The reactions of carboxylation, reduction, and regeneration cannot be called specific exclusively to the cell in which photosynthesis occurs. It is also impossible to say what a “uniform” course of processes is, since a difference still exists - during the reduction process, NADP*H is used, and not NAD*H.
The addition of CO 2 by ribulose diphosphate is catalyzed by ribulose diphosphate carboxylase. The reaction product is 3-phosphoglycerate, which is reduced by NADP*H2 and ATP to glyceraldehyde-3-phosphate. The reduction process is catalyzed by glyceraldehyde-3-phosphate dehydrogenase. The latter is easily converted into dihydroxyacetone phosphate. Fructose bisphosphate is formed. Part of its molecules takes part in the regenerating process of ribulose diphosphate, closing the cycle, and the second part is used to create reserves of carbohydrates in photosynthetic cells, that is, photosynthesis of carbohydrates takes place.
Light energy is necessary for phosphorylation and synthesis of substances of organic origin, and the energy of oxidation of organic substances is necessary for oxidative phosphorylation. That is why vegetation provides life for animals and other organisms that are classified as heterotrophic.
Photosynthesis occurs in a plant cell in this way. Its product is carbohydrates necessary for the creation of carbon skeletons of many substances of representatives of the flora world, which are of organic origin.
Substances of the organic nitrogen type are absorbed in photosynthetic organisms by reducing inorganic nitrates, and sulfur is absorbed by reducing sulfates to sulfhydryl groups of amino acids. Photosynthesis ensures the formation of proteins, nucleic acids, lipids, carbohydrates, and cofactors. It has already been emphasized that this “assortment” of substances is vital for plants, but not a word has been said about the products of secondary synthesis, which are valuable medicinal substances (flavonoids, alkaloids, terpenes, polyphenols, steroids, organic acids and others). Therefore, without exaggeration we can say that photosynthesis is the key to the life of plants, animals and people.
Photosynthesis is a rather complex process and includes two phases: light, which always occurs exclusively in the light, and dark. All processes occur inside chloroplasts on special small organs - thylakoids. During the light phase, a quantum of light is absorbed by chlorophyll, resulting in the formation of ATP and NADPH molecules. The water then breaks down, forming hydrogen ions and releasing an oxygen molecule. The question arises, what are these incomprehensible mysterious substances: ATP and NADH?
ATP is a special organic molecule found in all living organisms and is often called the “energy” currency. It is these molecules that contain high-energy bonds and are the source of energy in any organic synthesis and chemical processes in the body. Well, NADPH is actually a source of hydrogen, it is used directly in the synthesis of high-molecular organic substances - carbohydrates, which occurs in the second, dark phase of photosynthesis using carbon dioxide. But let's take things in order.
Light phase of photosynthesis
Chloroplasts contain a lot of chlorophyll molecules, and they all absorb sunlight. At the same time, light is absorbed by other pigments, but they cannot carry out photosynthesis. The process itself occurs only in some chlorophyll molecules, of which there are very few. Other molecules of chlorophyll, carotenoids and other substances form special antenna and light-harvesting complexes (LHC). They, like antennas, absorb light quanta and transmit excitation to special reaction centers or traps. These centers are located in photosystems, of which plants have two: photosystem II and photosystem I. They contain special chlorophyll molecules: respectively, in photosystem II - P680, and in photosystem I - P700. They absorb light of exactly this wavelength (680 and 700 nm).
The diagram makes it more clear how everything looks and happens during the light phase of photosynthesis.
In the figure we see two photosystems with chlorophylls P680 and P700. The figure also shows the carriers through which electron transport occurs.
So: both chlorophyll molecules of two photosystems absorb a light quantum and become excited. The electron e- (red in the figure) moves to a higher energy level.
Excited electrons have a very high energy, they break off and enter a special chain of transporters, which is located in the membranes of thylakoids - the internal structures of chloroplasts. The figure shows that from photosystem II from chlorophyll P680 an electron goes to plastoquinone, and from photosystem I from chlorophyll P700 to ferredoxin. In the chlorophyll molecules themselves, in place of electrons after their removal, blue holes with a positive charge are formed. What to do?
To compensate for the lack of an electron, the chlorophyll P680 molecule of photosystem II accepts electrons from water, and hydrogen ions are formed. In addition, it is due to the breakdown of water that oxygen is released into the atmosphere. And the chlorophyll P700 molecule, as can be seen from the figure, makes up for the lack of electrons through a system of carriers from photosystem II.
In general, no matter how difficult it may be, this is exactly how the light phase of photosynthesis proceeds, its the main point involves the transfer of electrons. You can also see from the figure that in parallel with electron transport, hydrogen ions H+ move through the membrane, and they accumulate inside the thylakoid. Since there are a lot of them there, they move outward with the help of a special conjugating factor, which in the figure orange color, is pictured on the right and looks like a mushroom.
Finally, we see the final step of electron transport, which results in the formation of the aforementioned NADH compound. And due to the transfer of H+ ions, energy currency is synthesized - ATP (seen on the right in the figure).
So, the light phase of photosynthesis is completed, oxygen is released into the atmosphere, ATP and NADH are formed. What's next? Where is the promised organic matter? And then comes the dark stage, which consists mainly of chemical processes.
Dark phase of photosynthesis
For the dark phase of photosynthesis, carbon dioxide – CO2 – is an essential component. Therefore, the plant must constantly absorb it from the atmosphere. For this purpose, there are special structures on the surface of the leaf - stomata. When they open, CO2 enters the leaf, dissolves in water and reacts with the light phase of photosynthesis.
During the light phase in most plants, CO2 binds to a five-carbon organic compound (which is a chain of five carbon molecules), resulting in the formation of two molecules of a three-carbon compound (3-phosphoglyceric acid). Because The primary result is precisely these three-carbon compounds; plants with this type of photosynthesis are called C3 plants.
Further synthesis occurring in chloroplasts is quite complex. Ultimately, a six-carbon compound is formed, from which glucose, sucrose or starch can then be synthesized. It is in the form of these organic substances that the plant accumulates energy. Only a small part of them remains in the sheet and is used for its needs. The rest of the carbohydrates travel throughout the plant and go exactly where energy is needed most, for example, at the growth points.
PHOTOSYNTHESIS is
photosynthesis is carbohydrates.
I Light phase
1. Photophysical stage
2. Photochemical stage
II Dark phase
3.
MEANING
4. Ozone screen.
Pigments of photosynthetic plants, their physiological role.
· Chlorophyll - This green pigment that provides color Green colour plant, with its participation the process of photosynthesis is determined. By chemical structure it is a Mg complex of various tetrapyrroles. Chlorophylls have a porphyrin structure and are structurally close to heme.
In the pyrrole groups of chlorophyll there are systems of alternating double and simple connections. This is the chromophore group of chlorophyll, which determines the absorption of certain rays of the solar spectrum and its color. D porphyry cores are 10 nm and the length of the phytol residue is 2 nm.
Chlorophyll molecules are polar, its porphyrin core has hydrophilic properties, and the phytol end is hydrophobic. This property of the chlorophyll molecule determines its specific location in the chloroplast membranes.
The porphyrin part of the molecule is associated with protein, and the phytol part is immersed in the lipid layer.
Chlorophyll of a living intact cell has the ability to reversibly photooxidize and photoreduce. The ability for redox reactions is associated with the presence in the chlorophyll molecule of conjugated double bonds with mobile p-electrons and N atoms with undefined electrons.
PHYSIOLOGICAL ROLE
1) selectively absorb light energy,
2) store it in the form of electronic excitation energy,
3) photochemically convert the energy of the excited state into the chemical energy of primary photo-reduced and photo-oxidized compounds.
· Carotenoids - This fat-soluble pigments of yellow, orange, and red colors are present in the chloroplasts of all plants. Carotenoids are found in all higher plants and many microorganisms. These are the most common pigments with a variety of functions. Carotenoids have maximum absorption in the violet-blue and blue parts of the light spectrum. They are not capable of fluorescence, unlike chlorophyll.
Carotenoids include 3 groups of compounds:
Orange or red carotenes;
Yellow xanthophylls;
Carotenoid acids.
PHYSIOLOGICAL ROLE
1) Light absorption as additional pigments;
2) Protection of chlorophyll molecules from irreversible photo-oxidation;
3) Quenching of active radicals;
4) Participate in phototropism, because contribute to the direction of shoot growth.
· Phycobilins - This red and blue pigments found in cyanobacteria and some algae. Phycobilins consist of 4 consecutive pyrrole rings. Phycobilins are chromophoric groups of globulin proteins called phycobilin proteins. They are divided into:
- phycoerythrins – red whites;
- phycocyanin – blue squirrels;
- alophycocyanin – blue squirrels.
All of them have fluorescent ability. Phycobilins have maximum absorption in the orange, yellow and green parts of the light spectrum and allow algae to more fully utilize the light penetrating the water.
At a depth of 30 m, red rays completely disappear
At a depth of 180 m - yellow
At a depth of 320 m – green
At a depth of more than 500 m, blue and violet rays do not penetrate.
Phycobilins are additional pigments; approximately 90% of the light energy absorbed by phycobilins is transferred to chlorophyll.
PHYSIOLOGICAL ROLE
1) The light absorption maxima of phycobilins are located between the two absorption maxima of chlorophyll: in the orange, yellow and green parts of the spectrum.
2) Phycobilins perform the functions of a light-harvesting complex in algae.
3) Plants have phycobilin-phytochrome; it is not involved in photosynthesis, but is a red light photoreceptor and performs a regulatory function in plant cells.
The essence of the photophysical stage. Photochemical stage. Cyclic and non-cyclic electron transport.
The essence of the photophysical stage
The photophysical stage is the most important, because carries out the transition and transformation of energy from one system to another (living from non-living).
Photochemical stage
Photo-chemical reactions of photosynthesis- these are reactions in which light energy is converted into the energy of chemical bonds, primarily into the energy of phosphorus bonds ATP. It is ATP that ensures the course of all processes; at the same time, under the influence of light, water decomposes and a reduced product is formed. NADP and stands out O2.
The energy of absorbed light quanta flows from hundreds of pigment molecules of the light-harvesting complex to one chlorophyll-trap molecule, giving an electron to the acceptor - oxidizing. The electron enters the electron transport chain; it is assumed that the light-harvesting complex consists of 3 parts:
main antenna component
· two photo fixing systems.
The antenna chlorophyll complex is immersed in the thickness of the thylakoid membrane of chloroplasts; the combination of antenna pigment molecules and the reaction center constitutes the photosystem in the process of photosynthesis 2 photosystems take part:
· it has been established that photosystem 1 includes light-focusing pigments and reaction center 1,
· photosystem 2 includes light-focusing pigments And reaction center 2.
Chlorophyll trap photosystem 1 absorbs light from a long wavelength 700nm. In the second system 680nm. Light is absorbed separately by these two photosystems, and normal photosynthesis requires their simultaneous participation. Transfer along a chain of carriers involves a series of redox reactions in which either a hydrogen atom or electrons are transferred.
There are two types of electron flow:
· cyclical
· non-cyclical.
With a cyclic flow of electrons from a chlorophyll molecule are transferred to the acceptor from the chlorophyll molecule and return to it back , with non-cyclic flow photo-oxidation of water occurs and electron transfer from water to NADP The energy released during redox reactions is partially used for the synthesis of ATP.
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 (P700). After excitation by a light quantum, it restores the primary acceptor - chlorophyll a, which restores the secondary one (vitamin K 1 or phylloquinone), after which the electron is transferred to ferredoxin, which reduces 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 intrathylakoid space and transfers an electron to the oxidized P700.
Photosystem II
A photosystem is a set of SSCs, a photochemical reaction center and electron carriers. Light-harvesting complex II contains 200 molecules of chlorophyll a, 100 molecules of chlorophyll b, 50 molecules of carotenoids and 2 molecules of pheophytin. The reaction center of photosystem II is a pigment-protein complex located in thylakoid membranes and surrounded by SSC. It contains a dimer of chlorophyll a with an absorption maximum at 680 nm (P680). The energy of a light quantum from the SSC is ultimately transferred to it, as a result of which one of the electrons moves to a higher energy state, its connection with the nucleus is weakened and the excited P680 molecule becomes a strong reducing agent (E0 = -0.7 V).
P680 reduces pheophytin, then the electron is transferred to quinones that are part of PS II and then to plastoquinones, transported in reduced form to the b6f complex. One plastoquinone molecule carries 2 electrons and 2 protons, which are taken from the stroma.
The filling of the electron vacancy in the P680 molecule occurs due to water. PS II includes a water-oxidizing complex containing 4 manganese ions in the active center. To form one oxygen molecule, two water molecules are required, giving 4 electrons. Therefore, the process is carried out in 4 cycles and for its complete implementation 4 quanta of light are required. The complex is located on the side of the intrathylakoid space and the resulting 4 protons are released into it.
Thus, the total result of the work of PS II is the oxidation of 2 water molecules with the help of 4 light quanta with the formation of 4 protons in the intrathylakoid space and 2 reduced plastoquinones in the membrane.
Photosynthetic phosphorylation. The mechanism of coupling of electron transport with the formation of a transmembrane gradient of electrochemical potential. Structural and functional organization and mechanism of operation of the ATP synthetase complex.
Photosynthetic phosphorylation- synthesis of ATP from ADP and inorganic phosphorus in chloroplasts, coupled with light-induced electron transport.
According to the two types of electron flow, cyclic and non-cyclic photophosphorylation are distinguished.
The transfer of electrons along the cyclic flow chain is associated with the synthesis of two high-energy ATP bonds. All light energy absorbed by the pigment of the reaction center of photosystem I is spent only on the synthesis of ATP. With cyclic F. f. no reducing equivalents for the carbon cycle are formed and no O2 is released. Cyclic f. f. described by the equation:
Non-cyclic f. f. associated with the flow of electrons from water through the transporters of photosystems I and II NADP +. Light energy in this process is stored in high-energy bonds of ATP, the reduced form of NADPH2 and molecular oxygen. The overall equation of a non-cyclic functional function. following:
The mechanism of coupling of electron transport with the formation of a transmembrane gradient of electrochemical potential
Chemosmotic theory. Electron carriers are localized asymmetrically in membranes. In this case, electron carriers (cytochromes) alternate with electron and proton carriers (plastoquinones). The plastoquinone molecule first accepts two electrons: HRP + 2e - -> HRP -2.
Plastoquinone is a derivative of quinone, in a fully oxidized state it contains two oxygen atoms connected to the carbon ring by double bonds. In a completely reduced state, the oxygen atoms in the benzene ring combine with protons: to form an electrically neutral form: PX -2 + 2H + -> PCN 2. Protons are released into the space within the thylakoid. Thus, when a pair of electrons is transferred from Chl 680 to Chl 700, internal space protons accumulate in thylakoids. As a result of the active transfer of protons from the stroma to the intrathylakoid space, an electrochemical potential of hydrogen (ΔμH +) is created on the membrane, which has two components: chemical ΔμH (concentration), resulting from the uneven distribution of H + ions on different sides of the membrane, and electrical, due to the opposite charge different sides of the membrane (due to the accumulation of protons from inside membranes).
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Structural and functional organization and mechanism of operation of the ATP synthetase complex
Structural and functional organization. The conjugation of proton diffusion through the membrane is carried out by a macromolecular enzyme complex called ATP synthase or coupling factor. This complex is shaped like a mushroom and consists of two parts - coupling factors: a round cap F1 protruding from outside membrane (the catalytic center of the enzyme is located in it), and the legs are immersed in the membrane. The membrane part consists of polypeptide subunits and forms a proton channel in the membrane through which hydrogen ions enter the conjugation factor F1. The F 1 protein is a protein complex that consists of a membrane, while it retains the ability to catalyze the hydrolysis of ATP. Isolated F 1 is not able to synthesize ATP. The ability to synthesize ATP is a property of a single F 0 -F 1 complex embedded in the membrane. This is due to the fact that the work of ATP synthase during ATP synthesis is associated with the transfer of protons through it. Directed transport of protons is only possible if ATP synthase is embedded in the membrane.
Mechanism of operation. There are two hypotheses regarding the mechanism of phosphorylation (direct mechanism and indirect). According to the first hypothesis, the phosphate group and ADP bind to the enzyme in active site complex F1. Two protons move through the channel along the concentration gradient and combine with the phosphate oxygen to form water. According to the second hypothesis (indirect mechanism), ADP and inorganic phosphorus combine spontaneously in the active site of the enzyme. However, the resulting ATP is tightly bound to the enzyme, and energy is required to release it. Energy is delivered by protons, which bind to the enzyme, change its conformation, after which ATP is released.
C4 photosynthesis pathway
C 4-pathway of photosynthesis or Hatch-Slack cycle
Australian scientists M. Hatch and K. Slack described the C 4 photosynthetic pathway, characteristic of tropical and subtropical plants of monocotyledons and dicotyledons of 16 families (sugar cane, corn, etc.). Most of the worst weeds are C4 plants, and most crops are C3 plants. The leaves of these plants contain two types of chloroplasts: ordinary ones in the mesophyll cells and large chloroplasts that do not have grana and photosystem II in the sheath cells surrounding the vascular bundles.
In the cytoplasm of mesophyll cells, phosphoenolpyruvate carboxylase adds CO 2 to phosphoenolpyruvic acid, forming oxaloacetic acid. It is transported to chloroplasts, where it is reduced to malic acid with the participation of NADPH (NADP+-dependent malate dehydrogenase enzyme). In the presence of ammonium ions, oxaloacetic acid is converted to aspartic acid (the enzyme aspartate aminotransferase). Malic and (or) aspartic acids pass into the chloroplasts of the sheath cells and are decarboxylated to pyruvic acid and CO 2 . CO 2 is included in the Calvin cycle, and pyruvic acid is transferred to mesophyll cells, where it is converted into phosphoenolpyruvic acid.
Depending on which acid - malate or aspartate - is transported into the sheath cells, plants are divided into two types: malate and aspartate. In the sheath cells, these C4 acids are decarboxylated, which occurs in different plants occurs with the participation of various enzymes: NADP+-dependent decarboxylating malate dehydrogenase (NADP+-MDH), NAD+-dependent decarboxylating malate dehydrogenase (malic enzyme, NAD+-MDH) and PEP-carboxykinase (PEP-KK). Therefore, plants are divided into three more subtypes: NADP + -MDG plants, NAD + -MDG plants, FEP-KK plants.
This mechanism allows plants to photosynthesize when closed due to high temperature stomata. In addition, the products of the Calvin cycle are formed in the chloroplasts of the sheath cells surrounding the vascular bundles. This promotes the rapid outflow of photoassimilates and thereby increases the intensity of photosynthesis.
Photosynthesis according to the type of Crassulaceae (succulents) is THE way.
In dry places, there are succulent plants whose stomata are open at night and closed during the day to reduce transpiration. Currently, this type of photosynthesis is found in representatives of 25 families.
In succulents (cacti and plants of the Crassulaceae family ( Crassulaceae) the processes of photosynthesis are separated not in space, as in other C4 plants, but in time. This type of photosynthesis is called the CAM (crassulation acid metabolism) pathway. The stomata are usually closed during the day, preventing loss of water through transpiration, and open at night. In the dark, CO 2 enters the leaves, where phosphoenolpyruvate carboxylase combines it with phosphoenolpyruvic acid, forming oxaloacetic acid. It is reduced by NADPH-dependent malate dehydrogenase to malic acid, which accumulates in vacuoles. During the day, malic acid passes from the vacuole into the cytoplasm, where it is decarboxylated to form CO 2 and pyruvic acid. CO 2 diffuses into chloroplasts and enters the Calvin cycle.
So, the dark phase of photosynthesis is divided in time: CO 2 is absorbed at night, and is restored during the day, malate is formed from PAL, carboxylation in tissues occurs twice: PEP is carboxylated at night, RuBP is carboxylated during the day.
CAM plants are divided into two types: NADP-MDG plants, PEP-KK plants.
Like C4, the CAM type is additional, supplying CO 2 to the C3 cycle in plants adapted to living in conditions of elevated temperatures or lack of moisture. In some plants this cycle always functions, in others it only functions under unfavorable conditions.
Photorespiration.
Photorespiration is a light-activated process of release of CO 2 and absorption of O 2. (NOT RELATED TO PHOTOSYNTHESIS OR RESPIRATION). Since the primary product of photorespiration is glycolic acid, it is also called the glycolate pathway. Photorespiration increases with low CO 2 content and high O 2 concentration in the air. Under these conditions, chloroplast ribulose bisphate carboxylase catalyzes not the carboxylation of ribulose-1,5-bisphosphate, but its cleavage into 3-phosphoglyceric and 2-phosphoglycolic acids. The latter is dephosphorylated to form glycolic acid.
Glycolic acid passes from the chloroplast to the peroxisome, where it is oxidized by glycolate oxidase to glyoxylic acid. The resulting hydrogen peroxide is decomposed by catalase present in the peroxisome. Glyoxylic acid is aminated to form glycine. Glycine is transported to the mitochondrion, where serine is synthesized from two glycine molecules and CO 2 is released.
Serine can enter the peroxisome and, under the action of aminotransferase, transfers the amino group to pyruvic acid to form alanine, and itself is converted into hydroxypyruvic acid. The latter, with the participation of NADPH, is reduced to glyceric acid. It passes into chloroplasts, where it is included in the Calvin cycle and 3 PHAs are formed.
Plant respiration
A living cell is an open energy system; it lives and maintains its individuality due to a constant flow of energy. As soon as this influx stops, disorganization and death of the body occurs. The energy of sunlight stored in organic matter during photosynthesis is again released and used for a variety of life processes.
In nature, there are two main processes during which the energy of sunlight stored in organic matter is released: respiration and fermentation. Respiration is the aerobic oxidative breakdown of organic compounds into simple inorganic compounds, accompanied by the release of energy. Fermentation is an anaerobic process of decomposition of organic compounds into simpler ones, accompanied by the release of energy. In the case of respiration, the electron acceptor is oxygen, in the case of fermentation, organic compounds.
The overall equation for the breathing process is:
С6Н1206 + 602 -> 6С02 + 6Н20 + 2824 kJ.
Respiratory pathways
There are two main systems and two main pathways for the transformation of the respiratory substrate, or the oxidation of carbohydrates:
1) glycolysis + Krebs cycle (glycolytic); This pathway of respiratory exchange is the most common and, in turn, consists of two phases. The first phase is anaerobic (glycolysis), the second phase is aerobic. These phases are localized in different cell compartments. The anaerobic phase of glycolysis is in the cytoplasm, the aerobic phase is in the mitochondria. Usually, the chemistry of respiration begins to be considered with glucose. At the same time, there is little glucose in plant cells, since the end products of photosynthesis are sucrose as the main transport form of sugar in the plant or reserve carbohydrates (starch, etc.). Therefore, to become a substrate for respiration, sucrose and starch must be hydrolyzed to form glucose.
2) pentose phosphate (apotomic). The relative roles of these respiratory pathways may vary depending on plant type, age, developmental stage, and depending on environmental factors. The process of plant respiration occurs in all external conditions under which life is possible. The plant organism does not have adaptations to regulate temperature, so the respiration process occurs at temperatures from -50 to +50°C. Plants also lack adaptations to maintain uniform distribution of oxygen throughout all tissues. It was the need to carry out the breathing process under various conditions that led to the development in the process of evolution of various respiratory metabolic pathways and to an even greater variety of enzyme systems that carry out individual stages breathing. It is important to note the interconnection of all metabolic processes in the body. Changing the respiratory metabolic pathway leads to profound changes in the entire metabolism of plants.
Energy
11 ATP is formed as a result of the work of CK and respiratory and 1 ATP as a result of substrate phosphorylation. During this reaction, one molecule of GTP is formed (the rephosphorylation reaction leads to the formation of ATP).
1 turnover of CK under aerobic conditions leads to the formation of 12 ATP
Integrative
At the level of the CK, the catabolism pathways of proteins, fats and carbohydrates are combined. The Krebs cycle is a central metabolic pathway that combines the processes of breakdown and synthesis of essential cell components.
Amphibolic
Metabolites of CK are key; at their level, they can switch from one type of metabolism to another.
13.ETC: Components localization. The mechanism of oxidative phosphorylation. Mitchell's chemiosmotic theory.
Electron transport chain- this is a chain of redox agents located in a certain way in the membrane of chloroplasts, carrying out photoinduced electron transport from water to NADP +. The driving force for electron transport through the ETC of photosynthesis is redox reactions in the reaction centers (RC) of two photosystems (PS). The primary separation of charges in the PS1 RC leads to the formation of a strong reducing agent A0, the redox potential of which ensures the reduction of NADP + through a chain of intermediate carriers. In RC PS2, photochemical reactions lead to the formation of a strong oxidizing agent P680, which causes a series of redox reactions leading to the oxidation of water and the release of oxygen. The reduction of P700 formed in the PS1 RC occurs due to electrons mobilized from water by photosystem II, with the participation of intermediate electron carriers (plastoquinones, redox cofactors of the cytochrome complex and plastocyanin). In contrast to the primary photoinduced reactions of charge separation in reaction centers, going against the thermodynamic gradient, electron transfer in other parts of the ETC occurs along the gradient of the redox potential and is accompanied by the release of energy, which is used for the synthesis of ATP.
The components of the mitochondrial ETC are arranged in the following order:
A pair of electrons from NADH or succinate is transferred along the ETC to oxygen, which, being reduced and adding two protons, forms water.
Definition and general characteristics of photosynthesis, the meaning of photosynthesis
PHOTOSYNTHESIS is the process of formation of organic substances from CO2 and H2O in the light, with the participation of photosynthetic pigments.
From a biochemical point of view, photosynthesis is redox process of transformation of stable molecules of inorganic substances CO2 and H2O into molecules of organic substances – carbohydrates.
general characteristics
6CO 2 + 6H 2 O → C 6 H 12 O 6 + O 2
The process of photosynthesis consists of two phases and several stages that occur sequentially.
I Light phase
1. Photophysical stage- occurs in inner membrane chloroplasts and is associated with the absorption of solar energy by pigment systems.
2. Photochemical stage- takes place in the inner membrane of chloroplasts and is associated with the conversion of solar energy into chemical energy ATP and NADPH2 and photolysis of water.
II Dark phase
3. Biochemical stage or Calvin cycle- takes place in the stroma of chloroplasts. At this stage, carbon dioxide is reduced to carbohydrates.
MEANING
1. Ensuring the constancy of CO2 in the air. The binding of CO 2 during photosynthesis largely compensates for its release as a result of other processes (respiration, fermentation, volcanic activity, industrial activity of mankind).
2. Prevents the development of the greenhouse effect. Some sunlight is reflected from the Earth's surface in the form of thermal infrared rays. CO 2 absorbs infrared radiation and thereby retains heat on the Earth. An increase in CO 2 content in the atmosphere can contribute to an increase in temperature, that is, create Greenhouse effect. However, the high content of CO 2 in the air activates photosynthesis and, therefore, the concentration of CO 2 in the air will decrease again.
3. Accumulation of oxygen in the atmosphere. Initially, there was very little oxygen in the Earth's atmosphere. Now its content is 21% by volume of air. Basically, this oxygen is a product of photosynthesis.
4. Ozone screen. Ozone (O 3) is formed as a result of photodissociation of oxygen molecules under the influence of solar radiation at an altitude of about 25 km. Protects all life on Earth from destructive rays.