Life on Earth is possible thanks mainly to light solar energy. This energy is converted into the energy of chemical bonds organic matter, formed during the process of photosynthesis.
All plants and some prokaryotes (photosynthetic bacteria and blue green algae). Such organisms are called phototrophs . The energy for photosynthesis comes from light, which is captured by special molecules called photosynthetic pigments. Since only a certain wavelength of light is absorbed, some of the light waves are not absorbed but reflected. Depending on the spectral composition of the reflected light, the pigments acquire color - green, yellow, red, etc.
There are three types of photosynthetic pigments - chlorophylls, carotenoids and phycobilins . The most important pigment is chlorophyll. The base is a flat porphyrin core formed by four pyrrole rings connected by methyl bridges, with a magnesium atom in the center. There are various type-a chlorophylls. U higher plants, green and euglenophyte algae have chlorophyll-B, which is formed from chlorophyll A. Brown and diatoms Instead of chlorophyll-B, they contain chlorophyll-C, and red algae contain chlorophyll-D. Another group of pigments is formed by carotenoids, which range in color from yellow to red. They are found in all colored plastids (chloroplasts, chromoplasts) of plants. Moreover, in the green parts of plants, chlorophyll masks carotenoids, making them invisible until the onset of cold weather. In autumn, the green pigments are destroyed and carotenoids become clearly visible. Carotenoids are synthesized by phototrophic bacteria and fungi. Phycobilins are present in red algae and cyanobacteria.
Light stage of photosynthesis
Chlorophylls and other pigments in chloroplasts form specific light-harvesting complexes . Using electromagnetic resonance, they transfer the collected energy to special chlorophyll molecules. These molecules, under the influence of excitation energy, give electrons to molecules of other substances - vectors , and then take away electrons from proteins and then from water. The splitting of water during photosynthesis is called photolysis . This occurs in the thylakoid cavities. Protons pass through special channels into the stroma. This releases the energy necessary for ATP synthesis:
2H 2 O = 4e + 4H + + O 2
ADP + P = ATP
The participation of light energy here is prerequisite, therefore this stage is called the light stage. Oxygen produced as a by-product is removed outside and used by the cell for respiration.
Dark stage of photosynthesis
The following reactions take place in the stroma of the chloroplast. From carbon dioxide and water, monosaccharides are formed. On my own this process contradicts the laws of thermodynamics, but since ATP molecules are involved, due to this energy the synthesis of glucose is real process. Later, polysaccharides are created from its molecules - cellulose, starch and other complex organic molecules. The overall equation for photosynthesis can be represented as follows:
6CO 2 + 6H 2 O = C 6 H 12 O 6 + 6O 2
Especially a lot of starch is deposited in chloroplasts during the day during intense photosynthetic processes; at night, starch is broken down into soluble forms and used by the plant.
Would you like to understand this or another biology topic in more detail? to the author of this article, Vladimir Smirnov.
The article is an excerpt from Vladimir Smirnov’s work “Genesis”; any copying and use of the material must include attribution.
We also suggest watching a video lesson about photosynthesis from our botanist Irina:
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Basic concepts and key terms: photosynthesis. Chlorophyll. Light phase. Dark phase.
Remember! What is plastic exchange?
Think!
Green color quite often mentioned in the poems of poets. So, Bogdan-Igor Antonich has the lines: “... poetry ebullient and wise, like greenery,” “... a blizzard of greenery, a fire of greenery,”
"...the green flood rises from the vegetable rivers." Green is the color of renewal, a symbol of youth, tranquility, and the color of nature.
Why are plants green?
What are the conditions for photosynthesis?
Photosynthesis (from the Greek photo - light, synthesis - combination) is an extremely complex set of plastic metabolic processes. Scientists distinguish three types of photosynthesis: oxygen (with the release of molecular oxygen in plants and cyanobacteria), oxygen-free (with the participation of bacteriochlorophyll under anaerobic conditions without the release of oxygen in photobacteria) and chlorophyll-free (with the participation of bacterial rhodopsins in archaea). At a depth of 2.4 km, green sulfur bacteria GSB1 were found, which, instead of sunlight use weak rays black smokers. But, as K. Swenson wrote in a monograph on cells: “The primary source of energy for living nature is the energy of visible light.”
The most common in living nature is oxygen photosynthesis, which requires light energy, carbon dioxide, water, enzymes and chlorophyll. Light for photosynthesis is absorbed by chlorophyll, water is delivered into cells through the pores cell wall, carbon dioxide enters cells by diffusion.
The main photosynthetic pigments are chlorophylls. Chlorophylls (from the Greek chloros - green and phylon - leaf) are green plant pigments, with the participation of which photosynthesis occurs. The green color of chlorophyll is an adaptation for absorbing blue rays and partially red ones. A green rays are reflected from the body of plants, enter the retina of the human eye, irritate the cones and cause colored visual sensations. That's why plants are green!
In addition to chlorophylls, plants have auxiliary carotenoids, and cyanobacteria and red algae have phycobilins. Greens
and purple bacteria contain bacteriochlorophylls that absorb blue, violet and even infrared rays.
Photosynthesis occurs in higher plants, algae, cyanobacteria, and some archaea, that is, in organisms known as photo-autotrophs. Photosynthesis in plants occurs in chloroplasts, in cyanobacteria and photobacteria - on internal invaginations of membranes with photopigments.
So, PHOTOSYNTHESIS is the process of formation of organic compounds from inorganic ones using light energy and with the participation of photosynthetic pigments.
What are the features of the light and dark phases of photosynthesis?
In the process of photosynthesis, two stages are distinguished - light and dark phases (Fig. 49).
The light phase of photosynthesis occurs in the grana of chloroplasts with the participation of light. This stage begins from the moment light quanta are absorbed by a chlorophyll molecule. In this case, the electrons of the magnesium atom in the chlorophyll molecule move to a higher energy level, accumulating potential energy. A significant part of the excited electrons transfers them to others chemical compounds for the formation of ATP and the reduction of NADP (nicotinamide adenine dinucleotide phosphate). This compound with such a long name is a universal biological carrier of hydrogen in the cell. Under the influence of light, the process of water decomposition occurs - photolysis. In this case, electrons (e“), protons (H+) and, as a by-product, molecular oxygen are formed. Hydrogen protons H+, adding electrons with a high energy level, are converted into atomic hydrogen, which is used to reduce NADP+ to NADP. N. Thus, the main processes of the light phase are: 1) photolysis of water (splitting of water under the influence of light with the formation of oxygen); 2) reduction of NADP (addition of a hydrogen atom to NADP); 3) photophosphorylation (formation of ATP from ADP).
So, the light phase is a set of processes that ensure the formation of molecular oxygen, atomic hydrogen and ATP due to light energy.
The dark phase of photosynthesis occurs in the stroma of chloroplasts. Its processes do not depend on light and can occur both in the light and in the dark, depending on the cell’s needs for glucose. The dark phase is based on cyclic reactions called the carbon dioxide fixation cycle, or the Calvin cycle. This process was first studied by the American biochemist Melvin Calvin (1911 - 1997), laureate Nobel Prize in chemistry (1961). In the dark phase, glucose is synthesized from carbon dioxide, hydrogen from NADP and ATP energy. CO 2 fixation reactions are catalyzed by ribulose bisphosphate carboxylase (Rubisco), the most common enzyme on Earth.
So, the dark phase is a set of cyclic reactions that, thanks to the chemical energy of ATP, ensure the formation of glucose using carbon dioxide, which is a source of carbon, and water, a source of hydrogen.
What is the planetary role of photosynthesis?
The importance of photosynthesis for the biosphere is difficult to overestimate. It is thanks to this process that the light energy of the Sun is converted by photo-autotrophs into chemical energy carbohydrates, which generally provide primary organic matter. This is where the food chains begin, through which energy is transferred to heterotrophic organisms. Plants serve as food for herbivores, which thereby obtain the necessary nutrients. Then herbivores become food for predators; they also need energy, without which life is impossible.
Only a small part of the sun's energy is captured by plants and used for photosynthesis. The sun's energy is mainly used to evaporate and maintain temperature regime earth's surface. So, only about 40 - 50% of the Sun's energy penetrates the biosphere, and only 1 - 2% of solar energy is converted into synthesized organic matter.
Green plants and cyanobacteria affect gas composition atmosphere. All the oxygen in the modern atmosphere is a product of photosynthesis. The formation of the atmosphere completely changed the state of the earth's surface, made possible appearance aerobic respiration. Later in the process of evolution, after the formation of the ozone layer, living organisms reached land. In addition, photosynthesis prevents the accumulation of CO 2 and protects the planet from overheating.
So photosynthesis has planetary significance, ensuring the existence of living nature on planet Earth.
ACTIVITY Matching task
Using the table, compare photosynthesis with aerobic respiration and draw a conclusion about the relationship between plastic and energy metabolism.
COMPARATIVE CHARACTERISTICS OF PHOTOSYNTHESIS AND AEROBIC RESPIRATION
Application of knowledge task
Recognize and name the levels of organization of the photosynthesis process in plants. Name the devices plant organism to photosynthesis different levels his organization.
RELATIONSHIP Biology + Literature
K. A. Timiryazev (1843 - 1920), one of the most famous researchers photosynthesis, wrote: “The microscopic green grain of chlorophyll is a focus, a point in cosmic space into which the energy of the Sun flows from one end, and from the other all manifestations of life on Earth originate. It is a real Prometheus, who stole fire from the sky. The ray of the sun he stole burns both in the flickering abyss and in the dazzling spark of electricity. A ray of sun sets in motion the flywheel of a giant steam engine, an artist’s brush, and a poet’s pen.” Apply your knowledge and prove the statement that the ray of the Sun sets the poet's pen in motion.
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Self-control tasks |
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1. What is photosynthesis? 2. What is chlorophyll? 3. What is the light phase of photosynthesis? 4. What is the dark phase of photosynthesis? 5. What is primary organic matter? 6. How photosynthesis determines aerobic respiration organisms? |
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7. What are the conditions for photosynthesis? 8. What are the features of the light and dark phases of photosynthesis? 9. What is the planetary role of photosynthesis? |
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10. What are the similarities and differences between photosynthesis and aerobic respiration? |
This is textbook material
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 a source of energy at any organic syntheses And chemical processes in organism. 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 thylakoid membranes - internal structures 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 the electrons after their removal, blue holes are formed with positive charge. 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 the five-carbon organic compound(which is a chain of five carbon molecules), resulting in two molecules of a three-carbon compound (3-phosphoglyceric acid). Because primary result These are 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 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, since there are several different paths 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. Everything that unites these different processes under the term “photosynthesis” - in all of them in total photon energy is converted into a chemical bond. For comparison: during chemosynthesis, energy is converted chemical bond some compounds (inorganic) to 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. U certain groups In 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), which already includes 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 that most of triose phosphate is spent on the regeneration of ribulose biphosphate (RiBP), which is again used to bind 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 carbon dioxide 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. Except TF important role FGK plays. However, such reactions occur not only in photosynthetic organisms. In this sense, the dark phase of photosynthesis is the same as the Calvin cycle.
From FGK by stepwise enzymatic catalysis six-carbon sugar is formed 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 environment, the less efficient the process of CO 2 binding. 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 ready for a large number 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 and binds to organic acids, in this form is transported into 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 lies in more efficient fixation of carbon dioxide, an increase in its concentration in the sheath cells and, therefore, more efficient work 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 the lining cells of the vascular bundles. 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.
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. Chemical reactions The dark phases of photosynthesis differ significantly from the reactions of the light phase, but the dark and light phases of photosynthesis depend on each other.
The 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 transform into active state, leave the pigment molecule and end up on outside 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 |
| Dependency 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