C3 and c4 plants comparison. C4 photosynthetic pathway (hatch and slack cycle)

Studies have shown that in plants in which the process of photosynthesis proceeds along the c4 pathway, there are two types of cells and chloroplasts:

1) small granal plastids in leaf mesophyll cells

2) large plastids, often lacking grana, in the sheath cells surrounding the vascular bundles.

The sheath cells have thickened cell walls, contain a large number of chloroplasts and mitochondria, and are located around vascular bundles in 1 or 2 layers. The combination of these features of the anatomical structure is called crown anatomy or crown syndrome (from the word kranz - crown). Chloroplasts of different cell types are characterized not only by structural features, but also by different types of phosphorylation. In mesophyll cells, predominantly non-cyclic phosphorylation occurs and NADPH is formed, which is necessary for the Calvin cycle, which occurs in the sheath cells. In the chloroplasts of the sheath cells, only cyclic phosphorylation occurs. This separation of the types of phosphorylation may be due to the fact that predominantly longer wavelength light penetrates the chloroplasts of the sheath cells located deep in the leaf, which is not absorbed by the photosystem responsible for the decomposition of H 2 0. At the first stage of the C 4 path, carbon dioxide , diffusing into the leaf through the stomata, enters the cytoplasm of mesophyll cells with small chloroplasts, in which the carboxylation reaction of phosphoenolpyruvic acid (phep) occurs

The reaction is catalyzed by the enzyme phosphoenolpyruvate carboxylase (fepcarboxylase) to form oxaloacetic acid (oxaloacetate). PIKE is converted into malic acid (malate) or aspartic acid (aspartate). Reduction to manat occurs in the presence of NADPH, and the formation of aspartate requires the presence of NH 4+. Then malic (or aspartic) acid apparently moves along plasmodesmata into the sheath cells. In the sheath cells, malic acid is decarboxylated by the enzyme malate dehydrogenase to pyruvic acid (pyruvate) and CO 2 . The decarboxylation reaction can vary among different groups of plants using different enzymes. CO2 enters the chloroplasts of the sheath cells and is included in the Calvin cycle - it joins RBF. Pyruvate returns to the mesophyll cells and turns into the primary CO 2 acceptor - PEP. Thus, in the C4 pathway, the carboxylation reaction occurs twice. This allows the plant to create carbon reserves in its cells. CO 2 acceptors (PEP and RBF) regenerate, which creates the possibility of continuous operation of the cycles. The fixation of CO2 with the participation of PEP and the formation of malate or aspartate serves as a kind of pump for the supply of CO2 to the sheath chloroplasts, functioning along the C3 pathway. Since this mechanism of photosynthesis involves two types of cells and two types of chloroplasts, this path is also called cooperative (Yu.S. Karpilov, 1970). It is suggested that the C 4 pathway arose in the process of evolution as an adaptation to changed environmental conditions. When photosynthesis arose, the atmosphere was significantly richer in CO 2 and poorer in 0 2 . This is why the most important enzyme of the Calvin cycle, Rubisco (Rubisco carboxylase/oxygenase), can only work at relatively high concentrations of CO 2 . Thanks to the activity of the plants themselves, the composition of the atmosphere changed: the content of C0 2 sharply decreased, and 0 2 increased. Under the changed conditions, a number of adaptive features appeared in the implementation of dark reactions of photosynthesis. In particular, the content of the Rubisco enzyme, which makes up almost half of the proteins in the chloroplast stroma, increased significantly. At the same time, some plants have developed a special, additional pathway for binding CO 2 using PEP carboxylase. This enzyme has a greater affinity for carbon dioxide and operates at CO 2 concentrations many times lower than Rubisco. It has been established that the mesophyll resistance to CO2 diffusion in C4 plants is more than 3.5 times less and amounts to 0.3-0.8 cm/s, while in C3 plants it is 2.8 cm/s .

Fixation along the C4 pathway has a number of other advantages. Plants of the C 3 pathway are characterized by a high intensity of the process called photorespiration. Photorespiration refers to the absorption of oxygen and the release of CO 2 in the light using intermediate products of the Calvin cycle as a substrate. Studies have shown that Rubisco (RBP carboxylase/oxygenase) has a dual function and can catalyze not only the carboxylation reaction of the Calvin cycle: Rubisco + CO 2 -> 2PGA. Rubisco is able to react with 0 2, carrying out an oxygenase reaction, and phosphoglycolic acid is formed:

RuBP + 0 2 -> PGA + phosphoglycolic acid.

Phosphoglycolic acid, through a series of transformations, decomposes with the release of CO 2. Thus, during photorespiration, part of the intermediate products of photosynthesis is lost due to the release of CO 2. Oxidation and carboxylation reactions compete with each other, and the implementation of Rubisco carboxylase or oxygenase function depends on the content of 0 2 and C0 2. Photorespiration requires an increased concentration of 0 2 . Meanwhile, as already mentioned, in the chloroplasts of the sheath cells the concentration of 0 2 is reduced, since only cyclic phosphorylation occurs in them, during which water is not decomposed and 0 2 is not released. At the same time, the concentration of CO2 in the sheath cells is increased. Such conditions inhibit the process of photorespiration in sheath cells and therefore C4-type plants are characterized by a very low loss of CO2 as a result of photorespiration.

Losses due to photorespiration in C 3 plants especially increase with increasing temperature and illumination. In this regard, it is clear that C 4 plants are mainly southern and even tropical, which receive additional benefits in terms of photosynthetic productivity. The optimum temperature for photosynthesis in C 3 plants is 20-25°C, while in C 4 plants it is 30-45°C. Light saturation of photosynthesis in C 4 plants also occurs at higher light intensities than in C 3 plants. Thus, in plants of the C 3 pathway, the intensity of photosynthesis stops increasing at 50% of full sunlight, while in C 4 forms this does not happen. Such features of C4 plants explain the high intensity of photosynthesis at elevated temperatures and light levels. A characteristic feature of C 4 -pathway plants is, finally, that the formation of Calvin cycle products occurs in chloroplasts located directly near the vascular bundles. This favors the outflow of assimilates and, as a result, increases the intensity of photosynthesis. The differences between C3 and C4 plants can be demonstrated by placing them side by side in the same chamber (for example, corn and beans) under high temperature and light. It turns out that CO 2, released during respiration, gradually passes to the corn and accordingly changes its growth rate. The corn seems to “eat” the bean plants.

It has been shown that C 3 -plants assimilate CO 2 in full sunlight at a rate of 1-50 mg/dm 2 h, and C 4 -plants at a rate of 40-80 mg/dm 2 h. Corn, sorghum, millet, sugar cane are one of the most productive crops. Thus, the intensity of photosynthesis in corn is 85 mg CO 2 /dm 2 h, sorghum - 55 mg CO 2 /dm 2 h, while in wheat it is only 31 mg CO 2 /dm 2 h. The high potential productivity of C 4 plants is most fully realized in full sunlight and high temperature. An important physiological feature of C 4 plants is their high drought and heat resistance. According to a number of researchers, the emergence of the C4 pathway of photosynthesis was facilitated by dry conditions environment. It has already been noted that the spatial separation of processes allows plants with C 4 through photosynthesis to fix carbon dioxide even with relatively closed stomata, since the chloroplasts of sheath cells use CO 2 accumulated in the form of CO 2 donors (malate or aspartate). Closing the stomata during the hottest part of the day is known to reduce water loss through transpiration. At the same time, C4 plants are characterized by more economical use of water. If C 3 plants spend 700-1000 g of water to form 1 g of dry matter, then C 4 plants spend 300-400 g. The main reason for the reduced water consumption of C 4 plants is that their stomata have high resistance to gas diffusion. When leaves wither and stomata close, this resistance increases many times over for water vapor and, to a lesser extent, for CO2. The low value of the diffusion resistance of mesophyll cells for CO 2 in C 4 plants with a higher resistance of stomata for H 2 0 favors an increase in the intensity of photosynthesis with reduced transpiration. It is therefore clear that C4 plants have an advantage over C3 plants in arid habitats due to the high intensity of photosynthesis even with closed stomata. In addition, they are practically not in danger of overheating of the leaves, which is associated with high heat resistance. The salt tolerance of some types of C 4 plants, for example, amaranth, and the possibility of their use for phytomeliorative purposes have been shown.

In 1966, Australian scientists M. Hatch and K. Slack established that photosynthesis has its own characteristics in some cereal plants of tropical and subtropical origin.

The peculiarity is that the first products of photosynthesis in this group of plants form not three, but four-carbon compounds. When 4-carbon compounds are formed, carbon dioxide combines not with ribulose diphosphate, but with * acid. The path of CO 2 assimilation through * acid with the formation of C4-dicarboxylic acids is called the C4-pathway of carbon assimilation, and organisms are called C4-plants.

In plants of tropical origin - sugar cane, sorghum, millet, cereals, corn, amaranth, etc., leaf vascular bundles are surrounded by large parenchyma cells with large, often devoid of granules, chloroplasts. These cells in turn are surrounded by smaller mesophyll cells with smaller chloroplasts. In the mesophyll cells of the leaf, the primary acceptance of CO 2 by * acid occurs, which involves CO 2 in carboxylation reactions even at very low concentrations of CO 2 in the surrounding air.

As a result of carboxylation, oxaloacetic, malic and aspartic acids are formed. Of these, malic and aspartic go into the parietal cells of the vascular bundles of the leaf, undergo decarboxylation there and create a high concentration of CO 2 inside the cells, which is absorbed through ribulose diphosphate carboxylase in the Calvin cycle. This is beneficial, firstly, because it facilitates the introduction of CO 2 into the Calvin cycle through the carboxylation of ribulose diphosphate using the enzyme ribulose diphosphate carboxylase, which is less active and requires higher concentrations of CO 2 for optimal operation than *-carboxylase. In addition, the high concentration of CO 2 in parietal cells reduces light respiration and associated energy losses.

Thus, high-intensity and cooperative photosynthesis occurs, free from unnecessary losses in light respiration, from oxygen inhibition and well adapted to an atmosphere poor in CO 2 and rich in O 2.

Plants with C4 photosynthesis are flowering plants from 19 families (3 families of monocots and 16 families of dicots). C4 grains predominate in areas with very high temperatures during the growing season. C4 dicotyledons are widespread in areas where the growing season is characterized by excessive aridity. 23 families of flowering plants are characterized by the metabolism of organic acids according to the Crassulaceae type, designated as CAM metabolism. CAM metabolism evolved in the leaves of succulent plants, including cacti and crassulas, but not all CAM plants are succulents, such as pineapples.

Succulents growing in arid areas (cactus) also fix atmospheric CO 2 with the formation of 4-carbon compounds. However, in their physiological behavior, these plants differ from other representatives of the C4 type. Their stomata are open at night and closed during the day. Usually the picture is the opposite: light stimulates the opening of stomata, but in the dark they remain closed.

This type of behavior is of undoubted benefit to desert plants. These plants absorb atmospheric CO2 at night, resulting in the fixation of 4-carbon organic acid, mainly malic acid. Malic acid is stored in vacuoles. In them, as in other C4 plants, PEP plays the role of the primary carbon acceptor. During the day, when chlorophyll is activated by light, malic acid is decarboxylated to form a 3-carbon compound and CO2, which then builds 6-carbon sugars in the Calvin cycle.

The alternation of two processes throughout the day: the accumulation of acids at night and their breakdown during the day is called CAM metabolism, according to the Crassulaceae family.

In CAM plants, primary carboxylation and formation of 6-carbon sugars occur in the same cells, but at different times. Whereas in other C4 plants these processes occur simultaneously, but can be confined to different cells. Time separation of CO 2 fixation and CO 2 processing the next day is economically beneficial. This way they provide themselves with carbon without suffering excessive water loss.

Research has shown that in plants in which the process of photosynthesis occurs along the C4 pathway, there are two types of cells and chloroplasts:

1) small granal plastids in leaf mesophyll cells;

2) large plastids, often lacking grana, in the sheath cells surrounding the vascular bundles.

C4 plants include a number of cultivated plants of predominantly tropical and subtropical origin - corn, millet, sorghum, sugar cane and many harmful weeds - pigweed, round grass, chicken millet, gumai, bristle grass, etc. As a rule, these are highly productive plants that sustainably produce photosynthesis at significant increases in temperature and in dry conditions.

Features of photosynthesis:

The CO 2 acceptor is PEP phosphoenolpyruvic acid;

Photosynthesis is separated in space

The final products are: organic acids, enzyme PEP carboxylase;

There is no process of photorespiration;

The carboxylation process is carried out twice and this allows CO 2 to enter while the stomata are closed.

A characteristic feature of C4-pathway plants is that the formation of Calvin cycle products occurs in chloroplasts located directly near the vascular bundles. This favors the outflow of assimilates and, as a result, increases the intensity of photosynthesis.

Stages of the C4 cycle:

1. carboxylation (occurs in mesophyll cells);

Phosphoenolpyruvic acid (PEP) undergoes carboxylation with the participation of PEP carboxylase and forms oxaloacetic acid (OA), which is reduced to malic acid (malate) or aminated to form aspartic acid.

PIKE, malate and aspartic acids are four-carbon compounds.

In the sheath cells, malic acid is decarboxylated by the enzyme malate dehydrogenase to pyruvic acid (pyruvate, PVK) and CO 2 . The decarboxylation reaction can vary among different groups of plants using different enzymes. CO2 enters the chloroplasts of the sheath cells and is included in the Calvin cycle - it joins the RDF. Pyruvate returns to the mesophyll cells and turns into the primary CO 2 acceptor - PEP. Thus, in the C4 pathway, the carboxylation reaction occurs twice. This allows the plant to create carbon reserves in its cells. CO 2 acceptors (PEP and RDF) regenerate, which creates the possibility of continuous operation of the cycles. The fixation of CO2 with the participation of PEP and the formation of malate or aspartate serves as a kind of pump for the supply of CO2 to the sheath chloroplasts, functioning along the C3 pathway.

The leaves of plants such as sugar cane, corn, sorghum, and amaranth are capable of fixing CO2 not only in the reactions of the Calvin cycle, but also in another way, during which C4 acids appear - oxaloacetic, malic and aspartic. This method of fixing carbon dioxide is called the C4 pathway of photosynthesis (Hatch and Slack pathway).

The leaves of C4 plants are characterized by a kranz-type anatomical structure (described by Haberlandt, 1884). The vascular bundles in such plants are surrounded by two layers of green cells of the assimilative parenchyma. The leaves of C4 plants are characterized by numerous air cavities through which air from the atmosphere comes directly to a large number of photosynthetic mesophyll cells, ensuring efficient absorption of carbon dioxide. A feature of the leaf structure of C4 plants is the presence of no more than 2-3 layers of mesophyll cells from the nearest sheath cells. Sheath cells, which easily exchange products of photosynthesis with mesophyll cells using large number plasmodesmata, densely packed around vascular bundles.

Mesophyll cells	Lining cells
Chloroplasts are small and there are many of them	Chloroplasts are large and few in number. Contains many starch grains
There are grains	They do not have a granal structure
There are photosystem 1 and 2	There is no photosystem 2, because PS2 requires thylakoid membrane junctions.
Non-cyclic flow of electrons, O2 is released	Cyclic flow of electrons, NADPH is NOT formed, O2 is NOT released, reduced partial pressure
There is photorespiration	No photorespiration (because there is no O2)
PEP carboxylase	-
Low activity RUBISCO	High activity RUBISCO
10-15 mesophyll cells per	1 lining square
Primary assimilation of CO2	Secondary assimilation of CO2

Those. the processes of primary and secondary assimilation of CO2 are spatially separated.

Carbon dioxide fixation in mesophyll cells occurs as a result of the addition of CO2 to phosphoenolpyruvic acid (PEP) and the formation of 4-carbon oxaloacetic acid, which is then converted to malic or aspartic acid.

PEP carboxylase, unlike RUBISCO (which binds only CO2), assimilates HCO3 - under conditions of very low partial pressure of CO2 and high -O2. Then C4 acids (malate or aspartate) are transported to the sheath cells, where they are decarboxylated and C3 acids are formed. After this, C3 acids return to the mesophyll cells, and carbon dioxide enters the Calvin cycle.

In the chloroplasts of the sheath cells, the activity of photosystem II is very low and therefore photolysis of water and the release of oxygen do not occur. Those. in the sheath cells where RUBISCO functions, a high concentration of CO2 and low -O2 are maintained. In these chloroplasts, light energy goes only to ATP synthesis as a result of the work of photosystem I and cyclic electron transport. NADPH, necessary for the synthesis of carbohydrates in the Calvin cycle in the chloroplast sheath, is formed by the oxidation of malic acid coming from mesophyll cells by malicenzyme.

There are three variants of the C4 photosynthesis pathway, which differ in the type of C4 acid that is transported to the sheath cells (aspartate or malate), the type of C3 acid (pyruvate or alanine) that is returned to the mesophyll cells for regeneration, and, finally, by the type of decarboxylation in the sheath cells.

The C4 pathway got its name because in the dark phase, the primary product of CO2 fixation in this case is organic compound not with three, but with four carbon atoms (oxaloacetic acid). Tropical plants of hot countries, for example, bromeliads, have this type of photosynthesis. It has long been noted that these plants absorb CO2 much better than C3 plants. In the anatomical structure of the leaves of C4 plants, along with normal ordinary chloroplasts, around the vascular bundles they have special kind very dense chloroplasts almost without thylakoids, but filled with starch. These chloroplasts are called parietal chloroplasts.

In ordinary chloroplasts of C4 plants, as expected, light phase photosynthesis, and CO2 fixation also occurs, but oxaloacetic acid is formed. This oxaloacetic acid is converted into malic acid, which enters the parietal chloroplasts, where it is immediately broken down with the release of CO2. And then everything goes the same as with normal C3 plants. In this case, the concentration of CO2 in the parietal chloroplasts as a result becomes significantly higher than in C3 plants, and the very dense arrangement of these chloroplasts ensures that almost no oxygen reaches them; there are no intercellular spaces. Therefore, since there is no oxygen, and carbon dioxide as much as you want, photorespiration does not occur.

Thus, in C4 plants, CO2 fixation occurs more efficiently in the form of other compounds, and the formation of sugars occurs in special chloroplasts, resulting in a reduction in the intensity of photorespiration and associated losses.

C4 plants can close their stomata in the heat and not lose such precious moisture. They usually have enough CO2 accumulated in the form of malic acid.

27. Photorespiration: biochemical reactions, their localization. Physiological role of photorespiration.

Photorespiration is a light-activated process of CO2 release and O2 absorption. The primary product of photorespiration is glycolic acid. Photorespiration increases with low CO2 content and high O2 concentration in the air. Under these conditions, chloroplast ribulose disphate carboxylase catalyzes not the carboxylation of ribulose-1,5-diphosphate, 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 to glyoxylic acid. Glyoxylic acid is aminated to form glycine. Glycine is transported to the mitochondrion, where serine is synthesized from two glycine molecules and CO2 is released.

Serine can enter the peroxisome and transfer the amino group to pyruvic acid to form alanine, and itself is converted to hydroxypyruvic acid. The latter, with the participation of NADPH, is reduced to glyceric acid. It passes into chloroplasts, where it enters the Calvin cycle

In C4 plants-type carbon dioxide released during photorespiration reacts in mesophyll cells with phosphoenolpyruvic acid to form oxaloacetic and malic acids. Malic acid passes into the sheath cells, where it serves as a CO2 donor. C3-path plants characterized by high intensity of photorespiration. Phosphoglycolic acid decomposes through a series of transformations, releasing CO2. Thus, during photorespiration, part of the intermediate products of photosynthesis is lost due to the release of CO2. Oxidation and carboxylation reactions compete with each other, and the implementation of the carboxylase or oxygenase function depends on the content of O2 and CO2

Photorespiration reduces the efficiency of photosynthesis and leads to losses of assimilated carbon, but has some synthetic significance. In the early stages of life, when there was little oxygen in the atmosphere, rubisco occupied a key position in photosynthesis, and its oxygenase function did not cause problems. As the oxygen content increased, losses due to photorespiration increased, and a number of plants developed mechanisms for the active delivery of rubisco carbon dioxide to the place of work (see C4 and CAM photosynthesis), increasing the share of its carboxylase activity to 100%.