Photosynthesis efficiency for different groups of plants. Kakhnovich L.V. Photosynthesis. Course of lectures - file n1.doc. Mechanism of the dark stage of photosynthesis

It is practically impossible to control photosynthesis directly, but indirectly it is possible.

Regulation of environmental factors (light, temperature, CO 2 , H 2 O, mineral nutrition, etc.)

Creation of crops with optimal parameters for photosynthesis:

Leaf area. ILP should be at least 4-5, i.e. per 1 ha, the leaf area should be 40-50 thousand m 2.
Optissic seeding density that allows for better use of light. This is achieved by seeding rates, which makes it possible to form either thickened (for example, in the seed plots of potatoes) or more sparse crops (for example, seed crops of cereals).
An important role is played by the shape of the leaves on the plant. It is bad both with horizontal and strictly vertical (bow) arrangement. Better - funnel-shaped like corn, cereals.

An important role is played by the flag leaf - the topmost leaf of cereals. Due to its work, about 50% of the products of photosynthesis are formed after flowering, during the grain filling period.

Prolongation of the period of active vegetation of plants by planting seedlings, germinated tubers, early sowing. It is necessary to keep the plants in an active physiological state.

Not only leaves are involved in the formation of the crop, but also ears, stems and even awns, in which photosynthesis takes place. Their share is different, but quite tangible.

The introduction of fundamentally new types of crops, in which the productivity of plants increases by 1.5-2 times due to higher photosynthetic activity. These are narrow-band crops: alternation of strips about 1 m wide of high-stem crops (cereals) and tilled crops (beets, potatoes, etc.). In such crops, the use of light is improved due to light side effects, the concentration of CO row crops and a number of other benefits.

Photosynthesis is the main highly profitable way to use solar energy.

Dependence of photosynthesis on environmental factors and plant characteristics

The dependence of FI on the content of CL is explained by the assimilation number (AN) or the Wilstetter number. ACh is the amount of CO 2 assimilated by the leaf in 1 hour per unit of contained chlorophyll. The higher the chlorophyll content, the lower the AP content. In plants with light green leaves, the value of AN is 60-80, in dark green - 5-7 mg CO 2 /hour mg CL.

Chlorophyll is the same throughout the plant world and its content, depending on the growing conditions, ranges from 0.7 to 9 mg / dm 2.

The more the plant absorbs light, the lower the content of CL in the leaves. In the temperate zone, for example, RB, the leaves are dark green, in the southern regions - light green. Plants usually synthesize chlorophyll in some excess. Its content in plants is from hundredths to tenths of a percent for natural humidity (0.05-0.32%).

Changes in photosynthesis in ontogeny.

To study this dependence, etiolated seedlings are usually used; grown in the dark. They do not contain chlorophyll. When illuminated, chlorophyll is formed in a few minutes, and after four hours, photosynthesis begins in them. In annual plants, a unimodal change in IF occurs in ontogeny. IF is set at a certain level two days after greening. The maximum value of IF is during the transition from vegetation to reproduction (flowering phase). In aging leaves, IF is lowered.

2. Photosynthesis intensity and environmental factors.

2.1 The FI depends both on the intensity of light (photon flux) and on its spectral composition. The dependence of the IF on the RI (light intensity) is described light curve photosynthesis, which has the form of a parabola, consisting of two phases. The first phase is a linear dependence of IF on IE co light compensation point (SKP). SKP is the light intensity at which IF = ID. The second phase is a decrease in the slope of the curve as the AI increases and its reaching a plateau. it light saturation photosynthesis.

The generalized light curve has the following form.

Light saturation in C 3 -plants occurs at IO values equal to 0.4-0.6 of PSO, and in C 4 it is practically not observed.

Solar radiation corresponding to the bending point of the light curve is called radiation fixture(RP). The efficiency of photosynthesis during RP reaches maximum values. However, in crops, due to mutual shading, the plants are in conditions of insufficient lighting.

In relation to light, plants are divided into light-loving (SR) and shade-tolerant (TR). They differ in their morphological, anatomical and physiological features. The leaves of SR are smaller, thicker, they have dense venation, have a lighter green color and a lower content of chlorophyll. In TR, the opposite is true: the leaves are large, thinner, sparse venation, dark green color, more chlorophyll, especially Chlv. SRs are more productive.

TR and SR differ in the course of the light curves of photosynthesis (Fig. 2). At low IE, FI is higher in TR than in SR, and with an increase in IE, FI is higher in TR ↓, and in SR.

The ability of individual plant species, hybrids, varieties to carry out photosynthesis at low values of AI is tried to be used in breeding work. Such a selection is possible even among C 4 -cultures - obligate light lovers.

Spectral composition of light. The IF is highly dependent on the quality of the light. According to quantum theory, 1 J of red rays (RC) contains 1.5 times more quanta than 1 J of blue-violet rays (SF). When aligning the SF and CS with respect to the incident quanta, the FI turns out to be higher on the CS than on the SF and white light (BS). However, in saturating light, the advantage passes to the SF. In plants grown on SF, FS saturation occurs at higher illumination and they use powerful radiant fluxes more efficiently than a plant on CL.

The quality of light does not affect the number and size of chloroplasts in a leaf that has completed growth; therefore, IFs are mainly due to the activity of a single chloroplast, which is higher in plants on CC.

The composition of synthesized substances depends on the quality of light. SF accumulates more proteins and lipids, while CS accumulates more soluble carbohydrates and starch. The effect of adding even 20% SF and RC is similar to that of monochromatic blue light. Note: SF refers to blue light. This is used in the construction of photosynthetic lamps.

The process of converting the radiant energy of the Sun into chemical energy, using the latter in the synthesis of carbohydrates from carbon dioxide. It's the only one a way to capture solar energy and use it for life on our planet.

Capturing and converting solar energy is carried out by diverse photosynthetic organisms (photoautotrophs). These include multicellular organisms (higher green plants and their lower forms - green, brown and red algae) and unicellular organisms (euglena, dinoflagellates and diatoms). A large group of photosynthetic organisms are prokaryotes - blue-green algae, green and purple bacteria. Approximately half of the work of photosynthesis on Earth is carried out by higher green plants, and the remaining half is mainly by unicellular algae.

The first ideas about photosynthesis were formed in the 17th century. In the future, as new data appeared, these ideas changed many times. [show] .

Development of ideas about photosynthesis

The beginning of the study of photosynthesis was laid in 1630, when van Helmont showed that plants themselves form organic substances, and do not receive them from the soil. Weighing the pot of earth in which the willow grew and the tree itself, he showed that within 5 years the weight of the tree increased by 74 kg, while the soil lost only 57 g. Van Helmont came to the conclusion that the plant received the rest of the food from water that was watered on the tree. Now we know that the main material for synthesis is carbon dioxide, which is extracted by the plant from the air.

In 1772, Joseph Priestley showed that the mint shoot "corrects" the air "spoiled" by a burning candle. Seven years later, Jan Ingenhuis discovered that plants can only "correct" bad air when they are in the light, and the ability of plants to "correct" the air is proportional to the clarity of the day and the length of time the plants stay in the sun. In the dark, plants emit air that is "harmful to animals."

The next important step in the development of knowledge about photosynthesis was the experiments of Saussure, carried out in 1804. By weighing the air and plants before and after photosynthesis, Saussure found that the increase in the dry mass of a plant exceeded the mass of carbon dioxide absorbed by it from the air. Saussure came to the conclusion that the other substance involved in the increase in mass was water. Thus, 160 years ago, the process of photosynthesis was imagined as follows:

H 2 O + CO 2 + hv -> C 6 H 12 O 6 + O 2

Water + Carbon Dioxide + Solar Energy ----> Organic Matter + Oxygen

Ingenhus suggested that the role of light in photosynthesis is the breakdown of carbon dioxide; in this case, oxygen is released, and the released "carbon" is used to build plant tissues. On this basis, living organisms were divided into green plants, which can use solar energy to "assimilate" carbon dioxide, and other organisms that do not contain chlorophyll, which cannot use light energy and are not able to assimilate CO 2 .

This principle of dividing the living world was violated when S. N. Vinogradsky in 1887 discovered chemosynthetic bacteria - chlorophyll-free organisms that can assimilate (i.e., convert into organic compounds) carbon dioxide in the dark. It was also violated when, in 1883, Engelman discovered purple bacteria that carry out a kind of photosynthesis that is not accompanied by the release of oxygen. At the time, this fact was not properly appreciated; meanwhile, the discovery of chemosynthetic bacteria that assimilate carbon dioxide in the dark shows that the assimilation of carbon dioxide cannot be considered a specific feature of photosynthesis alone.

After 1940, thanks to the use of labeled carbon, it was found that all cells - plant, bacterial and animal - are able to assimilate carbon dioxide, that is, include it in the molecules of organic substances; only the sources from which they draw the energy necessary for this are different.

Another major contribution to the study of photosynthesis was made in 1905 by Blackman, who discovered that photosynthesis consists of two successive reactions: a fast light reaction and a series of slower, light-independent steps, which he called the tempo reaction. Using high-intensity light, Blackman showed that photosynthesis proceeds at the same rate under intermittent illumination with flashes of only a fraction of a second, and under continuous illumination, despite the fact that in the first case the photosynthetic system receives half as much energy. The intensity of photosynthesis decreased only with a significant increase in the dark period. In further studies, it was found that the rate of the dark reaction increases significantly with increasing temperature.

The next hypothesis regarding the chemical basis of photosynthesis was put forward by van Niel, who in 1931 experimentally showed that photosynthesis in bacteria can occur under anaerobic conditions without being accompanied by the release of oxygen. Van Niel suggested that, in principle, the process of photosynthesis is similar in bacteria and in green plants. In the latter, light energy is used for the photolysis of water (H 2 0) with the formation of a reducing agent (H), which participates in the assimilation of carbon dioxide in a certain way, and an oxidizing agent (OH), a hypothetical precursor of molecular oxygen. In bacteria, photosynthesis proceeds in general the same way, but H 2 S or molecular hydrogen serves as a hydrogen donor, and therefore oxygen is not released.

Modern ideas about photosynthesis

According to modern concepts, the essence of photosynthesis is the conversion of the radiant energy of sunlight into chemical energy in the form of ATP and reduced nicotinamide adenine dinucleotide phosphate (NADP · N).

Currently, it is generally accepted that the process of photosynthesis consists of two stages, in which photosynthetic structures take an active part. [show] and photosensitive cell pigments.

Photosynthetic structures

In bacteria photosynthetic structures are presented in the form of an invagination of the cell membrane, forming lamellar organelles of the mesosome. Isolated mesosomes obtained by the destruction of bacteria are called chromatophores, they contain a light-sensitive apparatus.

In eukaryotes The photosynthetic apparatus is located in special intracellular organelles - chloroplasts, containing the green pigment chlorophyll, which gives the plant a green color and plays an important role in photosynthesis, capturing the energy of sunlight. Chloroplasts, like mitochondria, also contain DNA, RNA and an apparatus for protein synthesis, that is, they have the potential ability to reproduce themselves. Chloroplasts are several times larger than mitochondria. The number of chloroplasts varies from one in algae to 40 per cell in higher plants.

In the cells of green plants, in addition to chloroplasts, there are also mitochondria, which are used to generate energy at night due to respiration, as in heterotrophic cells.

Chloroplasts are spherical or flattened. They are surrounded by two membranes - outer and inner (Fig. 1). The inner membrane is stacked in the form of stacks of flattened bubble-shaped discs. This stack is called a facet.

Each grana consists of separate layers arranged like columns of coins. Layers of protein molecules alternate with layers containing chlorophyll, carotenes and other pigments, as well as special forms of lipids (containing galactose or sulfur, but only one fatty acid). These surfactant lipids seem to be adsorbed between individual layers of molecules and serve to stabilize the structure, which consists of alternating layers of protein and pigments. Such a layered (lamellar) grana structure most likely facilitates the transfer of energy during photosynthesis from one molecule to a nearby one.

In algae there is no more than one grain in each chloroplast, and in higher plants - up to 50 grains, which are interconnected by membrane bridges. The aqueous medium between the grana is the stroma of the chloroplast, which contains enzymes that carry out "dark reactions"

The vesicle-like structures that make up the grana are called thylactoids. There are 10 to 20 thylactoids in a grana.

The elementary structural and functional unit of photosynthesis of thylactic membranes, containing the necessary light-trapping pigments and components of the energy transformation apparatus, is called a quantosome, consisting of approximately 230 chlorophyll molecules. This particle has a mass of about 2 x 10 6 daltons and a size of about 17.5 nm.

	Stages of photosynthesis
	Light stage (or energy)	Dark stage (or metabolic)
Location of the reaction	In the quantosomes of thylactic membranes, it proceeds in the light.	It is carried out outside the thylactoids, in the aquatic environment of the stroma.
Starting products	Light energy, water (H 2 O), ADP, chlorophyll	CO 2, ribulose diphosphate, ATP, NADPH 2
The essence of the process	Photolysis of water, phosphorylation In the light stage of photosynthesis, light energy is transformed into the chemical energy of ATP, and energy-poor water electrons are converted into energy-rich NADP electrons. · H 2 . The by-product formed during the light stage is oxygen. The reactions of the light stage are called "light reactions".	Carboxylation, hydrogenation, dephosphorylation In the dark stage of photosynthesis, "dark reactions" occur in which the reductive synthesis of glucose from CO 2 is observed. Without the energy of the light stage, the dark stage is impossible.
end products	O 2, ATP, NADPH 2 Energy-rich products of the light reaction - ATP and NADP · H 2 is further used in the dark stage of photosynthesis.
The relationship between the light and dark stages can be expressed by the scheme The process of photosynthesis is endergonic, i.e. is accompanied by an increase in free energy, therefore, it requires a significant amount of energy supplied from outside. The overall photosynthesis equation is: 6CO 2 + 12H 2 O ---> C 6 H 12 O 62 + 6H 2 O + 6O 2 + 2861 kJ / mol.

Terrestrial plants absorb the water needed for photosynthesis through their roots, while aquatic plants obtain it by diffusion from the environment. The carbon dioxide necessary for photosynthesis diffuses into the plant through small holes on the surface of the leaves - stomata. Since carbon dioxide is consumed in the process of photosynthesis, its concentration in the cell is usually somewhat lower than in the atmosphere. The oxygen released during photosynthesis diffuses out of the cell, and then out of the plant through the stomata. Sugars formed during photosynthesis also diffuse into those parts of the plant where their concentration is lower.

For photosynthesis, plants need a lot of air, since it contains only 0.03% carbon dioxide. Consequently, from 10,000 m 3 of air, 3 m 3 of carbon dioxide can be obtained, from which about 110 g of glucose is formed during photosynthesis. Plants generally grow better with higher levels of carbon dioxide in the air. Therefore, in some greenhouses, the content of CO 2 in the air is adjusted to 1-5%.

The mechanism of the light (photochemical) stage of photosynthesis

Solar energy and various pigments take part in the implementation of the photochemical function of photosynthesis: green - chlorophylls a and b, yellow - carotenoids and red or blue - phycobilins. Only chlorophyll a is photochemically active among this complex of pigments. The remaining pigments play an auxiliary role, being only collectors of light quanta (a kind of light-collecting lenses) and their conductors to the photochemical center.

Based on the ability of chlorophyll to effectively absorb solar energy of a certain wavelength, functional photochemical centers or photosystems were identified in thylactic membranes (Fig. 3):

photosystem I (chlorophyll a) - contains pigment 700 (P 700) absorbing light with a wavelength of about 700 nm, plays a major role in the formation of products of the light stage of photosynthesis: ATP and NADP · H 2
photosystem II (chlorophyll b) - contains pigment 680 (P 680), which absorbs light with a wavelength of 680 nm, plays an auxiliary role by replenishing electrons lost by photosystem I due to water photolysis

For 300-400 molecules of light-harvesting pigments in photosystems I and II, there is only one molecule of the photochemically active pigment - chlorophyll a.

Light quantum absorbed by a plant

transfers the P 700 pigment from the ground state to the excited state - P * 700, in which it easily loses an electron with the formation of a positive electron hole in the form of P 700 + according to the scheme:
P 700 ---> P * 700 ---> P + 700 + e -
After that, the pigment molecule, which has lost an electron, can serve as an electron acceptor (capable of accepting an electron) and go into the reduced form
causes decomposition (photooxidation) of water in the photochemical center P 680 of photosystem II according to the scheme
H 2 O ---> 2H + + 2e - + 1/2O 2
The photolysis of water is called the Hill reaction. The electrons produced by the decomposition of water are initially accepted by a substance designated Q (sometimes called cytochrome C 550 because of its absorption maximum, although it is not a cytochrome). Then, from substance Q, through a chain of carriers similar in composition to the mitochondrial, electrons are supplied to photosystem I to fill the electron hole formed as a result of the absorption of light quanta by the system and restore the pigment P + 700

If such a molecule simply receives back the same electron, then light energy will be released in the form of heat and fluorescence (this is the reason for the fluorescence of pure chlorophyll). However, in most cases, the released negatively charged electron is accepted by special iron-sulfur proteins (FeS-center), and then

or is transported along one of the carrier chains back to P + 700, filling the electron hole
or along another chain of carriers through ferredoxin and flavoprotein to a permanent acceptor - NADP · H 2

In the first case, there is a closed cyclic electron transport, and in the second - non-cyclic.

Both processes are catalyzed by the same electron carrier chain. However, in cyclic photophosphorylation, electrons are returned from chlorophyll a back to chlorophyll a, whereas in acyclic photophosphorylation, electrons are transferred from chlorophyll b to chlorophyll a.

Cyclic (photosynthetic) phosphorylation

Non-cyclic phosphorylation

As a result of cyclic phosphorylation, the formation of ATP molecules occurs. The process is associated with the return of excited electrons through a series of successive stages to P 700 . The return of excited electrons to P 700 leads to the release of energy (during the transition from a high to a low energy level), which, with the participation of the phosphorylating enzyme system, accumulates in the phosphate bonds of ATP, and does not dissipate in the form of fluorescence and heat (Fig. 4.). This process is called photosynthetic phosphorylation (as opposed to oxidative phosphorylation carried out by mitochondria);

Photosynthetic phosphorylation- the primary reaction of photosynthesis - the mechanism for the formation of chemical energy (synthesis of ATP from ADP and inorganic phosphate) on the membrane of chloroplast thylactoids using the energy of sunlight. Necessary for the dark reaction of CO 2 assimilation

As a result of non-cyclic phosphorylation, NADP + is reduced with the formation of NADP · N. The process is associated with the transfer of an electron to ferredoxin, its reduction and its further transition to NADP +, followed by its reduction to NADP · H

Both processes occur in thylactics, although the second is more complex. It is associated (interrelated) with the work of photosystem II.

Thus, the lost P 700 electrons are replenished by the electrons of water decomposed under the action of light in photosystem II.

a+ into the ground state, are apparently formed upon excitation of chlorophyll b. These high energy electrons go to ferredoxin and then through flavoprotein and cytochromes to chlorophyll a. At the last stage, ADP is phosphorylated to ATP (Fig. 5).

Electrons needed to return chlorophyll in its ground state is probably supplied by OH - ions formed during the dissociation of water. Some of the water molecules dissociate into H + and OH - ions. As a result of the loss of electrons, OH - ions are converted into radicals (OH), which later give water molecules and gaseous oxygen (Fig. 6).

This aspect of the theory is confirmed by the results of experiments with water and CO 2 labeled with 18 0 [show] .

According to these results, all the gaseous oxygen released during photosynthesis comes from water, and not from CO 2 . Water splitting reactions have not yet been studied in detail. It is clear, however, that the implementation of all successive reactions of non-cyclic photophosphorylation (Fig. 5), including the excitation of one chlorophyll molecule a and one chlorophyll molecule b, should lead to the formation of one NADP molecule · H, two or more ATP molecules from ADP and F n and to the release of one oxygen atom. This requires at least four quanta of light - two for each chlorophyll molecule.

Non-cyclic electron flow from H 2 O to NADP · H 2 that occurs during the interaction of two photosystems and the electron transport chains connecting them, is observed despite the values of redox potentials: E ° for 1 / 2O 2 /H 2 O \u003d +0.81 V, and E ° for NADP / NADP · H \u003d -0.32 V. The energy of light reverses the flow of electrons. It is essential that during the transfer from photosystem II to photosystem I, part of the electron energy is accumulated in the form of a proton potential on the thylactoid membrane, and then into the energy of ATP.

The mechanism of formation of the proton potential in the electron transport chain and its use for the formation of ATP in chloroplasts is similar to that in mitochondria. However, there are some peculiarities in the mechanism of photophosphorylation. Thylactoids are like mitochondria turned inside out, so the direction of electron and proton transfer through the membrane is opposite to its direction in the mitochondrial membrane (Fig. 6). The electrons move to the outside, and the protons are concentrated inside the thylactic matrix. The matrix is charged positively, and the outer membrane of the thylactoide is negatively charged, i.e., the direction of the proton gradient is opposite to its direction in mitochondria.

Another feature is a significantly larger proportion of pH in the proton potential compared to mitochondria. The thylactoid matrix is highly acidic, so Δ pH can reach 0.1-0.2 V, while Δ Ψ is about 0.1 V. The total value of Δ μ H+ > 0.25 V.

H + -ATP synthetase, designated in chloroplasts as the "СF 1 +F 0" complex, is also oriented in the opposite direction. Its head (F 1) looks outward, towards the stroma of the chloroplast. Protons are pushed out of the matrix through СF 0 +F 1, and ATP is formed in the active center of F 1 due to the energy of the proton potential.

In contrast to the mitochondrial chain, the thylactoid chain apparently has only two conjugation sites; therefore, the synthesis of one ATP molecule requires three protons instead of two, i.e., the ratio 3 H + / 1 mol ATP.

So, at the first stage of photosynthesis, during light reactions, ATP and NADP are formed in the stroma of the chloroplast. · H - products necessary for the implementation of dark reactions.

Mechanism of the dark stage of photosynthesis

Dark reactions of photosynthesis is the process of incorporating carbon dioxide into organic substances with the formation of carbohydrates (glucose photosynthesis from CO 2). Reactions occur in the stroma of the chloroplast with the participation of the products of the light stage of photosynthesis - ATP and NADP · H2.

The assimilation of carbon dioxide (photochemical carboxylation) is a cyclic process, which is also called the pentose phosphate photosynthetic cycle or the Calvin cycle (Fig. 7). It can be divided into three main phases:

carboxylation (fixation of CO 2 with ribulose diphosphate)
reduction (formation of triose phosphates during the reduction of 3-phosphoglycerate)
regeneration of ribulose diphosphate

Ribulose 5-phosphate (a 5-carbon sugar with a phosphate residue at carbon 5) is phosphorylated by ATP to form ribulose diphosphate. This last substance is carboxylated by the addition of CO 2 , apparently to an intermediate six-carbon product, which, however, is immediately cleaved with the addition of a water molecule, forming two molecules of phosphoglyceric acid. Phosphoglyceric acid is then reduced in an enzymatic reaction that requires the presence of ATP and NADP · H with the formation of phosphoglyceraldehyde (three-carbon sugar - triose). As a result of the condensation of two such trioses, a hexose molecule is formed, which can be included in the starch molecule and thus deposited in reserve.

To complete this phase of the cycle, photosynthesis consumes 1 CO 2 molecule and uses 3 ATP and 4 H atoms (attached to 2 NAD molecules). · N). From hexose phosphate, by certain reactions of the pentose phosphate cycle (Fig. 8), ribulose phosphate is regenerated, which can again attach another carbon dioxide molecule to itself.

None of the described reactions - carboxylation, reduction or regeneration - can be considered specific only for the photosynthetic cell. The only difference found between them is that NADP is required for the reduction reaction, during which phosphoglyceric acid is converted to phosphoglyceraldehyde. · H, not OVER · N, as usual.

The fixation of CO 2 with ribulose diphosphate is catalyzed by the enzyme ribulose diphosphate carboxylase: Ribulose diphosphate + CO 2 --> 3-Phosphoglycerate Further, 3-phosphoglycerate is reduced with the help of NADP · H 2 and ATP to glyceraldehyde-3-phosphate. This reaction is catalyzed by the enzyme glyceraldehyde-3-phosphate dehydrogenase. Glyceraldehyde-3-phosphate readily isomerizes to dihydroxyacetone phosphate. Both triose phosphates are used in the formation of fructose bisphosphate (a reverse reaction catalyzed by fructose bisphosphate aldolase). Some of the molecules of the resulting fructose bisphosphate are involved, together with triose phosphates, in the regeneration of ribulose diphosphate (they close the cycle), and the other part is used to store carbohydrates in photosynthetic cells, as shown in the diagram.

It is estimated that 12 NADP is required to synthesize one molecule of glucose from CO2 in the Calvin cycle. · H + H + and 18 ATP (12 ATP molecules are spent on the reduction of 3-phosphoglycerate, and 6 molecules in the regeneration reactions of ribulose diphosphate). Minimum ratio - 3 ATP: 2 NADP · H 2 .

You can see the commonality of the principles underlying photosynthetic and oxidative phosphorylation, and photophosphorylation is, as it were, reversed oxidative phosphorylation:

The energy of light is the driving force of phosphorylation and synthesis of organic substances (S-H 2) during photosynthesis and, conversely, the energy of oxidation of organic substances - during oxidative phosphorylation. Therefore, it is plants that provide life to animals and other heterotrophic organisms:

Carbohydrates formed during photosynthesis serve to build the carbon skeletons of numerous organic plant substances. Nitrogen substances are assimilated by photosynthetic organisms by the reduction of inorganic nitrates or atmospheric nitrogen, and sulfur by the reduction of sulfates to sulfhydryl groups of amino acids. Photosynthesis ultimately ensures the construction of not only proteins, nucleic acids, carbohydrates, lipids, cofactors that are essential for life, but also numerous secondary synthesis products that are valuable medicinal substances (alkaloids, flavonoids, polyphenols, terpenes, steroids, organic acids, etc. .).

Chlorophilic photosynthesis

Chlorophilic photosynthesis was found in salt-loving bacteria that have a violet light-sensitive pigment. This pigment turned out to be the protein bacteriorhodopsin, which, like the visual purple of the retina - rhodopsin, contains a derivative of vitamin A - retinal. Bacteriorhodopsin, embedded in the membrane of salt-loving bacteria, forms a proton potential on this membrane in response to the absorption of light by retinal, which is converted into ATP. Thus, bacteriorhodopsin is a chlorophyll-free light energy converter.

Photosynthesis and the environment

Photosynthesis is possible only in the presence of light, water and carbon dioxide. The efficiency of photosynthesis is not more than 20% in cultivated plant species, and usually it does not exceed 6-7%. In an atmosphere of about 0.03% (vol.) CO 2, with an increase in its content to 0.1%, the intensity of photosynthesis and plant productivity increase, so it is advisable to feed plants with hydrocarbons. However, the content of CO 2 in the air above 1.0% has a harmful effect on photosynthesis. In a year, only terrestrial plants assimilate 3% of the total CO 2 of the Earth's atmosphere, i.e., about 20 billion tons. Up to 4 × 10 18 kJ of light energy is accumulated in the composition of carbohydrates synthesized from CO 2. This corresponds to a power plant capacity of 40 billion kW. A by-product of photosynthesis - oxygen - is vital for higher organisms and aerobic microorganisms. Preserving vegetation means preserving life on Earth.

Photosynthesis efficiency

The efficiency of photosynthesis in terms of biomass production can be estimated through the proportion of total solar radiation falling on a certain area in a certain time, which is stored in the organic matter of the crop. The productivity of the system can be estimated by the amount of organic dry matter obtained per unit area per year, and expressed in units of mass (kg) or energy (MJ) of production obtained per hectare per year.

The biomass yield thus depends on the area of the solar energy collector (leaves) operating during the year and the number of days per year with such light conditions when photosynthesis is possible at the maximum rate, which determines the efficiency of the entire process. The results of determining the share of solar radiation (in%) available to plants (photosynthetically active radiation, PAR), and the knowledge of the main photochemical and biochemical processes and their thermodynamic efficiency, make it possible to calculate the probable limiting rates of formation of organic substances in terms of carbohydrates.

Plants use light with a wavelength of 400 to 700 nm, i.e., photosynthetically active radiation accounts for 50% of all sunlight. This corresponds to an intensity on the Earth's surface of 800-1000 W / m 2 for a typical sunny day (on average). The average maximum efficiency of energy conversion during photosynthesis in practice is 5-6%. These estimates are based on the study of the process of CO 2 binding, as well as the accompanying physiological and physical losses. One mole of bound CO 2 in the form of a carbohydrate corresponds to an energy of 0.47 MJ, and the energy of a mole of red light quanta with a wavelength of 680 nm (the most energy-poor light used in photosynthesis) is 0.176 MJ. Thus, the minimum number of moles of red light quanta required to bind 1 mole of CO 2 is 0.47:0.176 = 2.7. However, since the transfer of four electrons from water to fix one CO 2 molecule requires at least eight photons of light, the theoretical binding efficiency is 2.7:8 = 33%. These calculations are made for red light; it is clear that for white light this value will be correspondingly lower.

Under the best field conditions, fixation efficiency in plants reaches 3%, but this is only possible in short periods of growth and, if calculated for the whole year, it will be somewhere between 1 and 3%.

In practice, on average per year, the efficiency of photosynthetic energy conversion in temperate zones is usually 0.5-1.3%, and for subtropical crops - 0.5-2.5%. The product yield that can be expected at a certain level of sunlight intensity and different photosynthesis efficiency can be easily estimated from the graphs shown in Fig. 9.

The Importance of Photosynthesis

The process of photosynthesis is the basis of nutrition for all living beings, and also supplies mankind with fuel, fibers and countless useful chemical compounds.
From the carbon dioxide and water bound from the air during photosynthesis, about 90-95% of the dry weight of the crop is formed.
Man uses about 7% of the products of photosynthesis for food, animal feed, fuel and building materials.

The uncontrolled consumption of fossil resources has brought the world to the brink of an ecological and energy crisis. In such a situation, a fundamentally different source of energy is needed, which, on the one hand, would fit into our oil world, and on the other hand, would be renewable, environmentally friendly and economically profitable. A possible solution is artificial photosynthesis (IF), thanks to which man-made installations for the synthesis of organic matter from electricity, light, as well as amazing semiconductor photosynthetic armor bacteria, have already been born.

The global energy crisis, or why artificial photosynthesis is needed

Today, the already large population of the planet is increasing by 1% annually. Mankind satisfies the energy needs growing every year primarily at the expense of fossil resources. But it is no longer a secret to anyone that oil and coal reserves are limited and in most cases non-renewable. When their volumes no longer correspond to the global pace of development (or even used up), the world will face an energy crisis of unprecedented proportions.

Already, one can observe a fierce struggle unleashed on the world stage for large sources of fossil fuels. In the future, there will be less and less fuel, and conflicts of interest will occur more and more often.

For the past two centuries, humanity has been blinded by the availability of fossil fuels and has developed many technologies based on them, without which life is simply unthinkable today. First there were coal and steam locomotives, then people learned to get electricity by burning the same coal, to produce gas stoves, private and public transport - all this requires the consumption of organic matter stored millions of years ago. Using the energy of these substances, humanity has made a leap in many areas of public life: the world population has exceeded 7 billion, flourishing cities and states have emerged in the deserts, production capacities and consumption levels are increasing year by year. Without a doubt, the modern world is unthinkable without coal, oil products and gas.

Here the dilemma of modern energy is manifested: on the one hand, the need to switch to renewable energy sources is absolutely obvious, on the other hand, the world is not adapted for consuming such energy. However, in the last decade, there has been an increasing development of an energy source that could solve this dilemma. We are talking about artificial photosynthesis (IF)- a way to convert the energy of the sun into a convenient form of organic fuel.

We must not forget that fuel combustion leads to massive emissions of CO 2 into the atmosphere, which negatively affects the state of the entire biosphere. In large cities, this influence is especially noticeable: thousands of smoking cars and enterprises form smog, and every citizen, having got out of the city, first of all admires the fresh air. Creating an energy source that, like plants, would absorb CO 2 and produce O 2, could stop the degradation of the environment going at full speed.

Thus, the IF is a potential solution to both the global energy and environmental crises. But how does IF work and how does it differ from natural?

Imperfect greenery

Figure 2. Non-cyclic photosynthesis in plants. The electron leaves the light-excited chlorophyll of photosystem II (PS-II), and the resulting "hole" is filled with electrons released during the splitting of water. The final electron receiver is not a photosystem pigment, as in purple bacteria, but NADP +. Another difference is that in plants, two photosystems (FS-I and PS-II) form a conjugated mechanism, and for one cycle of its operation, absorption of two photons is required. The figure does not show the b 6 f complex.

The resulting H+ gradient provides energy for ATP synthesis via the ATP synthase enzyme, similar to how falling water becomes the energy source for a water mill (Fig. 3). ATP is a universal carrier of chemical energy in the cell and is involved in the vast majority of energy-consuming reactions, including the reactions of the Calvin cycle, which ensure the conversion of CO 2 into reduced organic matter. In this cycle, most of the energy is spent on the fight against side reactions. There are other ways of carbon assimilation - for example, the Wood-Lyungdal path, which will be written about later.

Figure 3. Storage of light energy. During photosynthesis, photosystem proteins carry protons across the membrane at the expense of photon energy. The ATP synthase enzyme dumps the resulting H + concentration gradient and produces the universal energy carrier in the cell - ATP. The spinning water mill analogy is actually very close to reality.

Although photosynthesis ultimately provides the entire biosphere with energy, the efficiency of this process leaves much to be desired (Table 1). The photosynthesis record holder is sorghum grown for biofuel production, which has an efficiency of converting solar energy into chemical energy of 6.6%. For comparison: potatoes, wheat and rice have about 4%.

Table 1. Energy parameters of photosynthesis. Photosynthesis is a multi-stage process, and at each stage part of the energy of sunlight is lost. The low efficiency of photosynthesis is its main disadvantage in comparison with modern solar panels. The energy of sunlight falling on the leaf is taken as 100%. The table is based on data from .

Cause of energy loss	Loss of energy	Remainder
Absorption of photons only in the visible part of the spectrum	47%	53%
Only part of the light flux passes through the photosynthetic parts of the leaf	70%	37%
Although there are high- and low-energy photons in visible light, they are all absorbed by photosystems as low-energy (a kind of caravan principle)	24%	28%
Losses in glucose synthesis	68%	9%
Cleaning the leaf from the by-products of photosynthesis ( cm. photorespiration)	32%	6%

At the same time, the typical efficiency for modern solar batteries is 15-20%, and prototypes have reached a value of 46%,. Such a difference in the efficiency of man-made photocells and living plants is explained primarily by the absence of synthesis stages. But there is a more subtle difference: plant photosystems extract energy only from photons of visible light with wavelengths of 400–700 nm, and the output from high-energy photons is exactly the same as from low-energy ones. Semiconductors used in solar cells capture photons of a wider spectrum. And for maximum output, materials designed specifically for different parts of the sunlight spectrum are combined into one battery.

The ultimate goal of IF engineers is to create an installation (or an artificial organism) that would carry out photosynthesis better than plants. Today, bioengineering thought has reached a level where one can try to do this. And from year to year, the attempts of scientists are getting closer and closer to the cherished goal, making us marvel at incredible discoveries.

Such a different IF

The simplest IF scheme is fully abiotic synthesis of organics on a catalyst. In 2014, a ruthenium catalyst was discovered that synthesizes methane from H 2 and CO 2 when illuminated. Under optimal conditions, involving heating to 150 ° C and intense lighting, one gram of this catalyst creates one millimol of methane per hour, which, of course, is very small. The scientists themselves, who study the catalyst, admit that such a reaction rate at a rather high cost of the catalyst is too low for its practical application.

Real photosynthesis is a multi-stage process, at each stage of which there is a loss of energy. In part, this is even good, because it opens up a lot of room for optimization. In the case of abiogenic photosynthesis, all that can be done is to come up with a fundamentally new catalyst.

A completely different approach to IF - creation of bioreactors powered by solar energy. In such bioreactors, oddly enough, they use not photosynthetic microorganisms, which can still fix CO 2 using other energy sources.

Let's get acquainted with several types of designs of devices for IF using specific examples.

In 2014, the test results of a plant that converts current into biomass with a record efficiency of 13% were published. To get an IF-reactor, it is enough to connect a solar battery. This setup is essentially an electrochemical cell (Fig. 4 a), where two electrodes are placed in a nutrient medium with bacteria Ralstonia eutropha(they are - Cupriavidus necator). When an external current is applied, the catalyst on the anode splits water into oxygen and protons, and the catalyst on the cathode reduces protons to hydrogen gas. R. eutropha receives energy for the assimilation of CO 2 in the Calvin cycle due to the oxidation of H 2 by the enzyme hydrogenase.

Figure 4. Bioreactors for IF based on electrochemical cells. Current can be generated by photolysis of water at the anode using a solar battery (a) or without it (b) . In both cases, the electrons taken from the water provide autotrophic microbes with the recovery equivalents necessary for fixing CO 2 .

According to the developers' calculations, combining their installation with a typical solar battery (18% efficiency) will lead to a total photosynthesis efficiency of 2.5% if all light energy is converted into biomass growth, and 0.7% if genetically modified butanol-synthesizing bacteria are used. This result is comparable to the efficiency of photosynthesis in real plants, although it does not reach the level of cultivated plants. Ability R. eutropha Synthesizing organics in the presence of H 2 is very interesting not only in the context of IP, but also as a possible application of hydrogen energy.

In 2015, scientists from California created an equally interesting installation, where the stages of light absorption and synthesis are more closely related. The photoanode of the designed reactor, when illuminated, splits water into oxygen, protons and electrons, which are sent along the conductor to the cathode (Fig. 4 b). To increase the rate of photolysis of water at the phase boundary, the photoanode is made of silicon nanowires that multiply its surface.

The cathode of this setup consists of a “forest” of TiO2 nanorods (Fig. 5 a), among which bacteria grow Sporomus ovata. The electrons from the photoanode go exactly to these bacteria, which use them as reducing equivalents for the conversion of CO 2 dissolved in the medium into acetate.

Figure 5. Artificial photosynthesis is unthinkable without nanomaterials. a - In the IF-reactor from the article CO 2 fixed bacteria growing in the "nanoforest" of silicon rods coated with TiO 2 (layer 30 nm); this nanoscaffold creates the anaerobic conditions necessary for bacteria and increases the surface density of contacts between bacteria and the conductor. b - With a fundamentally different approach, not bacteria are placed on a semiconductor, but a semiconductor is placed on bacteria; thanks to the shell of CdS, bacteria dying in the light become photosynthetic.

TiO 2 nanoscaffold performs several functions at once: it provides a high density of bacteria at the contact, protects obligate anaerobic S.ovata from oxygen dissolved in the environment and can also convert light into electricity, helping bacteria to fix CO 2.

S.ovata- bacteria with a very flexible metabolism, which easily adapts to growth in the so-called electrotrophic mode. They fix CO 2 along the Wood-Ljungdal pathway, in which only 10% of acetate goes to biomass growth, and the remaining 90% is released into the environment.

But by itself, acetate is of little value. To convert it into more complex and expensive substances, genetically modified Escherichia coli synthesizing butanol, isoprenoids or polyhydroxybutyrate from acetate. Last Substance E. coli produces with the highest yield.

As for the efficiency of the entire installation, it is very low. Only 0.4% of solar energy can be converted to acetate, and the conversion of acetate to polyhydroxybutyrate proceeds with an efficiency of 50%. In total, only 0.2% of light energy can be stored in the form of organic matter, which can be further used as fuel or raw material for chemical production. The developers consider it their main achievement that the setup they have created can be used for completely different chemical syntheses without fundamental changes in the design. This shows an analogy with natural photosynthesis, where all kinds of organic substances are ultimately synthesized from the assimilation of CO 2 3-phosphoglycerate.

In both technologies described, the developers tried to combine the excellence of semiconductors as absorbers of light energy with the catalytic power of biological systems. And both of the resulting installations were “reverse” fuel cells, where current is used to synthesize substances.

With a fundamentally different approach, individual cells are combined with semiconductors into a single whole. So, at the very beginning of 2016, a work was published in which the bacterium-acetogen Moorella thermoacetica grown in a medium with a high content of cysteine and cadmium, . As a result, usually perishing in the light M. thermoacetica covered with a shell of CdS (semiconductor) and thus not only received protection from the sun, but also became a photosynthetic: electrons from CdS entered the Wood-Lyungdal path (Fig. 5 b).

Experiments on such an "armored" bacterium showed that CO 2 is fixed not only in the light, but also in the dark (subject to the daily cycle). The reason for this is the accumulation of photosynthesis metabolites in the light in such an amount that the cells do not have time to process them. The main advantage of such bacteria in comparison with the cells described above is self-organization. For cells, nanomaterials and catalysts must be prefabricated, and these parts themselves only wear out over time. When M. thermoacetica photosynthetic units divide, produce and repair everything they need if there is enough cadmium and cysteine in the environment. These bacteria have not yet been studied as a source of fuel, but in terms of the quantum yield of photosynthesis, they are not inferior to plants.

Not long to wait...

IF technologies are still at the prototype stage, but their developers see a lot of room for optimization. It is possible to optimize light-catcher semiconductors, microorganisms, the spatial organization of bacteria, and other catalysts. But first of all it is necessary to solve the problem of stability. The efficiency of the manufactured installations drops noticeably after a few days of operation. A fully prepared IF device, like any living system, must regenerate and reproduce itself. In this regard, of particular interest M. thermoacetica to which these properties apply in full measure.

And although the existing models are far from perfect, works in the field of IF are valuable primarily because they show the fundamental possibility of embedding solar energy in a world captured by an internal combustion engine. Windmills and solar panels, of course, have a high efficiency and already almost completely provide energy consumption in Uruguay and Denmark, and hydroelectric power stations are important nodes in the energy grid of many countries,. But the replacement of fuel with electricity in most cases requires a radical restructuring of energy networks and is not always possible.

Further development of the investment fund requires massive investments. It can be imagined that firms producing solar batteries, to which futurists predict world domination in the field of energy by 2030, will be interested in the development of this still young and inexperienced science at the intersection of bioenergy, materials science and nanoengineering. Who knows, maybe the IF will not become a daily routine of the future, or maybe work on it will give impetus to hydrogen energy or biophotovoltaics,. Not long to wait, wait and see.

Literature

Population Pyramids of the World from 1950 to 2100 . (2013). PopulationPyramid.net;
Korzinov N. (2007).

PHOTOSYNTHESIS EFFICIENCY

EFFICIENCY OF PHOTOSYNTHESIS 1) the proportion of light energy assimilated by plants; the calculation is based on either net production (net photosynthesis efficiency) or total production (total photosynthesis efficiency); 2) the rate of formation of primary production in plant formations under natural conditions. It is expressed as the percentage of incident visible radiation that is converted into net products during active photosynthesis. If there is enough water and nutrients and nothing limits plant production, then the maximum efficiency of photosynthesis is 1-2% of the available light energy (in highly productive varieties of cereals, sugar cane, etc.). see also Assimilation efficiency.

Ecological encyclopedic dictionary. - Chisinau: Main edition of the Moldavian Soviet Encyclopedia. I.I. Grandpa. 1989

TRANSPIRATION EFFICIENCY
PREDATOR EFFICIENCY

See what "EFFICIENCY OF PHOTOSYNTHESIS" is in other dictionaries:

EFFICIENCY OF ASSIMILATION- the proportion of energy consumed by the body compared to the energy absorbed (expressed as a percentage); the ratio of the amount of assimilated food to the amount of food ingested. The efficiency of assimilation of solar energy by green plants ... ... Ecological dictionary

PHOTOSYNTHESIS EFFICIENCY COEFFICIENT- the efficiency of the use of carbon dioxide absorbed in the process of photosynthesis for the construction of plant biomass. It changes during the vegetation of plants: in young plants it is 0.36 0.39, and by the end of the growing season it increases to 1.01 1.02 ... Glossary of botanical terms

CARBON DIOXIDE- carbon dioxide, carbonic anhydride, CO2, a necessary component for the construction of organic. in variant in the process of photosynthesis. It is formed during the respiration of a person and fats, the oxidation of organic. in va in organisms, burning, decay, some geological ... ...

- (from the Greek chlorós green and plastós fashioned, formed) intracellular organelles of the Plastid plant cell, in which photosynthesis takes place. They are colored green due to the presence of the main pigment of photosynthesis in them ... Great Soviet Encyclopedia

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