Energy conversion in chloroplasts. How the energy of sunlight is converted Chapter III

It is a widely known fact that the Sun is a celestial body (star), and solar energy is essentially the result of its vital activity. The processes occurring on it release a huge amount of energy, throwing it at incredible speed towards our planet. Harnessing solar energy happens to people both consciously and unconsciously. Bathing in the rays of the Sun, we do not think about the fact that the energy of this star triggers a number of important processes in our body (for example, vitamin D is produced in our skin); thanks to it photosynthesis occurs in plants; The water cycle in nature is also “her work.” We take it for granted. But this is only part of the role of solar energy in our lives.

Practical use of solar energy

The simplest and most familiar to everyone types of solar energy uses- its use in modern calculators (on very compact solar panels) and for household needs (dry fruit, heat water in the tank of an outdoor shower in the country). The movement of air heated by the heat of the sun ensures the operation of the ventilation system and chimneys. The sun's rays are used as an evaporator to desalinate seawater. The sun is one of the main sources of energy for the long-term operation of satellites, as well as devices used to study outer space. Cars powered by electric energy are increasingly being introduced into our lives.

Receiving and converting solar energy

Solar energy hits our planet in the form of three types of radiation waves: ultraviolet, light and infrared.

Use of solar energy primarily aimed at generating heat or electricity. It is infrared waves that fall on a special surface developed by scientists that turn into what we need.

Thus, to extract heat, a collector is used that absorbs infrared waves, a storage device that accumulates it, and a heat exchanger in which heating occurs.

When generating electrical energy, special photocells are used. They absorb light rays, and corresponding installations convert these rays into electricity.

Ways to use solar energy can be divided depending on the type of power plant for its processing. There are six of them in total.

First three: tower (design in the form of a black tower with water inside and mirrors around), parabolic (resemble satellite dishes with mirrors inside), dish-shaped (look like a metal tree with leaves made of mirrors). They can be combined, since they have the same principle of operation: they capture a certain amount of light, direct it to a reservoir of liquid, which heats up and releases steam, which in turn is used to produce electricity.

Fourth- equipment with photocells. The most famous type, since its dimensions can vary depending on the need. Small solar panels are used for the needs of private households, larger ones for industrial needs. The principle of operation is to generate electricity from the sun's rays absorbed by a photocell due to the potential difference inside it.

Fifth- vacuum. Structurally, it is a piece of land covered with a round glass roof, inside of which there is a tower with turbines at the base. The principle of operation is to heat the ground under this roof and create air draft due to the temperature difference. Turbine blades rotate and produce energy.

Many of us have encountered solar cells in one way or another. Someone has used or is using solar panels to generate electricity for domestic purposes, someone uses a small solar panel to charge their favorite gadget in the field, and someone has certainly seen a small solar cell on a microcalculator. Some were even lucky enough to visit.

But have you ever thought about how the process of converting solar energy into electrical energy occurs? What physical phenomenon underlies the operation of all these solar cells? Let's turn to physics and understand the generation process in detail.

From the very beginning, it is obvious that the source of energy here is sunlight, or, in scientific terms, it is obtained due to photons of solar radiation. These photons can be imagined as a stream of elementary particles continuously moving from the Sun, each of which has energy, and therefore the entire light stream carries some kind of energy.

From every square meter of the Sun's surface, 63 MW of energy is continuously emitted in the form of radiation! The maximum intensity of this radiation falls in the range of the visible spectrum - .

So, scientists have determined that the energy density of the flow of sunlight at a distance from the Sun to the Earth of 149,600,000 kilometers, after passing through the atmosphere and upon reaching the surface of our planet, averages approximately 900 W per square meter.

Here you can accept this energy and try to obtain electricity from it, that is, convert the energy of the light flux of the Sun into the energy of moving charged particles, in other words, into.

To convert light into electricity we need photoelectric converter. Such converters are very common, they are available for free sale, these are the so-called solar cells - photoelectric converters in the form of wafers cut out of silicon.

The best are monocrystalline, they have an efficiency of about 18%, that is, if the photon flux from the sun has an energy density of 900 W/sq.m, then you can count on receiving 160 W of electricity per square meter of a battery assembled from such cells.

A phenomenon called the “photo effect” is at work here. Photoelectric effect or photoelectric effect- this is the phenomenon of the emission of electrons by a substance (the phenomenon of electrons being ejected from the atoms of a substance) under the influence of light or any other electromagnetic radiation.

Back in 1900, Max Planck, the father of quantum physics, proposed that light is emitted and absorbed in individual portions or quanta, which later, namely in 1926, the chemist Gilbert Lewis called “photons.”

Each photon has energy, which can be determined by the formula E = hv - Planck's constant multiplied by the frequency of radiation.

In accordance with the idea of ​​​​Max Planck, the phenomenon discovered in 1887 by Hertz, and then thoroughly studied from 1888 to 1890 by Stoletov, became explainable. Alexander Stoletov experimentally studied the photoelectric effect and established three laws of the photoelectric effect (Stoletov’s laws):

With a constant spectral composition of electromagnetic radiation incident on the photocathode, the saturation photocurrent is proportional to the energy illumination of the cathode (in other words: the number of photoelectrons knocked out of the cathode in 1 s is directly proportional to the radiation intensity).

The maximum initial speed of photoelectrons does not depend on the intensity of the incident light, but is determined only by its frequency.

For each substance there is a red limit of the photoelectric effect, that is, a minimum frequency of light (depending on the chemical nature of the substance and the state of the surface), below which the photoelectric effect is impossible.

Later, in 1905, Einstein clarified the theory of the photoelectric effect. He will show how the quantum theory of light and the law of conservation and transformation of energy perfectly explain what happens and what is observed. Einstein wrote down the photoelectric effect equation, for which he received the Nobel Prize in 1921:

Work function A here is the minimum work that an electron needs to do to leave an atom of a substance. The second term is the kinetic energy of the electron after exit.

That is, a photon is absorbed by an electron of an atom, due to which the kinetic energy of the electron in the atom increases by the amount of the energy of the absorbed photon.

Part of this energy is spent on the electron leaving the atom, the electron leaves the atom and is able to move freely. And directionally moving electrons are nothing more than an electric current or photocurrent. As a result, we can talk about the occurrence of EMF in a substance as a result of the photoelectric effect.

That is, The solar battery works thanks to the photoelectric effect operating in it. But where do the “knocked out” electrons go in a photovoltaic converter? A photoelectric converter or a solar cell or a photocell is, therefore, the photoelectric effect in it occurs in an unusual way, it is an internal photoeffect, and it even has a special name “valve photoeffect”.

Under the influence of sunlight, a photoelectric effect occurs in the p-n junction of a semiconductor and an emf appears, but electrons do not leave the photocell, everything happens in the blocking layer, when electrons leave one part of the body, moving to another part of it.

Silicon in the earth's crust makes up 30% of its mass, which is why it is used everywhere. The peculiarity of semiconductors in general is that they are neither conductors nor dielectrics; their conductivity depends on the concentration of impurities, on temperature and on exposure to radiation.

The band gap in a semiconductor is several electron volts, and this is precisely the energy difference between the upper level of the valence band of atoms, from which electrons escape, and the lower level of the conduction band. In silicon, the bandgap has a width of 1.12 eV - just what is needed to absorb solar radiation.

So, p-n junction. The doped layers of silicon in a photocell form a p-n junction. Here an energy barrier is created for electrons; they leave the valence band and move only in one direction; holes move in the opposite direction. This is how current is generated in the solar cell, that is, electricity is generated from sunlight.

A Pn junction exposed to photons does not allow charge carriers - electrons and holes - to move other than in one direction; they separate and end up on opposite sides of the barrier. And being connected to the load circuit through the upper and lower electrodes, the photoelectric converter, when exposed to sunlight, will create in the external circuit.

This method of generating electricity is based on sunlight, named in textbooks as – Photons. For us, it is interesting because, just like a moving air flow, the light flow has energy! At a distance of one astronomical unit (149,597,870.66 km) from the Sun, where our Earth is located, the solar radiation flux density is 1360 W/m2. And having passed through the earth’s atmosphere, the flow loses its intensity due to reflection and absorption, and at the earth’s surface it is already ~ 1000 W/m2. This is where our work begins: to use the energy of the light flux and convert it into the energy we need in everyday life - electrical.

The mystery of this transformation occurs on a small pseudo-square with beveled corners, which is cut from a silicon cylinder (Fig. 2), with a diameter of 125 mm, and its name is . How?

The answer to this question was received by physicists who discovered such a phenomenon as the Photoelectric effect. The photoelectric effect is the phenomenon of electrons being ejected from atoms of a substance under the influence of light.

In 1900 German physicist Max Planck proposed a hypothesis: light is emitted and absorbed in separate portions - quanta(or photons). The energy of each photon is determined by the formula: E =h∙ ν (ash nude) where h- Planck’s constant equal to 6.626 × 10 -34 J∙s, ν - photon frequency. Planck's hypothesis explained the phenomenon of the photoelectric effect, discovered in 1887 by the German scientist Heinrich Hertz and studied experimentally by the Russian scientist Alexander Grigorievich Stoletov, who, by summarizing the results obtained, established the following three laws of the photoelectric effect:

1. With a constant spectral composition of light, the strength of the saturation current is directly proportional to the light flux incident on the cathode.
2. The initial kinetic energy of electrons ejected by light increases linearly with increasing frequency of light and does not depend on its intensity.
3. The photoelectric effect does not occur if the frequency of light is less than a certain value characteristic of each substance, called the red limit.

The theory of the photoelectric effect, which clarifies the mystery that reigns in FEP, was developed by the German scientist Albert Einstein in 1905, explaining the laws photoelectric effect using quantum theory of light. Based on the law of conservation and transformation of energy, Einstein wrote down the equation for the energy balance during the photoelectric effect:

Where: h∙ ν – photon energy, A– work function – the minimum work that needs to be done to leave an electron from an atom of a substance. Thus, it turns out that a particle of light - a photon - is absorbed by an electron, which acquires additional kinetic energy ½m∙v 2 and performs the work of leaving the atom, which gives it the opportunity to move freely. And the directed movement of electric charges is electric current, or, more correctly speaking, Electromotive Force - E.M.F. - arises in the substance.

Einstein was awarded the Nobel Prize for his equation for the photoelectric effect in 1921.

Returning from the past to the present day, we see that the “heart” of the Solar Battery is a FEP (semiconductor photocell), in which an amazing miracle of nature is realized - the Valve PhotoEffect (VPE). It consists in the appearance of an electromotive force in a p-n junction under the influence of light. VFE, or photoelectric effect in the barrier layer, - a phenomenon in which electrons leave the body, passing through the interface into another solid body (semiconductor).

Semiconductors- these are materials that, in terms of their specific conductivity, occupy an intermediate position between conductors and dielectrics and differ from conductors in the strong dependence of the specific conductivity on the concentration of impurities, temperature and various types of radiation. Semiconductors are substances whose band gap is on the order of several electron volts [eV]. Band gap is the difference in electron energies in a semiconductor crystal between the lower level of the conduction band and the upper level of the valence band of the semiconductor.

Semiconductors include many chemical elements: germanium, silicon, selenium, tellurium, arsenic and others, a huge number of alloys and chemical compounds (gallium arsenide, etc.) The most common semiconductor in nature is silicon, making up about 30% of the earth's crust.

Silicon was destined to become a material for, due to its widespread occurrence in nature, its lightness and suitable band gap of 1.12 eV for absorbing energy from sunlight. Today, crystalline silicon (about 90% of the global market) and thin-film solar cells (about 10% of the market) are the most prominent in the market for commercial terrestrial systems.

The key element in the design of crystalline silicon photovoltaic converters (PVCs) is the p-n junction. In a simplified form, a solar cell can be represented as a “sandwich”: it consists of layers of silicon doped to form a p-n junction.

One of the main properties of a pn junction is its ability to be an energy barrier for current carriers, that is, to allow them to pass in only one direction. It is on this effect that the generation of electric current in solar cells is based. Radiation incident on the surface of the element generates charge carriers with different signs in the volume of the semiconductor - electrons (n) and holes (p). Thanks to its properties, the pn junction “separates” them, allowing each type to pass through only its “own” half, and the charge carriers moving chaotically in the volume of the element end up on opposite sides of the barrier, after which they can be transferred to an external circuit to create voltage across the load and electric current in a closed circuit connected to a solar cell.

The history of the study of photosynthesis dates back to August 1771, when the English theologian, philosopher and amateur naturalist Joseph Priestley (1733–1804) discovered that plants can “correct” the properties of air that changes its composition as a result of combustion or animal activity. Priestley showed that in the presence of plants, “spoiled” air again becomes suitable for combustion and supporting the life of animals.

In the course of further research by Ingenhaus, Senebier, Saussure, Boussingault and other scientists, it was found that plants, when illuminated, release oxygen and absorb carbon dioxide from the air. Plants synthesize organic substances from carbon dioxide and water. This process was called photosynthesis.

Robert Mayer, who discovered the law of conservation of energy, suggested in 1845 that plants convert the energy of sunlight into the energy of chemical compounds formed during photosynthesis. According to him, “the sun’s rays propagating in space are “captured” and stored for later use as needed.” Subsequently, Russian scientist K.A. Timiryazev convincingly proved that the most important role in the use of sunlight energy by plants is played by chlorophyll molecules present in green leaves.

Carbohydrates (sugars) formed during photosynthesis are used as a source of energy and building material for the synthesis of various organic compounds in plants and animals. In higher plants, photosynthesis processes occur in chloroplasts, specialized energy-converting organelles of the plant cell.

A schematic representation of a chloroplast is shown in Fig. 1.

Under the double shell of the chloroplast, consisting of outer and inner membranes, there are extended membrane structures that form closed vesicles called thylakoids. Thylakoid membranes consist of two layers of lipid molecules, which include macromolecular photosynthetic protein complexes. In the chloroplasts of higher plants, thylakoids are grouped into grana, which are stacks of disc-shaped thylakoids flattened and closely pressed together. A continuation of the individual thylakoids of the grana are the intergranular thylakoids protruding from them. The space between the chloroplast membrane and the thylakoids is called the stroma. The stroma contains chloroplast molecules RNA, DNA, ribosomes, starch grains, as well as numerous enzymes, including those that ensure the absorption of CO2 by plants.

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Light and dark stages of photosynthesis

According to modern concepts, photosynthesis is a series of photophysical and biochemical processes, as a result of which plants synthesize carbohydrates (sugars) using the energy of sunlight. The numerous stages of photosynthesis are usually divided into two large groups of processes - light and dark phases.

The light stages of photosynthesis are usually called a set of processes as a result of which, due to the energy of light, adenosine triphosphate (ATP) molecules are synthesized and the formation of reduced nicotinamide adenine dinucleotide phosphate (NADP H), a compound with a high reducing potential, occurs. ATP molecules act as a universal source of energy in the cell. The energy of macroergic (i.e., energy-rich) phosphate bonds of the ATP molecule is known to be used in most biochemical processes that consume energy.

Light processes of photosynthesis occur in thylakoids, the membranes of which contain the main components of the photosynthetic apparatus of plants - light-harvesting pigment-protein and electron transport complexes, as well as the ATP synthase complex, which catalyzes the formation of ATP from adenosine diphosphate (ADP) and inorganic phosphate (P i) (ADP + Ф i → ATP + H 2 O). Thus, as a result of the light stages of photosynthesis, the energy of light absorbed by plants is stored in the form of high-energy chemical bonds of ATP molecules and the strong reducing agent NADP H, which are used for the synthesis of carbohydrates in the so-called dark stages of photosynthesis.

The dark stages of photosynthesis are usually called a set of biochemical reactions, as a result of which atmospheric carbon dioxide (CO 2) is absorbed by plants and carbohydrates are formed. The cycle of dark biochemical transformations leading to the synthesis of organic compounds from CO 2 and water is called the Calvin–Benson cycle, named after the authors who made a decisive contribution to the study of these processes. Unlike the electron transport and ATP synthase complexes, which are located in the thylakoid membrane, the enzymes that catalyze the “dark” reactions of photosynthesis are dissolved in the stroma. When the chloroplast membrane is destroyed, these enzymes are washed out of the stroma, as a result of which the chloroplasts lose the ability to absorb carbon dioxide.

As a result of the transformations of a number of organic compounds in the Calvin–Benson cycle, a molecule of glyceraldehyde-3-phosphate is formed from three molecules of CO 2 and water in chloroplasts, having the chemical formula CHO–CHOH–CH 2 O–PO 3 2-. In this case, per one molecule of CO 2 included in glyceraldehyde-3-phosphate, three molecules of ATP and two molecules of NADP H are consumed.

For the synthesis of organic compounds in the Calvin–Benson cycle, the energy released during the hydrolysis reaction of high-energy phosphate bonds of ATP molecules (reaction ATP + H 2 O → ADP + Ph i) and the strong reduction potential of NADP H molecules are used. The main part of the molecules formed in the chloroplast Glyceraldehyde-3-phosphate enters the cytosol of the plant cell, where it is converted into fructose-6-phosphate and glucose-6-phosphate, which during further transformations form sugar phosphate, the precursor of sucrose. Starch is synthesized from the glyceraldehyde-3-phosphate molecules remaining in the chloroplast.

Energy conversion in photosynthetic reaction centers

Photosynthetic energy-converting complexes of plants, algae and photosynthetic bacteria have been well studied. The chemical composition and spatial structure of energy-transforming protein complexes have been established, and the sequence of energy transformation processes has been clarified. Despite the differences in the composition and molecular structure of the photosynthetic apparatus, there are general patterns of energy conversion processes in the photoreaction centers of all photosynthetic organisms. In photosynthetic systems of both plant and bacterial origin, the single structural and functional unit of the photosynthetic apparatus is photosystem, which includes a light-harvesting antenna, a photochemical reaction center and associated molecules - electron carriers.

Let us first consider the general principles of the transformation of sunlight energy, characteristic of all photosynthetic systems, and then we will dwell in more detail on the example of the functioning of photoreaction centers and the electron transport chain of chloroplasts in higher plants.

Light-harvesting antenna (light absorption, energy migration to the reaction center)

The very first elementary act of photosynthesis is the absorption of light by chlorophyll molecules or auxiliary pigments that are part of a special pigment-protein complex called the light-harvesting antenna. A light-harvesting antenna is a macromolecular complex designed to efficiently capture light. In chloroplasts, the antenna complex contains a large number (up to several hundred) of chlorophyll molecules and a certain amount of auxiliary pigments (carotenoids) tightly bound to protein.

In bright sunlight, an individual chlorophyll molecule absorbs light quanta relatively rarely, on average no more than 10 times per second. However, since there are a large number of chlorophyll molecules per photoreaction center (200–400), even with a relatively weak intensity of light incident on the leaf under plant shading conditions, the reaction center is activated quite frequently. The ensemble of pigments that absorb light essentially acts as an antenna, which, due to its fairly large size, effectively captures sunlight and directs its energy to the reaction center. Shade-loving plants, as a rule, have a larger light-harvesting antenna compared to plants growing in high light conditions.

In plants, the main light-harvesting pigments are chlorophyll molecules. a and chlorophyll b, absorbing visible light with wavelength λ ≤ 700–730 nm. Isolated chlorophyll molecules absorb light only in two relatively narrow bands of the solar spectrum: at wavelengths of 660–680 nm (red light) and 430–450 nm (blue-violet light), which, of course, limits the efficiency of using the entire spectrum of incident sunlight on a green leaf.

However, the spectral composition of the light absorbed by the light-harvesting antenna is actually much wider. This is explained by the fact that the absorption spectrum of aggregated forms of chlorophyll that are part of the light-harvesting antenna shifts toward longer wavelengths. Along with chlorophyll, the light-harvesting antenna includes auxiliary pigments, which increase the efficiency of its operation due to the fact that they absorb light in those regions of the spectrum in which chlorophyll molecules, the main pigment of the light-harvesting antenna, absorb light relatively weakly.

In plants, auxiliary pigments are carotenoids that absorb light in the wavelength region λ ≈ 450–480 nm; in the cells of photosynthetic algae these are red and blue pigments: phycoerythrins in red algae (λ ≈ 495–565 nm) and phycocyanins in blue-green algae (λ ≈ 550–615 nm).

Absorption of a quantum of light by a chlorophyll (Chl) molecule or an auxiliary pigment leads to its excitation (the electron moves to a higher energy level):

Chl + hν → Chl*.

The energy of the excited chlorophyll molecule Chl* is transferred to molecules of neighboring pigments, which, in turn, can transfer it to other molecules of the light-harvesting antenna:

Chl* + Chl → Chl + Chl*.

The excitation energy can thus migrate through the pigment matrix until the excitation ultimately reaches the photoreaction center P (a schematic representation of this process is shown in Fig. 2):

Chl* + P → Chl + P*.

Note that the duration of existence of chlorophyll molecules and other pigments in an excited state is very short, τ ≈ 10 –10 –10 –9 s. Therefore, there is a certain probability that on the way to the reaction center P, the energy of such short-lived excited states of pigments may be uselessly lost - dissipated into heat or released in the form of a light quantum (fluorescence phenomenon). In reality, however, the efficiency of energy migration to the photosynthetic reaction center is very high. In the case when the reaction center is in an active state, the probability of energy loss is, as a rule, no more than 10–15%. This high efficiency of using solar energy is due to the fact that the light-harvesting antenna is a highly ordered structure that ensures very good interaction of pigments with each other. Thanks to this, a high rate of transfer of excitation energy from molecules that absorb light to the photoreaction center is achieved. The average time for a “jump” of excitation energy from one pigment to another, as a rule, is τ ≈ 10 –12 –10 –11 s. The total migration time of excitation to the reaction center usually does not exceed 10–10–10–9 s.

Photochemical reaction center (electron transfer, stabilization of separated charges)

Modern ideas about the structure of the reaction center and the mechanisms of the primary stages of photosynthesis were preceded by the works of A.A. Krasnovsky, who discovered that in the presence of electron donors and acceptors, chlorophyll molecules excited by light are able to be reversibly reduced (accept an electron) and oxidize (donate an electron). Subsequently, Cock, Witt and Duyzens discovered in plants, algae and photosynthetic bacteria special pigments of a chlorophyll nature, called reaction centers, which are oxidized under the action of light and are, in fact, the primary electron donors during photosynthesis.

The photochemical reaction center P is a special pair (dimer) of chlorophyll molecules that act as a trap for excitation energy wandering through the pigment matrix of the light-harvesting antenna (Fig. 2). Just as liquid flows from the walls of a wide funnel to its narrow neck, the energy of light absorbed by all the pigments of the light-collecting antenna is directed to the reaction center. Excitation of the reaction center initiates a chain of further transformations of light energy during photosynthesis.

The sequence of processes occurring after the excitation of the reaction center P and the diagram of the corresponding changes in the energy of the photosystem are schematically depicted in Fig. 3.

Along with the chlorophyll P dimer, the photosynthetic complex includes molecules of the primary and secondary electron acceptors, which we will conventionally designate as A and B, as well as the primary electron donor, molecule D. The excited reaction center P* has a low affinity for electrons and therefore it easily donates to its nearby primary electron acceptor A:

D(P*A)B → D(P + A –)B.

Thus, as a result of a very fast (t ≈10–12 s) electron transfer from P* to A, the second fundamentally important stage of solar energy conversion during photosynthesis is realized - charge separation in the reaction center. In this case, a strong reducing agent A – (electron donor) and a strong oxidizing agent P + (electron acceptor) are formed.

Molecules P + and A – are located asymmetrically in the membrane: in chloroplasts, the reaction center P + is located closer to the surface of the membrane facing the inside of the thylakoid, and the acceptor A – is located closer to the outside. Therefore, as a result of photoinduced charge separation, an electrical potential difference arises on the membrane. Light-induced charge separation in the reaction center is similar to the generation of an electrical potential difference in a conventional photocell. It should, however, be emphasized that, unlike all known and widely used energy photoconverters in technology, the operating efficiency of photosynthetic reaction centers is very high. The efficiency of charge separation in active photosynthetic reaction centers, as a rule, exceeds 90–95% (the best examples of solar cells have an efficiency of no more than 30%).

What mechanisms provide such a high efficiency of energy conversion in reaction centers? Why does the electron transferred to the acceptor A not return back to the positively charged oxidized center P + ? Stabilization of separated charges is ensured mainly due to secondary electron transport processes following the transfer of an electron from P* to A. From the restored primary acceptor A, an electron very quickly (in 10–10–10–9 s) goes to the secondary electron acceptor B:

D(P + A –)B → D(P + A)B – .

In this case, not only does the electron move away from the positively charged reaction center P + , but the energy of the entire system also noticeably decreases (Fig. 3). This means that to transfer an electron in the opposite direction (transition B – → A), it will need to overcome a fairly high energy barrier ΔE ≈ 0.3–0.4 eV, where ΔE is the difference in energy levels for the two states of the system in which the electron is respectively on the carrier A or B. For this reason, for the electron to return back, from the reduced molecule B - to the oxidized molecule A, it would take much more time than for the direct transition A - → B. In other words, in the forward direction the electron is transferred much more faster than in reverse. Therefore, after the electron is transferred to the secondary acceptor B, the probability of its return back and recombination with the positively charged “hole” P + decreases significantly.

The second factor contributing to the stabilization of separated charges is the rapid neutralization of the oxidized photoreaction center P + due to the electron supplied to P + from the electron donor D:

D(P + A)B – → D + (PA)B – .

Having received an electron from the donor molecule D and returning to its original reduced state P, the reaction center will no longer be able to accept an electron from the reduced acceptors, but now it is ready to fire again - to give an electron to the oxidized primary acceptor A located next to it. This is the sequence of events that occur in photoreaction centers of all photosynthetic systems.

Chloroplast electron transport chain

In the chloroplasts of higher plants there are two photosystems: photosystem 1 (PS1) and photosystem 2 (PS2), differing in the composition of proteins, pigments and optical properties. The light-harvesting antenna FS1 absorbs light with a wavelength λ ≤ 700–730 nm, and FS2 absorbs light with a wavelength λ ≤ 680–700 nm. Light-induced oxidation of the reaction centers of PS1 and PS2 is accompanied by their bleaching, which is characterized by changes in their absorption spectra at λ ≈ 700 and 680 nm. In accordance with their optical characteristics, the reaction centers of PS1 and PS2 were named P 700 and P 680.

The two photosystems are interconnected through a chain of electron carriers (Fig. 4). PS2 is a source of electrons for PS1. Light-initiated charge separation in the photoreaction centers P 700 and P 680 ensures the transfer of an electron from water decomposed in PS2 to the final electron acceptor - the NADP + molecule. The electron transport chain (ETC), connecting the two photosystems, includes plastoquinone molecules, a separate electron transport protein complex (the so-called b/f complex) and the water-soluble protein plastocyanin (P c) as electron carriers. A diagram illustrating the relative arrangement of electron transport complexes in the thylakoid membrane and the path of electron transfer from water to NADP + is shown in Fig. 4.

In PS2, from the excited center P* 680, an electron is transferred first to the primary acceptor pheophetin (Phe), and then to the plastoquinone molecule Q A, tightly bound to one of the PS2 proteins,

Y(P* 680 Phe)Q A Q B → Y(P + 680 Phe –)Q A Q B →Y(P + 680 Phe)Q A – Q B .

The electron is then transferred to a second plastoquinone molecule QB, and P 680 receives an electron from the primary electron donor Y:

Y(P + 680 Phe)Q A – Q B → Y + (P 680 Phe)Q A Q B – .

Plastoquinone molecule, the chemical formula of which and its location in the lipid bilayer membrane are shown in Fig. 5, is capable of accepting two electrons. After the PS2 reaction center fires twice, the plastoquinone Q B molecule will receive two electrons:

Q B + 2е – → Q B 2– .

The negatively charged Q B 2– molecule has a high affinity for hydrogen ions, which it captures from the stromal space. After protonation of the reduced plastoquinone Q B 2– (Q B 2– + 2H + → QH 2), an electrically neutral form of this molecule QH 2 is formed, which is called plastoquinol (Fig. 5). Plastoquinol acts as a mobile carrier of two electrons and two protons: after leaving PS2, the QH 2 molecule can easily move inside the thylakoid membrane, ensuring the connection of PS2 with other electron transport complexes.

The oxidized reaction center PS2 R 680 has an exceptionally high electron affinity, i.e. is a very strong oxidizing agent. Due to this, PS2 decomposes water, a chemically stable compound. The water-splitting complex (WSC), which is part of PS2, contains in its active center a group of manganese ions (Mn 2+), which serve as electron donors for P680. By donating electrons to the oxidized reaction center, manganese ions become “accumulators” of positive charges, which are directly involved in the water oxidation reaction. As a result of sequential quadruple activation of the P 680 reaction center, four strong oxidative equivalents (or four “holes”) accumulate in the Mn-containing active center of the VRC in the form of oxidized manganese ions (Mn 4+), which, interacting with two water molecules, catalyze the decomposition reaction water:

2Mn 4+ + 2H 2 O → 2Mn 2+ + 4H + + O 2.

Thus, after the sequential transfer of four electrons from the VRC to P 680, the synchronous decomposition of two water molecules occurs at once, accompanied by the release of one oxygen molecule and four hydrogen ions, which enter the intrathylakoid space of the chloroplast.

The QH 2 plastoquinol molecule formed during the functioning of PS2 diffuses into the lipid bilayer of the thylakoid membrane to the b/f complex (Fig. 4 and 5). When it encounters a b/f complex, the QH 2 molecule binds to it and then transfers two electrons to it. In this case, for each plastoquinol molecule oxidized by the b/f complex, two hydrogen ions are released inside the thylakoid. In turn, the b/f complex serves as an electron donor for plastocyanin (P c), a relatively small water-soluble protein whose active center includes a copper ion (reduction and oxidation reactions of plastocyanin are accompanied by changes in the valence of the copper ion Cu 2+ + e – ↔ Cu+). Plastocyanin acts as a link between the b/f complex and PS1. The plastocyanin molecule quickly moves inside the thylakoid, providing electron transfer from the b/f complex to PS1. From the reduced plastocyanin, the electron goes directly to the oxidized reaction centers of PS1 – P 700 + (see Fig. 4). Thus, as a result of the combined action of PS1 and PS2, two electrons from the water molecule decomposed in PS2 are ultimately transferred through the electron transport chain to the NADP + molecule, ensuring the formation of the strong reducing agent NADP H.

Why do chloroplasts need two photosystems? It is known that photosynthetic bacteria, which use various organic and inorganic compounds (for example, H 2 S) as an electron donor to restore oxidized reaction centers, successfully function with one photosystem. The appearance of two photosystems is most likely due to the fact that the energy of one quantum of visible light is not enough to ensure the decomposition of water and the effective passage of an electron along the chain of carrier molecules from water to NADP +. About 3 billion years ago, blue-green algae or cyanobacteria appeared on Earth, which acquired the ability to use water as a source of electrons to reduce carbon dioxide. Currently, it is believed that PS1 originates from green bacteria, and PS2 from purple bacteria. After, during the evolutionary process, PS2 was “included” in a single electron transfer chain together with PS1, it became possible to solve the energy problem - to overcome the rather large difference in the redox potentials of oxygen/water pairs and NADP + /NADP H. The emergence of photosynthetic organisms, capable of oxidizing water, became one of the most important stages in the development of living nature on Earth. Firstly, algae and green plants, having “learned” to oxidize water, have mastered an inexhaustible source of electrons for the reduction of NADP +. Secondly, by decomposing water, they filled the Earth's atmosphere with molecular oxygen, thus creating conditions for the rapid evolutionary development of organisms whose energy is associated with aerobic respiration.

Coupling of electron transport processes with proton transfer and ATP synthesis in chloroplasts

Electron transfer through the ETC is usually accompanied by a decrease in energy. This process can be likened to the spontaneous movement of a body along an inclined plane. A decrease in the energy level of an electron during its movement along the ETC does not mean at all that electron transfer is always an energetically useless process. Under normal conditions of chloroplast functioning, most of the energy released during electron transport is not wasted uselessly, but is used for the operation of a special energy-converting complex called ATP synthase. This complex catalyzes the energetically unfavorable process of ATP formation from ADP and inorganic phosphate P i (reaction ADP + P i → ATP + H 2 O). In this regard, it is customary to say that energy-donating processes of electron transport are associated with energy-acceptor processes of ATP synthesis.

The most important role in ensuring energy coupling in thylakoid membranes, as in all other energy-converting organelles (mitochondria, chromatophores of photosynthetic bacteria), is played by proton transport processes. ATP synthesis is closely related to the transfer of three protons from thylakoids (3H in +) to the stroma (3H out +) through ATP synthase:

ADP + Ф i + 3H in + → ATP + H 2 O + 3H out + .

This process becomes possible because, due to the asymmetric arrangement of carriers in the membrane, the functioning of the ETC of chloroplasts leads to the accumulation of an excess amount of protons inside the thylakoid: hydrogen ions are absorbed from the outside at the stages of NADP + reduction and plastoquinol formation and are released inside the thylakoids at the stages of water decomposition and plastoquinol oxidation (Fig. . 4). Illumination of chloroplasts leads to a significant (100–1000 times) increase in the concentration of hydrogen ions inside the thylakoids.

So, we have looked at the chain of events during which the energy of sunlight is stored in the form of the energy of high-energy chemical compounds - ATP and NADP H. These products of the light stage of photosynthesis are used in the dark stages to form organic compounds (carbohydrates) from carbon dioxide and water. The main stages of energy conversion leading to the formation of ATP and NADP H include the following processes: 1) absorption of light energy by pigments of the light-harvesting antenna; 2) transfer of excitation energy to the photoreaction center; 3) oxidation of the photoreaction center and stabilization of separated charges; 4) electron transfer along the electron transport chain, formation of NADP H; 5) transmembrane transfer of hydrogen ions; 6) ATP synthesis.

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