Natural and Artificial Photosynthesis: Solar Power as an Energy Source

By Reza Razeghifard

This technical book explores current and future applications of solar power as an unlimited source of energy that earth receives every day.  Photosynthetic organisms have learned to utilize this abundant source of energy by converting it into high-energy biochemical compounds.  Inspired by the efficient conversion of solar energy into an electron flow, attempts have been made to construct artificial photosynthetic systems capable of establishing a charge separation state for generating electricity or driving chemical reactions.  Another important aspect of photosynthesis is the CO2 fixation and the production of high energy compounds.  Photosynthesis can produce biomass using solar energy while reducing the CO2 level in air.  Biomass can be converted into biofuels such as biodiesel and bioethanol. Under certain conditions, photosynthetic organisms can also produce hydrogen gas which is one of the cleanest sources of energy.

• Language: English
• Publisher: Wiley
• Release date: Aug 23, 2013
• ISBN: 9781118659755

Book preview

CHAPTER 1

Physics Overview of Solar Energy

DIEGO CASTANO

1.1 INTRODUCTION

Undoubtedly the most important factor in the study of solar energy is the sun, the local star and the gravitational stake to which the earth is tethered. All forms of energy on earth, except for nuclear, can ultimately be traced to the sun. There are on the order of a hundred billion stars in the Milky Way galaxy and about a hundred billion galaxies in the known universe. Stars began forming several hundred thousand years after the Big Bang, which, based on current theories of cosmology, happened 13.7 billion years ago. The sun itself formed about 5 billion years ago and should shine for another 5 billion. The earth consequently finds itself in a somewhat special place in space and time. The low entropy state of the solar system ensures vital change for billions of years to come. In terms of thermodynamics, the sun represents an effective high temperature reservoir of temperature 6000 kelvins that bathes the earth (the low temperature reservoir at an average 287 kelvins) with a radiant intensity of approximately 1400 W/m² above the atmosphere. Attenuation due to absorption and scattering in the atmosphere reduces this value. The exact value of solar irradiance at the earth’s surface depends on the sunlight’s path through the atmosphere and is generally no greater than 1000 W/m². The solar irradiance translates into a naive, maximal power output of approximately 10¹⁷ watts (compare this to the world’s average energy consumption rate in 2010, of just over 10¹³ watts [1]). It would appear that the sunlight reaching just earth (the earth subtends a mere 4.6×10−8% of the whole solid angle at the sun; see Fig. 1.1) can supply earth’s needs 10, 000 times over. In fact, Freeman Dyson considered the possibility that advanced civilizations would surround their suns with enough orbiting artificial satellites (this system is referred to as a Dyson sphere) to harness most of the star’s solar energy. In the case of the sun, an optimal Dyson sphere would generate on the order of 10²⁷ watts, a significant fraction of the luminosity of the sun (L inline = 4.8×10²⁷ W).

FIGURE 1.1 The earth subtends six nanosteradians at the sun.

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In fact, about 30% of the sunlight reaching the earth is reflected back into space above the atmosphere. The albedo, or fraction of reflected sunlight, depends strongly on such factors as snow and cloud cover. The 70% of the incident sunlight not reflected is effectively transmitted through the atmosphere, which is mostly transparent to visible light, to the surface. The thermal, or infrared, radiation emitted by the surface of the earth is then absorbed by the atmosphere and reradiated. This atmospheric greenhouse effect is what leads to a global mean surface temperature of 287 kelvins as compared to a freezing 254 kelvins without an atmosphere.

1.2 THE SUN

The standard theories of particle physics and cosmology describe the Big Bang as the moment of creation of space, time, matter, and energy. At first, there was just one superforce to mediate the interactions between particles. The superforce eventually separated out into the four known fundamental forces of today, that is, gravity, electromagnetism, weak and strong nuclear. All four of these forces are important in the development and function of the sun.

Early on (about 20 minutes after the Big Bang) the universe is a cauldron containing, among other particles, electrons and various light nuclei. Recombination, which occurs around 300, 000 years after the Big Bang, is the term used to describe the moment in the history of the universe when electromagnetically neutral bound states of matter were first possible in large numbers. In this era, the universe is a rarified gas consisting mostly of hydrogen (75%) and helium (25%). Gravity starts to make its presence known and leads to clumping due to inhomogeneities in the matter distribution. As a hydrogen/helium gas clump shrinks, the gravitational energy is turned into kinetic energy, resulting in temperatures at the core high enough to fuse the hydrogen and helium. At a temperature of ten million kelvins, protons can tunnel through the Coulomb repulsion barrier, resulting in the first nuclear reaction of the so-called proton–proton cycle, which is prevalent in stars like the sun,

(1.1) numbered Display Equation

Note that the fusing of the protons involves the strong nuclear force, and the appearance of the neutrino indicates that the weak nuclear force was also involved in this process. This reaction is followed by the reaction

(1.2) numbered Display Equation

The last reaction in the cycle can be any one of four with the most common in the sun being

(1.3) numbered Display Equation

The net energy released along this sequence is 27 MeV. This is only one sequence of many possible ones, all of which lead to ⁴He. In stars like the sun this eventually leads to an inert helium core with the hydrogen fusion occurring in the shell surrounding the core. This causes the star to grow and become a red giant. The helium core continues to collapse gravitationally until the temperature increases to the point at which helium–helium fusion can occur. The fusion of heavier and heavier nuclei continues in stars that are massive enough until ⁵⁶Fe is reached. This isotope of iron has the largest binding energy per nucleon, and fusion reactions that produce heavier nuclei are consequently endothermic. Although heavy elements can still be produced in stars, most heavy elements are created in supernovas.

One way astronomers classify stars is by their luminosities and spectral types or surface temperatures. The resulting scatter plot is called a Hertzsprung–Russell diagram (Fig. 1.2). Most stars, including the sun, fall in the region referred to as the main sequence.

FIGURE 1.2 Sketch of Hertzsprung–Russell diagram.

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1.3 LIGHT

The history of the developments in the theory of light is a long one. The Greeks, from as far back as Pythagoras, believed that light emanated from visible bodies. Some even philosophized that it traveled at a finite speed. However, most people before the 17th century believed that light was instantaneous. Galileo is credited with being the first well-known scientist to attempt to measure the speed of light. His experiment involved him and an assistant on distantly separated hills with lanterns and some sort of time measuring device, perhaps a water clock. Due to the inherent lack of precision in the design of the experiment, his result was ambiguous. Of the speed of light, he is alleged to have said, If not instantaneous, it is extraordinarily rapid. He also concluded that the speed was at least ten times faster than sound. About a decade later, Ole Rømer used essentially the same idea as Galileo—that of measuring the time it takes a light signal to cover a spatial path—but Rømer did it with a longer path, the diameter of the earth’s orbit around the sun. His results were considerably better (c = 2×10⁸ m/s). It took about another 200 years to perfect an earth bound experiment, but Leon Foucault performed an experiment involving rotating mirrors (based on one designed by Hippolyte Fizeau using rotating toothed wheels) that was able to measure the speed of light accurately to good precision and that agrees with modern measurement to four significant figures (c = 2.998×10⁸ m/s).

The nature of light, whether it be a particle or wave phenomenon, was a topic of debate in the 17th century. Isaac Newton was on the particle side and referred to the constituent particles as corpuscles. On the wave side was Christian Huygens. Newton’s reputation helped the particle side, but the phenomena of interference and diffraction weighed heavily on the side of the wave theory. Today, at least in classical terms, light is recognized as a wave phenomenon, but it is interesting to note that quantum mechanics has forced reconsideration and Newton’s corpuscles can be thought resurrected as photons.

The unification of electricity and magnetism made by James Clerk Maxwell in the mid-19th century put the wave nature of light on firm theoretical footing. Maxwell’s correction to Ampere’s law led him to wave equations for electric and magnetic fields with a wave speed relation that involved electric permittivity and magnetic permeability constants

(1.4) numbered Display Equation

This calculable wave speed coincided with the experimentally measured speed of light. The conclusion was inescapable: light is a manifestation of electromagnetic waves. In 1800 Frederick William Herschel had ascertained the existence of an invisible form of light by noting that the shadow region beyond the red end of a prism-induced spectrum of sunlight registered a higher temperature than the red lighted region. This invisible light is recognized today as the infrared.

Although it is far afield from the main thrust of this overview, it is perhaps interesting to note that questions immediately arose concerning the nature of the supporting medium for these waves. Whatever the medium was, it pervaded all of space and was referred to as the ether. The possibility of anisotropies in the speed of light due to the ether would occupy the minds of theorists and the efforts of experimentalists, in particular, Albert Michelson and Edward Morley, until the beginning of the 20th century. The null result of the late 19th century Michelson–Morley experiment to detect the so-called ether wind was, in retrospect, consistent with Albert Einstein’s theory of relativity.

Along with the theory of relativity, the early 20th century saw the development of quantum mechanics. The understanding of various light phenomena was the catalyst for its inception. The story begins with the concept of a blackbody, an idealized body capable of absorbing all incident electromagnetic radiation. This is to be compared with a real body, which reflects and/or transmits some fraction of the incident radiation. A perfect absorber is also a perfect emitter by Kirchhoff’s law of radiation, so a blackbody is also a perfect emitter. A kiln with a small opening is a good realization of a blackbody emitter. The radiation emitted was measured (using a bolometer), and two important facts were discovered. The intensity radiated (or total radiant emittance) at all wavelengths depends only on the absolute surface temperature of the blackbody according to the Stefan–Boltzmann law

(1.5) numbered Display Equation

where σ = 5.67×10−8 W/(m²⋅K⁴) is the Stefan–Boltzmann constant. Also discovered was the fact that there was a peak to the intensity distribution, known as Wien’s law, at

(1.6) numbered Display Equation

Max Planck set out to theoretically derive the blackbody intensity distribution using the theory of electromagnetism and the laws of thermodynamics. He met with failure until he hypothesized the radiation quantum of energy

(1.7) numbered Display Equation

where h = 6.63×10−34 J⋅s and f is the frequency of the radiation. With this assumption, he was able to derive Planck’s law of blackbody radiation (see Fig. 1.3):

(1.8) numbered Display Equation

FIGURE 1.3 Characteristic blackbody curve.

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By fitting the sun’s radiation curve to this formula, its surface temperature can be deduced to be 6000 kelvins. The earth’s own effective radiant temperature is around 240 kelvins.

Planck’s idea was applied by Einstein to explain the photoelectric effect. In the late 19th century experiments conducted by Heinrich Hertz and Philipp Lenard showed that the energy of electrons ejected from a metal upon electromagnetic irradiation was independent of the radiation’s intensity but depended on its frequency. By assuming that the electromagnetic radiation transferred only a quantum of energy to the electrons in the metal, Einstein was able to explain the effect. The predicted maximum energy of a free photoelectron was

(1.9) numbered Display Equation

where Φ is the work function, the minimum energy required to strip the electron from the metal. This is the basis of the photoemissive cell or phototube (see Fig. 1.4).

FIGURE 1.4 A current produced by the photoelectric effect.

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1.4 THERMODYNAMICS

It is often stated that thermodynamics was developed in response to the desire to make a better steam engine. As such it can be thought of as the study of thermal energy conversion. So what is energy? The term is pervasive in the modern world, and the concept is often reified, especially in scientifically informal settings. Energy is, in fact, an abstract concept, and its conservation (constancy in time) is a powerful organizational principle in physics. It comes in an apparent myriad of forms, such as solar, chemical, electrical, and even dark, but they can all be placed into two broad categories, kinetic and potential. Perhaps the most intuitive, if not the most perceptively apparent, is the kinetic type: If there is motion in a system, then there is energy. Conversely, and naively to be sure, energy is what makes things go.

There is (thermodynamic) energy associated with matter due to its atomic nature and resulting from the randomized (thermal) motion and from the interactions of the constituent particles. One of the most important results from the kinetic theory of gases is that temperature is a measure of the average kinetic energy of the constituents of matter. The finite size of bodies implies that the kinetic energies of their constituents must be changing due to collisions and oscillations. It is known from atomic theory that matter contains charged particles, such as electrons and protons. Maxwell’s electromagnetic theory predicts that accelerated charged particles will radiate. Therefore all bodies at nonzero absolute temperature should radiate, and Eq. 1.8 predicts the spectrum of an ideal one.

In thermodynamics, the universe is divided into two parts, the system under consideration and its surroundings. All exchanges between system and surroundings are done across a boundary, the real or effective surface separating the two. The total energy of a thermodynamic system is referred to as its internal energy (U) and includes kinetic energies as well as the potential energies of particle interactions. In thermodynamics, heat (Q) is the process variable that represents the microscopically unobservable transfer of energy, whereas work (W) represents the macroscopically observable transfer of energy. Heat transfer is typically driven by diathermal, or unrestricted, contact between two bodies of differing temperatures. A boundary that does not permit the flow of heat is called adiabatic. The first law of thermodynamics is the statement of energy conservation. Any changes in the internal energy of a system must be the result of energy transfer across the boundary,

(1.10) numbered Display Equation

where d′ implies an inexact differential, and the work is considered done by the system.

Colloquially, the task of building an engine, for example, a steam engine, amounts to devising a way to turn internal into external energy, or how to use a source of heat to make something go. The first steam engine (the aeolipile; see Fig. 1.5) is credited to Hero of Alexandria, who lived in the 1st century A.D. A tropical hurricane is a natural example of a steam engine, effectively using the thermal energy in the surface water of the ocean to power wind. James Watt perfected the modern steam engine in the late 18th century. In all engines, some energy (in fact, generally quite a bit) is always wasted and in a form that is useless to the engine. The design details of the engine will affect the exact amount of wasted energy but there is always a nonzero amount, as surely as heat always flows spontaneously from higher to lower temperature. The second law of thermodynamics is the statement that the preceding empirical observation is indeed the case. The second law can be put into a mathematical form by noting that there exists a function of the extensive parameters of a system in a state of equilibrium that is maximized in any spontaneous process. The function is called the entropy (S) and for quasistatic processes

(1.11) numbered Display Equation

FIGURE 1.5 The aeolipile.

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Engines exploit a temperature difference to extract useful work. The ideal cyclic engine, given two working temperatures, is one for which the entropy change of the universe is zero (i.e., its operation is reversible). Such an engine is called a Carnot engine, named after Sadi Carnot who first conceptualized it. The Carnot cycle consists of two isothermal processes and two adiabatic ones. The Carnot engine’s efficiency depends on the two reservoir temperatures,

(1.12) numbered Display Equation

where TL(H) is the lower (higher) heat reservoir temperature. Unfortunately, the Carnot engine is an idealization. No heat transfer process is ever reversible in practice. Moreover, the sequence of processes associated with the Carnot cycle would be difficult to realize in a practical way. A more practical engine than Carnot’s, with a theoretical efficiency that nevertheless matches Carnot’s, is the Stirling engine (with regenerator). It is an external combustion engine that uses a single phase working substance (a gas, such as air). The Stirling cycle consists of two isothermal and two isochoric processes (see Fig. 1.6). Although they generally have low power outputs for their size, Stirling engines are relatively easy to make and can exploit even small temperature differences.

FIGURE 1.6 Stirling cycle.

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There are at least four obvious ways to use direct energy from the sun. The first involves the direct absorption of sunlight. Architecturally, living spaces can be warmed by designing them to take advantage of sunlight. Water can also be heated directly by the sun for different uses such as space heating. By using devices, such as Fresnel lenses or parabolic mirrors, the sun’s rays can be concentrated to increase input. Fresnel lenses have large aperture and dioptric power. Parabolic mirrors have the property that incident parallel light is focused without aberration. Both of these can collect light over a relatively large area and intensify it significantly.

The second way to exploit the thermal energy from the sun is to use an engine, perhaps in conjunction with some focusing system such as the ones discussed above. The Stirling engine can easily be adapted to exploit an external heat source and has consequently gained popularity in the solar energy business. Thermoelectric generators based on the Seebeck, or thermoelectric, effect can also be used to convert thermal energy into electricity, although these are usually less efficient than Stirling engines.

The third way is the direct conversion of sunlight into electricity and is the subject of Section 1.5.

1.5 PHOTOVOLTAICS

As discussed above, photoemission can be used to generate a current, but there is another related photovoltaic effect that involves semiconductors rather than metals. The discovery of the photovoltaic effect is credited to Alexandre-Edmond Becquerel in 1839. He discovered that illuminating one of the electrodes in an electrolytic cell caused the current to increase. The effect was also seen in solids, like selenium, in the late 19th century. In 1954, Bell Laboratories produced the first photovoltaic cell using a crystalline silicon semiconductor. In the photovoltaic effect, like in the photoelectric effect, light energy is absorbed by electrons, but, unlike in the photoelectric effect, the electrons are not ejected from the semiconductor.

Crystalline silicon, for example, has an extended, regular atomic pattern called a lattice. The basic unit of the lattice is a group of five silicon atoms. They form a geometrical array in which one is at the center of a tetrahedron and the other four at the centers of its faces. This structure is a consequence of the four outer valence electrons in silicon’s third, or M, shell. When many identical atoms in close proximity are considered together, such as in a lattice, their electron’s otherwise discrete energies (i.e., in isolation) are smeared into bands. In insulators, the highest band that is occupied by electrons at absolute zero is called the valence band. The next higher band, which is empty in this case, is called the conduction band. The energy gap between these bands in insulators is relatively large compared to characteristic thermal energies (0.025 eV at T = 300 K), so even above absolute zero an insulator has an effectively empty conduction band. In a conductor, the conduction band contains electrons even at absolute zero. In simplest terms, the electrons in the conduction band can be considered effectively free, that is, not bound to any one atom and therefore able to roam throughout the conductor. The electrons that are freed from covalent bonds leave a positively charged hole in the fixed lattice substratum. When a hole is filled by a neighboring electron, it in turn leaves behind a hole. The effective movement of the hole resembles that of a positively charged particle; the effect is similar to that of bubbles in a fluid. In a semiconductor, at absolute zero, the conduction band is empty like for an insulator. However, the energy gap is significantly smaller than in an insulator, and as the temperature rises thermal agitation is sufficient to lift electrons into the conduction band, making the material a conductor. Silicon has an energy gap of 1.1 eV at 300 K. In fact, every electron in the conduction band is accompanied by a hole in the valence band.

To change the balance of electrons in the conduction band and of holes in the valence band, a semiconducting material must be doped; that is, an impurity must be introduced into the otherwise homogeneous lattice. If phosphorus is introduced into the silicon lattice it will form the same four covalent bonds, but it will have an extra loosely bound electron. The energy level of these extra electrons lies in the energy gap just below the conduction band, so they readily become conduction electrons leaving behind a fixed positive ion. Impurities that have an extra valence electron, like phosphorus, are called donors. The semiconductor produced by donor doping is called n-type since the majority charge carriers are negative electrons. In an analogous fashion, silicon can be doped with an impurity that has one less valence electron, like boron. This type of impurity is called an acceptor. In this case the boron takes an electron from a neighboring silicon atom and creates a hole in the valence band. The energy level of these stolen (acceptor) electrons is just above the valence band. The holes are free to move as positive charge carriers, and the negative boron ions are fixed. The semiconductor produced by acceptor doping is called p-type since positive holes are the majority charge carriers.

Light incident on a semiconductor will generate electron–hole pairs if the photon energy is greater than the band gap. However, the pair will recombine unless the two carriers can be kept separated. Therefore to generate electrical power from incident light on these semiconductor materials requires a potential barrier. This barrier can be set up through the juxtaposition of p- and n-type materials. At the junction, both conduction and valance electrons move from the n-type side to the p-type side, thereby filling holes in the p-side and creating holes in the n-side. This process is finite and happens quickly. It creates a net fixed positive charge on the n-side of the junction and a net negative fixed charge on the p-side. This is known as the depletion zone and represents the potential barrier. It stops any further flow of electrons into the p-side and of holes into the n-side, but it does not prevent electrons from moving into the n-side and holes from moving into the p-side. Ideally a light-generated electron–hole pair will be separated by this mechanism and an emf generated.

If the two sides of the solar cell are connected through a load, a current will flow (see Fig. 1.7). Unfortunately, not all electron–hole pairs generated in this way will contribute to the current. The probability that a pair will contribute depends on many factors, one of which is the location within the cell where the pair forms. Those pairs created in the depletion zone will separate with certainty, then with high probability avoid combination and contribute to the current. Efficiency is typically a measure of output to input ratio. In the case of a solar cell, the input is solar energy and the output is electrical energy. The efficiency of a solar cell depends on many factors such as the one just described. Among other things, it also depends on the fraction of incident light absorbed and the nature (intensity and spectrum) of that light. The selenium cells of the late 19th century had 1% efficiency. The 1954 Bell Labs silicon cell had 4% efficiency. The maximum theoretical efficiency of a single p-n junction solar cell is given by the Shockley–Queisser limit of 31%. By using focusing devices and multijunction cells, that efficiency can be increased to 41%. A naive application of Eq. 1.12 would set an absolute upper limit of 95%. A more sophisticated calculation yields an upper limit of 86.6% [2].

FIGURE 1.7 A solar cell in action.

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1.6 PHOTOSYNTHESIS

This book is focused on the topic of photosynthesis. In this way, light energy generates an electron flow, which results in the production of energetic molecules that make reduced organic compounds. The photosynthetic mechanism absorbs light energy by using pigments, especially chlorophyll (Chl) molecules. Photosynthetic organisms only need 1% of the solar spectrum to provide enough biomass and oxygen to support life on earth. Two photosystems work in tandem to carry out oxygenic photosynthesis. They are very efficient systems, using most of their pigments as antennas to harvest the light energy and then to transfer it to a very few, special Chl molecules. Photochemistry begins when these special Chl molecules donate electrons, which travel through the electron transport chain, reaching nicotinamide adenine dinucleotide phosphate (NADP+) at photosystem I. NADPH is used for CO2 fixation into organic compounds. The ultimate electron donor in oxygenic photosynthesis is water, which is oxidized to oxygen by photosystem II.

For more detailed accounts of the subjects discussed in this overview, see [3–6], and [7].

REFERENCES

1. British Petroleum, Statistical Review of World Energy 2011, http://www.bp.com/sectionbodycopy.do?categoryId=7500&contentId=7068481 (accessed September 1, 2011).

2. M. A. Green, Third Generation Photovoltaics: Advanced Solar Energy Conversion, Springer, New York, 2006.

3. J. W. Rohlf, Modern Physics from α to Z0, 1st ed., Wiley, Hoboken, NJ, 1994.

4. M. L. Kutner, Astronomy: A Physical Perspective, 2nd ed., Cambridge University Press, Cambridge, U.K., 2003.

5. E. Hecht, Optics, 4th ed., Addison-Wesley, Reading, MA, 2002.

6. H. B. Callen, Thermodynamics: An Introduction to the Physical Theories of Equilibrium Thermostatics and Irreversible Thermodynamics, Wiley, Hoboken, NJ, 1960.

7. C. Kittel, Introduction to Solid State Physics, 8th ed., Wiley, Hoboken, NJ, 2005.

CHAPTER 2

Oxygenic Photosynthesis

DMITRIY SHEVELA, LARS OLOF BJÖRN, and GOVINDJEE

2.1 INTRODUCTION

2.1.1 Importance of Photosynthesis: Why Study Photosynthesis?

In a general sense the term photosynthesis is synthesis of chemical compounds by the use of light. In the more restricted sense, as we shall use it here, it stands for the process by which plants, algae, cyanobacteria, and phototrophic bacteria convert light energy to chemical forms of energy. Most photosynthesis is coupled to assimilation of carbon in the form of carbon dioxide or bicarbonate ions, but there exists also assimilation of CO2 that is not coupled to photosynthesis, as well as photosynthesis that is not coupled to assimilation of carbon.

All life on Earth, with some exceptions, is completely dependent on photosynthesis. Most organisms that do not live directly by photosynthesis depend on the organic compounds formed by photosynthesis and, in many cases, also on the molecular oxygen formed by the most important type of photosynthesis, oxygenic photosynthesis. Even much of the energy fueling the ecosystems at deep-water hydrothermal vents depends on photosynthesis, since it is made available to organisms using molecular oxygen of photosynthetic origin. In addition, photosynthesis is biologically important in a number of more indirect ways. The stratospheric ozone layer protecting the biosphere from dangerous ultraviolet radiation from the sun is formed from photosynthesis-derived oxygen by a photochemical process. The photosynthetic assimilation of CO2, and associated processes such as formation of carbonate shells by aquatic organisms, has (so far) helped to maintain the climate of our planet in a life-sustainable state. For basic descriptions of photosynthesis, see Rabinowitch [1] and Blankenship [2], and for reviews on all aspects of Advances in Photosynthesis and Respiration Including Bioenergy and Other Processes, see many volumes at the following web site: http://www.springer.com/series/5599.

Rosing et al. [3] speculate that photosynthesis has also caused the formation of granite and the emergence of continents. Granite is common, among bodies in the solar system, only on Earth. After oceans were first formed there were no continents, the surface of the Earth was completely aquatic. Granite with its lower density is, in contrast to the heavier basalt, able to float high on the Earth’s liquid interior. Thus photosynthesis is very important for life on Earth, and worth a thorough study just for its biological and geological importance. In recent years it has also attracted much interest in connection with the search for a sustainable energy source that can replace nuclear power plants and systems that release greenhouse gases to the atmosphere. There is much interest in solar fuels today: see http://blogs.rsc.org/cs/2012/09/25/a-centenary-for-solar-fuels/ for a special collection of articles and opinions to mark the centenary of Ciamician’s paper The Photochemistry of the Future [4].

2.1.2 Oxygenic Versus Anoxygenic Photosynthesis

The form of photosynthesis that first comes to mind when the term is mentioned is that carried out by the plants we see around us. It is called oxygenic photosynthesis because one of its products is molecular oxygen, resulting from the oxidation of water [5]. This form of photosynthesis is also carried out by algae and by cyanobacteria (formerly called blue-green algae) (for a perspective on cyanobacteria, see Govindjee and Shevela [6]). Photosynthesis by bacteria other than cyanobacteria, on the other hand, does not involve evolution of O2. Instead of water (H2O), other electron donors, for example, hydrogen sulfide (H2S), are oxidized. This latter type of photosynthesis is called anoxygenic photosynthesis [7, 8]. In addition to these processes, some members of the third domain of life, the Archaea, as well as some other organisms, carry out conversion of light into electric energy by carrying out light-dependent ion transport. Although this biological process, which strictly speaking is not photosynthesis, could also be a useful guide to technological applications, we shall not deal with it in this chapter (see, however, Oesterhelt et al. [9]).

The reactions of oxygenic photosynthesis in algae and plants take place within a special cell organelle, the chloroplast (see Fig. 2.1). The chloroplast has two outer membranes, which enclose the stroma. Inside the stroma is a closed membrane vesicle, the thylakoid, which contains the lumen. The stroma is the site where the CO2 fixation reactions occur (the dark reactions of photosynthesis; described in Section 2.5); the thylakoid membrane is the site for the conversion of light energy into energy of the chemical bonds (the light reactions; discussed in Sections 2.2 and 2.3). In cyanobacteria, however, the thylakoid membrane is within the cytoplasm.

FIGURE 2.1 Three-dimensional diagrammatic view of a chloroplast.

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2.1.3 What Can We Learn from Natural Photosynthesis to Achieve Artificial Photosynthesis?

Natural photosynthesis is characterized by a number of features, which are useful to keep in mind when trying to construct useful and economically viable artificial systems [10]:

1. Use of antenna systems that concentrate the energy.

2. Regulation of antenna systems by light.

3. Use of quantum coherence to increase efficiency.

4. Connection, in series, of two photochemical systems to boost electrochemical potential difference.

5. Protective systems and safety valves to prevent overload and breakdown.

6. Self-repair of damaged components.

We must consider constructing artificial systems from common and cheap materials that may be available everywhere. One should also bear in mind that perhaps plants do not optimize the process toward the same goal as we wish them to do. Maximizing energy conversion is not always the best strategy for an organism; they have evolved for survivability.

Attempts are being made on many fronts including mimicking the manganese–calcium cluster of PSII for energy storage (for more details see Chapters 3 and 4, and Refs. [11–18]).

2.1.4 Atomic Level Structures of Photosynthetic Systems

By means of X-ray diffraction studies of protein crystals and other methods, the detailed atomic structure of some photosynthetic systems are now available (for recent reviews on the structures of photosynthetic complexes, see Refs. [19–22]). They have revealed great similarity between the cores of the two photosynthetic systems (PSI and PSII) present in oxygenic organisms, and the cores of photosystems present in various groups of bacteria carrying out anoxygenic photosynthesis, indicating a common evolutionary origin of all photosynthesis (e.g., see Refs. [23–25]). Among many other structures of photosynthetic systems, we mention, at the very outset, that we now have available atomic level structure of the PSII at 1.9 Å resolution [26].

2.1.5 Scope of the Chapter

This chapter is intended as a background on natural photosynthesis for those interested in artificial photosynthesis. We start with a description of how light is used for creating positive and negative charges, and continue with how these charges are transferred through the molecular assemblies in the membranes. Next, we describe how the charge transport leads to creation of a pH difference across the photosynthetic membrane, and how charge and pH differences lead to the production of high-energy phosphate that can be used in chemical synthesis. Finally, we deal with the time dimension, how the type of photosynthesis present today has evolved over billions of years, and what can we expect of the future that we are ourselves able to influence. In addition, in the end, we consider some interesting photosynthesis-related questions relevant to whole land and aquatic plants.

2.2 PATH OF ENERGY: FROM PHOTONS TO CHARGE SEPARATION

2.2.1 Overview: Harvesting Sunlight for Redox Chemistry

The initial event in photosynthesis is the light absorption by pigments: chlorophylls (Chls), carotenoids (Cars), and phycobilins (in cyanobacteria and in some algae), contained in antenna protein complexes (for overviews of light-harvesting antenna, see Green and Parson [27], for Chls, see Scheer [28] for Cars, see Govindjee [29], and for phycobilins, see O’hEocha [30]). The absorbed energy is transferred from one antenna pigment molecule to another in the form of excitation energy until it reaches reaction centers (RCs), located in two large membrane-bound pigment–protein complexes named photosystem I (PSI) and photosystem II (PSII) (see Figs. 2.2 and 2.3). Due to the primary photochemistry, which takes place after trapping of the excitation energy by special photoactive Chl molecules in the RCs of these two photosystems, light energy is converted into chemical energy. This energy becomes available for driving the redox chemistry of the stepwise extraction of electrons from water and their transfer to NADP+ (oxidized form of nicotinamide adenine dinucleotide phosphate) (for further details, see Section 2.3). In this section we briefly describe how photosynthetic organisms capture light energy and how this energy migrates toward the RC Chl molecules, where the primary photochemical reactions occur.

FIGURE 2.2 A schematic view of the photosynthetic thylakoid membrane and the protein complexes involved in the light-induced electron transfer (black solid arrows) and proton transfer (black dashed arrows) reactions in the thylakoid membrane of chloroplast in plants and algae. The end result of these light reactions is the production of NADPH and ATP. NADPH and ATP drive the dark reactions (grey arrows) of CO2-fixation in stroma of the chloroplast via a cyclic metabolic pathway, the so-called the Calvin–Benson–Bassham cycle (also called by some as Calvin cycle, or Calvin-Benson cycle). This results in the reduction of CO2 to energy-rich carbohydrates (e.g., sucrose and starch). See text for abbreviations and further details. Adapted from Messinger and Shevela [247].

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FIGURE 2.3 Excitation energy transfer in light-harvesting antenna that leads to primary photochemistry (charge separation) at the reaction center of photosynthetic organisms. Light energy is transferred through photosynthetic pigments of the light-harvesting antenna until it reaches reaction centers, where primary charge separation takes place. Abbreviations: P, reaction center Chl a molecule; e−, electron; A, an electron acceptor; D, an electron donor. For further description see text. Adapted from Messinger and Shevela [247].

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2.2.2 Light Absorption and Light-Harvesting Antennas

The function of all light-harvesting antennas in photosynthetic organisms is common to all, that is, capture of light energy through absorption of photons of different wavelengths, and its transfer to RC complexes where photochemistry (the primary charge separation) takes place (see Fig. 2.3).

The process of photosynthesis starts in femtosecond time scale (∼10−15 s) by light absorption in pigments, located in the light-harvesting antenna. Within less than a second, thylakoid membranes release O2 and produce reducing power (reduced form of nicotinamide adenine dinucleotide phosphate or NADPH) and adenosine triphosphate (ATP). Kamen [31] used a pts (negative log of time) scale, analogous to the pH scale, to describe this process that spans pts of +15 to −1. The process of light absorption in any pigment molecule in the antenna, say, a Chl a molecule, implies that when a photon has the right energy (E = hc/λ, where h is Planck’s constant, c is velocity of light, and λ is the wavelength of light), the molecule, which is in the ground state, will go to its excited state (Chl*): one of the two outermost electrons, spinning in the opposite directions, is transferred to the higher excited states. An excited singlet state is produced (¹Chl a*). This process is very fast: it occurs within a femtosecond, as mentioned above. Figure 2.4 shows the relation between the absorption spectrum of a Chl a molecule and its energy level diagram, the Jabłonski–Perrin diagram [32]. It shows that blue light (440 nm) will take the molecule to the nth excited state, whereas the red light (672 nm; or 678 nm, depending on the Chl a species) will take the molecule to its first excited state. The higher excited state is very unstable and within a pts of +14 to +13, the electron falls down to the lowest excited state; the extra energy is lost as heat. No matter what color of light is absorbed, the photochemical processes begin from this lowest excited state.

FIGURE 2.4 A Jabłonski–Perrin diagram of the energy levels in a Chl molecule with spectral transitions between them (vertical arrows) and absorption spectrum (turned 90° from the usual orientation) of Chl a corresponding to these levels. Diagram shows heat loss as radiationless energy dissipation (downward-pointing wiggly arrow): other radiationless energy dissipation processes such as fluorescence emission and intersystem crossing are not shown here. Note that the short- (blue) and long-wavelength (red) absorption bands of Chl absorption spectrum correspond to the absorption by this molecule of blue and red photons, respectively. Thus the red absorption band corresponds to the photon that has energy required for the transition from the ground state to the lowest excited state, while the blue absorption band reflects the transition to a higher excited state.

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Plants and green algae have major and minor antenna complexes in both the photosystems (I and II). In PSII, there is a major complex, the LHCII (light-harvesting complex II), with many subcomplexes, and minor complexes that include CP43 (Chl–protein complex of 43 kDa mass) and CP47 (Chl–protein complex of 47 kDa mass). LHCII contains both Chl a and Chl b (the latter has absorption maxima at 480 nm and 650 nm), whereas CP43 and CP47 contain only Chl a. Chl b transfers energy to Chl a with 100% efficiency as has been known for a very long time (e.g., see Duysens [33, 34]). In addition, there are Cars that also transfer excitation energy to Chl a, with different efficiencies (see Govindjee [29]); the Cars, in general, are of two types: carotenes and xanthophylls (the mechanism of their energy transfer to Chl a is, however, unique and different; e.g., see Zigmantas et al. [35] and Zuo et al. [36] for a discussion).

Brown algae, yellow-brown and golden-brown algae, and diatoms contain, in addition to Chl a, fucoxanthin as a xanthophyll, and various forms of Chl c, instead of Chl b [37]. Chls c1 and c2 have absorption maxima at ∼630 nm; and Chl c3 at 586 nm. Fucoxanthin absorbs in the green (535 nm) and gives the organisms brown color; cryptomonads and dinoflagellates contain peridinin (absorption peak at 440–480 nm [38]) instead of fucoxanthin. On the other hand, red algae have water-soluble red and blue pigment-proteins, the phycobilins: phycoerythrins (absorption peak at 570 nm), phycocyanins (at 630 nm), and allophycocyanins (at 650 nm), absorbing green to orange light [39, 40].

The oldest oxygenic photosynthesizers are cyanobacteria (they were called blue-green algae before their prokaryotic nature was realized) (for a perspective, see Govindjee and Shevela [6]). Being prokaryotes they do not have chloroplasts. They contain Chl a and phycobilins like the red algae, and phycoerythrin, the red pigment, is also present in some cyanobacteria; these cyanobacteria capture light that is not absorbed by green algae [41, 42], and thus they have different ecological niches in nature [43, 44]. The major LHCs of cyanobacteria are the phycobilisomes (PBS) that are made of the phycobiliproteins attached to the cytoplasmic surface of thylakoid membrane (for further details on the cyanobacterial PBS, see Mimuro et al. [45] and Sidler [46]). Interestingly, Chl b, that is, with some exceptions, not present in wild type cyanobacteria, can be introduced by genetic engineering into cyanobacteria [47]. For recent overviews on the LHCs of photosynthetic organisms, see Collines et al. [48] and Neilson and Durnford [49].

2.2.3 Excitation Energy Transfer: Coherent Versus Incoherent or Wavelike Versus Hopping

2.2.3.1 A Bit of History

In 1936, Gaffron and Wohl [50] were the first to discuss excitation energy transfer (or migration) among hundreds of Chl molecules, in what we now call antennas before it reaches what we now call RCs, and what Emerson and Arnold in 1932 [51] had called a unit (that could be interpreted as a photoenzyme). The concept of the photosynthetic unit serving a photoenzyme (RC in today’s language) was born in the experiments of Emerson and Arnold [51, 52], who found that a maximum of only one oxygen molecule evolved per thousands of Chl molecules present (for a review, see Clegg et al. [53]). In 1943, Dutton et al. [54] and in 1946 Wassink and Kersten [55] were among the first to demonstrate efficient excitation energy transfer from fucoxanthin to Chl a in a diatom (Nitzschia sp.) using the method of sensitized fluorescence: excitation of fucoxanthin led to as much Chl a fluorescence as excitation of Chl a did (see Govindjee [29]). Using the same sensitized fluorescence method, Duysens (in 1952) [33] showed 100% excitation energy transfer from Chl b to Chl a in the green alga Chlorella, and about 80% transfer from phycocyanin to Chl a in the cyanobacterium Oscillatoria. In 1952, both French and Young [39] and Duysens [33] showed efficient excitation energy transfer, in red algae, from phycoerythrin to phycocyanin and from phycocyanin to Chl a (however, later, it was realized that a distinct kind of phycobiliprotein, allophycocyanin, carries energy from phycocyanin to Chl a). Such excitation energy transfers from one type of pigment to another may be dubbed heterogeneous excitation energy transfer. On the other hand, Arnold and Meek [56] showed for the first time that when Chl a molecules in Chlorella cells were excited with polarized light, an extensive depolarization of fluorescence was observed; this was evidence of excitation energy migration among Chl a molecules. Such energy migration can be dubbed as homogeneous energy transfer since it is between the same type of pigment molecules. One of the first measurements of the time of excitation energy transfer was performed by Brody in 1958 [57] when he observed a delay of about 500 ps for energy transfer from phycoerythrin to Chl a, using a home-built instrument for measuring lifetime of fluorescence. Furthermore, excitation energy transfer from various pigments to Chl a, and one spectral form of Chl a to another, was found to be temperature dependent down to 4 K (see Refs. [58–60]). For further historical details, see Govindjee [61].

2.2.3.2 Mechanism of Excitation Energy Transfer

There are two extreme cases [1]: (1) When there is a very strong coupling between the neighboring pigment molecules, the net result is that the exciton (excitation energy) formed from the absorbed photon belongs to all the pigment molecules, but not to one. There is thus quantum coherence in the system, and the motion of the exciton has a wavelike character. The exciton is delocalized [53, 62, 63]. (2) When there is a very weak (or even weak) coupling between the neighboring pigment molecules, the net result is that the excited pigment molecule formed from the absorbed photon belongs to that specific molecule only. There is thus quantum incoherence in the system, and the motion of the excitation energy has a hopping character. At one specific time, the excitation energy is said to be localized on a specific molecule [64–66]. Excitation energy transfer in this case is by the Förster resonance energy transfer (FRET), the magnitude of which depends (i) inversely on R⁶, where R is the distance between the donor and the acceptor molecules; (ii) on the overlap integral of the absorption spectrum of the acceptor molecule and the emission spectrum of the donor molecule; and (iii) the so-called orientation factor, κ² [67, 68]. In photosynthetic systems, both the coherent (delocalized, wavelike) and incoherent (localized, hopping) mechanisms exist (for basics on fluorescence spectroscopy, see Colbow and Danyluk [69] and Lakowicz [70]).

Other sophisticated theories, besides the Förster theory, have evolved, which incorporate additional details and concepts and are applicable to several photosynthetic systems. They are the Redfield theory, the modified Redfield theory, and the generalized Förster theory [71–75].

When the pigment–pigment excitonic interaction coupling is weak, but the pigment–protein excitonic interaction–vibrational coupling is strong, and the excitation energy is localized, the classical FRET [66] mechanism applies (see Kleima et al. [76] as, for example, in the case of peridinin–Chl a system). However, when the pigment–pigment interaction coupling is strong, and the pigment–protein interaction coupling is weak, and the excitation is delocalized, a different mechanism called the Redfield theory applies (see Redfield [77] and Renger et al. [78]). On the other hand, when both the pigment–pigment and pigment–protein interaction couplings are strong, a modified Redfield theory applies [73]. For the case of energy transfer in LHCII, see Novoderezhkin et al. [79].

In PSII and PSI complexes, and in anoxygenic bacterial photosystems, we have pigment–protein domains that have strong coupling within them, but weak coupling between the domains. Thus interdomain excitation energy transfer would be by Förster theory, but the integrated mechanism would require extension of this theory to include coherence within individual domains (using Redfield or modified Redfield theory); the final description is called generalized Förster theory (see, e.g., a description of excitation energy transfer in PSII core complexes [80]).

In our opinion, the Förster theory must be applicable to excitation energy transfer within the phycobiliproteins, when excitation energy is transferred from phycocyanin to allophycocyanin and then from allophycocyanin to Chl a [41]. For examples and discussion of Förster energy transfer in other photosynthetic systems, see Şener et al. [81] and Jang et al. [82] and, for a historical perspective, see Clegg et al. [53]. On the other hand, Ishizaki and Fleming [62] and Ishizaki et al. [83] discuss the ramifications of coherent energy transfer in photosynthetic systems, whereas Collini et al. [84] show coherent energy transfer in marine algae at room temperature. Quantum coherence is inferred when oscillations of exciton state populations, lasting up to a few hundred femtoseconds, are observed. This happens when pigment–pigment interaction coupling is very strong. The reality is that both coherent and incoherent mechanisms occur in natural systems. However, there are many open questions that remain to be answered!

2.2.4 Concluding Remarks and Future Perspectives for Artificial Photosynthesis

Sunlight is a dilute form of energy traveling at a speed that is beyond our comprehension. Photosynthetic organisms have learned over billions of years of trial and error how to catch it, concentrate it, and convert it in an efficient way to electrical energy for further use. We have attempted here to describe what is known about this process. An urgent task for humanity is to explore this further and adapt the process for solving our present technological energy crisis by what we may call artificial photosynthesis.

2.3 ELECTRON TRANSFER PATHWAYS

2.3.1 Overview of the Primary Photochemistry and the Electron Transfer Chain

The photosynthetic electron transfer chain (ETC) from water to NADP+ is energized by two membrane-bound photosystems. Thus the end result of the steps of light harvesting by the antenna complexes of PSI and PSII (or by phycobilisomes in cyanobacteria) is the capture of excitation energy by the ensemble of unique photoactive Chl molecules, denoted P, in the RC of photosystems (see Fig. 2.3). In PSII, this special photoactive RC is composed of several Chl a molecules, dubbed P680, and in PSI, the special RC is a heterodimeric complex of Chl a and Chl a′, dubbed P700 (the numbers 680 and 700 are based on the wavelengths of the absorption maxima of these Chls in the red region). The singlet excited states of these RC Chls (¹P*) is where the primary photochemistry begins. This is followed by fast charge separation between ¹P* and the neighboring primary electron acceptor (symbolized as A in Fig. 2.3) and thus the formation of the radical pair P•+ A•−. The cation radical P•+ is reduced by the electron donor (denoted D in Fig. 2.3). An important point to realize here is that as soon as the cation radicals P680•+ (in PSII) and P700•+ (in PSI) are formed, the light energy has already been converted into chemical energy; this process has a very high quantum efficiency [85].

However, the steps involved in photosynthetic energy conversion are catalyzed not only by PSII and PSI, but also by two other protein complexes that are also located in the thylakoid membrane. These complexes are the cytochrome (Cyt) b6f and the ATP synthase (ATP-ase) (see Fig. 2.2). PSII, PSI, and Cyt b6f complexes contain almost all the redox active cofactors that allow light-induced transfer of electrons from H2O to NADP+ through the thylakoid membrane in the following sequence: H2O → PSII → Cyt b6f → PSI → NADP+. PSII is linked with the Cyt b6f complex via the mobile lipophilic hydrogen atom carrier plastoquinone (PQ), in the membrane, while the Cyt b6f is linked with PSI via a mobile water-soluble redox carrier plastocyanin (PC) in the thylakoid lumen (Fig. 2.2). This copper-containing protein PC can, in some cases, be substituted by the iron-containing carrier Cyt c6 (also sometimes called Cyt c533) [86, 87]. In addition to these mobile electron transfer carriers, there is a soluble [2Fe-2S]-containing protein ferredoxin (Fd), a one-electron carrier connected with PSI. Upon receiving an electron from PSI, Fd reduces NADP+ to NADPH. This reduction is catalyzed by the membrane-associated flavoprotein called ferredoxin-NADP+ reductase (FNR) [88, 89]. The scheme that depicts the ETC from water to NADP+ via redox-active cofactors is called the zig-zag or the Z-scheme (Fig. 2.5). Its origin and development have been described recently by Govindjee and Björn [90].

FIGURE 2.5 The zig-zag or Z-scheme of oxygenic photosynthesis representing the energetics of linear electron transfer from H2O to NADP+ plotted on redox midpoint potential (Em, at pH 7) scale. The diagram also shows a cyclic electron transfer around PSI, Q-cycle, and half-times of several linear electron transfer steps. The two black vertical arrows symbolize the excitation of RC Chl a molecules (P680 and P700 in PSII and PSI, respectively); these lead to electrons in the ground state to be raised into a higher (singlet) excited state in response to the absorption of excitation energy from the light-harvesting antenna or by direct absorption of photons (wiggly white arrows). For further details and abbreviations of the components involved in the electron transfer see