**2. Photosynthesis**

## **2.1 Overview**

Photosynthesis is the world's most abundant process with an approximate carbon turnover number of 300- 500 billion tons of CO2 per year. In this vital process, green plants, algae and photosynthetic bacteria are converting CO2 with water into carbohydrates and oxygen (in oxygenic photosynthesis), both of which are essential for sustaining life on earth. Oxygenic photosynthesis is believed to be started 2.5 billion years ago by the ancestors of cyano bacteria. In this remarkable process, energy need for converting stable compounds (CO2 and H2O) into comparably less stable arranged molecules ((CH2O)n and O2) is supplied from solar energy in which highly sophisticated protein complexes embedded in an internal chloroplast membrane (called thylakoid membrane) are major players.

���� + ������ + ������������ � ������� + ��� + ����

ΔG0= 2880 kJ /mol C6H12O6

Harnessing solar energy into chemical bonds in this process is achieved by light absorption and sequential electron and proton transport processes in which a great deal of number of light harvesting pigments, protein complexes and intermediate charge carriers are involved. CO2 is being reduced with the indirect products of water oxidation; supplying required energy in the form of redox free energy (from NADPH) and high energy Pi bonds (from ATP).

Overall process can be shown in the reaction scheme below where D: electron donor, A: electron acceptor and T: energy trap (Govindjee, 1975).

Water oxidation: �� ����+ � � ���������+ � � �� + �� *NADP+* reduction: �� ����� <sup>+</sup> � � ����� + �� � �� ����+ � � ����� + � � �� Cyclic Photophosphorylation: �� ����� + ��� + �� � � � � � � + ��� CO2 reduction: ��� + ������� + ����� � ������ + ������ + ����� + ���

#### **2.2 Reactions**

14 Artificial Photosynthesis

observation that we make in this table is the following: when compared per mole of hydrocarbon formed, the Gibbs free energies of formation increase with increasing carbon chain length. But when the Gibbs free energy formation values are normalized per mole O2 formed, one can compare the energy demand of the reactions on a common basis. A close examination of the data in the last column reveals the fact that energetically almost all of the reactions are similar. The second conclusion we can arrive at is that once the water splitting reaction is possible, the formed hydrogen can drive the subsequent reduction reactions,

**Reaction Gf (kJ/mol HC product) Gf (kJ/mol O2)** 

CO2+2H2O CH4+ 2 O2 801.0 400.5 CO2+2H2O CH3OH+ 1½ O2 689.2 459.5 2 CO2+3H2O C2H5OH+ 3 O2 1306.6 435.5 H2O + CO21/6 C6H12O6 + O2 2880.0 480.0 H2O H2+ ½ O2 228.6 457.2

Table 1.1. The thermodynamics of the reactions involved in carbon dioxide reduction

chloroplast membrane (called thylakoid membrane) are major players.

electron acceptor and T: energy trap (Govindjee, 1975).

Water oxidation: �� ����+ �

*NADP+* reduction: �� ����� <sup>+</sup> �

Photosynthesis is the world's most abundant process with an approximate carbon turnover number of 300- 500 billion tons of CO2 per year. In this vital process, green plants, algae and photosynthetic bacteria are converting CO2 with water into carbohydrates and oxygen (in oxygenic photosynthesis), both of which are essential for sustaining life on earth. Oxygenic photosynthesis is believed to be started 2.5 billion years ago by the ancestors of cyano bacteria. In this remarkable process, energy need for converting stable compounds (CO2 and H2O) into comparably less stable arranged molecules ((CH2O)n and O2) is supplied from solar energy in which highly sophisticated protein complexes embedded in an internal

���� + ������ + ������������ � ������� + ��� + ����

ΔG0= 2880 kJ /mol C6H12O6 Harnessing solar energy into chemical bonds in this process is achieved by light absorption and sequential electron and proton transport processes in which a great deal of number of light harvesting pigments, protein complexes and intermediate charge carriers are involved. CO2 is being reduced with the indirect products of water oxidation; supplying required energy in the form of redox free energy (from NADPH) and high energy Pi bonds (from

Overall process can be shown in the reaction scheme below where D: electron donor, A:

���������+ �

����� + �� � �� ����+ �

� �� + ��

�

����� + �

� ��

�

Cyclic Photophosphorylation: �� ����� + ��� + �� � � � � � � + ���

�

CO2 reduction: ��� + ������� + ����� � ������ + ������ + ����� + ���

almost spontaneously.

**2. Photosynthesis** 

**2.1 Overview** 

ATP).

Photosynthesis includes a series of photophysical, photochemical and chemical reactions realized by highly sophisticated protein complexes, energy carriers and enzymes. With all the complexity of their mechanisms, reactions involved in photosynthesis are mainly divided into two stages: (i) light dependent reactions including water oxidation and chemical energy generation through electron and proton transport and (ii) light independent reactions including CO2 fixation, reduction and regeneration of ribulose 1,5 biphosphate (Calvin Cycle).

#### **2.2.1 Light induced reactions**

The light induced reactions occur in a complex membrane system (thylakoid membrane) via electron transfer through light induced generation of cation- anion radical pairs and intermediate charge carriers such as plastoquinone, plastocyanin and ferrodoxin. Light dependent reactions in green plants follow a Z scheme which was first proposed by Hill & Bendall, 1960 (Figure 2.1). In this scheme, light energy is absorbed by light harvesting molecules and funneled to two special reaction center molecules; P680 and P700 which are acting as major electron donors in PS II and PSI respectively. Electron transport from PSII to PS I is realized by intermediate charge carriers and electron need of P680+ (strong oxidant with E0 = 1.1 eV) in PSII is compensated from water molecules (water oxidation).

Electron transport through thylakoid membrane and water oxidation reactions results in a proton concentration gradient across the thylakoid membrane. Energy created by proton electrochemical potential resulting from this proton gradient is used by ATP synthase to produce ATP from ADP and Pi. The net reaction in light dependent reaction system is the electron transport form a water molecule to a NADP+ molecule with the production of ATP molecules (Figure 2.2).In this complex electron transport system, PS II alone is composed of more than 15 polypeptides and nine different redox components including chlorophylla and b, pheophytin, plastoquinone.

Fig. 2.1. Z scheme electron transfer in terms of redox potentials (Ke, 2001)

Artificial Photosynthesis from a Chemical Engineering Perspective 17

This high energy requiring water oxidation reaction with four- proton, four- electron extraction and an oxygen- oxygen bond formation (with a standard free energy requirement of 312 kJ/mol of O2) necessitates the regeneration of the oxygen evolving complex at every half an hour in order to repair the damage caused by the oxygen

In electron transfer from the oxygen evolving complex (OEC) to P680+ molecule, tyrosine (Yz\*) acts as intermediate electron carrier. Protons evolved from OEC are deposited in lumen phase contributing proton concentration gradient (ΔpH) mentioned in ATP synthesis part. Excited electron upon light absorption is transferred to the cytochrome b6f complex through a pheophytin, a tightly bound phylloquinone (QA) and a mobile phylloquinone (QB). Subsequently reduced phylloquinol (PQH2) (reduced with electrons from P680\* and two protons from stromal phase) releases two additional protons into lumen phase as it binds to cyctochrome b6f complex after diffusion through thylakoid membrane. Electron transfers from cyctochrome b6f complex to PS I (through lumen phase) and from PS I to NADP+ molecule (through stromal phase) are achieved by plastocyanin and ferrodoxin respectively. ATP synthesis reaction in light dependent reactions is driven by the proton electrochemical and charge potential across the membrane resulted from proton concentration difference and charge separation during illumination respectively. NADPH and ATP molecules produced as such are used as energy and proton sources in carbon dioxide reduction

The reactions that do not involve solar energy directly are somewhat roughly called the dark reactions. These reactions take place in outer space of thylakoid membrane which is also known as stromal phase. CO2 enters the leaf structure through small holes called stomata and diffuses into stromal phase in the chloroplast where it is being reduced with reactions in series that are catalyzed by more than ten enzymes. Driving force for the reduction reaction is supplied from NADPH and ATP molecules; hence, the *'catalytic'* reaction sequence does not require light as an energy source and called as light independent reactions. However, recent findings indicate light activation of enzymes due to regulatory

Melvin Calvin and his collaborators were the first to resolve the photosynthetic CO2 reduction mechanism with studies involving radioactively labeled CO2. The Calvin Cycle,

1. CO2 fixation by carboxylation of rubilose 1,5- bisphosphate to two 3-phosphoglycerate

3. Regeneration of rubilose 1,5- bisphosphate from triose phosphate molecules (Figure

The key reaction in photosynthetic CO2 reduction is the fixation of a CO2 molecule to rubilose 1,5- bisphosphate to two phosphoglycerate molecules with a standard free energy of -35 kJ/mol indicating its irreversibility. This reaction is catalyzed with the Ribulose biphosphate Carboxylase/Oxygenase (RubisCO) enzyme which is one of largest enzymes in nature with its 8 large, 8 small subunits (with molecular weights changing from 12 to 58 kDa). This enzyme also catalyzes a side reaction, oxygenation, to give a 3-phospho glycerate and a 2- phosphoglycolate instead of two 3- phosphoglycerates for CO2 fixation. Although

also known as reductive pentose phosphate pathway consists of three sections:

2. Reduction of 3-phosphoglycerate to triose phosphate, and

production (Meyer, 2008).

reactions in Calvin Cycle.

processes (reductive pentose phosphate).

**2.2.2 Dark reactions** 

molecules,

2.4).

Photosystem II is the only protein complex with the capability of oxidizing water into O2 and protons. In PS II, water is oxidized with an Oxygen Evolving Complex whose components are revealed to be in the form of Mn4CaO5 (Umena et al., 2011). This inorganic core oxidizes two water molecules in Kok cycle, comprised of five oxidation states (S states) of PSII donor site. In this model, oxygen formation requires successive four light flashes for four-electron and fourproton release. Recently, presence of an intermediate S4' state and kinetics of completion of final oxidation cycle responsible for O- O bond formation was revealed with time resolved X ray study of Haumann et al. (Figure 2.3) (Haumann et al., 2005).

Fig. 2.2. Schematic illustrations of electron and proton transport processes and ATP synthesis in light dependent reactions (Hankamer et al., 1997)

Fig. 2.3. Extension of classical S state cycle of the manganese- calcium complex (Haumann et al., 2005)

This high energy requiring water oxidation reaction with four- proton, four- electron extraction and an oxygen- oxygen bond formation (with a standard free energy requirement of 312 kJ/mol of O2) necessitates the regeneration of the oxygen evolving complex at every half an hour in order to repair the damage caused by the oxygen production (Meyer, 2008).

In electron transfer from the oxygen evolving complex (OEC) to P680+ molecule, tyrosine (Yz\*) acts as intermediate electron carrier. Protons evolved from OEC are deposited in lumen phase contributing proton concentration gradient (ΔpH) mentioned in ATP synthesis part. Excited electron upon light absorption is transferred to the cytochrome b6f complex through a pheophytin, a tightly bound phylloquinone (QA) and a mobile phylloquinone (QB). Subsequently reduced phylloquinol (PQH2) (reduced with electrons from P680\* and two protons from stromal phase) releases two additional protons into lumen phase as it binds to cyctochrome b6f complex after diffusion through thylakoid membrane. Electron transfers from cyctochrome b6f complex to PS I (through lumen phase) and from PS I to NADP+ molecule (through stromal phase) are achieved by plastocyanin and ferrodoxin respectively. ATP synthesis reaction in light dependent reactions is driven by the proton electrochemical and charge potential across the membrane resulted from proton concentration difference and charge separation during illumination respectively. NADPH and ATP molecules produced as such are used as energy and proton sources in carbon dioxide reduction reactions in Calvin Cycle.

#### **2.2.2 Dark reactions**

16 Artificial Photosynthesis

Photosystem II is the only protein complex with the capability of oxidizing water into O2 and protons. In PS II, water is oxidized with an Oxygen Evolving Complex whose components are revealed to be in the form of Mn4CaO5 (Umena et al., 2011). This inorganic core oxidizes two water molecules in Kok cycle, comprised of five oxidation states (S states) of PSII donor site. In this model, oxygen formation requires successive four light flashes for four-electron and fourproton release. Recently, presence of an intermediate S4' state and kinetics of completion of final oxidation cycle responsible for O- O bond formation was revealed with time resolved X

Fig. 2.2. Schematic illustrations of electron and proton transport processes and ATP

Fig. 2.3. Extension of classical S state cycle of the manganese- calcium complex

(Haumann et al., 2005)

ray study of Haumann et al. (Figure 2.3) (Haumann et al., 2005).

synthesis in light dependent reactions (Hankamer et al., 1997)

The reactions that do not involve solar energy directly are somewhat roughly called the dark reactions. These reactions take place in outer space of thylakoid membrane which is also known as stromal phase. CO2 enters the leaf structure through small holes called stomata and diffuses into stromal phase in the chloroplast where it is being reduced with reactions in series that are catalyzed by more than ten enzymes. Driving force for the reduction reaction is supplied from NADPH and ATP molecules; hence, the *'catalytic'* reaction sequence does not require light as an energy source and called as light independent reactions. However, recent findings indicate light activation of enzymes due to regulatory processes (reductive pentose phosphate).

Melvin Calvin and his collaborators were the first to resolve the photosynthetic CO2 reduction mechanism with studies involving radioactively labeled CO2. The Calvin Cycle, also known as reductive pentose phosphate pathway consists of three sections:


The key reaction in photosynthetic CO2 reduction is the fixation of a CO2 molecule to rubilose 1,5- bisphosphate to two phosphoglycerate molecules with a standard free energy of -35 kJ/mol indicating its irreversibility. This reaction is catalyzed with the Ribulose biphosphate Carboxylase/Oxygenase (RubisCO) enzyme which is one of largest enzymes in nature with its 8 large, 8 small subunits (with molecular weights changing from 12 to 58 kDa). This enzyme also catalyzes a side reaction, oxygenation, to give a 3-phospho glycerate and a 2- phosphoglycolate instead of two 3- phosphoglycerates for CO2 fixation. Although

Artificial Photosynthesis from a Chemical Engineering Perspective 19

The vesicular thylakoid membrane structure defines a closed space separating outside water phase (stromal phase) and inside water phase (lumen phase). CO2 fixation reactions occur in the stromal phase while majority of light dependent reactions are realized in the complex membrane system with embedded protein complexes and intermediate charge carriers. As mentioned in light dependent reactions, electron and proton transport processes through protein complexes and intermediate charge carriers like plastoquinone, plastocynanin and ferrodoxin molecules play an important role in controlling photosynthetic rates. Within a protein complex such as PSII or cytochrome bf complex, electron transfer and pathway is controlled by polypeptide chains of the protein. However between protein complexes, electron transfer via electron carriers is controlled by distance and free energy. Below, electron and proton transport processes taken place in light dependent reactions are

Fig. 2.6. Conversion of 3- Phosphoglycerate to triose phosphate (Diwan, 2009)

illustrated with particle sizes of protein complexes given by Ke, 2001.

Fig. 2.7. Distribution of photosynthetic complexes in thylakoid membrane and the

Presence of the membrane affects reaction rates in an aspect that it limits electron and proton transport to two dimensions which increases the random encounters. Furthermore, electron transport reactions and special structure and orientation of the membrane and protein complexes contribute to a proton electrochemical potential difference which drives

**2.3 Transport processes** 

corresponding Z scheme (Ke, 2001)

oxygenation occurs with a ratio of 1:4 to 1:2 (oxygenation : carboxylation), oxygenation ratio decreases as CO2 concentration in the atmosphere is increased. This regulatory measure of photosynthesis is worth appreciation.

#### Fig. 2.4. The Calvin Cycle

In carboxylation reaction catalyzed by RubisCO, rubilose 1,5- bisphosphate (RuBP) accepts CO2 to form a keto intermediate after keto-enol isomerization (Figure 2.5). For the synthesis of glyceraldehydes 3- phosphates, firstly 3- phosphoglyerates are phospholyrated to 1,3 bisphosphoglycerate with phosphoglycerate kinase enzyme. Afterwards, 1,3 biphosphoglycerate is reduced with NADPH to glyceraldehydes 3- phosphate with glyceraldehydes phosphate dehydrogenase enzyme. Redox potential difference between the aldehyde and carboxylate is overcome with the consumption of ATP (Figure 2.6).

Fig. 2.5. Reaction sequence of carboxylation of RuBP by RubisCO (Diwan, 2009)

After production of glyceraldehyde 3- phosphates, out of six aldehydes produced by fixation of three CO2 molecules, five of them are used in regeneration of three RuBP molecules together with ATP consumption. Remaining one molecule of glyceraldehyde 3 phosphate is transported into the cytosol for utilization in glucose synthesis.

Fig. 2.6. Conversion of 3- Phosphoglycerate to triose phosphate (Diwan, 2009)

#### **2.3 Transport processes**

18 Artificial Photosynthesis

oxygenation occurs with a ratio of 1:4 to 1:2 (oxygenation : carboxylation), oxygenation ratio decreases as CO2 concentration in the atmosphere is increased. This regulatory measure of

In carboxylation reaction catalyzed by RubisCO, rubilose 1,5- bisphosphate (RuBP) accepts CO2 to form a keto intermediate after keto-enol isomerization (Figure 2.5). For the synthesis of glyceraldehydes 3- phosphates, firstly 3- phosphoglyerates are phospholyrated to 1,3 bisphosphoglycerate with phosphoglycerate kinase enzyme. Afterwards, 1,3 biphosphoglycerate is reduced with NADPH to glyceraldehydes 3- phosphate with glyceraldehydes phosphate dehydrogenase enzyme. Redox potential difference between the

aldehyde and carboxylate is overcome with the consumption of ATP (Figure 2.6).

Fig. 2.5. Reaction sequence of carboxylation of RuBP by RubisCO (Diwan, 2009)

phosphate is transported into the cytosol for utilization in glucose synthesis.

After production of glyceraldehyde 3- phosphates, out of six aldehydes produced by fixation of three CO2 molecules, five of them are used in regeneration of three RuBP molecules together with ATP consumption. Remaining one molecule of glyceraldehyde 3-

photosynthesis is worth appreciation.

Fig. 2.4. The Calvin Cycle

The vesicular thylakoid membrane structure defines a closed space separating outside water phase (stromal phase) and inside water phase (lumen phase). CO2 fixation reactions occur in the stromal phase while majority of light dependent reactions are realized in the complex membrane system with embedded protein complexes and intermediate charge carriers.

As mentioned in light dependent reactions, electron and proton transport processes through protein complexes and intermediate charge carriers like plastoquinone, plastocynanin and ferrodoxin molecules play an important role in controlling photosynthetic rates. Within a protein complex such as PSII or cytochrome bf complex, electron transfer and pathway is controlled by polypeptide chains of the protein. However between protein complexes, electron transfer via electron carriers is controlled by distance and free energy. Below, electron and proton transport processes taken place in light dependent reactions are illustrated with particle sizes of protein complexes given by Ke, 2001.

Fig. 2.7. Distribution of photosynthetic complexes in thylakoid membrane and the corresponding Z scheme (Ke, 2001)

Presence of the membrane affects reaction rates in an aspect that it limits electron and proton transport to two dimensions which increases the random encounters. Furthermore, electron transport reactions and special structure and orientation of the membrane and protein complexes contribute to a proton electrochemical potential difference which drives

Artificial Photosynthesis from a Chemical Engineering Perspective 21

Fig. 2.9. PS I electron transport pathways and transfer times with midpoint potentials of

Water oxidation and CO2 reduction reactions are the slowest processes in photosynthesis. Scycle taking place in PS II for water oxidation is completed with a total of 1.59 ms, which is equivalent to production of 630 molecule of O2/site/s. On the other hand, turnover frequency given for a subunit of RubisCO for CO2 reduction is given as 3.3 s-1 (Heldt, 2010), which is much slower than oxygen evolution. Average photosynthesis rate of a sunflower was given as 13.5 µmol/m2/s by Whittingham, 1974 and as 12 µmol/m2/s for Brassica pods with an internal CO2 concentration of 292 ppm by Singal et al., 1995 where rate of dark CO2

In Table 2.1, time characteristics are unraveled for photosynthesis and artificial photosynthesis which indicates similarity in photochemistry but significant difference in

**Photosynthesis Artificial Photosynthesis (on TiO2)**  Charge carrier generation ps Charge carrier generation ps Charge trapping ps-ns Charge trapping 10 ns Electron transport ns- µs Interfacial charge transfer 100 ns Water oxidation 1.59 ms Water oxidation 670 msa CO2 reduction 300 ms CO2 reduction 14950 sb Table 2.1. Time characteristics of major processes realized in photosynthesis and artificial

Since the pioneering work of Inoue et al., 1979, CO2 is being reduced with H2O photocatalytically to mainly one carbon molecules like methane and methanol in the

a. Oxygen evolution with cobalt ITO electrode (Kanan & Nocera, 2008) b. Considering 200 µmole/gcat/h activity and 50 m2/gcat and 1015 sites/cm2

electron carriers (Whitmarsh & Govindjee, 1999)

fixation was given as 400 nmol/ mg protein/h.

time characteristics of chemical reactions.

photosynthesis

**3. Artificial photosynthesis** 

ATP synthesis reaction; i.e., plays a significant role in energy supply of photosynthesis. The proton electrochemical potential difference across the membrane is created by two main contributions; i. proton concentration gradient (pH difference), and ii. electric potential difference.

The processes contributing *proton concentration difference (ΔpH)* across the membrane can be listed as below:


On the other hand, vectoral electron transfer process in PS II and PS I initiated by photon absorption could be accounted as the reason for *electric potential difference (ΔΨ)*. Whitmarsh & Govindjee, 1999 gave the proton electrochemical potential difference with Equation (1).

$$
\Delta\mu\_{H+} = F\Delta\Psi - 2.3RT\Delta\text{pH} \tag{1}
$$

Where F is the Faraday constant, R is the ideal gas constant and T is temperature in Kelvin. They reported that although electric potential difference can be as large as 100 mV, pH difference has a dominating effect in overall electrochemical potential. For a pH difference of 2 (with inner pH 6 and outer pH 8, ΔpH equivalent to 120 mV), the free energy difference across the membrane is about -12 kJ/mol of proton.

In photosynthesis, fastest reactions taking place are the photophysical reactions like light absorption and charge separation in picoseconds orders. They are followed by rapid photochemical processes like electron transfer reactions and with slower biochemical reactions like water splitting and CO2 reduction.

Since photosynthesis is a series of reactions including photophysical, photochemical and chemical reactions, reaction rates of particular reactions are dependent upon transfer rates of reaction intermediates like electrons or protons. In Figures 2.8 and 2.9, electron transfer times in PS II and PS I are given to illustrate characteristic times of different processes.

Fig. 2.8. PS II electron transport pathways and transfer times with midpoint potentials of electron carriers (Whitmarsh & Govindjee, 1999)

ATP synthesis reaction; i.e., plays a significant role in energy supply of photosynthesis. The proton electrochemical potential difference across the membrane is created by two main contributions; i. proton concentration gradient (pH difference), and ii. electric potential

The processes contributing *proton concentration difference (ΔpH)* across the membrane can be

1. Proton release to the lumen phase as a consequence of water oxidation reaction at PS II.

On the other hand, vectoral electron transfer process in PS II and PS I initiated by photon absorption could be accounted as the reason for *electric potential difference (ΔΨ)*. Whitmarsh & Govindjee, 1999 gave the proton electrochemical potential difference with Equation (1).

Where F is the Faraday constant, R is the ideal gas constant and T is temperature in Kelvin. They reported that although electric potential difference can be as large as 100 mV, pH difference has a dominating effect in overall electrochemical potential. For a pH difference of 2 (with inner pH 6 and outer pH 8, ΔpH equivalent to 120 mV), the free energy difference

In photosynthesis, fastest reactions taking place are the photophysical reactions like light absorption and charge separation in picoseconds orders. They are followed by rapid photochemical processes like electron transfer reactions and with slower biochemical

Since photosynthesis is a series of reactions including photophysical, photochemical and chemical reactions, reaction rates of particular reactions are dependent upon transfer rates of reaction intermediates like electrons or protons. In Figures 2.8 and 2.9, electron transfer times in PS II and PS I are given to illustrate characteristic times of different processes.

Fig. 2.8. PS II electron transport pathways and transfer times with midpoint potentials of

߂Ɋுା ൌ ߖ߂ܨ െ ʹǤ͵ܴܶܪ߂) 1 (

3. Proton release into lumen phase during PQH2 oxidation at cytochrome b6f complex.

2. Proton uptake from stromal phase for PQ reduction.

across the membrane is about -12 kJ/mol of proton.

reactions like water splitting and CO2 reduction.

electron carriers (Whitmarsh & Govindjee, 1999)

4. NADP+ reduction at stromal phase.

difference.

listed as below:

Fig. 2.9. PS I electron transport pathways and transfer times with midpoint potentials of electron carriers (Whitmarsh & Govindjee, 1999)

Water oxidation and CO2 reduction reactions are the slowest processes in photosynthesis. Scycle taking place in PS II for water oxidation is completed with a total of 1.59 ms, which is equivalent to production of 630 molecule of O2/site/s. On the other hand, turnover frequency given for a subunit of RubisCO for CO2 reduction is given as 3.3 s-1 (Heldt, 2010), which is much slower than oxygen evolution. Average photosynthesis rate of a sunflower was given as 13.5 µmol/m2/s by Whittingham, 1974 and as 12 µmol/m2/s for Brassica pods with an internal CO2 concentration of 292 ppm by Singal et al., 1995 where rate of dark CO2 fixation was given as 400 nmol/ mg protein/h.

In Table 2.1, time characteristics are unraveled for photosynthesis and artificial photosynthesis which indicates similarity in photochemistry but significant difference in time characteristics of chemical reactions.


Table 2.1. Time characteristics of major processes realized in photosynthesis and artificial photosynthesis

