In addition to a suitable environment, appropriate pigments, whose cumulative light-absorbing properties determine the range of wavelengths over which photosynthesis occurs, a reaction center where the excited pigments emit
electrons, and an electron transfer chain that generates the high-energy phos- phorylating agent adenosine triphosphate (ATP) by photophosphorylation are necessary for ambient C02 reduction. The pigment chlorophyll absorbs light and is oxidized by ejection of an electron. The electron is accepted by ferredoxin (Fd), a nonheme iron protein, to form reduced ferredoxin (Fd+2), which through other electron carriers generates ATP and the original oxidized ferre­doxin (Fd+3). Chlorophyll functions as both a light absorber and a source of electrons in the excited state, and as the site of the initial photochemical reaction. Accessory pigments function to absorb and transfer light energy to chlorophyll.

Two photochemical systems are involved in these “light reactions”: photo­systems II (PS II) and I (PS I). PS II consists of the first series of reactions that occur in the light phase of photosynthesis during which the excited pigment participates in the photolysis of water to liberate free oxygen, protons, and electrons. PS I is the second series of reactions that occur in the light phase of photosynthesis; they result in the transfer of reducing power to nicotinamide adenine dinucleotide phosphate (NADP) for ultimate utilization by C02. The light reactions yield ATP, and the reduced form of nicotinamide adenine dinucleotide phosphate (NADPFI2), both of which facilitate the dark reactions that yield sugars. Hydrogen is transferred by NADPH2. The low — energy adenosine diphosphate (ADP) and NADP produced in the dark reactions are reconverted to ATP and NADPH2 in the light reactions.

The chemistry of the light reactions was elucidated in 1954 by the U. S. biochemist Daniel Arnon and co-workers (Arnon, Allen, and Whatley, 1954). Light energy is absorbed by the chlorophyll pigments in plant chloroplasts and transferred to the high-energy bonds in ATP, which is produced in noncy­clic and cyclic photophosphorylation reactions. Noncyclic photophosphoryla­tion occurs in the presence of light, requires Fd catalyst, and yields ATP and oxygen. NADP is then reduced by Fd+2 in the absence of light:

4Fd+3 + 2ADP + 2P, + 2H20 -> 4Fd+2 + 2ATP + 02 + 4H+

4Fd+2 + 2NADP + 4H+ -* 4Fd+3 + 2NADPH2.

For each molecule of oxygen evolved, two molecules each of ATP and NADPH2 are formed. Cyclic photophosphorylation requires Fd catalyst and produces ATP only:

ADP 4- P, —» ATP.

This process provides the additional molecule of ATP needed for assimilation of one molecule of C02.

PS I appears to promote cyclic photophosphorylation and proceeds best in light of wavelengths greater than 700 nm, whereas PS II promotes noncyclic photophosphorylation and proceeds best in light of wavelengths shorter than
700 nm. PS II and PS I operate in series in the chloroplast membranes and transfer reducing power from water to Fd and NADP by an electron chain that includes plastoquinone and three proteins in chloroplasts—cytochrome b cytochrome f, and the copper protein plastocyanin. To overcome the potential difference between the carbon dioxide-glucose couple and the water-oxygen couple, two photons are absorbed, one by PS II and one by PS I. In the traditional “Z scheme” concept of photosynthesis first proposed in 1960 (Hill and Bendall, 1960), and which is now well accepted, the strong oxidizing agent, oxygen, is at the bottom, and the strong reducing agent, reduced ferredoxin, is at the top (Fig. 3.1) (Bassham, 1976). By transfer of electrons from water to ferredoxin, the chemical potential for reduction of C02 is created. PS II is depicted as absorbing one photon to raise an electron to an intermediate energy level, after which the electron falls to an intermediate lower energy level while generating ATP. PS I then absorbs the second photon and raises the electron to a still higher intermediate energy level, and subsequently generates NADPH2 via reduced ferredoxin, after which C02 is reduced to yield sugars. In effect, the Z scheme transfers electrons from a low chemical potential in water to a higher chemical potential in NADPH2, which is necessary to reduce C02.

It has been discovered, however, that both photosystems do not seem to be necessary as depicted in the Z scheme to reduce C02. PS II seems to be adequate alone to generate the chemical potential for reduction, at least in

biomass species under certain conditions. PS I is absent in a mutant of the alga Chlamydomonas reinhardtii, but the organism has been found to be capable of photoautrophic assimilation of C02 and the simultaneous evolution of oxygen and hydrogen (Greenbaum et ah, 1995). The investigators interpre­ted their results to mean that a single-photon light reaction has the potential of increasing the efficiency of photosynthesis by overcoming the thermodynamic limitations of converting light energy into chemical energy. This will be referred to later in the discussion of photosynthesis efficiency and biomass yield. In any case, this observation tends to verify Amon’s argument that water oxidation and NADP reduction can be driven by PS II alone (Barber, 1995).

In summary, ambient C02 fixation by photosynthesis involves the photo­chemical decomposition of water to form oxygen, protons, and electrons; the transport of these electrons to a higher energy level via PS II and I and several electron transfer agents; the concomitant generation of NADPH2 and ATP; and reductive assimilation of C02 to monosaccharides. The initial process is the absorption of light by chlorophyll, which promotes the decomposition of water. The ejected electrons are accepted by the oxidized form of Fd. The reduced Fd then starts a series of electron transfers to generate ATP from ADP and inorganic phosphate, and NADPH2 in the light reactions. The stoichiometry, including the reduction in the dark reactions of 1 mol C02 to carbohydrate, represented by the building block (CH20), is illustrated in simplified form as follows:

2H20(1) -> 4H+ + 4e + 02(g)

4Fd+3 + 4e -» 4Fd+2 3ADP 4- ЗР, 3ATP

4Fd+2 -> 4Fd+3 + 4e 2NADP + 4H+ + 4e ^ 2NADPH2 C02(g) + 3ATP + 2NADPFP, -> (CH20) + 3ADP + 2NADP + 3P,

+ H20(1)

Overall: C02(g) + H20(1) -► (CH20) + 02(g).

For each of the two light reactions, one photon is required to transfer each electron; a total of eight photons is thus required to fix one molecule of C02. Assuming C02 is in the gaseous phase and the initial product is glucose, the standard Gibbs free energy change at 25°C is +0.48 MJ (+114 kcal) per mole of C02 assimilated and the corresponding enthalpy change is +0.47 MJ (+112 kcal).