Green Plant Photosynthesis

Oxygen oeeurs in the atmosphere in vast quantities as the free element O2 (and O3, p. 607) and there are also suhstantial amounts dissolved in the oeeans and surfaee waters of the world. Virtually all of this oxygen is of hiologieal origin having been generated by green-plant photosynthesis from water (and earbon dioxide).The net reaetion ean be represented by ... 

A simpler and better-understood process of the primary photosynthetic reaction and charge separation occurs in bacterial photosynthesis, which has only one photosystem instead of the two photosystems of green-plant photosynthesis. 

We examine here possible structural effects that may result from or accompany the generation of the primary photoproducts, and speculate about the consequences of concomitant changes in distances,conformations, relative orientations and charges on the electronic profiles of and interactions between the BChls, BPheos and their radicals. Because the primary events in green plant photosynthesis also involve a series of chlorophyll donors and acceptors ( ), similar trends should therefore prevail for chlorophyll radicals as well. Furthermore, radicals of porphyrins and hydroporphyrins (saturated porphyrins such as chlorins and isobacteriochlorins) have been... 

The opposite direction of transfer is also perfectly exploitable if required by the designer. Such PET processes are of course responsible for green plant photosynthesis [32-36], In the present simple instance the electron transfer is essentially reversed immediately following excitation. Such self-repair is essential following a potentially lethal PET process so that the molecular system is reasonably photostable. The important net result for us is that no luminescence is seen following excitation. This is the off state of the molecular switch. 

Green plant photosynthesis, which feeds the world, runs on photoinduced electron transfer (PET). 121 This principle was developed in chemical contexts by Albert Weller over three decades ago, 131 and became adapted for use in fluorescent switching contexts in the late 1970s and early 1980s. 14-211 A general design principle emerged soon afterwards. 221... 

R = Me, PhCH2, CH2=CH, HOCH2CH2, CH3CH=CH] are unique in their ability to undergo two reversible one-electron reductions in almost any medium (Table 12.3).15 This property makes the viologens especially useful mediators between one-electron and two-electron processes. This property also is the basis for the herbicidal activity of methyl viologen (paraquat), which disrupts the electron-transport chain in green-plant photosynthesis. 

By far the most important role of manganese in nature is its direct involvement in the photocatalytic, four-electron oxidation of water to dioxygen in green plant photosynthesis, an essential process for the maintenance of life. Pirson, in 1937, first discovered the requirement of manganese in photosynthesis by showing that plants grown in a Mn-deficient medium lost their water oxidation capacity (184). During the next four decades, several researchers showed that two photosystems, photosystem I (PSI) and photosystem II (PSII), were involved in photosynthesis and that 02 evolution and Mn were localized at PSII (for a review, see Ref. 185). 

Figure 4. The Z-scheme of green plant photosynthesis coupling of the two pigment systems, 1 and II. Peso and p7(Ki = chlorophyll pQ = plastoquiiionc Cyt = cytochrome pC = plastocyaniiie Fdb, Fds = ferredoxin.

One important consequence of van Niel s formulation is that in green-plant photosynthesis, the O2 evolved should originate from H2O, not CO2. This was soon confirmed experimentally by Ruben, Randall, Kamen and Hyde by mass-spectrometric examination of oxygen evolved by the alga Chlorella suspended in 0-labeled water. In retrospect, van NieTs simple and elegant postulate had provided one of the most important cornerstones for our understanding of the mechanism of photosynthesis during the past half century. For an in-depth discussion of the evolution of van NieTs concept, see the review by Clayton ". 

In 1965 Hill elaborated the two-photosystem scheme further as shown in Fig. 15 (B). In this Z-shaped scheme, two groups of chloroplast components with known redox potentials were placed at the bends of the Z Cyt/, plastocyanin and P700, close to -1-0.4 V, and plastoquinone and Cyt b( close to 0 V. Ferredoxin, with a potential of-0.43 V, is close to the midpoint potential ofhydrogen electrode. For oxygen production, the midpoint potential of the unknown component must exceed that of the oxygen electrode. Over the past thirty years, a variety of Z-schemes have been published in the literature to illustrate the electron-transfer processes in green-plant photosynthesis, but their basic features have not deviated from that shown in Fig. 15 (B). For instance, we show a currently accepted, concise Z-scheme in Fig. 15 (C) it includes many more individual components than were originally envisioned, plus a representation of the operation of the so-called Q-cycle in the Cyi-b(,f complex. 

We have seen the Z-scheme for the two photosystems in green-plant photosynthesis and the electron carriers in these photosystems. We have also described how the photosystems of green plants and photosynthetic bacteria all appear to function with basically the same sort ofmechanisms of energy transfer, primary charge separation, electron transfer, charge stabilization, etc., yet the molecular constituents of the two reaction centers in green plants, in particular, are quite different from each other. Photosystem I contains iron-sulfur proteins as electron acceptors and may thus be called the iron-sulfur (FeS) type reaction center, while photosystem 11 contains pheophytin as the primary electron acceptor and quinones as the secondary acceptors and may thus be called the pheophytin-quinone (0 Q) type. These two types of reaction centers have also been called RCI and RCII types, respectively. 

Even before the fluorescence-quencher hypothesis was proposed, however, it had been suggested that plastoquinone may be an important electron carrier in green-plant photosynthesis. Bishops reported in 1959 the significant finding that extraction of PQ from chloroplasts results in the loss ofthe ability to evolve oxygen but that upon reconstitution with plastoquinone, oxygen-evolution activity is restored. 

Fig. 3. (A) EPR spectrum of P700 at 77 K (B) Kinetic correlation between the time course of light-induced P700 signal changes measured by EPR spectroscopy and by optical spectroscopy near 700 nm at ambient temperature in D144 particles (top) and TSF-1 particles (bottom). Figure source (B) Warden and Bolton (1972) Simultaneous optical and electron spin resonance detection ofthe primary photoproduct in green plant photosynthesis. J Am Chem Soc 94 4352.

JT Warden and JR Bolton (1973) Simultaneous optical and electron spin resonance detection of the primary photoproduct P700 in green plant photosynthesis. J Am Chem Soc 94 4351-4353... 

Green-plant photosynthesis utilizes water as a source of reducing equivalents... 

Hence, the phosphorylation cycles represent a poised system for the reversible transfer of electrons from oxy anions [(HO)RO -> (HO)RO- + e ] to hydronium ions (H3O+ + e- —> H- + H2O), which is facilitated by (1) the coupling of the respective products to form H2O [(HO)RO- + H- —> H2O + R(O) -AGgp, 111 kcal mol ] and (2) the nucleophilic condensation reaction [ADP3- + R(O)]. Biological systems such as cytochrome-c oxidase and Photosystem 11 of green-plant photosynthesis produce net proton fluxes during turnover and thereby drive oxidative phosphorylation to store 5 kcal per mole of ATP produced from one mole of hydronium ions. 

Current ideas on photosystem II in green plant photosynthesis suggest that the resting system contains Mn and that in conversion of two molecules of water into oxygen, Mn and/or Mn are involved. A study of [Mn (GH3)2] as a possible model for this system yielded electrochemical evidence for Scheme 1. ... 

All cytochromes c are oxidation-reduction proteins involved in either respiration or photosynthesis. In eukaryotic organisms—cells with both nuclei and mitochondria— there are only three c-type cyctochromes c and Cl from the mitochondrial respiratory chain, and Ca or / from green plant photosynthesis. These are compared in Table II and discussed in Section II. The field in prokaryotes— bacteria and blue-green algae— is much more diverse, and one is left with the impression that c, Ci, and / in eukaryotes are only the most successful survivors of a very stiff biochemical competition. Prokaryotic cytochromes c are the subject of Section III. 

Fig. 27. Electron flow in green plant photosynthesis. Vertical wavy arrows represent excitation of chlorophyll molecules by absorbed light. Reaction intermediates are as follows Z , unknown intermediate donating electrons to photocenter II Q , unknown intermediate accepting electrons from excited chlorophyll PQ, plasto-quinone, structurally similar to coenzyme Q or ubiquinone of Fig. 4 bss>, haa,, cytochrome components PC, plastocyanin, a copper-containing nonheme protein FR8, unknown ferredoxin reducing substance FD, ferredoxin FP, flavoprotein mediating reduction of NADP. ...

Two h-type cytochromes are present in green plant photosynthesis 6559 and buz or bt. The former has traditionally been placed between plastoquinone and cytochrome / in the electron chain, and 69 has been placed between ferredoxin and cytochrome / in cyclic photophosphorylation (291). Both of these assignments have been challenged recently (292-295), and we must simply note the controversy and drop the issue for the purposes of this chapter. 

The measurement of reduction potential is an important step in establishing the order of chain elements. Reduction potentials vary with relative concentrations of the two redox species, of course, with an order of magnitude increase in ratio of reduced to oxidized form leading to a 60-mV increase in reduction potential in one electron reactions. Differences of a few tens of millivolts between the standard reduction potentials of cytochrome / and plastocyanin in green plant photosynthesis are not enough to establish their relative order since relative concentrations could reverse the actual reduction potentials. Still, a cytochrome c with a standard reduction potential of -(-50 mV is unlikely to donate electrons to... 

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