4.4.2—
Two Photosystems and the Z-Scheme of Electron Transport

Investigations in the 1950's and early 1960's established that the thylakoid membrane contains two types of photosynthetic units, designated photosystem 1 (PS–1)and photosystem 2 (PS–2), which cooperate in a sequential manner to transfer electrons from water to NADP⁺ (Boardman, 1968). Figure 4.4 depicts the electron transport pathway known as the Z-scheme first put forward by Hill and Bendall (1960). Both photosystems contain chlorophyll a , chlorophyll b and the four carotenoids, but in different proportions. Each unit of PS–1 or PS–2 contains 200 light-harvesting chlorophylls and one reaction centre.

Quanta of light absorbed by PS–2 are transferred to a form of chlorophyll absorbing at 682 nm and termed chl a– 682. Excitation of chl a– 682 catalyses

the transfer of an electron from Y to Q, giving a strong oxidant, Y⁺ , and a weak reductant, Q^– . Oxidation of water and the release of a molecule of O₂ requires the sequential absorption of four quanta and the accumulation of four oxidizing equivalents (Cheniae, 1970). The mechanism of water oxidation is unknown, although it is established that manganese, probably in the form of a manganeseprotein complex, and Cl^– are required (Boardman, 1975). The primary acceptor of PS–2, (Q), has a redox potential around zero volts, and is incapable of reducing NADP⁺ , without an imput of energy into PS–1. Quanta absorbed by PS–1 are transferred to P–700, a form of chlorophyll a absorbing at 700 nm. On excitation, P–700 donates an electron to an acceptor Z, resulting in the formation of P–700⁺ , a weak oxidant with a mid-point potential of + 0.43V, and Z^– , a strong reductant with a redox potential in the vicinity of –0.6V. P–700⁺ interacts with the weak reductant, Q^– , generated by PS–2 via an electron transport chain, which includes plastoquinone A, cytochrome ¦ and plastocyanin. The reduction of NADP⁺ by Z^– is mediated by soluble ferredoxin and ferredoxin-NADP reductase. It seems possible that the primary acceptor of PS–1, Z, is identical to bound ferredoxin, since the latter is photoreduced at very low temperatures (25°K) (Bearden & Malkin, 1974).

Cytochrome b₆ appears to function on a cyclic electron transport pathway around PS–1 and it may play a role in cyclic phosphorylation. There is conflicting evidence concerning the role of cytochrome b– 559_HP (Boardman, 1975). This cytochrome is oxidized by PS–2 at liquid nitrogen temperature and under certain conditions at room temperature, but at high light intensity it can also be reduced by PS–2. The function of cytochrome b– 559_LP is unknown at present.

Much evidence for the scheme of photosynthetic electron transport depicted in Fig. 4.4 has come from difference spectroscopy of chloroplasts. In this method, the change in the absorbance of various chloroplast components is measured on illumination of the chloroplasts with monochromatic light of various wavelengths. By following the change in the spectrum of individual components e.g. cytochrome ¦ or P–700, the role of these components can be deduced. For example, if chloroplasts are illuminated with far-red light of wavelength 720 nm, which is only absorbed by PS–1, there is a decrease in the absorbance of the chloroplasts in the region of 554 nm, due to the oxidation of cytochrome ¦ . If the chloroplasts are then illuminated with 650 nm light, absorbed by PS–2 (as well as by PS–1)there is an increase in absorbance due to the reduction of cytochrome ¦ . The oxidation of P–700 is followed by a decrease in absorption at 700 nm.

Electron transport from water to NADP⁺ in isolated chloroplasts may be intercepted by the addition of artificial electron acceptors (oxidants), which accept electrons from Z^– in PS–1 or from PQ in PS–2. For example, methyl viologen accepts electrons at PS–1 while p -phenylenediamine intercepts the chain at PQ. Ferricyanide or dichlorophenolindophenol can interact at either PS–1 or PS–2, depending on the experimental conditions (Trebst, 1974).

Figure 4.4
Z-scheme for photosynthetic electron transport and photophosphorylation. The
number beside a component indicates the number of molecules of that component per
photosynthetic unit of 400 chlorophyll molecules. Two sites of ATP formation are located
on the pathway between water and PS–1 (see text); one between water and plastoquinone
(PQ) and the other between PQ and PS–1. A scale of redox potentials is shown on the left.

Photosynthetic electron transport is readily monitored by illuminating isolated chloroplasts in the presence of an electron acceptor, and measuring either the oxygen evolved or the amount of acceptor reduced (Hall & Rao, 1972). This reaction is known as the Hill reaction (Hill, 1939).

The herbicides 3(3,4-dichlorophenyl) 1,1-dimethylurea (DCMU) and 3(p -chlorophenyl)-1,-1-dimethylurea (CMU) inhibit electron transport between

Q and PQ. Photoreduction of NADP⁺ can be restored in the inhibited chloroplasts by the addition of an artificial electron donor such as reduced 2,6-dichlorophenolindophenol. Electrons from the artificial donor enter the electron transport chain between the light reactions, and photoreduction of NADP⁺ is then driven by PS–1.

PS–1 is known as the far-red system because its absorption spectrum extends to longer wavelengths than that of PS– 2. At wavelengths beyond 700 nm, PS– 1 receives a high fraction of the quanta absorbed by chloroplasts. Chloroplasts contain one molecule of cytochrome ¦ and one molecule of P–700 per 430 chlorophyll molecules, from which it is concluded that the photosynthetic unit contains about 400 chlorophyll molecules. As shown in Fig. 4.4 the chlorophyll molecules appear to be distributed about equally between the two photosystems.