Photoelectron transport system

Another class of compounds of importance in electron transport systems and found in chloroplast material are the quinones. Plastoquinone is thought to play an important role in both photoelectron transport and photophosphorylation (Crane et al., 1960). It has sometimes been suggested that vitamin K and other quinones may be components of the photoelectron transport system (Kegel et al., 1962). 

Fig. 3. Scheme of photoelectron transport in photosynthesis. The path of photoelectron flow associated with noncyclic photophosphorylation through the two postulated light reactions mediated by Pigment Systems 1 and 2 is indicated by the heavy lines. Redox potentials of electron-carrying cofactors along this path is indicated by scale on the left. Further explanation in text. 

Levine and Smillie (1962) used mutants of Chlamydomonas reinhardi to show clearly the separation of photoelectron transport into two phases, each with its own light reaction. Two mutant strains which are incapable of carrying out the Hill reaction are able to photoreduce NADP+ without oxygen evolution, provided they are supplied with electrons from an electron donor. Both mutants had an increased content of cytochrome f and a decreased content of plastoquinone. Presumably they are capable of performing the reactions carried out by System 1 only. 

From these experiments and many others, it seems established that photoelectron transport in green plants can occur via two photochemical steps, together with some intermediate dark steps. In one photochemical step (System 2), plus one or more dark steps, electrons are removed from water and transferred to an electron acceptor which may be plastoquinone. Dark reactions plus a second photochemical step transfer electrons to another electron acceptor, which becomes a reducing agent of at least the strength of hydrogen gas or reduced ferredoxin. Coupled to one or more... 

The photoelectron transport of electrons from water to the intermediate electron transport system (plastoquinones, cytochromes, etc.. Fig. 3) has not been resolved into its component reactions. These must include the primary quantum conversion act of Pigment System 2, the oxidation of water, the evolution of O2 from the immediate product of this oxidation, and the reduction of some stable intermediate electron carrier, such as plastoquinone, by the electrons from the oxidation of water. It is known that the... 

The main charge transport pathway in [PcSiO] is via the phthalocyanine 7c-system. This is proved by photoelectron spectra of a dinuclear unit R3Si-0(SiPc)-0-(SiPc)-0 SiR3 [34d, 34h]. Its crystal structure is in good agreement with the results given for [PcSiO] [34b, 34d]. 

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