Cycles and Q-cycles in plant photosynthesis
The classical description of electron transfer in chloroplasts linked photosystem II to photosystem I with a simple hydrogen-carrying plastoquinone shuttle which transferred one H/sup +/ per electron travelling through the chain. An additional H/sup +//e/sup -/ liberated inside the thylakoid with the splitting of water provided an overall H/sup +//e/sup -/ stoichiometry of 2. Cyclic electron transfer around photosystem I via cytochrome b/sub 563/ was considered to reenter the main chain at plastoquinone, translocating one H/sup +/ per electron completing the cycle. Such a scheme has had to be modified, however, following the discovery of a further charge translocation that is observed, following a single turnover flash, as a slow rise in the electric-field indicating bandshift termed P518. According to the earlier scheme for electron transfer in chloroplasts, charge separation across the thylakoid membrane took place only in the reaction centers of photosystems I and II, giving a rapid (submicrosecond) appearance of the P518 bandshift. In the last few years, however, several groups have shown a slow (millisecond) additional component of the P518 rise following sngle turnover flash illumination of algae or chloroplasts. The maximum amplitude of this slow component is equal to that of the fast phase generated by electron transfer in photosystem I reaction centers, and its appearance is dependent on the redox poise of the electron transfer chain. If this field-indicating slow rise in P518 (P518/sub s/) reflects an electron crossing the thylakoid membrane from inside to out if a dark step, then additional complexities must exist in the electron transport pathway between the photosystems.
- Research Organization:
- Brookhaven National Lab., Upton, NY (USA)
- DOE Contract Number:
- AC02-76CH00016
- OSTI ID:
- 6721510
- Report Number(s):
- BNL-28684; CONF-8010149-1
- Resource Relation:
- Conference: Function of quinones in energy conserving systems conference, Hanover, NH, USA, 26 Oct 1980
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
PHOTOSYNTHESIS
BIOCHEMICAL REACTION KINETICS
AFFINITY
ATP
BINDING ENERGY
CHLOROPLASTS
CYTOCHROMES
ELECTRON EXCHANGE
PLANTS
QUANTITY RATIO
REDOX REACTIONS
TRANSLOCATION
CELL CONSTITUENTS
CHEMICAL REACTIONS
ELECTRON TRANSFER
ENERGY
KINETICS
NUCLEOTIDES
ORGANIC COMPOUNDS
PHOTOCHEMICAL REACTIONS
PIGMENTS
REACTION KINETICS
SYNTHESIS
550200* - Biochemistry