BCH/PPA 503 -- Plant
Lecture Six

Organization of the PSII and PSI reaction centers


Electron Transport and H+ Pumping



The Z scheme and photophosphorylation were overviewed in Lecture 2.  The Z scheme is a diagrammatic representation of photosynthetic e- flow from H2O to NADP+.  The overall equation again for this is:

2H2O + 2NADP+ + xADP + xPi + nhv -> O2 + 2NADPH + 
2H+ + xATP + xH2O

ATP is generated by the chemiosmotic potential generated by the accumulation of H+ on the lumen side of the thylakoid membranes during this process.  This overall process is known as acyclic photophosphorylation.  There is also a cyclic e- transfer pathway that operates under certain conditions during photosynthesis that only involves PSI and the cytochrome b6/cytochrome f complex.

In this process photons excite P700 in PSI causing e- derived from P700* to be transferred to the cytochrome b6/cytochrome f complex and ultimately be cycled back to P700+.  This cyclic e- flow does not result in the splitting of H2O and yields no O2 or NADPH but can lead to ATP synthesis via cyclic photophosphorylation.  This photophosphorylation is carried out by a 4th polypeptide complex of the photosynthetic apparatus embedded in the thylakoid membranes, the ATP synthase complex.


An overall model of these photosynthetic structures in the thylakoid membrane is given in the following figure from M.J. Farabee's web site:

[Image of model of 
photosynthetic structures, part one]
This image is from M.J. Farabee's web site.

The 5 major structural units required for the light reactions of photosynthesis are segregated in thylakoid membranes.  PSII and the light harvesting complexes are mainly located in the appressed grana and PSI and ATP synthase complexes are restricted to the non-appressed areas.  The cytochrome b6f complex is assumed to be uniformly distributed.  This is illustrated in Plate 5 of Dey and Harborne's Plant Biochemistry and Fig. 3.4 of the class text.

O2 evolution requires the accumulation of 4 oxidizing equivalents in PSII.  The PSII reaction center is cycled through 5 redox states, So to S4.  One e- is removed photochemically with each photon, hn, moving the reaction center successively through S1, S2, S3, and S4S4 decays spontaneously to So by oxidizing 2 H2O to O2 (see Fig. 3.20 of the Heldt text).


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    This page was last modified January 17, 2000.