Electron Transport and H⁺ Pumping

1)Q cycle

a)stoichiometry and molarity

b)Energetics

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 4^th polypeptide complex of the photosynthetic apparatus embedded in the thylakoid membranes, the ATP synthase complex.

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, S_o to S₄.  One e^- is removed photochemically with each photon, hn, moving the reaction center successively through S₁, S₂, S₃, and S₄.  S₄ decays spontaneously to S_o by oxidizing 2 H₂O to O₂ (see Fig. 3.20 of the Heldt text).