Plastoquinone as a mobile redox carrier in the photosynthetic membrane

The role of plastoquinone as a mobile redox carrier linking photosystem II and the cytochrome b 6 − f complex is considered. It is proposed that plastoquinone is located primarily within the fluid bilayer‐midplane region of the thylakoid membrane and thus can move laterally at very fast rates corresponding to a microscopic diffusion coefficient of 10 −6 cm 2 ·s −1 . Because of the presence of integral proteins the diffusion path will be tortuous and extended, giving rise to a sub‐macroscopic diffusion coefficient which is lower than the above value. Even so it is concluded that within the half‐time for electron donation to the cytochrome b 6 − f complex there is adequate time for plastoquinone to diffuse over a distance equivalent to the radius of a granum membrane.