Lateral Electron Transport in Thylakoids of Higher Plants

Abstract

Photosynthetic electron transport in higher plants from H2O to NADP+ involves three multisubunit protein complexes which are embedded in the thylakoid membrane. These complexes, the photosystem II (PSII) complex, the cytochrome (cyt) b6-f complex and the photosystem I (PSI) complex operate in series. They interact via small electron carriers. Plastoquinone links the PSII complex with the cyt b6-f complex, and plastocyanin links the cyt b6-f complex with the PSI complex. A considerable impact for the concept of linear electron transport came from the investigations showing an extreme lateral heterogeneity in the distribution of the two photosystems with PSII preferentially in appressed grana and PSI in stroma exposed membranes (l). Although grana formation is not prerequisite for the functioning of photosynthetic electron transport it is generally observed in chloroplasts of higher plants and therefore may be essential for optimal photosynthetic efficiency. The photosynthetic membrane provides a possibility to study reactions of laterally separated components in vivo which may not be obvious in membranes with a random distribution of the integral complexes. Questions related to the long-range lateral electron transport between the two regions of the thylakoid membrane concern the constraints and the requirement of the lateral mobility of electron transport components imposed by the membrane composition, membrane topography, and the kinetics of the reactions.