Spectral Heterogeneity and Energy Equilibration in Higher Plant Photosystems

Abstract

Higher plant thylakoids contain a number of pigment binding proteins which are organised into two photosystems known as photosystem I (PSI) and photosystem II (PSII). The major light harvesting pigments are chlorophylls (chls) of which about 200-250 are associated with each photosystem, though this number is somewhat variable depending on the growth conditions. Light energy is absorbed by these antenna pigments and transferred rapidly to the reaction centre primary electron donor, a special chl dimer or monomer, where photochemistry occurs. Energy transfer times between neighbouring antenna chls occur in the range of 0.2-5ps (for review see Jennings et al., 1995). The time between light absorption and primary photochemistry is in the range 100-500ps with the time constant for reduction of the primary pheophytin electron acceptor probably around 3ps. The reader is referred to chapters by Bassi et al. for details of pigment-protein biochemistry, Borisov for descriptions of energy transfer mechanisms, and Mathis for details on reaction centres.