Structural and functional organization of the photosystems in spinach chloroplasts. Antenna size, relative electron-transport capacity, and chlorophyll composition

Abstract

The structural and functional organization of the spinach chloroplast photosystems (PS) I, II α and II β was investigated. Sensitive absorbance difference spectrophotometry in the ultraviolet (▵ A 320) and red (▵ A 700) regions of the spectrum provided information on the relative concentration of PS II and PS I reaction centers. The kinetic analysis of PS II and PS I photochemistry under continuous weak excitation provided information on the number ( N) of chlorophyll (Chl) molecules transferring excitation energy to PS II α, PS II β and PS I. Spinach chloroplasts contained almost twice as many PS II reaction centers compared to PS I reaction centers. The number N α of chlorophyll (Chl) molecules associated with PS II α was 234, while N β = 100 and N PS I = 210. Thus, the functional photosynthetic unit size of PS II reaction centers was different from that of PS I reaction centers. The relative electron-transport capacity of PS II was significantly greater than that of PS I. Hence, under light-limiting green excitation when both Chl a and Chl b molecules are excited equally, the limiting factor in the overall electron-transfer reaction was the turnover of PS I. The Chl composition of PS I, PS II α and PS II β was analyzed on the basis of a core Chl a reaction center complex component and a Chl a b- LHC component. There is a dissimilar Chl a b- LHC composition in the three photosystems with 77% of total Chl b associated with PS II α only. The results indicate that PS II α, located in the membrane of the grana partition region, is poised to receive excitation from a wider spectral window than PS II β and PS I.