Changes in Electron Transport Chain Composition, and Activities of Photosystems and Photophosphorylation during Leaf Ontogeny

Abstract

Photosynthesis sensu stricto is a series of biophysical and biochemical processes driven by radiant energy in the range of ca. 400–700 nm resulting in the formation of NADPH₂ and ATP. Both these compounds are necessary for the fixation of CO2 to the appropriate acceptor and the redution of the primary product to the primary saccharide, triose phosphate. The formation of NADPH₂ and ATP takes place in thylakoid membranes situated in the chloroplast (see Chapter 2). Absorption of radiant energy by chlorophylls and other photosynthetie pigments located in the thylakoid membrane produces a separation of positive and negative charges in special pigments of the reaction centres of Photosystems 1 and 2 (PS 1 and PS 2), i.e. P700 and P680. The functionning of PS 2 is bound with the splitting of water: oxygen is evolved, and the electrons obtained pass through a series of acceptors, located in various parts of the thylakoid membrane, to NADP. The redox components involved form the photosynthetie electron transport chain. Its formulation with two photoreactions in series suggested by Hill and Bendall (I960), the so- called Z-scheme, is generally accepted today. Nevertheless, alternative hypotheses (of oxygenic and anoxygenic photosystems—see Arnon et al. 1981) still exist.