The Primary Photochemical Reactions in Systems I and II of Photosynthesis

Abstract

In plant photosynthesis the electron transport from water to carbon dioxide is light-driven at two different sites: the photosystems I and II. In both systems the light energy is absorbed by antenna molecules, such as chlorophylls and carotenoids and thereby converted to the energy of a singlet excited state. The excitation is then transferred among the various pigment molecules until the reaction center is reached. In the reaction center the excitation of the primary electron donor chlorophyll, called P700 in PS I and P680 in PS II, leads to electron transfer to the primary acceptor, followed by secondary electron transport steps which stabilize the charge separation. In principle the rates of electron transfer as well as the identity of the various ‘early’ electron acceptors can be studied by means of fast optical absorption spectroscopy, but in practice these measurements are seriously complicated by the large absorbance of antenna chlorophylls. Much of our knowledge on the primary reactions in plant photosynthesis was therefore based on indirect kinetic measurements (e.g. antenna fluorescence) and spectral studies by ‘photoaccumulation’ experiments. Recently the measurement of picosecond absorbance changes in systems with large antennae became possible and allowed a direct verification of some earlier conclusions. This paper summarizes the main results of recent picosecond absorption studies on relatively intact PS I and PS II from spinach (70–100 Chi a/reaction center), as well as on the recently isolated reaction center of PS II.