Oxygen Evolution and Uptake as a Measure of the Light-Induced Electron Transport in Spinach Chloroplasts

Abstract

The light-induced electron transport in thylakoid membranes of higher plant chloroplasts uses H₂ O as electron donor and is able to reduce components with a potential as low as -0.55 V while producing at least one molecule of ATP. Thus 1.65 V/electron are required for the process. One quantum of red light, however, has not more than 1.3 V available. This leads to the conclusion that the energy of at least two photons must be utilized in order to provide sufficient energy to drive an electron through the complete photosynthetic electron transport system. The manner by which this could be achieved was first shown in the observation of the red drop phenomenon by Emerson (Emerson and Lewis, 1943). Today the proposed Z-scheme is still generally accepted (Hill and Bendall, 1960). According to this model, the electron goes through two photoreactions (PS II and PS I), which are linked together by an electron transport chain. This scheme and a possible arrangement in the thylakoids proposed by Trebst (1974) is summarized in Figure 1. In Figure 2 some possible electron donors, acceptors, and inhibitors are introduced which will be used in the experiments of this course. With these artificial components it is possible to elucidate the individual steps of the electron transport.