I. Intrazellulärer Transport von Zwischenprodukten der Photosynthese im Photosynthese-Gleichgewicht und im Dunkel-Licht-Dunkel-Wechsel

Abstract

1. The distribution — before, during and after photosynthesis — of different phosphate esters in chloroplasts and cytoplasm of leaf cells of spinach and Elodea has been investigated. In steady state experiments intact leaves were fed, during illumination, with 14CO2. The kinetics of the distribution of labelled phosphate esters between chloroplasts and cytoplasm were determined. In further experiments with intact leaves fluctuations in the pool sizes of phosphate esters in chloroplasts and in the cytoplasm were recorded in the dark/light and the light/dark transient. Independent fluctuations served as an indication that little or no exchange between chloroplasts and cytoplasm takes place. Concomitant fluctuations suggest rapid exchange. 2. Although labelling takes place in the chloroplasts, a number of labelled phosphorylated intermediates appear rapidly in the cytoplasm during steady state photosynthesis of intact leaves in the presence of 14CO2. This is particularly true for phosphoglyceric acid, glucose-6-phosphate, fructose-6-phosphate and fructose-1,6-diphosphate. On the other hand, labelled ribulosediphosphate, sedoheptulosediphosphate and sedoheptulosemonophosphate remain largely in the chloroplasts. This agrees with earlier work on the behaviour of phosphorylated intermediates during the induction period of photosynthesis. 3. In the dark the level of the bulk of the sugar diphosphates is lower in the chloroplasts than in the cytoplasm of intact leaves. On illumination, a large accumulation occurs only in the chloroplasts. This behaviour suggests impermeability of the chloroplast membrane towards at least some of the sugar diphosphates. In contrast, concomitant large fluctuations in the levels of dihydroxyacetonephosphate and fructose-1,6-diphosphate have been observed during the transients from dark to light and vice versa in chloroplasts and cytoplasm alike indicating that at least one of these compounds functions as a transport metabolite. Changes in the concentrations of glucose-6-phosphate and fructose-6-phosphate were much smaller under the influence of light than those of other sugar phosphates. 4. The results demonstrate the role of phosphorylated transport metabolites in carbon metabolism in chloroplasts and cytoplasm. Implications of these findings in relation to photosynthesis, respiration and the regulation of metabolism are discussed.