Ion exchange in chloroplasts with special reference to the effect of light

• Main Author: Whitehouse, David George
• Published: Imperial College London 1967
• Subjects: 581.4
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.622984

The absorption of chloride by detached leaves of Elodea densa and the absorption of chloride and calcium by isolated pea chloroplasts was investigated using radioactive isotopes. Chloride absorption by Elodea is stimulated by light and dependent upon photophosphorylation as the energy source for accumulation. Both photosystems could support uptake although photosystem II was more efficient than photosystem I. DCMU severely inhibited uptake. Accumulation in the light could proceed in the absence of oxygen and was stimulated under C02-free conditions, whereas uptake in the dark was dependent upon oxygen and C02. Bicarbonate inhibited both uptake in the light and dark. When detached pea leaves were fed with radioactive chloride the isolated chloroplasts contained high levels of chloride. Preliminary experiments with isolated chloroplasts indicated that ATP was involved in chloride uptake. Using a faster method of separation of chloroplasts from experimental solutions, very rapid and large chloride movements occurred in the light with slower movements taking place in the dark. These results indicated that the chloroplast membrane in vitro was permeable to chloride. The accumulation of calcium was stimulated by very low light intensities, The mechanism was very labile and showed a high degree of specificity. Uptake was not associated with phosphorylative processes but with electron flow. Inhibitor experiments indicated that the accumulatory mechanism probably lies close to the electron flow chain but not directly upon it. Approximately 60% of the accumulated calcium was present in a water-soluble fraction, though not associated with protein. A further 30% was present associated with a component that could exchange with nonradioactive calcium. A small fraction of the accumulated calcium was present in a lipid-chlorophyll fraction. The calcium accumulation reported here shows several new features. The results are discussed in terms of a mechanism of ion exchange induced by light in chloroplasts.