The influence of the thylakoid membrane surface properties on the distribution of ions in chloroplasts

Thylakoid membranes isolated from peas have been subjected to ionic analyses using the technique of neutron activation. This has allowed the analyses of K⁺, Na⁺, Mg²⁺, Ca²⁺ and Cl^? to be measured simultaneously on the same sample. By varying the ionic composition of the suspending medium it has been shown that these chloroplast membranes have no obvious chemical specificity for the inorganic cations studied and that the major controlling factor is the electrostatic neutralization of the surface negative charges. In agreement with the Gouy-Chapman theory and for the conditions used, divalent cations were preferentially attracted to the membrane surface. This finding, together with the ionic analysis of the unwashed thylakoids and of isolated intact chloroplasts, indicated that the major physiological surface cation is Mg²⁺ and that K⁺ is probably the main inorganic cation of the stroma. This conclusion is discussed in terms of counterion movement in response to light induced proton pumping at the thylakoid membrane.