An ion-exchange model for thylakoid stacking in chloroplasts

Abstract

1. Introduction Chloroplasts of higher plants contain stacks of internal membranes (thylakoids). These stacks (grana) can be reversibly unstacked in vitro by changing the salt composition ln the chloroplast suspension [I]. Attempts have been made to explain the mechanism of stacking in terms of the classical theory of colloidal particle interactions which involve the interplay of electrostatic and van der Waals’ forces [2]. In a [3] a quantitative analysis of these forces led to the conclu- sion that an additional short range force [4] was required to explain stacking. It was also estimated that for the stacking to occur the net surface charge density on the grana membranes should be <l electronic charge (1 e)/3000 A’. Such a density is low when compared with the density expected from the full dissociation of surface acidic groups of proteins and lipids [3]. The required low net surface charge density in the stacked regions of the membranes can arise from one of two possible mechanisms [3]: (a) cation binding [S--8]; or (b) redistribution of charges between grana (stacked) and stroma (unstacked) portions of the thylakoid mem- brane [9-l 11. Here, we present a model involving binding of cations which explains the mechanism of stacking. The model is inspired by recent studies on the nature of short range repulsive forces between lipid bilayers [4,12,13] and mica surfaces [13-161. 2. Balance of forces For phospholipid or lecithin bilayers in water an additional repulsive force arises at distances below -30 A [4,12]. This so called ‘hydration’ force domi- nates the interactions between the bilayers at the short distances. The origin of this additional force between mica surfaces has been studied in detail