A light-dependent mechanism that results in a slow, high-amplitude swelling of spinach chloroplasts in vitro has been discovered. The swelling is readily observed by optical and gravimetric methods, and by the use of an electronic particle counter; all show a 100 per cent increase of chloroplast volume in the light with an approximately 10-minute half-time. The existence of an osmotic mechanism for chloroplast swelling in the dark is confirmed. The volume of illuminated chloroplasts versus NaCl concentration represents the addition of osmotic and light effects. The action of light is enhanced by electron flow cofactors, such as phenazine methosulfate (PMS). However, neither conditions for ATP hydrolysis or synthesis nor NH4Cl influence the time course and extent of swelling. Hence, high-amplitude chloroplast swelling is light- (or electron flow), but not energy-dependent. A remarkable inhibitory effect of inorganic phosphate on chloroplast swelling is observed in the light, but not in the dark. Another action of light on chloroplasts is known to result in a shrinkage of chloroplasts which is rapid, reversible, energy-dependent, and requires phosphate. Thus phosphate determines the action of light on chloroplast volume. Since shrinkage is reversible, but swelling is not, it may be that they reflect physiological and deteriorative processes, respectively. Chloroplasts and mitochondria appear to control their volume by similar mechanisms.

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