Anion Permeability of Chloroplasts

The relative anion permeability of chloroplasts was measured by utilizing three techniques: (a) measurement of light induced NH₄+ movements, (b) measurement of the light induced osmotic swelling in the presence of NH₄+ salts and (c) measurements of valinomycin induced dark swelling in the presence of K+ salts. All were found to be dependent on the nature of the anion in the solution. Impermeant anions have been found specially in the group of zwitterions (tricine, glycine, ɛ‐amino‐caproate and aspartate) or polyvalent ions (citrate, succinate, AMP, ADP, and ATP). The permeant anions could be placed in the following order: I‐ > NO₃‐ > Cl‐ > SO₄=. The uncoupling by ammonium salts was independent of the nature of the anion in solution. Thus, in chloroplasts, uncoupling by ammonium salts was not necessarily accompanied by massive ammonium uptake or dependent on the presence of a permeant anion. The relevance of these studies to the chemiosmotic hypothesis is discussed, and it is suggested that a membrane potential by itself is not sufficient in chloroplasts to provide the driving force for ATP synthesis.