Electric light scattering measurements of thylakoid membranes from wild type and two mutant forms (JB67 and LK3) of Arabidopsis thaliana have shown that application of external electric pulses induces electric dipole moments of different origin. The asymmetric surface charge distribution and electric polarizability are significantly altered by the lipid modification. Mild trypsin treatment of Arabidopsis thylakoids leading to digestion of small polypeptides from the light-harvesting chlorophyll a/b protein complex of photosystem II (LHCP II) gives evidence for a lower content of LHCP II in the mutant forms. The results demonstrate the significance of the level of thylakoid lipid unsaturation in determining the surface charge distribution through changes either in the pigment-protein content and membrane appression induced by the lipid modification or in the exposure of charged polypeptides on the thylakoid membrane surface(s) arising from alteration of the lipid geometry.