Electrical properties of Valonia ventricosa

The cytoplasmic electrical potential and membrane resistance of mature cells of Valonia ventricosa have been measured by inserting a microelectrode concentric with another electrode into the vacuole of the cell. The cytoplasmic region was investigated by advancing the microelectrode into the cell wall from the vascuolar side. The results revealed a unique region where the vascuolar electric potential and membrane resistance changed in a simultaneous single step to values close to zero. The measured potential always remained positive immediately after the step. At no time was a highly negative potential region encountered. Further penetration of the microelectrode revealed a low resistance negative potential region of --12.6 +/- 1.1 mV associated with the cell wall. Experiments were also carried out on aplanospores of V. ventricosa to compare mature and immature cells. The chemical composition of the vacuolar and protoplasmic phases of mature cells was determined. The results agreed with previous results except that the C1- ion content of the protoplasm was significantly higher at 381 +/- 20 mmoles/liter (H2O). It was concluded that mature cells of Valonia are significantly different from immature cells in that no highly negative potential cytoplasmic region was found in mature cells. It was considered that the measured step change in electric potential and membrane resistance occurred at the plasmalemma and that the tonoplast was a region of very low resistance. The implications of these findings in terms of models of ion transport into Valonia are discussed.