In semicells of the unicellular green algaClosteriumthat are undergoing elongation, transcellular ion currents enter the elongating region of the cell and leave via the non-elongating region of the cell, as in the case of many other tip-growing organisms. The density of the inwardly and outwardly directed currents was 142.5±63.7 nA cm−2(n=42) and 109.3±46.5 nA cm−2(n=33), respectively, at the respective regions of the cells. Both currents clearly decreased with decreases in the external concentration of Ca2+ions, and they were completely blocked by addition of Ca2+-channel blockers, such as 20μM LaCl3, to the external medium. Increases in pH up to 10.2 had no effect on the currents, but a decrease in pH from 7.5 to 5.7 or 4.5 resulted in an explosive increase in the currents. Removal of external K+and Cl−ions induced some increases in the currents, but removal of external Na+Mg2+plusSO42-andNO3-ions had little effect on the currents. A major part of the currents may be carried by Ca2+ions, while H+, K+and Cl−ions may play a minor role as members of the group of ions that carry the currents. Thus, there is a clear relationship between Ca2+ion currents and elongation in
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