The current—voltage (I–V) relationship of single K+ channels present in the Chara gymnophylla droplet membrane was studied. The channel presumably contains large mouths at both pore ends which are sufficiently wide to accomodate TEA+ as evidenced by internal and external TEA+ blockade. The voltage dependence of blockade by external Cs+ and Na+ reveals the multi-ion occupancy of the channel. The value of streaming potential (4.0 mV/osmol) measured in the Chara K+ channel indicates that the channel contains up to nine water molecules in the narrow region. It is concluded that the length of this region is around 28 Å.
