Potassium ion channels in the plasmalemma.

The potassium ion is an indispensible cytosolic component of living cells and a key osmolyte of plant cells, crossing the plasmalemma to drive physiological processes like cell growth and motor cell activity. K(+) transport across the plasmalemma may be passive through channels, driven by the electrochemical gradient, K(+) equilibrium potential (E(K) ) - membrane potential (V(m) ), or secondary active by coupling through a carrier to the inward driving force of H(+) or Na(+) . Known K(+) channels are permeable to monovalent cations, a permeability order being K(+) > Rb(+) > NH(4) (+) > Na(+) ≥ Li(+) > Cs(+) . The macroscopic K(+) currents across a cell or protoplast surface commonly show rectification, i.e. a V(m) -dependent conductance which in turn, may be controlled by the cytosolic activity of Ca(2+) , of K(+) , of H(+) , or by the K(+) driving force. Analysis by the patch clamp technique reveals that plant K(+) channels are similar to animal channels in their single channel conductance (4 to 100 pS), but different in that a given channel population slowly activates and may not inactivate at all. Single-channel kinetics reveal a broad range of open times (ms to s) and closed times (up to 100 s). Further progress in elucidating plant K(+) channels will critically depend on molecular cloning, and the availability of channel-specific (phyto)toxins.