Table I.

Estimated parameters for the permeation of K⁺, Na⁺, and Ca²⁺ through the pore of the VIC channel in the plasma membrane of wheat roots

Parameter	K+	Na+	Ca2+
G1	6.97 ±  0.080	5.95 ±  0.138	9.18 ±  0.315
G2	3.11 ±  0.323	7.49 ±  0.110	6.48 ±  0.990
G3	5.00*	5.00*	8.92 ±  0.228
U1	−5.39 ±  0.140	−5.50 ±  0.136	−6.69 ±  0.302
U2	−3.76 ±  0.776	−3.25 ±  0.159	−5.00*
A		5.00*
D1		0.07 ±  0.021
D2		0.31 ±  0.087
D3		0.61 ±  0.049
D4		0.66*
D5		0.70*
Rstrans		16.27 ±  0.390
Rscis		11.59 ±  0.550

Permeation was modeled as single-file movement through a free-energy profile with three energy barriers and two ion-binding sites that could be occupied simultaneously. Parameters were estimated from unitary current versus voltage relationships collected across a wide variety of ionic conditions. The total no. of observations was 624 and the weighted residual sum of squares was 200.51. ses of the parameters are indicated. Reliable estimates of the ses of values marked with asterisks are not available. These parameters could not be estimated with any precision and were fixed to their optimal values. The parameters G1, U1, G2, U2, and G3 define the free energies at positions D1, D2, D3, D4, and D5, respectively. They are expressed in (dimensionless) multiples of thermal energy (RT). The solution reference state is 55.5 m. The postscript refers to their position relative to the trans- (cytoplasmic) compartment. The parameter A defines the magnitude of ionic interactions, which raise the free energy profile by A(z₁z₂)/d, where z₁ and z₂ are the valencies of the interacting cations and d is the electrical distance from the occupied well. The parameters R_strans and R_scis (expressed in Å) correspond to the radii of circles containing one electron charge in the trans- and cis-vestibules of the pore, respectively.