||"Diffusion of HCO3- across the plasmalemma is given by: V16=P[pH]A([HCO3-]e-[HCO3-]p) (eq. 5) where A=4p[r4]^2 (for [r4] see Table l BNID 102674), and P[pH] is the permeability coefficient of the plasmalemma for HCO3-. Eq. 5 somewhat underestimates V16 as it omits the electric field across the plasmalemma (see Stein 1986, p.45). However, the inaccuracy involved in this approximation will have been small in comparison with the uncertainty necessarily involved in estimating P[pH]. The value of the latter has been taken as 0.3X10^-2µm/sec."