Figure 2.

Schematic of the reaction and diffusion processes involved in facilitated diffusion of: HCO₃⁻ (A); H⁺ (B); B⁻ (C); and HB (D). We set up the four examples as in Figure 1, except that the component that we add to Region 0 is different. At time zero, we establish a gradient for HCO₃⁻ in (A) from Region 0 (high [HCO₃⁻]) to Region 1 (low [HCO₃⁻]), for H⁺ in (B), for B⁻ in (C), and for HB in (D). The solid arrows identify reactions, and the dashed arrows identify free solute diffusion. For details, see text. CA, carbonic anhydrase.