Figure 1.

Schematic of the reaction and diffusion processes involved in facilitated CO₂ diffusion. Before time zero, we assume that Regions 0, 1, and 2 have identical compositions, with all reactions being in equilibrium and no concentration gradients existing between Regions. At time zero, we establish a CO₂ gradient from Region 0 (high [CO₂]) to Region 1 (low [CO₂]) in the presence of HB/B⁻, a generic non-CO₂/HCO₃⁻ buffer. Although CO₂ is the only species moving from Region 0 to Region 1, all species can move between Region 1 and Region 2. Some CO₂ moves from Region 1 to Region 2 by free diffusion. The shaded grey area identifies facilitated CO₂ diffusion (i.e., HCO₃⁻ and H⁺ moving in the same direction as CO₂) from Region 1 to Region 2. The pink shaded area identifies facilitated H⁺ diffusion (i.e., the antiparallel movements of HB and B⁻, with HB [the weak acid] moving in the same direction as CO₂ [the potential weak acid]). The solid arrows identify reactions, and the dashed arrows identify free solute diffusion. These reaction and diffusion events greatly accelerate the transfer of carbon (in the guise of CO₂ or HCO₃⁻) from Region 1 to Region 2. For details, see text. CA, carbonic anhydrase.