Figure 2.

Venting of metabolically produced acids. (a) H⁺-lactate efflux across the membrane is facilitated by H⁺-monocarboxylate transport (MCT). Diffusion of H⁺ and lactate away from the cell-surface is necessary for sustained MCT activity. Mobile H⁺-buffers can facilitate H⁺ diffusion and support H⁺-lactate venting. (b) CO₂ can permeate the cell membrane through the lipid bilayer or gas channels. Spontaneous CO₂ hydration is slow, but can be accelerated by exo-facial carbonic anhydrase (CA) enzymes. Diffusion of the hydration products alongside CO₂ represents a form of facilitated CO₂ diffusion.