Figure 1.

Model of an oocyte exposed to ${\text{CO}}_2/{\text{HCO}}_3^{-}$. As CO₂ enters the cell, the concentration of CO₂ near the outer surface of the oocyte falls. The lost CO₂ can be replenished by 1) diffusion from the bulk extracellular fluid (BECF) and 2) the reaction(s) ${\text{HCO}}_3^{-}+{\text{H}}^{+}\to {\text{CO}}_2+{\text{H}}_2\text{O}$ at the cell surface. This reaction, which consumes H⁺, causes pH_S to rise. Conversely, the entry of CO₂ into the cell leads to the generation of intracellular H⁺ via the reaction(s) ${\text{CO}}_2+{\text{H}}_2\text{O}\to {\text{HCO}}_3^{-}+{\text{H}}^{+}$, therefore causing a fall in pH_i. (Cell model adapted from ref (Boron, 2010). The inset at the bottom right shows experimental pH_S and pH_i records for a H₂O-injected oocyte exposed to 1.5% CO₂/10mM ${\text{HCO}}_3^{-}$. Data kindly provided by Dr. Musa-Aziz).