Figure 2

Possible fate of gases produced during fermentation. CO₂ freely diffuses across the cell membrane and reacts with water to form carbonic acid, which dissociates to form bicarbonate and a H⁺. The high pH inside the cell relative to the medium shifts this equilibrium towards bicarbonate, while the opposite occurs extracellulary. Excess H⁺ are pumped out of the cell at the cost of ATP or are reduced to H₂ by the hydrogenase. Bicarbonate may be transported out of the cell or used in biosynthetic reactions. Adapted from Dixon et al. [38].