[Potassium transport in yeast]

K+ transport is vital for all cells. The first model to explain K+ transport in yeast was a redox potential driven H+/K+ antiporter. Later, a H+-ATPase was described which generates a H+ gradient, acid outside. This proton gradient is used by a secondary transporter, which transports K+ to the inside. These transporters are separate proteins, that have been isolated and their genes have been cloned. Furthermore, in Saccharomyces cerevisiae two different K+ transporters have been described. K+ exit occurs through K+ channels and through H+/K+ antiporters that seem to anticipate in the regulation of internal pH. In addition, yeast produces a large amount of CO2 during the fermentation process. CO2 accumulation in turn results in the establishment of a Donnan potential due to the accumulation of bicarbonate, which retains large quantities of K+. In S. cerevisiae, bicarbonate is responsible for a large accumulation of K+. Also, the H+-ATPase, by pumping protons to the outside, favors the formation of bicarbonate which cannot diffuse to the extracellular environment leading to cytoplasmic pH values near 7.0. Bicarbonate accumulation allows yeast to maintain large pH gradients across the plasma membrane.