ASER ROTHSTEIN 97 



iralions inside the cell are relatively constant at i X 10 ' and 10^^ m/1. for 

 K+ and H"*", respectively. In the outer compartment however, the concentra- 

 tions are dependent on the rate of ion-equilibration with the medium and on 

 the rate of ion-transport into or out of the cell. Because certain of the fermenta- 

 tion reactions in the outer compartment are influenced by H+ and K+, the 

 concentrations of these ions in the medium can markedly alter the overall 

 rate of fermentation. In contrast, reactions located in the central compart- 

 ment of the cell are Httle affected by the concentrations of H"*" and K+ in the 

 medium. 



The glycolytic reactions of the outer zone are directly responsible for the 

 uptake of inorganic phosphate by an esterification reaction, probably at the 

 phosphoglyceraldehyde dehydrogenase step in fermentation. The same re- 

 actions are responsible for the uptake of bivalent cations which enter the cell 

 combined with orthophosphate in a complex. 



In the case of the monovalent cations a special pumping mechanism must 

 be invoked. The metabolic reactions are only indirectly concerned as a source 

 of energy. A lipid soluble carrier system would seem to be the most reasonable 

 mechanism. The type described in detail for Na"*" and K+ transport in red blood 

 cells (65) seems equally applicable for yeast cells. Briefly, the K+ is carried 

 across a lipid phase of the cell membrane as an undissociated complex with a 

 lipid soluble carrier molecule. At the inner face of the membrane, the K+ is 

 dissociated and moves into the cytoplasm. Back diffusion of K+ from the 

 cytoplasm to the cell is largely prevented by metabolic reactions which convert 

 the carrier at the inner membrane into an inactive substance (poor complexor 

 of K+). The inactive substance diffuses back to the outer face of the mem- 

 brane and is there reconverted to carrier. The reactions which convert the 

 carrier to inactive form and back might be redox reactions, or they might be 

 phosphorylation reactions, or others. The lipid-carrier system could account 

 for the active transport of K+ into the cell and Na+ out of the cell, wdth a 

 simple coupling to metabolic reactions occurring at the cell surface. It can 

 explain the high degree of specificity between K+ and Na+ (20 to i), which 

 would be hard to accomplish by any system working in an aqueous phase 

 (model systems of the lipid carrier system can be set up which discriminate 

 between Na and K+ to an even greater degree). Finally, it explains why the 

 binding of K"*" by the cell is not a prerequisite for its inward transport. 



Our understanding of many aspects of electrolyte metabolism in living cells 

 is poorly understood, particularly with respect to the nature of the mechanisms 

 involved. Cellular structure plays an important if not a central role. Therefore, 

 studies on isolated systems often give only partial or deceiving answers. But 

 studies with intact cells involve so many variables, known and unknown, that 

 the data obtained can be interpreted in many ways. 



