ASER ROTHSTEIN 



73 



In summary, it can be stated that the surface layer of the cell contains lixed 

 anionic groups of two species, polyphosphate and carboxyl. These groups are 

 in ionic equilibrium with the cations of the medium rather than those of the 

 interior cytoplasm. The cell therefore acts like a cation-exchange resin. It 

 differs from exchange resins only in the nature of the fixed anions and in the 

 density of the negative charges. In the exchange resin, the negative charges are 

 close packed, whereas in the cell, they are relatively far apart and behave 

 independently of each other. If the surface groups of the cell are completely 

 tilled with bivalent cations, then only a few per cent of the total surface will be 

 occupied. The tixed anions of the cell-surface are of interest not only because 

 they are capable of binding cations, but also because they are intimately in- 

 volved in the uptake of sugars by the cell. 



TRANSFER OF lOXS IN THE PRESENCE OF SUBSTRATE 



In contrast to the relative isolation of the cytoplasm of the resting cell from 

 the electrolyte environment, the actively metabolizing cell transfers ions at 

 high rates and in large quantities. In considering the problem of ion transport it 

 is well to remember that electrical potentials are established when one species 

 of ion is moved, or moves from one phase or compartment to another. For these 

 reasons ions must move in pairs. A cation can pass from the medium to the 

 cytoplasm in exchange for another cation given up by the cell, or a cation and 

 anion can pass into the cell at about the same time. Such must be the case even 

 when the two ions are transported by independent pathways. It is therefore 

 difficult in many cases to determine which of a pair of ions is actively trans- 

 ported by the cell, and which is moving passively to maintain electrical balance. 

 Furthermore, the overall electrolyte balance involves not only ions transported 

 into the cell from the medium, but also ions created by the breakdown of sub- 

 strates. In the case of the yeast cell, in which cellular H+ enters into exchanges 

 with extracellular cations, the overall acid-base balance of the cell must also 

 be considered. 



K+-TRANSPORT 



Of the cations, K+ has been most intensively studied. In the absence of sub- 

 strate, yeast cells lose K+ at a slow rate (page 67). During active metabolism, 

 however, relatively large quantities of K+ move into the cell, even though the 

 concentration of K+ in the medium may be considerably lower than that in 

 the cell. Although active metabolism is a prerequisite for K"*" uptake, there is 

 no specificity in regard to the type of metabolism or the nature of the substrate. 

 For example, in the earliest study, that of Pulver and V'erzar (42), the respira- 

 tion of glucose served as a source of energy. Later it was found that anaerobic 

 fermentation of sugars would also induce K+-uptake, although at a lower rate 

 (51). Of a large variety of potential substrates tested, all which could be re- 

 spired (15 in number) also induced an uptake of K+ (40). 



