go ELECTROLYTES IN BIOLOGICAL SYSTEMS 



Menten kinetics. Such a system would deliver sugar unchanged into the in- 

 terior of the cell. It could be energized by the inward diffusion gradient of the 

 sugar, or it could be coupled to metabolic reactions to achieve an 'active trans- 

 port'. The observed actions of extracellular ions on sugar uptake might then be 

 attributed to their action on the carrier system. There are, however, a number 

 of facts which are not entirely consistent with the carrier hypothesis. For 

 example, uranyl ion in low concentrations completely blocks the uptake of 

 sugars under anaerobic conditions. The carrier is presumably completely 

 blocked. Yet, if O2 is admitted, sugar can again be taken up (at 40% of the 

 normal rate) even though the O2 has no influence on U02"^-binding by the cell 

 (59). The aerobic uptake of glucose is completely inhibited only at higher 

 concentrations of U02"*^, associated with its interaction with a second species 

 of binding site. It appears that there are two modes of entry of glucose under 

 aerobic conditions and only one under anaerobic conditions. Yet under anaero- 

 bic conditions sugar is delivered into the cell at a considerably more rapid rate. 

 To explain the above observations, a complicated system involving specific 

 aerobic and anaerobic carriers coupled to feedback from the metabolic systems 

 would have to be postulated. 



A second objection to the carrier hypothesis is based on the observation that 

 extracellular cations can alter the end products of sugar metabohsm. Thus at 

 alkaline pii, or at neutral pn with the addition of K+ or NH4+, there is a dra- 

 matic increase in glycerol production and a diminution of polysaccharide 

 formation (38, 49). If sugar is delivered unchanged into the interior of the 

 cell where it is then degraded into end products, it is difficult to visualize just 

 how extracellular factors can bring about large changes in the relative amounts 

 of the end products. 



A third objection is based on the stimulating action of K"*" on the surface 

 reaction (table 8). At pn 2.6, extracellular K+ stimulates the fermentation of 

 glucose by 84%, but enhances respiration by only 26%. Furthermore, the 

 aerobic fermentation is stimulated 146% (48). If the surface reaction were 

 simply a system for delivering glucose to intracellular enzymes, then it is 

 difficult to explain how the increased delivery of sugar induced by K+ under 

 aerobic conditions would result in an increase primarily in the rate of the 

 fermentative pathway of metabolism rather than in the respiratory pathway. 

 At the pH of the experiments, the rate of respiration is considerably below the 

 maximal rate, so the failure of respiration to respond to the same extent as 

 fermentation cannot be attributed to the saturation of the respiratory appara- 

 tus. 



The third hypothesis is based on the concept that sugar does not pass into 

 the cell unaltered, but that it is glycolyzed by enzymes located in a peripheral 

 zone of the cell. The action of cations on sugar uptake would be due perhaps in 

 part to a direct effect on enzymes of the outer surface of the cell exposed to the 

 extracellular environment, and in part to an effect on the surface structure in 



