3° 



SUBCELLULAR PARTICLES 



-60 - 



^=—0.001 M 



D 



C 



O 



Fig. 3. Zeta potentials of B. 

 coll as a function of pH and sodium 

 chloride concentration (9). 



Phosphate is actively transported into yeast by a mechanism involving glycoly- 

 sis reactions and the pHi, optimum for the phosphate uptake of 6.5 is shifted to 

 the acid by nearly two units by 0.02 m K^ (18). A possible explanation is that an 

 increase in extracellular K"^ results in alkalinization of the cell surface by exchange 

 reaction (cf. ecj. i). 



Burstrom (3) has observed a splitting of sucrose by wheat roots in external 

 solution 20 times as rapidly as the absorption of hexoses under the same condi- 

 tions. The hydrolysis proceeds by enzyme action at the root surface and is a de- 

 creasing function of pH between 3.7 and 7.4 at high salt concentrations where 

 pHi,~pHs. The root surface is negatively charged, and one way of decreasing 

 ApH is by increasing the external salt concentration at constant pH),. A com- 

 parison of columns ^ and 4 in table 3 shows that the rate of hydrolysis closely 

 follows the hydrogen ion concentration on the root surface, independendy of the 

 external pH (3). 



It is worth noting that Burstrom suggested that probably one could adapt the 

 rate of sucrose inversion to a quantitative method of determining the charge of 

 the root surface. 



Table 3. Influence of A pH on inversion of sucrose by wheat roots (3) 



