34 SUBCELLULAR PARTICLES 



those in solution, and one such difference may be revealed by study of pH optima 

 for enzyme reactions. In gels and at interfaces in cells the terms 'concentration,' 

 'diffusion" and 'pH' cannot be readily applied and must eventually be replaced by 

 conceptually clear notions amenable to mathematical treatment before the cell 

 physiologist can hope to deal quantitatively with the microheterogeneity of the 

 cell. 



The authors are indebted to Dr. G. V. F. Seaman of Cambridge University and Dr. Eva 

 Estermann for the elcctrophoretic data on the kaolin-lysozyme systems, and for many stimulating 

 discussions. 



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