SCOPE OF THE PROBLEM 



ally the production and release of control substances, the hormones, 

 which serve to integrate various other metabolic steps. Others are 

 the elaboration of the special environments represented by secre- 

 tions; in this case groups of cells may spend most of their metabolic 

 energy in transporting solutes into or out of such a secretion. 



An overwhelming part of the problem of segregating biochemi- 

 cal reactions, however, is relegated to the subcellular level, and up to 

 the present time has been only dimly perceived. Apparently all cells, 

 even those of a complex organism, synthesize their own proteins 

 and nucleic acids, generate their own power, and in general carry 

 out many other metabolic steps independently. Steps that have been 

 diminished or lost by one tissue or another during specialization of 

 function do not necessarily seem to be the less compatible ones. 

 In short life appears to have made very little use of the possible 

 structural economy of the cell that seemingly could result from 

 a high degree of specialization of cell function. Accordingly, the 

 problem of segregation and transport undoubtedly is dealt with 

 more at the subcellular level than at the plasma membrane or, for 

 secretions, at a barrier formed by close-lying cells. 



At the same time we may possibly exaggerate the extent of 

 internal compartmentalization necessary to cellular function. As long 

 as concentration gradients can be maintained by localized action, a 

 wide range of conditions can be anticipated without having the cell 

 be as compartmentalized as an onion. Even in cells containing very 

 high proportions of organelles, 40 to 70 per cent of the volume may 

 well be represented by a single-solution phase. In cells of certain 

 types, electron microscopy shows reticulum lying layer-on-layer; 

 but the observation of protoplasmic streaming and of small resist- 

 ance to movement of objects argues against a universal, rigid or- 

 ganization of the cytoplasm of other cells. The similarity of the 

 transport behavior of the adult human erythrocyte (or its ghost) 

 to that of more complex cells indicates that our observations of the 

 uptake and release of solutes by cells more often than not concern 

 the behavior of the plasma membrane. 



Secretion 



It is in the case of secretion that concentrative phenomena be- 

 come most apparent. Aqueous solutions are produced that differ 

 entirely from those from which their components come. A multi- 



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