Cell Structure and Metabolic Regulation 29 



is reformed into much smaller units than is the membrane 

 material having the DPNH-cytochrome c reductase activity. 



Here, then, are the two presently known electron-transport 

 chains of the cell, and both seem to be an intimate part of a 

 membrane structure, one in the mitochondria, and one in the 

 microsomes. We have given the possible significance of this 

 localization in the case of the mitochondrial enzymes (Sieke- 

 vitz and Watson, 1956). I would suggest that one of the 

 functions of the microsomal enzyme may be in transporting 

 ions across the membrane, in a manner similar to that 

 visualized by Conway (1951) by the use of a redox pump 

 scheme. Conway's scheme could be used to explain the active 

 transport of hydrogen ions, of cations, and with modifications, 

 of anions, across membranes. Its attractiveness lies in the 

 fact that the active carrier, being a reduced, metal-containing 

 catalyst, binds the cation and is the same system as the 

 energy-providing system. Its particular enticement in the 

 morphological sense lies in that not only does it extend the 

 idea of the ER system being an adjunct of an active cell 

 membrane system, but that it could also provide for intra- 

 cellular differences in ion concentrations and for pH variations 

 deep within the cell. It is clear what this would mean for 

 enzymic activities. Furthermore, the ER system not only 

 divides the cell by its magnificent virtue of being, but also, if 

 our aforementioned ideas are correct, by acting enzymically 

 in such a way as to produce differences in ion concentrations 

 between the two sides of the membranes and perhaps also 

 differences between localities contiguous and remote from the 

 membranes. In this general connexion, it has already been 

 suggested (Porter and Palade, 1957) that the ER in the muscle 

 cell might act as an internal conducting system, the membranes 

 serving as conductors of the excitatory impulse. The idea of 

 a flavoprotein-mediated electron-transport and cation-carrier 

 system being a part of the ER membrane certainly does no 

 harm to this concept. 



Concerning the phospholipid composition of membranes, 

 while it is true that the mitochondrial membranes isolated 



