CHAPTER 10 



EPILOGUE 



There is nothing makes a man suspect much, more 

 than to know httle. 



Bacon. 

 Essay XXXI, Of Suspicion 



The idea that salt absorption is mainly an active process has now 

 been firmly established for more than 30 years, but precise details 

 of the mechanism through which respiratory energy is used to bring 

 about accumulation is still far from certain. Many hypotheses have 

 been proposed but none has received widespread acceptance for one 

 reason or another. All have one feature in common, namely the 

 dependence at some stage on the formation of a specific complex 

 between the ion transported and an organic constituent of the 

 cytoplasm. There is great diversity of opinion about the likely 

 nature of the "carrier" substances. In my view, cytoplasmic proteins 

 may act as carriers, in the manner outlined below. There is not to my 

 knowledge any evidence to disprove this hypothesis, and much 

 which is consistent with it. 



Salts diffuse as ions across the cellulose cell wall — the water- 

 filled spaces of which comprise the "free space" of the cell — to the 

 surface of the cytoplasm, where they become attached to protein 

 molecules located in the surface membrane. The ion-binding 

 capacity of proteins is best known in the case of enzymes, which 

 are the only proteins yet isolated from cytoplasm in a fairly natural 

 state, but doubtless other proteins possess the same ability. Whether 

 all the proteins which bind ions at the cell surface are in fact enzymes 

 is not yet certain but this is quite likely to be the case. As a result of 

 protein synthesis, new sites are created to which salts may be bound, 

 and uptake from the medium continues as long as newly synthesized 

 protein is being exposed at the external surface. 



Cytoplasm is in a constant state of flow (cyclosis), which means 



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