1916] on Chemical Change in Living Organisms 643 



If the rate at which B is converted into C is greater than that at 

 which A changes into B, it is obvious that the amount of B present 

 at any moment may be extremely small, although the whole of the 

 final products have passed through the stage. The fact warns us 

 from estimating the importance of any particular constituent of the 

 cell by the quantity to be obtained. 



The second point is this. Suppose that there are two independent 

 reversible reactions, both leading to the same product, C, 



A<. 

 C 

 B<^ 



and that A — ^C is more rapid or easier that B — ^C. This latter 

 reaction will be practically absent, being balanced by the excess of C. 

 But, if the former reaction is abolished by the removal of A, then 

 B — >C will take place in proportion as C is used up in other 

 reactions. Thus, under special conditions, a reaction may take place 

 Avhich is not detectable under normal conditions, although capable 

 of taking place. 



One of the most difficult questions is the manner in which the 

 various components of the cell are prevented from entering into 

 chemical reaction except when required. Enzymes, for example, are 

 not always in activity. The conception which states that the cell 

 consists of numerous minute "reaction chambers," separated from 

 one another by membranes, seems to present most possibilities. These 

 membranes must be regarded as capable of removal and of recon- 

 struction, or reversible as regards their permeability. The food 

 vacuoles of an Amoeba may serve as an illustration of such chambers 

 on a comparatively large scale. In these vacuoles digestion processes 

 are going on independently of other reactions in various parts of the 

 same cell protoplasm, although this latter behaves as a liquid. 



The general conclusion to which w^e arrive is that velocity of 

 reaction plays an exceedingly important part in the regulation of cell 

 mechanics. I venture to think that the conception is destined to 

 replace static points of view, such as that of " lock and key " or the 

 fitting together of molecular groupings. That there is still very 

 much to be discovered is obvious. We have to find out how the 

 living cell is able to modify and adjust together the large number of 

 reactions known to the chemist. The study of the methods by which 

 the rate of these reactions is affected is one of the most valuable of 

 those accessible to us. 



[W.M.B.] 



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