PROTOPLASM AS A PHYSICAL' SYSTEM 53 



extracts or in the residue remaining after complete 

 mechanical or other disintegration of the protoplasm; 

 and (B) those which continue only while the protoplasm 

 remains structurally intact and 'living." The former 

 group (A) includes a large number of hydrolyses and 

 some oxidations; e.g., those due to oxidases; but, as 

 already indicated, the physiologically significant oxida- 

 tions, especially of sugar and other energy-yielding 

 compounds, cannot be accomplished, at least with 

 anything like the normal velocity and completeness, 

 under the influence of enzymes or cell-extracts. Yeast 

 cells which have been mechanically destroyed, or even 

 simple watery extracts of yeast, rapidly hydrolyze cane 

 sugar, just as does the living cell, and autolyzing yeast 

 cells split proteins rapidly into amino-acids. It has been 

 found, however, that the alcoholic fermentation of sugar 

 proceeds much more slowly in the press-juice of yeast 

 than it does under the influence of the living protoplasm.^ 

 Many other cases are known where biochemical reactions, 

 although proceeding in dead cells or under the influence 

 of cell-extracts, do so at a slower rate than in living 

 protoplasm. 



The latter group of reactions (B) include the specific 

 syntheses, i.e., of protein, together with those syntheses 

 which require the expenditure of considerable energy, 

 like the building up of fats from carbohydrate, or of 

 amino-acids and other compounds of high chemical 

 potential from compounds of lower potential. The 

 energy required for these syntheses is apparently de- 



^ Cf . Harden, "Alcoholic Fermentation," in Monographs on Bio- 

 chemistry, edited by Plimmer and Hopkins. 



