YEAST METABOLISM 33 



Water H 2 O 83 . 76 per cent. 



Cane sugar (C^H^On) 15 . oo per cent. 



Ammonium tartrate (NH^C^^j i .00 per cent. U ^ 



Potassium phosphate K 3 P04 20 per cent. 



Calcium phosphate Ca 3 (PO 4 )2 02 per cent. 



Magnesium sulphate MgSC>4 02 per cent. 



Bearing in mind the essential elements found in all protoplasm 

 C, H, N, O, P, S, and some salts, it will be seen that the ingredi- 

 ents of Pasteur's solution contain all of the needed elements. 

 On a priori grounds it would be possible to leave out some of the 

 ingredients without seriously affecting the vital reactions. If 

 sugar, for example, is omitted all of the necessary elements 

 which it contains are found in the other ingredients and the 

 cells continue to grow and multiply but fermentation of course 

 ceases. If some salts are left out, growth is much retarded and 

 vital actions are slow. The one absolutely essential ingredient 

 is the ammonium tartrate. If this is omitted life processes 

 cease altogether and a glance at the chemical symbols shows 

 that this alone contains nitrogen. 



By means of this simplified medium it is demonstrated that 

 yeast cells are much less complex in their nutritive processes 

 than plants on the one hand and animals on the other. Green, 

 or chlorophyll-bearing plants, by photosynthesis (see p. 117), 

 manufacture food from far simpler elements than .proteids; 

 animals require proteids ready made and these, as we have seen, 

 are highly complex substances. Yeast survives and thrives 

 on a nitrogenous compound much less complex than proteid and 

 more complex than CO% and H^O which serve as food elements 

 of plants; they are, therefore, as regards nutrition at least, 

 intermediate between plants and animals. 



Enzymes. Even with this simplified nutrition, however, the 

 finer processes of assimilation and the upbuilding of protoplasm, 

 are as obscure as elsewhere in the living world, but, largely 

 through the study of yeast cells some good working hypotheses 

 have been formulated and many vital activities have been 

 traced to unstable chemical compounds termed enzymes or 

 ferments. The first hint of these illusory agents in reactions, 



