YEAST METABOLISM 33 



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able, by omitting one substance after another, to determine 

 what elements are necessary for the vital activities of yeast. 

 The fluid, known as Pasteur's fluid, has the following percentage 

 composition: 



Water H 2 O 83 . 76 per cent. 



Cane sugar (C^H^On) 15 .00 per cent. 



Ammonium tartrate (NH^C-iHUOe i .00 per cent. 



Potassium phosphate K 3 PO4 20 per cent. 



Calcium phosphate Ca 3 (PO4)2 02 per cent. 



Magnesium sulphate MgSO 4 02 per cent. 



Bearing in mind the essential elements, C, H, N, O, P, S, 

 and some salts, found in all protoplasm it will be seen that the 

 ingredients of Pasteur's solution contain all of the needed ele- 

 ments. 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 elements 

 which it contains are found in the other ingredients and the 

 yeast cells continue to grow and multiply although fermentation 

 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 green plants on the one hand and animals on the other. 

 Green, or chlorophyll-bearing plants, by photosynthesis (see 

 p. 119), manufacture food from far simpler elements than 

 proteins; animals require proteins ready made, and these, as 

 we have seen, are highly complex substances. Yeasts survive 

 and thrive on a nitrogenous compound much less complex than 

 protein and more complex than CO2 and H 2 O which serve as 

 food elements of green plants; they are, therefore, as regards 

 nutrition at least, intermediate between such plants and 

 animals. 



Even with this simplified nutrition, however, the finer 



