2o8 



METABOLISM 



the alcohol formed in the manufacture of wine, beer, and brandy is almost 

 entirely a product of the activity of this organism. 



Yeasts belong for the most part to the genus Saccharomyces, a genus rich 

 in species and in varieties (Fig. 37), Ascomycetes of the simplest structure 

 increasing by budding. Under certain culture conditions they do not exhibit 

 any capacity for forming alcohol and behave just like other Fungi, respiring 

 organic substance into carbon-dioxide and water. Yeast behaves in this way 

 when grown in a nutritive solution in which peptone serves as a source both of 

 carbon and of nitrogen, or in solutions which in addition to some appropriate 

 source of nitrogen, contain quinic acid or lactose to supply the carbon required. 

 As might be expected the yeast under these conditions dies at once if oxygen 

 be withheld. If the milk-sugar be replaced by cane sugar, alcohol is formed 

 whether oxygen he present or not. It is obvious that fermentation begins only 

 when an appropriate fermentable substance is present, so that we must first 

 of all inquire which substances are fermentable and which not. Yeasts can form 

 alcohol only from carbohydrates, and at the same time they exhibit a remarkable 

 capacity for distinguishing between bodies nearly related, as we have elsewhere 



Fig. 37. Yeast Fungi, a. Saccharomyces cerevisiae \ b. S. pasteurianiis\\\ ; c. 5'. elHpsoideus I ; 

 A. elUpsoideus \\. (After Fischer, Vorles. ii. Bacterien. and ed.) 



discovered. The proof of this fact, as far as yeasts are concerned, we owe chiefly 

 to E. C. Hansen (1888) and E. Fischer (1898), who have established the fact 

 that individual species and varieties behave in entirely different ways. 



Fermentable carbohydrates are recognized by possessing three carbon 

 atoms, or a multiple of that number, and are directly fermentable trioses, 

 hexoses and nonoses, whilst the more complicated di-, tri-, and polysac- 

 charides must be first of all hydrolysed and transformed into hexoses before they 

 can be fermented. In nature, only the hexoses and those higher sugars which 

 may be split into hexoses are to be considered as forming the material of fer- 

 mentation. Amongst these we may distinguish aldohexoses and ketohexoses, 

 the former exhibiting four and the latter three asymmetric carbon atoms. These 

 asymmetric carbon atoms are indicated (in the following formulae) by heavy 

 type, and the first of these we may draw attention to is ordinary grape sugar 

 i.e., ^-glucose : — 



OH OH H OH OH H,OH 



C — C 



I I I 



c — c — c 



c 



I I I I 



H OH H H 



Let us now consider in this formula the four H and OH groups 

 united to the four asymmetric carbon atoms to be arranged in all possible 

 configurations, we shall then have sixteen ' stereoisomeric ' hexoses and 

 these are all optically active; eight rotate polarized light to the right 

 and the other eight rotate it to the left. The enantiomorphs, dis- 

 tmguished by their different rotatory powers, are characterized by the 

 mterchange of H and OH groups around the asymmetric carbon atoms; 



