MECHANISM OF COMPETITION IN YEAST CELLS 



73 



almost in proportion to the increase of the volume of yeast. In other 

 terms, a proportionality exists between the metabolism of the yeast 

 cells and the growth of their volume. Later on, conditions arise in 

 which the growth of the yeast ceases, but alcohol continues to accu- 

 mulate. Therefore, at the moment when the growth ceases there are 

 still unutilized resources of sugar in the medium. The life activity 

 of the yeast cells and the accumulation of alcohol continue after the 

 biomass has ceased growing. 



The microscopical study of the population of yeast cells made by 

 Richards at the moment when growth was ceasing, has shown the 

 following facts. The yeast cells continue to bud actively, but as soon 



0% 1% z% 3» /o yo /o 



Additional alcohol 



Fig. 12. The effect of additional alcohol upon the level of saturating popula- 

 tion in Saccharorriyces cerevisiae in test tubes. 



as a bud separates from the mother cell it perishes. In this way, 

 unfavorable chemical changes in the medium destroy the most sensi- 

 tive link in the population, and lead to a cessation of its growth. 

 According to Richards ('28a) the accumulating ethyl alcohol is just 

 the factor which kills the young buds and inhibits the growth of the 

 population. He showed this experimentally: with an addition of 1.2 

 per cent of ethyl alcohol to the nutritive medium, the maximal yield 

 of population was 65 per cent from that of the control population 

 (acidity kept constant). Therefore, with the additional alcohol the 

 critical concentration of waste products at which growth ceases was 

 reached with a smaller quantity of accumulated yeast volume. 



