Lecture IV. 31 



enzymes, leaving over a slime containing a few active enzymes 

 among which is invertase. A muddy sediment settles from this 

 slime and if the supernatent clear liquid is decanted off, it is 

 found to be an active preparation of invertase. The usual 

 destructive effect of a high temperature may easily be verified 

 with this preparation. When raised above 70 C. it will be found 

 that the enzyme has become inactive. The addition of alcohol 

 to the preparation precipitates, or coagulates, the enzyme ; but 

 this, however, recovers its activity when redissolved in water. 



Invertase is one of those enzymes, which, working under the 

 ordinary laboratory conditions, effects an apparently complete 

 change of sucrose (cane sugar) into dextrose (or glucose) and 

 levulose : but it is quite possible that in the spaces of cells, 

 where all the sucrose or water is continually removed from the 

 sphere of activity of the enzyme, that a synthesis of sucrose from 

 dextrose and levulose may also be effected. 



Slow penetration of the diffusible peptones, amino-acids, sugars 

 and salts makes up for those which are absorbed by the proto- 

 plasm ; within the protoplasm they are further modified and 

 built up into new protoplasm. This action, which is called 

 assimilation, is a characteristic of protoplasm and is not ex- 

 hibited by dead matter. Probably in many of the stages of 

 assimilation enzymes are used as agents. 



Evidently, if assimilation alone is considered, new additions 

 to the protoplasm, vacuoles and substance of the cell-wall are 

 continually being made. These additions become visible in 

 the growth of the cells and their reproduction. It is to be 

 noted in the case of yeast that the carbon as well as the nitrogen 

 is supplied from organic sources. Carbon is present in the 

 medium round the cells in the form of proteins and carbohydrates, 

 and nitrogen principally in the form of proteins. It has been 

 found, however, that yeast can draw its supplies of nitrogen 

 from simpler organic compounds such as ammonium tartrate. 



For this assimilation, or building up process, in which more 

 complex bodies are formed from simpler ones, and bodies which 

 contain more energy from those which possess less, evidently a 

 supply of energy is needed. 



This supply of energy is furnished by combustion. 



We noticed already that when yeast is immersed in a sugar- 

 solution gas is evolved and the smell of alcohol may be observed. 

 As a matter of fact the change indicated by these products may 

 be expressed chemically as follows : 



Q, H 12G = 2C 2 H 5 OH + 2CO 2 



Glucose or dextrose = ethyl alcohol + carbon dioxide. 



