180 MICRO-ORGANISMS AND FERMENTATION. 



class of albuminoids. They have a most important biological 

 significance, owing to the fact that the nuclei, as micro-chemical 

 investigations have shown, are mostly composed of nuclein, 

 substance. In the case of yeast in which the reactions have 

 been more closely studied, it was found that by treating them 

 with pepsin the albuminoids in the cell were dissolved, but 

 the nuclein compounds remained unchanged. 



Our knowledge in regard to the other albuminoids occurring 

 in the cells of fungi is confined at present mainly to the investi- 

 gations which have been made on yeast. 



Closely associated with the albuminoids, and, indeed,, 

 derived from them, are the enzymes, which doubtless occur 

 in all forms of life. They may be separated from the living 

 cell, and can still produce their characteristic effects in aqueous 

 solutions. Each enzyme has the property of bringing about 

 a definite and particular form of chemical change, and the 

 process is further characterised by the fact that a small amount 

 of enzyme is able to decompose a relatively large quantity of 

 organic substance. 



The result of the changes brought about by the action 

 of the enzyme can be recorded in the form of a simple equation. 

 According to H. Fischer, the enzymes may be classified as 

 follows : 



I. Dissociating or splitting enzymes, by means of which 

 a complex substance is split into its component parts. The 

 action may be regarded as a form of hydrolysis, and is accom- 

 panied by the absorption of water. The biological signifi- 

 cance of this enzyme consists in enabling the organism to 

 convert substances which would otherwise be useless into 

 simpler and soluble substances capable of diffusion. 



The group includes : (1) Carbohydrate-splitting enzymes, 

 such as invertase, which converts saccharose into equal mole- 

 cules of dextrose and laevulose ; maltase, converting maltose 

 into two molecules of dextrose ; and lactase, which converts 

 milk sugar (lactose) into equal molecules of dextrose and 

 galactose. The sugars saccharose, etc. can only be converted 

 by fermentation into alcohol and carbonic acid after undergoing 

 inversion. The melibiases convert melibiose (a product of 

 hydrolysis from raffmose) into one molecule of galactose and 



