46 PHYSIOLOGY OF THE FUNGI 



and the action of microorganisms, see Stephenson (1939) and Harden 

 (1932). 



It was not until late in the nineteenth century that Buchner (1897) 

 succeeded in releasing certain enzymes from yeast cells and demonstrating 

 that the endoenzyme(s) in yeast causing fermentation was also active 

 entirely apart from the living yeast cells. While yeast juice prepared 

 according to the method of Buchner contained a variety of enzymes, it 

 contained fermentative enzymes never before obtained apart from the 

 living cell. These enzymes cleaved sugar into alcohol and carbon dioxide. 

 This was truly a monumental step in the science of enzymes, for it 

 afforded a way of studying "life" processes apart from the terrible com- 

 plexity of the living organism. The study of isolated enzyme systems 

 has led to important advances in our knowledge and understanding of 

 life processes; yet the student should be reminded that life is more com- 

 plex than its parts. Leibowitz and Hestrin (1945) say: 



... it has become clear that the risk involved in translating results from lifeless 

 to living systems is a two-way one: not only may mechanisms which operate 

 in vivo be absent in vitro; mechanisms may be present in vitro and yet not neces- 

 sarily function in vivo. In fermentative physiology, as in biology generally, 

 selective and restrictive activity by the living organism must always be taken into 

 account. 



The rate of many chemical reactions is changed by the presence of 

 traces of substances which do not appear to enter into permanent chemi- 

 cal combination with the reactants and which appear unchanged when 

 the reaction has come to equilibrium. Substances which alter the rates 

 of chemical reactions are called catalysts, and the process catalysis. 

 Enzymes are catalysts of a very special kind, and many of them catalyze 

 but a single reaction. For example, lactose reacts with water to form 

 glucose and galactose. Unless a catalyst is present, this reaction occurs 

 at a very slow rate. Even at 100°C. a long time is required for an appreci- 

 able amount of lactose to react with water. If, however, some acid is 

 added to the lactose solution, the rate of the reaction is greatly increased, 

 varying in degree with the amount and kind of acid used. This same 

 reaction is catalyzed w^hen the enzyme, lactase, produced by some yeasts 

 and certain other fungi, is added to a solution of lactose. In general, 

 enzymes are specific catalysts. There is no stoichometric relation 

 between the amount of catalyst (acid or enzyme) and the amount of sub- 

 strate decomposed. Within limits, the amount of substrate decomposed 

 per unit of time is dependent upon the amount of catalyst present. 



For a given set of conditions there is a position of equilibrium where the 

 rate of reaction of the reactants is equal and opposite to the rate of com- 

 bination of the products. The position of equilibrium is not changed by 



