6 BACTERIOLOGICAL AND ENZYME CHEMISTRY 



conditions, but ceases when a certain definite proportion of 

 the ester has been broken up. Such a reaction is known as a 

 reversible reaction, and is generally written thus : 



CH 3 COOC 2 H 5 + H 2 + C 2 H 5 OH + CH 3 COOH 



The changes above described are typical of a series of 

 reactions characterised by the absorption of the elements of 

 water ; such a process is generally referred to as hydrolysis. 



A great many fermentative changes are hydrolytic in 

 their character and consequently of a very simple order. 

 It was at one time considered that under natural conditions 

 only changes took place which were essentially of this order, 

 and in which there was always a liberation of heat as a result. 

 Recent research has, however, shown that this generalisation 

 does not hold, it being possible to build up substances by the 

 action of enzymes, as well as to break them down, It is 

 probably more correct to say that enzyme actions are, strictly 

 speaking, reversible, but that the reaction takes place in both 

 directions only under special conditions. 



Besides the multiplicity of chemical agents already 

 mentioned, the chemist has at his disposal means for varying 

 at will within wide limits the physical conditions of reaction. 



Temperature and concentration have already been 

 mentioned, but it is also possible to remove one or more of 

 the reacting bodies from the sphere of action by distillation, 

 either at the ordinary or at reduced pressure. Filtration 

 through various kinds of filtering media is possible, or separa- 

 tion by varying solubilities. 



Under natural conditions the choice of methods is obviously 

 much more restricted, and therefore before going farther it 

 will be well to consider more closely the conditions under 

 which chemical actions actually do take place in nature, and 

 for this purpose to devote some attention to what may be 

 termed nature's ultimate laboratory, that is, a living cell. 



The unit of all living matter is the cell. Broadly speaking, 



