6 PHYSIOLOGY OF BACTERIA 



heat formation in the body. We must distinguish, there- 

 fore, between dependent and independent functions. If 

 the death of a cell by poison is independent of the rate 

 of growth, or of the rate of fermentation, we may call 

 it an independent function. Growth, however, depends 

 upon the amount of available energy. Further progress 

 in knowledge may disclose to us that the assumedly 

 independent functions are, after all, dependent ones. 



A most natural question arises in this connection: 

 Which functions are found in all living beings, i.e., which 

 functions are essential for life? This question leads us 

 immediately to the simplest forms of life. The most 

 characteristic quality of a living thing which distin- 

 guishes it from all dead matter, is growth. Growth, 

 meaning the construction of complex molecules from 

 less complex molecules, is, however, a dependent func- 

 tion and requires energy, and energy is, with chlorophyl- 

 free organisms, provided by a function of the body which 

 we call respiration, or, in the case of bacteria, fermenta- 

 tion; here we are using the term in its widest meaning. 



Growth is not identical with multiplication. How- 

 ever, bacteriologists commonly use these terms as 

 synonyms, probably because growth of a culture is 

 identical with multiplication of bacteria. The difference 

 is very great cytologically, but in these biochemical 

 studies, no emphasis shall be placed upon the difference, 

 and the two functions shall be treated as one because 

 we have hardly any means to differentiate them chem- 

 ically (see also Introduction to ^^ Growth'' p. 162). 



Besides energy formation and growth, all living organ- 

 isms share one more property: they all must die. Star- 

 vation, poison, excessive heat will kill all living cells, 

 and we know that organisms will die of old age in a 

 natural way. Though there are great variations in 



