THE UNIT OF BIOLOGICAL ACTIVITY 153 



differs with different microorganisms. The capacity to utilize certain 

 foods in the presence of others, to acquire certain ingredients, and to 

 leave others behind makes for great variability in the functioning of 

 microbial cells. 



Spores of some microorganisms are exceedingly resistant, with- 

 standing over twenty hours of heat at 100. Spores of other species 

 are destroyed in from one to five minutes at 100 under the same con- 

 ditions. Wide differences are found among vegetative cells likewise. 

 The molecular stability which embodies the life of the cell against heat 

 must be as variable as the dissolution of chemical substances under 

 the influence of heat. Life then can be associated with many kinds of 

 molecular mechanisms although for each species there is a more or 

 less constancy. 



In the process of selecting, digesting and assimilating their food, 

 certain microorganisms develop an unusual amount of heat so much that 

 many microorganisms cannot live in its presence, yet these heat- 

 producers (thermophile microorganisms) are dependent upon this 

 excessive heat for their proper growth and functioning. 



So far evidences of varied processes existing in different species 

 and strains have been set forth in the products and energy resulting 

 from growth and development, or, more briefly, in the processes of 

 metabolism. These clearly convey the very noticeable variation which 

 must exist in the mechanisms which are responsible for such differences. 



There is another approach which will contribute force to what has 

 already been said concerning the mechanisms of living cells. 



Chemists have repeatedly shown that cells differ in their chemical 

 compositions. When chemists begin their destructive laboratory 

 manipulations for the purpose of ascertaining what is present in a cell, 

 they may not be working with the real substances or bodies making the 

 living protoplasm. However, the bodies which they do study and 

 which are fairly constant in the same kind of cells or in the same species 

 are doubtless products which in one form or another enter into the 

 complexity of protoplasm. The chemical substances studied in nervous 

 tissue differ from those of muscles; the chemical substances as products 

 of certain species of microorganisms differ from the products of other 

 species. Some of these substances are definitely known to differ and 

 others, although they cannot be satisfactorily determined, are suspected 

 to differ in different tissues and different species of microorganisms. 



