258 PHYSIOLOGY OF BACTERIA 



lies primarily in the fact that the problem is not only one 

 of physics and chemistry, but also one of cytology, of 

 systematic arrangement. 



We cannot understand growth without first under- 

 standing the energy forming processes, for we can hardly 

 imagine growth without a supply of energy. The 

 liberation of energy in bacterial cells is fairly well 

 understood, comparatively speaking. 



The form of energy made available through cell 

 activity is unknown. It may be heat, or it may be 

 radiant energy; or we may call it ^'chemical energy" 

 which is merely a good word for an unknown form. 



Number of Enzyme Molecules per Cell. — While the 

 form of energy is unknown, data on its amounts are 

 available. The average cell of Streptococcus lactis, during 

 its period of most rapid growth at 20°C., ferments about 

 12 X 10~^^ mg. of sugar per hour (p. 106). The weight 

 of one molecule of glucose is 



g-^^3gm. = 30XlO-2«mg. 



Therefore the number of molecules of sugar used per 

 hour by a single cell is 



12 X 10-10 



30 X 10-20 



0.4 X lO^o 



or four billion molecules per hour, or about one million 

 molecules per second. This number is so large that we 

 must consider the fermentation to be for all practical 

 purposes a continuous process. The number of enzyme 

 molecules required to handle one million sugar molecules 

 per second is not known, but it must be fairly large. 



Most cells can use more than one source of energy, 

 e.g., streptococci and yeasts can grow without sugar. 



