96 Growth and Form 



a culture of bacteria because it is the easiest to understand, and conclu- 

 sions reached from studies of it are applicable to all organisms, plant and 

 animal alike. 



Imagine that we introduce a few hundred bacterial cells into a 

 cotton-stoppered flask containing sterile, nutrient broth (called the "me- 

 dium"). Every hour or so we abstract one cubic centimeter of the broth 

 and determine the number of living bacterial cells. Figure 44c shows the 

 complete curve that would be obtained. 



The curve is divided into three parts: I— the period called the lag 

 phase during which the cells prepare for growth; II— the period of actual 

 growth, called the exponential or logarithmic phase; III— the period in 

 which growth ceases and the population enters the stationary phase. None 

 of these phases is of set duration. That is to say, the time a culture spends 

 in each phase depends on the particular species of bacteria and the condi- 

 tions under which they are grown. 



LAG PHASE 



The lag phase is a period of rapid protoplasmic growth and a period 

 of preparation for the cell division that is to come. Thus, it is a time when 

 the cells become larger but remain constant in number. 



We may ask why cells must prepare for cell division. Why can they 

 not enter the exponential phase immediately? The answer is that a cell 

 in the exponential phase is somewhat like an assembly line operating full- 

 blast. Parts are fabricated on the line in consecutive stages or are received 

 from outside contractors and these are ultimately put together to yield 

 the finished product. The prime requirement for an efficient assembly line 

 is that all the operations mesh harmoniously. Supplies must arrive in the 

 right place, at the right time, and in the right amounts, and the operations 

 along the assembly line must proceed at equivalent speeds. If, then, we 

 start a culture with cells that are already in the exponential phase (i.e., the 

 "assembly lines" are already set up and operative ) and supply them with 

 everything they need, the lag phase should be eliminated, and this is just 

 the result that is obtained. In contrast, if we start a culture with cells that 

 are not in the exponential phase, they require time to prepare for growth. 

 We will now take up the preparations that are necessary to generate the 

 exponential phase. 



RESYNTHESIS OF ENZYME SYSTEMS REQUIRED FOR GROWTH. A CCll that 



has already passed the exponential phase is no longer faced with the prob- 

 lems of growing, only with those of survival; it must protect itself against 

 metabolic waste products and other toxic substances that may have built 

 up in the environment and must conserve whatever raw materials and 

 energy that it still may have left. In so doing, it may lose some enzymes 

 that are no longer needed and synthesize others that are required for its 



