How the Cell Lives 27 



we have no clear evidence in favour of this possibility. If life has 

 evolved from simple unicellular organisms to complex ones, there 

 does not seem to have been any great improvement in the enzymes. 

 There has merely been an increase in specialization and in the possi- 

 bilities of cooperation and of division of labour between cells. 



In fact, as we have seen, it is usually the case that the simpler 

 organisms, in order to live, have to carry out a greater range of 

 chemical actions, so that they are very rich in enzymes — perhaps 

 richer than the more highly developed animals. 



Let us take a look at a specimen of a very simple organism — a 

 single cell of a small bacterium, of which the solid parts weigh 10~" 

 of a gram (i.e. about 300 million million will make up one ounce). 

 In this tiny volume it can be calculated that there will be about 

 200,000 protein molecules altogether. A moderate estimate suggests 

 that the bacterium must have several hundred distinct enzymes at 

 least for all its activities, or perhaps a thousand or more. Obviously 

 there cannot be very many molecules of each enzyme — an average of 

 200 or so of each, but in some cases perhaps there will only be a 

 few molecules of a particular enzyme. 



Our picture of the smallest living ceUs is thus already one of a 

 complexity which the mind finds difficulty in grasping. How all 

 these separate and complex enzyme molecules are packed away in a 

 tiny fragment of protoplasm, how they work in harmony with each 

 other, producing that result which we recognize as life, we hardly 

 know. How they are made out of the food which the organism lives 

 on, we shall discuss in a later chapter. 



