Life: Its Nature and Origin 37 



are "fed" to the enzyme C, and presumably AB is removed in some 

 fashion. 



This example is an extremely simple one used to illustrate the 

 general principle of enzyme action. In the cell many of these re- 

 action chains are extremely complex. Theorell ( 1956 ) gave an 

 interesting account of the mode of action of several enzymes and 

 pointed out that in some cases the chain of reactions may go through 

 as many as nine steps before the enzyme is free again. He also 

 pointed out that in certain reactions a second enzyme must first 

 activate or condition the primary enzymes. These accessory co- 

 enzymes act like a self-starter on a car, a little motor starting the 

 big motor. Energy is always either released or consumed in a chemi- 

 cal reaction. If, therefore, one of the enzymatic reactions consumes 

 energy, it must be linked in some fashion with another reaction 

 which produces energy. The energy-giving reaction itself will be a 

 machine-like one involving an enzyme system, and may indeed be 

 a whole system of systems, such as the ATP-ADP system (Fig. 16). 



We can make a simplified analogy between one of these com- 

 plicated situations and an automobile engine. The battery supplies 

 the energy to rotate the starter (a co-enzyme) which in turn starts 

 the motor, which then turns the generator to produce electric 

 power which recharges the battery. This recharging is a by-product 

 of the main work done by the motor. The entire system, however, is 

 dependent on receiving from the external environment fuel in the 

 form of gasoline and oxygen. However, not any indiscriminate 

 amount will work; the motor is a machine in such delicate balance 

 that the correct amounts of its fuels must be applied and at the 

 right time and intervals. This is accomplished by accessory ma- 

 chines, the carburetor and valving system. Also like a living cell, 

 the automobile motor must dispose of its waste products. 



Bearing in mind the vast number of compounds in the cell, the 

 complexities of the enzyme systems, and the meticulously accurate 

 organization of the cell, it is possible to visualize it as an intermeshed 

 series of molecular machines. From the original intake of raw 

 materials from the environment to the duplicating of the last 

 molecule in the mature cell, these tiny machines must lead from 

 one to the other, the last dependent on the first and the first on the 

 last. A failure of even one could conceivably bring the whole chain 

 to a halt, resulting in the death of the organism. For one fact is true 

 about life: it must keep going to stay alive (Ruzicka, 1919; Nilsson, 

 1953). 



The ultimate and critical products of cell growth would seem 



