38 PHYSICAL FORCES AND CHEMICAL BONDS 



and cause it to do some mechanical work. Thus the water energy is 

 partly converted into usable mechanical energy. We say "partly" be- 

 cause it took some energy to turn the wheel itself, some energy is lost in 

 friction, and some in the bouncing off the wheel. Then the water falls a 

 little more before going through a second water wheel. The water must 

 fall somewhat, or it will have no motion to impart to the wheel ; each 

 time it hits a wheel, some of the energy is lost in the three processes 

 mentioned. Thus, in a series of interactions with a set of wheels, the 

 initial energy of the water can be fairly efficiently converted to usable 

 energy. 



One way of storing the energy extracted by each water wheel is to 

 have it turn a magnet to create electric energy which could then, if 

 desired, be stored in a battery for later use. 



The same process, in spirit, exists in biological events. If a complex 

 molecule exists, there is energy stored in its bonds, as described previ- 

 ously. The molecule exists because work was done in creating it, and 

 this energy is now to be extracted by taking the molecule apart. If the 

 molecule were taken apart all at once, the energy would be primarily 

 released as heat, for it is difficult to couple an explosion efficiently to an 

 energy storage device. Instead, the complex molecule is taken apart one 

 atom (or a very few atoms) at a time, and some of the energy released 

 is used to build up a small molecule. These small molecules play the role 

 of storage batteries, and may serve subsequently to provide the energy 

 for needs of the cell, such as movement, replication, etc. 



In this stepwise fashion, the energy originally stored in the complex 

 molecules can be used, with reasonable efficiency, to create energy storage 

 molecules which then are of general use to effect the biochemical re- 

 actions. In practice, for example, the complex molecule could be a sugar, 

 and the small battery-like molecule could be ATP. The device of 

 storing energy in special molecules like ATP avoids the problem which 

 would otherwise arise in coupling energy-yielding dissociation of complex 

 molecules to energy-requiring reactions. There would otherwise be prob- 

 lems of different locales of the two processes, as well as problems in 

 timing. 



