CHAPTER II 

 THE STORAGE OF ENERGY 



" The energy available for each man is his income. Stored energy is a 

 legacy deposited in Nature's bank." 



ALL life processes demand for their continuation and mainten- 

 ance a continuous supply of energy. In metabolism, as far as 

 matter is concerned, there is a closed cycle. Animals feed on 

 plants, and plants feed on the products of animal metabolism 

 and disintegration. The net result is practically nil. Energy, 

 however, must be supplied from outside. The one essential 

 physical factor that makes the process possible is the supply 

 of energy as sunlight to the plant. 



The ultimate source of all the energy upon which existence 

 on this planet depends is the sun. (One need not here enter 

 on the interesting question of how the sun evolves energy ; see 

 Soddy, Matter and Energy, Chap. X.) As far as we know, the 

 higher forms of life are unable directly to use, either heat or 

 light as sources of bodily energy. Some of the lower forms of 

 animal life may have this power ; plants certainly have. As 

 we shall see later, light energy may act as the trigger, setting 

 free potential energy and causing work to be done. 



All living matter may be divided into two distinct classes : 

 (a) that subsisting upon the materials which they take from 

 the earth and the air, and (b) that dependent upon other organisms 

 either living or dead. Examination shows that the main chemical 

 difference between these two classes is that the former contains 

 a green pigment called chlorophyll as a regular functioning con- 

 stituent, while the latter does not. It is evident that chlorophyll 

 is correlated with the state of independence that is typical of 

 plant life. 



Chlorophyll. 



(Revise : Physics of light, especially that of absorption spectra, 

 page 71 ; chemistry of chlorophyll and its relation to xantho- 



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