PART B 



ENERGY SUPPLY OF THE CELL 

 I. PHYSICAL CONSIDERATIONS 



(a) THE TWO SOURCES OF ENERGY FOR LIFE 



In the entire living world, only two sources of energy- 

 are known to be available to living cells, namely light 

 and chemical energy. Light can be utilized only by the 

 chlorophyl plants, the bluegreen algae and a small 

 number of prototrophic bacteria. Chemical energy is 

 utilized by all living organisms. 



Light is of importance in the growth of certain plants which 

 possess no chlorophyl; but it acts there only as a stimulus. Rhizopus 

 grows better in diffuse light than in dark, but it requires the same 

 amount of food to produce the same amount of mycelium; no energy 

 of light is ''stored" in the cells of the mold. Such stimulation by 

 light causes the daily rings in cultures of bacteria and molds (Hutchin- 

 son, 1907). 



Heat, electricity, or other forms of energy cannot be utilized by 

 organisms as available energy. A higher temperature may cause a 

 more rapid functioning of the cell, yet it is merely a difference in the 

 rate of the process, and a cell cannot economize on food by utilizing 

 the heat energy of the surrounding medium. A starving cell is not 

 benefited by an increase of temperature. 



Probably the cells lack the ^transformers" or ^^acti- 

 vators" for all these forms of energy. Electricity, 

 heat, and probably even light are not used as such by 

 the cells. They must be transformed into another kind 

 of energy, and we know only the transformer for light, 

 i.e. the chlorophyl. 



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