Energy transfers 



and biological oxidations 



All life depends upon the ability to utilize energy from the environ- 

 ment for the physiological processes of the living system. The useful 

 energy is available in three general forms. Light is used directly by 

 the photosynthetic plants and microorganisms employing the mech- 

 anism outlined on page 213. Chemical energy is utilized in all 

 species by the metabolic reactions of the sections beginning on page 

 166. Heat serves indirect roles in providing for the liquid internal 

 phases of cells and suitable external environments for species living in 

 water and soil. In addition, the temperature must be high enough to 

 allow a suitably rapid metabolic rate but low enough to avoid dena- 

 turation of the proteins and other sensitive elements of the cells. 



All cells require heat from the environment, and all probably 

 supplement this heat by additional heat energy derived from the 

 reactions carried out. Furthermore, all cells also require environ- 

 mental energy in one of the other two forms. The photosynthetic 

 plants, for example, convert light energy into stored chemical energy 

 and are thus able to utilize it for the activities of life. Animals 

 and the other species unable to convert light to chemical energy must 

 secure the latter directly from the environment. The different living 

 forms are characterized by the kinds of compounds capable of supply- 

 ing the required chemical energy, some needing a variety of substances 

 for special purposes, some subsisting on a very few. 



The rate at which an organism processes the energy it receives 

 influences the rate of growth, ultimate size, mechanical activity, rate 



158 



