Energy 



21 



All living things withdraw energy from their environments and 

 use it lor their various physiological operations, including growth, 

 movement, chemical transformations, temperature control, and lu- 

 minescence. Thus cells are always at the mercy of their environments 

 in this fundamental regard. Without energy there can be no life. 

 However, evolution has led to the development of species capable of 

 lUilizing different forms of energy. At least indirectly all life depends 

 on heat, primarily in the range between the freezing and boiling 

 points of water. Most species are limited to narrow parts of this range 

 and have quite definite optimum environmental temperatures. All 

 plants and the great group of cold-blooded animals possess internal 

 temperatures approximating the external temperature. Such or- 

 ganisms cannot function when the environmental temperatme drops 

 below characteristic levels. Not all die, of course, since the species of 

 temperate and cold climates become dormant or possess reproductive 

 stages that are cold resistant and do not complete the life cycle until 

 the return of warm weather. 



Mammals and birds have evolved with mechanisms for the control 

 and maintenance of their internal temperatures. These specialized 

 forms can thus sujjply their own needed heat energy, providing that 

 a suitable supply of chemical energy is available in their food mate- 

 rials. Many plants and microorganisms have similar requirements for 

 such chemical energy, but some classed as photosynthetic species do 

 not. This last group is adapted for the absorption of radiant energy 

 and its conversion to necessary chemical energy. 



Since the energy needed in physiological processes is ultirr<ate|y con- 



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