42 



FUNDAMENTAL PRINCIPLES 



produce urea, a characteristic excretion of animals. In photosynthesis 

 plants use up the available carbon dioxide and give off an excess of oxygen, 

 while animals always utilize all the oxygen they can get and give off 

 carbon dioxide as waste. 



Eli minoifion 



In spira fio n 

 (oxygen) 



Organic food 



Expl ration ^ 

 (COz) 



■Eg est ion ^ 

 (feces) 



a. Animcil 



and CO 2 



Excess of 



oxygei 



•n 



b. Plant 



Fig. 10. — Diagrams contrasting the metabolic and food-manufacturing processes in plants 



with the metabolic processes in animals. 



Because of the difference in metabolism of plants and animals plants 

 have often been referred to as predominantly anabolic in their activities, 

 while animals are characteristically katabohc. Also, since in the presence 

 of hght the processes concerned with photosynthesis outweigh those 

 concerned in the metabolism of protoplasm, during the day plants use 

 more carbon dioxide than they produce and produce more oxygen than 

 they use. At night when photosynthesis is arrested they are, from a 

 metabohc standpoint, on the same plane as animals. 



