PLANTS AND ANIMALS 



43 



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 utiUze all the oxygen they can get and give off 

 carbon dioxide as waste. 



Eli rninafion 

 Water and Urea 



Expi ration 

 (COz) 



Eotestion ^ 

 (feces) 



a. Animal 



Excess of 



Fig. 13. 



oxygen 



and CO 2 



b. Plo(n+ 

 ■Diagrams contrasting the metabolic and food-manufacturing processes in plants 

 with the metabolic processes in animals. 



Because of the difference in metabohsm 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 light the processes concerned with photosynthesis outweigh those 

 concerned in the metabohsm 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. 



