TWO PATTERNS OF LIFE 13 



carbon compound. The rapid oxidation when fuel is burned in 

 a furnace or an engine is combustion; the slower oxidation taking 

 place during cell metabolism is termed respiration. Just as few 

 people think of plants as needing food, it is commonly not appre- 

 ciated that plants need air and must breathe. Cell breathing or 

 respiration in the plant world is a less vigorous and less obvious 

 process than among animals, since plants need less oxygen and 

 carry on a less rapid rate of respiration than do the more active 

 animal organisms. With the energy of the fuel made available 



DESTRUCTIVE CONSTRUCTIVE 



PHASE PHASE 



ENERGY . 

 RELEASED 



PROTOPLASM 



ENEfiGY 

 STORED 



PROT£:iN 



SYNTHESIS 

 RESPIRATION PHOTOSYNTHESIS 



CO^ ^-^ H^O CO^ --' H^O 



Fig. 6. — Metabolism is a dual process, with a constructive and a destructive 



phase. 



by respiration, all of the vital activities of protoplasm can take 

 place. 



In the constructive phase of metabolism, there is a marked 

 difference between the plant and the animal cell. The two king- 

 doms separated along diverging pathways millions of years ago, 

 in their methods of securing food. Ancestors of both may have 

 been simple unicellular organisms capable of extracting inor- 

 ganic materials from the environment and synthesizing them 

 into organic ones as food for protoplasm. Such a type of metab- 

 olism is termed autotrophic, indicating an ability to transform 

 common inorganic substances such as water, air and a few 



