48 



FOUNDATIONS OF BIOLOGY 



carbohydrates and fats are resolved into carbon dioxide and 

 water, and the proteins are reduced to carbon dioxide, water, 

 and ammonia (NH 3 ) or free nitrogen, while the nitrogenous 

 waste (urea, etc.) of animals is similarly broken down. Prac- 

 tically all of these long series of chemical reactions are carried 

 on by different kinds of Bacteria. Most green plants, how- 

 ever, take their nitrogen chiefly in the form of nitrates and 



Dead 

 Organisms 



Living 

 Animals 



Bacterial 

 Decay 



Carbohydrates, 

 Proteins, Fats, 

 in Green Plants 



\ 



Fermentation 

 and Animal 

 Respiration 



Intermediate 

 Decomposition 

 Products 



FiG. 15. The Carbon Cycle. A schematic representation of the circulation 

 of carbon in nature. 



accordingly we find that another type of Bacteria (NITRITE 

 BACTERIA) acts upon the ammonia and transforms it into 

 nitrous acid (HNO 2 ). After certain chemical reactions in 

 the soil, forming, e.g., potassium nitrite or ammonium nitrite, 

 still another type (NITRATE BACTERIA) oxidizes the nitrites 

 into nitrates (e.g., KNO 3 or NH^Oa), so that again this 

 nitrogen is in a form which is available for green plants. 



But, still confining our attention to the nitrogen, it is 

 obvious that there is a leak from this cycle, since some of the 



