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 



Carbohydrates, 

 Proteins, Fats, 

 in Green Plants 



Dead 

 Organisms 



Living 

 Animals 



Fermentation 

 and Animal 

 Respiration 



Bacterial 

 Decay 



Intermediate 

 Decomposition 



Atmosphere Products 



Fermentation 

 and Plant 



Living^-^Respiration 



Green Plants\ I > 



>s^ >^r^r 



PiO. 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 4 N0 3 ), 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 



