THE CELL 45 



intimately associated with the nitrogen salts the soil may contain 

 and that nitrogen fixation, that is, its capture and combination in 

 usable form, is one of the most important of all chemical processes. 



From these relations flows a tremendous influence on human 

 society. The harvesting and removal of plants from fields year after 

 year eventually robs the soil of its nitrogen salts. Consequently, 

 olants that house nitrogen-fixing bacteria in their roots, fertilizing 

 crops, must be planted in order to restore soil nitrogen. Thus agri- 

 culture is involved in a constant battle to maintain the usefulness of 

 soils. Out of this struggle has grown methods of artificial nitrogen 

 fixation, transportation chains for importing nitrogen salts from 

 foreign sources, struggles by war and diplomacy for new and as yet 

 fertile areas, and the manufacturing and transportation of the 

 engines and machinery that are required. 



The Carbon Cycle. In a similar fashion a cycle of carbon 

 may be traced (Fig. lo). All organic compounds, as was shown 

 earlier (p. 9), contain one or more carbon atoms. In the synthesis 

 of carbohydrates by chlorophyll in the metabolism of green plants 

 the carbon atoms are introduced into organic compounds, carbo- 

 hydrates, proteins, and fats, as well as various other substances. By 

 the oxidation of these substances carbon is combined with two 

 atoms of oxygen and eventually appears again as the gas, carbon 

 dioxide. The oxidative processes are the energy-releasing reactions 

 whereby the potential energy accumulated from sunlight is con- 

 verted into kinetic energy, that is, becomes available to do the work 

 carried on by protoplasm. Not all carbon-containing compounds are 

 oxidized with equal ease. Carbohydrates are the most easily oxidized 

 in the protoplasm. So carbon may be returned to gaseous carbon 

 dioxide by the breakdown of carbohydrates, or it may enter into 

 much more complex molecules that are oxidized less readily and less 

 completely. The carbon cycle and the nitrogen cycle are in contact 

 at the point where carbon dioxide is utilized in the manufacture of 

 carbohydrate. 



