212 BIOLOGICAL CHEMISTRY 



a chemical process is used to reduce carbon dioxide and to form 

 energy-yielding food materials. 



This completes our survey of the cycle of Biological 

 Chemistry. We have traced our materials from simple 

 substances through their synthesis into complex compounds 

 and the decomposition of these compounds to the simple 

 substances from which we started. During the cycle we have 

 seen the accumulation of energy and the use of this stored 

 energy for muscular movement, growth, secretion, etc. 



Various branches of chemistry are included. For some 

 purposes one branch is more important than another, but all 

 are valuable and the particular line of work taken by an 

 investigator depends upon his taste and training. 



The main outstanding feature is that life is a continual 

 struggle for a supply of energy and to expend that energy in 

 the most advantageous manner. The former process involves 

 the First Law of Thermodynamics that energy cannot be 

 created or destroyed, therefore it must be obtained from some 

 supply of energy. The latter involves the Second Law of 

 Thermodynamics, that the free or available energy tends to a 

 minimum and that a certain proportion of energy is trans- 

 formed into a form which is not available for other purposes 

 (entropy). This is due to the fact that all forms of energy 

 can be converted into heat, but that heat by itself cannot 

 always be quantitatively transformed into other forms of 

 energy. 



GENERAL REFERENCES 



A. HARDEN : Alcoholic Fermentation, Longmans, Green & Co., 1911. 

 E. J. RUSSELL : Soil Conditions and Plant Growth, Longmans, Green & Co., 

 1912. 



