'^^ Introduction 



Biosynthesis begins with photosynthesis. Green plants 

 and other photosynthetic organisms use the energy of ab- 

 sorbed visible light to make organic compounds from in- 

 organic compounds. These organic compounds are the 

 starting point for all other biosynthetic pathways. 



The products of photosynthesis provide not only the 

 substrate material but also chemical energy for all subsequent 

 biosynthesis. For example, nonphotosynthetic organisms 

 making fats from sugars would first break down the sugars 

 to smaller organic molecules. Some of the smaller molecules 

 might be oxidized with O2 to CO2 and water. These reac- 

 tions are accompanied by a release of chemical energy, be- 

 cause O2 and sugar have a high chemical potential energy 

 toward conversion to CO2 and H2O. In a biochemical system 

 only part of this energy would be released as heat. The rest 

 would be used to bring about the conversion of certain 

 enzymic cofactors to their more energetic forms. These co- 

 factors would then enter into specific enzymic reactions in 

 such a way as to supply energy to drive reactions in the 

 direction of fat synthesis. Fats would be formed from the 

 small organic molecules resulting from the breakdown of 



