INTRODUCTION 



The biosphere is the seat of a continuous transformation of matter and 

 energy. Despite influences tending constantly to convert the chemical 

 elements into the highly oxidized state characteristic of inanimate matter, 

 this activity maintains the living material and replaces it continuously 

 throughout its more or less prolonged, but finite, life-span. The metabolism 

 of the organisms Vv^hich make up the biosphere shows certain general 

 characteristics common to all organisms, besides those applying only to 

 particular t\'pes or classes. The organism makes pyrophosphate bonds ready 

 for the performance of various types of cellular work by means of a chemical 

 machine whose essential character is everywhere the same. This machine 

 can be fed by the chief types of chemical structure which are present to a 

 high proportion in all types of protoplasm : sugars, fatty acids and amino 

 acids. This applies also to those organisms which receive their energy from 

 outside the biosphere and are called autotrophes (see Part Five). In the 

 autotrophes, which can build up organic macromolecules from simple 

 inorganic materials, and in the heterotrophes (organisms depending on a 

 supply of protoplasm from another organism, which can be broken down), 

 the supply of cellular nutrients (sugars, fatty acids and amino acids) in 

 most cases requires a corresponding breakdown of macromolecules formed 

 by the association of these nutrients. Here we have an important generaliza- 

 tion which has already been mentioned by Herbert Spencer in his Principles 

 of Biology. The macromolecules which form the plastic structure pre- 

 dominant in living beings should in fact have a certain stability, which will 

 allow them to act as a reserve for nutritional purposes and which will assure 

 the persistence of form, despite what we now know to be a dynamic state 

 which, in the course of time, renews these molecules more or less quickly, 

 completely or partially. On the other hand these polymers should be 

 endowed with a certain instability permitting them to deliver up the whole 

 of their monomer content to the turnover of cellular nutrients when 

 protoplasm is acting as food. This, we know now, is not due to the in- 

 stability of the macromolecules but to the presence of specific enzymes, the 

 hydrolases, which can cleave peptide, oside and ester bonds. There are not 

 very many diff"erent kinds of hydrolases, but they are found in extremely 

 diverse locations and employed in a variety of different ways; in fact their 

 study is of interest both to the biochemist and to the comparative physio- 

 logist. The final result of their action is the liberation of fatty acids, sugars 

 and amino acids, which become available to the metabolism of the cell and 

 form its actual nutrient. 



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