CHAPTER I 



INTRODUCTION 



The chemist who turns his attention to biological problems 

 meets at the start a seemingly insurmountable barrier. All living 

 matter being composed of cells, and the surface of the cell in 

 such an unstable condition that it is changed by very mild physical 

 or chemical treatment, the rough treatment necessary for chem- 

 ical analysis is out of the question. This surface layer of the 

 protoplasm has been called by physiologists the plasma mem- 

 brane. If this plasma membrane is destroyed the entire proto- 

 plasm undergoes rapid changes (cytolysis). The protoplasm, 

 therefore, is excluded from the ordinary chemical methods of 

 investigation, the methods that may be applied to the interior 

 of living cells being at present very few, and concerned chiefly 

 with the inorganic constituents. 



Modern biochemistry is therefore not yet concerned directly 

 with the composition of normal living cells, but with their de- 

 composition products and the exchange between the cell and its 

 surroundings. While the entire cell is treated as a unit and 

 sometimes called protoplasm' or the living substance, we have 

 many reasons to believe that it is composed of a large number 

 of different chemical substances, some distributed throughout the 

 cell and others confined to certain regions, some in solution and 

 others in the form of jellies (gels) or solids. The living cell 

 shows a visible structure under the microscope, and chemical 

 differences between parts of the structure may be detected after 

 death of the cell. 



From our knowledge of the decomposition products of cells 

 and the exchange with the medium, we may speculate on the 

 composition of the cell and the changes that go on in it during 

 functional activity. But before doing this the most exact quanti- 

 tative data on the exchange with the environment are desirable. 

 It is the purpose of this introduction to show that these data 



