^^The Chemical and Physical 

 Structure of Protoplasm 



VARIOUS microscopic techniques provide 

 many clues to the [unctions of the different 

 parts of the cell. But below the reach even of 

 the electron microscope there lies a realm of 

 smaller things: the world of molecules and 

 atoms, which compose the ultimate struc- 

 ture of all matter. Biologists now realize that 

 the visible parts of the cell are very impor- 

 tant, but full attention must also be given to 

 the submicroscopic molecular pattern of 

 protoplasm. This means that an irreducible 

 minimum of biochemistry and biophysics has 

 become basic in general biology. The cell, 

 essentially, is a delicate physicochemical 

 mechanism; and the merest trace of a foreign 

 chemical such as cyanide, or the slightest ex- 

 cess of a physical factor such as heat, may de- 

 range the finely poised molecular structure 

 of the protoplasm, killing the cell immedi- 

 ately. 



Each cell faces the problem of obtaining a 

 quantity and variety of substances sufficient 

 to maintain and operate its existing proto- 

 plasmic structure, as well as to increase the 



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protoplasm when growth occurs. But before 

 the chemistry of growth, maintenance, and 

 operation can be considered, it is necessary 

 to know the general composition of living 

 matter. What substances are always present? 

 How much of each of these essential compo- 

 nents must there be? And how are these ac- 

 tive molecules assembled and arranged with 

 reference to visible structures in the cell? 



No two samples of protoplasm are ever 

 precisely identical; but the man)' differences 

 found among various cells are mainly matters 

 of detail. One cell utilizes a certain kind of 

 sugar that differs slightly from the sugar used 

 by another. Plant cells synthesize starch; 

 whereas animal cells build glycogen, a closely 

 related compound. But above and beyond 

 these specific variations, there is a general 

 pattern, and all protoplasm conforms to this 

 pattern. Always the same tvpes of chemical 

 compounds are present, and these compounds 

 play similar roles in the protoplasm of plants 

 and animals generally. 



Merely to list the specific compounds that 



