THE STRUCTURE OF PROTOPLASM 



251 



spectrogam indicate symmetry in the substance photographed, 

 but by studying the distances (identity periods) between these 

 points, the precise arrangement of parts in the crystalHne frame- 

 work can be determined. 



It is necessary first to ascertain the so-called elementary, or 

 unitf cell of a crystal. This space is the smallest parallelopiped 

 within a crystal that still has the properties of the material as a 

 whole (Fig. 130). The number of molecules in the elementary 

 cell of a simple crystal, such 

 as sodium chloride, is four. 

 In the case of cellulose, it is 

 not molecules but anhydrous 

 glucose groups (CeHioOs) that 

 build up the elementary cell 

 to the number of four. 



The "unit cell of cellulose has 

 the dimensions 8.35 A. U. X 

 10.3 A. U. X 7.9 A. U. The 

 length, 10.3 A. U., is the 

 equivalent of two anhydrous 

 glucose groups of which 40 to 

 100 or more constitute a chain. 

 A model of a unit cell would 

 preferably contain 10 such 

 (CeHioOs) groups, i.e., that 

 part of Fig. 131 contained 

 within the heavy lines form- 

 ing a cube; but as each unit 

 cell when entirely surrounded by others shares each vertical 

 edge with three other unit cells, then the number of anhydrous 

 glucose groups that can be allocated to each individual unit 

 cell is four, i.e., two on one edge and two on the center axis which 

 are not shared. Because of the small number of radicals con- 

 tained within an elementary cell, early work on cellulose led to 

 the belief that the cellulose molecule was of low molecular weight ; 

 but the elementary cell determines the character of the crystal 

 and not necessarily that of the molecule. Organic chemists 

 found it very hard to reconcile the known properties of cellulose 

 with the idea of a low molecular weight. The long chain mole- 

 cule was therefore postulated. 



Part of a cellulose micelle or 

 crystallite. 



