CONSTITUTION AND ARCHITECTURE IN THE CELL WALL 15 



and parallel to the axis at 8.3 and 7.9 A intervals. The crystallo- 

 graphic unit cell is rhomb ohedral, measures 10.3x8.3x7.9 A; 

 and has a characteristic angle of 84° between the shorter sides. 

 From its dimensions, we may calculate that the unit cell has a 

 volume of 675 A 3 . This volume multiplied by the density of cellulose 

 (taken at 1.6 g or 1.6xl0- 24 g/A 3 ), gives a unit cell mass of 

 1.08 X 10- 21 . This mass divided by the weight of the anhydroglucose 

 residue (C 6 Hi O 5 ) of about 162 gives moles of residue/unit 

 cell, 6.7xl0- 24 . The latter multiplied by Avogadro's number 

 6.06 x 10 23 , yields a number of glucose residues very close to 4. The 

 10.3 A identity period corresponds to the distance between the num- 

 ber one carbon of one residue and the oxygen at the number four 

 carbon of the second residue in cellobiose. The 7.9 A spacing agrees 

 satisfactorily with the maximum width of the cellobiose residue, 

 7.5 A. Thus, the unit cell contains cellobiose units lying in planes 

 8.3 A apart. The unit cell is best depicted with four cellobiose res- 

 idues lying on the four edges of a parallelepiped, each such dimer 

 being shared by the four unit cells to which that edge is com- 

 mon. From the number of glucose residues per unit cell, an ad- 

 ditional cellobiose chain must extend through the center of the 

 parallelepiped, but is out of phase by one-half residue with the 

 other and antiparallel to them. 



We have now examined the main features of crystalline struc- 

 ture, but the unit so defined gives no indication of the length of 

 the individual polymer chains. In the absence of oxygen, cellulose 

 may be dissolved in cuprammonium solution, that is, dispersed 

 into individual molecular chains, without appreciable degradation. 



The molecular weights of these chains have been determined by 

 viscometry or sedimentation measurements in the ultracentrifuge. 

 From such determinations, the molecular weight of cellulose is 

 found to vary with the source, and methods of preparation and 

 measurement, but is consistently high: 



Reported variations notwithstanding, cellulose has a degree 

 of polymerization of 1400-10,000, hence molecules ranging in 

 length from 7000-50,000 A, (0.7-5 my). 



The ordered structure of cellulose is also revealed in its optical 

 properties. Isolated cellulose exhibits intrinsic double refraction and 



