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PLANT BIOCHEMISTRY 



Cellulose 



The principal polysaccharide louiul in Lhe cell wall ol higher plants 

 is cellulose (Chapter 3). Generally it is found in association with 

 other polysaccharides, lignin, and inorganic silicates. However, cellu- 

 lose may be found in almost pure form in cotton and ramie bast 

 fibers. Plant fibers are bundles of high-molecular-weight, linear cellu- 

 lose molecules held together by physical chemical forces at intervals 

 along the chains. The cellulose fiber, therefore, consists of repeating 

 crystalline areas interspersed with non-crystalline (amorphous) areas. 

 X-ray analysis has been used in determining the structure of the crys- 

 talline regions called micelles. From X-ray data it has been possible 

 to calculate the volume of the unit micelle (670 A.-^) and to determine 

 the average glucose content. Each micelle contains an average of four 

 glucose units. From this information and from the calculated micel- 

 lular dimensions (10.3 X 7.9 X 8.3 A.), a structure for the crystalline 

 region of cellulose has been proposed as shown in Figure 8-2. 



The unit cell has a length of two D-glucose units (one cellobiose 

 unit). Thus each cell contains an average of two cellobiose units with 

 the corner units shared with the neighboring micelles. Note also that 



FIGURE 8-2. Unit cell of cellulose. (From Cellulose and Cellulose Derivatives, Vol- 

 ume V, Part 1, of High Polymer Series, copyright 1954. Published with permission 

 of Interscience Publishers, New York.) 



