Cellulose

Objectives: understand the structure of cellulose from its monomeric constituent to its crystalline forms.

 

Important Concepts
  1. Crystallinity and Amorphous Zones. Cellulose is polymerized within a cell by an extrusion process similar to what one would observe with a pasta maker. Stands are extruded out into the extracellular matrix. As these pores (or rosettes) are next to one another, the cellulose chains hydrogen bond with one another forming a crystalline matrix. The crystallization process is not perfect, leading to crystalline and non-crystalline regions. The non-crystalline regions are termed the amorphous zones. Due to the lack of crystallinity, the amorphous zones are more susceptible to degradation than the crystalline regions. For a set of images that have recently been obtained dealing with cellulose biosynthesis, click here.
  2. Concept of a Unit Cell and repeat unit. The unit cell describes the dimensions of the cellulose crystallite and the repeat unit is the shortest chain length needed to describe the polymer of cellulose. The dimer, cellobiose, is the repeat unit of crystalline cellulose.
  3. Cellulose degree of polymerization. See the ranges below.
  4. Major Crystalline Forms of Cellulose. There are two major kinds, cellulose I (native) and cellulose II (non-native, regenerated cellulose or rayon). More discussion below.
  5. Chain Orientation (parallel and anti-parallel). Cellulose I chains are parallel, II is antiparallel.

Degree of Polymerization of Several Celluloses

 Cellulose Degree of Polymerization
Native Cellulose 3,500---12,000
Purified Cotton Linters 1,000---3,000
Commercial Wood Pulps 600---1,500
Regenerated Cellulose (Cellulose II) 200---600

Cellulose Content of Various Plant Materials
 Plant Material Percent Cellulose
Cotton 95-99
Ramie 80-90
Bamboo 40-50
Wood 40-50
Wood Bark 20-30
Mosses 25-30
Bacteria 20-30

FOUR CRYSTALLINE FORMS OF CRYSTALLINE CELLULOSE
Cellulose I: native form of cellulose. Only form found in nature.
Cellulose II: regenerated cellulose. Cannot be reconverted to Cellulose I.
Cellulose III: formed on treatment with liquid ammonia.
Cellulose IV: formed on heating to a high temperature in a polar liquid, and is very similar to Cellulose I.

Structure of Cellulose.
Cellulose Morphology.
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