Cellulose is often considered to be one of the most abundant biopolymers on earth. Although the composition of cellulose, the [3-1,4 linked linear glucose polymer (glucan), is relatively simple compared to other plant cell wall polysaccharides and the physical structure of cellulose is complicated. Celluloses can be crystalline, sub-crystalline, and even amorphous, depending on their tissue source in native plant, or the way that cellulose is isolated. The structural integrity of cellulose is believed to be one of the major causes of resistance to chemical and enzymatic hydrolysis.

Until recently, the crystal structures of native cellulose (cellulose I) were elucidated using algae Glaucocystis (Ia, triclinic) (15) and the tunicate Halocynthia roretzi (Ip, monoclinic) (16) celluloses. It has been suggested that the Ia and Ip allomorphs naturally coexist in various proportions in different organisms (17). These celluloses obtained from algae or tunicate; however, do not necessarily represent the cellulose in plant cell walls. The cellulose crystallite in higher plants is reported to be much smaller (3-5 nm in diameter) (9, 17), whereas they can be approximately 20 nm in diameter in some algae. Plant cell wall cellulose is embedded in a complex polymer matrix forming the microfibrils. Indeed, the interactions between cellulose and other polysaccharides are ubiquitous. The plant cell walls are dynamic and their compositions and properties may differ for different tissues, cell types, as well as different developmental stages. While the molecular structure of plant cell wall cellulose remains unclear, some scientists believe that cellulose in higher plant cell walls exists pri­marily in the Ip form with a small proportion of Ia form (13, 18-20). Other researchers have suggested that there is only the Ip form, with some disorder on the crystallite surface, based on solid-state 13C NMR spectroscopy studies (21). The challenges for characterizing plant cell wall celluloses stem not only from the limited resolution of available measurement techniques, but also from the processes commonly used to isolate celluloses, as well as sam­ple preparation methods. Typically, sequential extraction processes using acid and alkaline incubations are used, sometimes at high temperature. Such extensive sample processing can result in fiber aggregation (22). The isolated “microfibrils” are thought to be cellulose crys­tallites surrounded by a small pocket of partially hydrolyzed non-crystalline polysaccharides (13, 23). The question remains, how does this extensively treated “microfibril” relate to its native form in the plant cell wall? The plant cell wall cellulose crystallites can be summarized as being too small for traditional microscopy (3-5 nm in cross section) and containing a large proportion of disordered surface glucan chains (20, 24, 25). The detailed molecular structure of plant cell wall cellulose remains unknown.