Primary cell wall major structural polymers

In order to think about the nature and consequences of cell wall polymer phenolic cross-linking, we need a working model of the mode of assembly and the final structure of the primary cell wall. Unfortunately, there is no universally acceptable model that proposed by Albersheim and co-workers (3) is not now widely accepted because the postulated interpolysaccharide glycosidic bonds have not been demonstrated (4) and the warp-weft model of Lamport (5) rests on the assumptions that extensin (i) forms a defined-porosity network (not proven) (ii) is orientated anti-clinally to the cell surface [some evidence against (6)] and (iii) is a major component of all primary cell walls (not true). 

Cellulose is the most abundant polysaccharide in nature and is probably the most studied cell wall polymer. Most of the structural studies of cellulose have been carried out with material from secondary cell walls little data is available for primary cell wall cellulose. The chemistry of cellulose has been reviewed numerous times (56, 78, 107), The present review considers only the major aspects of cellulose structure and assumes that the cellulose of primary walls is similar to that of secondary walls. 

Pectin is a mixture of complex polymers (1-3). These are formed during primary wall formation while the cell wall is expanding in surface area. At these early stages of growth the pectin polymers contribute in a major way to the texture of the wall especially to its ability to expand and stretch. The wall is much more a fluid structure at this time and water is an extremely important constituent. The primary wall when the matrix is non-lignified can be considered as a fluid plastic structure so that any load applied to the wall is transmitted to the microfibrils by the viscous drag of the plastic deformation of the matrix. This can alter very much with the composition and physical state of the matrix materials, especially with that of the pectin complex ( 3). 

Cellulose microfibrils are usually embedded in a continuous phase of lignin, pectin, and hemicellulose hemicellulose usually predominates (Whistler and Richards, 1970). Most of the hemicellulose is found mixed with cellulose in both the primary and secondary cell walls. Hemicellulose is now known not be a precursor to cellulose. In general, hemicellu-loses are the cell wall polysaccharides other than cellulose and pectin. These polymers are classified on the basis of the type of sugar residues present. For example, o-xylan is made up of D-xylose, D-mannan of D-mannose, and D-galactan of D-galactose units (Whistler and Richards, 1970). In practice, few hemicelluloses contain only one sugar, most contain two to four. Furthermore most hemicelluloses have branched structures. The major types of hemicellulose and their distribution have been reviewed (Whistler and Richards, 1970). Methods for the analysis of mannans have been reviewed (Matheson, 1990). 

---