Primary cell wall framework

Modern knowledge of cell wall structure comes from detailed chemical analyses, X-ray diffraction studies and work with the electron microscope. The dry primary cell wall contains hemicelluloses (up to 50%), cellulose (up to 25%) and smaller amounts of pectic substances, fats and protein. Hemicelluloses, cellulose and pectins are all polysaccharides with molecules built up from the linking together in chains of sugars or uronic acid residues. The morphology of cell walls is, however, not destroyed by extracting the hemicelluloses and pectins, indicating that the structural framework of the wall is... 

Pairs of unbranched cellulose molecules, which may contain as many as 12,000 glucose units each, are held together by hydrogen bonding to form sheetlike strips called microfibrils (Figure 7.34). Each bundle of microfibrils contains approximately 40 of these pairs. These structures are found in both plant primary and secondary cell walls, where they provide a structural framework that both protects and supports cells. 

The primary wall also contains relatively small proportions of randomly oriented, cellulosic microfibrils that have a low and non-uniform degree of polymerization (see p. 333). Miihlethaler suggested (see p. 343) that these microfibrils are synthesized by particles loosely scattered over the plasmalemma, at the beginning of formation of the cell wall. As development of the wall proceeds, the number of these particles increases until they aggregate into regular arrays this results in the production of densely packed and parallel-oriented microfibrils of the secondary wall, instead of the loose, microfibrillar framework of the primary wall. 

Cellulose microfibrils make up the basic framework of the primary wall of young plant cells (3), where they form a complex network with other polysaccharides. The linking polysaccharides include hemicellulose, which is a mixture of predominantly neutral heterogly-cans (xylans, xyloglucans, arabinogalactans, etc.). Hemicellulose associates with the cellulose fibrils via noncovalent interactions. These complexes are connected by neutral and acidic pectins, which typically contain galac-turonic acid. Finally, a collagen-related protein, extensin, is also involved in the formation of primary walls. 

---