Size, Shape, and Arrangement of Native Cellulose Fibrils in Maize Cell Walls

Higher plant cell walls are the major source of the cellulose used in a variety of industries. Cellulose in plant forms nanoscale fibrils that are embedded in non-cellulosic matrix polymers in the cell walls. The morphological features of plant cellulose fibrils such as the size, shape, and arrangement, are still poorly understood due to its inhomogeneous nature and the limited resolution of the characterization techniques used. Here, we sketch out a proposed model of plant cellulose fibril and its arrangement that is based primarily on review of direct visualizations of different types of cell walls in maize using atomic force microscopy at sub-nanometer scale, and is also inspired by recent advances in understanding of cellulose biosynthesis and biodegradation. We propose that the principal unit of plant cellulose fibril is a 36-chain cellulose elementary fibril (CEF), which is hexagonally shaped and 3.2 × 5.3 nm in cross-section. Macrofibrils are ribbon-like bundles containing variable numbers of CEFs associated through their hydrophilic faces. As the cell expands and/or elongates, large macrofibril may split to become smaller bundles or individual CEFs, which are simultaneously coated with hemicelluloses to form microfibrils of variable sizes during biosynthesis. The microfibrils that contain one CEF are arranged nearly parallel, and the hydrophobic faces of the CEF are perpendicular to the cell wall surface. Structural disordering of the CEF may occur during plant development while cells expand, elongate, dehydrate, and die, as well as during the processing to prepare cellulose materials.