Cellulosic Microfibril and Its Embedding Matrix within Plant Cell Wall
Cellulose is the homopolymer of (1→4)-β-D-glucopyranose. Glucan chains are aggregated to form microfibrils with cross dimensions ranged from 2 to 20 nm. The variation in each of microfibrilluar dimension, hydrogen-bonding network and molecular orientation depends on their parent nature. Cellulosic microfibrils isolated from bacteria and certain algae (10–25 nm in diameter) were found to be thicker than those extracted from the primary cell walls in higher plants ( 3–10 nm in diameter).Three biopolymers, namely hemicellulose, lignin and/or pectin contribute to the embedding matrix enveloping the cellulosic microfibrils.The secondary cell wall was found to be differentiated into three consecutive layers (S1, S2, S3). Within each layer, cellulose microfibrils are found parallel and of high order with a different microfibrilluar angle. The bundling process of cellulose microfibrils in both primary and secondary cell walls may involve the aggregation of the closely arranged cellulosic microfibrils. Some models for the microfibril construction have been proposed. Along the longitudinal microfibril’s axe, crystalline regions are alternated with amorphous ones. The most important methods of estimating the crystallinity of cellulose microfibrils such as Xray diffraction (XRD), 13C solid-state NMR spectroscopy (13C-NMR), small-angle neutron scattering (SANS) and sum frequency generation (SFG) spectroscopy were concluded.