Cellulose
TCs, create fibrils that have very small cross-sectional size (Nobles et al. 2001). The organization of fibrils outside the TC has been suggested to be a self- organizing process (Emons and Mulder 2000) possibly guided by the microtubules (Paradez et al. 2006). The environment into which the glucan chains are secreted will influence the microfibril architecture. Specifi- cally, it was shown that it is possible to influence the ratio between cellulose I a and I b produced by Acetobacter by the addition of polymers to the culture medium (Yamamoto et al. 1996), or even to make it produce cellulose II (Shibazaki et al. 1998). These observations suggest that native cellulose is not created by spontaneous self-organization, but rather that during biosynthesis the glucan chains are guided to associate into a regular intermolecular arrangement within which they remain trapped. Hence, H-bonds do not alone cause the regular intermolecular arrangement of as-biologically-pro- duced cellulose chains within fibrils but are merely consistent (in form of a local free energy minima) with the chains being guided into a specific spatial arrangement, thereby contributing to the stabilization of the metastable state. But even in this respect, they are not working alone. An often-neglected fact in the case of cellulose is that the molecular packing of chains is to a large extent a joint result of steric repulsion and attractive dispersion interactions and their total contribution to the cohesive energy of cellulose may be as high as 70% (Nishiyama 2018).
van der Waal’s interaction, or H-bonding, in combi- nation with a regular chemical structure. However, the cellulose I structure is not merely a result of spontaneous processes. It is well known that cellulose, when precipitated from solution, crystal- lizes in the form of cellulose II. This indicates that this structure is lower in energy than cellulose I, and thereby that native cellulose is in a metastable state (Sto¨ckmann 1972). Moreover, over the scale of organisms, cellulose fibrils have, with retained inter- chain arrangements within the fibrils, an extreme variation in cross-sectional fibril dimensions that are regular and reproducible within a given species (Tsekos 1999). Fibril dimensions vary from thin (two to three nanometers) and isotropic (i.e. the width approx. equal to the height) in most land plants, via large (up to 20 nm) isotropic shapes in some species (e.g. Oocystis , Valonia and Hypoglossum ), to the highly anisotropic ribbon-like structures produced by, e.g., Acetobacter and Erythrocladia . The cross-sec- tional variation is correlated with the spatial arrange- ment of the linear terminal complexes (TC) that secrete the glucan chains and hence the fibrils: lateral TC sizes in land plants are much smaller than those in Valonia or Oocystis and in Acetobacter the TC shape is elongated. The only possible way of having such correlation is that cellulose association into fibrils and ribbons is not random but directed by the regular arrangement of cellulose synthase units. Curiously, cyanobacteria, which seems to lack regularly arranged
Fig. 3 The size and spatial arrangement of the cellulose-synthesizing complexes are responsible for the large variation in fibril dimensions among different species
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