Ultrastructural localization of cellulose Iα and Iβ allomorphs in one microfibril from algal
sources was investigated using electron microdiffraction. Both cellulose Iα and Iβ were characterized as
one-chain triclinic and two-chain monoclinic unit cells, respectively, in agreement with previous studies.
These two structures coexisted in each microfibril, alternating either longitudinally or laterally. The
transition zone between the two phases was found to be the interface between adjacent H-bonded molecular
sheets (i.e., 0.39-nm lattice planes).
The crystalline ultrastructure and orientation of cellulose microfibrils in the cell wall of Valonia macrophysa were investigated by means of high-resolution electron microscopy of ultrathin (approx. 28 nm) sections. With careful selection of imaging conditions, ultrastructural aspects of the cell wall that had remained unresolved in previous studies were worked out by direct imaging of crystal lattice of cellulose microfibrils. It was confirmed that each microfibril is a single crystal having a lateral dimension of 20·20 nm(2), because lattice images of 0.39 nm resolution were clearly recorded with no major disruption in the whole area of the cross section of the microfibril. There was no evidence for the existence of 3.5-nm elementary fibrils which have been considered to be basic crystallographic and morphological units of cellulose in general. It was also confirmed that the axial directions (crystallographic fiber direction) of adjacent microfibrils in each single lamella of the cell wall are opposite to each other.
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