Cellulose Elementary Fibrils Assemble into Helical Bundles in S₁ Layer of Spruce Tracheid Wall

The ultrastructural organization of cellulose elementary fibrils (EFs) in wood cell wall is considered to be the prime factor regulating the material characteristics of wood in micro to macro levels and the conversion of delignified wood fibers into various products. Specifically, the complex assembly of EFs in wood cell wall limits its swellability, solubility, and reactivity, for example, in dissolution of cellulose for regeneration of textile fibers, fibril separation for the manufacture of nanocellulose, and enzymatic hydrolysis of cellulose into sugars for their subsequent fermentation to various products, like ethanol for future fossil fuels replacement. Here cryo-transmission electron tomography was applied on ultrathin spruce wood sections to reveal the EF assembly in S₁ layer of the native cell wall. The resolution of these tomograms was then further enhanced by computational means. For the first time, cellulose in the intact cell wall was visualized to be assembled into helical bundles of several EFs, a structural feature that must have a significant impact on the swelling, accessibility, and solubility of woody biomass for its conversion into the aforementioned value added products.