Cellulose is the most abundant organic molecule on earth due to its presence in plant cell walls. Cotton is almost pure cellulose at 98%; flax is 80%, and wood is 40-50%. It is a mechanically strong molecule (unlike starch and glycogen) so not surprisingly it provides structural strength for the plant cell.
Cellulose is formed when β-glucose monomers are linked together through glycosidic bonds by condensation.
Cellulose is a straight chain polymer so, unlike starch, no coiling occurs. The multiple hydroxyl groups on the cellulose strands hydrogen bond with neighboring strands, holding the chains firmly together and contributing to their high tensile strength. The cellulose molecules become tightly cross-linked and form bundles called microfibrils.