Cellulose I and II are the two most common polymorphs of cellulose, with Cellulose I being the naturally occurring form found in plant cell walls, exhibiting a parallel chain orientation, while Cellulose II is the more stable form obtained through chemical treatment or regeneration, characterized by an antiparallel chain orientation. The transformation from Cellulose I to Cellulose II involves significant changes in hydrogen bonding and crystallinity, impacting their mechanical and chemical properties, which are critical for applications in materials science and bioengineering.