We investigate the influence of adsorbed water on amorphous cellulose structure and properties, within the full range of moisture content from the dry state to saturation, by molecular dynamics simulation. Increasing water content results in overall swelling, a substantial decrease in stiffness, and higher diffusivity of the water molecules. The obtained sorption curve as well as the range of swelling and weakening are confirmed by experiments. The measured properties undergo a noticeable change at about 10% of moisture content, which suggests that a transition occurs in the porous system, indicating that the sorption process is stepwise. Our analysis of water network formation reveals that the onset of percolation coincides with the moisture content at which a transition in the material properties is observed. An in-depth analysis of the molecular mechanism of hydrogen bonding, van der Waals interactions, and water network in the two regimes enhances the understanding of the adsorption process.