The interaction of water with many biopolymers is known to rearrange their internal structure, make therm moisture sensitive, and influence their physical properties. We study amorphous cellulose and hemicellulose, two hydrophilic biopolymers, using molecular dynamics simulations, and we analyze their structural and physical properties over the full range of moisture content. We find a quasi-linear dependence of volumetric strain on moisture content, and a linear scaling between volumetric strain and porosity, showing that swelling is directly related to the space created by adsorbed water molecules. The interaction of water with the polymer structure results in a weakening of the mechanical properties, leading to rubber-like behavior at high moisture content. Weakening is caused by a decrease in the number of hydrogen bonds that follow exponential scaling. Breaking of the hydrogen bonds system is found to control not only the mechanical response but also the evolution of porosity and the volumetric strain.