The use of lignocellulosic material to produce ethanol presents a number of challenges relative to the use of starch containing sources such as corn, which can be digested with amylases. Cellulose molecules are actually crystalline in nature with very little water hydration. Plant material is a complex structure that contains a number of polysaccharide polymers. Primary and secondary walls contain cellulose, hemicellulose and pectin, albeit in different proportions. The secondary walls of woody tissue and grasses are composed predominantly of cellulose, lignin, and hemicellulose. The cellulose fibrils are embedded in a network of hemicellulose and lignin. Cross-linking of this network is believed to result in the elimination of water from the wall and the formation of a hydrophobic composite that limits accessibility of hydrolytic enzymes and is a major contributor to the structural characteristics of secondary walls. Xylan, which accounts for up to 30% of the mass of the secondary walls in wood and grasses, contributes to the recalcitrance of these walls to enzymatic degradation.