Hemicellulose is a large polysaccharide which helps give structure to plant cells. Along with pectin, it forms an amorphous matrix in which cellulose fibrils from the plant cell walls are embedded. It has the chemical formula of C6H10O5 and consists of a chain of D-oxylose sugar units which has side chains of units of other sugars branching off it.

From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.