Heteropolysaccharide of plants

Hemicellulose refers to a chemically ill-defined group of heteropolysaccharides of plant cell-walls that are closely associated with cellulose. They are composed of different hexoses and pentoses, and uronic acids. The hemicelluloses may be separated into two fractions, A and B however, there is no clear demarcation of structure between the two fractions. Hemicellulose B usually contains a higher proportion of uronic acid, chiefly 4-0-methyl-D-glucuronic acid, than the A fraction. The methyl ethers of D-aldobiouronic acids containing D-glucuronic acid, D-galacturonic acid, or D-mannuronic acid are isolated chiefly as the 4-O-methylaldobiouronic acids, and are hydrolyzed with difficulty by acid. 

---