Some monomers of hemicellulose are shown in the sketch :

Usually, all of the pentoses are present. There may even be small amounts of L- sugars. Note that there are hexoses as well as acids formed by oxidation of sugars. Mannose and mannuronic acid tend to be present, and there can be galactose and galacturonic acid.The groups that are different from glucose are shown in red on the six-carbon structures. The pentoses are also present in rings (not shown) that can be 5-membered or 6-membered. Xylose is always the sugar present in the largest amount.

In contrast to cellulose that is crystalline, strong, and resistant to hydrolysis, hemicellulose has a random, amorphous structure with little strength. It is easily hydrolyzed by dilute acid or base, but nature provides an arsenal of hemicellulase enzymes for its hydrolysis. These enzymes are commercially important because they open the structure of wood for easier bleaching, and older methods of bleaching consume larger amounts of chemicals such as chlorine that are bad for the environment.

while on sabbatical leave, ESB, Porto, Portugal July 1996