When the cyclic form of glucose forms, 2 different versions of it are produced. One is alpha, the other beta.
Starches like amylose and amylopectin link only alpha-type glucose molecules together. With cellulose, it is the beta molecules that link together.
The glycosidic bonds in cellulose have a different orientation from those in starch.
As a result of this, human digestive enzymes can break down starch but not cellulose.
Cellulose consists of linear molecules each made up of at least 3,000 glucose units linked together.