Starch is a polysaccharide comprising glucose monomers joined in α 1,4 linkages.It consists of two types of molecules: the linear and helical amylose and the branched amylopectin. Amyloseis almost unbranched and helical, but could contain a few branch points. Amylose is a much smaller molecule than amylopectin, which is fully brunched molecule. Starch molecules arrange themselves in the plant in semi-crystalline granules.
Cellulose is a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is a straight chain polymer, so unlike starch, no coiling or branching occurs and the molecule adopts an extended and rather stiff rod-like conformation, aided by the equatorial conformation of the glucose residues. The multiple hydroxyl groups on the glucose from one chain form hydrogen bonds with oxygen atoms on the same or on a neighbor chain, holding the chains firmly together side-by-side and forming microfibrils with high tensile strength. Compared to starch, cellulose is also much more crystalline.
Glycogen is a branched biopolymer consisting of linear chains of glucose residues with an average chain length of approximately 8–12 glucose units. Glucose units are linked together linearly by α(1→4) glycosidic bonds from one glucose to the next. Branches are linked to the chains from which they are branching off by α(1→6) glycosidic bonds between the first glucose of the new branch and a glucose on the stem chain.