Starch (C6H10O5)n: Energy storage in plants. Two components — amylose (20%, unbranched, alpha-1,4 linkages, helical structure, gives blue color with I2) and amylopectin (80%, branched at alpha-1,6 linkages every 24-30 glucose units, gives violet color with I2).

Cellulose (C6H10O5)n: Structural polymer in plant cell walls. Unbranched, beta-1,4 linkages. The beta configuration causes straight chains that form strong intermolecular H-bonds → insoluble, rigid fibers. Humans cannot digest (lack cellulase enzyme). Cellulose acetate (rayon) and cellulose nitrate (gun cotton) are semi-synthetic derivatives.

Glycogen: Energy storage in animals (liver, muscles). Structure similar to amylopectin but MORE highly branched (alpha-1,6 branch every 8-12 glucose units). More branching = more non-reducing ends = faster glucose release when needed.

Iodine test: Amylose → deep blue (I2 fits in helix); Amylopectin → violet-red; Glycogen → reddish-brown; Cellulose → no color (no helix, beta linkages).