Iodine is a polysaccharide made up of many glucose molecules. Glucose is a monosaccharide or simple sugar and is only one of the many glucose components that make up starch. Do to starch's long structure, a shorter polymer (and component of starch) is stuck in the middle of starch. The Iodine then binds to this amylose in the starch molecule and turns it black. Glucose is not altered by iodine because it is a single molecule. There needs to be hundreds if not thousands of glucose molecules to create these starch-amylose-amylopectin molecule complexes to which iodine will bind. Hope this helps.