Both are macromolecules with molecular weights in the hundreds of thousands. Because of their six carbon atoms, each is a hexose . Substances such as these three, which have identical molecular formulas but different structural formulas, are known as structural isomers . All sugars are very soluble in water because of their many hydroxyl groups. Although not as concentrated a fuel as fats, sugars are the most important source of energy for many cells.
Starches are polysaccharides.
Polysaccharides Starches Starches are polymers of glucose. The total number of glucose residues in a molecule of amylopectin is several thousand. Starches are insoluble in water and thus can serve as storage depots of glucose. Plants convert excess glucose into starch for storage. Rice, wheat, and corn are also major sources of starch in the human diet.