Both cellulose and starch are polysaccharides found in plants, but they serve different purposes and have different structures. Starch is used by plants as a way to store energy, and humans can digest it to use that energy. On the flip side, cellulose provides structural support to plant cell walls. Structurally, the glucose units in starch are connected in a way that is digestible by humans, whereas the bonds in cellulose cannot be broken down by human digestive enzymes.