The key is the glycosidic bonds between individual sugar molecules that make up the sugars. (beta 1-4 for cellulose, whereas alpha 1-4 for starch/glycogen). The different anomers means that only certain enzymes can break them down. Of course, you want starch to be decomposable for food, while you don't want that of cellulose. Also, starch is branched (alpha 1-6 I believe). Assuming starch is like glycogen in animals, the organism can only chew at the starch chain from one end, thus, branching the chain allows for more ends to chew from, allowing for faster glucose release if needed. Cellulose, unbranched, is more suitable for tensile strength and structure that it's typically used for.
