Both glycogen and starch are polysaccharides of glucose units, formed by alpha(1-4) glycosidic linkages. Cellulose is also a glucose polysaccharide and makes the strong cell walls of plants. It is formed by glycosidic linkages of the opposite orientation, beta (1-4) instead of alpha(1-4). The former two are easily digestable, but the latter is not. Thinking about the overall structure of the two polysaccharides, can you devise an explanation as to why the former is easily digested and the latter not?