Cellulose is a complex carbohydrate, specifically a polysaccharide, that is a key structural component of the plant cell wall. It provides rigidity and strength to the plant, allowing it to maintain its shape and resist external pressures. Unlike starch and glycogen, which serve as energy storage molecules, cellulose is not used for energy by plants but is crucial for their structural integrity. Chitin, on the other hand, is a polysaccharide that provides structure to the exoskeletons of insects and the cell walls of fungi. Glycogen is the storage form of glucose in animals, while starch serves a similar purpose in plants. Therefore, the correct answer to the fill-in-the-blank question regarding the function of cellulose is that it makes up the plant cell wall.