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Hama1 is correct that the functional difference is that starch is for energy storage and cellulose is for structural strength. The differing structures of these two classes of molecule help demonstrate how structure is related to function.
In starch molecules the base structure is a linear chain. These chains join end to end to form long branching molecules where the chains are all relatively parallel to one another (see this link for good diagrams). These branched starch chains form into concentric rings which are stored within the plant as starch granules. Because the starch molecules are chemically bonded only at the ends of the chain, they can easily come apart and are soluble in water. This allows the plant ready access to its food stores.
Cellulose also begins as carbohydrate chains, but in this case the chains form hydrogen cross-bridges along their lengths, converting the strands into a fibrous mesh, which is insoluble and lends cellulose its strength and flexibility. You can see diagrams of cellulose's structure here.
The basic functional difference is that Starch is for energy storage and Cellulose is for Cell Wall formation.
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