How do carbons bonding properties contribute to the existence of a wide variety of biological molecules?

Both starch and cellulose are made by stringing together many glucose molecules, but starch is easily digested by humans, while cellulose is indigestible. the starch polysaccharide has every bond occurring below the sugar rings while the bonds in cellulose alternate between one bond above the ring and the next below the ring. how can you explain humans' inability to digest cellulose?