Both Starch and Cellulose are homopolysaccharides of $\text{D-glucose}$, but the linkage geometry differs:

• Starch: Consists of Amylose (linear) and Amylopectin (branched) with $\alpha$(1→4) glycosidic linkages. This makes it digestible by human amylase.
• Cellulose: Consists of linear chains with $\beta$(1→4) glycosidic linkages. Human enzymes cannot break the $\beta$-linkage, so cellulose serves as roughage/fiber in our diet.