The primary structural difference that accounts for the functional differences between starch and cellulose lies in the types of glycosidic linkages between their glucose monomers.
1. **Glycosidic Linkages**: Starch is composed mostly of amylose and amylopectin, which have α(1→4) and α(1→6) glycosidic bonds. In contrast, cellulose consists of β(1→4) glycosidic bonds. The difference in these linkages significantly affects the three-dimensional structure of the polymers.
2. **Structural Implications**:
- **Starch**: The α-linked structure of starch leads to a helical shape, making it easier for enzymes in animals (like amylase) to break down the glucose units for energy. This is why starch is digestible by animal enzymes.
- **Cellulose**: The β-linked structure of cellulose results in straight, rigid chains that can form strong hydrogen bonds with neighboring chains, creating a stable and insoluble structure. This structure makes cellulose resistant to digestion by most animal enzymes, as they cannot easily break the β(1→4) bonds.
Therefore, the correct answer to highlight the structural difference that accounts for the functional differences is: **Starch and cellulose differ in the glycosidic linkages between their glucose monomers.**