Advanced Placement (AP) Biology Practice Exam

What differentiates starch from cellulose in terms of molecular structure?

1. Presence of nitrogen groups

2. Position of the hydroxyl group on Carbon 1

3. Chain length

4. Number of carbon atoms

The correct answer is: Position of the hydroxyl group on Carbon 1

Starch and cellulose are both polysaccharides made from glucose units, but they differ significantly in their molecular structure, which influences their biological functions. The key differentiator lies in the position of the hydroxyl group on Carbon 1 of the glucose molecule. In starch, the hydroxyl (-OH) group on Carbon 1 of the glucose unit is in a downward position when viewed in its standard orientation. This leads to the formation of alpha (α) linkages between the glucose molecules, specifically α-1,4-glycosidic bonds in the linear portions and further branching in amylopectin, one of its components. These alpha linkages allow starch to be easily broken down by enzymes, making it a readily accessible form of energy storage for plants. In contrast, cellulose consists of glucose units linked by beta (β) linkages, specifically β-1,4-glycosidic bonds, where the hydroxyl group on Carbon 1 of each glucose is in an upward position. This structural difference results in a straight, rigid chain that forms structures called microfibrils, contributing to the strength and rigidity of plant cell walls. Since the human digestive system lacks the enzymes to break down these β linkages, cellulose