Exemplar Problems

Question:

Explain the structural differences between cellulose and starch and their respective functions in plants.

Answer:

Cellulose and starch are both polysaccharides composed of glucose monosaccharide units, but they have distinct structures and functions in plants.

• Structural Differences:

  • Cellulose is a linear polysaccharide with glucose units linked by β-1,4-glycosidic bonds. The glucose molecules are oriented alternately upside down, resulting in a straight, unbranched chain. This linear structure allows cellulose to form a strong, rigid, and insoluble structure, making it a major component of plant cell walls.
  • Starch, on the other hand, is composed of glucose units linked by α-1,4-glycosidic bonds with occasional α-1,6 branch points. This branching creates a more compact, helical structure. Amylose is unbranched, while amylopectin is branched. Starch serves as an energy storage molecule in plants.

• Functions in Plants:

  • Cellulose provides structural support to plant cells and is a crucial component of plant cell walls. It gives plants their rigidity and allows them to withstand environmental pressures.
  • Starch serves as a storage polysaccharide in plants. It stores excess glucose produced through photosynthesis and can be converted back into glucose when the plant needs energy, such as during periods of low light or growth.