Describe structural differences among glycogen, starch and cellulose and elaborate their functions. (IFS 2021, 15 Marks)

Introduction

Glycogen, starch, and cellulose are all polysaccharides composed of glucose units, but they differ in their structural arrangements and functions. These differences is crucial in understanding their roles in biological systems.

Structural Differences

• Glycogen
  
  • Monomer Unit: Composed of glucose units.
  • Linkage: The glucose molecules are linked by alpha-1,4-glycosidic bonds with branches formed by alpha-1,6-glycosidic bonds.
  • Branching: Highly branched structure with frequent branch points (about every 8-12 glucose units).
  • Shape: Compact, highly branched, and globular.
  • Storage Site: Primarily stored in the liver and muscles of animals.
• Starch
  
  • Monomer Unit: Composed of glucose units.
  • Linkage: Mainly consists of two components—amylose (linear chains with alpha-1,4-glycosidic bonds) and amylopectin (branched with alpha-1,4 and alpha-1,6-glycosidic bonds).
  • Branching: Less branched than glycogen, with amylose being unbranched and amylopectin having some branching.
  • Shape: Amylose forms helical structures, while amylopectin forms a more branched structure.
  • Storage Site: Stored in plants, primarily in roots, tubers, seeds, and fruits.
• Cellulose
  
  • Monomer Unit: Composed of glucose units.
  • Linkage: The glucose units are linked by beta-1,4-glycosidic bonds.
  • Branching: No branching; forms long, straight chains.
  • Shape: The chains are linear and form microfibrils through hydrogen bonding, giving it strength.
  • Storage Site: Found in the cell walls of plants.

Functions of Glycogen, Starch, and Cellulose

• Glycogen
  
  • Energy Storage: It serves as a primary energy reservoir in animals. When energy is needed, glycogen is broken down into glucose via glycogenolysis.
  • Rapid Mobilization: Its highly branched structure allows for quick release of glucose molecules, making it ideal for short-term energy needs, such as during exercise.
  • Regulation of Blood Sugar: Glycogen stored in the liver helps maintain blood glucose levels.
• Starch
  
  • Energy Storage: Starch acts as the main energy storage form in plants, stored in roots, tubers, and seeds.
  • Digestion and Energy Supply: Starch is broken down into glucose by enzymes like amylase in the digestive system of animals, providing a long-term energy source.
  • Flexibility: The mixture of amylose and amylopectin allows plants to regulate the rate of digestion and energy release.
• Cellulose
  
  • Structural Role: Cellulose provides structural support to plant cells. It is a major component of the plant cell wall, giving rigidity and strength to plant tissues.
  • Indigestibility: While it is a polysaccharide like starch, the beta-1,4-glycosidic linkage in cellulose makes it indigestible by most animals (except for some herbivores with specific bacteria in their digestive system).
  • Cell Wall Integrity: Helps maintain the shape of the plant cell and prevents the cell from bursting under osmotic pressure.

Conclusion

Glycogen, starch, and cellulose are important polysaccharides with distinct structural differences and functions. Glycogen and starch serve as energy storage molecules in animals and plants, respectively, while cellulose provides structural support in plant cell walls.