Polysaccharides and Their Structures

Polysaccharides

Overview of Polysaccharides

• Definition: Polysaccharides are long chains (polymers) made of monosaccharide units, primarily D-Glucose.
• Types: Major types include:
  • Amylose
  • Amylopectin
  • Glycogen
  • Cellulose

Starch

• Function: Starch is the main storage form of glucose in plants.
• Sources: Found in insoluble granules in foods, such as rice, wheat, potatoes, beans, and cereals.
• Composition:
  • Composed of two types of polysaccharides:
  • Amylose
  • Amylopectin

Amylose

• Composition:
  • Accounts for about 20% of starch.
  • Consists of 250 to 4000 α-D-glucose molecules.
  • Bonding: Connected by α(1 → 4) glycosidic bonds, forming a continuous unbranched chain.
• Structure:
  • Polymer chains are coiled in a helical fashion, contributing to the structural properties of starch.

Amylopectin

• Composition:
  • Comprises as much as 80% of starch.
  • Consists of α-D-glucose molecules.
  • Bonding: Connected by both α(1 → 4) and α(1 → 6) glycosidic bonds, resulting in a branched structure.

Glycogen

• Function: Glycogen serves as the storage form of α-D-Glucose in animals, primarily in the liver and muscles.
• Structure:
  • Similar to amylopectin but is more highly branched.
  • Utilizes α(1 → 6) linkages at the branches, leading to rapid mobilization of glucose when needed.

Cellulose

• Composition:
  • Made of glucose units linked in unbranched chains.
  • Bonding: Connected by β(1 → 4) glycosidic bonds.
• Digestibility:
  • Humans cannot digest cellulose because they lack the enzyme necessary to break β(1 → 4) linkages.
• Function:
  • Provides structural support to plant cell walls, contributing to the rigidity of wood and fiber.

Practice Questions

• Identify polysaccharides and types of glycosidic bonds in various examples to reinforce understanding of the material.

• Example Considerations:
  A. Starch (amylose, amylopectin)
  B. Glycogen
  C. Cellulose