Introduction to Polysaccharides

• Following disaccharides, polysaccharides are formed by the repetition of sugar units.
• Each disaccharide retains four hydroxyl (–OH) groups and a hemiacetal group, allowing for further sugar linkages.

Cellulose vs. Starch

• Cellulose:

• Formed by beta-1,4 glycosidic bonds.

• Not digestible by humans; our enzymes can’t break these bonds.

• Structure:

  • It has a stable chain of sugar molecules linked by beta-glycosidic bonds.
  • Each sugar molecule is in a six-membered ring causing strong hydrogen bonding between chains.
  • This makes cellulose flexible, strong, and stable.

• Function:

  • Structural component of plant cell walls (e.g., wood).

• Starch:

• Formed primarily by alpha-1,4 glycosidic bonds, making it digestible.

• Types of Starch:

  • Amylose:
  • Linear chain with only alpha-1,4 links.
  • Forms helices (similar to DNA), relatively stable.
  • Typically constitutes about 20% of starch and can reach molecular weights of 500,000 Daltons.
  • Amylopectin:
  • Branched structure with both alpha-1,4 and alpha-1,6 bonds.
  • Accounts for about 80% of starch.
  • Contains 100,000 + glucose units, branched approximately every 25 glucose units, creating a hierarchical structure.
  • Results in a micelle structure visible under an optical microscope.

Cooking and Starch:

• Cooking alters the granular structure of starch, making it digestible.

Glycogen:

• Primary form of energy storage in animals.
• Similar to amylopectin but has random branching, making it hyperbranched.
• Can contain up to 1,000,000 glucose units.
• Readily converted back into glucose when the body needs energy.

Summary of Key Concepts in Carbohydrate Chemistry:

• Different types of sugars: monosaccharides, disaccharides, polysaccharides.
• Structural and functional differences between cellulose, starch, and glycogen.
• Importance of glycosidic bonds (beta vs alpha) in determining digestibility and function.
• Basic understanding of sugar projections and isomer configurations (D/L forms, anomers).

Exam Preparation Tips:

• Know definitions and structural differences among sugars.
• Practice converting between Fischer and Haworth projections.
• Be familiar with the configurations: alpha and beta anomers, and how these impact functionality of sugars.
• Understand the implications of polysaccharide structure on digestibility and energy storage.