3. Polysaccharides

Storage:

• Starch (plants): α-glucose polymer, stored in plastids
• Glycogen (animals): highly branched α-glucose, liver/muscle

Structural:

• Cellulose (plants): β-glucose polymer, cell walls
  • Most abundant organic polymer on Earth
  • Humans cannot digest (lack cellulase)
• Chitin: Modified glucose with N-groups
  • Fungal cell walls, arthropod exoskeletons

Key Concepts

α-glucose vs. β-glucose:

• α: OH on C1 below ring → forms starch/glycogen (digestible)
• β: OH on C1 above ring → forms cellulose (indigestible)

Glycosidic bonds:

• Formed by dehydration synthesis
• Broken by hydrolysis
• Enzyme specificity determines digestibility

Glycogen:

• α-1,4 bonds with α-1,6 branches every 8-12 glucose units (highly branched)

Cellulose:

• β-1,4 glycosidic bonds (linear, unbranched)

(c) Structure:

Starch: Helical coils (amylose) and branched structure (amylopectin)

Glycogen: Highly branched, compact globular structure

Cellulose: Long, straight, unbranched chains that form hydrogen bonds with adjacent chains → microfibrils

(d) Biological Function:

Starch:

• Energy storage in plants
• Found in roots, tubers, seeds
• Easily digestible by amylase enzymes

Glycogen:

• Energy storage in animals
• Stored in liver and muscle cells
• Highly branched for rapid glucose release

Cellulose:

• Structural support in plant cell walls
• Provides rigidity and strength
• Most humans cannot digest (lack cellulase enzyme)
• Dietary fiber in human nutrition

$\boxed{\text{Storage: starch/glycogen (α-glucose); Structural: cellulose (β-glucose)}}$