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