Overview
This lecture covers the three main polysaccharides—starch, glycogen, and cellulose—including their structures, functions, and how these relate to their biological roles in plants and animals.
Structure and Formation of Polysaccharides
- Polysaccharides are formed by condensation reactions linking glucose monomers with glycosidic bonds.
- The three key polysaccharides are starch, glycogen, and cellulose—all polymers of glucose.
Starch
- Starch is found in plants and serves as an insoluble store of glucose.
- Made from two polymers of alpha glucose: amylose (unbranched, helical) and amylopectin (branched).
- Amylose has 1,4 glycosidic bonds; amylopectin has both 1,4 and 1,6 glycosidic bonds.
- Stored in plant cells such as chloroplasts.
Glycogen
- Glycogen is found in animals (muscle and liver cells) and stores glucose.
- Also made from alpha glucose, with both 1,4 and more frequent 1,6 glycosidic bonds than amylopectin.
- Highly branched structure allows rapid hydrolysis to glucose for energy needs.
Cellulose
- Cellulose is found in plant cell walls and provides structural strength.
- Made from beta glucose with only 1,4 glycosidic bonds, forming long, straight chains.
- Chains align in parallel, held by many hydrogen bonds, forming microfibrils for cell wall strength.
Structure-Function Relationships
- Starch and glycogen’s branched and helical shapes make them compact and suitable for storage.
- Multiple ends in branched molecules allow enzymes to quickly release glucose.
- Insolubility in water prevents these polysaccharides from affecting osmosis in cells.
- Cellulose's many hydrogen bonds collectively provide rigidity to plant cell walls.
Key Terms & Definitions
- Polysaccharide — large carbohydrate molecule made from many monosaccharides linked by glycosidic bonds.
- Glycosidic bond — covalent bond that links carbohydrate molecules together.
- Alpha glucose — glucose isomer where the OH on carbon 1 is below the ring.
- Beta glucose — glucose isomer where the OH on carbon 1 is above the ring.
- Amylose — unbranched, helical polymer of alpha glucose in starch.
- Amylopectin — branched polymer of alpha glucose in starch.
- Microfibril — bundle of cellulose chains held by hydrogen bonds for structural strength.
Action Items / Next Steps
- Complete the comparison table of polysaccharides covering monomer, bond type, location, function, and structure.
- Review and practice questions on polysaccharide structure-function relationships.
- Prepare for a test on carbohydrates.