Overview
This lecture explains polysaccharides, focusing on the structure and function of starch, glycogen, and cellulose, and compares their features and biological roles.
Polysaccharides: Definition and Roles
- Polysaccharides are complex carbohydrates composed of many monosaccharides linked by glycosidic bonds.
- Primary examples include starch, glycogen, and cellulose.
- Main functions: energy storage and structural support in living organisms.
Starch: Structure and Function
- Starch exists in two forms: amylose (unbranched) and amylopectin (branched); both are made of alpha-glucose monomers.
- Amylose forms via 1,4 glycosidic bonds, leading to a coiled, helical structure.
- Amylopectin has both 1,4 and 1,6 glycosidic bonds, creating side branches.
- Starch stores glucose in plants; its insolubility and size prevent loss from cells.
- Amylose's compact coil allows dense energy storage; amylopectin's branches aid rapid glucose release.
Glycogen: Structure and Function
- Glycogen is the animal starch, also made of alpha-glucose monomers.
- Structure is highly branched due to 1,4 and 1,6 glycosidic bonds.
- Stores glucose in animals, with more branches for faster energy release due to higher activity levels.
Cellulose: Structure and Function
- Cellulose consists of beta-glucose monomers forming long, unbranched chains.
- Every second beta-glucose is flipped to enable 1,4 glycosidic bond formation.
- Chains are cross-linked by hydrogen bonds into strong bundles (microfibrils and macrofibrils).
- Provides structural support and rigidity in plant cell walls.
Comparison of Starch, Glycogen, and Cellulose
- Starch (amylose, amylopectin) and cellulose are found in plants; glycogen in animals.
- Starch and glycogen are made of alpha-glucose; cellulose of beta-glucose.
- Amylose: only 1,4 bonds, unbranched. Amylopectin/glycogen: 1,4 and 1,6 bonds, branched. Cellulose: 1,4 bonds, unbranched, with cross-links.
- In cellulose, adjacent beta-glucose units are inverted.
Key Terms & Definitions
- Polysaccharide — Large carbohydrate made of many monosaccharides linked by glycosidic bonds.
- Glycosidic bond — Covalent bond joining carbohydrate monomers.
- Amylose — Unbranched starch polymer with 1,4 glycosidic bonds.
- Amylopectin — Branched starch polymer with 1,4 and 1,6 bonds.
- Glycogen — Highly branched animal carbohydrate from alpha-glucose.
- Cellulose — Structural plant polysaccharide from beta-glucose, forming strong bundles.
- Microfibril — Bundle of cellulose chains held by hydrogen bonds.
Action Items / Next Steps
- Review differences between starch, glycogen, and cellulose structures.
- Practice identifying types of glycosidic bonds in each polysaccharide.
- Prepare for questions comparing functions and features of polysaccharides.