Overview
This lecture covers the types, structures, and roles of carbohydrates in living organisms, emphasizing their energy, structural, and storage functions.
Introduction to Carbohydrates
- Carbohydrates are the most abundant biomolecules on Earth and are a major energy source for animals and plants.
- Carbohydrates also function as structural components, e.g., ribose in DNA and cellulose in plant cell walls.
- Basic formula of carbohydrates: carbon, hydrogen, and oxygen in a 1:2:1 molar ratio.
Types of Carbohydrates
- Carbohydrates are classified into monosaccharides, disaccharides, oligosaccharides, and polysaccharides.
- The term "saccharide" comes from the Greek word for sugar.
Monosaccharides
- Monosaccharides are simple sugars and the building blocks of carbohydrates.
- Main dietary monosaccharides: glucose, galactose, and fructose.
- Glucose has six carbons and exists in alpha (opposite OH and C6) and beta (same direction OH and C6) forms.
- Alpha and beta configurations also apply to galactose and fructose.
Disaccharides
- Disaccharides consist of two monosaccharides linked by glycosidic bonds.
- Maltose: two alpha-glucose molecules, linked by alpha 1,4 glycosidic bond.
- Lactose: galactose and glucose, linked by beta 1,4 glycosidic bond; found in milk.
- Sucrose: glucose (alpha) and fructose (beta), linked by a bond between glucose C1 and fructose C2; found in plants.
Oligosaccharides
- Oligosaccharides are short chains of monosaccharides, usually fewer than 20 units.
- Example: maltotriose, made from three glucose units linked by alpha 1,4 glycosidic bonds.
Polysaccharides
- Polysaccharides (glycans) are long chains of monosaccharides; can be homopolysaccharides (one type) or heteropolysaccharides (multiple types).
- Polysaccharides can be branched or unbranched.
- Starch (plants): storage form of glucose, exists as amylose (unbranched, alpha 1,4 bonds) and amylopectin (branched, alpha 1,4 and alpha 1,6 bonds).
- Glycogen (animals): similar to starch but more highly branched (branching every 8-12 glucose units).
- Dextrans: structural polysaccharides in bacteria/yeast, with various alpha glycosidic bonds.
- Cellulose: unbranched, beta glucose polymer in plants; linked by beta 1,4 bonds, not digestible by humans.
Key Terms & Definitions
- Monosaccharide — Single sugar molecule (e.g., glucose, fructose).
- Disaccharide — Two monosaccharides joined by a glycosidic bond.
- Oligosaccharide — Short chain of monosaccharides (less than 20).
- Polysaccharide — Long chain of monosaccharides (more than 20).
- Glycosidic bond — Linkage between carbohydrate molecules.
- Alpha/Beta configuration — Orientation of the hydroxy group at carbon 1 relative to carbon 6.
Action Items / Next Steps
- Review the structures and differences between alpha and beta configurations in glucose.
- Memorize examples of each carbohydrate type and their biological roles.