Overview
This lecture covers the structure, types, and biological importance of carbohydrates, focusing on their classification and key examples relevant to biochemistry.
Introduction to Carbohydrates
- Carbohydrates are the most abundant biomolecules and are essential for energy and structure in organisms.
- They are primarily composed of carbon, hydrogen, and oxygen in a 1:2:1 ratio.
- Carbohydrates are major energy sources and structural components (e.g., cellulose in plants, ribose in DNA).
Classification of Carbohydrates
- Carbohydrates are classified into four types: monosaccharides, disaccharides, oligosaccharides, and polysaccharides.
- The term “saccharide” refers to sugar.
Monosaccharides (Simple Sugars)
- Monosaccharides are the smallest carbohydrate units and building blocks for other types.
- Major dietary monosaccharides: glucose, galactose, and fructose.
- Glucose contains six carbon atoms; alpha and beta forms differ by the orientation of the hydroxy group on carbon 1.
Disaccharides
- Disaccharides are formed by linking two monosaccharides through a condensation reaction (releasing water).
- Maltose: two alpha-glucose molecules linked by an alpha 1,4-glycosidic bond.
- Lactose: galactose and glucose linked by a beta 1,4-glycosidic bond; found in milk.
- Sucrose: glucose (alpha) and fructose (beta) linked; main plant sugar (table sugar).
Oligosaccharides
- Oligosaccharides are short chains of less than 20 monosaccharides.
- Maltotriose: three glucose units linked by alpha 1,4-glycosidic bonds.
- Additional glucose units can extend oligosaccharides.
Polysaccharides (Glycans)
- Polysaccharides are long chains of monosaccharides (over 20 units), can be branched or unbranched.
- Homopolysaccharides have one type of monosaccharide; heteropolysaccharides have multiple types.
- Starch: plant energy storage, made of glucose; unbranched (amylose) or branched (amylopectin).
- Glycogen: animal energy storage; more frequent branches than starch.
- Dextrans: structural polysaccharides in bacteria and yeast with various branching patterns.
- Cellulose: unbranched beta-glucose chains in plants; humans cannot digest due to beta 1,4-glycosidic bonds.
Key Terms & Definitions
- Monosaccharide — Basic single sugar unit, e.g., glucose.
- Disaccharide — Two linked monosaccharides, e.g., maltose, lactose, sucrose.
- Oligosaccharide — Short chain of monosaccharides (less than 20).
- Polysaccharide — Long chain of monosaccharides, often with branches.
- Glycosidic Bond — Linkage between sugar molecules (alpha or beta orientation).
- Condensation Reaction — Process forming a bond with the release of water.
- Hydrolysis — Breaking a bond by adding water.
- Homopolysaccharide — Polysaccharide with identical monosaccharide units.
- Heteropolysaccharide — Polysaccharide with different types of monosaccharides.
Action Items / Next Steps
- Review structural differences between alpha and beta glycosidic bonds.
- Practice drawing and identifying glucose, galactose, and fructose structures.
- Study the branching patterns in starch and glycogen.