Overview
This lecture covers the structure, classification, and biological importance of carbohydrates, including their types and roles as energy sources and structural components.
Carbohydrates: Roles and Composition
- Carbohydrates are the most abundant biomolecules and provide energy and structural support in living organisms.
- They are composed of carbon, hydrogen, and oxygen in a 1:2:1 molar ratio.
Types of Carbohydrates
- Carbohydrates are classified into monosaccharides, disaccharides, oligosaccharides, and polysaccharides.
- "Saccharide" means sugar, originating from Greek.
Monosaccharides (Simple Sugars)
- Monosaccharides are the simplest carbohydrates and serve as building blocks.
- Key monosaccharides: glucose, galactose, and fructose (all with C, H, and O).
- Glucose has six carbon atoms and can exist as alpha or beta forms depending on the direction of the hydroxy group on carbon 1.
- Alpha and beta configurations are defined by the position of the hydroxy group relative to carbon 6.
Disaccharides
- Disaccharides are formed by linking two monosaccharides via glycosidic bonds in condensation reactions (water released).
- Hydrolysis is the reverse reaction, breaking bonds by adding water.
- Maltose: two alpha glucose units, linked by an alpha 1,4 bond.
- Lactose: galactose (beta) and glucose (beta), linked by a beta 1,4 bond; found in milk.
- Sucrose: alpha glucose and beta fructose, linked by a glucose alpha 1 to fructose beta 2 bond; found in plants.
Oligosaccharides
- Oligosaccharides consist of 2–20 monosaccharides.
- Maltotriose: three glucose units linked by alpha 1,4 bonds; can be broken by hydrolysis.
Polysaccharides
- Polysaccharides (glycans) are long chains of monosaccharides.
- Homopolysaccharide: composed of one type of monosaccharide (e.g., all glucose).
- Heteropolysaccharide: composed of two or more different monosaccharides.
- Polysaccharides can be branched or unbranched.
Important Homopolysaccharides
- Starch (plants): glucose storage, can be unbranched (amylose, alpha 1,4 bonds) or branched (amylopectin, alpha 1,4 and alpha 1,6 bonds).
- Glycogen (animals): glucose storage, more frequent branching (alpha 1,4 and alpha 1,6 bonds) than starch.
- Dextrans: found in bacteria and yeast, made of glucose with various alpha bonds (1,2; 1,3; 1,4; 1,6).
- Cellulose (plants): structural, made of beta glucose with beta 1,4 bonds; humans can’t digest due to lack of necessary enzymes.
Key Terms & Definitions
- Monosaccharide — Single sugar unit (e.g., glucose) that serves as a carbohydrate building block.
- Disaccharide — Carbohydrate formed by two monosaccharides linked together.
- Oligosaccharide — Short chain of 2–20 monosaccharides.
- Polysaccharide — Large chain of many monosaccharides (can be branched or unbranched).
- Glycosidic Bond — Covalent bond joining carbohydrate molecules (e.g., alpha 1,4 or beta 1,4).
- Condensation Reaction — Chemical reaction where two molecules join and release water.
- Hydrolysis — Reaction breaking bonds by adding water.
Action Items / Next Steps
- Review the structures and configurations (alpha vs. beta) of glucose, galactose, and fructose.
- Memorize the types and examples of disaccharides and polysaccharides.
- Study the key bonds (alpha 1,4, alpha 1,6, beta 1,4) and their biological significance.