Lecture: Carbohydrates in Biochemistry

Introduction

• Carbohydrates: Most abundant biomolecules on Earth.
  • Found everywhere.
• Functions:
  • Major source of energy for living organisms (animals and plants).
  • Structural components (e.g., DNA contains ribose, plant cell walls consist of cellulose).
• Composition:
  • Contain carbon, hydrogen, and oxygen (C:H:O in a 1:2:1 ratio).

Types of Carbohydrates

1. Monosaccharides
2. Disaccharides
3. Oligosaccharides
4. Polysaccharides

• Saccharide: Derived from Greek for sugar.

Monosaccharides

• Also known as simple sugars.
• Smallest units of carbohydrates, building blocks.
• Main Monosaccharides in Human Diet:
  • Glucose: Main energy source, contains six carbon atoms.
    • Alpha and Beta Glucose:
      • Alpha Glucose: Hydroxy group on carbon 1 opposite to carbon 6.
      • Beta Glucose: Hydroxy group on carbon 1 same direction as carbon 6.
  • Galactose: Similar structural variations.
  • Fructose: Often in beta configuration.

Disaccharides

• Formed by linking two monosaccharides.
• Examples:
  • Maltose: Two glucose molecules linked by alpha 1-4 glycosidic bond.
    • Formed via condensation (water released).
    • Broken by hydrolysis (water added).
  • Lactose: Galactose and glucose linked by beta 1-4 glycosidic bond.
    • Found in milk.
  • Sucrose: Glucose and fructose linked (glucose alpha 1 and fructose beta 2).
    • Table sugar, not formed by animals.

Oligosaccharides

• Consist of short chains of monosaccharides (<20).
• Disaccharides are a type of oligosaccharide.
• Example:
  • Maltotriose: Glucose linked by alpha 1-4 glycosidic bonds.
• Oligosaccharides grow to polysaccharides when exceeding 20 monosaccharides.

Polysaccharides

• Known as glycans.
• Types:
  • Homopolysaccharide: Single type of monosaccharide.
  • Heteropolysaccharide: Two or more different monosaccharides.
  • Can be branched or unbranched.

Homopolysaccharides

• Serve as storage forms of monosaccharides.
• Examples:
  • Starch: Storage in plants; main carbohydrate in human diet.
    • Amylose: Unbranched.
    • Amylopectin: Branched.
  • Glycogen: Storage in animals; similar to starch but more frequent branching.
  • Dextrans: Structural in bacteria and yeast.
  • Cellulose: Structural in plants; unbranched beta-glucose chains.
    • Humans cannot digest cellulose due to lack of enzyme for beta 1-4 glycosidic bonds.

Key Points

• Starch vs. Glycogen:
  • Both can be branched or unbranched.
  • Glycogen has more frequent branching compared to starch.
• Cellulose:
  • Strong structure due to hydrogen bonding between chains.
  • Important for plant structure.

This summary provides an overview of the types, structures, and roles of carbohydrates in biological systems as discussed in the lecture.