A-Level Biology: Disaccharides with Mr. Egg

Introduction

• Topic: Disaccharides (2nd lesson in Biological Molecules series)
• Activity: Mini quiz to recall previous lesson (monosaccharides)

Mini Quiz Recap (Previous Lesson)

• Draw Alpha Glucose: Key difference - hydroxyl group on carbon 1 is on top.
• Three Monosaccharides: Glucose, Fructose, Galactose.
• Definitions:
  • Monomer: Smaller unit forming larger molecules.
  • Polymer: Made of monomers bonded together.

Disaccharides

• Definition: Two monosaccharides bonded together by a glycosidic bond.
• Formation Reaction: Condensation reaction (joins molecules, releases water).

Important Disaccharides

• Maltose: Glucose + Glucose
• Lactose: Glucose + Galactose
• Sucrose: Glucose + Fructose
• Commonality: All release a molecule of water during formation.

Chemical Reactions

• Condensation Reaction

  • Joins two molecules by removing water.
  • Example: Involves hydroxyl groups on carbon 1 and 4.
  • Forms a glycosidic bond (e.g., 1 to 4 linkage).

• Hydrolysis

  • Reverse of condensation.
  • Splits molecules by adding water.
  • Mnemonic: Hydro (water) + Lysis (split apart).

Application and Challenge Questions

• Processes:

  • Digestion: Hydrolyzes polymers (e.g., starch) into monomers (e.g., glucose).
  • Glycogenolysis: Splitting glycogen into glucose (occurs when blood sugar drops).

• Catalysts:

  • Enzymes are biological catalysts driving hydrolysis reactions.

Summary

• Disaccharide: Two monosaccharides bonded by a glycosidic bond.
• Main Disaccharides: Maltose, Sucrose, Lactose.
• Reactions:
  • Condensation: Joins molecules, removes water.
  • Hydrolysis: Splits molecules, adds water.

Next Steps

• Review monosaccharide video if needed.
• Proceed to polysaccharides or practice/test stages.