Overview
This lecture explains how disaccharides are formed from monosaccharides, the type of bonds involved, and important related reactions and examples.
Monosaccharides & Isomers
- Glucose has two isomers: alpha and beta, differing in the position of the carbon one hydroxyl group.
- In alpha glucose, the carbon one hydroxyl points below the ring; in beta glucose, it points above.
Formation of Disaccharides
- Disaccharides are created when two monosaccharides chemically react together.
- Two alpha glucose molecules react to form the disaccharide maltose.
- The reaction produces a disaccharide and a water molecule.
- The water molecule is formed from a hydrogen atom on one monosaccharide and a hydroxyl group from the other.
- This process is called a condensation reaction.
Glycosidic Bonds
- A glycosidic bond forms between two monosaccharides during disaccharide formation.
- In maltose, the glycosidic bond is between carbon 1 of one glucose and carbon 4 of the other (a 1-4 glycosidic bond).
- Learning to recognize and draw this bond may be required in exams.
Hydrolysis Reaction
- Adding water to a disaccharide breaks the glycosidic bond (hydrolysis), converting it back to monosaccharides.
- Hydrolysis in cells is usually performed by enzymes.
Other Important Disaccharides
- Sucrose is formed from glucose and fructose.
- Lactose is formed from glucose and galactose.
Key Terms & Definitions
- Monosaccharide — single sugar molecule (e.g., glucose, fructose, galactose)
- Disaccharide — molecule formed by two monosaccharides joined together
- Condensation reaction — chemical reaction that forms a larger molecule with the release of water
- Glycosidic bond — the bond linking two monosaccharides in a disaccharide
- Hydrolysis — reaction where water breaks a bond, splitting a molecule
Action Items / Next Steps
- Learn to draw and label the formation of maltose and the 1-4 glycosidic bond.
- Memorize examples of disaccharides and their constituent monosaccharides.
- Next video: study the structure and formation of polysaccharides.