• Definition: The monomer of carbohydrates, containing carbon, hydrogen, and oxygen in a ratio of (CH₂O)n.

• Monosaccharides are soluble in water and serve as fundamental components in metabolism and biosynthesis.

• Triose sugars contain 3C, Pentose sugars contain 5C and Hexose sugars contain 6C.

• Examples:

  • Hexoses

    • α-glucose (used in energy storage and respiration)

    • β-glucose (used in structural molecules like cellulose)

    • Fructose (found in fruits, an alternative energy source)

    • Galactose (part of lactose, important in milk digestion)

  • Pentoses

    • Ribose (used to make nucleic acids: RNA)

    • Deoxyribose (used to make nucleic acids: DNA)

Structure of Glucose:

• • Both α- and β-glucose have six carbon atoms (hexose sugars).
  • α- and β-glucose are isomers of each other.
  • The carbons are numbered starting clockwise from the O in the ring.
  • The difference between α- and β-glucose is the position of the hydroxyl (-OH) group on carbon 1.

The hydroxyl group (OH group) is below carbon 1.

The hydroxyl group (OH group) is above carbon 1.

• You need to be able to recognise and draw both of these isomers.

• Definition: Carbohydrates formed when two monosaccharides join via a glycosidic bond.
• Examples:

  • Maltose = α-glucose + α-glucose (important in digestion and germination)

  • You need to be able to draw the structure of maltose.

• • Sucrose = glucose + fructose (common table sugar, transported in plants)
  • Lactose = glucose + galactose (found in milk, important for infant nutrition)
    
  • You do not need to be able to draw sucrose and lactose from memory, however, when given the monosaccharides you should be able to show how they join together in a condensation reaction to form a glycosidic bond and the subsequent disaccharides.

• Formation & Breakdown:
  • Condensation Reaction: Two monosaccharides join, forming a glycosidic bond, with water produced.
  • Hydrolysis Reaction: Water is added to break the glycosidic bond, separating the monosaccharides. Enzymes like maltase, sucrase, and lactase help catalyse these reactions in living organisms.

• The glycosidic bond can form on any of the carbons. At A-level the important ones are the 1,4-glycosidic bond (which forms between carbon 1 of one monosaccharide and carbon 4 of the next) and the 1,6-glycosidic bond (which forms between carbon 1 of one monosaccharide and carbon 6 of the next)
• The diagram above shows the formation of a 1-4 glycosidic bond. Many of these bonds will produce a straight chain of monosaccharides.