Disaccharides in Chemistry

Disaccharides

• Disaccharides are formed by joining two monosaccharides with a glycosidic linkage.
• Examples of disaccharides include:
  • Maltose (malt sugar) = glucose + glucose
  • Sucrose (table sugar) = glucose + fructose
  • Lactose (milk sugar) = glucose + galactose
• Sucrose has no reducing properties due to the lack of a free anomeric carbon.

Reducing Sugars

• Reducing sugars are capable of reducing Benedict's solution, which is an alkaline cupric solution.
• A sugar is a reducing sugar if it possesses a free anomeric carbon.
• Examples of reducing sugars include:
  • All monosaccharides
  • Disaccharides: maltose, lactose
• Examples of non-reducing sugars include:
  • Sucrose

Oligosaccharides

• Oligosaccharides are saccharide polymers containing a small number (typically three to ten) of simple sugars (monosaccharides).
• Examples of oligosaccharides include:
  • Maltotriose (3 glucose units, α-1,4 linkage)
  • Dextrin (6-8 glucose units)
• Oligosaccharides also occur in glycoproteins and glycolipids, which are proteins and lipids to which oligosaccharides are covalently attached.

Polysaccharides

• Polysaccharides are high molecular weight polymers composed of more than 10 monosaccharide units linked together.
• Polysaccharides are insoluble in water.
• Polysaccharides can be polymers of hexose or pentose sugars.
• Homopolysaccharides are composed of a single monosaccharide building block, while heteropolysaccharides are composed of more than one type of monosaccharide.
• Polysaccharides can be linear or branched polymers.
• Examples of homopolysaccharides include:
  • Starch (storage form of glucose in plants)
  • Glycogen (storage form of glucose in animals)
  • Cellulose (structural element of plants)
  • Chitin (structural element in the exoskeletons of insects)
  • Inulin (found in roots of some plants)

Starch

• Starch is a storage polysaccharide in plants composed of alpha glucose.
• Its structure is identical to glycogen, except for a much lower degree of branching (every 20-30 residues).
• Starch consists of two types of molecules:
  • Unbranched chains called amylose (15-20%)
  • Branched chains called amylopectin (80-85%)
• Amylose is a continuous, unbranched chain of glucose units linked by α-1,4 glycosidic bonds.
• Amylopectin is a branched-chain polysaccharide with α-1,4 glycosidic bonds between the glucose units and α-1,6 bonds to branches.

Glycogen

• Glycogen is the storage form of glucose in animals.
• It is stored in the liver and muscle.
• Glycogen is a homopolymer of glucose in α-1,4 linkage.
• It is highly branched, with α-1,6 branch linkages occurring every 8-11 residues.
• Glycogen has the same overall structure as amylopectin but with more branching in the molecule.
• Glycogen is a very compact structure that results from the coiling of the polymer chains.

Structural Polysaccharides

• Cellulose is a structural polysaccharide in plants composed of beta glucose units.
• It is a major constituent of plant cell walls and provides structure and support.
• Cellulose is indigestible to humans because it cannot be broken down by human enzymes.
• Chitin is a structural polysaccharide in the exoskeletons of insects, providing protection and support.
• It is a polymer of N-acetylglucosamine units linked by β-1,4 glycosidic bonds.
• Inulin is a structural polysaccharide found in the roots of some plants.

Learn about disaccharides, a type of carbohydrate formed by two simple sugars joined together. This quiz covers maltose, sucrose, and lactose, including their composition and properties.