Disaccharides

Disaccharides

Definitions and General Information

• Disaccharides: Composed of two monosaccharide units joined by a carbon–oxygen-carbon linkage known as a glycosidic bond.

  • Glycosidic Bond: A type of covalent bond that connects two monosaccharides.

• Formation: Disaccharides are formed through a condensation reaction that combines two monosaccharides into a single molecule with the loss of water.

  • Condensation Reaction: A chemical reaction where two molecules combine to form one molecule while losing water.

Structure and Characteristics

• Disaccharides differ in their monosaccharide constituents and in the specific location and direction of the bonds connecting them.

  • Types of Bonds:

  • α (alpha) Bonds: Face “down.”

  • β (beta) Bonds: Face “up.”

• Common disaccharides include:

  • Maltose

  • Lactose

  • Sucrose

  • All three are white crystalline solids at room temperature and soluble in water.

Individual Disaccharides

Maltose

Structure:

• Composed of two glucose units joined in a head-to-tail manner through a 1,4 linkage.

• Specifically, the bond connects carbon 1 of one glucose molecule to carbon 4 of the second glucose molecule with the bond oriented down (α) from carbon 1 and 4.

• Function:

  • Maltose serves as an intermediate in the formation and digestion of larger polysaccharides starch and glycogen.

  • Formed during digestion by the hydrolysis of starch and glycogen.

  • Also found in sprouting grains (grains germinating into plants).

Sucrose

• Structure:

  • Contains an α-1,β-2 glycosidic bond between glucose and fructose.

  • Features a downward-facing bond off of carbon 1 of glucose and an upward-facing bond off of carbon 2 of fructose.

• Function:

  • Commonly known as sugar, sucrose serves as a primary energy storage in plants (produced in roots, fruits, and nectars).

  • Consumed by many mammals, birds, insects, and bacteria, and is a significant food source for some.

• Commercial Production:

  • Most sucrose sold commercially is derived from sugar cane and sugar beets, containing 14%-20% sucrose in their juices.

  • The production process includes crushing the cane/beets, washing raw sugar crystals, dissolving it in sugar syrup, filtering, carbon treatment, concentrating, boiling under vacuum, and crystallization to produce pure sucrose (clear, odorless, sweet).

Lactose

• Structure:

  • Composed of galactose and glucose with a β-1,4 glycosidic bond between them.

  • The bond faces upwards between carbons 1 of galactose and 4 of glucose.

• Function:

  • Known as milk sugar, lactose occurs in the milk of mammals and is synthesised naturally in mammary tissue.

Digestion of Disaccharides

• The human body must hydrolyze any consumed disaccharides into their monosaccharide units.

  • Enzymatic Hydrolysis: The reaction is catalyzed by enzymes such as maltase, sucrase, and lactase.

• Lactose Intolerance:

  • Many adults and some children suffer from a lactase deficiency, leading to the inability to digest lactose. These individuals are termed lactose intolerant.

Conclusion

• Disaccharides play crucial roles in biological systems, serving as energy sources and structural units while participating in various metabolic processes throughout living organisms.