2.9 Properties and Roles of Disaccharides

A disaccharide is a type of carbohydrate formed when two monosaccharides are joined together. Disaccharides are a specific kind of oligosaccharide.

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General Properties

• Formed by the combination of two monosaccharides through a condensation reaction.
• Less sweet in taste compared to monosaccharides.
• Comparatively less soluble in water.
• Can be hydrolyzed (broken down with water) to yield their constituent monosaccharides.
• Serve as a familiar energy source for living organisms.
• The general chemical formula is ${\mathrm{C}}_{12}{\mathrm{H}}_{22}{\mathrm{O}}_{11}$.

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Types of Disaccharides

Sucrose

• Common Name: Cane sugar.
• Primary Use: Widely used as a sweetener in homes.
• Role in Plants: Known as the transport disaccharide. Food prepared by plants (via photosynthesis) is transported throughout the plant in the form of sucrose.
  • Properties for Transport: It is very soluble, allowing it to be moved efficiently in high concentrations, and it is relatively unreactive chemically.
• Hydrolysis: Upon hydrolysis, sucrose yields one molecule of α-glucose and one molecule of β-fructose.
• Reducing Nature: It is a non-reducing sugar because the glycosidic bond involves the anomeric carbons of both glucose and fructose.

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Maltose

• Common Name: Malt sugar.
• Formation: It is an intermediate product formed during the breakdown of complex carbohydrates like starch and glycogen.
• Role: Acts as an energy-producing molecule.
• Specific Linkage: It consists of two glucose units joined by an $\alpha (1\to 4)$ glycosidic linkage.
• Sources:
  • Industrial: Produced in the brewing industry through the malting process, where the enzyme amylase breaks down barley starch.
  • Natural: Found in germinating seeds and sprouting potatoes, where it serves as an energy source for the growth of young seedlings.

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Lactose

• Common Name: Milk sugar.
• Source: Found in the milk of mammals. The concentration varies between species.

• Role:
  • Serves as a direct energy source for infants.
  • Acts as a substrate for the synthesis of essential macro-biomolecules involved in vital nervous and immunological processes.
• Industrial Byproduct: It is produced as a byproduct during the cheese-making process.

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Possible Questions and Answers

• Q: Why do potatoes taste sweet when stored for a long time?

  A: During long-term storage, especially as they begin to sprout, the complex carbohydrate starch (which is not sweet) within the potato is broken down by enzymes into maltose (malt sugar), which is sweet. This maltose provides energy for the growing sprout.

• Q: How do initially tasteless or sour young fruits become sweet when they ripen?

  A: Young fruits store energy as complex, non-sweet carbohydrates like starch or acids. The ripening process involves enzymes that break down these storage molecules into simple, sweet-tasting sugars like sucrose and fructose.

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Biological Significance

Disaccharides are crucial for energy metabolism and transport. Lactose is vital for neonatal nutrition, while sucrose is the primary form of carbohydrate transport in plants, and maltose is a key intermediate in the digestion of starch.