Disaccharides — Maltose, Sucrose, Lactose and Glycosidic Bonds

When two monosaccharides join together, they form a disaccharide. The covalent bond between them is called a glycosidic bond, and its formation releases a molecule of water. This lesson covers OCR specification point 2.1.2 (b)(iii): the synthesis and breakdown of disaccharides, and the formation of glycosidic bonds.

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1. The Glycosidic Bond

A glycosidic bond is the covalent bond formed between two monosaccharide units in a condensation reaction. When the bond forms between C1 of one monosaccharide and C4 of the next, it is called a 1,4-glycosidic bond. Bonds can also form between other carbons (e.g., 1,6 in branching points of amylopectin and glycogen, 1,2 in sucrose).

Key Definition — Condensation reaction: A reaction in which two molecules are joined by the formation of a new covalent bond, with the elimination of a water molecule.

Key Definition — Hydrolysis reaction: A reaction in which a covalent bond is broken by the addition of a water molecule, splitting one molecule into two.

1.1 Condensation: Forming a Glycosidic Bond

Glucose + Glucose → Maltose + H₂O

   HO-C1 ...  +  HO-C4 ...   →   ... C1-O-C4 ...  +  H₂O

The hydroxyl group (–OH) on C1 of one glucose reacts with the hydroxyl group on C4 of another glucose. One –OH provides the oxygen that bridges the two rings; the other –OH (and an H from the first) leave as water.

1.2 Hydrolysis: Breaking a Glycosidic Bond

Maltose + H₂O → Glucose + Glucose

Hydrolysis is the reverse of condensation. A water molecule is added across the glycosidic bond: the –OH is added to one monosaccharide and the –H to the other. Hydrolysis reactions are catalysed by specific enzymes (e.g., maltase, sucrase, lactase).

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2. The Three Most Important Disaccharides

2.1 Maltose — α-glucose + α-glucose

α-glucose — 1,4-glycosidic bond — α-glucose

• Formed by a 1,4-glycosidic bond between two α-glucose molecules.
• Found in germinating seeds (e.g., barley), produced by the hydrolysis of starch by the enzyme amylase.
• A reducing sugar — the C1 of the second glucose remains free and can open to reveal an aldehyde group.
• Hydrolysed by the enzyme maltase to yield two α-glucose units.

2.2 Sucrose — α-glucose + fructose

α-glucose — 1,2-glycosidic bond — fructose

• Formed by a 1,2-glycosidic bond between α-glucose and fructose.
• The main transport sugar in the phloem of plants — it is soluble, chemically less reactive than glucose, and osmotically less active than an equivalent mass of separate hexoses.
• Extracted commercially from sugar cane and sugar beet; the familiar "table sugar".
• A non-reducing sugar — both anomeric carbons (C1 of glucose and C2 of fructose) are locked up in the glycosidic bond, so neither monosaccharide can open to reveal a reducing group.
• Hydrolysed by the enzyme sucrase (also called invertase) to yield glucose and fructose.

2.3 Lactose — β-galactose + α-glucose

β-galactose — 1,4-glycosidic bond — α-glucose

• Formed by a 1,4-glycosidic bond between β-galactose and α-glucose.
• The principal sugar in mammalian milk; provides energy for young mammals.
• A reducing sugar — the anomeric carbon of the glucose remains free.
• Hydrolysed by the enzyme lactase in the small intestine. Adults with low lactase activity suffer from lactose intolerance, as undigested lactose is fermented by gut bacteria producing gas and osmotic discomfort.

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3. Summary Table

Disaccharide	Monomers	Bond	Reducing?	Enzyme	Biological Role
Maltose	α-glucose + α-glucose	1,4-glycosidic	Yes	Maltase	Product of starch digestion; germinating seeds
Sucrose	α-glucose + fructose	1,2-glycosidic	No	Sucrase	Main transport sugar in phloem
Lactose	β-galactose + α-glucose	1,4-glycosidic	Yes	Lactase	Energy source for young mammals

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4. Reducing Sugars — A Closer Look

A reducing sugar is one that can donate electrons to (reduce) another chemical. In the Benedict's test, reducing sugars reduce blue Cu²⁺ ions (copper(II) sulfate) to brick-red Cu⁺ ions (copper(I) oxide precipitate).