MCAT Content / Carbohydrates / 1d Disaccharides

Disaccharides

Topic: Carbohydrates

Disaccharides are sugars composed of two monosaccharide units that are joined by glycosidic linkages and have the chemical formula C12H22O11.

Disaccharides form when two monosaccharides undergo a dehydration reaction. During this process, the hydroxyl group of one monosaccharide combines with the hydrogen of another monosaccharide, releasing a molecule of water and forming a covalent bond. A covalent bond formed between a carbohydrate molecule and another molecule (in this case, between two monosaccharides) is known as a glycosidic bond. In some cases, it’s important to know which carbons on the two sugar rings are connected by a glycosidic bond. Each carbon atom in a monosaccharide is given a number, starting with the terminal carbon closest to the carbonyl group (when the sugar is in its linear form). This numbering is shown for glucose and fructose, above. In a sucrose molecule, the 1 carbon of glucose is connected to the 2 carbon of fructose, so this bond is called a 1-2 glycosidic linkage. Glycosidic bonds are also categorized based on whether the monosaccharide is in the α form or β form.

Common disaccharides include maltose, lactose, and sucrose, as shown below. These disaccharides differ from one another in their monosaccharide constituents and the specific type of glycosidic linkages connecting them. Maltose, or malt sugar, is a reducing sugar joined in an α-1-4-glycosidic linkage, meaning that an α-linkage is made between the first carbon atom of one glucose molecule to the fourth carbon atom of the second glucose molecule. Lactose is a reducing sugar made from β-galactose and α/β-glucose joined by a β-1-4-glycosidic bond. It is found naturally in milk. Sucrose, or table sugar, is a nonreducing sugar made from α-glucose and β-fructose joined at the hydroxyl groups on the anomeric carbons. It is unique among the common disaccharides in having an α-1,β-2-glycosidic linkage.


Key Points

• Disaccharides form when two monosaccharides undergo a dehydration reaction to form a covalent bond known as a glycosidic bond.

• Maltose is composed of two molecules of glucose joined by an α-1,4-glycosidic linkage. It is a reducing sugar that is found in sprouting grain.

• Lactose is composed of a molecule of galactose joined to a molecule of glucose by a β-1,4-glycosidic linkage. It is a reducing sugar that is found in milk.

• Sucrose is composed of a molecule of glucose joined to a molecule of fructose by an α-1,β-2-glycosidic linkage. It is a nonreducing sugar that is found in sugar cane and sugar beets.


Key Terms

Disaccharides: A carbohydrate consisting of two monosaccharides connected to one another through a glycosidic linkage.

Monosaccharides: The basic unit of carbohydrates that cannot be hydrolyzed to simpler chemical compounds with the general formula (CH2O)n.

Glycosidic bonds (linkages): A type of covalent bond that joins two carbohydrate molecules.

Dehydration reaction: A chemical reaction in which two molecules are covalently linked in a reaction that generates a water molecule as a second product.

α form: The configuration of a cyclic monosaccharide where the oxygen attached to the anomeric carbon is on the opposite side of the ring from the CH₂OH group.

β form: The configuration of a cyclic monosaccharide where the oxygen attached to the anomeric carbon is on the same side of the ring as the CH₂OH group.

Maltose: A disaccharide composed of two glucose molecules connected through an α-1-4-glycosidic linkage.

Lactose: A disaccharide composed of β-galactose and α/β-glucose connected through a β-1-4-glycosidic bond.

Sucrose: A disaccharide composed of α-glucose and β-fructose connected through an α-1,β-2-glycosidic linkage.

α-1-4 glycosidic linkages: Linkages in which carbon atoms 1 and 4 of the two monomers form a glycosidic bond.

β-1-4 glycosidic bonds: Linkages in which carbon atoms 1 and 4 of two glucose monomers in their β form connect through a glycosidic bond.

α-1,β-2-glycosidic linkage: Linkages in which carbon atoms 1 and 2 of an α-glucose and β-fructose connect through a glycosidic bond.



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