Carbohydrates are major biological macromolecules, serving primarily as the body’s preferred source of energy. These compounds are chemically defined as saccharides, or sugars, and are categorized by the number of basic sugar units they contain. Disaccharides are a fundamental type of sugar composed of two units.
Defining the Two-Sugar Structure
The term “disaccharide” literally translates to “two sugars.” A disaccharide is created when two smaller, single sugar molecules, known as monosaccharides, chemically link together. Monosaccharides, such as glucose, are the building blocks for all more complex carbohydrates.
The two monosaccharide units are joined by a strong covalent bond called a glycosidic linkage. This bond forms between the hydroxyl (OH) group of one sugar unit and the hydroxyl group of the second. The chemical formula for most common disaccharides is C12H22O11, indicating they are formed from two six-carbon sugars with the loss of one water molecule.
The specific atoms involved determine the type of disaccharide formed. For instance, a bond might form between the first carbon atom of one monosaccharide and the fourth carbon atom of the second sugar, creating a 1,4-glycosidic linkage. The orientation of the bond (alpha or beta) introduces structural differences, which affects how the body’s enzymes can break the bond down.
Formation and Breakdown
Disaccharides are formed through dehydration synthesis, or a condensation reaction. During this process, two monosaccharide molecules bond covalently, and a molecule of water (H2O) is removed, forming the glycosidic linkage.
The reverse process, called hydrolysis, is how the body digests these sugars. Hydrolysis means “splitting by water.” The digestive system uses specific enzymes, known as disaccharidases, to break the glycosidic bond.
To break the bond, a molecule of water is added back into the disaccharide. This restores the two original, single sugar units, which are then absorbed through the small intestine wall and into the bloodstream.
Common Types and Food Sources
The three most common disaccharides in the human diet are sucrose, lactose, and maltose, each composed of a unique pairing of monosaccharides.
Sucrose
Sucrose, commonly known as table sugar, is the most frequently consumed disaccharide. It is composed of one glucose molecule and one fructose molecule. It is found naturally in high concentrations in sugar cane, sugar beets, maple syrup, and many fruits and vegetables.
Lactose
Lactose is often called milk sugar because it is the primary carbohydrate found in milk and dairy products. This disaccharide is formed by the bonding of one glucose molecule and one galactose molecule. An inability to break down this specific bond due to a lack of the enzyme lactase causes lactose intolerance.
Maltose
Maltose, or malt sugar, consists of two glucose molecules joined together. Although it exists in nature in small amounts, it is most often created when starches are partially broken down during digestion or food preparation. Maltose is found in foods like beer, cereals, malted beverages, and cooked sweet potatoes.