Lactose is made up of:

Question

Accepted Answer

The correct option is (3):
Lactose is commonly known as "milk sugar" and is a primary carbohydrate found naturally in the milk of nearly all mammals. As a disaccharide, it is fundamentally composed of two simpler sugar units, or monosaccharides, joined together: one molecule of **Glucose** and one molecule of **Galactose**. These two monosaccharides are covalently linked by a specific chemical bond known as a $\beta$-1,4 glycosidic bond. This unique pairing and bond structure are why the digestive enzyme **lactase** is specifically required to break down lactose in the small intestine, allowing the resulting glucose and galactose to be absorbed and used for energy. This composition is essential to the definition and biological role of milk sugar.

The incorrect option is (1):
Glucose + Fructose represents the exact chemical composition of **Sucrose**, which is commonly referred to as table sugar, cane sugar, or beet sugar. This disaccharide is structurally distinct from lactose. In sucrose, glucose and fructose are joined by an $\alpha$-1, $\beta$-2 glycosidic bond, not the $\beta$-1,4 bond found in lactose. Sucrose is typically sourced from plants like sugarcane and sugar beets, and it does not occur naturally in milk. Therefore, while sucrose is a major dietary disaccharide, its molecular composition is different from that of lactose.

The incorrect option is (2):
Glucose + Glucose is the chemical composition of **Maltose**, often colloquially called malt sugar. Maltose is primarily known as a degradation product of starch, such as occurs in the malting of grains for brewing. This disaccharide is formed by two identical glucose molecules linked by an $\alpha$-1,4 glycosidic bond. Because its structure involves two glucose units, it is fundamentally different from the structure of lactose, which is an asymmetric pairing of glucose and galactose.

The incorrect option is (4):
Fructose + Fructose does not form a major, naturally occurring disaccharide that is commonly found in food or possesses a recognized biological role comparable to lactose, sucrose, or maltose. Fructose is a monosaccharide found in high concentrations in fruits and honey, but it generally exists in solution as a single unit or as part of the sucrose molecule. A stable disaccharide composed solely of two fructose molecules is not recognized as a principal carbohydrate in the context of physiological and nutritional science.