Lactose is made-up of the following units:

Question

Accepted Answer

The correct option is 2:

Lactose, often called "milk sugar," is a vital and naturally occurring disaccharide found exclusively in the milk of mammals, making it a critical carbohydrate in infant and early childhood nutrition. A disaccharide is a carbohydrate formed when two simpler sugar molecules, known as monosaccharides, are chemically bonded together. The formation of lactose specifically involves a dehydration reaction between one molecule of the monosaccharide **Glucose** and one molecule of the monosaccharide **Galactose**. These two units are linked by a $\beta$-1,4-glycosidic bond. The chemical composition is crucial for understanding its digestion; the body requires the enzyme lactase to break this $\beta$-1,4 bond, releasing the individual glucose and galactose units for absorption. This definitive composition of Glucose + Galactose is unique to lactose, distinguishing it from other dietary disaccharides and confirming Option 2 as the correct structural unit of milk sugar.

The incorrect option is 1:

The combination of Glucose + Glucose does not form Lactose; instead, this specific linkage of two glucose molecules forms the disaccharide **Maltose**, which is commonly referred to as malt sugar. Maltose is a very important disaccharide in biology, primarily acting as an intermediate product during the digestion of large polysaccharides like starch. In the maltose structure, the two glucose units are typically joined by an $\alpha$-1,4-glycosidic bond, which allows it to be easily broken down by the enzyme maltase. While glucose is a required component of lactose, bonding two molecules of glucose together creates a completely different sugar with distinct properties, sources, and biological roles, such as its presence in grains and the malting process. Therefore, selecting Glucose + Glucose would incorrectly identify the composition of malt sugar, not the milk sugar, lactose.

The incorrect option is 3:

The combination of Fructose + Galactose does not result in any of the three major, common dietary disaccharides that you should know: sucrose, lactose, or maltose. It is a non-standard, theoretical sugar. For context, the combination of Fructose and **Glucose** forms the disaccharide **Sucrose**, which is ordinary table sugar and is sourced from sugar cane or sugar beets. Galactose is an aldohexose that is structurally similar to glucose, and while it is part of lactose, its combination with fructose is not naturally or physiologically significant in the same way as lactose. This option incorrectly substitutes the critical glucose unit of lactose with fructose, which would create a compound with different chemical properties and metabolic pathways altogether. Thus, Fructose + Galactose is chemically incorrect for the structure of milk sugar.

The incorrect option is 4:

The pairing of Galactose + Galactose is also not the correct building block for lactose, nor does it form a common, naturally abundant disaccharide in the human diet. Lactose, as we established, is a heterodimer, which means it is necessarily made of two **different** types of monosaccharide units (glucose and galactose). If two identical monosaccharides were to combine, like two glucose units to form maltose, it would be called a homodimer. Two identical units of galactose would form a theoretical sugar called a digalactose, which is neither naturally occurring nor functional as milk sugar in the body. Therefore, to form lactose, the structure strictly requires the presence of one glucose unit along with one galactose unit, making this option definitively incorrect.