Galactose is a monosaccharide, a simple sugar, and is classified as a hexose because it has six carbon atoms. It is structurally similar to glucose, which shares the same molecular formula (C6H12O6). However, the detailed structure of galactose involves a different arrangement of functional groups.

Structural Formula

In its Haworth projection, which provides a three-dimensional view of the cyclic structure, people often depict the structural formula for galactose.

In this representation:

The horizontal lines represent bonds from the plane towards you (wedges).

The vertical lines represent bonds going into the plane away from you (dashes).

The “C” represents carbon atoms.

The “H” represents hydrogen atoms.

The “OH” represents hydroxyl groups.

Cyclic Structure

Galactose, like glucose and fructose, forms a six-membered ring structure. The hydroxyl group on carbon 4 (C4) and the aldehyde group on carbon 1 (C1) react to form a hemiacetal linkage, resulting in a six-membered oxygen-containing ring.

The orientation of the hydroxyl group at the anomeric carbon (C1) determines the configuration of galactose. In the galactose molecule, this hydroxyl group is positioned above the plane of the ring.

Fischer Projection

Using Fischer projection, one can also represent galactose.

Function

Galactose, which is less sweet than glucose and is not commonly found in free form in nature, constitutes a component of lactose, a disaccharide sugar found in milk. Lactose comprises one molecule of galactose and one molecule of glucose. In the body, the lactase enzyme breaks down lactose into its component monosaccharides, allowing for their absorption in the digestive system. Like other monosaccharides, galactose is a energy source and participates in various metabolic processes within cells.

These monosaccharides serve as fundamental building blocks for more complex carbohydrates and play essential roles in various biological processes, including energy metabolism.