Difference Between Glucose and Galactose

Main Difference – Glucose vs Galactose

Carbohydrates are the major components of all living organisms. Glucose and galactose are carbohydrates. Monosaccharides are the building blocks of larger carbohydrate molecules. The main difference between Glucose and Galactose is the position of each hydroxyl group in the 4^th carbon; the position of hydroxyl group (-OH) in the 4^th carbon is horizontal in the chair confirmation of the Glucose whereas the position of hydroxyl group (-OH) in the 4^th carbon is upward in the chair confirmation of the Galactose.

Key Areas Covered

1. What is Glucose
      – Definition, Molecular Formula, Chemical Structure, Properties
2. What is Galactose
      – Definition, Molecular Formula, Properties
3. What are the Similarities Between Glucose and Galactose
      – Outline of Common Features
4. What is the Difference Between Glucose and Galactose
      – Comparison of Key Differences

Key Terms: Aldohexose, Carbohydrates, D-Glucose, Galactose, Glucose, Hexose, Molecular Formula, Monosaccharides

What is Glucose

Glucose is a simple sugar. It is a monosaccharide. The molecular formula of Glucose is C₆H₁₂O₆. Its chemical structure has four –OH groups along with a -CH₂OH group. Since it has six –OH groups, glucose is called a Hexose. In the Fischer projection, glucose has a terminal aldehyde group. Therefore, glucose is called an aldohexose.

Difference Between Glucose and Galactose

Figure 1: The chair confirmation of D-Glucose

The above image shows the general structure of glucose. This image indicates that glucose has four chiral centers. (A chiral center is a carbon atom having four different groups attached to it.) Therefore, glucose has stereoisomers.

The naturally occurring glucose is known as D-Glucose. The isomer of D-Glucose is L-Glucose. But the L-Glucose does not occur naturally. Glucose is water soluble and is colorless when dissolved in water. Glucose can occur as an open chain or as a cyclic structure. But the open chain is typically unstable and spontaneously converts into cyclic form. The melting point of glucose is about 146⁰C (for alpha-D-Glucose).

Aqueous solutions of glucose are able to rotate plane polarized light. D-Glucose rotates light clockwise and L-Glucose rotates light anticlockwise. Glucose act as a monomer for the formation of a number of important polysaccharides. Some polymers that have glucose as the monomer include starch, cellulose, and glycogen.

What is Galactose

Galactose is a simple sugar and a monosaccharide. The molecular formula of galactose is also C₆H₁₂O₆. Its chemical structure has four –OH groups along with a -CH₂OH group. Same as glucose, galactose is also an aldohexose due to the presence of six carbon atoms and a terminal aldehyde group.

Figure 2: Fischer Projection of D-Galactose

Galactose can exist either in the open chain form or the cyclic form. Galactose contributes to the formation of some disaccharides such as Lactose (Glucose + Galactose) and Galactan (composed of Galactose). The melting point of galactose is about 167⁰C. Galactose is also soluble in water, but the solubility is less than that of glucose.

Similarities Between Glucose and Galactose

Glucose and Galactose have six carbon atoms.
The molecular formula is the same for both molecules.
Both are Monosaccharides and simple sugars.
Both are aldohexose molecules.

Definition

Glucose: Glucose is a simple sugar composed of C, H, and O atoms and tastes sweet.

Galactose: Galactose is a simple sugar composed of C, H, and O atoms and is less sweeter than glucose.

Position of –OH Group

The main difference between glucose and galactose is the chemical structure; the position of each –OH group present in both molecules. This difference occurs in the 4^th carbon atom. The following image shows this in detail.

Difference Between Glucose and Galactose - 3

Figure 3: Glucose vs. Galactose

Glucose: The –OH group of the 4^th carbon (from the top) of D-Glucose is directed towards right side.

Galactose: The –OH group of the 4^th carbon of D-Galactose is directed towards the left side.

Stability

Glucose: Glucose is more stable.

Galactose: Galactose is less stable.

Sweetness

Glucose: Glucose is sweeter.

Galactose: Galactose is less sweeter than Glucose.

Melting Point

Glucose: The melting point of glucose is about 146-150^oC.

Galactose: The melting point of galactose is about 167^oC.

Conclusion

Glucose and Galactose are very important simple sugars as monosaccharides as well as polysaccharides, which are formed after their polymerization. The chemical formula and the molar mass of both molecules are the same, but the difference between glucose and galactose lies in the arrangement of –OH groups along the carbon chain.

References:

1. Steane, Richard. “Molecular structure of glucose and other carbohydrates.” Structure of glucose and other carbohydrate molecules. N.p., n.d. Web. Available here. 29 June 2017.
2. “Galactose.” Chemistry LibreTexts. Libretexts, 04 Mar. 2017. Web. Available here. 29 June 2017.

Image Courtesy:

1.”Beta-D-Glucose” By Yikrazuul – Own work (Public Domain) via Commons Wikimedia
2. “Methyl-α-D-galactose” By Andromeas – Own work (CC0) via Commons Wikimedia
3. “GlucoseGalactose”By Lord Biro – Glucosetatuetata.jpg (Public Domain) via Commons Wikimedia