What is the Difference Between Hexose and Pentose

Hexoses and pentoses are both types of monosaccharides, which are the simplest forms of carbohydrates. The main difference between hexose and pentose is in the number of carbon atoms in their molecular structure. This difference in carbon count leads to variations in their chemical properties and biological roles, as well.

What is the difference between hexose and pentose? Hexose contains six carbon atoms in its molecule, whereas pentose contains five carbon atoms.

Key Areas Covered

1. What is Hexose
      – Definition, Features 
2. What is Pentose
      – Definition, Features
3. Similarities Between Hexose and Pentose
      – Outline of Common Features
4. Difference Between Hexose and Pentose
      – Comparison of Key Differences
5. FAQ: Hexose and Pentose
      – Answers to Frequently Asked Questions

Key Terms

Hexose, Pentose

What is Hexose

Hexose is a simple sugar with six carbon atoms. It is a monosaccharide with the formula C₆H₁₂O₆. There are two main forms of hexoses. One is the open-chain form, and the other is the cyclic form.  The open chain form resembles a straight chain with a carbonyl group at one end and hydroxyl groups attached to most carbons. Crystallization can be seen when hexoses are added to water (in aqueous solutions). A hydroxyl group on a later carbon reacts with the carbonyl group, forming a ring structure with five or six carbons (pyranose or furanose, respectively). This ring formation creates two stereoisomers, alpha (α) and beta (β), depending on the hydroxyl group’s orientation at the newly formed ring junction.

Hexoses show isomerism. For example, glucose and its mirror image, galactose, are both hexoses but have different metabolic fates.

Figure 1: Glucose

There are five hydroxyl groups in each hexose molecule. Hence, hexoses become highly reactive and even form hydrogen bonds. Additionally, the presence of a hemiacetal or hemiketal group (depending on the cyclic form) allows hexoses to react with other molecules, forming glycosidic bonds. These bonds are the foundation of complex carbohydrates like starches and cellulose.

What is Pentose

Pentoses are monosaccharides. They are a type of carbohydrate. They have five carbon atoms in the molecule, and its chemical formula is C₅H₁₀O₅. Pentoses have a backbone of five carbon atoms, each bonded to a hydroxyl group (OH) except for one carbon that forms a carbonyl group (C=O). There are two types of pentoses, namely, open chain form and closed ring form. In the cyclic form, a covalent bond forms between the carbonyl carbon and a hydroxyl group on another carbon atom within the chain. This creates a five-membered ring structure called a furanose. Examples of pentoses are ribose and deoxyribose.

Figure 2: Fructose

Ribose is found as a main component in RNA while deoxyribose is found as a main component in DNA. The other examples of pentoses include xylose and arabinose.

Similarities Between Hexose and Pentose

1. Both are monosaccharides.
2. They share a similar chemical formula pattern.
3. Both hexoses and pentoses have the same functional groups.
4. Both can exist in open-chain and cyclic forms.

Difference Between Hexose and Pentose

Definition

• Hexose is a simple sugar with six carbon atoms and the molecular formula C₆H₁₂O₆, while pentose is a simple sugar with five carbon atoms and the molecular formula C₅H₁₀O₅.

Carbon Atoms

• Hexoses have six carbon atoms (C₆), while pentoses have five carbon atoms (C₅).

Energy

• Due to the extra carbon atom, hexoses can store more potential energy than pentoses.

Role

• Hexoses are more commonly involved in energy metabolism and cellular processes, while pentoses are often important in genetic information storage and transfer.

Examples

• Examples of hexoses include glucose, fructose, and galactose, while examples of pentoses include ribose, deoxyribose, and xylose.

Conclusion

Hexoses and pentoses are both types of simple sugars, differing primarily in the number of carbon atoms in their molecular structure. Hexoses, as the name suggests, contain six carbon atoms, while pentoses contain five. This difference between hexose and pentose also results in different chemical properties and uses.

FAQ: Hexose and Pentose

1. Is fructose a pentose or hexose?

Fructose is a hexose as it has 6 carbon atoms in its molecule. Its hexose nature contributes to its sweetness and its role as a primary energy source in the body.

2. Which test distinguishes pentoses and hexoses?

Bial’s test is used to distinguish pentoses from hexoses. The compound present (either hexose/pentose) is found based on the colour that develops in the presence of orcinol and iron (III) chloride.

3. Is galactose a pentose or a hexose?

Galactose is a hexose because it contains six carbon atoms. This places it in the same category as fructose and glucose, which are also hexoses.

4. Is maltose a hexose or a pentose?

Maltose is composed of two hexoses, specifically two glucose molecules bonded together. Each glucose molecule contains six carbon atoms, so when two glucose molecules combine in maltose, they create a total of 12 carbon atoms.

5. What are the sugars in DNA and RNA, respectively?

In DNA, the pentose sugar is deoxyribose, while in RNA, it’s ribose. These sugars form the backbone of the nucleic acid molecules. Both these sugars are classified as pentoses, meaning they contain five carbon atoms.

Reference:

1. “Hexose – An Overview.” Science Direct.
2. “Pentose – An Overview.” Science Direct.

Image Courtesy:

1. “Glucose Chain Structure” By Acdx – Own work based on: D-glucose-chain-2D-skeletal.png (Public Domain) via Commons Wikimedia
2. “Skeletal Structure of D-Fructose” By Darkness3560 – Own work (CC0) via Commons Wikimedia