Difference Between Hexose and Pentose

Hexose vs Pentose
 

Carbohydrates are a group of compounds that are defined as “polyhydroxy aldehydes and ketones or substances that hydrolyze to yield polyhydroxy aldehydes and ketones.” Carbohydrates are the most abundant type of organic molecules on earth. They are the source of chemical energy for living organisms. Not only this, they serve as important constituents of tissues. Carbohydrate can be again categorized into three as monosaccharide, disaccharides and polysaccharides. Monosaccharides are the simplest type of carbohydrate. Monosaccharide has the formula of C_x(H₂O)_x. These cannot be hydrolyzed into simpler carbohydrates. They are sweet in taste. All monosaccharides are reducing sugars. Therefore, they give positive results with benedicts’ or Fehling’s reagents.  Monosaccharides are classified according to,

• The number of carbon atoms present in the molecule
• Whether they contain an aldehyde or keto group

Therefore, if the monosaccharide has an aldehyde group, it is called as aldose. A monosaccharide with a keto group is called a ketose. Among these, the simplest monosaccharides are glyceraldehyde (an aldotriose) and dihydroxyacetone (a ketotriose).  Glucose is another common example for a monosaccharide. For monosaccharides, we can draw a linear or a cyclic structure. In solution, majority of the molecules are in cyclic structure. For example, when a cyclic structure is forming in glucose, the -OH on carbon 5 is converted into the ether linkage, to close the ring with carbon 1. This forms a six member ring structure. The ring is called as hemiacetal ring, due to the presence of carbon that has both an ether oxygen and an alcohol group

Hexose

As described above, one way of classifying monosaccharides is using the number of carbon atoms present in the molecule. Therefore, hexose is the group of monosaccharides with six carbon atoms. It has the chemical formula of C₆H₁₂O₆. For example, glucose, galactose, fructose are some of the common molecules with six carbon atoms. For example, glucose has four hydroxyl groups and has the following structure.

These are further divided based on whether they have an aldehyde group or a ketone group.  For example, glucose has an aldehyde group; therefore, it is an aldohexose. Allose, altrose, glucose, mannose, gulose, idose, and talose are other types of aldohexoses. All these have four chiral centers, thus have 16 stereoisomers.  When they form cyclic molecules, they form hemiacetals. Fructose has a ketone group, so it is a ketohexose. Other than fructose, sorbose, tagtose, and psicose are some other ketohexoses. They have three chiral centers and, therefore, eight stereoisomers.

Pentose

Pentoses are monosaccharide molecules with five carbon atoms. As hexoses, pentoses can also be further divided into two groups as aldopentoses and ketopentoses.  Ribose, xylose, arabinose, lyxose, are aldopentoses. They have three chiral centers, thus eight stereoisomers.  Ribulose, xylulose are ketopentoses, and they only have two chiral centers.