Question

Which of the following is a ketohexose?
(a) Fructose
(b) Glucose
(c) Ribose
(d) Starch

Answer 1

Hint: We know that carbohydrates are classified as monosaccharides, disaccharides, and polysaccharides. The monosaccharides are further classified into aldoses and ketoses. Ketohexoses are those carbohydrates that have six carbon atoms and the main functional group is a ketone which is present on the terminal carbon atom.

Complete step by step answer:
Carbohydrates are known as the compounds having polyhydroxy groups either having aldehyde or ketone as the main functional group at its terminal carbon atom. We know that carbohydrates are classified as monosaccharides, disaccharides, and polysaccharides.
Monosaccharides are those which cannot be hydrolyzed further into simpler molecules and these are further classified into two:
Aldoses: The main functional group at the terminal carbon atom is an aldehyde.
Ketoses: The main functional group at the terminal carbon atom is a ketone.
The aldoses and ketoses are specified with the number of carbon atoms in the molecule as trioses, tetroses, pentoses, hexoses. For example, aldotriose are those which have three carbon atoms and the main functional group is an aldehyde, ketotetrose are those which have four carbon atoms and the main functional group is a ketone.
So, ketohexoses are those which have six carbon atoms and the main functional group is a ketone.
Fructose is ketohexoses having six carbon atoms with 5 having a hydroxyl group and the main functional group is a ketone.

Glucose is aldohexoses having six carbon atoms with 5 having a hydroxyl group and the main functional group is an aldehyde.
Ribose is aldopentoses having five carbon atoms with 4 having a hydroxyl group and the main functional group is an aldehyde.
Starch is a polysaccharide.
Therefore the correct answer is an option (a) Fructose.

Note: Some other examples of ketohexoses are sorbose, tagatose, etc. Examples of ketopentoses are ribulose, xylulose, etc. Examples of aldohexoses are mannose, galactose, iodse, allose, etc.