11.1 Introduction
Carbohydrates are polyhydroxy aldehydes or ketones. They are primarily produced by plants and form a very large group of naturally occurring organic substances. Some common examples are cane sugar, glucose, starch, etc. They have general molecular formulas that make them appear to be hydrates of carbon, Cn(H2O)n , from where the name carbohydrate was derived. Carbohydrates are formed in the plants by photosynthesis from carbon dioxide and water in the presence of sunlight (Scheme 1).
Scheme 1
11.2 Classification
Carbohydrates are classified into two main classes, sugars and polysaccharides.
11.2.1 Sugars
Sugars are sweet crystalline substances that are soluble in water. These are further classified on the basis of their behavior on hydrolysis.
11.2.1.1 Monosaccharides
The simplest form of carbohydrates is the monosaccharide. 'Mono' means 'one' and 'saccharide' means 'sugar'. Monosaccharides are polyhydroxy aldehyde or ketone that cannot be hydrolyzed further to give simpler sugar. They may again be classified on the basis of the nature of carbonyl group.
- Polyhydroxy aldehydes are called aldoses. Example: Glucose
- Polyhydroxy ketones are called ketoses. Example: Fructose
The aldoses and ketoses are further divided based on the number of carbons present in their molecules, as trioses, tetroses, pentoses, hexoses etc. They are referred to as aldotrioses, aldotetroses, aldopentoses, aldohexoses, ketohexoses etc.
