Carbohydrates are naturally occurring organic compounds that are composed of the elements carbon, hydrogen, and oxygen. They have the general formula Cx(H2O)y. The carbon and hydrogen in carbohydrates are present in a ratio of 2: 1. Carbohydrates are also called saccharides. By definition, carbohydrates are polyhydroxy-aldehydes or polyhydroxy ketones or compounds, which produce polyhydroxy-aldehydes or polyhydroxy ketones on hydrolysis.
Classification of Carbohydrates
Carbohydrates are classified as simple sugars (which are monosaccharides and disaccharides) and complex sugars – the polysaccharides.
Monosaccharides are the simplest group of carbohydrates and are referred to as simple sugars. They taste sweet. They cannot be further hydrolyzed to simpler compounds. They have the general formula Cn(H2O)n. Examples: Glucose and fructose.
Polysaccharides are complex carbohydrates. They give many units of monosaccharides on hydrolysis. They are usually amorphous, insoluble in water, and tasteless and are called non-sugars.
Classification of Carbohydrates as Reducing or Non-Reducing Sugars
Carbohydrates may also be classified as either reducing or non-reducing sugars. All those carbohydrates which contain a free aldehyde or ketonic group and reduce Fehling’s solution and Tollen’s reagent are referred to as reducing sugars. All monosaccharides whether aldose or ketose are reducing sugars because they have free aldehyde and free ketone groups. In disaccharides, if the reducing group of monosaccharides i.e., aldehydic or ketonic groups are bonded (not free), such disaccharides are non-reducing sugars e.g., sucrose, while others in which these functional groups are free are reducing sugars. Examples: Maltose and lactose.
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