Carbohydrates are the most abundant organic molecules in nature. They are primarily composed of the elements carbon, hydrogen and oxygen. The name carbohydrate literally means ‘hydrates of carbon’. Some of the carbohydrates possess the empirical formula (C.H2O)n where n 3, satisfying that these carbohydrates are in fact carbon hydrates. However, there are several non-carbohydrate compounds (e.g. acetic acid, C2H4O2; lactic acid, C3H6O3) which also appear as hydrates of carbon. Further, some of the genuine carbohydrates (e.g. rhamnohexose, C6H12O5; deoxyribose, C5H10O4) do not satisfy the general formula. Hence carbohydrates cannot be always considered as hydrates of carbon.
Carbohydrates may be defined as polyhydroxy aldehydes or ketones or compounds which produce them on hydrolysis. The term ‘sugar’ is applied to carbohydrates soluble in water and sweet to taste.
Functions of carbohydrates
Carbohydrates participate in a wide range of functions
1. They are the most abundant dietary source of energy (4 Cal/g) for all organisms. 2. Carbohydrates are precursors for many organic compounds (fats, amino acids).
3. Carbohydrates (as glycoproteins and glyco- lipids) participate in the structure of cell membrane and cellular functions such as cell growth, adhesion and fertilization.
4. They are structural components of many organisms. These include the fiber (cellulose) of plants, exoskeleton of some insects and the cell wall of microorganisms.
5. Carbohydrates also serve as the storage form of energy (glycogen) to meet the immediate energy demands of the body.
CLASSIFICATION OF CARBOHYDRATES
Carbohydrates are often referred to as saccharides (Greek: sakcharon–sugar). They are broadly classified into three major groups— monosaccharides, oligosaccharides and poly- saccharides. This categorization is based on the number of sugar units. Mono- and oligo- saccharides are sweet to taste, crystalline in character and soluble in water, hence they are commonly known as sugars.
Monosaccharides
Monosaccharides (Greek : mono-one) are the simplest group of carbohydrates and are often referred to as simple sugars. They have the general formula Cn(H2O)n, and they cannot be further hydrolysed. The monosaccharides are divided into different categories, based on the functional group and the number of carbon atoms .
Aldoses : When the functional group in H monosaccharides is an aldehyde C O , they are known as aldoses e.g. glyceraldehyde, glucose.
Ketoses : When the functional group is a keto C O group, they are referred to as ketoses e.g. dihydroxyacetone, fructose. Based on the number of carbon atoms, the monosaccharides are regarded as trioses (3C), tetroses (4C), pentoses (5C), hexoses (6C) and heptoses (7C). These terms along with functional groups are used while naming monosaccharides. For instance, glucose is an aldohexose while fructose is a ketohexose
Oligosaccharides:
Oligosaccharides (Greek: oligo-few) contain 2-10 monosaccharide molecules which are liberated on hydrolysis. Based on the number of monosaccharide units present, the oligo- saccharides are further subdivided to disaccharides, trisaccharides etc.
Polysaccharides
Polysaccharides: (Greek: poly-many) are poly- mers of monosaccharide units with high mole- cular weight (up to a million). They are usually tasteless (non-sugars) and form colloids with water. The polysaccharides are of two types – homopolysaccharides and heteropolysaccharides.
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