Carbohydrates- Their Importance and Classification

Carbohydrates-

Importance-

The carbohydrates are often termed as sugars are the 'staff of life' for most organisms. On the basis of mass, they are the most abundant class of biomolecules in nature. Carbohydrates are also known as saccharides (sakcharon = Sugars or sweetness) since many of those of relatively small molecular weight have a sweet taste, although this is not true of those with large molecules. They are widely distributed molecules (moles = mass) in both plant and animal tissues. They are indispensable for living organisms, serving as a skeletal structures in plants and also in insects and crustaceans. They also occur as food reserves in the storage organs of plants and in the liver and muscles of animals. In addition, they are an important source of energy required for the various metabolic activities of the living organisms; the energy being derived as a result of their oxidation. They also serve to lubricate skeletal joints, to provide adhesion  between cells and to confer  biological specificity on the surface of animal calls.
          Plants are  considerably richer in carbohydrates in comparison to the animals. In fact, animal and plant tissues differ widely in the relative abundance of the various major classes of constituent chemicals.

Nomenclature and definition-

The term carbohydrate was  originally coined  for this class of compounds has most of them were 'hydrates of carbon' or could be represented by the general formula Cx(H2O)y. Later it was found that some of them such as deoxyribose (C5H10O4) and Rhamnose (C6H12O5) do not have the required ratio of hydrogen to oxygen. In addition certain other carbohydrates are now known to possess nitrogen (e.g.,glucosamine C6H13O5N), phosphorus or sulphur also and obviously do not coincide with the above general formula.
        Hence the continued uses of the term carbohydrate is for convenience rather than exactness.
          To accommodate a wide variety of compounds the carbohydrates are nowadays broadly defined as polyhydroxy aldehyde or  ketones and their derivatives or as  substances that yield  one of these compounds on hydrolysis.

Classification-

Carbohydrates are usually classified in three groups:-

(1) Monosaccharides-
( Mono=one ; sakcharon = sugar)

The monosaccharides often called simple sugars, are compounds which possesses a free aldehyde (-CHO) or Ketone (=CO)group and two or more hydroxyl (-OH)groups. They are, in fact the simplest sugars and cannot be hydrolysed into smaller units. Their general formula is Cn(H2O)n.
          The monosaccharides may be subdivided into trioses, tetroses, pentoses, hexoses heptoses etc., depending upon the number of carbon atoms they possess; and as aldoses or ketoses, depending upon whether they contain aldehyde or Ketone group.  some important examples are-
  Name   |   Aldoses  |      Ketoses 

                |                   |
  Trioses | Glycerose|Dihydoxy-
                |                   |  acetone
Tetroses |Erythrose |Erythrulo-                                                    se  Pentoses|Ribose       |Ribulose                         
Hexoses |Glucose     |Fructose                               
Heptoses|Gluco-       |Sodo-                                                |heptulose                         heptose
                |                   |
                |                   |

(2) Oligosaccharides-
( Oligo=few  ; sakcharon = sugar)

  These are compound Sugars that yield 2 to 10 molecules of the same or different monosaccharides on hydrolysis.  Accordingly, an oligosaccharide yielding two molecules of monosaccharide on hydrolysis is designated as a disaccharide, and the one yielding three molecules of monosaccharide as a trisaccharide and so on. The general formula of disaccharides is Cn(H2O)n-1 and that of trisaccharides is Cn(H2O)n-2 and so on. A few examples are-

Disaccharides -
 sucrose, Lactose , maltose etc.

Trisaccharide -
Raffinose, Rhamninose, etc.

Tetrasaccharides -
Scorodose, Stachyose etc.

Pentasaccharides-
Verbascose

(3) Polysaccharides-
( Poly =many  ; sakcharon = sugar)

 These  are also compound Sugars and yield more than 10 molecules of monosaccharides on hydrolysis. These may be further classified depending on whether the monosaccharide molecules produced as a result of the hydrolysis of polysaccharides are of the same type(Homopolysaccharides) or of different types(Heteropolysaccharides). Their general formula is (C6H10O5)x.
   Some common examples are-

Homopolysaccharides -
starch, glycogen, inulin, cellulose, pectin, chitin etc.

 Heteropolysaccharides-
 Hyalueonic acid,  Chindrotin etc.