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Carbohydrates

Written By profitgoonline on Monday 10 June 2013 | 16:40

Carbohydrates 


Pure  carbohydrates  have  the  empirical  formula (CH2O)n  .  The  smallest  carbohydrates  are  simple
 sugars,  or  monosaccharides.  Glucose  is  the  six carbon  monosaccharide  (hexose)  used  as  a  basic
source of energy by most heterotrophic cells. Ribose and deoxyribose are the five-carbon sugars (pentoses) that serve a structural role in the nucleic acids RNA and DNA, respectively. Oligosaccharides are small polymers of two to six monosaccharides. Sucrose is a disaccharide of the two monosaccharides glucose and fructose (an isomer of glucose). Sucrose is the major sugar transported between plant cells, whereas glucose is the primary sugar transported between animal cells. Lactose, the major sugar in milk, is a disaccharide of glucose and galactose (an epimer of glucose). Most of the carbohydrate molecules in nature are composed of hundreds of sugar units and are referred to as polysaccharides.
     The monomers of polysaccharides become covalently connected by glycosidic bonds (see Figure 2-1).
     Carbohydrates  serve  several  major  functions  in  living  systems. Monosaccharides and ligosaccharides serve as readily utilizable energy sources.  Starch  and  glycogen  act  as  macromolecular  energy  stores  in
plants and animals, respectively. Carbohydrates perform structural roles, such  as  cellulose  in  plant  cell  walls  and  chitin  in  the  exoskeletons  of arthropods. Surface carbohydrates are often complexed with proteins as glycoproteins or with lipids as glycolipids in the plasma membrane. The great potential for structural diversity and thus, specificity, makes these molecules very useful as cell-recognition markers in cellular communication and in cell-to-cell attachments.

                                  Note! 

   Glycogen  consists  of  polymers  of  glucose  units joined  by  a (1→4)  linkages  and  forms  branched
   chains  by  a (1→6)  linkages.  Starch  has  fewer a (1→6) linkages than glycogen.


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