Introduction
Carbohydrates consist of carbon, hydrogen and oxygen with the last two elements hydrogen and oxygen usually present in the same proportion as in water. Carbohydrates arose as one of the products of photosynthesis. Carbohydrates constitute a large proportion of the plants biomass and are responsible as cellulose for rigid framework, and as starch for providing an important food reserve.
Biosynthesis of monosacchrides
Various monosaccharides arise from the photosynthetic cycle. D- Fructose-6-phosphates and D-glucose-6-phosphates are universal in their occurrence. Free sugars may accumulate as a result of hydrolysis of phosphorylated sugars or the later maybe utilized in respiration and converted to sugar nucleotides (e.g. Uridine diphosphate glucose (UDP glucose) ) or by the action of epimerases giving rise to the other monosaccharides (e.g. galactose ) .
Biosynthesis of Di- , Tri- and Tetrasaccharides
These are derived from two, three or four monosaccharides respectively with the elimination of one , two or three molecules of water. One of the commonest plant disaccharides is sucrose (sugar cane sugar). It is formed in photosynthesis by the reaction of UDGP (Uridine diphosphate glucose) with Fructose 6 phosphate (Fig.1 below).
Control mechanisms for the build up of sucrose and its breakdown for transport to storage organs are achieved by metabolite effector control of the appropriate enzymes.
Biosynthesis of sucrose (a disaccharide)
Biosynthesis of polysaccharides
By condensation involving sugar phosphates and sugar nucleotides, polysaccharides are derived from monosaccharides in an exactly similar manner to the formation of di, tri, and tetra saccharides.