Question

Both oligosaccharides and polysaccharides are formed from monosaccharide units involving glcocidic bond .Then why are pligosaccharides soluble in water but not the polysaccharides

Answer 1

Most polysaccharides (sugar polymers) are far less soluble in water than their monomers (simple sugars). This happens because the polymer linkage between sugars ties up two of the sugar's reactive groups, which prevents those two groups from interacting with water. The resulting glycosidic ether linkage between sugars has less hydration than the alcohol and carbonyl (aldehyde or ketone) groups from which it is made. Also, there is molecular crowding from polymerization (steric hindrance) that decreases the ability of adjacent hydroxy groups from interacting with water. The glucose polymer starch is linked between the #1 carbon on one glucose and the #6 carbon on the adjacent glucose. This linkage maximally separates the glucose residues and maintains some substantial solubility in water. The glucose polymer cellulose is linked between the #1 carbon and the #4 carbon, which crowds the glucose residues tightly together and minimizes solubility.