Final answer:
Polysaccharides do not give a positive reducing sugar reaction due to their complex structure and large size, which cause the aldehydic or ketonic groups to be unavailable for reducing actions.
Step-by-step explanation:
Polysaccharides do not give a positive reaction for reducing sugars because, despite having at least one free aldehydic or ketonic group, they are non-reducing sugars due to their larger molecular size and the complexity of their structure. This results in the aldehydic or ketonic group being unavailable for the reducing action. Polysaccharides, or glycans, are large polymers composed of hundreds of monosaccharide monomers linked together by glycosidic bonds, making them very large and not able to participate in the reactions typical of smaller reducing sugars.
The three most abundant polysaccharides are starch, glycogen, and cellulose. These are all homopolymers that yield only glucose upon complete hydrolysis. In contrast to monosaccharides and some disaccharides which are reducing sugars and can change the oxidation state of other molecules, polysaccharides do not have this capability because their glycosidic bonds do not permit the necessary aldehydic or ketonic groups to participate in such reactions.