Why are polysaccharides insoluble while their monomers or their disaccaride units are soluble when dissolved separately?
Why is the solubility so different when you put soluble building blocks together?
First, note that not all polysaccharides are insoluble, although many polysaccharides are less soluble than their monomer counterparts. Starch, for example, is not completely insoluble.
The main reason for insolubility of polysaccharides, such as cellulose, is the extended intermolecular interactions among polysaccharide molecules. Solubility depends on a solvent's ability to separate solutes into each individual molecule by surrounding it.
When the attraction between solute molecules is great, such as the case of cellulose in water, the molecules are insoluble.
When sugar molecules form a polymer, intramolecular interactions, which are usually hydrogen bonds, allow formation of secondary structures, which in turn may allow intermolecular interactions, leading to an even larger structure. Each hydrogen bond interaction may be weak, but when many hydrogen bonds form within and between polymers, the total attraction force can be great.
The following are links that discuss the intra- and intermolecular interactions of cellulose and starch: www.lsbu.ac.uk/water/hycel.html
These sites can help you visualize the secondary structures and intermolecular interactions that lead to insolubility in water.