Why do glucose, galactose and mannose differ in solubility in water?
The solubility of these sugars will depend on the relative energies of the crystalline form versus the dissolved form of the sugar. Without knowing the specifics, one could surmise that the crystal structures of these three sugars may differ substantially from each other - one may be more tightly or efficiently packed into a crystal lattice than another due to the specific shape of the molecule, resulting in a lower energy state than that of the other sugar's crystal form, and hence giving a lower solubility in water.