Hydration Shell

∞ generated and posted on 2016.12.06 ∞

The surrounding of especially a polar substance by water molecules within an aqueous solution.

A Hydration Shell is a more or less continuous cluster of water molecules surrounding and to a degree attached to some other substance that the water molecules are attracted to, because this other substance is water loving, that is, hydrophilic.

The hydration shell, or more generally the solvation shell, represents water's attraction or adhesion to a substance and the greater the attraction the greater the number of water molecules involved in the hydration shell.

Here greater attraction is typically seen as a consequence of the charge to which water is attracted where partial charges result in lesser water molecule involvement whereas multiple full charges results in much greater water involvement.

The consequence of hydration shells is that polar substances are hydrated, that is, they normally exist, within aqueous solutions, not just of themselves but of substantial numbers of water molecules.

The starch, glycogen, is a good example of a biomolecule that is associated with hydration shells such that there exist important physiological consequences. In particular, while glycogen is a source of readily available glucose, each of those glucose moieties found within glycogen is associated with a hydration shell. As a consequence, starch molecules, as colloidal substances, are fully wetted within aqueous solutions, resulting in substantial bulk and mass associated with glycogen molecules.

Fats, by contrast, are minimally hydrated so represent a more compact energy storage molecule (also, per gram of molecule, that is, ignoring hydration, fats store more energy than do starches). A consequence of this is that fats are used for longer-term energy stores particularly by organisms that are highly mobile, such as animals or seeds. An additional consequence is that dieting can results in substantial weight loss early on as glycogen stores are depleted along with their associated hydration shells.