An individual water molecule which has become
fixed into place or has become arranged in a certain position by virtue of its tendency to form
hydrogen bonds. This term is most often used to refer to
water in biological systems, as the chemical properties of
every biological molecule on earth are dependent on this particular property of water. Yes, its one of those
big, big, important things that we all take for granted.
Much of this is covered in hydrogen bond and in water, but as a review: Water is, of course, dihydrogen monoxide, formed by two hydrogen atoms covalently bonded to a single atom of oxygen. Relative to the oxygen, the bond angle between the two hydrogens is approximately 109.47 degrees, meaning that water molecules are asymmetric. This asymmetry in form causes the molecule to be an electrochemical dipole : the end of the molecule closest to the two hydrogens is slightly positive in charge, while the end farthest away is slightly negative. Thus, water molecules will obey their dipole nature and form hydrogen bonds -- with each other as well as with any other polar or ionic molecules.
This has many implications. For instance, hydrogen bonding of water with itself causes frozen water molecules to form a rough lattice structure which occupies a greater volume than when in liquid form; thus, ice floats in water.
However, for biologists and biochemists the most exciting implication is that water is not just a solute for biological molecules, but in facts plays an active role in biochemical reactions. For example, enzymes are typically highly selective for specific substrates, and for some reactions, a great deal of effort is expended on determining exactly how an enzyme and its substrate(s) physically interact with one another during the course of a reaction. Often it is determined that the only way for the molecules to interact properly is in the presence of water -- specifically the presence of individual molecules of water that form hydrogen bonds with the enzyme and the substrate and facilitate their physical interaction. In other words, many biological reactions require water molecules as a reaction co-factor. In this way ordered water directly facilitates the reactions that form the basis of life on this planet.