Water, treated as the continuous liquid in and around cells. 
In the quantitative treatment of non-covalent inter- and intra-cellular interactions taking place in water, in vitro as well as in vivo, it is essential to treat the surrounding and pervading liquid medium as the continuous medium.  In the close vicinity of inter- and intra-cellular surfaces and of biopolymers the various different non-covalent forces may locally alter the structure of water in a number of ways, but these local structural changes can be quantitatively taken into account.  The operative forces are: Lifshitz-van der Waals (LW) forces.  Lewis acid-base (AB) forces and electrostatic (EL) forces.  Of these, the AB forces are generally the preponderant ones, in aqueous media.  This is due, inter alia, to the strong cohesive and adhesive hydrogen-bonding interactions typically occurring in and by water.  Among the strong AB interactions occurring in water are hydrophobic attraction (the hydrophobic effect) and hydrophilic repulsion (hydration pressure).  Also treated is the function of LW, AB and EL forces in: hydration; in the stability of particle and cell suspensions, the solubility of biopolymers, small organic solutes, and electrolytes; and in specific ligand-receptor (such as antigen-antibody) interactions.