Friends and Neighbors—Interactions in a System: Phase Equilibria

Abstract

So far we have considered only homogeneous solutions. Such systems, while composed of a number of constituents, contain them in a single homogeneous phase. A phase is considered to be a state in which there is both chemical and physical uniformity. In a biological system, such a presumption is not realistic. Biological systems have many heterogeneous characteristics. Many of the processes in cells and living organisms involve the transfer of chemical species from one phase to another. For example, the movement of ions across a membrane such as the cell membrane or an intracellular organelle is often treated as transport between two phases, one inside and the other outside. The nature of the equilibria that can exist between phases will be the focus of this section. When different phases come into contact with each other, an interface between them occurs. This interface is a surface, and the properties of a surface are different from those of either of the phases responsible for creating it.