Coupling as a way of life.

More than three decades years ago, Aharon Katchalsky and Ora Kedem (1958, 1962) pointed out that the essence of biological transport is coupling. This insight was not so much a deduction as a concise summary of observations. At an elementary level, movement of a solute in the simplest aqueous solution implies movement of a hydration shell, and therefore movement of water, thence coupling of water movement to solute movement. In biological systems, for our present purposes specifically in biomembranes, more complex coupling mechanisms were clearly perceived, albeit largely without mechanistic understanding. The necessary response of these and other physically oriented biologists was to seek an understanding of coupling processes, along with formal descriptions, in the familiar language of thermodynamics. Their effort produced an elegant systematic statement of transport coupling, in the so-called Kedem equation (Kedem, 1961; see Gerencser and Stevens, 1994):