Thus, at equilibrium the concentration of sodium ions on either side of a colluding membrane separating a solution of sodium chloride in water is equal. If protein, to which the membrane is impermeable, be added to the solution on one side of the membrane is expressed by the ratio of the Donnan equilibrium. A similar distribution occurs between the sodium – ion content of the vascular and extra vascular compartments of the extracellular fluid, which accordingly are in true physicochemical equilibrium. On the other hand, the sodium content whit in the cells is less and the potassium content higher then in the surrounding extracellular fluid. These ions accordingly cannot be considered as being in physicochemical equilibrium across the cell membrane; energy must be expended by the cell to maintain this constant state of “disequilibrium”. Only with death of the cell and a cessation of this expenditure of energy does equalization of the ionic activities of the sodium and potassium across the cell membrane occur. Also in disease, when the cell can no longer maintain its normal activity, there will be a tendency for it to lose potassium and gain sodium, as has been observed in many pathologic conditions.

A system in equilibrium has no capacity to do work; its free energy is at a minimum; its entropy is at a maximum, i.e. it is in its most probable state. The living organism, on the other hand, is in constant chemical activity, is capable of doing work and is endowed with compounds potentially capable of entering into reactions. Even a single cell apparently at rest utilizes oxygen and liberates heat, since it must expend energy to maintain the heterogeneity oh its interior, the semi permeability of its surrounding membrane, and other manifestations of cellular integrity. The active cell must also synthesize compounds which require an expenditure of energy.