![]() This localization not only serves the purpose of mechanical function, as in muscle, but also the purpose of metabolic channeling ( Saks et al., 1994). Proteins are also specifically localized within the cell. Based on hydration estimates of serum albumin, it has been suggested that, if it did not occur, all of the water of the cell could be osmotically inactive ( Cameron et al., 1997). The association serves the purpose of minimizing the water of hydration associated with all proteins. Rather, they are directed by the biological machinery of the cell to form close associations with each other and the cytoskeleton and membranes of the cell. The proteins of the cell are neither distributed randomly nor do they impart the aqueous phase separation in a manner analogous to that observed with macromolecules in a test tube, as suggested by Walter and Brooks (1995). The two-phase hypothesis contains important implications for understanding the aqueous cytoplasm. It remains surprising to me that this powerful, but conceptually simple, approach has not been applied to other biological systems. The decisive results came from a novel, but perfectly straightforward, experimental approach, in which the effects of matrix volume on nonelectrolyte distribution coefficients were compared with distribution coefficients measured at constant volume. Nevertheless, the abnormal phase dissolves small solutes, with solute activity coefficients that are very different from those in bulk aqueous solutions. The abnormal aqueous phase is osmotically inactive and comprises the water of hydration of mitochondrial membranes and proteins. A thermodynamically complete description of matrix water requires no more than two different phases, 1 which I call “normal” and “abnormal.” The normal aqueous phase behaves in every respect like a bulk aqueous solution of similar composition. The mitochondrial matrix is a single, membrane-bounded compartment that is very rich in proteins. Isolated rat liver mitochondria were used as the experimental model for this investigation. This experimental evidence will be reviewed. Thus, a straightforward application of the scientific method conclusively excluded the alternative hypothesis that biological water comprises a single, homogeneous phase. This hypothesis was both unusual and, it still seems to me, inescapable. In the early 1970s, I put forward a novel hypothesis for the macroscopic state of water in biological systems: that biological water spontaneously separates into two (or more) phases with distinct solvent properties ( Garlid, 1976, 1978, 1979). In this domain, one is dealing with familiar macroscopic properties such as osmotic activity and solute activity coefficients. This review deals with the time-averaged, equilibrium properties of water in cells. Garlid, in International Review of Cytology, 1999 I Introduction *
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