This paper addresses the issue of how the aqueous cytoplasm is organized on a macroscopic scale. Mitochondria were used as the experimental model, and a unique experimental approach was used to probe the properties of water in the mitochondrial matrix. The results demonstrate aqueous phase separation into two distinct phases with different osmotic activity and different solute partition coefficients. The larger phase, designated "normal water," is osmotically active and behaves in every respect like a bulk, dilute salt solution. The smaller phase, designated "abnormal water," is osmotically inactive and comprises the water of hydration of matrix proteins. It is, nevertheless, solvent water, with highly selective partition coefficients, and behaves like a Lewis base.

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