Properties and metabolism of the aqueous cytoplasm and its boundaries.

The nucleoplasm, the interiors of cytoplasmic membrane-bound organelles, and the aqueous cytoplasm make up the aqueous compartments of animal cells. The extent to which these compartments are concentrated solutions of macromolecules, metabolites, ions, and other solutes is a matter of some importance to current thinking about cell structure and function. This paper will focus on the aqueous cytoplasm. It will show that the composition and metabolic activities of the cytosol, obtained by methods of cell disruption and fractionation, bear almost no resemblance to those of the aqueous cytoplasm in intact cells. The consequences of this to contemporary views on cell structure and function are considered. A closely related topic concerns the physical properties of the dominant component of these compartments, water: Are these properties the same as those of water in aqueous solutions, or are they altered as a result of interaction with cell architecture? Available evidence strongly suggests that at least a large fraction of the total cell water exhibits properties that markedly differ from those of pure water. Selected examples of these studies will be reviewed, and the roles of cell water will be discussed, notably as they relate to metabolism and cell ultrastructure. Although dimly perceived at present, it appears that living cells exhibit an organization far greater than the current teachings of cell biology reveal.