The cytoplasmic matrix: its volume and surface area and the diffusion of molecules through it.

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Abstract
The problem of why proteins, unlike small molecules, diffuse in the cytoplasm much more slowly than in aqueous solutions was examined. In order to examine whether the cytoplasmic matrix could, by simple obstruction, retard protein diffusion to such an extent, a method was developed to measure semiquantitatively the fractional volume occupied by the cytoplasmic matrix (which includes the microfilaments, intermediate filaments, microtubules, and the microtrabeculae of the cytoplasmic matrix). This method yielded values in the range of only 16-21%. A more elaborate model is suggested in which the diffusing proteins bind and dissociate constantly from the surfaces in the cytoplasmic matrix. From this model, the diffusion coefficients and the measured values of the fractional volumes, binding constants were calculated. Most of the diffusing proteins are bound to the matrix at any given time, in spite of the possibility that they may bind and dissociate very rapidly. The surface area of structures within the cytoplasmic matrix are estimated to be in the range of 69,000-91,000 .mu.m2 per cell.