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

In this work we look into the problem of why proteins, unlike small molecules, diffuse in the cytoplasm much more slowly than in aqueous solutions. In order to examine whether the cytoplasmic matrix could, by simple obstruction, retard protein diffusion to such an extent, we developed a method 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%. Thus, 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, we calculated the corresponding binding constants. These values indicate that 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. In addition, from our measurements, we estimate the surface area of structures within the cytoplasmic matrix to be in the range of 69,000-91,000 micron 2 per cell.