FIG 5.

Quantitative illustration of effector (p-cresol) availability at 10 nM. (A) Upon effector addition, the cell density is approximately 1 cell (ellipsoid, 3 μm³) in a cubic volume of 1,300 μm³. At 10 nM, such a cubic space element contains 8,000 solute particles (red dots), with the green-shaded zoom-in representing a 0.2-μm-wide slice. At equilibrium, the solute concentration in the cells equals that of the surrounding medium, while solute molecules are markedly enriched in the cell envelope. (B) Stages of the equilibration process. The initial concentration gradient of the effector across the cell envelope is enhanced by the lipophilic character of this aromatic solute and drives transmembrane diffusion until an equilibrium between intra- and extracellular spaces is reached ($\ll 1\text{s}$). In the final state of equilibrium, a gradient no longer exists and the elevated concentration of the solute in the cell envelope just reflects the lipophilic character of the latter. (C) Timeline from effector equilibration to full-scale gene expression.