When is the outer membrane of Escherichia coli rate-limiting for uptake of galactosides?

During inflow into Escherichia coli substrates must first diffuse through the porin pores in the outer membrane by simple, passive, diffusion and then be translocated across the inner membrane by a specific (active) carrier protein, or permease. A graphical procedure is outlined whereby it is possible to estimate the concentration drop across the outer membrane from simple kinetic measurements of net inflow velocity. Experiments confirm that the concentration drop across the outer membrane is proportional to rate of inflow, as expected from Fick's law. The expected rate of diffusion of 2-nitrophenylgalactoside through the outer membrane was calculated from reported values of pore radius, length and number, and the rate was found to correspond closely with the experimental results. It is pointed out that at low substrate concentrations the outer membrane is rate-limiting and that a large increase in the amount of permease in the inner membrane will cause very little change in net inflow velocity.