During inflow into Escherichia coli substrates must first diffuse through the porin pores in the outer membrane by simple, passive, diffusion and then be transiocated 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.
Nikaido & Rosenberg (1981) demonstrated that a large part of the flux of solutes such as arabinose, glucose and lactose across the outer membrane of E. coli is via the porin pores, that the effective radius of these pores is close to 0.58 nm, and that at maximal growth rates the high rate of sugar flux across the outer membrane demands that at least 105 pores per cell are available if an appreciable concentration drop across the diffusion barrier of the outer membrane is to be avoided. Koch & Wang (1982) recently raised the question of whether, in Escherichia coli, evolution had succeeded in matching the effective permeabilities of the outer and the inner membranes. In other words, the question is whether the flow of nutrient into the cell under typical growth conditions is limited by the outer membrane pores or the inner membrane permeases, or by both about equally. Koch and Wang argued that a great