Diffusion, passive across living membranes

Maintenance of unequal concentrations of ions across membranes is a fundamental property of living cells. In most cells, the concentration of K+ inside the cells is about 30 times that in the extracellular fluids, while sodium ions are present in much higher concentration outside the cells than inside. These concentration gradients are maintained by the Na+-K+-ATPase by means of the expenditure of cellular energy. Since the plasma membrane is more permeable to K+ than to other ions, a K+ diffusion potential maintains membrane potentials which are usually in the range of -30 to -90 mV. H+ ions do not behave in a manner different from that of other ions. If passively distributed across the plasma membrane, then the equilibrium intracellular H+ concentration can be calculated from the Nernst equation via... 

There have been several reports of passive diffusion of NH3 in free-living microbial cells as well Suzuki et al. (56) measured the effect of pH on whole-cell oxidation rates of NH4 to NO2" by Nitrosomonas europaea and concluded that NH3 was the form that moved across the cellular membrane to the intracellular oxidation site. This conclusion was based on the questionable assumptions of equal concentrations across... 

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