## 6 Results and Discussion  
To directly compare how two general diffusion porins--Omp-Pst2 from Providencia stuartii and OmpF from _Escherichia coli_--facilitate ammonium (NH4\({}^{+}\)) passage, each porin was reconstituted into planar lipid bilayers and applied well-controlled bi-ionic gradients [35; 36; 53]. By measuring zero-current reversal potentials under defined salt conditions and fitting the resulting I-V relationships to the Goldman-Hodgkin-Katz equation, permeability ratios (P\(-\)/P+) and extrapolated single-channel flux rates [35; 36; 53] at physiologically relevant (micromolar) NH4\({}^{+}\) concentrations were obtained. This head-to-head approach reveals both quantitative transport efficiencies and selectivity differences that underlie ammonium uptake in bacterial outer membranes. When Omp-Pst2 was assayed with an ammonium chloride gradient (200 mM cis/50 mM trans), the observed reversal potential (V_rev) was 22.46 mV Table 1. From its single-trimer conductance (0.53 nS at 50 mM NH4 Cl) and the fitted permeability ratio (P_Cl\({}^{-}\)/P_NH4\({}^{+}\)\(\approx\) 5:1), the flux rate was calculated to roughly \(\sim\)6000 NH4\({}^{+}\)\(\cdot\)s\({}^{-1}\)-monomer\({}^{-1}\) at a 1 \(\upmu\)M transmembrane NH4\({}^{+}\) gradient. This rate indicates that, even under low micromolar conditions, Omp-Pst2 demonstrated efficient NH4\({}^{+}\) entry. Substituting chloride with sulfate (ammonium sulfate: 200 mM cis/50 mM trans) modestly reduced the driving force (V_rev = 20.06) and increased the anion-to-cation permeability ratio (P_SO4\({}^{2-}\)/P_NH4\({}^{+}\)\(\approx\) 6:1), yielding an extrapolated flux of \(\sim\)5500 NH4\({}^{+}\)\(\cdot\)s\({}^{-1}\)-monomer\({}^{-1}\) at 1 \(\upmu\)M. The roughly \(\sim\)15% decrease in NH4\({}^{+}\) throughput when sulfate is the counterion might possibly be due to sulfate's larger size and higher hydration energy [80; 81], but Omp-Pst2 still maintains a comparatively high transport rate. Under identical NH4 Cl conditions (200 mM cis / 50 mM trans), OmpF produced V_rev = 18.5, with a fitted permeability ratio (P_Cl\({}^{-}\)/P_NH4\({}^{+}\)\(\approx\) 9:1). Using its single-trimer conductance (0.40 nS at 50 mM NH4 Cl), this translates into an extrapolated NH4\({}^{+}\) flux of \(\sim\)4000 NH4\({}^{+}\)\(\cdot\)s\({}^{-1}\)-monomer\({}^{-1}\) at 1 \(\upmu\)M--roughly 30% lower than Omp-Pst2 under the same conditions. When ammonium sulfate was substituted (200 mM cis / 50 mM trans), V_rev remained around 18 mV and the P_SO4\({}^{2-}\)/P_NH4\({}^{+}\) ratio stayed near 9:1, giving a slightly higher flux (\(\sim\)5000 NH4\({}^{+}\)\(\cdot\)s\({}^{-1}\)-monomer\({}^{-1}\) at 1 \(\upmu\)M) but still in a same range of Omp-Pst2's flux rates. Structurally, Omp-Pst2's constriction zone is marginally wider and lined with acidic residues that create an electrostatic environment conducive to NH4\({}^{+}\) passage with moderate anion exclusion. By contrast, OmpF's tighter eyelet and more densely arranged charges trade flux for discriminative power: its permeability ratio against anions is -9:1 anion to cation for NH4\({}^{+}\) but the maximum NH4\({}^{+}\) flux is only -70-80% of what Omp-Pst2 achieves. These observations underscore how pore diameter, residue distribution, and local electrostatics balance throughput versus selectivity in general diffusion porins [82; 83]. Biologically, passive NH4\({}^{+}\) diffusion through general porins complements active transport under nutrient-limited conditions [7; 19; 63; 65]. Extracellular NH4\({}^{+}\) often falls to low micromolar levels in soils, aquatic micronuclehes, or host niches such as the urinary tract [3; 74; 84]. In such contexts, energy-intensive transporters (e.g., AmtB) may be downregulated or saturated, whereas porin-mediated entry provides an energy-efficient entry route [5; 6; 7].