Packaging Life: The Origin of Ion-Selective Channels Clay M. Armstrong  Biophysical Journal  Volume 109, Issue 2, Pages 173-177 (July 2015) DOI: 10.1016/j.bpj.2015.06.012 Copyright © 2015 Biophysical Society Terms and Conditions

Figure 1 A mitochondrion. The outer wall is derived from the mitochondrion’s bacterial ancestry, and is relatively freely permeable. Permeability of the bilipid layer is severely restricted: it is impermeable to most ions. The internal membrane voltage (−140 mV) is relative to the voltage of the surrounding cytoplasm. To see this figure in color, go online. Biophysical Journal 2015 109, 173-177DOI: (10.1016/j.bpj.2015.06.012) Copyright © 2015 Biophysical Society Terms and Conditions

Figure 2 A chemiosmotic bacterium in sea water. Equilibrium ion concentrations are shown for a freely permeable membrane and (in italics) for a membrane selectively permeable to K+ and Cl−. To see this figure in color, go online. Biophysical Journal 2015 109, 173-177DOI: (10.1016/j.bpj.2015.06.012) Copyright © 2015 Biophysical Society Terms and Conditions

Figure 3 Approximate ion concentrations outside and inside an animal cell, drawn as a neuron. The dark ovals are mitochondria that produce ATP. The Na/K pump uses ATP to pump 3 Na+ ions outward and 2 K+ ions inward in each cycle. Efflux of K+ down its concentration gradient through K+ channels establishes a membrane voltage (Vm) of ∼−70 mV. The value Vm is positive relative to the equilibrium potential of K+ (∼−92 mV) because of inward leakage of Na+ and other cations. Pumps are essential to keep the internal concentrations of the leaking cations low. To see this figure in color, go online. Biophysical Journal 2015 109, 173-177DOI: (10.1016/j.bpj.2015.06.012) Copyright © 2015 Biophysical Society Terms and Conditions