Why do ions flow from high concentration to low concentration?

by sameeralord
Tags: concentration, flow, ions
 Sci Advisor P: 5,523 The free energy change for ion transport goes something like: dG = -D dC + Z*C*E, where dC is the concentration gradient, Z the electric charge of the ion, and E the electric field. At equilibrium, dG = 0 and so there is a balance of concentration graident and electric field (hence the membrane potential): $$E_{eq,K^+} = \frac{RT}{zF} \ln \frac{[K^+]_{o}}{[K^+]_{i}}$$ For many ions (protons across the mitochondrial membrane, Na+, Ca++, K+, etc) the concentrations follow the setpoint of the membrane potential (60 mV for the cell membrane, 220 mV for the mitochondrial membrane). That's one way to think of the membrane potential- it stores energy by segregating ions. The change in free energy as an ion crosses the membrane (via ATP synthase, for example), is used by the cell to do something useful. http://en.wikipedia.org/wiki/Membrane_potential http://en.wikipedia.org/wiki/Chemiosmotic_theory http://en.wikipedia.org/wiki/Atp_synthase