Development of resting membrane potential

When permeable, the movement of Na+ and K+ ions in and out of the cell depends on two factors  

The K+ ions that moved out of the cell down their concentration gradient have caused an excess of (+) charges to accumulate on the external surface 

Ion Extracellular fluid Intracellular fluid Relative permeability 

The Na+-K+ pump also plays a vital role in this process. For each molecule of ATP expended, three Na+ ions are pumped out of the cell into the ECF and two K+ ions are pumped into the cell into the ICF. The result is the unequal transport of positively charged ions across the membrane such that the outside of the cell becomes more positive compared to its inside in other words, the inside of the cell is more negative compared to the outside. Therefore, the activity of the pump makes a small direct contribution to generation of the resting membrane potential. 

The other, even more important effect of the Na+-K+ pump is that it maintains the concentration differences for sodium and potassium by accumulating Na+ ions outside the cell and K+ ions inside the cell. As previously discussed, the passive diffusion of these ions down their concentration gradients is predominantly responsible for generating the resting membrane potential. Sodium diffuses inward and potassium diffuses outward. The continuous activity of the pump returns the Na+ ions to the ECF and the K+ ions to the ICF. Therefore, it can be said that the pump also makes an indirect contribution to generation of the resting membrane potential. 

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