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There are other considerations.If you placed a lump of sugar in a glass of water, the sugar will dissolve and then diffuse in an attempt to create a uniform solution. This is driven by entropy and the second law; maximize entropy. The ions, in your example, are segregated at different concentration on each side of the membrane. This creates an concentration gradient and entropy potential for each ion. The entropy potential acts, side-by-side, with the charge potentials. If you do just a charge balance, you will not expect sodium to spontaneously lead and go inside the cell, since there is already too much positive inside the cell. However, the sodium ions have a concentration gradient from outside to inside, so there is nevertheless a potential to flow inside, driven by entropy. The charge and entropy potential act in opposite directions, with the net total, the net push to act. There is one more layer of potential. Potassium ions are called chaotropes, which means they create chaos/disorder in water. While sodium ions and calcium ions are kosmotropes and will create order in water. The chloride ions are in the middle, being of low impact on water. See the diagram below;All the ions around the membrane, need to move through water. Therefore, the state of the water; induced by the ions, will add a third leg of potential. Ordered water has lower structural entropy, than disordered water, which defines higher entropy. The net affect is the water on the inside has higher entropy while the outside water is lower entropy. The higher internal water entropy is needed to loosen and potentiate the cellular water for the enzymes. The water outside is more ordered by the sodium. What that order water does is create the analogy is higher surface tension. This makes the outside membrane amiable; lure, for organic to collect on the membrane; food attractant. If potassium ions were to flow outside, they will carry their chaotropic nature and can help the external water gain entropy. There is a push by the water, on potassium ions, to go outside to cell to help the water gain entropy. The needs of the water allows the potassium ions to most feely flow across the membrane water, since it maximizes the second law for the water. Potassium has three legs of potential. However, the movement of ions is regulated by protein channels. The net affect is one can start the discharge of potential using a less likely spontaneous ion, like calcium. Early cells, before all the bells and whistles, would be water and potassium driven. They would still make use of all the same ions, but with the channels less specific, potassium would lead the swapping; more cell division and less rest time.
However, the movement of ions is regulated by protein channels.