FIGURE 11-25 Transmembrane forces acting on Na⁺ ions. As with all ions, the movement of Na⁺ ions across the plasma membrane is governed by the sum of two separate forces: the ion concentration gradient and the membrane electric potential. At the internal and external Na⁺ concentrations typical of mammalian cells, these forces usually act in the same direction, making the inward movement of Na⁺ ions energetically favorable.