FIGURE 11-3 Multiple membrane transport proteins function together in the plasma membrane of metazoan cells. Gradients are indicated by triangles with the tip pointing toward lower concentration. The Na⁺/K⁺ ATPase in the plasma membrane uses energy released by ATP hydrolysis to pump Na⁺ (red circles) out of the cell and K⁺ (blue squares) inward; this creates a concentration gradient of Na⁺ that is greater outside the cell than inside, and one of K⁺ that is greater inside than outside. Movement of positively charged K⁺ ions out of the cell through membrane K⁺ channels creates an electric potential across the plasma membrane—the cytosolic face is negative with respect to the extracellular face. A Na⁺/lysine transporter, a typical sodium/amino acid cotransporter, moves two Na⁺ ions together with one lysine from the extracellular medium into the cell. “Uphill” movement of the amino acid is powered by “downhill” movement of Na⁺ ions, which in turn is powered both by the outside-greater-than-inside Na⁺ concentration gradient and by the negative charge on the inside of the plasma membrane, which attracts the positively charged Na⁺ ions. The ultimate source of the energy to power amino acid uptake comes from the ATP hydrolyzed by the Na⁺/K⁺ ATPase, since this pump creates both the Na⁺ ion concentration gradient and, via the K⁺ channels, the membrane potential, which together power the influx of Na⁺ ions.