brane: a chemical force (the ion's concentration gradient) and an electrical force (the effect of the membrane potential on the ion's movement). This combination of forces acting on an ion is called the electrochemical gradient. In the case of ions, then, we must refine our concept of pas-sive transport: An ion diffuses not simply down its concentration gradient but, more exactly, down its electrochemical gradient. For example, the concentration of sodium ions (Na +) inside a resting nerve cell is much lower than outside it. When the cell is
stimulated, gated channels open that facilitate Na + diffusion. Sodium ions then "fall" down their electrochemical gradient, driven by the concentration gradient of Na + and by the attrac-tion of these cations to the negative side of the membrane. In this example, both electrical and chemical contributions to the electrochemical gradient act in the same direction across the membrane, but this is not always so. In cases where electrical forces due to the membrane potential oppose the simple diffu-sion of an ion down its concentration gradient, active transport may be necessary. In Chapter 48, you'll learn about the impor-tance of electrochemical gradients and membrane potentials in the transmission of nerve impulses. Some membrane proteins that actively transport ions con-tribute to the membrane potential. An example is the sodium-potassium pump. Notice in Figure 7.16 that the pump does not translocate Na + and K+ one for one, but pumps three sodium ions out of the cell for every two potassium ions it pumps into the cell. With each "crank" of the pump, there is a net transfer of one positive charge from the cytoplasm to the extracellular fluid, a process that stores energy as voltage. A transport pro-tein that generates voltage across a membrane is called an electrogenic pump. The sodium-potassium pump seems to be the major electrogenic pump of animal cells. The main elec-trogenic pump of plants, fungi, and bacteria is a proton pump, which actively transports hydrogen ions (protons) out of the cell. The pumping of H+ transfers positive charge from the cy-toplasm to the extracellular solution (Figure 7.18). By gener-ating voltage across membranes, electrogenic pumps store energy that can be tapped for cellular work. One important use of proton gradients in the cell is for ATP synthesis during cel-lular respiration, as you will see in Chapter 9. Another is a type of membrane traffic called cotransport. Cotransport: Coupled Transport by a Membrane Protein A single ATP-powered pump that transports a specific solute can indirectly drive the active transport of several other solutes in a mechanism called cotransport. A substance that has been pumped across a membrane can do work as it moves back across the membrane by diffusion, analogous to water that has been pumped uphill and performs work as it flows back down.