The plasma membranes of animal cells contain many open K+ channels but few open Na+, Cl−, or Ca2+ channels. As a result, the major ionic movement across the plasma mem brane is the movement of K+ from the inside outward, powered by the K+ concentration gradient. This movement leaves an excess of negative charge on the cytosolic face of the plasma membrane and creates an excess of positive charge on the exoplasmic face, as in the experimental system shown in Figure 1c. This outward flow of K+ ions is the major determinant of the inside-negative membrane potential. The channels through which the K+ ions flow, called resting K+ channels, alternate, like all channels, between an open and a closed state (see Figure 2), but since their opening and closing is not affected by the membrane potential or by small signaling molecules, these channels are referred to as non gated. In contrast, the various gated channels in neurons and other excitable cells open only in response to specific ligands or to changes in membrane potential.
Fig1. Generation of a transmembrane electric potential (voltage) depends on the selective movement of ions across a semipermeable membrane. In this experimental system, a membrane separates a 15 mM NaCl/150 mM KCl solution (left) from a 150 mM NaCl/15 mM KCl solution (right); these ion concentrations are similar to those in cytosol and blood, respectively. If the membrane separating the two solutions is impermeable to all ions (a), no ions can move across the membrane, and no electric potential is registered on the potentiometer connecting the two solutions. If the membrane is selectively permeable only to Na+ (b) or only to K+ (c), then diffusion of these ions through their respective channels leads to a separation of charge across the membrane. At equilibrium, the membrane potential caused by the charge separation becomes equal to the Nernst potential ENa or EK registered on the potentiometer. See the text for further explanation.
Fig2. Overview of membrane transport proteins. Gradients are indicated by triangles with the tip pointing toward lower concentration, electric potential, or both. 1 Channels permit movement of specific ions (or water) down their electrochemical gradient. 2 Trans porters, which fall into three groups, facilitate movement of specific small molecules or ions. Uniporters transport a single type of molecule down its concentration gradient 2A. Cotransport proteins (symporters, 2B , and antiporters, 2C catalyze the movement of one molecule against its concentration gradient (black circles), driven by movement of one or more ions down an electrochemical gradient (red circles).3 Pumps use the energy released by ATP hydrolysis to power movement of specific ions or small molecules (red circles) against their electrochemical gradient. Differences in the mechanisms of transport by these three major classes of proteins account for their varying rates of solute movement.
Quantitatively, the usual resting membrane potential of –60 to –70 mV is close to the potassium equilibrium potential, calculated from the Nernst equation and the K+ concentrations in cells and surrounding media (see Table 1). Usually the resting membrane potential is slightly lower (less negative) than that calculated from the Nernst equation because of the presence of a few open Na+ channels. These channels allow the net inward flow of Na+ ions, making the cytosolic face of the plasma membrane more positive—that is, less negative—than predicted by the Nernst equation for K+. The K+ concentration gradient that drives the flow of ions through resting K+ channels is generated by the Na+/K+ ATPase described previously (see Figures 3 and 4). In the absence of this pump, or when it is inhibited, the K+ concentration gradient cannot be maintained, the membrane potential falls to zero, and the cell eventually dies.
Fig3. Multiple membrane transport proteins function together in the plasma membrane of metazoan cells. Gradients are indicated by triangles with the tip pointing toward lower con centration. 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 mem brane, 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 mem brane potential, which together power the influx of Na+ ions.
Fig4. Operational model of the plasma-membrane Na+/K+ ATPase. Only one of the two catalytic α subunits of this P-class pump is depicted. It is not known whether just one or both subunits in a single ATPase molecule transport ions. Ion pumping by the Na+/K+ ATPase involves phosphorylation, dephosphorylation, and conformational changes similar to those in the muscle Ca2+ ATPase. In this case, hydrolysis of the E2–P intermediate powers the E2 → E1 conformational change and concomitant transport of two K+ ions inward. Na+ ions are indicated by red circles; K+ ions, by purple squares; high-energy acyl phosphate bond, by ~P; low-energy phosphoester bond, by –P.
Although resting K+ channels play the dominant role in generating the electric potential across the plasma membranes of animal cells, this is not the case in bacterial, plant, and fun gal cells. The inside-negative membrane potential in plant and fungal cells is generated by transport of positively charged protons (H+) out of the cell by ATP-powered proton pumps, a process similar to what occurs in lysosomal membranes lacking Cl− channels (see Figure 5a): each H+ pumped out of the cell leaves behind a Cl− ion, generating an inside-negative electric potential across the membrane. In aerobic bacterial cells, an inside-negative potential is generated by outward pumping of protons during electron transport, a process simi lar to proton pumping in mitochondrial inner membranes.
Fig5. Effect of V-class proton pumps on H+ concentration gradients and electric potential gradients across cellular membranes. (a) If an intracellular organelle contains only V-class pumps, proton pumping generates an electric potential across the membrane (the cytosolic face becomes negative and the luminal face positive), but no significant change in the intraluminal pH. (b) If the organelle membrane also contains Cl− channels, anions passively follow the pumped protons, resulting in an accumulation of H+ and Cl− ions in the lumen (low luminal pH) but no electric potential across the membrane.
The electric potential across the plasma membrane of a cell can be measured with a microelectrode inserted into the cell and a reference electrode placed in the extracellular fluid. The two electrodes are connected to a potentiometer capable of measuring small potential differences (Figure 6). The potential across the plasma membrane of most animal cells generally does not vary with time. In contrast, neurons and muscle cells—the principal types of electrically active cells— undergo controlled changes in their membrane potential.
Fig6. The electric potential across the plasma membrane of a live cell can be measured. A microelectrode, constructed by filling a glass tube of extremely small diameter with a conducting fluid such as a KCl solution, is inserted into a cell in such a way that the plasma membrane seals itself around the tip of the electrode. A reference electrode is placed in the extracellular medium. A potentiometer connecting the two electrodes registers the potential—in this case, –60 mV, with the cytosolic face negative with respect to the exoplasmic face of the membrane. A potential difference is registered only when the microelectrode is inserted into the cell; no potential is registered if the microelectrode is in the extracellular fluid.
