The Membrane Potential in Animal Cells Depends Largely on Resting K Channels

The plasma membranes of animal cells contain many open K+ channels but few open Na+, CP, or Ca2+ channels. As a result, the major ionic movement across the plasma membrane is that of K+ from the inside outward, powered by the K+ concentration gradient, leaving an excess of negative charge on the inside and creating an excess of positive charge on the outside, similar to the experimental system shown in Figure 7-13c. Thus the outward flow of K+ ions through these channels, called resting K+ channels, is the major determinant of the inside-negative membrane potential. Like all channels, these alternate between an open and a closed state, but since their opening and closing are not affected by the membrane potential or by small signaling molecules, these channels are called nongated. The various gated channels discussed in later sections open only in response to specific ligands or to changes in membrane potential.

Quantitatively, the usual resting membrane potential of — 70 mV is close to but lower in magnitude than that of the potassium equilibrium potential calculated from the Nernst equation because of the presence of a few open Na+ channels. These open Na+ 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

▲ EXPERIMENTAL FIGURE 7-14 The electric potential across the plasma membrane of living cells 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 surface membrane seals itself around the tip of the electrode. A reference electrode is placed in the bathing medium. A potentiometer connecting the two electrodes registers the potential, in this case —60 mV. A potential difference is registered only when the microelectrode is inserted into the cell; no potential is registered if the microelectrode is in the bathing fluid.

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 Figure 7-9). In the absence of this pump, or when it is inhibited, the K+ concentration gradient cannot be maintained and eventually the magnitude of the membrane potential falls to zero.

Although resting K+ channels play the dominant role in generating the electric potential across the plasma membrane of animal cells, this is not the case in plant and fungal cells. The inside-negative membrane potential in these cells is generated by transport of H+ ions out of the cell by P-class proton pumps (see Figure 7-10a).

The potential across the plasma membrane of large cells can be measured with a microelectrode inserted inside the cell and a reference electrode placed in the extracellular fluid. The two are connected to a potentiometer capable of measuring small potential differences (Figure 7-14). In virtually all cells the inside (cytosolic face) of the cell membrane is negative relative to the outside; typical membrane potentials range between — 30 and —70 mV. The potential across the surface 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 that we discuss later.

▲ Figure 7-15 Structure of resting K+ channel from the bacterium Streptomyces lividans. All K+ channel proteins are tetramers comprising four identical subunits each containing two conserved membrane-spanning a helices, called by convention S5 and S6 (yellow), and a shorter P or pore segment (pink). (a) One of the subunits, viewed from the side, with key structural features indicated. (b) The complete tetrameric channel viewed from the side (left) and the top, or extracellular, end (right). The P segments are located near the exoplasmic surface and connect the S5 and

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