Semipermeability also forms the condition of another fundamentally important property of the living cell, namely, its possession of highly characteristic electrical properties. Physical chemists find in general that if a solid film or other partition consisting of any sufficiently impermeable material—e. g., glass, an organic membrane or a precipitation membrane of copper ferrocyanide or similar material—is interposed between two different solutions containing electrolytes which are thus prevented from mixing, a permanent difference of electrical potential arises between the two solutions. The same appears also to be true of the protoplasmic surface-films or membranes. Apparently, so long as the plasma membrane preserves its normal semi-permeability there exists a considerable difference of electrical potential between its external and internal surfaces—i. e., between the exterior and the interior of the cell—dependent on the difference in composition between the protoplasm and its surroundings. Thus the exterior of a resting muscle cell or nerve fibre is always found positive relatively to its interior. But with the loss of semipermeability at death this potential difference—or demarcation-current potential—also disappears. It thus evidently depends upon the semipermeability of the plasma membrane; and since this electrically polarized condition of the membrane is undoubtedly a factor of prime importance in many cell activities, including stimulation, we see again how physiologically essential a property this semipermeability of the plasma membrane may be.[1]
Further, there is little doubt that this property is one of the essential conditions on which the possibility of stimulation depends. Nernst has shown that an electric current stimulates by changing the concentration of ions at the semipermeable membranes of the irritable tissue; this is equivalent to producing a potential-difference or electrical polarization between the outer and inner surfaces of the membrane. The normal preexisting or physiological potential difference is thus altered—in stimulation is typically diminished—and when this change is sufficiently extensive and rapid the tissue gives its characteristic response. Now these polarization-effects depend on semipermeability, since if the membrane allowed all ions to pass freely the differences of concentration in which the polarization depends evidently could not arise. We find in fact that the cell whose membranes have lost their semipermeability does not respond to stimulation. Such a cell is "dead"; this however need not mean that all vital manifestations have ceased; many metabolic processes may in fact continue in dead cells and lead to far-reaching chemical transformation of the cell-constituents; such changes are called "autolytic." What is lost is the power of responding to stimulation; hence the automatic regulation of the vital processes ceases, and presently these
- ↑ Cf. my article on the rôle of membranes in cell processes in The Popular Science Monthly for February, 1913.
