All biologic membranes are characterized by transmembrane electrical potential differences, which are, in almost all instances, oriented so that the cell interior is electrically negative with respect to the extracellular compartment. The size of the electrical (membrane) potential difference, Vm, ranges from —10 mV to as high as about —100 mV. Furthermore, in a number of cell types, Vm is variable, and this variation is responsible for the propagation of signals by nerve tissue, contraction of muscle, and stimulus-secretion coupling in exocrine and endocrine secretory cells.
What Is the Origin of Vm?
To appreciate how transmembrane electrical potential differences arise as a consequence of the interaction between pumps and leaks, let us consider a hypothetical cell, such as that illustrated in Fig. 22, which contains a coupled potassium acetate (KAc) pump, energized by the hydrolysis of ATP, and leak pathways for K+ and Ac—. Let us assume that this cell is initially filled with distilled water and is then dropped into a solution with a concentration [KAc]o = 10 mmol/L. Initially, KAc will enter the cell, some via the pump and some via diffusion through the leaks. When the intracellular concentration reaches 10 mmol/L, diffusion of K+ and Ac— into the
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