Membrane Potential Ionic Equilibrium

The central topics in Chapter 3 were the factors that influence the distribution of water across the plasma membrane and the strategies by which cells can attain osmotic equilibrium. For clarity, all the examples so far have used only uncharged particles; however, a glance at Table 2-1 in Chapter 2 shows that all the solutes of both ICF and ECF are electrically charged. For charged particles, movement across the membrane will be determined not only by their concentration gradients, but also by the electrical potential across the membrane. This chapter will consider how cells can achieve equilibrium in the situation where both diffusional and electrical forces must be taken into account.

To illustrate the important principles that apply to ionic equilibrium, it will be useful to work through a series of examples that are increasingly complex and increasingly similar to the situation in real animal cells. At the end of the series of examples, we will see how a model cell, with internal and external compositions like those given in Table 2-1, could be in electrical and chemical equilibrium. However, we will also see that this equilibrium model of the electrochemical state of cells does not apply to real animal cells. Instead, real cells must expend energy to maintain the distribution of ions across the plasma membrane.

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