Ion Channels and Membrane Potential

Cells are demarcated by the cell membrane. The cell membrane is a lipid bilayer in which various protein molecules are embedded (see Figure 1). In all animal cells, there is a large difference in ionic concentration between inside and outside the cell (Table 1). Na concentration is higher outside the cell and the K concentration is higher inside the cell. Intracellular Ca concentration is kept extremely low compared to Na and K. The cell membrane itself, being made of lipids, has low ionic permeability and may be considered an insulator from an electrical standpoint. There are two natural questions that arises at this point. The first is how this concentration differential is maintained. The second is the reason why the cell may want to maintain such a concentration differential. The answer to the first question is that the ionic concentration difference across the cell membrane is maintained by ionic pumps. Ionic pumps are molecular machines that are embedded in the cell membrane, and use chemical energy (from ATP hydrolysis) to pump ions against a possibly adverse transmembrane electrochemical gradient. The most important of these is the Na-K ATPase, which pumps Na ions out of the cell and pumps K ions into the cell. The above concentration gradients of Na and K are largely maintained by this pump. The answer to the second question is that this is necessary for the cell to control its volume. Before we can begin to understand this (which we shall