Ionic Steady State

The Goldman equation represents the actual situation in animal cells. The membrane potential of the cell takes on a steady value that reflects a fine balance between competing influences. It is important to keep in mind that neither sodium ions nor potassium ions are at equilibrium at that steady value of potential: sodium ions are continually leaking into the cell and potassium ions are continually leaking out. If this were allowed to continue, the concentration gradients for sodium and potassium would eventually run down and the membrane potential would decline to zero as the ion gradients collapsed. It is like a flashlight that has been left on: the batteries slowly discharge.

To prevent the intracellular accumulation of sodium and loss of potassium, the cell must expend energy to restore the ion gradients. Here again is an important role for the sodium pump. Metabolic energy stored in ATP is used to extrude the sodium that leaks in and to regain the potassium that was lost. In this way, the batteries are recharged using metabolic energy. Viewed in this light, we can see that the steady membrane potential of a cell represents chemical energy that has been converted into a different form and stored in the ion gradients across the cell membrane. In Part II of this book, beginning with Chapter 6, we will see how some cells, most notably the cells that make up the nervous system, are able to tap this stored energy to generate signals that can carry information and allow animals to move about and function in their environment.

0 0

Post a comment