11.2. Ion pumping[[Student version, January 17, 2003]] 417
Figure 11.3: (Metaphor.) If water is continuously pumped to the upper reservoir, the fountain will come to a
nonequilibrium steady state. If not, it will come to a quasisteady state, which lasts until the reservoir is empty.
electrostatic and osmotic equilibrium could place a cell under large internal pressure, bursting or
at least immobilizing it.
We get a big clue that we’re finally on the right track when we put our nerve cell in the
refrigerator. Chilling a cell to just above freezing doesn’t change the absolute temperature very
much.But it does shut down the cell’s metabolism. Suddenly the cell loses its ability to maintain a
nonequilibrium sodium concentration difference. Moreover, the shut-down cell also loses its ability
to control its interior volume, orosmoregulate. When normal conditions are restored, the cell’s
metabolism starts up again and the interior sodium falls.
Certain genetic defects can also interfere with osmoregulation. For example, patients with
hereditary spherocytosis have red blood cells whose plasma membrane is much more permeable to
sodium than that of normal red cells. The affected cells must work harder than normal cells to pump
sodium out. Hence they are prone to osmotic swelling, which in turn triggers their destruction by
the spleen. Entropic forces can kill.
Alookahead This section raised two puzzles: Eukaryotic cells maintain a far-from-equilibrium
concentration drop of sodium, and they don’t suffer from the immense osmotic pressure predicted
byDonnan equilibrium. In principle both of these problems could be solved if, instead of being in
equilibrium, cells could constantlypumpsodium across their membranes, using metabolic energy.
Such active pumping could create a nonequilibrium, but steady, state.
Here is a mechanical analogy: Suppose you visit your friend and see in his garden a fountain. The
fountain could be supplied by a high tank of water (Figure 11.3). In that case it flows, converting
the gravitational potential energy of the water in the tank to kinetic energy (and ultimately heat),
until the tank is empty; that is, it drives to equilibrium. But if you watch the fountain for many