11.3. Mitochondria as factories[[Student version, January 17, 2003]] 433
- The need to give macromolecules an overall net negative charge, in order to avert a clumping
catastrophe (Section 7.4.1 on page 229), and - The need to maintain osmotic balance, or osmoregulate, in order to avoid excessive internal
pressure (see Section 11.2.1).
This chain of logic may well explain why ion pumps evolved in the first place: to meet a challenge
posed by the physical world.
But evolution is a tinkerer. Once a mechanism evolves to solve one problem, it’s available to
bepressed into service for some totally different purpose. Ion pumping implies that the resting,
or steady, state of the cell is not in equilibrium, and so is not a state of minimal free energy.
That is, the resting state is like a charged battery, with available (free) energy distributed all over
the membrane. We should think of the ion pumps as a “trickle charger,” constantly keeping the
battery charged despite “current leaks” tending to discharge it. Section 11.3.3 showed one useful
cellular function that such a setup could perform: the transmission of useful energy among machines
embedded in the mitochondrial membrane. In fact, the chemiosmotic mechanism is so versatile that
it appears over and over in cell biology.
Proton pumping in chloroplasts and bacteria Chapter 2 mentioned a second class of ATP-
generating organelles in the cell, the chloroplasts. Chloroplasts capture sunlight and use its free
energy to pump protons across their membrane. From this point on, the story is similar to that in
Section 11.3.3: The proton gradient drives a “CF0CF1” complex similar to F0F1 in mitochondria.
Bacteria, too, maintain a proton gradient across their membranes. Some ingest and metabolize
food, driving proton pumps related to, though simpler than, the ones in mitochondria. Others, for
example the salt-lovingHalobacterium salinariumcontain a light-driven pump, bacteriorhodopsin.
Again, whatever the source of the proton gradient, bacteria contain F0F1 synthases quite similar
to those in mitochondria and chloroplasts. This high degree of homology, found at the molecular
level, lends strong support to the theory that both mitochondria and chloroplasts originated as
free-living bacteria. At some point in history, they apparently formed symbiotic relations with
other cells. Gradually the mitochondria and chloroplasts lost their ability to live independently, for
example losing some of their genomes.
Other pumps Cells have an array of active pumps. Some are powered by ATP: For example,
the calcium ATPase, which pumps Ca++ions out of a cell, plays a role in the transmission of nerve
impulses (see Chapter 12). Others pull one molecule against its gradient by coupling its motion to
the transport of a second speciesalongits gradient. Thus for example the lactose permease allows
aproton to enter a bacterial cell, but only at the price of bringing along a sugar molecule. Such
pumps, where the two coupled motions are in the same direction, are generically called symports.
Arelated class of pumps, coupling an inward to an outward transport, are called antiports. An
example is the sodium–calcium exchanger, which uses sodium’s electrochemical potential gradient
to force calcium out of animal cells (see Problem 11.1).
The flagellar motor Figure 5.9 on page 157 shows the flagellar motor, another remarkable molec-
ular device attached to the power busbar ofE. coli.Like F0, the motor converts the electrochemical
potential jump of protons into a mechanical torque; Section 5.3.1 on page 153 described how this
torque turns into directed swimming motion. The flagellar motor spins at up to 100 revolutions per