430 Chapter 11. Machines in membranes[[Student version, January 17, 2003]]
Figure 11.9: (Schematic.) Mechanism of oxidative phosphorylation. Electrons are taken from NADH molecules
and transferred down a chain of carriers (black dots), ultimately ending up on an oxygen atom in water. Two of
the membrane-bound enzymes shown couple this process to the pumping of protons across the inner mitochondrial
membrane, seen in cross section. Protons then flow back through the F0F1 complex (right), which synthesizes ATP.
See also the more realistic depiction of these objects in Figure??on page??.[From Goodsell, 1993.]
Membrane as electrical insulation It is possible to rip apart the mitochondrial membrane into
fragments (using ultrasound), without damaging the individual proteins embedded in it. Ordinarily
these fragments would reassemble into closed vesicles, because of the high free energy cost of a bilayer
membrane edge (see Section 8.6.1), but this reassembly can be prevented by adding a detergent.
The detergent, a one-chain amphiphile, protects the membrane edges by forming a micelle-like rim
(Figure 8.8 on page 285). When such fragments were made from the mitochondrial inner membrane,
they continued to oxidize NAD+,but lost the ability to synthesize ATP.The loss of function is easy
to understand in the light of the chemiosmotic mechanism: In a membrane fragment, the electrical
transmission system is “short-circuited”; protons pumped to one side can simply diffuse to the other
side.
Similarly, introducing any of a class of membrane channel proteins, or other lipid-soluble com-
pounds known to transport protons short-circuits the mitochondrion, cutting ATP production.
Analogous to the electric heater shown in Figure 11.7, such short-circuiting converts the chemical
energy of respiration directly into heat. Some animals engage this mechanism in the mitochondria of
“brown fat” cells, when they need to turn food directly into heat (for example, during hibernation).
Operation of the ATP synthase Wehave seen that an elaborate enzymatic apparatus accom-
plishes the oxidation of NADH and the associated proton pumping. In contrast, the ATP synthase
turned out to be remarkably simple. As sketched in Figure 11.10a, the synthase consists of two
major units, calledF0andF1. The F0 unit (shown as the elements a, b, and c in the figure) is
normally embedded in the inner mitochondrial membrane, with the F1 unit (shown as the elements