Biological Physics: Energy, Information, Life

11.3. Mitochondria as factories[[Student version, January 17, 2003]] 425

generation:

a

b

coal firebox

∆T

boiler

water

steam

turbine

torque

torque

• generator

high-p.e.

electrons

low-p.e.

electrons

distribution and utilization:

heater

electrolytic cells

H 2 ,O 2

H 2 O

motor

drive

shaft

Figure 11.7: (Schematic.) An imagined industrial process. (a)Chemical fuel is burned, ultimately creating a
difference in the electrical potential of electrons across two wires. The difference is maintained by electrical insulation
(in this case air) between the wires on the far right. (b)Inside a factory, the electrons are used to drive an uphill
chemical process, converting low-energy molecules to ones with high stored chemical energy. The latter can then be
loaded into an automobile to generate torque and do useful work. If desired, some of the electrons’ potential energy
can be converted directly to thermal form by placing a resistor (the “heater”) across the power lines.

mechanical energy to the various machines attached to it. Later, the invention of electrical tech-
nology allowed a more flexible energy currency, the potential energy of electrons in a conductor.
With this system, the initial conversion of chemical energy (for example, in coal) to electricity could
occur many kilometers away from the point of use in the factory. Within the factory, distribution
could be accomplished using abusbar,alarge conducting bar running through the building, with
various machines attached to it.
Figure 11.7 sketches a factory of a sort that may one day supply hydrogen-powered automobiles.
Some high-energy substrate, like coal or uranium, comes in at the left. A series of transductions
converts the incoming free energy to the potential energy of electrons; the electrons themselves
are recirculated. In the factory, a busbar distributes the electricity to a series of electrolytic cells,
which convert low-energy water molecules to high-energy hydrogen and oxygen. The hydrogen gets
packaged and delivered to cars, which burn it (or convert it directly to electricity) and generate
useful work. In winter, some of the electricity can instead be sent through a resistor, doing no
mechanical work but warming up the factory for the comfort of those working inside it.
The next subsections will discuss the close parallels between the simple industrial process just
described and the activity of mitochondria.

11.3.2 The biochemical backdrop to respiration

The overall biochemical process we wish to study is one ofoxidation.Originally this term referred
to the chemical addition of oxygen to something else, and indeed you breathe in oxygen, attach it to
high-energy compounds with carbon and hydrogen, and exhale low-energy H 2 Oand CO 2 .Chemists
have found it useful, however, to generalize the concept of oxidation, in order to identify individual
subreactions as oxidation or the opposite process,reduction. According to this generalization,