Page 80 - MODERN ELECTROCHEMISTRY

24 CHAPTER 1

glucose oxidase. The result was duly electrifying—the enzyme became radically more
conducting than before.

1.10. SPECULATIVE ELECTROCHEMICAL APPROACH TO
UNDERSTANDING METABOLISM
We eat and various biochemical processes produce glucose from some of the food
molecules. We breathe and the oxygen changes the glucose to gluconic acid, and
finally by way of the ubiquitous enzymes, to On the way, we get mechanical
energy to operate our muscles, including the heart pump, which drives the fuel-carry-
ing blood around the body circuit. This is metabolism, and one thing about it that is
not well understood is why the energy conversion (chemical energy of the oxidation
of food to mechanical energy of the body) is so efficient (50%), compared with the
efficiency of a normal heat engine (~25%).
The reaction that gives chemical energy to heat engines in cars (hydrocarbon
oxidation) has to obey the Carnot cycle efficiency limitation With
around body temperature (37 °C), the metabolic would have to be 337 °C to
explain this metabolic efficiency in terms of a heat engine. Thus, the body energy
conversion mechanism cannot use this means to get the energy by which it works.
However, there are electrochemical energy converters (fuel cells), such as the one
shown in Fig. 1.6. An electrochemical energy converter is not restrained by the Carnot
efficiency of 25% and can have efficiencies up to for the heat content and free
energy changes in the oxidation reactions involved in digesting food. This ratio is often
as much as 90% (cf. Chapter 13).
Hence, to explain the high metabolism of > 50%, we are forced into proposing an
electrochemical path for metabolism. How might it work? Such a path was proposed
by Felix Gutmann in 1985 and the idea is shown in a crude way in Fig. 1.13.
Mitochondria are tiny systems found in every biological cell, and they are known
to be the seat of the body’s energy conversion. Suppose one could identify certain
organic groups on the mitochondria as electron acceptors and other groups as electron
donors—microelectrodes, in fact. Glucose diffuses into the cell and becomes oxidized
at the electron acceptors. At the electron donor groups, is reduced using the
electrons provided by the glucose. The mitochondrion has made millions of micro fuel
cells out of its two kinds of electrodic groups and now has electrical energy from these
cells to give—and at an efficiency typical of fuel cells of ~50%.
There is much more to the story—how energy in living systems is stored, for
example, and finally how it is transported to all the body parts which use it—this is
discussed in Chapter 14. This electrochemical (and vectorial) approach to metabolism,
which was proposed in 1986, is not yet widely accepted by tradition-bound biologists,
but it has one tremendous thing going for it—it solves the problem of why the
efficiency of the conversion of the chemicals in food to mechanical energy is so much
higher than it can be in alternative energy conversion pathways.