ENERGETICS AND THERMODYNAMICS IN BIOTECHNOLOGY
In a recent meeting on a Swiss alp, the thermodynamic
efficiency of microbial growth was rediscussed
and compared to efficiencies of other machines.
The issue of defining efficiency
in terms of Gibbs energy, in terms of enthalpy came
up, and so did the issue of the reference state to be taken.
For choices of the reference state outside
the system itself (such as the state of complte combustion
of all substrates and products),
the efficiency becomes a number that does depend
on that reference state.
This is done in the definition of Roels.
Battley takes the product of the
catabolic reactions as the reference state. This at least
is defined in terms of the growth reactions itself, hence
depends on the system itself. It is metabolically
irrealistic in many cases however,
as the anabolic substrate (say glucose) is not
usually degraded all the way to the catabolic product (say CO2)
before it is converted to biomass.
Westerhoff takes the difference between catabolic substrate and
catabolic product as the input free energy, and the difference between
anabolic product (=biomass) and anabolic substrate (say glucose)
as the output free energy difference.
This is certainly independent of any external
refernec state (because differences are considered),
and biochemically a bit more realistic, BUT
something is undefined
in cases of single substarte groth in
the sensne of chosing whether
one makes the oxygen balance with oxygen or with CO2.
This gives two different numbers (both of which may
make snese by the way).
Moreover, they may give rise to the situation
that in the calculations oxygen ppears to be produced in the
anabolic reaction, which is again biochemially
irrealistic.
Also the term exergy was discussed in this relation ship.
This is H-To.S where T0 is then lowest temperature in the system.
With this choice, exergy is a funciton of the system itself and
indicates the free energy one may do at the "lowest point in the system
with any heat stream at a higher temperature".
This corresponds somewhat with Battleys definition, or at least with a
definition
where one would take the free energy of the lowest free energy
compound in the system as the reference state.
It is also of interest to define a different Exxergy, which uses for T0
some temp[erature outside the system,
of fystem, for instance of the house one
wishes to heat
with heat flowing in the
factory. This corresponds to the Roels definition of growth efficiency.
If there is any further interest in these issues, please
let me know:
Hans V. Westerhoff
HW at NKI.NL
+31 20 512 2029 (FAX)
