Thermodynamics of Bioreactions
109
-A G = 219-12.8*0.260+ 0.0048-401-0.02-541 -0.240*808 = 12.9 kJfm oleH ;)'1
It is seen that the heat of reaction is 5 times higher than the free energy change, and this is quite unusual.
The reason for the small AG is that much of the free energy contained in the substrate (H:) is recovered in
the product
(CH
4
).
The entropy change by (4.1) is consequently large and negative.
When
A H
and A G are calculated on the basis of biomass formed
-A H -
3160 kJ (C-mole biomass)1,
(-AG) = 645 kJ (C-mole biomass)'1
(4)
for a yield coefficient of
= 0.02 the heat of reaction is about 9 times higher than for respiratory growth of
yeast on glucose when compared on a C-mole biomass basis (see Example 4.5) while - AG is only 70%
higher than for respiratory growth of yeast on glucose. One must conclude that
Methanobacterium
thermoautotrophicum
grows on CO; and H3 with a very poor thermodynamic efficiency - and also that
anaerobic processes are certainly not always associated with a small heat of reaction.___________________
43
Non-equilibrium Thermodynamics
As mentioned earlier cellular systems are open systems where many processes operate far from
equilibrium. If they were at equilibrium there would be no flow through the many different
cellular pathways
and
the
cells
would
stop
functioning.
For many cellular processes
thermodynamic driving forces are used directly, e.g. in passive diffusion of substrates across the
cytoplasmic membrane, and even if the transport is mediated by a carrier as in facilitated
diffusion the driving force is still based on thermodynamics. When the flow is in the direction of
the thermodynamic force gradient it is referred to as
conjugate flow .
There are, however, also
some important cellular processes where the flow is
non-conjugate
, i.e. the flow is against a
thermodytiamic driving force. Clearly non-conjugate flow does not occur on its own, but through
tight coupling with a conjugate flow it is possible to drive processes against a thermodynamic
driving force. Non-equilibrium thermodynamics is an extension of classical thermodynamics to
non-equilibrium states. It supplies relationships between flows and thermodynamic driving
forces,
for
both
conjugate
and
non-conjugate
flows.
Additionally,
non-equilibrium
thermodynamics allows for a description o f processes where conjugate flows are tightly coupled
to non-conjugate flows.
An example o f a cellular process where conjugate and non-conjugate flows are coupled is the
oxidative phosphorylation (see Fig. 4.1), which is an essential life process for all animals and an
option for many microorganisms. In this process protons are transferred across the mitochondrial
membrane (the cytosolic membrane in bacteria) against a proton gradient. This transfer of
protons is driven by the oxidation o f NADH by oxygen, and there is consequently a tight
coupling of the non-conjugate flow o f protons across the membrane and the oxidation of the co-
factor NADH. Furthermore, the process is closely coupled to the phosphorylation of ATP, which
takes place by an ATPase that converts ADP and free phosphate into ATP. The process of ATP
generation is thermodynamically driven by the translocation of protons down the concentration
gradient.
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