264
Chapter 7
Fig. 7.11 The influence of pH on the maximum specific growth rate of the filamentous fungus
Aspergillus oryzae.
The line is simulated using eq, (7.31) with £i=4T0'3, AT2=2'10'8, and &e,o,=0.3 h'1.
where
e
is taken to be the active form of the enzyme while the two other forms are assumed to be
completely inactive. With AT
, and
K2
being the dissociation constants for
e
and
e
respectively.
The fraction of active enzyme
e
is calculated to be:
e
_
1
e,„
~
(7.30)
and the enzyme activity is taken to be
k^k^e.
If the cell acticity is determined by the activity of
the enzyme considered above the maximum specific growth rate will be:
~
i+J/t J /a:,
+ k 2
/[tt]
(7‘31^
Although the dependence of cell activity on pH cannot possibly be explained by this simple model
it is, however, found that Eq. (7.31) gives an adequate fit for many microorganisms, as illustrated in
Fig. 7.11, where data are shown for
Aspergillus oryzae.
From the profiles of the maximum specific
growth rate as a function of pH it is observed that the optimum is relatively broad. Thus the cell
activity does not change much when one moves one pH unit away from the optimum, but larger
deviations from the optimum pH leads to drastic reductions in the microbial activity.
Microbial cells have a remarkable ability to maintain the intracellular pH at a constant level even
with large variations in the pH of the extracellular medium, but only at the expense of a significant
increase in the maintenance demands, since Gibbs free energy has to be used for maintaining the
proton gradient across the cell membrane. This ability of the microbial cells to function at
suboptimal environmental pH may, however, be destroyed by the presence of
uncoupling agents
,
e.g., organic acids (see Example 2.1). At a low extracellular pH, organic acids present in the
medium may be transported by passive diffusion into the cell, where they are dissociated at the
higher intracellular pH. The protons have to be pumped back to the extracellular medium, and this
requires a considerable amount of ATP, which would otherwise be utilized for growth or other
useful purposes. At high concentrations of the undissociated acids, the cell machinery breaks down
with fatal outcome for the cell.
previous page 287 Bioreaction Engineering Principles, Second Edition  read online next page 289 Bioreaction Engineering Principles, Second Edition  read online Home Toggle text on/off