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Chapter 7
growth rate of the original strain. Occurrence of spontaneous mutants is a phenomenon often observed for
filamentous fungi, especially when they are grown in continuous cultures for long periods, and the resulting
mutants are normally referred to as
colonial mutants.
_____________________________________________
In case the growth of the individual morphological forms is to be described by an intracellularly
structured model the complexity of morphologically structured models increases substantially. It is
possible to derive a general mass balance for the concentration of the intracellular variables in the
different morphological forms (Nielsen and Villadsen, 1994), but as these equations have limited
practical use we will not elaborate on this topic. Instead we will focus on two different applications
of morphologically structured models: quantitative description of oscillating yeast cultures and
growth of filamentous microorganisms.
7.6.1 Oscillating Yeast Cultures
Whereas the division of unicellular bacteria is symmetric, with the formation of two almost
identical cells, the cell division of yeast is asymmetric, with formation of a so-called mother and a
so-called daughter cell (see Fig. 7.17). The daughter cell is converted to a mother cell within the
time fd. After a period of maturation
(tm)
a new bud emerges on the mother cell, and after a further
time period rb the bud has obtained a critical size, resulting in cell division. At cell division a
bud
scar
is formed on the cell envelope of the mother cell, and it is believed that the cell cannot form a
new bud at this position.
td, tm,
and
ib
are functions of the cellular composition (and therefore also of
the environmental conditions).
daughter cell
mother cell
bud emerges
cell division
G1
+—
+—
S
-----------------------1
-------------1
---------1
G2
M
Figure 7.17 The cell cycle of budding yeast. The period of maturation (i.e.,
td + tm
for a daughter cell and 4.
for a mother cell) is called the
Gl-phase,
and in this phase the cells prepare for budding (e.g., carbohydrate
storage is built up). The budding itself (lasting fb) consists of three phases. First the chromosomes are
duplicated in the
S-phase.
Thereafter the cells prepare for cell division (the
G2-phase),
and finally they enter
the
M-phase
(M for mitosis), where cell division occurs.
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