538
Bacterial Growth and Division
around the genome to the opposite end of
thegenomewherereplicationterminates
.
3.3
Surface Synthesis during the Division Cycle
3.3.1
Peptidoglycan Synthesis during the
Division Cycle
The cell surface is made to perfectly
enclose, without excess or deFcit, the
cytoplasm synthesized by the cell. Cell
cytoplasm increases continuously during
the division cycle (described above), and
therefore the cell surface is made contin-
uously. How is the bacterial cell surface
madeanddup
l
ica
teddu
r
ingthed
iv
is
ion
cycle? What is the rate and topography of
cell-surface synthesis during the division
cycle?
Consider an imaginary cell in which the
cytoplasm is enclosed in a cell-surface tube
that is open at each end; the cytoplasm
remains within the bounds of the tube.
T
h
ec
y
t
o
p
l
a
smi
nt
h
en
ew
b
o
r
nc
e
l
li
s
thus
encased in the cylinder of cell
surface that is made up of membrane
and
peptidoglycan.
As
the
cytoplasm
increases exponentially (see above), the
tube length increases to exactly enclose
the newly synthesized cytoplasm. In this
imaginary case, the cell surface increases
exponentially in the same manner as
the cytoplasm. When the cell cytoplasm
doubles, the tube divides into two new cells
and the cycle repeats. In this imaginary
cell, the cytoplasm increases exponentially,
the internal volume of the cell increases
exponentially, the surface area increases
exponentially, and the density of the cell,
that is, the total weight of the cell per cell
volume, is constant during the division
cycle.
However, a real rod-shaped cell does
have ends, and therefore the pattern of cell-
surface synthesis during the division cycle
is not simply exponential. If the cell sur-
face were synthesized exponentially, the
cell volume cannot increase exponentially.
Since the cytoplasm increases exponen-
tially, there would have to be a change in
cell density. (In theory, it may be that
major changes in cell shape could al-
low a pure exponential increase in cell
mass and cell surface, but such changes
are clearly not observed during the cell-
division cycle.) Cell density, however, is
constant during the division cycle. A pro-
posal for cell-surface synthesis that allows
an exponential increase in cell volume,
and therefore a constant cell density, is
presented in ┬▒ig. 4. Before invagination,
the cell grows only in the cylindrical side-
wall. After invagination, the cell grows
in the pole area and the sidewall. Any
volume required by the increase in cell
cytoplasm that is not accommodated by
pole growth is accommodated by an in-
crease in sidewall. The cell is considered a
pressure vessel, and when the pressure in
the cell increases, there is a corresponding
increase in cell surface area. The pole is
preferentially synthesized and any resid-
ual pressure due to new cell cytoplasm
that is not accommodated by pole growth
is relieved by an increase in cylindrical
sidewall area.
The resulting pattern of synthesis is
approximately exponential. The formula
describing surface synthesis during the
division cycle is a complex one, including
terms for the shape of the newborn cell,
the cell age at which invagination starts,
the pattern of pole synthesis, and the age
of the cell. One consequence of this model
of surface synthesis is that at no time
is surface synthesis exponential, since the
rate of surface synthesis is not proportional
to the amount of surface present over any
period of time.
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