Biology Reference
In-Depth Information
increasingly serious problem in transporting the actin-profilin complexes from the main
body of the cell to the tip of the acrosomal process, where they are needed. There seems to
be no specific transport system in the acrosomal process, so transport has to rely on diffusion.
The flow of unpolymerized actin by diffusion can be approximated by the one-dimensional
form of Fick's law of diffusion, since the acrosomal process is essentially one-dimensional in
form once it has grown out some way:
dx
where J is the flow of monomer, D is the diffusion constant, C is the local concentration of
monomer, and x is the distance along the process. If the system were at steady state (which
it will not be
d
see below) the flow of actin when the acrosomal process had reached a length
L would be:
J
¼
D
:
dC
=
J
steady state
¼
D
: ð
C
cell
C
tip
Þ=
L
where C
tip
is the concentration of monomer at the tip of the process, and C
cell
is the concen-
tration in the reservoirs in the cell. The key point made by this equation is that, as the acro-
somal process grows longer, the rate of monomer delivery falls. In fact, the problem is worse
than this, because although C
tip
will remain approximately constant, being set by the associ-
ation constant of actin-profilin with the barbed end of the filament, C
cell
will decrease as the
cell's reserves of actin are used up (more precise models of the diffusion, which take this into
account, have been worked out).
54
T. briareus
seems to have evolved an adaptation to fight
back against the diminishing returns of the diffusion equation, by using water flows to main-
tain the effective concentration of actin monomers in the cellular stores. At the very begin-
ning of the acrosomal reaction, ion channels open and allow salt water into the
periacrosomal region, which doubles in volume (this halves the concentration of actin, which
may be important in preventing addition to the pointed end even in the presence of profilin).
As acrosome process formation proceeds, the ion channels close and the pumps that are nor-
mally active in the cell remove the salts, which action also draws out the water by osmosis.
The dwindling pool of unpolymerized actin at the base of the growing acrosomal process is
therefore confined in a diminishing volume of cytoplasm, so that its concentration is kept
higher than would otherwise be expected.
54
This solves the problem of diminishing effective
concentrations, though the limiting effect of increasing length on actin transport remains.
The fact that microfilaments are used as compression structures in the acrosomal process
creates a second potential problem for actin-profilin complexes even when they have reached
the tip: if the barbed ends of the filament are pushing against the plasma membrane to force it
forwards, how can new monomers fit on to those ends? The probable solution makes elegant
use of the random thermal movements inherent in all matter at micro-scales and gains from
movements that happen to be directed in the forward direction while blocking those in the
reverse direction. This action is analogous to that of an electronic rectifying diode, which
makes pulses of direct current that flows only one way by accepting phases of an alternating
current source that are polarized one way while rejecting those polarized the other way. For
this reason, the action mechanism of filament elongation is sometimes called 'rectification',
although it is more usually known by the term used in the rest of this topic: the Brownian
ratchet.
55
Brownian motion is the random movement of a small object in suspension and it
results from the averaged impacts of small molecules of water, and so on, that collide
