||"...if [researchers] approximate the geometry of the membrane
where polymerization occurs as a sphere of radius r=25 nm, a minimum value for the flux of proteins reaching its
surface can be estimated from the diffusion equation as
4pDArCmax, where D is the diffusion coefficient, A is Avogadro’s
number, and Cmax is the bulk concentration of each
protein. If [researchers] use a low estimate for D=2µm^2/sec in a
crowded environment, the following numbers of proteins
collide with the surface of the sphere each second: ~8000
actin monomers, 500 Arp2/3 complexes, 300 capping proteins,
and 15,000 cofilins. In the worst case, this is 2–30 times more than the numbers of proteins consumed in the reactions.
Thus [researchers'] assumption regarding diffusion is valid even
if crowding reduces diffusion up to one order of magnitude.
However, this is unlikely, because [they] estimate that the
actin network occupies <3% of the volume of the patch.
This idea agrees well with estimates of diffusion and actin
assembly in similar actin networks (Plastino et al., 2004
Rafelski et al., 2009)."