  Liquid-liquid phase separation (LLPS) is currently of great interest in cell
biology. LLPS is an example of what is called an emergent phenomenon -- an idea
that comes from condensed-matter physics. Emergent phenomena have the
characteristic feature of having a switch-like response. I show that the Hill
equation of biochemistry can be used as a simple model of strongly cooperative,
switch-like, behaviour. One result is that a switch-like response requires
relatively few molecules, even ten gives a strongly switch-like response. Thus
if a biological function enabled by LLPS relies on LLPS to provide a
switch-like response to a stimulus, then condensates large enough to be visible
in optical microscopy are not needed.
