Summary: of a brownian ratchet. Thermal fluctuations
of the flexible actin filaments expose the tip
long enough for a new monomer to attach.
Once the tip springs back, it ratchets the
membrane forward by the length of one
monomer. Brownian ratchets have found a
life of their own in the physics literature; it is
Granted that cells move, how do they
know where to go? Under some circum-
multicoloured fluorescent proteins seems
to indicate a more or less random walk (R.
Lansford, California Inst. Technol.). But in
vitro studies of nerve cells known as astro-
cytes seem to show directional streaming
motions into unpopulated areas (A. Czirók,
Eötvös Univ., Budapest). M. Abercrombie