All substances exhibit constant
random motion at the
microscopic scale. This is a direct consequence of
thermal agitation, and leads to
diffusion of molecules and small particles in a
liquid. In addition to this nondirected motion,
living cells also use
active transport mechanisms, such as motor activity and
polymerization forces that depend on linear
biopolymers and are therefore fundamentally directed in nature. Nevertheless, it has become increasingly clear that such active processes can also drive significant
random fluctuations that can appear surprisingly like
thermal diffusion of particles, but faster. Here, we discuss recent progress in quantifying this behavior and identifying its origins and consequences. We suggest that it represents an important and biologically tunable mechanism for transport in
living cells.
