Intracellular transport by active diffusion
Entry: Chia Wei Hsu, AP 225, Fall 2010
Clifford P. Brangwynne, Gijsje H. Koenderink, Frederick C. MacKintosh and David A. Weitz, "Intracellular transport by active diffusion" , Trends in Cell Biology 19 (9), 423-427 (2009).
This article concerns cell mechanics. The main point of this article is that active transports in cells can result in significant random fluctuations of particles that resemble thermal fluctuations. These particles undergo an enhanced diffusion, which the authors refer to as "active diffusion".
Background: Thermal diffusion and random intracellular motion
Thermal agitation causes molecules or small particles to perform random walk in a solution. This is referred to as Brownian motion or diffusive motion. Although diffusion is not directional, it acts as an important mechanism for short-distanced transports in cells and provides the basis for signal transduction networks.
Thermal-driven diffusion follows well-known physical laws. The diffusion constant increases with temperature, and decreases with the particle size and the medium viscosity. However, a number of intracellular motions appear to be random and diffusive-like, but do not follow these basic properties of thermal diffusion. This suggests that there must be other mechanism that contributes to the random transport of particles in cells.
In cells, there is another kind of transport. These are active, directional transports driven by either the motion of motor proteins along cytoskeletal filaments (kinesins and dyneins run on microtubules; myosins run on actin), or the polymerization/de-polymerization of these filaments. These directed transports are distinct from the random diffusive transport.
This article discusses how the active transports can drive random diffusive-like transports.