Thermal diffusion is the mediator for cellular computation
Brownian motion in a cell due to thermal fluctuations is the main mediator, the “substrate” of cellular computation.
References:
Bray, Wetware (2009), p. 55-56
The molecules of a fluid move continually at high speeds, colliding with each other frequently so that they are continually knocked off course. Their movements are due to heat energy, and their average speed is directly related to the temperature. At very low temperatures the molecules of water hardly move, being locked into the rigid structure of ice. But as the temperature rises, their agitation increases and they break free to form first a liquid and then, if the heating continues, a gas. A large particle suspended in water at room temperature experiences many tiny impacts at every instant, but their effects are relatively small and mostly cancel out: the particle barely stirs. But for a particle the size of a bacterium or smaller, the effects are far greater, and the particle is displaced this way and that. For an even smaller particle, say the size of a protein molecule, the motion becomes insanely vigorous. Dancing violently, the molecule skips along a meandering path from its starting point. The result is diffusion.
The interior of living cells is a strange environment-rather like a chunky soup or semisolid slurry. There are membranes and large particles of all sizes, including the nucleus, and the bacterium-sized mitochondria that generate energy. If these large structures move at all, it is with a relatively slow and orderly motion driven by molecular reactions. But the watery contents of the cell, the dissolved salts and unattached molecules, are in continual motion. They are constantly stirred by thermal diffusion. If you introduce a fluorescent marker in one region of a cell and then watch it under a special microscope, it will instantly bleed into the rest. A small molecule such as a sugar can cross from one side of a human cell to the other in a tenth of a second; a protein might make the same journey in a second. Diffusion is a very effective way to move over small distances.
Because of thermal diffusion, any two molecules in a cell have a good chance of meeting each other within a short space of time. Although their encounter is usually fleeting and inconsequential, it can lead to a meaningful change. For example, molecules might agglomerate into a larger body. Or if one molecule is an enzyme (a protein that performs a specific reaction), it might provoke a chemical change in the other molecule. From the standpoint of cellular computations, diffusion provides the connections. It is a miniature World Wide Web that allows every molecule to address every other molecule in a short space of time.